Lay the tapes out on the surface and cut them to the proper length. Leave a little extra for precise trimming later on.

Application of finishing tapes is the next step in the covering process. BY RON ALEXANDER

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Tapes come in several widths. The most common widths are pictured: 2 inch, 3 inch and 4 inch.

Poly-Brush is used to precoat the area where the tape will be applied. Thin the Poly-Brush as directed and brush one coat between the pencil lines you have drawn, indicating where the tape should be placed.

The last step we discussed in the fabric-covering process was attaching fabric to the wings and control surfaces. This attachment of fabric to wingribs, etc. is necessary to prevent the fabric from “ballooning up” during flight. Lift on a wing or control surface will create an upward force on fabric sufficient to actually raise the fabric off of the ribs. For this reason fabric must be adequately attached to the ribs. This is normally accomplished using rib-lacing cord to tie the fabric to the ribs. Alternate methods of attachment include screws, rivets and fabric clips. Whichever method you use, this step is important and must be done properly to ensure safety of flight. Once the attachment is made, it is necessary to place a strip of fabric tape over the area. You can picture how unsightly screws, rivets or rib-lacing cord would be if left uncovered. Adding tape also provides additional strength to the area. What material is used to make this tape, what widths of tape are available, where should they be applied, and how do you apply them to a surface? Those are the subjects of this month’s installment.

Roll Tape When an airplane is covered, numerous areas require fabric tape, referred to as “finishing tape.” The tape serves to reinforce or strengthen an area, protect fabric from abrasion and cuts, and is applied for cosmetic reasons. Tapes are made from the same polyester fabric that you will use to cover your airplane. The fabric-covering manufacturer cuts the fabric into tapes of varying widths, ranging

from 1 to 6 inches. The most common tape used on airplanes is 2 inches wide. The tape is actually cut leaving pinked edges, which are used to increase the amount of surface that can adhere to the aircraft fabric when the tape is cemented in place. The actual edge area that will be cemented in place increases over 40% using pinked-edge tape versus straightedge tape. Pinking allows the tape to be held more firmly in place than would be possible without it. Finishing tapes are cut from two different weights of fabric—light weight and medium weight. Light weight fabric weighs about 1.7 ounces per square yard, and medium weight about 2.7 ounces per square yard. Either of these tapes may be used with any weight of fabric. In other words, if you are using medium weight fabric, you may use either light

Just prior to applying the tape, brush one more coat of Poly-Brush over the marked area, and then lay the tape in place.

Photos: Richard VanderMeulen

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Fabric continued weight tapes or medium weight tapes. A number of people with covering experience find that the light weight tapes are easier to work with and provide a more pleasing appearance on a surface. The choice is yours. Tapes are normally cut lengthwise with the roll of fabric. These are referred to as “linear tapes.” After being cut they are then rolled into lengths of 25 to 50 yards, so when you buy a roll of tape from a supply house it will be either 25 or 50 yards long.

Curves Ahead It is often necessary to tape a curved area. How do you get the tapes to lay down neatly and adhere to the surface? There are a few tricks we will outline later, but you should be aware that special cut tapes, called “bias tapes,” are available just for curved surfaces. These special tapes are cut at 45° angles to the fabric weave, and they are available in

After laying the tape in place, you can go back over the tape lightly with a brush to make it smooth and brush out wrinkles.

various widths just as regular tapes. Fabric is milled in widths of approximately 70 inches, so it’s impossible to cut long lengths of tape on a bias. Consequently, bias tapes will have a sewn seam every 70

inches or so. This has obvious cosmetic disadvantages when you’re trying to use a bias tape over a long area. If you are taping the leading edge of a wing, for example, you can use linear cut tape for

This picture shows a tape that has been partially completed to illustrate the difference in appearance.

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If the Poly-Brush is drying quickly, you can lightly recoat an area as you work the tape.

most of the leading edge and then place bias tape around the wingtip area. More on this later. To summarize, finishing tapes are cut from light weight or medium weight fabric. Two-inch tapes are the standard width for most ribs and longerons, and you will use more 2-inch tape than any other width. Three-inch-wide tapes are used for leading or trailing edges of wings and control surfaces; 4- and 6-inch widths are used for larger leading edges of wings.

Where to Use Them Finishing tapes will be applied over all ribs that have been rib-laced or where screws or other attachment methods are used to secure the fabric. Any fabric seam that has been sewn together or glued using fabric cement requires a finishing tape to reinforce the area. Remember earlier discussions where we talked about overlapping and cementing fabric? The resulting seam where the fabric was overlapped should have at least a 2-inch tape placed over it. Other areas need to be reinforced for added strength, and using finishing tapes accomplishes this objective. For example, the very aft portion of a metal or wood wing’s leading edge requires a finishing tape. If this area is left without it, a spanwise crack will often develop. Trailing edges of wings and controls should be reinforced with finishing tapes. Leading edges also require taping. Longerons and stringers on a fuselage are usually taped. The leading edge of gear legs is another area where tape should be applied. In short, any area that is subject to abrasion or any area you think needs to be reinforced should be taped.

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Fabric continued

Selecting and Applying Tapes Selection of the right size and type of finishing tape is important. You will want to cover the area completely and neatly. As mentioned, 2-inch tape is normally used over wingribs. Leading and trailing edges of wings and control surfaces will often require a wider tape such as the 3- or 4-inch. Some leading edges will need to have 6-inch wide tapes applied for optimum protection. It depends entirely upon the size of the wing, but 2- or 3-inch tape is satisfactory for most trailing edges. Sewn seams should have at least 2-inch tapes applied over them. Where should you use bias-cut tapes? This is a personal choice. Some experienced coverers do not use bias-cut tapes at all. They work with linear tapes to cover curved areas. If the curve is fairly pronounced, such as on a rudder, I would recommend using bias tapes. Bias tapes are often used on wingtips as well. Remember that the seam found on a bias tape should be removed for cosmetic reasons. A bias cut tape will also become somewhat narrower when pulled tight around a curve. For this reason, you will normally want to use 3-inch bias tapes. A 3-inch tape pulled tight around a curve will narrow to about 2 inches or by roughly a third. Bias cut tapes will lay flat around a curved area, whereas linear-cut tapes will leave wrinkles when pulled around a curve. Use bias-cut tapes only on curved areas. Now let’s get down to the business of applying tapes. With the Poly-Fiber process, tapes are cemented in place using Poly-Brush. Do not use Poly-Tak fabric cement to hold tapes to the fabric. PolyTak cement dries much too brittle for the flexible bond tapes need. Also, do not mix Poly-Tak with Poly-Brush. As you will recall, one of the warnings we have discussed throughout this series is to be sure you follow the instructions in the procedure manual. Let’s take a wing that has been riblaced and apply the finishing tapes over the ribs. I recommend that you draw pencil lines where you want to apply a 54

KITPLANES January 2009

Use a single-edge razor blade to carefully cut the tape for a precise fit.

Application of a tape on a leading-edge area is very similar to taping a wingrib. Again, coat the area with thinned Poly-Brush and work the tape on both sides of the leading edge.

tape. Using a straight edge to accomplish this task only takes a few minutes. Mark the area just a bit wider than the tape to ensure proper edge adhesion. After marking where the tape will go, you then pre-coat only that area with Poly-Brush. Thin the Poly-Brush with reducer—three parts of Poly-Brush to one part of reducer. It is important that you pre-coat the area prior to placing the tape. This will ensure proper adhesion to the fabric. In taping a wing, you have two choices. You can either cut the tape for

one side of the surface then cut another piece for the other side and then butt them together at the leading edge, or you can cut a piece of tape long enough to wrap around the entire wing surface. I think the latter is easiest to do. So cut a piece of 2-inch tape long enough to wrap around the whole wing, leaving a small amount as extra. Attach the tape to the top of the wing first. You can roll up the tape at the leading edge and clip it with a clothespin to keep it off the floor. After you have pre-coated the area to be taped, you will then brush another www.kitplanes.com

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Completing the taping of the curved edge. We’ll talk more about curved edges in the next part of this series.

very wet strip of Poly-Brush over the area just pre-coated. (Be careful not to get excess Poly-Brush outside of the lines drawn.) You will need to work fairly rapidly in higher temperatures, as the Poly-Brush will dry quickly. Immediately lay the tape onto the area just brushed. The tape will begin soaking up the Poly-Brush right away. Use your brush to then go back over the tape to make it smooth and work out any air bubbles. Little brushing is needed, and be sure to complete any brushing before the Poly-Brush dries. If you don’t, there will be brush marks that are difficult to remove. It is also important that you have the first one-half inch of each side of the tape thoroughly wetted out with Poly-Brush, thus ensuring that the edges of the tape will not lift.

Don’t worry about small wrinkles, air bubbles or an occasional pinked edge that is not in place. We will work those out later. As soon as the Poly-Brush dries, apply one more coat over the tape itself. Again, watch those brush marks. Turn the wing over and repeat this process on the bottom side of the wing. You may notice some wrinkles where the tape wraps around the leading edge of the wing. This will be more of a problem on tapered-wing aircraft. We will talk about working these out in the next installment of the series. You have now learned what tapes are available, where to use them, and the first steps in applying them. Next month we will continue this discussion with taping curved areas, leading edges and more. 

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Either bias tape or straight tape may be used around a curved area. With a very abrupt curve, a bias tape is sometimes preferable.

A further look at curved surfaces and how to deal with inspection rings, drain gromments and gussets. BY RON ALEXANDER 50

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In last month’s installment, we began a discussion of finishing tapes and how to apply them. Finishing tapes are required over any fabric seam and over wingribs after the fabric has been attached. They should also be placed on leading and trailing edges of surfaces, over the stringers on a fuselage and over any area exposed to wear or that may need additional reinforcement. Now let’s continue the discussion by outlining the methods required to attach inspection rings and drain grommets on a fabric surface.

Taping Curved Areas Compound curves found on wingtips and tail surfaces are more easily taped using bias-cut tapes. These specially cut tapes will more readily conform to a curved surface than will linear or straight tapes. Remember two important points: A bias-cut tape has a seam about every 70 inches, and the tape will reduce in width about one-third when stretched around a curved area. So when you’re selecting the proper width of tape, remember to order a size wider to allow for the shrinkage. You will probably want at least a 4-inch-wide tape on a wingtip bow. That will allow you to

Begin taping a curved area by securing the tape at one end. Be sure to cut the tape to the proper length before applying.

You can work a straight tape around a curved area as shown. The advantage of a linear or straight tape, as opposed to a bias-cut tape, is that there are no seams to deal with during the application.

All of the wrinkles that may occur in the tape can be worked out of the fabric by using the heat from an iron to smooth and form the material.

Photos: Richard VanderMeulen, Marc Cook

begin on the leading edge of the wing and extend the tape around the bow, finishing with the proper width on the bow itself. You can actually overlap the bias tape on top of the leading-edge tape that is in place. The first step is to cut the length of tape needed. Don’t forget the seams. Start your cut right after a seam. Cut the end that will be overlapped in the shape of a teardrop, as this will provide a more

pleasing appearance. After cutting the tape, use a pencil (no pens) and carefully mark a centerline along its entire length. You can again mark the area where you will be applying the tape. Mark about a 3-inch width on the wingtip bow. Precoat this area with Poly-Brush that is thinned with one part of reducer to three parts of Poly-Brush. You will now attach the end of the bias tape cut in the shape of a teardrop over the leading edge KITPLANES February 2009

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Fabric continued tape that is in place. Apply Poly-Brush to a small area on the leading edge where you will overlap the tapes, and then apply the tape with Poly-Brush, holding it in place with a spring clamp or your fingers. Allow it to dry for a few minutes until it remains in place on its own. The idea is to cement a small part of the bias tape in place, let it dry, and then come back after about an hour to complete the taping. You now have the tape held in place over about a 3-inch area, allowing you to apply tension to the tape. Next pull the tape around the wingtip, applying it to the entire bow. Use the pencil line to keep the tape centered over the bow. Keep pulling on the tape until all of the wrinkles are gone and the tape is in place. You now have a tape that has spanned a curved area without leaving any puckers or wrinkles. Linear-cut tapes may also be pulled around curved areas if the curve is fairly shallow. Many people prefer using linear tapes because there are no seams to deal with. To use a linear tape, first attach the tape over a small area and allow it to dry. Then stretch the tape tightly around the curved area over a coat of Poly-Brush. You will probably end up with a few wrinkles and tape edges that are not cemented in place. You can then take an iron, calibrate it to 225° and use it to smooth out the wrinkles. Again, bias tapes are more easily placed around a tight curve, but linear tapes may be used around normal curves of wingtips, etc. The choice is yours. I would suggest practicing on a curved part if at all possible before going to the actual part you are covering.

After applying the tape, it can be formed to the curve using heat smoothing with an iron set at 225°. Make sure your iron is calibrated, as any hotter will deform the tapes.

Place a piece of smooth fabric over each inspection ring. Cut to fit using pinking shears. You can trace the outline of this patch using a 1-gallon can as a pattern. Place the can on the fabric and trace around the bottom of the can. The size will be just right to cover the inspection plate.

Heat Smoothing This step makes the job look professional. When you are taping (linear or bias tapes) you will probably end up with a few wrinkles and raised edges on the tapes. These can be smoothed down and removed with the application of heat from an iron. First, calibrate your iron to 225°. A small hobby iron is best for this task. The Poly-Brush used to cement the 52

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The finished patch, as shown here, should have a smooth appearance.

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tapes in place will begin to soften at about 200°. (This step can be effectively accomplished only with the vinyl coatings used in the Poly-Fiber process.) This means you can use the iron to actually smooth out any imperfections that exist, particularly along the edges of tape. If you have pinked edges that are not cemented in place, you can use the tip of your iron to literally melt them down. This is much easier than sanding out imperfections. You must be careful not to heat the iron to above 225°. Anything hotter will

All wrinkles can be worked out using heat smoothing and forming.

shrink and deform your tapes, and you will then have tapes with a snakelike appearance. All wrinkles, bubbles and raised areas can be fi xed using the iron as discussed. Go over every tape edge on the fabric surface. Use your fingers to tell when they are smooth enough. When they feel smooth to the touch, they will appear smooth in the final finish. This one step will save many hours of sanding later.

