PART IV

Aircraft Coverings And Their Applications By Martin C. Haedtler, EAA 4846 4920 Stone Circle, Oak Lawn, 111.

"RAZORBACK" FIBERGLAS OEVERAL BRANDS and methods of covering aircraft 0 with fiberglas have been tried in recent years, with varying results. One class of fiberglas cover is based on applying glass cloth over a rejected covering of cotton or other natural fiber—it might be described as a "retread" job. The advent of glass cloth resistant to all molds, acids, mildew and sun rot has encouraged the development of another method which will be described here. In it, the glass cloth is applied to the bare aircraft framework in a manner similar to that used for other fabrics, but with details worked out to suit the properties of glass cloth. This method has the advantage of not involving the hazard of completely "killing" an underlying natural-fiber cloth due to entrapment of acids, smoke deposits and other destructive agents. I have personally inspected various aircraft covered with the all-glass method, and on the oldest examples having two summers and two winters of outdoor exposure, could not find any indication of deterioration, rot or cracking. All that could be detected was a slight fading of color. The fiberglas method to be described here bears the trade name "Razorback" and was developed by Planters, Inc. of Manila, Ark. Glass cloth is like Dacron in that it will not absorb aircraft dopes, so this created a stumbling block for all methods of glass cloth-and-dope application to aircraft until Planters developed a method of predoping cloth at their factory in a special way which assures the adhesion of subsequent coats applied after the cloth is on an airframe. It was not a problem of chemical but of mechanical adhesion of the dope film to the nonabsorbent cloth. "Razorback" glass cloth weighs 3.92 oz. per sq. yd. as compared to 4 oz. for Grade A cotton cloth, so there is a slight over-all reduction in weight on a "Razorback"-covered plane. Tensile strength undoped is in the region of 150 Ibs., and when doped tests at 200 Ibs. Two-inch wide overlapped doped seams have a pull test of over 100 Ibs., so no stitching is required at edges

and on patches. It is quite essential that manufacturer's instructions be followed faithfully. Cellulose acetate butyrate dope,

Fig. 1.

often referred to by the abbreviation CAB, must be used; no other dope—such as nitrate—is compatible with the factory pre-doping. Because glass cloth sews poorly, the usual wing covering method of sewing together several strips to make a "blanket" is not used. The strength of the cloth is so great that it has been found safe and feasible to deviate from the usual custom of applying cloth so the warp of its weave is parallel to the wing chord. Instead, glass cloth supplied by "Razorback" comes in 72 inch wide strips which are unrolled spanwise to cover the whole top or bottom of a wing with no chordwise seams. Before starting to cover a wing, make sure all frameto-cloth contacting points are free of sharp edges. Lightly sandpaper edges of rib capstrips to remove the edge from capstrip varnish or dope-proof paint. All leading edge metal seams which overlap should be taped to keep the cloth from snagging, and also put tape over edges which butt to keep the cloth from sagging between them. All wood surfaces such as wing catwalks, plywood sheathing, wing spar tops and bottoms close to fabric, etc., should be given two coats of dope-proof paint so that their protective varnish won't soften and affect the bond of dope to glass cloth. Metal surfaces such as longerons, stringers and fittings should preferably be coated with epoxy type primers, such as Glidden's "Glid-Plate", because it won't "lift" when wetted by dope and will give metal very durable protection to match the very long life expectancy of the covering job. Starting on the bottom of the wing, spread the 72 inch fabric spanwise as in Fig. 1. Adjust it in place so that a strip two inches wide falls over the leading edge metal covering, Fig. 2. This is the recommended method, but on some wings such as those having metal on the top surface of the leading edge only, it may be necessary to have the two-inch overlap located directly on the leading edge. With the cloth laying smooth and true on the wing, glue it to the metal with full strength butyrate dope, Fig. 3. If the leading edge is of wood, coat the bare wood (Continued on page 12)

A five-yard length of 72 in. "Razorback" is be-

ing put into position on the underside of a Stearman lower wing panel.

Fig. 2.

The fabric is adjusted so that a two-inch

strip overlaps the rear edge of the leading-edge metal. SPORT AVIATION

11

AIRCRAFT COVERINGS . . . (Continued from page 11)

Fig. 3 The overlapped fabric is now bonded to the metal with full-strength butyrate dope.

