PART IV

Aircraft Coverings And Their ApplicationsBy Martin C. Haedtler, EAA 4846

4920 Stone Circle, Oak Lawn, 111.

"RAZORBACK" FIBERGLAS

OEVERAL BRANDS and methods of covering aircraft0 with fiberglas have been tried in recent years, withvarying results. One class of fiberglas cover is based onapplying glass cloth over a rejected covering of cottonor other natural fiber—it might be described as a "re-tread" job. The advent of glass cloth resistant to allmolds, acids, mildew and sun rot has encouraged the de-velopment of another method which will be describedhere. In it, the glass cloth is applied to the bare aircraftframework in a manner similar to that used for otherfabrics, but with details worked out to suit the propertiesof glass cloth. This method has the advantage of not in-volving the hazard of completely "killing" an underlyingnatural-fiber cloth due to entrapment of acids, smoke de-posits and other destructive agents.

I have personally inspected various aircraft coveredwith the all-glass method, and on the oldest exampleshaving two summers and two winters of outdoor expos-ure, could not find any indication of deterioration, rotor cracking. All that could be detected was a slight fadingof color.

The fiberglas method to be described here bears thetrade name "Razorback" and was developed by Planters,Inc. of Manila, Ark. Glass cloth is like Dacron in that itwill not absorb aircraft dopes, so this created a stumblingblock for all methods of glass cloth-and-dope applicationto aircraft until Planters developed a method of pre-doping cloth at their factory in a special way which as-sures the adhesion of subsequent coats applied after thecloth is on an airframe. It was not a problem of chemicalbut of mechanical adhesion of the dope film to the non-absorbent 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 isin the region of 150 Ibs., and when doped tests at 200Ibs. Two-inch wide overlapped doped seams have a pulltest of over 100 Ibs., so no stitching is required at edgesand on patches.

It is quite essential that manufacturer's instructionsbe followed faithfully. Cellulose acetate butyrate dope,

often referred to by the abbreviation CAB, must be used;no other dope—such as nitrate—is compatible with thefactory pre-doping.

Because glass cloth sews poorly, the usual wing cov-ering method of sewing together several strips to makea "blanket" is not used. The strength of the cloth is sogreat that it has been found safe and feasible to deviatefrom the usual custom of applying cloth so the warp ofits weave is parallel to the wing chord. Instead, glasscloth supplied by "Razorback" comes in 72 inch widestrips which are unrolled spanwise to cover the whole topor bottom of a wing with no chordwise seams.

Before starting to cover a wing, make sure all frame-to-cloth contacting points are free of sharp edges. Lightlysandpaper edges of rib capstrips to remove the edge fromcapstrip varnish or dope-proof paint. All leading edgemetal seams which overlap should be taped to keep thecloth from snagging, and also put tape over edges whichbutt to keep the cloth from sagging between them.All wood surfaces such as wing catwalks, plywood sheath-ing, wing spar tops and bottoms close to fabric, etc.,should be given two coats of dope-proof paint so thattheir protective varnish won't soften and affect the bondof dope to glass cloth. Metal surfaces such as longerons,stringers and fittings should preferably be coated withepoxy type primers, such as Glidden's "Glid-Plate", be-cause it won't "lift" when wetted by dope and will givemetal very durable protection to match the very longlife expectancy of the covering job.

Starting on the bottom of the wing, spread the 72inch fabric spanwise as in Fig. 1. Adjust it in place sothat a strip two inches wide falls over the leading edgemetal covering, Fig. 2. This is the recommended method,but on some wings such as those having metal on the topsurface of the leading edge only, it may be necessary tohave the two-inch overlap located directly on the leadingedge. 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)

Fig. 1. A five-yard length of 72 in. "Razorback" is be-ing put into position on the underside of a Stearmanlower wing panel.

Fig. 2. The fabric is adjusted so that a two-inchstrip overlaps the rear edge of the leading-edge metal.

