Reviving rag-wings, bolt by bolt
For pilots and owners of metal airplanes — called, with a sneer by the rag-wing enthusiasts, Spam cans — a tube-and-fabric model is nearly uncommon enough to be exotic. While fabric-covered airplanes continue to be produced, both as turnkey airplanes and as kits for the do-it-yourself crowd, it's been almost 40 years since the last of the truly mass-produced rag-wings came off the line.
A long and illustrious history of Piper Cubs, Taylorcrafts, Aeronca Champs, and fabric-winged Cessna 120s made the fabric airplane the most commonplace of designs through the 1950s. And while current Maules, Citabrias, Pittses, and Huskys come off the line wearing fabric, aluminum has been the material of choice for mainstream airplanes since the last days of the short-wing Piper. Durability — particularly compared to the fabric methods of the time — and greatly reduced labor costs led the manufacturers to turn to aluminum skins. Also, metal can be made into monocoque structures, which are simpler and can be lighter than the steel-tube underpinnings required of fabric models. That fabric's days were numbered for production-line airplanes, where ease of construction is everything, should surprise no one.
If you're planning to buy a fabric-covered airplane or restore one as we are doing with our Timeless Tri-Pacer, you'll need to know something about the medium. At the core is a polyester fabric, sometimes called Dacron — this is actually a DuPont trademark and, in fact, describes one of the fibers that may be present in the fabric. Better to use the generic term, polyester. All current covering systems use polyester. You would have an extremely difficult time finding Grade-A cotton for a restoration today, and linen is similarly rare. Polyester, while certainly unattractive in Uncle Joe's pants, is actually a very good material for airplanes. It's easy to work with and can be shrunk with heat in a predictable fashion; it is also more resistant to tearing than cotton. Start a rip in polyester and it will not propagate the way it would in cotton.
Although polyester will not rot like cotton, it has a critical flaw: It will virtually dissolve with exposure to ultraviolet light. Leave a skein of polyester out in the sun and it will deteriorate to dust within a year. So a nonpermeable coating to block UV light is imperative. One company's version is a relatively thick, sandable coating that is sprayed on and, by virtue of its high content of aluminum powder, presents a physical barrier to UV light.
Although there are several covering systems on the market, they are essentially the same at the level of the basic process. At the urging of Clarksburg Air Repair in Sacramento, California, we chose the Poly-Fiber system for the restoration of our Timeless Tri-Pacer (see " Timeless TriPacer: Airframe Arrangements," p. 101). We'll follow the Poly-Fiber process, much simplified, for illustration. Incidentally, new vinyl-based covering systems have essentially replaced more conventional cellulose-based coatings like nitrate and butyrate dope. Nitrate is highly flammable, butyrate somewhat less so, but both continue to tauten the fabric after applications. Even more recent iterations of these dopes are said to be stable only for a few years, after which time the anti-shrinking agents wick out and let the dope do what comes naturally: shrink.
In his seminal manual, "How to Cover an Aircraft Covering Using the Poly-Fiber System," the company's president, Jon Goldenbaum, makes a big deal about preparation. The structural and support surfaces must be clean and free of rust or corrosion. Moreover, the new vinyl-based covering systems, like Poly-Fiber, require that any surface in contact with the fabric be covered with an epoxy-based coating; the fabric coatings will dissolve plain zinc-chromate primer, for example.
Also as part of the preparation phase, it's imperative that the structure be carefully inspected and repaired as necessary. A disadvantage of fabric-covered airplanes is the inability to inspect a large portion of the airframe with the covering in place. So every time the covering is off, it's important that any surface rust has been dusted off the steel tubes; that they are inspected for cracks and weld failures; and that all the cables and pulleys be checked. (A good portion of the prep time on our Tri-Pacer involved just these tasks.) This is also the best time to smooth pockmarked wing leading edges and perform any other metal work — a properly applied fabric skin will reveal even the smallest of imperfections underneath.
