Aircraft Coverings 1ml Their Applications By Martin C. Haedtler, EAA 4846 4920 Stone Circle, Oak Lawn, 111.

O OTHER branch of aircraft manufacturing requires N greater care and closer attention to technique than covering and finishing. These following methods have been

proven successful by their manufacture and use, and complete adherence to the manufacturers' suggestions and instructions will result in a finished product that will be more than satisfactory. The various coverings will be covered in separate articles, as each has peculiar and particular requirements that do NOT pertain to each other. Therefore, at no time should a practice known to be proven for one type of fabric be used for another type. The first of the series will deal with the oldest and possibly best known . . . COTTON; then follows LINEN; DACRON (under trade names such as Ceconite, Eonite, etc.), and lastly FIBERGLAS. These make up the flexible coverings, leaving only aluminum which merely requires painting. I. COTTON. The cotton fabric was used because of its absorption qualities and flexibility. Also, the fact that it shrank to a very smooth finish through water or dope applications makes it easy to use. The aircraft quality seal will be in the "Mil Spec" found on the edge. Most aircraft supply companies will also have available the "Pull Strength" readings on the bill of sale. The method of fastening cotton to the basic structure can be done in several ways. 1. Envelope—where a "slip cover" is sewn to fit the part, or over the shape (wing, fuselage, stabilizer, etc.), and then sewn closed before shrinking. (See CAM 18). 2. Blanket—various widths are laid on the shape, clamped in place, then sewn together or "glued" to the shape. "Glue'' is usually extra thick dope or special dope, i.e.. "Air-Lac," which dries rapidly and holds very well. After the fabric is fastened to the structure, the shrinking process can begin. Two schools of thought are available: 1. Water shrink—water sprayed or brushed with a sponge will dampen the cotton and it will shrink to a perfect smooth, wrinkle-free condition. 2. Dope shrunk—the first coats of dope, usually thinned, will shrink the cotton to the desired beginning tautness. The first coat will be a brush coat, if clear dope, brushed into the fabric at full body or "as it comes"; an exception is when the dope is so thick it would prevent penetration into the fabric. It is recommended that a full brushful of dope be used, and that the dope be applied evenly over the surface. Do not attempt to get too much surface with one brushful of dope. It should be remembered that the first coat is the "bonding" coat, consequently, dry brushing or stretching of dope must be avoided. Proper saturation of the fabric and penetration calls for an evenly applied wet coat of dope. Allow the first coat of dope to dry 30 to 45 minutes, depending on whether or not thinning was necessary. The reinforcing tape, patches, drain grommets, and inspection rings may be applied with the second coat or as a separate operation immediately prior to the applica-

tion of the second coat of clear dope. The most common practice is to apply the tapes and patches with the second coat. This is easily accomplished by doping up to the rib or section to be taped, lay the tape on the doped surface rubbing it down to remove air and insure bonding, and proceeding with the application of the dope up to the next area to be taped. After the second coat of dope has been applied and all the tapes are in place, allow this coat to dry 30 to 45 minutes before lightly scuffing or sanding with No. 000 (Triple 0) sandpaper or its equivalent. All coats after the second should be applied by spray equipment il possible to insure even application. Howe%'er, brush application may be used if applied carefully and evenly. The third, fourth, and fifth coats of clear dope are applied, allowing a drying interval of 30 to 45 minutes between coats. If these later coats of clear dope are applied by brush, it may be necessary to thin with the proper dope thinner. Also, if smaller spray equipment is used, usually thinning is necessary. Therefore, more of these thin coats will be needed to produce the "build" necessary. After the fifth coat has been applied and allowed to dry thoroughly, the surface should be sanded lightly with No. COO paper. IT IS RECOMMENDED THAT COMPONENTS BEING SANDED BE GROUNDED TO PREVENT STATIC DISCHARGE AND RISK OF FIRE.

