The shrinking Art of fabric covering

when heat is applied. Fabric. Covering. The Shrinking ... perfected many of these tips over the years, and we'll present .... Too much heat will release the cement ...
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The Shrinking Art of

Fabric Covering Techniques and tricks from the pros Ron Alexander

Five decades ago,

most homebuilts and antique/classic aircraft

being restored were covered with fabric. Today’s kits are mostly composite or sheet metal, but most plans-built airplanes and some kits still call for a traditional aviation skill. At first, covering an airplane with fabric can seem a daunting task, but if you take it one step at a time, it’s relatively simple. No matter what brand of fabric covering system you use, the basics are the same because almost everyone uses a polyester fabric—Ceconite or PolyFiber—that’s designed to shrink approximately 10 to 12 percent

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when heat is applied.

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At first, covering an airplane with fabric can seem a daunting task, but if you take it

one step at a time,

it’s relatively simple. Getting Started Before you can attach the fabric, the underlying structure must be cleaned, repaired, inspected, and then primed or varnished. After you’ve applied anti-chafe tape to protect the fabric from sharp edges and jagged ridges, you’re ready to attach the fabric. Attaching and properly shrinking the fabric is the first major covering task, and it’s the foundation of all that follows. You can attach fabric two ways: you slide a pre-sewn envelope over the part like a sock, or you can cover the part by cutting and gluing individual pieces (blankets) of fabric in place. Using a pre-sewn envelope designed for the specific component you’re covering will save you time because you attach the open end to the structure and start shrinking,

but it will cost you more money. The blanket method will save you money but cost you more time. If you can’t find a pre-sewn envelope for your airplane, the blanket is your leading option, and we’ll use it to cover a wing in the following paragraphs. A number of shortcuts and tips can make fabric covering easier, and you can learn them first hand at an EAA SportAir Workshop (and make any mistakes on the training panel and not your airplane), or by watching the fabric covering video or DVD available from EAA (see box). EAA SportAir instructor Lynn Zaro perfected many of these tips over the years, and we’ll present several of them. Let’s assume your wing is ready for covering. If you’re using PolyFiber, you must apply two coats of

Covering sharp areas before attaching the fabric.

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Poly-Brush on the leading edge and any other large plywood or metal surfaces. This provides a bed that helps the fabric adhere to large surfaces and reduces the possibility of pinholes in later steps. Ceconite and Poly-Fiber fabric is 70 inches wide. Because the chord of most small airplane wings measures less than this, you should be able to cover a wing with just a top and bottom blanket and have the required fabric overlap. When cutting the blankets, include enough to cover the wing’s butt end (the wing root) and have about a foot hanging off the wingtip. Before gluing the fabric in place, think about the visual appeal of the finished product. When standing next to the airplane, what side of the wing do you see, top or bottom? If you’re of average stature, you’ll see the bottom of a high wing, the top of a low wing, and one of each on a biplane. If looking good is important to you, locate the glue seams on the sides you don’t normally see. On the wing’s leading edge the top and bottom blankets must overlap by at least 2 inches. In other areas you must have at least a 1-inch overlap. To get a straight line on the leading edge snap a line on it using a carpenter’s chalk line. Then measure 1 inch above and below the centerline and snap parallel lines at these marks. (Blue carpenter’s chalk lines will disappear later and won’t bleed through the final coatings— don’t use red.) With the wing topside up at a comfortable working level, lay the fabric on the wing, ensuring that it rests smoothly on the surface, and then hold it in place with clothespins and spring clamps. Usually,

you’ll have to futz with the fabric and reposition the clamps at least once before you’re ready to glue the fabric in place, but keep at it until the blanket rests smoothly on the surface. You need not pull the blanket taut when clamping it in place. Gravity provides the appropriate tension. The same goes for the gluing process. Do not pull and tug on the fabric as you’re cementing it in place. The blanket should look the same after you’ve glued it in place as it did before—resting smoothly on the surface. This provides the correct tautness to achieve the proper shrinking of the fabric later. Cementing the Fabric Using the system’s recommended cement, such as Poly-Fiber’s PolyTak, attach the fabric to the wing. Where you start doesn’t matter— just as long as it is not the wingtip.

You do that last. We’ll start with the trailing edge and aileron well. Lift the fabric, apply a wet bed of cement to the surface, and then press the fabric into the wet PolyTak. Do not go over the top of the fabric with more cement. You want the solvents to evaporate through the fabric. Work in 12-inch sections, and go over the fabric with a small plastic or rubber squeegee to remove the excess cement, which makes for smooth seams. Where you can’t use the squeegee, use your fingers (protected by latex gloves or Invisible Glove hand cream). When you reach the wing’s butt end, glue the fabric on the outside of the wing-root rib (the side that’ll be next to the fuselage). This helps hold the fabric in position when you turn the wing over to cement the leading edge. You can prevent the fabric from dropping away from it while you turn it over by “spot

gluing” it to the leading edge. Now, we are looking at the bottom side of the wing where we will glue the fabric to the leading edge. Before cementing the fabric to the leading edge, cut the fabric using the chalk line that is 1 inch from the centerline toward the top of the wing (this ensures the necessary 2inch overlap with the other blanket). You can see the blue chalk line through the fabric. Use it as a guide to snap a duplicate chalk line on the fabric. Before you cut on the line, hold the fabric away from the leading edge and go over the line with a 1inch brush dipped in Poly-Tak cement. This seals the fabric weave and keeps it from fraying when you cut it. If you glue fabric with frayed ends, you’ll forever see them in the finished product. (Do this anytime you cut fabric.) When the cement is dry, cut the