Once the airframe has been tended to, it's time to start laying out the fabric. Unlike fiberglass cloth, aircraft fabric has no up or down, no proper orientation; and while it's desirable to have the weave running within a few degrees of the airplane's major axes, it's not crucial. Lay out the fabric over the work and trim; for a wing, you should trim the excess to about 6 inches on each edge, being sure to leave at least this amount at the butt rib and wing tip. While most re-covering professionals use raw cloth on the bolt — which, at 72 inches wide, gives plenty of leeway for most smaller airplanes — there are pre-sewn envelopes available, too. The envelopes reduce some of the work by reducing the number of seams and by being nearly pre-fit to some difficult sections like the wing tip.
In the Poly-Fiber system, the next step is cementing the still-dry fabric to the structure at the leading and trailing edges and around the wingtips — but not to the ribs. A part of the system called Poly-Tak, a fast-drying adhesive, is used here. Because the cement dries within 15 minutes in moderate climates — and as fast as five minutes in hot weather — it's important to work carefully and in small segments. Fortunately, the Poly-Tak can be cut with methyl ethyl ketone (MEK), even after it's cured, so pulling up a corner of the fabric to get rid of a nasty wrinkle is permissible.
One critical step during this part of the process is to ensure that the fabric has the proper overlap, specified by the supplemental type certificate approval that governs the fabric system you're using. For Poly-Fiber, this overlap is one inch at the trailing edge and two inches at the leading edge. With the edges are cemented in place, it's time to tauten the fabric with an iron. One of polyester's amazing qualities is the way it shrinks in response to heat; it reacts to certain levels of heat, not the amount of time that heat is applied. For example, when heated to 250 degrees Fahrenheit, polyester will shrink a predictable 5 percent. Heat it to 350 degrees, and it'll shrink another 5 percent. But let the fabric get above 375 degrees and it will lose its tension forever and will melt at about 415 degrees.
A calibrated clothing iron is used to shrink the fabric and make it taut between the glued-on members. The Poly-Fiber manual warns to never use a heat gun because you can't control the temperature accurately enough. This step takes some practice and patience — getting the fabric to the right tautness doesn't happen instantly, and the skilled fabric tech will know when to stop the shrinking process. Small hobby irons are used for hard-to-reach areas.
After the fabric is made taut, the first layer of the vinyl-based chemicals is brushed on. In Poly-Fiber-speak, this is called Poly-Brush, a pink-tinted coating that does two things: seal the fabric and penetrate its fibers to help attach the skin to the structure underneath. This first coat is a thick one but dries in about 15 minutes.
Once the first coat of Poly-Brush has been applied, it's time to tackle rib stitching. Most airplanes use a polyester cord that's run through the skin and around the wing ribs to keep the fabric from flapping in the breeze. To watch someone work the complicated knots and keep track of which part of the cord goes where, one might think that rib stitching is the hardest part of the job. It's not, according to accomplished fabric layers; in fact, after the first rib or two, the process is more tedious than trying. Again, the most difficult part is the preparation. Reinforcing strips need to be placed along the rib lines under the stitching to help distribute the load and a template is helpful to mark the stitch-hole locations top and bottom; the extra curvature of the top of the wing complicates matters. Some airplanes use other methods of attaching the fabric to the ribs: blind rivets, self-tapping "PK" screws, or fabric clips, for example. It's customary to repeat whatever the original manufacturer did when re-covering a production airplane. Experimental-airplane builders are basically on their own.