Inspection Access Rings

An example of a completed patch over an inspection ring. When needed, the fabric may be cut out of the inside of the ring so that an inspection plate can be added.

Tapes may be cut so that they are rounded on one end to make for a more pleasing finished appearance.

After you have covered your airplane you may have to gain access to certain areas inside the wings or the fuselage. This may be accomplished using plastic inspection rings. These hole reinforcements are cemented in place over every drag-wire junction, wing fitting, cable guide, control bell crank or over any other area that will need to be inspected regularly. Access may also have to be gained into one of these areas during assembly of the airplane. These inspection holes are usually installed on the bottom side of a surface. If you are recovering an airplane, use the old fabric as a guide to locate placement of the inspection rings. The plastic rings are cemented in place using Poly-Tak fabric cement. Use a small brush to apply a coat of PolyTak onto the ring itself and then lay it onto the fabric (flat side down) in the area desired. The fabric cement will dry within a few minutes. Be sure to clean up any cement that works out from under the ring using MEK. After cementing the ring in place, you will then want to cover it using a piece of fabric. Failure to do this will provide an opportunity for the ring to separate from the base fabric at a later time. This separation is often the result of vibraKITPLANES February 2009

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Fabric continued tions and air loads encountered when flying the airplane. A simple piece of fabric cut to fit over the ring will prevent this from occurring. The first step in cutting a piece to fit over the ring is to smooth the fabric that will be used. You don’t want any wrinkles in it. Light weight fabric works best for this purpose. You can use an iron set at 250° and iron out all of the wrinkles on a piece of scrap fabric. Then take one of your 1-gallon cans and carefully set it on top of the piece of fabric. Take a pencil and draw a circle on the fabric using the outline of the can as a guide. This size is just about perfect to cover the inspection ring. You will want the fabric to extend beyond the ring itself. Cut the fabric with a pair of pinking shears. This will ensure you have pinked edges for proper cementing. Using Poly-Brush that has been thinned appropriately, brush on a coat both inside and outside the inspection ring large enough to wet the fabric overlay. Set the piece of cut fabric over the ring and allow the Poly-Brush to soak through the fabric. Use a brush to work the Poly-Brush through the fabric. After it dries, brush on another coat of PolyBrush. After this coat has thoroughly dried, you can use your iron to smooth out any imperfections. When you have completed the air-

Fabric or other reinforcing material may be cut to the size and shape needed. Be sure to follow fabric system guidelines when applying.

plane you may want to cut the fabric out of a few of these inspection rings. Cut out only those where access is needed for assembly of the airplane. Leave the others uncut. Those that remain can be opened at a later date if the need arises. Once the fabric inside the inspection ring is removed, it will be covered with a metal inspection plate. These plates are made to fit over the plastic inspection ring and are easily removed to provide access to the area. Another tip: When you are painting your airplane be sure to paint a number of metal inspection plates the final finish color. You will then have them available to place on the inspection rings when needed at a later time.

Drain Grommets Every fabric-covered airplane must have a way for moisture to escape. Condensation can introduce moisture, or rain or water from washing your airplane may leak into a wing or fuselage. These areas need to be able to breathe and allow moisture to escape. This is accomplished using drain grommets. These small grommets are cemented in place on the underneath side of surfaces. Any place where you think water may collect should have a grommet. Most wings, for example, will have a drain grommet located next to the outboard side of each rib at the trailing edge. Some people will place a grommet on each side of a rib. Obviously, you will want to place drain grommets on the underside of a fuselage to allow water to drain. Drain grommets are cemented in place using Poly-Tak just like the inspection rings. They are then covered with a piece of fabric. They will vibrate loose if not covered with fabric. You can use a roll of fabric and draw a pencil mark on the fabric to be cut using the inside of the roll of fabric as a guide. This will provide the ideal size piece of fabric to cover a drain grommet. After you have completed the final finish, you can use a pencil soldering iron and melt the fabric Cut a fabric patch to the specific shape that will be covered. Be sure it fits smoothly, and then put it in place using the same procedure you would use for a finishing tape.

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Grommets are preferred to reinforce drain holes, but some builders elect to go without. This hole was made with a pointedtip soldering iron.

out of the drain hole, which will allow water to drain out. Three types of drain grommets are used: plastic, aluminum and seaplane grommets. I prefer to use aluminum grommets because they are thin and cover easily with a piece of fabric. Seaplane grommets feature a small vented hood that will help them siphon water out. They are normally reserved for use on seaplanes.

Fabric Gussets There may be areas of the fabric that need to be reinforced where a finishing tape is not satisfactory. The area may be oddly shaped or too large to accommodate a tape. When this occurs you will want to cut a piece of fabric to the exact size and shape needed, and then apply it to the area using Poly-Brush. An example would be over a wingstrut fitting that is protruding through the fabric. The fabric area around the protrusion should be reinforced. This can be accomplished using a fabric patch cut to the proper shape and size. Be sure that you iron out all of the wrinkles on the fabric you will be using to cut out the patch. Use of light weight fabric is also recommended for these gussets or patches, trimmed using pinking shears. After completing the steps outlined, we are now ready to begin spraying on our chemical coats. Next month we will review the fabric covering steps and then begin the spraying process. 

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The second coat of Poly-Brush is being applied to the prepared fabric. The first coat is brushed on after being properly thinned.

Once you have your structures covered and the surfaces prepared, it’s time to spray on the chemical coats. BY RON ALEXANDER

The steps involved in covering an airplane are essentially the same for all available fabric-covering systems until you are ready to apply the chemicals. Differences among the systems do exist, but they are somewhat minor until reaching this stage. We’ll review the steps we have discussed that must occur prior to spraying the chemical coats, and then we will begin this application process. To begin with, the surface you are covering must be properly prepared. This is accomplished using an epoxy primer or an epoxy varnish following a thorough cleaning. After preparing the surface, it should then be carefully inspected for any defects. Remember that you will probably not have access to the inside of the surface for a number of years. After the inspection it is time to select the proper weight of fabric and then attach it to the structure using fabric cement. Once the fabric is secured in place on a component 50

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part, it is then heat shrunk using a regular household iron. The iron is calibrated to the required temperature, and then the fabric is ironed until all areas have received the proper heat, ensuring that the fibers are adequately shrunk. With the Poly-Fiber covering system, the next step is to seal the fabric by brushing on a coat of Poly-Brush. This encapsulates all of the fibers and provides the necessary adhesion for all subsequent coats. This is an important step because the bond of this coat is essential for the remaining coats to properly adhere. Next we attach the fabric to the wings and to all surfaces that provide lift. This important step prevents the fabric from www.kitplanes.com

“ballooning up” during flight due to the low pressure created on the top of the wing as a result of lift. The inspection rings and drain grommets are then cemented in place. Finally, several areas are covered, using tapes that are precut from the fabric into various widths. The most common width is 2 inches. This last step brings us to the spraying of the chemical coats.

The First Step When you reach this stage of fabric covering most of the hard work is done. Many individuals are unsure whether they have the ability to spray paints and dopes on their airplane. Let me assure you that you can learn easily. It is not difficult to spray. This is particularly true when spraying the fabric-covering chemicals. They are more viscous and have fewer tendencies to run than regular enamels. This way you can learn the spraying techniques with a chemical that is easy to spray before you get to the color coats. Don’t be afraid to spray your own surfaces. With practice you will become proficient. The first step in spraying is to select the proper spray outfit. Perhaps you already have a spray gun and compressor. If so, be sure it is a high-quality gun. Do not begin the spraying process with a cheap gun. Not using proper equipment will cause you untold grief as you spray. If you do not have a spray rig, I would suggest you consider purchasing a highvolume-low-pressure (HVLP) setup. These systems are easy to use, especially for those who have little or no spraying experience. They also plug right into a 110-volt outlet and come complete with everything you need. In addition, they will save you money by using fewer chemicals, because this type of outfit creates little overspray. In other words, the chemicals go on the fabric surface and not into the air. Before you begin to spray, make sure you take the proper safety precautions. This involves purchasing a high-quality charcoal filtered face mask or a forced-

Spraying with a high volume low pressure (HVLP) system such as the one shown here reduces the amount of overspray.

air breathing system. If you will be painting your airplane with polyurethane paints, a forced-air breathing system is absolutely necessary. If you are not going to be using polyurethane paints, a good quality charcoal mask is sufficient. When mixing chemicals be sure to protect your eyes and skin. You should also protect your eyes and skin while spraying the various chemical coats.

Where to Spray? A clean, well-ventilated area is necessary for spraying. Do not spray outside in direct sunlight. (The photos that accompany this story were taken at various hands-on clinics using non-flying components.) Try to find an area where you can construct a small spray booth using PVC pipe and clear plastic sheets. If this is not possible, find an area that is free from dust and dirt. Cover the floor if you want to keep it clean. Overspray will coat a floor over time. Good lighting is mandatory. Otherwise, you will miss areas, and create runs and sags, with the overall result being a poor finish. Be sure you have adequate ventilation and then begin to practice by spraying chemicals on pieces of cardboard, fabric panels, etc. A little practice will save you problems when you actually begin to spray your surface. Another point to remember is that you should start out by spraying a small control surface. Don’t begin by spraying a wing. Start with a small surface so you can gain experience. You will be

painting your entire airplane one piece at a time while it is apart. This is much easier than painting the entire airplane while it is assembled, which requires a lot more experience and practice.

Preparation Now that you have practiced, you are ready to spray the first coat of PolyBrush on a control surface. There are several considerations before you begin. First of all, the temperature of the area should be at least 60° to 65° F. Anything lower than 60° will impede the drying process. If the temperature is above 85° F, you must use a reducer that contains a retarder. If the temperature is above 95°, or if the humidity is high, you will have to use a retarder to reduce the drying time. Again, do not spray in direct sunlight or if the wind is blowing. Before you begin spraying you must

The first coat of Poly-Spray will highlight imperfections that will need attention prior to additional coats. (We’ll talk more about this in the next installment of this series.)

Photos: Richard VanderMeulen, Rick Lindstrom, Marc Cook

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Fabric continued be sure the surface of the structure is clean, which is best accomplished by passing a commercial tack rag over the area just prior to spraying. This slightly sticky rag is designed to pick up small pieces of lint, dirt, etc. Lightly pass the cloth over the surface. Do not rub the surface, though, because it could transfer unwanted chemicals to the area. Next you must prepare the PolyBrush for spraying. As you recall, we applied one coat of Poly-Brush after the fabric was shrunk using a brush. We will now spray a coat of Poly-Brush over the entire surface we are covering. Again, this is done after we have completely taped and prepared the structure we are going to spray. Poly-Brush must be properly thinned before applying. Use R65-75 Reducer if the ambient temperature is less than 85°. If the temperature during the spraying operation will be higher than 85°, then you must use R8500 Reducer. Thin the Poly-Brush using the formula of one part thinner to three parts of Poly-Brush. Mix it thoroughly and then pour it through a paint strainer. Use a 60x48-mesh paint strainer available from Poly-Fiber distributors. You can pour the mixture from the can through a strainer directly into your spray cup.

Play Misty for Me You are now ready to spray. Start out by spraying a “mist” coat over the entire surface. This coat should put a fine layer of Poly-Brush over the area. It will not look wet when completed, because you

have placed so little chemical over it that it does not create a uniform film. This mist coat will allow the next full coat to adhere to the surface properly. Trying to spray a thick coat on first will only cause runs and other imperfections. Wait about 15 minutes after the mist coat, and then spray a wet coat over the surface. This coat will be sprayed on heavier and will appear wet and shiny. Be careful not to spray too much chemical onto the surface, or runs and sags will result. If runs and sags occur, they should be immediately removed using reducer and a brush. Lightly brush out the runs before they dry. Poly-Brush is normally tinted slightly to a pinkish color to allow you to apply it more easily. Still, you must have a good source of light when spraying. The PolyBrush will appear to have a deeper pink color than the coat that was brushed on. As it dries it will become glossy. Poly-Brush is also available without the pinkish tint. The untinted product is used where the backside of the fabric will be visible. An example of this would be on the fuselage of an open-cockpit airplane where you will actually see the backside of the fabric. You would not want the pinkish color showing in this area. The untinted Poly-Brush is a bit more difficult to apply.

A gravity-fed spray gun may be used with an HVLP system.

Poly-Brush is the first chemical that is brushed onto the prepared fabric. An additional two spray coats should follow.

Problem Areas You may observe some problems while spraying. If the Poly-Brush sprays filaments that look like cotton candy, or if the surface dries rough, it has probably not been adequately thinned. Be sure you thin the mixture as directed. If the temperature is excessive, you may have to add BR-8600 Blush Retarder. This retarder simply slows the drying process so that the chemical has a chance to adhere to the surface before it begins to dry. Of course, runs and sags may be a problem. Simply use a brush with thinner to brush out

The R8500 retarder-reducer is used to thin both Poly-Brush and Poly-Spray. It is used when the temperature is above 75° F.

the run as soon as you discover it. Spray Poly-Brush over the area again. Do not try to sand out a run or sag at this point. Poly-Brush will not sand. Sanding will

You must wear some type of respirator when spraying chemicals. The charcoal filter respirator pictured here is adequate for both Poly-Brush and Poly-Spray.

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KITPLANES March 2009

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be done at a later stage of the process. Pinholes may appear. These are tiny areas that have the appearance of a small hole. Insufficient filling of the fabric or tape weave during the brush coat may cause this problem. Spraying in direct sunlight can also cause pinholes. At this point the only way to cover a pinhole is to rub the area with a soft cloth and reducer. This will soften the Poly-Brush and force it into the unfilled weave. You must then spray the area again with thinned PolyBrush with retarder added. You may notice an “orange peel” appearance. This is often the result of

Second Spray Coat After allowing the first coat of PolyBrush to dry thoroughly, you then should spray on a second coat. I recommend waiting a few hours between coats. If you spray on the first coat in the morning, wait until the afternoon to spray on the second coat. It should also be thinned according to our formula of one part thinner to three parts of Poly-Brush. Normally, one coat of Poly-Brush brushed on the surface followed by two spray coats of Poly-Brush will be adequate. You will then be ready to apply the Poly-Spray. We will discuss this important step in next month’s installment of the series.