with two coats of butyrate before attempting to glue the cloth in place. When the leading edge dope is firm, attach cloth to the trailing edge in the same way, Fig. 4. Pull the cloth just tight enough to remove wrinkles and give a snug fit. When dry, trim off the surplus cloth. Now turn the wing right side up and spread the top cloth onto it so that there is a two-inch overlap, Fig. 5. Dope top and bottom fabric together along this two-inch overlap. Do not dope the fabric to the entire leading edge metal cover as this can cause troublesome wrinkles and blisters. The two pieces of pre-doped cloth become as one when thus cemented together. Fig. 6 shows the wing with fabric almost all on, and Fig. 7 shows the wing right side up with the fabric not glued to the leading edge metal. Next dope the top cloth to the trailing edge, using wooden clothespins to hold it in place if necessary while the butyrate dope firms up. Again, only enough tension in the cloth to remove wrinkles and sags. Fig. 8 gives an

idea of how the cloth is cut to fit around a fitting at the aileron cutout. The wing root is treated in essentially the

same manner, using a two-inch overlap, pulling just tight enough to remove wrinkles and sags, and cementing with butyrate dope. Figs. 8, 9, 10 and 11 show doping and clamping of the cloth to the wing tip bow. If trouble is experienced in getting rid of puckers, it is possible to minimize the difficulty by making inward cuts about an

Fig. 4. The "Razorback" fabric is now attached to the trail ing-edge in the same manner.

Fig. 5. The wing is then turned over, and the cloth brought around and over the leading-edge, and overlapped two inches, then glued down.

Fig. 6. The wing panel is now almost completely covered with "Razorback", with the tip cloth not yet fastened down. 12

MAY 1964

inch long in the edge of the cloth. There is no stitching at the wingtip so be sure the cloth is really saturated with dope and securely glued together. To cover an aileron, use a strip of 44 inch "Razorback" cloth. Glue to the trailing edge, wrap over the leading edge and glue together with the standard two-inch overlap. Use two pieces to cover a control surface shaped like a typical rudder. Cover one side, wrapping the cloth around the tubing. When dry, turn over and cover the other side, lapping this cloth about an inch over the cloth on the first side. Make one-inch cuts around the edge where needed to help work out any puckers. Rib-stitching is not done until the surface has been doped smooth and taut, instead of after the first coat of dope as is common with cotton, linen, etc. Therefore, doping procedure will be explained at this point. If at all possible use a pressure pot spray gun with 18 Ibs. on the pot and 60 Ibs. on the line. Spray the first coat of clear butyrate dope as lightly as possible; note how far from the work the gun is being held in Fig. 12. Do not thin the dope more than 10 percent and even then, only when absolutely necessary and as little as possible to get satisfactory spraying. Do not, under any circumstnces, blow dope through the weave or let it build up so much that it leaks through, otherwise the dope will "orange peel" and the cloth won't tauten up. The predoping takes care of adhesion; your only concern is to start the build-up process as directed. The second coat may be sprayed on a little heavier and so on with each additional coat. The first coats will cause the "Razorback" cloth to sag. Tightening action is often a little slower than normal. Full tautening action is usually achieved after the fourth or fifth coat or when the weave is filled. At this point rib-stitching can be done. Use the spe-

cial plastic-covered glass cord supplied by the firm from which the cloth is obtained. Over the rib capstrips, use slightly wider reinforcing tape to prevent corners of capstrips from chafing the cord. Do not pull excessively hard on the cord when knotting so as not to damage it. Double stitching is recommended on ribs in the propeller blast area. Cover rib-stitching and all edges and seams

Fig. 7. With the fabric not yet glued to the leadingedge metal, care is taken to remove the possibility of blisters on the leading-edge before the bond takes place.

Fig. 10. The fabric, cut to shape, is glued down around the wing tip bow.

Fig. 8. The cloth is cut and brought around the aileron fitting and then fastened down.

Fig. 11. After being glued down, the fabric on the wing tip bow is then clamped tight.

Fig. 9. Clamp-type clothes pins are used to hold the fabric tight to the trailing edge.

with two-inch or three-inch glass tape obtained from the supplier, in the same manner as with other kinds of covering. Don't try to install it until the weave of the covering cloth is filled and the cloth is taut and wrinkle-free. Dope inspection rings in place at this time, too. Then apply two more coats of clear butyrate dope, by brush, if desired. Allow to dry thoroughly and sand lightly with No. 280 paper, using care not to sand over ribs, rivets, etc. One coat of aluminum-pigmented butyrate dope is then sprayed on and when dry, wet-sand

with No. 320 paper. Sand off practically all of it save for that which remains in low spots. The purpose of this aluminum coat is for smooth sanding and not sunlight protection. To finish up for average service, spray on two coats of the final color. For a finer finish, wet-sand with No. 320 paper and spray on a wash coat (thinned) which will dry with a good gloss. (Continued on top of page 14)

Fig. 12. The first coat of clear butyrate dope Is sprayed on lightly, the gun held about this far from the work. SPORT AVIATION

13

AIRCRAFT COVERINGS . . . (Continued from poge 13)

Fuselage work is done with 44 inch "Razorback" cloth, four strips normally being used, though, of course, smaller or narrower-than-average fuselages might be done with less. The usual procedure is to hang cloth on a side, dope to the lower longeron and then to the top longeron, pulling just tight enough to remove wrinkles and sags. After the sides are on, top and bottom cloth is added, doping it to the sides with the usual two-inch overlap.

