nuts & bolts

building basics

Fun With Fabric Covering your air frame the old-fashioned way R I C HARD K OEHL ER

square yard and had a minimum strength of 80 pounds per inch. For small, light aircraft there was also a lighter cotton that had a strength of 65 pounds per inch, but here I will only discuss the 80-pound stuff. Aircraft were covered by gluing the cotton material around the edge of a component, shrinking it in place with water, sealing with nitrate dope, and finishing with butyrate dope,

Up to the mid-1950s, fabric aircraft were covered with grade-A cotton or, rarely, linen. Today the most popular fabric covering is synthetic polyester.

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have been teaching a class in fabric covering, which is a whole bunch of fun, and I thought I would share a simplified description of modern fabric finishing. First off, let’s look briefly at how it used to be done. Back in the old days (up to the mid-1950s) fabric aircraft were covered with grade-A cotton or, rarely, linen. The standard cotton material weighed about 4.5 ounces per

at least one coat of which had aluminum dust added to block UV sunlight from the cotton. Note that this “dope” is not the illegal drug, but rather a specialized form of lacquer paint for aircraft. The bad news about this process is that in harsh sunlight conditions, the covering might last only three or four years! How do you know when the covering needs replacing? The FAA standard says the fabric is no longer airworthy when its strength is less than 70 percent of its original strength. So, for 80-pound-perinch cotton fabric, 80 × 0.70 = 56 pounds per inch. You can either cut a 1-inch-wide piece of fabric out of the covering on your aircraft and pull-test it to see if it holds at least 56 pounds, or you can use a nondestructive tester, such as the one created and sold by Maule. It has a specialized blunt tip that is pressed into the fabric. The tip is mounted on a spring scale that shows how much you are pressing. You press up to 56 pounds, and then examine the point of pressure to see if the fabric has broken. Often the paint will break, but the real question is the condition of the fabric. If the fabric fails, you must re-cover the aircraft.

Today the most popular covering fabric is man-made polyester, which is most commonly called Ceconite or Poly-Fiber. There are also processes that uses a special fiberglass, and a few folks do historic aircraft with cotton, but most of us use a type of polyester (switching to a polyester covering on an antique or classic aircraft will not lower your aircraft’s score for judging purposes). The polyester is just like the material used for your clothing, except it comes out of the mill prior to being shrunk, so heat will shrink it. The neat thing about polyester is that it is relatively impervious to most of the things that attack and age our aircraft. It will not significantly rot, corrode, or age in the sun. In theory, it will last longer than the wood, aluminum, and steel of the primary aircraft structure. Also, it is much stronger than cotton for equal weights, or put another way, it is much lighter than cotton for the same strength. We usually compromise today, using a grade of polyester that is both lighter and stronger than the original cotton. Covering with polyester starts out the same as for cotton. The fabric is glued down to the frame around the edges. Once

The neat thing about polyester is that it is relatively impervious to most of the things that attack and age our aircraft.

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building basics attached it is then shrunk in place with heat. Ideally, the heat should be from a calibrated iron that is set to 350°F. Although not legal, I have known folks who’ve used a hair dryer or heat gun. After shrinking, the stitching and finishing process is essentially the same with polyester as with cotton. Let me pause here and discuss finish processes. It is extremely important to follow a complete and correct process. Many of the early problems with polyester fabric covering jobs were due to the paint, and not the fabric. First off, you should never use the same dopes that you used on cotton. If you use a shrinking dope on the already heat-shrunk fabric, you will grossly over-shrink it. I have seen ailerons and wingtips turned into pretzels, ruined beyond repair by putting shrinking dope on polyester. I see crushed ribs on a couple of aircraft that routinely visit our flyins. So, you must at least use a nontautening dope. Unfortunately, even non-tautening dope is not a good match for the properties of polyester. The polyester fabric is tough and flexible, but the dope tends to dry hard and brittle. As a result it’s common for the dope to crack wherever the polyester flexes. The dope can be rejuvenated with a special rejuvenator, which is a clear dope with a lot of solvent and plasticizers, but the result is usually a thicker (heavier) coat of paint that will soon dry out and crack again. A better finish approach is to use the modern finishes with the modern fabric. Since you don’t need to shrink the fabric with the paint finish, you can use just about any modern finish that will stick. The best seems to be polyurethane. Again, let me emphasize, I highly recommend you follow the entire fabric and finish process of one manufacturer. That way you are assured everything is compatible, both chemically and structurally. Systems 108

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such as the Poly-Fiber process work well and are worth the few extra dollars. Remember, polyester fabric covering can last longer than an aluminum wing skin, so it pays to finish it properly the first time. To review, whatever covering process you use—linen, cotton, polyester, or fiberglass—you still follow the same basic procedures of protecting the fabric. After getting the material attached to the airframe, and shrunk with heat if polyester, the first coats of paint, be they nitrate dope, PolyBrush, or otherwise, should contain

If you use a shrinking dope on the already heat-shrunk fabric, you will grossly over-shrink it. I have seen ailerons and wingtips turned into pretzels, ruined beyond repair by putting shrinking dope on polyester. a fungicide to minimize rot and mold problems on the fabric. After the fabric has been encapsulated, the weave is built up with additional layers of paint that contain fine aluminum dust. This sandable layer also provides a UV block so the sun does not attack the fabric. A good check for the correct amount of this layer of paint is to look inside the covered part with a bright light on the outside. The aluminum dust should block all of the light. If you can see the light, you need to add more. This defines the minimum thickness of the layer of aluminum-impregnated paint. After this, the finish color can be applied. Even with all these precautions, under worse case environments cotton-fabric-covered aircraft that are parked outside have been known to fail a strength test after

only three to four years, particularly if the fungicide or aluminum coats were skimped on. Obviously, hangaring and care with painting will produce better results. Notwithstanding, polyester will vastly outlast cotton, no matter how you finish it. Bottom line: follow one finish process completely, using only its products to ensure compatibility and years of service.

Aviation Maintenance Careers My day job is as a professor of aviation maintenance technology at the University of the District of Columbia, located at Reagan Washington National Airport. I thought EAA Sport Aviation readers might be interested in knowing that enrollment at our school is up. We like to think that’s because we are doing a better job of recruiting, but the reality is that there is a huge shortage of mechanics right now. Our average students start with the commuters at about $15 to $16 per hour or with contract maintenance operations or airlines at $19 to $23 per hour, with growth to near $30 within two to three years. With overtime, that is about $60,000 per year, all within a few years of finishing your two years of school. This means you, as an aircraft owner, can expect the maintenance rates to rise at your local FBO, if it can keep mechanics. We smart homebuilders can do our own maintenance, of course, so this may not have much beyond a tangential effect. Back to fabric covering—there are more fabric-covered airplanes than airliners, and interestingly, because of the shortage of people certificated to do this sort of work, the pricing has gone way up. The going rate to re-cover a Cub-size plane, at least in urban markets, has reached $30,000. Let’s see, go to school for two years, do three or four re-covers each year at $30,000, minus your costs, shop, etc. Sounds like a nice way to make a living…

Answer: Yes you do!* *Question: Do I need one of these? Find out why at: www.ellison-tbi.com

Carburetors You Can Bank On! Ellison Fluid Systems Inc. 350 Airport Way • Renton, WA 98055 • 425-271-3220 EAA Sport Aviation

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