Tubing!

Spruce!

Hardware!

Paint!

Chemical Coat Application Poly-Brush

Application

First coat

Brush on

Second coat (initial mist)

Spray on

2 to 4 hours

Second full coat

Spray on

15 minutes

Third coat

Spray on

2 to 4 hours

WWW.WICKSAIRCRAFT.COM

Poly-Spray is applied by spraying at least three cross coats over the Poly-Brush. We will discuss this in our next segment.

using too much air pressure if you are using a pressure spray gun. It can also be caused by the coat being too thick. If you notice some of the small edges of pinked tape lifting up, this is normal. The solvents sometimes cause small areas to release. This is easily resolved using a small iron calibrated to 225° F. Simply iron the edges down. They will then stay in place. Be careful not to leave the iron in one place too long.

Time Between Coats

Helpful Hints Spray in well-ventilated area, and use a respirator. Thin the Poly-Brush 3:1 (Poly-Brush to thinner) before application. Use R65-75 Reducer if temps are less than 85°; R8500 if temps are higher than 85°. The “wet coat” will appear wet and shiny. Remove any runs or sags immediately, before the coat dries. Use non-tinted Poly-Brush in areas where the backside of the fabric will be visible. An “orange peel” texture is the result of either too much air pressure in the pressure spray gun or a too-thick coat. Areas with pinholes may be rubbed with a soft cloth and reducer, and then resprayed with thinned Poly-Brush with retarder added.

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Fabric continued

Superflite: Hittin’ the Books Wandering around Oshkosh last year, with my shopping bag becoming increasingly heavy, my thoughts turned to the prospect of adding a couple of half-inch thick, 10-inch square books to it. I’ve learned over the years to take a half-filled suitcase, but never learned to say “No, thanks” when offered bits of printed material or a sample. A week after getting home, the pile diminished as the trash can filled. Fat books appeal to my readaholic nature, so they easily made it to the “keep” pile. With winter upon us, it was time to delve into them. I’m a neophyte here. I’ve built a riveted aluminum airplane and a composite one, but the next project will be tube and fabric, so I was interested in every word in the Superflite Recovering Handbook. As I watched the accompanying DVD, I made notes. And Now the Movie About 4 minutes in, the narrator said “two parts MEK plus one part U-500…” and I did a double take. The screen showed two cans of U-500 and one of MEK, the opposite of the narration. I immediately called Superflite and was told, “Follow the narration, not the graphic!” The books have the same problem. On Page 5.3, it should be two parts MEK plus one part U-500. Despite the musical score, I would encourage anyone interested in building or owning a fabric aircraft to watch the DVD. It is a gold mine of information and will leave you enthusiastic about an area of aviation that is fast becoming, if not extinct, at least a dark art. Not all of the useful lessons are intentional. For instance, they use an excellent glue gun that I want even if I’m not doing fabric. Also, it’s nice to see the work being done, and I’d never have guessed that bubbles and small distortions in the fabric could be removed during the shrinking process. I was also surprised to see that inside corners could be cut to a sharp, zero radius; on my previous projects, I’d have used a paper punch to get a curve to eliminate those stress concentrations. Not here, though. I also learned that shrinking is a three-stage/temp process, so you’ll likely tire of it before you overdo it. Says Superflite, “Don’t go over 375° F, and you’ll be OK. It shrinks just so far and no further.” Does the weight of the iron affect the process? Although that question is not addressed, watching the technique illustrated that this is not like ironing your shirt before going out Typical of the books is this clear illustration with concise do’s and dont’s. on a Saturday night. It’s easier—except for those glued 54

KITPLANES March 2009

areas, which should not be retouched. Armed & Ready I had read elsewhere that a heat gun has no place in this fabriccovering process, but I wondered why. The DVD shows a large hole quickly being burned through the fabric in a scene that only Freddy Krueger would enjoy. Rib-stitching. Old movies of aircraft construction showed legions of ladies intent upon tying knots that would make Nelson’s sailors blush. I’d seen several printed examples showing the thread’s path, but this demonstration made it look downright easy! Safety One area needing improvement might be safety. Tyvek suits are emphasized, but what about MEK? I looked up the MSDS on it and found that it causes droopy eyelids and lack of coordination, but does not inhibit reproduction. Tyvek suits are those white, protective suits used to protect the skin from airborne chemicals. Forget using your old jeans and long-sleeve shirt. Superflite offers two options for painting your aircraft, and both are fairly toxic. You’ll need a filtered room for the project, filtered air for you to breathe and something to keep all that airborne particulate off your skin. MEK, methyl ethyl ketone, is a manufactured organic chemical sometimes known simply as ketone, less often by its parent group butanone. Working with all chemicals, wear rubber gloves, no open flame, don’t drink it, and dispose of it properly. MSDS, material safety data sheets. These are government approved fact sheets that all chemicals must have available. They’re not difficult to interpret, and if you’re going to be around MEK and similar liquids, you should become familiar with them. Google the chemical name and MSDS, and you’ll eliminate one cause of a shortened flying career. Circling Back Around I’d like to offer the expert painters out there an opportunity to agree, disagree or elucidate. The Superflite video shows spraying of the various coats, be they hardeners, UV protectants or color coats, by hitting the major flat surfaces first, and then the corners. I’m a proponent of doing the corners and difficult surfaces first, then the easy parts. What say you? (Your thoughts to [email protected], please.) If It’s in Print, It Must Be True Having watched the 50-minute DVD, I moved more diligently into the books. Titled System I - Butyrate Dope and System VI – Urethane, the differences are tough to spot; they use many of the same illustrations, they’re laid out in identical fashion, and even the text is identical, so how to determine which process is right for your project? www.kitplanes.com

Molly McNamara, the author of both books and daughter of Superflite’s owner, answered that question. “The books are identical in many ways if for no other reason than both address the issue of recovering and finishing fabric aircraft,” McNamara said. “The real differences come after Chapter Six, where we talk about the finishing.” Fair enough. But how does someone choose which process? “Well, System I Butyrate Dope is usually used on classic aircraft. It produces a matte-satin finish,” she said. “It’s also about 5 pounds lighter for a typical aircraft. The System VI Urethane process is quicker and easier to apply, and is what we recommend for show planes that want a super high-

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gloss. It can be used on metal and fiberglass as well as fabric.” Having started my first airplane in a condo with a bedroom above the garage, I was interested in smell and toxicity issues. “Dope smells strong and urethane less so, but it’s still not recommended for application in your basement,” McNamara said. So what’s the story on Systems II through V? Are there other books or videos? “We’d made a lot of revisions over the years and when we decided to publish these books. Systems I and VI were, and are, current,” McNamara added. Both books are well illustrated, easy to follow, spiral bound to lay flat, well organized so finding what you need is easy, and colorful so they’re fun. Superflite also sent a project kit; look for a report in a future issue. For more information, visit www.super flite.com. —Bob Fritz 

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Now that you have your first chemical coats applied, it’s time to protect the fabric from the sun before applying color. BY RON ALEXANDER

Most of the hard work of fabric covering has been completed once you begin spraying chemicals on the fabric. As you may recall, the first coat of chemical used with the Poly-Fiber covering system is called Poly-Brush. This is initially brushed on the fabric to provide the base for all subsequent coats. Two additional coats are then sprayed onto the surface to further build up the layers of chemicals necessary to provide protection and smoothness for the final finish. After spraying on two coats of Poly-Brush, we now must spray on a coating that will protect the fabric from the ultraviolet rays of the sun. Polyester fabric, like most fabrics, will deteriorate in sunlight if not properly protected, and this will occur within a very short time— perhaps six months or less. Polyester fabric is not as susceptible to this problem as cotton; however, if bare Photos: Richard VanderMeulen, Marc Cook

polyester is left in direct sunlight for 12 months, it will lose more than 85% of its strength. Cotton fabric exposed to the same sunlight for the same period of time will deteriorate almost completely. Polyester is protected from the UV rays of the sun by applying chemical coatings containing aluminum pigment. Application of the recommended number of aluminum “silver” coats will provide adequate protection for a number of years. In the Poly-Fiber system the chemical used to block the sun’s UV rays is called Poly-Spray, and its application is the subject of this last installment of the fabric series. KITPLANES April 2009

55

Fabric continued

Applying Poly-Spray After allowing the last coat of PolyBrush to dry for at least a day, you are now ready to apply the Poly-Spray. These coats are often referred to as the silver coats. You need to apply enough PolySpray to prevent any light from shining through the fabric. Three cross-coats are generally sufficient, but this depends on the amount of sanding you do between coats. A certain amount of Poly-Spray will be removed when you sand, and it will need to be replaced to ensure proper protection of the fabric. Before we go any further, let’s explain the term cross coat. A cross coat is defined as moving the spray gun north and south followed by east and west— hence, one cross coat. This means you will paint a surface moving the gun up and down over the item and then immediately go back to where you first started and move the gun from side to side as you spray. This process ensures total coverage over the area you are spraying. So, in effect, you are actually putting two coats on the surface during a cross-coat application. Now, let’s discuss a few basics. First, Poly-Spray is always thinned, using one part of reducer to four parts of PolySpray. The type of reducer will depend

upon the temperature where you are spraying. If the temperature is above 85° F, you should use 8500 reducer; below 85°, use 65-75 reducer. The aluminum pigment in Poly-Spray settles to the bottom of the can quickly. This means that when you first open the can you must stir it completely. The best thing to do is to take the can to a local paint store and have them shake it for a few minutes. Even if you do that, you must still thoroughly mix the pigment just prior to spraying. Failure to do so will lessen the protection the chemical provides. Often you will find the pigment has settled to the bottom of the can to the extent that it feels like a hard layer. You must use a stirring stick or some other means to break that loose and mix it throughout the can. This must be done just prior to spraying. After thoroughly mixing the PolySpray you should pour it through a paint strainer as you fill your spray cup. A 60 x 48 mesh strainer is best. This is a necessary step. Otherwise, particles of the silver pigment will pass through your spray gun onto the surface, requiring additional sanding. Don’t use anything finer than a 60x48 mesh or you will strain out part of the silver pigment. Next, make sure the surface is clean and free of lint and other contaminants. The best way to ensure this is to wipe

After the two coast of Poly-Brush have been applied, it’s time for sun protection.

the surface down using a paint cleaning solvent such as Poly-Fiber’s C-2210. This will remove skin oils and such that may be on the surface. Then, just before spraying, wipe the area using a tack cloth to remove fine particles of dust.

Choose the Time and Place Be sure your conditions are right for spraying. The temperature should be between 65° and 90° F. If the temperature is high or the air is humid, you may need to add a retarder to slow the drying process. BR-8600 Retarder is the product to use. You will know that retarder is needed if the Poly-Spray has a rough appearance or feel. If it looks or feels like sandpaper, it is drying before it reaches the surface. Add the retarder as directed in the manual. (This assumes you have properly thinned the Poly-Spray. Failure to thin it will also cause this problem.) Poly-Spray should go on easily. You can see how much and where it is being applied. As you spray you will also notice every imperfection that begins to appear on the fabric. This coat will help you find problem areas that may A spray coat in one direction, side to side, is followed by another coat that is applied from top to bottom for a cross coat. (Incidentally, before you ask if this Sport Air workshop is demonstrating a new water-based system, here’s your answer: That garden hose is actually carrying the heated, low-pressure air supply for an HVLP gun.

56

KITPLANES April 2009

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Make sure that the conditions are right for spraying. Too high temperatures or too much humidity can adversely affect the drying process.

need attention. It is simply the result of a darker color being applied to the surface. Again, spray one cross coat and then allow it to dry. You should give it at least 3 to 4 hours. When the imperfections are highlighted, this is a good time to fi x them. You may notice that some of the edges of

your pinked tape are curling up. You can use your iron to melt these areas down. It does, however, require careful attention. Your iron must not directly touch the Poly-Spray. You must use a thin piece of Teflon, an oven-cooking bag, or baker’s parchment paper as a shield to protect the silver. You must also be sure the iron is calibrated to a temperature of 225° F.

Dealing with Pesky Pinholes If pinholes appear, you will need to fill them. (Pinholes are tiny little areas that have the appearance of a small hole. Insufficient filling of the fabric or tape weave during the initial brush coat of Poly-Brush may cause this problem.) The only way to fi x pinholes is to fill them. This is done with Poly-Brush (not Poly-Spray). If you continue to spray over Poly-Spray is always thinned at one part reducer to four parts of Poly-Spray. You can also use the retarder to slow drying.

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Fabric continued the pinholes, they will simply appear to deepen. Mix three parts Poly-Brush to one part reducer and add a small amount of retarder. Sand the affected area using 400-grit sandpaper. Wipe the area with paint cleaning solvent, and then brush the Poly-Brush into the area to fill the pinholes. Once it has dried, you can then apply more Poly-Spray over the area. Let it dry, and then sand it smooth.

Drying Time Between Coats A good rule of thumb is to apply one cross coat in the morning followed by another in the afternoon, but you can allow the Poly-Spray to dry much longer if you desire. If you do not apply the next coat within one week, however, you will need to lightly sand the surface before applying the next coat. I recommend that once you begin applying Poly-Spray that you continue until you have the required number of coats on the surface. After fi xing any imperfections that may have appeared, you are now ready to spray on the second cross coat of PolySpray. After spraying the second coat you may notice more problem areas similar to the ones we just discussed. If so, now is the time to fi x them. After this second coat of Poly-Spray you should have a fairly good buildup of silver on the fabric. It is now time for the fabric coverer’s favorite activity—sanding.

Have a paint store shake the can so that the aluminum pigments are evenly distributed. Then stir it again just before use.

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KITPLANES April 2009

Allow each cross coat to dry thoroughly before applying the next one. If too much time has passed between coats, you may need to do a light sanding of the surface before spraying.

Unfortunately, no matter what type of fabric covering system you decide to use, you will be sanding if you want a nice finish. No system provides a smooth finish without this step.