Be sure to saturate cloth completely with dope to get reliable seams. Finish with "Razorback" tape over seams,

as seen in Fig. 13, after building up the surface with sprayed coats of dope as was done on the wings. The operator at the left is pulling on a length of tape to lay it over a stringer true and straight, while the one at the right presses it firmly into place. Because the glass cloth does not deteriorate from

mildew, acids, sunlight, etc., no punch testing is needed in service. Write in the log book a notation saying that only visual inspection of the dope for cracks and deterioration is required at periodic inspections. In time the dope may need rejuvenation, at which time butyrate dope-

rejuvenator is sprayed on, followed by a wash coat of pigmented dope to restore like-new appearance. The

Planters firm supplies with its material decals for application to aircraft and components covered with the "Ra-

TWO KIWIS . . .

(Continued from poge 10)

Just after midnight, on Friday morning after this busy day, we were roused out by Paul Poberezny with the news that a storm was on its way, so we went out to help tie down aircraft on the lines. After the storm

passed, we helped clear up the damage at the display

tent, which was blown down. I spent the morning finishing up the BT-13 and then taking aircraft back to the

hangars when the second storm threatened. Later in the afternoon I went up with Tony Spezio in his "Tu-Holer" to get some aerial shots of "Hawk Pshaw", the "Delta

Kitten", "Chuparosa" and the "Mini-Mustang." A lot of people were skeptical that I'd be able to get into "TuHoler", since I am 6 ft. 2 in. tall and weigh 200 Ibs. But I made it, and was up for about a half hour. When Tony installs his new tank, the front seat should be comfortable for anyone my size.

On Saturday morning I went flying with Bob Lake in

his Luscombe 8A over to Machesney Airport and a tour around Rockford city and environs. Later in the morning I went up with Prof. Lesher in his "Nomad" and took more photos of "Chuppy" and Dr. Linn Cockroft's "Tailwind." That evening we appeared on the stage with other overseas visitors. Calvin Hoth of Rockford very kindly invited us to stay the night at his house, which we gratefully accepted.

On Sunday morning I took more photos of departures

and made tape recordings with Paul Poberezny, the FAA weather and flight safety officers, Captain John Louck and several others. Then George Hardie took all overseas visitors to the control tower for a visit with "Smoky" Smolla and his boys. Again I took more photos and made tape recordings of the tower operations. You may wonder why so many photos and the tapes—I knew that if I didn't take back some sort of record of this marvelous meet, my fellow members in New Zealand wouldn't believe what I told them. 14

MAY 1964

Fig. 13. The "Razorback" finishing tape is applied after the cloth has been sealed by spraying.

zorback" method, so that service personnel can readily tell what material is on the aircraft. As far as experience thus far indicates, this covering material can be considered permanent.

Patching is done with "Razorback" cloth and butyrate dope, using the usual two-inch overlap. However, emergency patches may be made with any available cotton or other aircraft fabric which can be applied with butyrate dope. A

After a quick lunch, we loaded up George Hardie's

station wagon with our gear and turned northwards to Hales Corners, taking with us many, many memories of the friendships and hospitalities extended to us at the Fly-In. We'll never forget the wonderful people who took us flying, and the capabilities and performances of the aircraft themselves. We left with many vivid impressions —the marvelous aerobatics, the fine static displays and exhibits, the interesting and informative lectures. The unstinting and uncomplaining devotion and service given by the girls in the Headquarters tents, the public relations trailer, the food tent. The friendliness of the exhibitors like John Thorp and his crew, Mr. R. G. Huggins, B. & F. Aircraft, Aerial Blight Control, the engraver, EAA Chapter members, and all others too numerous to mention. Particularly impressive to both of us was the excellent finish and workmanship on the homebuilt aircraft, and the oft-times immaculate restoration jobs on the antiques. Of course, all of this was not possible without the painstaking and involved job of organizing the Fly-In with all its myriad details—the sound system, picketing ropes, registration tents, trash bins, etc., and the tower operators, flag men, gas men and announcers. The FAA cannot go without mention, as without their enthusiastic

support, nothing could be done — hats off to those

boys! To all of you, no matter how small or large your job, we both would like to extend our thanks to you for the marvelous job, and we are proud that we are also part of the brotherhood of homebuilders who can go to such lengths to get together and show the efforts of their own

hands, who have the enthusiasm for the pure "grass roots" type of aviation to pursue their ideas and ideals to those ends. We are proud to have been a part of this great show. Next month I'll bring you an account of the rest of our trip as we headed back for home. A