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Your iron should be set at around fabric with a good pair of straight scissors. Don’t use pinking shears; we want a straight edge because we want to hide this cut under the second piece of fabric. A pinked edge has about 40 percent more edge area than a straight cut, which makes it much more difficult to hide. With the leading edge cut, we’re ready to glue it in place. Starting in the middle and using the chalk line as a guide, cement the fabric to the leading edge 12 inches at a time. Brush on a wet bed of Poly-Tak at least 2 inches wide, press the fabric into it, and remove the excess cement with a squeegee. 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 point where the wingtip starts, make a small vertical cut down the leading edge material and cement it in place. Leave the tip fabric loose. We’re going to use a process called “heat forming” to mold the fabric around the wingtip. We will actually pull the fabric tightly around the tip and iron it to the contour. Your iron should be set at around 250°F to 300°F. (We’ll discuss iron calibration later.) To shape the fabric to the tip, pull the fabric and iron toward the hand you are using to pull the fabric. Always iron perpendicular to the tip bow. Never pull sideways—always straight. Do a little at a time, and keep at it until the fabric lies perfectly flat on the tip bow. Don’t worry about how wrinkled it is past the area you will cement. Carefully cut off the excess fabric along the tip’s curve and start cementing in the center of the area being attached. Put down a wet bed of Poly-Tak under the fabric and then press the fabric into the PolyTak. Use your hand to squeegee out the excess. Smooth out any lumps or excess 66

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250°F to 300°F.

cement, otherwise they will appear in the final product. Remove the Poly-Tak with a clean rag and MEK (protect your hands). Do not scrub the seams. That will fuzz up the edge. Always rub in the direction that causes the fibers to lie down in the seam edge. Now smooth out any wrinkled areas with your iron set at 250°F. If you have a wrinkle that just won’t go away at that temperature, you may increase the iron temperature 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. Before we can cement the second blanket in place we need to snap a couple more chalk lines. We need a 1-inch overlap on the wing’s trailing edge, so snap a line 1-inch inboard of the trailing edge on the fabric. On the leading edge, snap a line 2 inches past the edge of our first piece of

Calibrating an iron before the shrinking process.

fabric. Repeat the smoothing and clamping process as before, and you’ll have to cut the fabric so it will properly fit into an aileron cutout. (Remember to first coat the area to be cut with Poly-Tak and to use straight scissors.) Always mark the area you are going to cut with a pencil—not a pen or marker. You can cut into corners at a 45-degree angle removing excess fabric. Because we want our leading edge and trailing edge cemented seams on the topside of the wing, turn the wing over to glue these seams. Through the fabric you will be able to see the chalk lines previously snapped on both the leading and trailing edges. Gluing to this line on the leading edge will ensure the 2-inch overlap and, on the trailing edge, the 1-inch overlap. Snap a chalk line onto the fabric itself corresponding to each line underneath. Apply Poly-Tak to the line and carefully cut the fabric.

With the trailing edge clamped in place, cement the fabric to the leading edge. This next step is important to the overall look of your finished surface. You have a blue chalk line on both the leading and trailing edges. You have a corresponding blue chalk line you have just cut 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 you make these two lines meet, you will have a perfectly straight seam. As before, work in 12-inch increments starting from the center to lay down a 2-inch wide bed of cement. Press the fabric into it and use the squeegee to smooth the seam. Use a slight sideways motion as you squeegee, and be sure to mate the cut edge of the fabric to the blue line on the surface. Cement the trailing edge using the same technique. It does not matter whether you cement the leading or trailing edge first. With the leading and trailing edges glued in place, heat form the tip just like you did before pulling the iron toward your hand. Keep going until there is 1 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 Poly-Tak to cement this edge in place. You now have successfully covered the entire wing. Go back over any rough areas and smooth them as mentioned earlier. Let the fabric cement dry for a couple of hours while you calibrate your iron for the shrinking process. Iron Calibration Before shrinking fabric, calibrate and mark your household iron for four temperature settings: 200°F, 225°F, 250°F, and 350°F. Your iron should hold the desired temperatures within plus or minus 10 degrees. Never use a heat gun to shrink fabric. ■ Use an accurate thermometer Sport Aviation

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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 from your distributor. ■ Build a one-half-inch thick stack of dry paper towels. You will use this as a heat sink to place the iron on while calibrating it. ■ Next, place a small amount of

heat sink 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 and 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°F. ■ Using a piece of masking tape applied over the rheostat, place a

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