Sand, Sand, Sand Be sure the Poly-Spray is dry before sanding. The best way to sand is to wet the area with water and use wet or dry sandpaper, starting with 320 grit. You can use a bucket of water and dip the sandpaper in the water frequently to wet the area. A word of caution: Sand only the large open areas of fabric. You must be careful not to sand over rivet heads or rib laces. Anything that protrudes out

of the fabric should be avoided. Sanding over the top of ribs will cut the fabric over rib laces, screws or rivets. Avoid the area. Use of a sanding block is often helpful. Also, protect your fingers when sanding or the sandpaper will abrade your skin. The other area to concentrate on when sanding is the pinked edges of tapes. If any of the edges continue to stand up, simply sand them down at this point. After sanding you will need to remove the sanding residue. This is best done with a wet sponge, or simply get out your hose and wash the area completely with clean water. Remember, the PolySpray that has been removed must be

Fix any problem areas or imperfections that show up after the initial coat before applying subsequent coats. You will almost certainly have to sand between coats to accomplish a nice finish.

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KITS! Trying too hard to hide tapes could mean excess weight and coats cracking.

replaced to ensure adequate UV protection. You can check for this by using a 60-watt light bulb held inside the wing or fuselage. If you have adequate UV protection you will not be able to see the light on the surface of the fabric. You must be very careful when accomplishing this test. Solvents exist inside the wing or fuselage, and if the bulb accidentally breaks, it could cause the fumes to ignite. To prevent this, be sure the bulb is protected. A protected droplight works well for this purpose. You can also do this test by cutting a small hole in an inspection hole area and looking through it while holding the light above the surface. If you see light with either of these tests, you do not have adequate UV protection.

One More Coat or Two Regardless of the outcome of the light bulb test, you should apply a third coat of Poly-Spray to the surface. This cross coat will ensure that you have an adequate amount of coverage. You may want to spray on an additional coat. Some people sand after the third coat and apply a

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It’s OK if the pinked edges of the fabric tape show in some areas.

fourth. This will sometimes result in a smoother surface to which the topcoat is applied. But do not continue to build up the Poly-Spray beyond four coats, because it will reduce the flexibility of the chemicals and may cause a cracking problem in years to come. Understand that you will see the pinked edges of tapes and you will see the weave in the fabric. To hide those areas requires too many coats and may result in cracking of the coatings. Only the old covering method of using nitrates and butyrates will hide the weave of the fabric, but the price paid for hiding the tapes is a heavy airplane that has excessive coats of chemicals on the fabric. The bottom line is this: Check for proper UV protection using the light bulb test. This is most important, and it should occur after three cross coats of Poly-Spray have been properly applied. You are now ready to spray on the color coats, so get to work and have fun finishing your fabric-covered airplane. 

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Do not continue to build the Poly-Spray beyond four coats, because the chemicals will lose their flexibility and may cause future cracking problems.

KITPLANES April 2009

59

Choosing the “right stuff” for your project. BY RON ALEXANDER

Selecting the right type and weight of fabric to cover your airplane project can be confusing. Advice on the correct fabric is plentiful, varied and sometimes inaccurate. Years ago, aviation pioneers used a wide variety of fabrics on their aircraft and finally chose different types of linen and cotton. Grade A cotton, as it is referenced, became the standard fabric for use on most aircraft until the 1950s. However, cotton material was and remains susceptible to deterioration from sunlight and mildew. Polyester fabric was later developed and has become the fabric of choice for aircraft covering. Different styles and weights of polyester are available, and determining the correct weight is an important consideration. As you will see, the aircraft’s mission, engine size, aircraft speed and other factors influence the fabric weight you need to use.

A Bit More History The early pioneers in aviation realized that a better alternative to muslin and cotton fabric needed to be developed. Cotton fabric must be coated with nitrate dope—a flammable chemical. Nylon was tested only to find that it stretches too easily, and after being shrunk returns to its original state. 30

KITPLANES August 2008

Polyester material was developed during the 1940s in England. DuPont obtained the rights to manufacture this product in the United States under the trade name Dacron. DuPont published information on shrinking this material by applying heat, and Colonel Daniel Cooper began testing polyester fabric for aircraft applications. In 1958, Cooper termed his new material Ceconite and began selling it to cover aircraft. By this time butyrate dope had been developed and was being used as a coating. During the early 1960s, Ray Stits developed a fabric covering system using polyester fabric, which used chemicals other than nitrate and butyrate dope. He wanted a system that would not support combustion and that would not continue to shrink fabric over time. (Nitrate and butyrate dopes will continue to www.kitplanes.com

shrink both cotton and polyester fabrics through the years. This shrinkage is not critical if the fabric is properly applied.) The Stits Poly-Fiber covering system was introduced in 1965 and rapidly found widespread use.

Fabric Options You have limited choices regarding fabric. Polyester is the main type available today, as Grade A cotton has all but faded away. To my knowledge, there is no Grade A fabric available today that meets FAA requirements. If you find Grade A fabric, you must ensure it meets the proper specifications listed in the Technical Service Order C-15d. All types of polyester are basically the same regardless of their name. Both Poly-Fiber and Ceconite contract with a mill to have their fabrics loomed to the same specifications. Cooper Superflite is similar. Polyester fabric for aircraft will heat shrink approximately 10 to 12%, which allows us to apply the fabric loosely to the aircraft structure and then shrink it using a properly calibrated household iron. This provides the aerodynamic tension needed for flight. Fabrics are also identified by their weight, which is measured in ounces per square yard. Light weight fabric weighs 1.7 ounces per square yard, medium

Tapes are system-specific. Poly-Fiber tapes should not be used on Ceconite fabrics and vice versa.

Photos: Marc Cook

A light weight polyester is 1.7 ounces per square yard of material and is good for very light and slow aircraft.

Fabric Type

Weight

Heavier cloth, at 3.4 oz./sq. yd., is suitable for faster aircraft. It’s marked for reference, but the feel is distinctive.

Thread Count

Breaking Strength

Light

1.7 oz./sq. yd.

92 x 76/in.

67 lb./in.

Medium

2.7 oz./sq. yd.

68 x 62/in.

102 lb./in.

Heavy

3.4 oz./sq. yd.

68 x 56/in.

125 lb./in

Chart 1.

2.7 ounces, and heavy 3.4 ounces. The thread count of each fabric varies among types with the light weight fabric having a higher thread count than heavy weight; the light weight fabric will appear smoother. Strength of the fabric also varies with weight. Light weight fabric has a lower breaking strength than heavier weights (see Chart 1). Fabrics are certified by the FAA for use on production aircraft through Technical Service Orders (TSO), which apply to all aircraft fabrics including Grade A cotton. Fabrics are certified for use on production airplanes and should have a Parts Manufacturing Authorization (PMA) stamp, which specifies weight and type. Certified fabric has been inspected for flaws prior to being shipped for use on aircraft; non-certified fabric has not. If you are covering a production airplane, you must use only certified fabric. Ultralights and Experimental aircraft may use non-certified, but it is not recommended. Covering systems must possess a Supplemental Type Certificate (STC) to be used as replacement fabric covering systems on production aircraft. The inspected and certified fabric mentioned is part of this STC process. If you do not see PMA stamps on the fabric you purchase, do not use it on a production airplane. The PMA stamp appears at 3-foot intervals on certified fabrics. The ink is a

special type that will not bleed through the final coats of paint on your airplane. A point to remember: Do not use ink pens to mark on fabric while re-covering your aircraft; use only pencils. Most inks (except the one used for the stamp) will bleed through the final color coats. Certified fabrics are regularly tested to comply with FAA requirements. The U.S. Testing Company, an independent testing entity, conducts the tests, which include bursting strength, tension and tear strength. The results are published.

Down to the Requirements With all of this in mind, what factors influence selection of fabric for your covering project? One of the major problems associated with fabric covering has to do with the fabric flexing when the airplane is flying. Flexing problems are most prevalent in the prop-wash area of an airplane. Excessive flexing cracks the coatings that have been applied to the fabric, which results in rapid deterioration of the entire covering, and causes premature repairs or even a complete re-covering of the airplane. Light weight fabrics are more prone to this problem than heavier fabrics, which damp the flexing motion simply due to their heft. A contributing problem results from large spaces between structural members such as wingribs, which cause more flexing and bending of the fabric. Add KITPLANES August 2008

31

Fabric continued to this a high horsepower engine and a propeller that is beating the area, and the problem becomes even more acute. Another problem is on gear legs, where the engine exhaust may be blasting directly on the area. Again, light weight fabrics are more susceptible to damage here. The underside of a fuselage is another area where problems can occur due to rocks and other debris being thrown against the surface. So let’s get to the point. What fabric should you use? First of all, we have determined that you should use polyester fabric. Even if you can find Grade A cotton, it may not meet FAA requirements, and it is a more difficult system to apply.

Tapes come in varying widths and weights for certain applications on the aircraft.

At Poly-Fiber, large rolls of fabric are brought to the warehouse. Subsequently the fabric is rolled out, inspected and placed in measured rolls for sale.

Chart 2.

32

Airplane Type

Fabric Type

Ultralights, very light aircraft, gliders, aircraft less than 65 hp, most LSA.

Light Weight

All normal service aircraft—kit aircraft, antiques, classics, newer production aircraft—in short, most airplanes using normal airports.

Medium Weight

Aerobatic aircraft, ag aircraft, warbirds, all larger aircraft.

Heavy Weight

KITPLANES August 2008

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Weighty Matters What about proper weights and styles? On light aircraft with a wing loading of less than 9 pounds per square foot with never-exceed speeds of less than 160 mph, you can use light weight (1.7 ounces/square yard) fabric. The aircraft found in this category are ultralights, very light aircraft, gliders, most Light Sport Aircraft and kit aircraft with engines of less than 65 horsepower. It should be noted that both Poly-Fiber and Ceconite light weight fabric is uncertified, which means it can only be used for Experimental aircraft, not for production airplanes—with one exception. If you were covering a plywood surface on a production airplane, you would be able to legally use the light weight uncertified fabric. For aircraft not in the above category, medium weight fabric (2.7 ounces/ square yard) is considered the standard. In other words, medium weight fabric may be used for all normal service aircraft including antiques, classics, kit aircraft and all contemporary designs that anticipate normal airport operations. Heavy weight (3.4 ounces/square yard) fabric is recommended for more severe operating conditions and for highwing-loading aircraft. The polyester filaments in heavy fabric are larger and the strength is greater, resulting in excellent resistance to damage and to tearing. Heavy weight should be used where a tough, durable, high-tension fabric is needed, including aerobatic planes, ag planes, warbirds and larger aircraft. (Chart 2 shows proper selection.) Poly-Fiber fabrics are identified as light uncertified (1.7 ounce), medium (2.7 ounce), and heavy duty (3.4 ounce). Ceconite fabrics have uncertified light (1.7 ounce), Ceconite 102 (2.6 ounce) and Ceconite 101 (3.4 ounce). All of the fabrics are 70 inches wide with the exception of the light weight fabric, which is 60 inches wide. Fabrics are sold by the running yard, so that you can calculate the amount needed for your project. Both the Poly-Fiber covering manual and the Ceconite manual contain yardage estimates for most popular homebuilt aircraft. KITPLANES August 2008

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Fabric continued

A simple ink roller marks the fabric as inspected and approved.

Let’s Go to the Tape Any weight of tape may be used with any style of fabric. Most experienced builders use light weight tapes on all weights of fabric. A light weight tape is simply light weight fabric that has been cut into different widths of tape, typically 1, 2, 3, 4 and 6 inches wide. Medium weight tapes are cut from medium weight fabric. Do not use cotton tapes on polyester fabric. Do not use Ceconite tapes on Poly-Fiber fabric or vice versa.

Doing so will void the STC of each system. Ceconite and Poly-Fiber tapes will work on the other fabric, but it is not legal to use them on production aircraft. We will discuss more about fabric tapes later in the series. All types of fabric will deteriorate in direct sunlight unless properly protected. There is no so-called “lifetime” fabric. Polyester fabric is not as susceptible to this problem as cotton; however, if bare polyester fabric is left in direct sunlight for 12 months it will lose over 85% of its strength. Cotton fabric exposed to the same sunlight for the same period will deteriorate almost completely. Polyester fabric is protected from UV rays with chemical coatings containing aluminum pigment. Application of the recommended number of aluminum coats will provide adequate protection for years. (We will discuss how to do this later.) A properly applied covering system should have a 15-year life even if the plane is left outside. The same system on a hangared airplane should result in more than 20 years of service. After selecting the proper weight of fabric for your covering project, how you attach it to the structure is equally important, especially achieving the correct tension. Next time we’ll discuss how to apply the fabric and the proper way to shrink it for final tautness. 

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www.quadcitychallenger.com KITPLANES August 2008

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Students at a SportAir fabric covering workshop learn the steps involved in rib-lacing a surface.

Securing the fabric to the wings. BY RON ALEXANDER

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Last month we began our discussion of attaching fabric to an airplane’s wing. As a quick review, we must ensure that the fabric on a wing will not balloon as a result of the lift created by the wing during flight, which would cause the fabric to try to separate from the upper surface of the ribs. To ensure that this doesn’t happen, you must use some form of mechanical attachment when securing fabric to ribs. There are several options including rib-lacing, screws, pop rivets and fabric clips. Several aircraft plans call for the builder to glue the fabric in place on each rib. As mentioned previously, it is OK to do that, but you must also secure the fabric through a mechanical means. Fabric cement is not adequate protection against the possibility of the fabric lifting during flight, as it has little tensile strength; it is designed to have shear strength. Properly attaching the fabric costs little in money and time, and it will help to ensure safety in the long run. Don’t take the chance. Remember that with a production airplane you must secure the wing fabric to the ribs using the same method employed by the factory when the plane was originally built. If the manufacturer rib-laced, you must do the same. If screws or other specific types of fasteners were used, you must also use the same method. Experimental-aircraft builders may use any method desired—just be sure to mechanically attach the fabric. Also, you will probably want to secure the fabric on any surface that will create lift, such as all control surfaces.

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The Steps As we discussed, prior to attaching the fabric to the ribs you will have shrunk the fabric and then coated it with one coat of Poly-Brush. You are then ready to begin the next step. Before installing the fabric on the wing you will want to be sure the ribs are all parallel to each other. This step must be done prior to placing any fabric on the surface. A twill tape called interrib bracing tape is used to keep the ribs straight up and down when the fabric is tautened with heat. The tape is looped Photos: Richard VanderMeulen

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Fabric continued around the top capstrip of one rib to the bottom capstrip on the next rib until all are secured. This simply keeps all of the ribs straight and parallel with each other until the fabric is mechanically attached. When rib-lacing or some other form of attachment takes place, the inter-rib bracing really serves no further function. Even so, it will not be removed.