SPORT AVIATION 11

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

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

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

Fig. 6. The wing panel is now almost completely cov-ered with "Razorback", with the tip cloth not yet fasteneddown.12 MAY 1964

AIRCRAFT COVERINGS . . .(Continued from page 11)

with two coats of butyrate before attempting to glue thecloth in place. When the leading edge dope is firm, attachcloth to the trailing edge in the same way, Fig. 4. Pullthe cloth just tight enough to remove wrinkles and givea snug fit. When dry, trim off the surplus cloth.

Now turn the wing right side up and spread the topcloth onto it so that there is a two-inch overlap, Fig. 5.Dope top and bottom fabric together along this two-inchoverlap. Do not dope the fabric to the entire leadingedge metal cover as this can cause troublesome wrinklesand blisters. The two pieces of pre-doped cloth become asone when thus cemented together. Fig. 6 shows the wingwith fabric almost all on, and Fig. 7 shows the wingright side up with the fabric not glued to the leadingedge metal.

Next dope the top cloth to the trailing edge, usingwooden clothespins to hold it in place if necessary whilethe butyrate dope firms up. Again, only enough tensionin the cloth to remove wrinkles and sags. Fig. 8 gives anidea of how the cloth is cut to fit around a fitting at theaileron cutout. The wing root is treated in essentially thesame manner, using a two-inch overlap, pulling just tightenough to remove wrinkles and sags, and cementing withbutyrate dope. Figs. 8, 9, 10 and 11 show doping andclamping of the cloth to the wing tip bow. If trouble isexperienced in getting rid of puckers, it is possible tominimize the difficulty by making inward cuts about aninch long in the edge of the cloth. There is no stitchingat the wingtip so be sure the cloth is really saturatedwith dope and securely glued together.

To cover an aileron, use a strip of 44 inch "Razor-back" cloth. Glue to the trailing edge, wrap over the lead-ing edge and glue together with the standard two-inchoverlap. Use two pieces to cover a control surface shapedlike a typical rudder. Cover one side, wrapping the clotharound the tubing. When dry, turn over and cover theother side, lapping this cloth about an inch over the clothon the first side. Make one-inch cuts around the edgewhere needed to help work out any puckers.

Rib-stitching is not done until the surface has beendoped smooth and taut, instead of after the first coat ofdope 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 with18 Ibs. on the pot and 60 Ibs. on the line. Spray the firstcoat of clear butyrate dope as lightly as possible; note howfar from the work the gun is being held in Fig. 12. Donot thin the dope more than 10 percent and even then,only when absolutely necessary and as little as possibleto get satisfactory spraying. Do not, under any circum-stnces, blow dope through the weave or let it build up somuch that it leaks through, otherwise the dope will"orange peel" and the cloth won't tauten up. The pre-doping takes care of adhesion; your only concern is tostart the build-up process as directed. The second coatmay be sprayed on a little heavier and so on with eachadditional coat. The first coats will cause the "Razor-back" cloth to sag. Tightening action is often a littleslower than normal. Full tautening action is usuallyachieved after the fourth or fifth coat or when the weaveis filled.

At this point rib-stitching can be done. Use the spe-cial plastic-covered glass cord supplied by the firm fromwhich the cloth is obtained. Over the rib capstrips, useslightly wider reinforcing tape to prevent corners of cap-strips from chafing the cord. Do not pull excessivelyhard on the cord when knotting so as not to damage it.Double stitching is recommended on ribs in the propellerblast area. Cover rib-stitching and all edges and seams

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

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

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

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

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

with two-inch or three-inch glass tape obtained from thesupplier, in the same manner as with other kinds of cover-ing. Don't try to install it until the weave of the cover-ing 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 sandlightly with No. 280 paper, using care not to sand overribs, rivets, etc. One coat of aluminum-pigmented buty-rate dope is then sprayed on and when dry, wet-sandwith No. 320 paper. Sand off practically all of it savefor that which remains in low spots. The purpose of thisaluminum coat is for smooth sanding and not sunlightprotection. To finish up for average service, spray ontwo coats of the final color. For a finer finish, wet-sandwith 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 sprayedon 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 donewith 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 getreliable seams. Finish with "Razorback" tape over seams,as seen in Fig. 13, after building up the surface withsprayed coats of dope as was done on the wings. The op-erator at the left is pulling on a length of tape to lay itover a stringer true and straight, while the one at theright presses it firmly into place.