After rib stitching — or use of the equivalent methods — it's time to install inspection panels and reinforcements. Additional layers of fabric are used to beef-up the areas cut out of inspection plates. Next come the reinforcing tapes, which are used, as the name implies, to add strength to areas of the fabric that carry extra loads, such as over the rib (and stitching), along the leading edges, and wherever the fabric rides atop structural members. (If you didn't tape these areas, normal flight loads and the normal, constant fluttering of the fabric would wear the surfaces through in a short time.) Pinked tapes — those cut with a sawblade-style edge — are throwbacks to the days of cotton that would unravel easily; today, the pinked edges help to provide additional surface area for the adhesives to "grip." Finally, drain grommets are installed at the appropriate points in the wing, fuselage, and control surfaces. It's important that small, quarter-sized "doilies" are installed atop the grommets because otherwise the drains will cut themselves through the fabric in a few years.
With the basic mechanical work done, the Poly-Fiber system calls for two additional coats of Poly-Brush to be applied with a spray gun. After that, three coats of Poly-Spray, which is the critical UV blocker, are applied by spray gun, followed by the color coats. Naturally, the metal parts of the airplane should be treated to the normal types of preparation, including, as recommended by Poly-Fiber, an epoxy primer coat.
Which type of color coat you use determines the sheen of the airplane and its reparability. For example, the Poly-Tone we are using on the Timeless Tri-Pacer will have a satin-gloss finish, not a highly lustrous, deep-as-your-fist kind of surface. It's a tradeoff for Poly-Tone's far easier reparability and long-term durability. It comes down to this: Fabric is always moving and is far more flexible than metal. Even paints with flexative agents can't keep up with fabric and will eventually crack off the surface. The more flexible the paint, the longer it will last on a fabric airplane. Poly-Fiber also sells a product called Aero-Thane, a two-part polyurethane paint that's far glossier than Poly-Tone, but much harder to match and blend when you have to make a fabric repair. It's also not as durable. We're willing to give up some shine in the present to give the winner of our Tri-Pacer a longer-lasting finish that's easier to repair.
To the untrained individual, inspecting a fabric airplane may seem quite daunting and unfamiliar. But there are a few guidelines to follow that will give you a pretty fair read on the quality of an airplane's fabric job and how well it's been maintained. Ron Alexander, owner of Alexander Sportair Workshops, offers these tips.
Look for proper tautness. Too-loose fabric will beat up on the structure underneath and will be prone to cracking along the intersection of the flailing polyester and the nearest supporting structure.
Conversely, fabric that's too tight can do serious damage to the structure, including scalloping of the trailing edge, buckling of lightweight skins, and distortion of wing ribs. It probably won't make the structure unairworthy, but too-tight fabric will create a bundle of repairs for you before the next covering.
Check the condition of fastening systems. Rib lacing, PK screws, and fabric clips all should be in good condition, holding the fabric to the structure without tears or bunching up.
Examine the outer coat for evidence of cracking, "ringworm," or flaking. (Ringworm shows up as small, dime-sized cracks where the outer layers have hardened and exposed the underlying coatings.) A too-thick or too-stiff outer coating can crack in just a few years and, although not strictly an airworthiness issue, certainly creates an eyesore. Usually the final coats will crack, leaving intact the UV barrier, which is the most important layer on the whole airplane.
To determine if the UV block is still doing its job, fabric-savvy mechanics say to roll the airplane out of the hangar and look through an inspection hole. If you see sunshine through the top of the wing, for example, walk away from the airplane unless you intend to do an immediate re-covering job. (Our Tri-Pacer passed this test just fine, even though the fabric was more than 20 years old. A complete and well-maintained topcoat, along with a properly applied UV barrier, made this so.)
Inspect the fabric according to industry specifications for integrity. At the annual inspection, fabric airplanes are supposed to be "punch tested" to determine if the fabric is airworthy. Today, almost all shops use the nondestructive Maule fabric tester.
Yes, it's true that fabric has been with us since the beginning of aviation and, thanks to escalating interest in restoring older airplanes, doesn't show signs of retreating into obsolescence. It's nice to know that the state of the art continues to be pushed forward and that there are still shops and individual restorers who take pride in keeping this time-honored craft aloft.
E-mail the author at [email protected].