Determine Proper Spacing The first step is to determine how far apart you should space the attach points. This means going to the chart presented in Advisory Circular 43-13 or in the fabric-covering manual. Notice that you base the distance of the spacing on the never-exceed airspeed of the airplane. The Poly-Fiber chart also requires a closer spacing in the prop-wash area, which is defined as all of the wing or control surface included within the diameter of the propeller plus one rib. In the interest of cosmetics, most builders will take the more restrictive prop-wash distance and apply it throughout the entire surface. Otherwise, you end up with staggered attach points. There’s nothing wrong with that except that you will not have

Use a cardboard template to help locate the rib spacing on the bottom of the wing. This is necessary because most wings are not symmetrical.

neat looking rows of rib-lacing, screws, etc. when you look down the wing. It is also easier to lay out the spacing if you use the same distance. If you are re-covering a production airplane and know the original spacing, you can use that distance. Most light aircraft will end up with 2- to 3-inch spacing within prop-wash areas. Of course, you can use tighter spacing if you so desire. Spacing requirements for tail surfaces are not as restrictive. You can use twice the wing prop wash spacing in this area.

Use the template on the butt rib to ensure adequate spacing on the top and bottom of the wing.

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Ailerons should use the same spacing as on the wings.

Marking the Spacing The next step is to measure and mark the spacing using a pencil. Do not use anything other than a pencil or blue chalk to mark fabric. Ink may bleed through the final finish. You will begin measuring at the butt rib on top of the wing. Begin at the aft edge of the leading-edge fairing and measure aft toward the trailing edge of the wing. The first point is always placed at one half the distance of the regular spacing. So if our spacing is 3 inches, the first measurement would be at 1.5 inches. Then the next mark would be at 3 inches, and so on. In this case, you would be sure the final mark is no greater than 3 inches from the wing’s trailing edge. After marking the butt rib, pick a rib near the center of the wing and near the outboard end and place the same marks. Rather than mark each rib independently, you can use a common chalk line and stretch it across the marks, snap, and you then have a mark at each rib. Blue chalk line will not bleed through the final finish. Do not use red chalk, though, as it may bleed. Next, we want to measure and mark the bottom of the wing. Unless the wing is perfectly symmetrical, you will have different marks on the bottom. You want any rib-lacing to be as parallel as possible to the wingspars. If the www.kitplanes.com

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After measuring the proper rib-lacing spacing, you can mark the entire surface using a chalk line.

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Mark the spacing in a couple of places and then draw the chalk line very tight.

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Have someone snap the chalk line in the center to properly mark the line. Use only blue chalk.

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wing were symmetrical, you could flip it over and mark the same spacing. But most wings have an airfoil where the top surface has a greater curve than the bottom surface, so you must use a different method to measure and mark. You can keep your lacing parallel to the spar by making a cardboard template. Hold a piece of cardboard next to the butt rib and trace its shape. Mark on the cardboard the location of the forward spar, and cut out this template. Now place the template against the butt rib and transfer to the template the marks you have made on the top of the butt rib. Then draw a line from each mark down to the bottom of the template, keeping the line parallel with the spar mark. After the lines are drawn, KITPLANES December 2008

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Fabric continued you can transfer these marks to the other side of the template. This will give you a template for both wings. Place the template on the butt rib and mark the position of all attach points to the bottom of the wing. Turn the wing over and, using the bottom marks on the template, transfer the spacing to a middle and end rib. You can now use a chalk line to snap marks across all ribs on the underside of the wing.

Reinforcing Tape You must place a piece of polyester reinforcement tape over each rib prior to riblacing or before using whatever means of attachment you decide on. Without this tape to reinforce the fabric, the lacing, screw, etc. will cut right through the fabric and defeat the purpose of this entire step. Reinforcement tape comes in various widths to accommodate the size of your wingribs. Use the width that exactly matches the width of your rib. The tape should be placed on each rib, both top and bottom. Align the tape carefully with the rib as you apply it, and use only approved reinforcing tape. There have been instances where non-approved tapes have been cut by rib-lace cord.

Pre-punch Holes If you are going to rib-lace, the next step is to pre-punch all holes. This will make it much easier for you to accomplish the rib-lacing process. Use a straight rib-lacing needle and punch a hole in the fabric on the rib-lace mark right next to the reinforcement tape. Do this on the top and bottom of the surface. You will use these holes for needle placement during the rib-lacing process.

Place reinforcing tape over each rib. This will protect the fabric from the pulling of the rib-lacing cord.

Rib-Lacing If you are going to use rib-lacing as a means of attachment, be sure that you use only approved polyester rib-lacing cord. It is available as a round or flat After placing the tape on the surface, the end may be neatly trimmed using a single-edge razor blade.

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Notice that the reinforcing tape has an adhesive backing for ease of attachment.

cord. Many people prefer to use the flat cord, because it will lay flat on the surface of the wings and cover nicely with finish tapes. There are two ways to rib-lace. You can put the wing on sawhorses for this process, or you can support it vertically in a wing stand. With the latter method you will need a helper to pass the needle back and forth as you lace. This method is often easier and faster. You can start rib-lacing at the leading edge of the wing or the trailing edge. You can perform the rib-lacing process on the top or the bottom of the wing. It does not matter, because the knots will be concealed on the inside of the wing. Use a curved-tip rib-lacing needle to tie the approved knot. This will allow you to pass the cord under the fabric from one hole to the other. Start with a piece of cord about 6 to 8 feet in length. Only two knots are approved for a production airplane: The modified seine knot is described in Advisory Circular 43-13, and the hidden modified seine knot is found in the Poly-Fiber manual. The hidden knot is much easier to tie and looks better as a finished product. Although I won’t attempt to explain these knots, they are not complicated, but they do require a little practice. Go to workshops for some practical experience. Most major fly-ins will have a fabric-covering workshop where the knot is presented. EAA technical counselors may also assist you.

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Other Methods of Attachment Pop rivets, screws and fabric clips will often be approved and used on metal ribs. Again, you will start by measuring, marking and applying reinforcement KITPLANES December 2008

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Fabric continued

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VË2¬Ë͝ˤɴË+ˉ™WÁj?Äjˉ™Ëĝ”jË?¬¬‰W?͉™ÄË wÁ”Ë?ËÄÍ?™a?Áa˔ÖwzjÁËÄßÄÍj”± VË0†jÄjˏ‰ÍÄË?ÝËߝÖË͝Ëajĉ~™Ë?™aËM։aËߝÖÁËÝ™Ë jކ?ÖÄÍË݉͆Ë?ËÍÁÖjË|ˉ™ÍË¤Ë”jÁ~jËWjW͝Á± build a fixture around it and Kit comes with 1 or 2 fabricate an identical system collectors and 4 or 6 stub risers, depending on Engine using polished stainless steel. Kits come complete with size. Components are hardware, clamps and cabin sized to fit 1-1/2” schedule or carb heat shrouds. When 40 PVC, which is same the system arrives customers OD (1-3/4”) as will be in direct contact with finished header a fabricator to avoid tubes used. Some any communication Continental and issues. Rotax may use different size PVC. Kit comes with full instructions and tips to obtaining the most Finished PVC Mockup horsepower for your engine style. After you send your completed Finished mock up to us, we *deposit required for kits*

tape. If you are re-covering an airplane, then you will have holes where the rivets or screws were previously used. Make sure they are not oversized. If they are, drill a new hole as close as possible to the old one. If you are using pop rivets, buy the ones that have a broad head for use on metal ribs. Standard hardware-store rivets will not work. Place a small 0.016 aluminum washer under each rivet. Pop rivets are certainly easy to install but may be difficult when you want to re-cover, as drilling them out can be a problem. PK screws are another method of attachment. Again, start with the reinforcement tape and the required spacing. Use a 0.016 aluminum washer under the screw, and use self-tapping screws. You should not use PK screws on wood ribs, as they can introduce moisture into the wood over time. Fabric clips are often used for this covering step. They are pieces of wire formed into self-locking barbs that are snapped into holes or slots on metal ribs. Cessna and Taylorcraft use them. They are also difficult to remove without damage. There you have the common methods of attaching fabric to wingribs and control surfaces. The important thing to remember is to mechanically attach the

After the reinforcement tape is in position, you can use a rib-lacing needle to prepunch holes in the fabric along the chalk line. This will make the rib-lacing process much easier.

fabric. Do not risk the consequences of not doing this step properly. If you have an ultralight or other small Experimental aircraft, you can even simplify this process by tying a square knot every few inches. (Any knot is better than none, though you can’t do this on a production airplane.) Just make sure that each knot you tie is independent of all the other knots. That way if one breaks loose, the others will remain in place. Next month we will continue with our covering steps by examining installation of finishing tapes. 

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This photo shows how to place reinforcing tape on a control surface.

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We begin our fabric series with a bit of a history lesson. BY RON ALEXANDER

Aircraft covered with fabric have been in existence since the very beginning of flight. During the late 1800s, kites and gliders were constructed and flown. The Wright brothers’ curiosity resulted in their study of several glider and kite designs, each using fabric material as a covering. They discovered that without fabric on a wing it was useless as a lifting surface. In 1899, the Wrights constructed and flew their first kite from 30

KITPLANES July 2008

which further knowledge of flight was gained, and it was then used in the design of their first powered aircraft. All of these designs were covered with a fabric material to hold the structure together in addition to providing the foundation for lift. Fabric also provided www.kitplanes.com

Builders have the option of working from “raw” fabric in rolls or, for some designs, pre-sewn envelopes that closely resemble the final shape of the piece.

for flexibility of the wing structure, allowing control of the aircraft using the wing-warping technique. The first fabric the Wright brothers employed was “Pride of the West” muslin. It was a tightly woven cotton material used largely for women’s undergarments. Strips of fabric were sewn together to provide covering for the wings and control surfaces. For later model aircraft they used cotton infused with rubber, making it airtight and waterproof.

Along Comes Cotton and Linen Aviation pioneers largely used cotton and linen material for covering aircraft through WW-I. The fabric used on aircraft was later termed Grade A cotton, a name that is still recognized today, and it became the standard for builders of early airplanes. They covered their aircraft with fabric because it was light and easy to apply. But just covering the wings with fabric wasn’t enough. Just as the Wright brothers learned, early aviators found that unless they tightened and sealed the fabric, the wings would not create lift, so they used varnish to seal the fabric. However, they discovered that the varnish would soon turn yellow and crack. They tried furniture lacquers next, with limited success. Finally, they developed furniture lacquer into nitrate dope, and the dope we use today is still made from the same raw material used in lacquer.

Good Dope The term dope as often used in our society today has an obvious and negative connotation. But to people who cover their airplanes with fabric, it has an entirely different meaning. Used for decades, the term describes a liquid that seals and tightens an aircraft’s fabric. Nitrate dope is flammable, and problems associated with this hazard were an issue during the early days of flying. A crash with any type of spark would almost certainly cause a destructive fire. Photos: Richard VanderMeulen and Arnold Greenwell

The Equipment You’ll Need Other than the spraying equipment, the tools required for fabric covering are not expensive. Several of them you probably already have around the house or shop. An ideal list of fabric covering tools includes the following: Sturdy sawhorses about 3 feet high covered with carpet scraps Fuselage turning jig Snag-free table for cutting fabric Electric clothing iron (1100 watts minimum) Small hobby iron Thermometers and heat sink for calibrating the irons An effective respirator Fresh air breathing source if using polyurethanes Half-inch glue brushes Paint brushes Rib lacing needles Sharp scissors Good quality pinking shears Single-edge razor blades Chalk snap line Measuring tape Paint-stirring paddles and straining cones Soup ladle for dipping paint Spring clamps with 2-inch throats to clamp fabric Wood spring clothes pins to hold fabric T-head pins Tack cloths Cotton rags Paint cans for use in applying chemicals A word of caution concerning fabric covering tools. I do not recommend the use of a heat gun. If you have one—hide it during this stage. It is impossible to calibrate the heat emitted by a heat gun. Also, the temperature changes as the gun’s distance from the fabric changes. Your fabric-covering manual will explain the importance of proper temperature when shrinking polyester fabric. A calibrated iron must be used —not a heat gun. We’ll discuss spraying equipment when that step is presented in a later article. —R.A.

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Fabric continued Mechanics doing welding repairs or a careless cigarette smoker could cause major destruction of aircraft covered in cotton and nitrate dope. Polyester material was developed in England during the 1940s. DuPont obtained the rights to manufacture this product in the United States under the trade name Dacron. This fabric, first manufactured in the U.S. in 1953, was widely used in many industries, including the garment trade. During this time, DuPont published information on shrinking this material through the application of heat. Colonel Daniel Cooper, a military officer, began testing polyester fabric for aircraft applications. In 1958, Cooper termed his new material Ceconite and began selling it to cover aircraft. Butyrate dope had also been developed and was being used as a coating over aircraft fabric. It has the advantage of being less flammable than nitrate, but the disadvantage of not adhering to polyester fabrics. The old nitrate dope was found to stick on polyester fabric very well. Colonel Cooper later began applying nitrate dope to polyester fabric for the initial coats followed by finishing with butyrate dope. This process lessened the fire hazards associated with an all nitrate system, and it’s still in use today.