Because the glass cloth does not deteriorate frommildew, acids, sunlight, etc., no punch testing is neededin service. Write in the log book a notation saying thatonly visual inspection of the dope for cracks and deteriora-tion is required at periodic inspections. In time the dopemay need rejuvenation, at which time butyrate dope-rejuvenator is sprayed on, followed by a wash coat ofpigmented dope to restore like-new appearance. ThePlanters firm supplies with its material decals for appli-cation to aircraft and components covered with the "Ra-

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

zorback" method, so that service personnel can readilytell what material is on the aircraft. As far as experi-ence thus far indicates, this covering material can beconsidered permanent.

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

TWO KIWIS . . .(Continued from poge 10)

Just after midnight, on Friday morning after thisbusy day, we were roused out by Paul Poberezny withthe news that a storm was on its way, so we went outto help tie down aircraft on the lines. After the stormpassed, we helped clear up the damage at the displaytent, which was blown down. I spent the morning finish-ing up the BT-13 and then taking aircraft back to thehangars when the second storm threatened. Later in theafternoon I went up with Tony Spezio in his "Tu-Holer"to get some aerial shots of "Hawk Pshaw", the "DeltaKitten", "Chuparosa" and the "Mini-Mustang." A lot ofpeople were skeptical that I'd be able to get into "Tu-Holer", since I am 6 ft. 2 in. tall and weigh 200 Ibs. ButI made it, and was up for about a half hour. When Tonyinstalls his new tank, the front seat should be comfort-able for anyone my size.

On Saturday morning I went flying with Bob Lake inhis Luscombe 8A over to Machesney Airport and a touraround Rockford city and environs. Later in the morningI went up with Prof. Lesher in his "Nomad" and tookmore photos of "Chuppy" and Dr. Linn Cockroft's "Tail-wind." That evening we appeared on the stage with otheroverseas visitors. Calvin Hoth of Rockford very kindlyinvited us to stay the night at his house, which we grate-fully accepted.

On Sunday morning I took more photos of departuresand made tape recordings with Paul Poberezny, the FAAweather and flight safety officers, Captain John Louck andseveral others. Then George Hardie took all overseasvisitors to the control tower for a visit with "Smoky"Smolla and his boys. Again I took more photos and madetape recordings of the tower operations. You may wonderwhy so many photos and the tapes—I knew that if I didn'ttake back some sort of record of this marvelous meet,my fellow members in New Zealand wouldn't believewhat I told them.

After a quick lunch, we loaded up George Hardie'sstation wagon with our gear and turned northwards toHales Corners, taking with us many, many memories ofthe friendships and hospitalities extended to us at theFly-In. We'll never forget the wonderful people who tookus flying, and the capabilities and performances of theaircraft themselves. We left with many vivid impressions—the marvelous aerobatics, the fine static displays andexhibits, the interesting and informative lectures. Theunstinting and uncomplaining devotion and service givenby the girls in the Headquarters tents, the public rela-tions trailer, the food tent. The friendliness of the ex-hibitors 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 tomention.

Particularly impressive to both of us was the excel-lent finish and workmanship on the homebuilt aircraft,and the oft-times immaculate restoration jobs on the an-tiques. Of course, all of this was not possible without thepainstaking and involved job of organizing the Fly-In withall its myriad details—the sound system, picketing ropes,registration tents, trash bins, etc., and the tower oper-ators, flag men, gas men and announcers. The FAA can-not go without mention, as without their enthusiasticsupport, nothing could be done — hats off to thoseboys!

To all of you, no matter how small or large your job,we both would like to extend our thanks to you for themarvelous job, and we are proud that we are also partof the brotherhood of homebuilders who can go to suchlengths to get together and show the efforts of their ownhands, who have the enthusiasm for the pure "grassroots" type of aviation to pursue their ideas and idealsto those ends. We are proud to have been a part of thisgreat show.

Next month I'll bring you an account of the rest ofour trip as we headed back for home. A

14 MAY 1964