Today’s Covering Processes The aircraft builder or restorer has three different types of covering systems available to cover an airplane. The first is polyester or Grade A cotton fabric using nitrate and butyrate dope as a chemical process. The most common polyester cloth is Ceconite, using Randolph dopes. Legal Grade A cotton is difficult to obtain. There are a couple of sources of cotton or linen material, but you need to ensure that these fabrics meet the Supplemental Type Certificate (STC) requirements of the dope manufacturer. Also, certain Technical Service Orders (TSOs) apply to fabric. For example, medium weight Ceconite meets the requirements of TSO C-15d. If you are covering a production air32

KITPLANES July 2008

The Wright brothers learned early on that unless they tightened and sealed the fabric, the wings would not create lift.

craft, you must stay within the STC guidelines. The Ceconite STC allows the use of aircraft grade nitrate and butyrate dope. The next type of system is polyurethane, and there are two systems to choose from: Air Tech and Superflite. The Air Tech covering system uses polyurethane products throughout, including primers, coatings and topcoats. Air Tech uses any C-15d polyester fabric including both Ceconite and Poly-Fiber fabrics. Superflite also has its own STC using Superflite polyester fabric. The most widely used covering system today is the Poly-Fiber system. This process was formerly known as the Stits covering process named after the inventor of the system, Ray Stits, who developed it in the mid-1960s. His system uses vinyl-based chemicals that do not shrink the fabric after they are applied, nor do they support combustion. This system is entirely different from the others. It is easy to use and is supported by a detailed manual and video/DVD. First-time coverers should investigate each process and choose the one that they want to work with. The cost of each system is within the same range. Issues to consider are flammability, ease of use, longevity, final appearance and quality. Another important issue to consider is availability of instruction. The Experimental Aircraft Association (EAA) presents fabric covering workshops throughout the country on a regular

While there are few restrictions for Experimental aircraft, the vast majority of builders elect to use certified materials.

basis. They are called EAA/SportAir workshops. More information and a schedule may be found at www.sportair. com. Workshops and forums are also conducted at major fly-ins including Sun ’n Fun and AirVenture. Regardless of the process you choose, you must adhere to the materials and instructions specific to that system. In other words, do not mix and match covering systems. Pick one and stay with it. Do not use one type of fabric, another type of coating and a different type of topcoat. This not only voids the STC, but several of these products are not chemically compatible. You will have problems mixing systems—guaranteed.

Legality of Covering Your Own Airplane Can you legally do the covering work on your airplane project without a mechanic’s license? If you are building an Experimental airplane, doing the fabric work is legal. There are no restrictions. Should www.kitplanes.com

Working with fabric is something you can learn easily, though attending a class will help immensely. This is the EAA SportAir workshop hosted by Aircrafters in Watsonville, California, and given by Mark Lightsey.

you be restoring an antique or classic production airplane that requires fabric work, you are allowed under FAR Part 43 to cover the airplane under the supervision of a licensed mechanic. Now, if you are covering a production aircraft, you have a further restriction requiring you to apply a fabric covering process that meets or exceeds the specifications of the original fabric process that was installed by the manufacturer. To do this you must use a covering system that has been issued an STC by the FAA. In addition, the materials included in the STC must be manufactured according

to a Parts Manufacturing Authorization (PMA) also issued by the FAA. Your particular type of airplane must be listed under the STC of the respective fabric process for you to use it as a replacement for the original fabric. Each company maintains a listing of all aircraft that are approved to use their respective systems. Each approved fabric process must have an instruction manual outlining how to apply the fabric and coatings. This manual is an integral part of the STC and must be adhered to during the covering procedure. To use different fabrics or chemicals that are not part of the process voids the STC. What does this mean? If you are going to mix one type of fabric with another type of tape that is not part of the manufacturer’s STC, your airplane will not be considered airworthy. If you mix and match different chemicals, in the same way the final product will not be legal. Again, the best advice is to choose a covering system and follow the manual explicitly. Each company has tested its products for years and knows what works and what will cause problems. Shrinking fabric to fit is part science, part art and a lot of patience. As we move through this series, we’ll offer tips and hints learned over decades of working with the stuff.

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Fabric continued If you are covering an Experimental aircraft, you are free to choose any type of fabric and chemicals you desire to complete this process. However, common sense should prevail. If you do not follow the manufacturer’s covering manual, you will have the same disastrous results that occur from mixing systems. Your Experimental aircraft flies through the same airspace as a production airplane, and from a maintenance standpoint it should be treated with the same care. Just because a custom aircraft builder can legally mix systems and experiment with products, the temptation to do so should be avoided. Otherwise, you may be re-covering your airplane within a short period of time. Often people are unsure about whether they are capable of covering their airplanes. Let me assure you that you can do it. Manuals, videos and/or DVDs help explain the covering process, and workshops provide you with a weekend of hands-on covering experience. You will save a considerable amount of money if you do this work yourself. Typically, a shop will charge $10,000 or more to cover an airplane depending upon the type. This along with approximately $3500 for materials adds up. If you follow the manual you can install fabric and coatings on your

Rib stitching, yes…the often-feared process used to secure the fabric to the wing structure. We’ll tell you all about the best methods in this series.

airplane that will not only result in a quality appearance but will also provide a service life of over 15 years. So let’s get to work and learn the steps of fabric covering.

Tools and Workshop Space At this point you will need to decide where to do the actual covering and spraying. I recommend having your workshop as close to home as possible. If you have a garage, you have a workshop. Having your workshop in or close to your home will provide you with the opportunity to work more often and also to involve your family. Fumes will be a problem. If you are working in your garage, you may want to cover the surfaces there and then take them to another location for spraying. You can, however, minimize the fumes and overspray by building an inexpensive paint booth with a wood or PVC frame that is large enough to house a wing or a fuselage and has room for you to walk around while you are spraying. You can hang the frame from your shop ceiling and then lower it with a set of pulleys. Cover the roof and sides with cheap plastic sheeting stapled to the frame. We’ll have more on constructing a paint booth in a later article.

The Basic Steps We’ll be getting into much more detail as this series unfolds, but here are the basics. Removal of old fabric (restorations only) is followed by the preparation and inspection of surfaces. Then comes the selection of fabric type, attachment of

that fabric, followed by shrinking the fabric. Next up are the chemical coats, taping, making inspection holes and drain grommets, spraying UV protection, color coats and trim. Yes, that’s a shorthand outline, but we’ll be getting into great detail as the process unfolds. But let’s first talk about preparing the surfaces, which may vary somewhat depending upon the material involved. Usually, you will encounter aluminum, steel, wood or fiberglass as the underlying structure. Always make sure you use epoxy primer on metal and fiberglass parts and epoxy varnish on wood parts. The majority of other primers and varnishes will be lifted from the aircraft surface by the chemicals found in covering processes. MEK and reducers found in covering process chemicals will not affect most epoxy primers and varnishes. But they will lift some paints and varnishes like a paint stripper, allowing moisture to collect in the metal or wood with obvious consequences. If you have already primed or varnished with some other product, test it before applying the fabric. Soak a rag with MEK and leave it on the surface for about 30 minutes. If it lifts the paint or varnish, you need to redo the surface. Often you can simply spray epoxies over the existing finish without having to strip them. This, too, can be tested. Spray the epoxy over the existing surface on a small area to be sure it does not act as a paint stripper prior to applying it to the entire piece. Before priming or varnishing a surface, be sure it is completely clean and

One of the most-used items in your fabric toolbox will be a calibrated iron.

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KITPLANES July 2008

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free of all oil and other contaminants. Do not prime over rusted pieces. Remove the rust and immediately prime. A piece of bare steel will rust within hours if a primer is not in place. Make sure you fill all dents in leading edges, etc. I would recommend a product called SuperFil rather than Bondo, which is a polyester filler that will shrink with age. I do not recommend using Bondo on aircraft surfaces unless you are prepared to redo the filled portion of the surface after it shrinks and cracks the topcoat. SuperFil is an epoxy filler. That means it will not shrink over time. It may be used on wood, fiberglass or metal with equally good results. You may want to use cloth padding on a really dented leading edge. Polyester padding is often used between the leading edge aluminum or plywood and the fabric itself. All of the sharp edges that could potentially cut the fabric should be covered with anti-chafe tape. This usually includes rivet heads and metal seams. Let your sense of touch be your guide. If you feel something sharp, cover it with the anti-chafe tape. Do not use masking tape for this purpose. It will retain moisture and cause problems later on. Also, paper masking tape will turn brown with age and possibly show through a light colored paint. That’s it for the background and some of the prep work. In the next issue, we’ll continue with a detailed presentation of the covering steps and processes you’ll be using on your own project. 

While the basis of a fabric covering is, indeed, the fabric, there’s much more to it, including glues, “dope,” UV protectants, primer and paint. We’ll cover it all over the next few months.

KITPLANES July 2008

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Cutting and fitting the fabric prior to shrinking is essential for a good fit.

A calibrated lib t d iron i isi iintegral t l tto th the shrinking h i ki process. BY RON ALEXANDER

The next step in our ongoing discussion of fabric covering is to shrink the fabric to its proper tautness. This is a critical step in the covering process. Proper shrinkage is essential to a long-lasting covering job. Polyester fabric will shrink approximately 10% to 12%. When using the PolyFiber covering system, and all other covering systems except nitrate and butyrate dope, the tautening process ends after you shrink the fabric with a household iron. Covering an 32

KITPLANES October 2008

aircraft using Ceconite fabric along with nitrate and butyrate dope is a different story. Nitrate and butyrate (even the non-tautening variety) will continue to shrink fabric throughout the life of the airplane, with both Grade A cotton and polyester fabric. As the dopes shrink, they tighten the fabric they have encapwww.kitplanes.com

sulated. Proper heat-shrinking procedures for Ceconite fabric may be found in the Ceconite covering manual. When using Ceconite fabric, use only non-tautening dopes to cover your airplane even though they will shrink (to a lesser degree than regular dopes) with age. When using this process, you will not shrink the fabric to its maximum prior to applying the dopes, because you must allow for the resulting shrinkage caused by the dopes themselves. There are several reasons why an aircraft has to be re-covered before the full life of the covering system has been achieved. One of them concerns improper tautening of the fabric when it is placed on an aircraft structure. If the fabric is too loose, it will create a drumming effect in flight, causing the chemical coats to crack and peel. If the fabric has been tightened too much, structural damage may result. Either of these problems will contribute to having to prematurely re-cover your aircraft. This in mind, it is important that you use the proper procedures to tighten the fabric. The fabric tautening process is much simpler today than in the days of Grade A cotton fabric. Cotton has to be initially shrunk with water and then allowance made for the fabric to continue to tighten through the years as a result

The iron can be used to heat-form the fabric around curved areas. Slowly working and shrinking the fabric allows it to be perfectly formed with no wrinkles.

of the shrinking of the dopes that are applied. Attaching the fabric properly prior to the tightening process is essential. If it is too loose when attached and glued, you will never be able to achieve the desired tautness. If it is too tight when attached you will risk damaging a structure from the subsequent tightening through the years. Today’s polyester fabric is shrunk to its optimum tightness by applying heat with a regular household iron. With the iron properly calibrated and set at 225°, wrinkles in the fabric can be worked out where necessary. Be careful not to iron over areas that are cemented, as 250° will loosen the cement bond.

Any additional tautening is not only unnecessary, but it may be harmful to the underlying structure. Too much tightening can distort or damage the components of the airplane. We will use a regular household iron to shrink the fabric to proper tautness, using the Poly-Fiber covering system for our illustration. With any system, be sure you follow the procedures that are outlined in the manual. Note that when the iron is properly set at a maximum temperature of 350° F, you can leave it in one place and it will not burn through the fabric.

Photos: Richard VanderMeulen

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Fabric continued The initial tautness of the fabric prior to the shrinking process was discussed in a previous article. As a brief review, if you are covering a wing, the fabric should be attached loose enough to allow you to pull the fabric above the top of the structure about 1 inch. As you gain experience you will be able to properly judge the amount of tautness prior to shrinking.

Iron It Out The iron you use must be properly calibrated prior to using it on your fabric. You will want to calibrate and mark at least four temperature settings: 200°, 225°, 250° and 350°. Your iron should be of a high enough quality to hold the desired temperatures within +/-10°. Let’s look at what actually occurs when you tighten polyester fabric. Once again, the fibers have the capability of shrinking about 10% to 12%. This will occur at a maximum temperature of 350° F. At 250° F the fabric will shrink about 5%. At less than 250° F (200° to 225°) the fabric will smooth but shrink little. At a temperature of 370° F the fibers will actually begin to loosen. At 425° F the fibers will melt. As you can see, it is extremely important for you to know the temperature of the device you are using to apply the heat. The household iron you use should have a rating of at least 1100 watts or it will not reach 350°. Never use a heat gun to shrink fabric. You have absolutely no idea what temperature is being emitted and furthermore, the temperature being applied will vary according to how far away from the fabric you hold the gun.

The Calibration Process Obtain an accurate thermometer with a stem that can be placed under your iron. The best thermometer is a glass bulb type that is pre-marked for the temperatures outlined above. This is available through all Poly-Fiber distributors. Purchase some silicone heat-sink compound. Build a half-inch-thick stack of dry paper towels. You will use this as a heat 34

KITPLANES October 2008

Stewart Systems A tour of the displays at the Golden West Air Show in Northern California in early June gave me a chance to interview Doug Stewart, President of Hangar 21. You might be more familiar with the company under the name of Stewart Systems, maker of eco-friendly fabric coverings and paint for aircraft. Stewart was demonstrating the company’s fabric covering system and started by handing a bottle of the adhesive around as he described it as non-flammable, non-toxic and suf- Doug Stewart of Stewart Systems demficiently non-aromatic that, “It can be onstrates his STC’d method of applying applied in the bedroom with no out- fabric. The non-toxic glue is brushed onto side ventilation.” A sniff of the bottle clean tubing. bore that out. In addition to keeping the neighbors and spouse happy, these benign characteristics mean that it can be shipped UPS/FedEx with no additional HazMat expenses. The product is supplied in plastic bottles and requires dilution with distilled, not filtered, water. Stewart was clear about this: Filtration devices are not all the same in what they remove, and with age they may change in their effectivity. “You only need a gallon bottle or so to do an entire airplane, so that’s not a big expense,” he said. Application was equally easy. After cleaning the tubing, you need only brush a liberal coat on. There’s no need to rush, as you could come back the next day to apply the fabric. After coating a length of tubing, Stewart simply draped some fabric across it. The glue is tacky, so you can lift the fabric and adjust out the wrinkles. Once it was smooth, he applied a hot iron to activate the glue. As it soaked through the fabric, a simple wipe with a rag was sufficient to remove any excess. Gluing the tapes and rib-stitching, if needed, is done on bare fabric with no preparatory coatings required. Stewart pointed out that, “You can paint the glue onto the tape and lay it on the base fabric, then apply the iron to activate it.” He added that the glue doesn’t stick to the iron, so it makes a very smooth edge. The last step is to apply a second coat of the glue, wipe off the excess, and call it done. No secondary hot iron is needed. Stewart was also happy to point out that the fabric system is approved by the FAA under STC SA01734SE. Stewart Systems is located in central Washington, Cashmere to be precise, right on the Cashmere Airport (HS2). “Drop by to see us,” he said. “It’s a short field, so if you prefer to land at Pangborn, we’ll come pick you up.” Stewart Systems also has an eco-friendly group of paints to finish off your project. Look for a lengthy report on it in an upcoming issue. For more information, call 888/356-7659, or visit www.stewartsystems.aero. —Bob Fritz

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THE VELOCITY TXL sink on which to place the iron while calibrating it. Next, place a small amount of heatsink compound on the bulb of the thermometer and lay it in the middle of the paper towels. Place your iron on top of the thermometer that is on the paper towels. Turn the iron on, advance the heat control to the wool setting, and watch the temperature rise. Let it stabilize and then vary the control to reach a temperature of 200°. Using a piece of masking tape, place a mark over the temperature dial at the 200° setting. Vary the temperature until you have the iron calibrated and marked for 200°, 225°, 250° and 350°. Turn the iron off and allow it to cool. Then thoroughly remove the silicone heat sink on the bottom of the iron.

Shrinking the Fabric

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After attaching the fabric to the structure and prior to shrinking the fabric, you must allow the fabric cement to completely dry. Failure to do so will often result in the fabric being pulled loose from the structure.

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Always use a good quality iron rated for at least 1100 watts.

KITPLANES 2008

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Fabric continued Normally, you will do two separate iron passes on the fabric. The first ironing is done at 250°, followed by a final ironing at 350°. If you are covering an ultralight or a lighter structure aircraft, shrinking beyond 250° could result in structural damage. Also, if you are using nitrate and butyrate dope then you will only shrink to 250°. If you have used a fabric envelope with sewn seams, you will want to shrink the area immediately along the seam first. Failure to do so will cause the seam to be crooked. You will want to be sure that the seam is perfectly straight. Oth-

The iron is being used to smooth out wrinkles in a tape that is covering a curved surface.

erwise, you will have difficulty placing a finishing tape over the seam. As you begin the shrinking process, the fibers of the fabric will shrink uniformly and evenly. If you are doing a large surface, such as a wing, begin in the bay near the wingroot. Shrink that at 250° and then do the bay at the wingtip. You can then work your way toward the middle of the wing. This will help prevent any warping of the airframe. Let the iron glide over the surface—no pressure is necessary. Do not worry about leaving the iron in one place temporarily. It will not scorch the fabric, nor will the fabric get any tighter. The amount of shrinkage is due totally to the temperature, not the pressure or time. Don’t worry about removing all of the wrinkles on the first pass. They will come out with the higher temperature setting you will use on the next pass. Iron over the hard surfaces such as the leading edges. Realize these areas may act as a heat sink and require a little extra time to properly shrink. Be careful not to allow the tip of the iron to penetrate protrusions or rivets and cut the fabric. Also, and this is important, do not place the iron over cemented

A cardboard “ironing board” is used to help shrink the fabric inside the curve of the wingtip to give it the correct shape. The cardboard is simply a tool that will provide stability to the loose fabric.

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KITPLANES October 2008

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seams. A temperature of about 250° will loosen fabric cement. After shrinking the fabric at 250°, you will then turn the heat up to the 350° setting and iron everything again. Be sure you shrink all of the fabric at this setting. The 350° setting will shrink the fabric to its maximum of 10% to 12%. Failure to shrink to this setting on

The edges of a patch on fabric covering can be smoothed using an iron set at 225° F.

most airplanes (other than ultralights) will cause the fabric to be loose forever. When you pass the iron over the fabric at this temperature you will often see steam rising off of the fabric. This is normal. Moisture from the fabric is escaping. Also, it is a good idea to mark each wing bay or area that has been shrunk to 350° using a pencil, not a pen. That way, if you are distracted, you will know which areas are completed. You will want to purchase a small heat-sealing iron in addition to your regular iron. This smaller iron will be used in some of the non-load-carrying areas that are inaccessible with a larger iron. Use this type of iron only in areas where exact fabric tension is not critical. Removal of wrinkles and smoothing of tapes can be accomplished this way. Calibrate the smaller iron just as you do the larger iron. If you are a beginning fabric coverer, I recommend learning the techniques on a small surface such as an elevator. Perfect your procedures before going to the wing or fuselage. Ideally, learn the basics from the EAA SportAir workshop, or the company manuals and videos. The video and DVD provide an excellent visual image of the procedure discussed in this article. 

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The initial shrinking of a fabric surface is done at 250°. After this ironing, a final shrinking should be accomplished using 350° F.

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Make M k a lasting l ti attachment. tt h t BY RON ALEXANDER

Covering an airplane with fabric can seem a daunting task. Taken one step at a time, though, it is actually relatively simple. No matter what brand of fabric covering system you decide to use, the basics are the same. The surface being covered must be cleaned, repaired, inspected and then primed or varnished before the fabric can be attached to the structure. After accomplishing those steps, you then protect the fabric from sharp edges, ridges, etc. by using anti-chafe tape. The next step is to attach the fabric to the part.

Let’s Get Started Attaching the fabric is the first major task involved in the actual covering process. It is imperative that this step be done correctly. Proper attachment provides the foundation for the remainder of the process. There are two ways to attach fabric to a part, either by using a pre-sewn envelope or by simply cutting the fabric and attaching it. If you elect to purchase a fabric envelope, it will slide over the part being covered like a sock. You would then 32

KITPLANES September 2008

attach one end and begin the shrinking process. You can save time by using an envelope, but the cost is higher. They are available for most aircraft. Envelopes have a sewn seam that needs to be properly placed and hidden. This seam will be located on the outside of the envelope. You must then turn the envelope inside out to make sure the seam will be on the inside of the envelope against the structure, which will help hide the seam. Can you buy the material and sew an envelope yourself? You could do this, providing you use the approved machine sewing thread. However, I do not recommend sewing your own envelope unless you have a commercial-grade machine. The fabric will wreak havoc www.kitplanes.com

on a regular sewing machine needle. Save yourself time and grief and order an envelope rather than attempting to sew it yourself. And if you are going to do any stitching at all, even by hand, you must use the proper polyester thread.

A Blanket Endorsement If you want to save money and not worry about dealing with a sewn seam, you can cover the airplane surfaces using what we call the “blanket method.” The blanket method simply means you cut the fabric to fit and then attach it with fabric cement to the surface. With this in mind, let’s go through the steps involved in attaching the fabric with this method, using a wing to illustrate the process. We’ll assume that the wing is ready for cover. For a new fabric covering—that is, not a replacement or restoration— it’s essential that all of the surfaces are properly prepped. The preparation steps are explained in the manuals and videos, and we have touched on them in the previous installments of this series. If you have elected to use the PolyFiber covering system, it is essential that you apply two coats of Poly-Brush on the leading edge of the wings and any other large plywood or metal surfaces prior to placing the fabric. This step provides a bed that will help the fabric adhere to large surfaces and also reduces the possibility of pinholes in later steps. A few considerations before we begin. Fabric is generally attached using fabric cement. This is a special type of glue that has high strength in shear loads. PolyTak cement is used with the Poly-Fiber process. This cement is applied initially to the structure itself, and the fabric is then placed over the cement. The cement is worked up through the fabric to provide a bond. This step is used only to hold the fabric in place on the structure. You will then wrap the fabric around the surface you are covering and overlap it where the fabric joins, which provides the necessary strength to secure the fabric to the surface. Again, fabric cement is applied to the fabric that you initially glued in place, and then the overlap is placed over the top of the cement and worked Photos: Richard VanderMeulen, Marc Cook

through the fabric. The overlap of fabric should be a minimum of 1 inch on all surfaces except the leading edge of wings, where it needs to be a minimum of 2 inches.

The First Cut Is the Deepest Now it is time to cut the fabric. Most small aircraft wings have a chord that measures less than 70 inches. The width of both Ceconite and Poly-Fiber fabric is 70 inches. This means that you should be able to lay a piece of fabric on both the top and bottom of the wing and glue them in place, overlapping them as described later. Roll out a piece of fabric over the top of the wing, leaving enough extra to cover the entire butt end of the wing along with about 1 foot at the tip. (You will see later why we need the extra length at the tip.) Cut another piece the exact same length for the other side of the wing. Before we actually attach the fabric, let’s consider an issue that has to do strictly with the visual appeal of the finished product. Which side of the wing can you see when you are standing next to the airplane—top or bottom? If it is a high-wing airplane, you’ll more than likely be looking at the bottom. If it’s a low-wing airplane or the bottom wing of a biplane, you will see the top side of the wing. Once you’ve considered which side you can see, plan to glue the seams on the opposite side. This way, if the final cemented seam isn’t perfect, no one will notice. For example, on the lower wing of a biplane you will want to cover the bottom of the wing first, so that the top piece of fabric is overlapped and cemented to the bottom piece on the underside of the wing. This is purely cosmetic, but it is good to be thinking from the start about ways to make the finished piece not only technically correct but also visually attractive. Now let’s proceed with the attachment of the fabric to the wing. When applying the first piece of fabric, you should cement a 2-inch-wide area of fabric to the leading edge of the wing. Where this is located is up to you. The only requirement is that the second piece

When using the blanket method of covering, the first step is to lay the fabric over the structure to be covered.

Use only a new, sharp razor to cut and trim fabric. Take your time!

For larger cuts, use a pair of sharp scissors to cut the fabric along the pencil line you’ve already drawn. Remember to never use a pen or Sharpie.

Use cloth adhesive tape to cover any sharp structure that will be under the fabric. If you don’t, the sharp edges will eventually cut or wear through the fabric. KITPLANES September 2008

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Fabric continued of fabric you install reaches far enough around the leading edge so that you can cement it to the first piece with a 2-inch overlap. This is for strength purposes. In other areas the overlap requirement is only 1 inch. We will make our overlap on the centerline of the wing’s leading edge. To ensure that it’s straight use a chalk line and snap a line along the centerline as a marker. Then measure 1 inch above the centerline and 1 inch below the centerline and snap parallel lines at these marks. (Regular blue carpenter’s chalk line will disappear later and won’t bleed through the final coatings—do not use red chalk). These chalk lines will be used as a guide later when cutting and cementing the fabric to the leading edge. When marking fabric for cutting, always use a pencil—never an ink pen. Ink will bleed through the final paint coat.

Poly-Tak fabric cement is one of the chemicals that can be used to secure the fabric to the structure. Be aware that certain cements must be used with certain fabric types and systems.

Over You Go...

Here is an example of properly protecting the fabric from sharp edges; use only enough tape to do the job.

Fabric cement should be used to completely coat the structure to which the fabric will be bonded.

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KITPLANES September 2008

Next, position the wing topside up. You will now need lots of clothespins and spring clamps to clamp the fabric in place. Lay the fabric on the wing, ensuring that it rests smoothly on the surface, and then clamp it in place exactly in the position where it will be cemented. You will usually have to reposition the clamps at least once before you are ready to cement. Keep repositioning the fabric until it rests smoothly on the surface. Do not start to cut or glue until the fabric is clamped in position. This is true when you are covering any structure. How tight should the fabric be when you initially glue it in place? The tightness here will affect the final tautness of the fabric. Heat tightening will shrink the fabric about 10%, so on a 50-inchwide wing, the fabric will shrink about 5 inches. You do not want the fabric so loose that it will not properly tighten and, conversely, you do not want it so tight that structural damage results from the heat-shrinking process. You can actually bend or warp light structures during heat shrinking if the fabric is too taut to begin with. For example, if you are covering a

wing, the fabric should be attached loose enough to allow you to pull the fabric above the top of the structure about an inch. This is a rule of thumb. As you gain experience with the process you will be able to properly judge the amount of tautness required. The first question is where to begin gluing the fabric to the wing, and the answer is that it doesn’t really matter. Do whatever seems most practical—with one exception: Always do the wingtip bow last. We will begin with the trailing edge and aileron. Apply a wet bed of cement to the surface using a 1-inch brush and then press the fabric into the wet cement. It’s important that you don’t go over the top of the fabric with more cement. You want the solvents to evaporate out through the fabric. Work with no more than a 12- to 18-inch strip of cement at a time. It is advisable to go over the fabric using a small plastic or rubber squeegee to remove the excess cement so that there are no lumps in the seams. In some areas you will not be able to use a squeegee, and you will have to use your fingers. (Protect your hands using latex gloves or Invisible Glove hand cream.)

Apply the fabric cement in small sections, as it will dry rapidly, particularly in hot climates.

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Next, we’ll cement the fabric to the butt or wingroot rib. This is done on the side of the rib that will be next to the fuselage. You are not actually attaching fabric to the capstrip area of the rib, but doing this now will help secure the fabric in position before turning the wing over to cement the leading edge. We now have the fabric glued in position on the trailing edge and at the butt rib where the wing attaches to the fuselage. Now we are looking at the bottom side of the wing where we will glue the fabric to the leading edge. (Again, we are assuming the wing we are covering will be visible from the bottom side, so we will want our overlap on the top of the wing.) The next step is to cut the fabric along the snapped chalk line that is 1 inch from the centerline toward the top

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Poly-Tak is the preferred cement to be used with the Poly Fiber system.

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of the wing. You should be able to see the chalk line through the fabric, and this will show you where to cut. Now snap a chalk line on the piece of fabric that exactly corresponds to the chalk line already on the wing. Before cementing the fabric to the leading edge it must be cut to the proper size so that the seam follows our chalk line on the leading edge. Don’t make the cut yet. The fabric will want to fray badly when cut. If this happens, and you glue it down with the frayed ends, you will KITPLANES September 2008

35

Fabric continued forever see them in the finished piece. Remember, in fabric covering, if you can feel it, you’ll see it in the final product. There is a trick to keeping the fabric from fraying, and you should do this every time you’re about to cut a piece of fabric. With a 1-inch brush, coat the area you are about to cut with a small amount of cement. This seals the fabric weave and will keep the fabric from fraying. Always use a good pair of straight scissors (not pinking shears) to cut the fabric. We want a straight edge for this seam. Why not pinking shears? We want to hide this cut under the second piece of fabric. A pinked edge has about 40% more edge area than a straight cut, so it is much more difficult to hide. Let’s cut the fabric carefully along the blue chalk line. Now we are ready to glue the fabric in place. Starting in the middle, cement the fabric to the leading edge using the chalk line previously snapped as a guide. Work in sections of 12 to 18 inches. Brush at least a 2-inchwide wet bed of cement on the leading edge, press the fabric down into it and, using a squeegee, remove the excess. The squeegee accomplishes two things: It firmly presses the fabric into the cement and also keeps the glued area free from lumps. As you squeegee, put a slight side load on the fabric in the direction you’re cementing, but do not pull and tug on the fabric. When you arrive at the tip end of the

leading-edge skin where the wingtip starts, make a small vertical cut down the leading edge material and cement that area. Leave the tip fabric loose for the moment. We are going to use a process called “heat forming” to mold the fabric around the wingtip. We will actually pull the fabric tight around the tip and iron it to the contour. First, set your iron to 250 to 300® F—this is for reference at this point. We’ll get into the details of how much heat to use when, and how to calibrate your iron in the next installment. Then begin ironing toward the hand that’s pulling, but always perpendicular to the tip bow. Pull straight, and do a little at a time until the fabric lies perfectly flat. Carefully cut off the excess fabric along the curve, and start cementing in the center. Apply a bed of cement and then press the fabric into it. Hand-squeegee out the excess.

Bottom’s Up Now we have fabric attached to the top side of our wing. We are ready to glue the other piece we have cut to the bottom side. Before we begin this process, take a look at your cemented areas. Any lumps or excesses? If so, they should be smoothed out, otherwise they will show in the final product. Remove the cement with a clean rag and MEK (protect your hands). Be sure not to scrub the seams, as that will fuzz up the edge. Always rub in the direction that causes the fibers to lie down in the seam edge.

Anytime you are overlapping fabric and cementing it in place, you must have a minimum of 1 inch overlap as illustrated.

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KITPLANES September 2008

This picture illustrates fabric cement being applied just prior to placing the fabric onto the structure.

After gluing the fabric to the structure, you can carefully trim it using a single edge razor blade.

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If you are overlapping fabric onto the leading edge of a wing surface, the minimum amount is 2 inches.

Now smooth out any wrinkled areas with your iron set at about 200°. If you have a wrinkle that won’t go away, you may turn the temperature of the iron up a bit, but be careful. Too much heat will release the cement bond. The objective here is to clean up and smooth out all areas where you will be cementing the second piece of fabric onto the first one. Let’s glue the other piece and complete the attachment step. Initially, position the wing with the covered side up. First, snap a line 1 inch inboard of the trailing edge on the fabric. This will give you a guide for gluing the fabric, ensuring the proper 1-inch overlap. On the leading edge, snap a line 2 inches past the edge of our first piece of fabric. Now for the bottom piece. Turn the wing over and clamp the fabric in place as before, making sure it rests smoothly on the surface. Begin cementing in the aileron well (if there is one). You will have to cut the fabric to get it to properly fit. Remember to coat the fabric with cement before cutting. You can cut into corners at a 45° angle to remove the excess fabric. Next, cement the fabric onto the butt or root rib. We now need to turn the wing over to glue the seams. You can spot-glue areas of the leading fabric to hold it in place while rotating the wing. You will be able to see the chalk line previously snapped on both the leading and trailing edges through the fabric. Snap a chalk line onto the fabric itself that corresponds to

each line underneath. Apply cement to the line and carefully cut the fabric. You can now use fabric clamps to hold the trailing edge fabric in position while you cement the leading-edge seam. Important: You have a blue chalk line on both the leading and trailing edges, and you have a corresponding blue chalk line on the second piece of fabric that is about to be cemented to the first. These two lines should meet when you cement the seams. If they do, you will have a perfectly straight seam. Cement the fabric to the leading edge with a 2-inch-wide glue area. Start in the middle and, with the squeegee, work your way to each end, using a slight sideways motion as you go. Cement the trailing edge using this same technique. It really does not matter whether you cement the leading edge or the trailing edge first. The last step is the wingtip bow. Heat form the tip just as you did before. Pull and iron toward your hand. Keep going until there is an inch of fabric resting flat around the tip. Using your pencil and a ruler, carefully draw a line 1 inch in from the tip bow radius. Cement the line and make the cut. Now use the cement to secure this edge in place. You now have successfully covered the wing. Go back over any rough areas, and smooth them. Next time, we’ll discuss how to calibrate your iron and take a detailed look at shrinking the fabric to the airplane structures.  KITPLANES September 2008

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During this procedure, the fabric should appear glossy and wet, and there should be no dry areas.

We’ll apply the first chemical coat and look at the rationale for rib-lacing. BY RON ALEXANDER 32

KITPLANES November 2008

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Previously, we’ve discussed selection of the proper type of fabric, attaching the fabric to the surface and heat-tautening the fabric. This month the chemical coats are applied to the fabric, and we’ll discuss securing it to the top surface of a wing.

First Chemical Coat After you have attached the fabric to the airplane surfaces and tightened it properly, you are ready to apply the first coat of chemical. In the Poly-Fiber process, this chemical is called Poly-Brush. Application of this initial coat is generally referred to as sealing the fabric. The first coat of Poly-Brush must bond to the fabric or all subsequent coats are in jeopardy of peeling. Some of you may have witnessed this firsthand. It is a common problem and is a major reason that aircraft must be prematurely re-covered. If the first coat does not bond, you can literally peel off all subsequent coats. I have seen aircraft where you could take

Care must be taken when applying PolyBrush around the leading and trailing edges to avoid getting a large buildup on these areas.

Photos: Richard VanderMeulen, Marc Cook

an air gun and blow the coatings off of the fabric in sheets. The problem is usually caused by one of three things: 1) The fabric was contaminated with dirt or oil; 2) the chemical was not applied properly; or 3) the chemical was applied in temperatures that were too cold. The first coat of Poly-Brush also provides a secondary function: It acts as cement that soaks through the fabric and secures the fabric to the airframe. This is particularly important on a plywood surface such as a wing. I recommend brushing the first coat of Poly-Brush onto the prepared fabric rather than spraying it. This should be done only after you have thoroughly cleaned the fabric using a clean cotton rag and reducer or MEK. All traces of oil and dirt must be removed. A tack rag should then be used to wipe down the fabric just prior to applying the first coat. Failure to remove dirt and oil will result in an inadequate bond. With the first coat we are working to literally encapsulate the fibers of the fabric. This means the chemical should flow under the fibers and grip them for a good bond. Polyester does not readily absorb liquid, so care must be taken to ensure proper penetration of the chemical through the fabric. This is why I recommend brushing on the first coat. You must penetrate the fabric with the liquid to get it to the back or the underside of the surface. Unless you have experience covering aircraft, it is difficult to tell when you have an adequate penetration if you are spraying this first coat. Though it certainly may be sprayed, do so only if you are an experienced fabric person. Use a high quality polyester or natural-bristle brush for the first coat. Poly-Brush must be thinned prior to application. It should be three parts of Poly-Brush to one part reducer. Two reducers are available, and you will select one based upon the temperature when you apply the coat. If the temperature is less than 85°, you will use the 65-75 reducer. If the temperature is higher than 85°, use the 8500 reducer. Do not attempt to brush or spray any coats of Poly-Brush if the temperature is less than 60° F.

When applying Poly-Brush, be sure to use a good quality brush, and use even strokes. It is not necessary to continue over the same area more than once.

Poly-Brush should be thinned one part of reducer to three parts of Poly-Brush.

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Fabric continued Poly-Brush has a pinkish tint, so you can see where it has been applied. If you are covering an open-cockpit airplane where the inside of the fabric will be visible, a clear Poly-Brush is available for use on those areas. Find an old reducer can or some sort of gallon can, and cut the side off to make a trough. This makes a nice container to pour your chemical into before applying it to the fabric. You can place a piece of welding rod through holes in the can to wipe your brush on or to hold the brush out of the liquid. Brush on a wet coat. Be sure the chemical is penetrating the fabric and flowing to the underneath side. The fabric should appear glossy and wet, with no dry areas. You must be careful not to allow it to drip through to the other side of the surface, as the drips may show up on the final color coats as a difference in gloss. You also need to be careful to work quickly and not leave any brush marks. In normal temperatures the coating will dry rapidly. You will probably see runs on the inside of the fabric. This is perfectly normal, and they will

Applying Poly-Brush to the leading edge of a control surface. Note how the brush is being held.

not show on the final product. Just don’t let them drip through to the other side of the piece you’re working on. If you do get drips on the other side, immediately turn the surface over and wipe them off with a clean rag and reducer. This is not necessary if the drip occurs on a surface that has a coat of Poly-Brush already applied. In other words, after completing the first side of the surface, this warning is not important. The basic idea of this step is to thoroughly brush on a wet coat and then make only one more pass with your brush to level out any small bubbles or marks that may have formed. Work steadily so that you are not still brushing when the surface dries. It will dry fairly rapidly in temperatures above 85°—about 5 minutes or so. In cooler temperatures drying takes about 15 minutes. Brushing when the surface is just about dry will leave serious brush marks. Do take some time with this initial step. Unless you thoroughly encapsulate the fabric with the chemical, you will be faced with a major problem within a few years. What is the solution if you have all of the coatings peeling off of your airplane? Usually the fabric is good. If so, and you can easily peel the coatings to the bare fabric, you can often solve the problem. If the fabric has not been exposed to the sun or lost its strength in any other way, you can reapply the first coat of the

process and continue the build-up of the coatings. Seek the advice of someone with fabric experience prior to doing this. If you cannot easily remove all coatings to reach bare fabric, do not try to use a paint stripper. The stripper itself is difficult to remove from the fabric. The best solution is to apply Poly-Brush properly in the first place.

Attaching Fabric to Wingribs When fabric is used to cover a wing, certain precautions must be taken to ensure the fabric does not “balloon-up” in flight. Lift on a wing will cause the fabric to attempt to rise up on the top surface and separate from the ribs, and it could be disastrous. You certainly do not want the fabric to separate from the ribs and spoil the lift. If this occurs in flight, an accident is almost certain to follow. What normally occurs in this situation is that the fabric will separate on one rib followed by adjacent ribs as the pressure builds. This acts as a spoiler on one wing with the other wing creating full lift. An uncontrolled spiral results. You can think of the lift created by normal flight as a giant vacuum cleaner that is trying to peel your wing fabric off the top surface. Beginning with the first aircraft flown in the early 1900s, fabric has been mechanically secured to the ribs. The Wright brothers used a sewn pocket in the fabric itself, into

The tinted coating allows you to see where the product has been applied.

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which they inserted the wingribs. (This method is still being used successfully by several ultralight manufacturers.) Bleriot aircraft used a piece of wood physically attached on top of the fabric to the rib below. So our next step in the covering process is to mechanically attach the fabric to the wingribs. This will also be accomplished on the control surfaces of most aircraft, or any surfaces that create lift. Seldom will you find a fuselage that needs any rib-lacing. A rib-lacing cord is the most common method of securing the fabric to the ribs of a surface. This cord is made out of polyester material and is extremely strong. A special needle is used to tie a specific knot in this process. Other methods of attaching fabric to ribs include the use of screws, pop-rivets and special fabric clips.

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Mechanical Attachments With the advent of the ultralight, a number of people have used alternative KITPLANES November 2008

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Fabric continued methods of fabric attachment including cementing the fabric to the ribs. This has become somewhat common recently, using cements designed for fabric, and the ribs are made a bit wider to accommodate it. At Poly-Fiber, fabric cements have been designed to hold two pieces of fabric together against the shear forces that may try to pull them apart. The fabric on the leading edge of a wing is overlapped by 2 inches, and that overlap is secured by applying fabric cement to the two pieces of fabric, thereby holding them in place. However, the fabric cement is not designed for peel forces that result from the lift created during flight. I recommend using a form of mechanical attachment when securing the fabric to the ribs. Gluing without a mechanical attachment is simply asking for potential problems. Why take the chance when the fabric may be secured properly with a little extra effort? On production aircraft, the method used to secure the wing fabric to the wingribs should be the same one used at the factory when the airplane was manufactured. If you want to use an alter-

One pass can be made after the initial coating to work out any bubbles or brush marks.

native method you must obtain FAA approval. On Experimental aircraft, you may use whatever means you desire. I would suggest rib-lacing. If your plans call for cementing the fabric to the ribs, you should also rib-lace as an added precaution. If you are concerned about tying the proper type of knot, you might attend one of the EAA/SportAir fabric workshops or visit one of the workshops

held at various airshows. The knot is demonstrated and practiced at these events. The knot may appear to be challenging when in fact it is quite simple. If you have an Experimental aircraft and you are having problems with the riblacing knot, simply tie square knots with each lace spaced properly. The proper spacing requirements may be found in the Poly-Fiber Covering Manual or in

This gallon empty can has been cut away to be used as a container. It also has a piece of welding rod placed through it to allow you to wipe the brush.

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Although Poly-Brush comes tinted so that you can see it while brushing or spraying it on, it also comes untinted for certain applications where you do not want the color to show through the fabric.

FAA Advisory Circular 43-13. The distance between the mechanical attachments depends on the never-exceed speed of the aircraft. As an example, if the Vne of your airplane is 150 mph, the distance between fabric attachment points on each rib will be 2.5 inches within the slipstream of the propeller and 3.5 inches outside the slipstream. Remember to attach the fabric mechanically, using the method recommended by the manufacturer. If you insist on cementing the fabric to the ribs, add an additional safety margin by rib-lacing. If you are covering an ultralight or a slow, low-horsepower aircraft, simply tie a square knot every 3 to 4 inches to secure the fabric properly. It takes only a few hours and a few dollars to ensure that you will never have problems. By the way, use only approved riblacing cord. I have seen everything from string to fishing line used. Don’t do that! You will be placing yourself in unnecessary jeopardy. Next month we will continue with a complete discussion on proper techniques of rib-lacing and other methods of fabric attachment. 

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