On completion of the sanding of the clear dope, the next step will be two heavy, wet coats of aluminum-pigmented dope. Allow the same drying time between coats; light sanding here will insure the smoothness of the final work. Aluminiim-pigmented dope is used after clear and before color for three reasons: 1. To protect the fabric and clear dope from the Actinic or destructive rays of the sun. 2. To provide an effective filling or building coat. 3. It is easier to sand, and actually see the amount removed each time it is re-sprayed. Aluminum-pigmented dope is usually mixed on the job. It is prepared by adding 1.5 Ibs. of fine aluminum powder to each 5 gals, of full-bodied dope. Should aluminum paste be used, it should be mixed at 1.75 Ibs. per 5 gals. The aluminum should be mixed with dope thinner to a loose paste, then added to the dope to insure easier and thorough mixing. After the second coat of aluminum dope has been applied, allow 8 hrs. to "overnight" drying time before sanding to a smooth surface with No. 280 wet or dry sandpaper and water. NOTE: The thoroughness or degree of sanding at this point will determine the smoothness of the final finish, since no amount of sanding later on will accomplish the same result. After the aluminum dope has been sanded, wipe off all dust, dirt, and sanding mud. It is recommended that a sponge dampened in clean water be used to completely remove all foreign matter before applying succeeding coats of color (Continued on page 31) SPORT

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25

AIRCRAFT COVERINGS . . . (Continued from preceding page)

Apply three or more coats of colored dope. These coats should be sprayed, and are usually thinned 20-30 percent to spraying consistency, allowing 30 to 45 minutes between coats. After the final coat of color has dried for 8 to 12 hrs., only a light sanding with No. 400 wet or dry paper will be required. It is then rubbed out with commercial rubbing compound to insure a deep luster. It should be noted that most "dope" finishes will "lift" ordinary paint and varnishes. All painted surfaces, which will be touched by the doped fabric, might have this other paint "lifted." Therefore, some caution and extra steps must be taken. Formerly, adhesive tape was applied to the steel longerons of the fuselage to prevent "lifting" the paint. Now zinc chromate and the new Epoxy types of protective paint are used, namely, Glidden's "Glid-plate." Along this line is the action of the dope finish on the varnish used on wooded ribs in the wings. Adhesive tape is commonly used, as is household aluminum foil. Also, all sharp edges should be covered with tape. This prevents the finish from cracking due to the pressure from beneath. It is assumed that temperature control will be taken into consideration to maintain the quality of the work. Dope should be brought to room temperature at least 24 hrs. prior to application. Room temperature is from 75-80 deg. Blushing is common in humid weather where a small amound of water condenses in the top surface of the wet dope. This must be removed. This can be prevented by adding the special thinner known as "Retarder." If blush occurs (which is nitrocellulose "out of solution") the finish is ruined. It can be removed after occurrence by washing with a rag liberally wet with dope thinner. PLYWOOD COVERED WITH COTTON

Several builders have tested and adopted the following method, which results in the strongest and most durable (for plywood) ever devised. As a matter of fact, the strength and longevity of the wood is increased 15 percent. Sand the plywood evenly to obtain a smooth surface. Be sure to remove glue and interior varnish or preservative from the surface. Brush or spray two coats of thinned dope on the bare wood. These will both tend to soak into the plywood. Now brush or spray two full-bodied coats, allowing them to dry approximately 45 minutes between coats. When the dope is thoroughly dry, tack the cotton covering in place, securing it firmly.

Now two methods can be followed: 1. Water shrink the cloth and let dry, or— 2. Go directly to the thinner dope application. A libersl wetting of the cloth by thinner (retarder if possible) to the leading edge to start with, working back to the trailing edge, push the thinner into the cloth with a bristle brush or the hands, until the dope underneath starts to work up to the surface. Push all air bubbles to the edge or relieve with a pin. Apply the second coat, composed of dope added to thinner in equal parts, before the original thinner coat has had a chance to dry. Scuff sanding can be done if the dope is completely dry. Follow with two or more coats of aluminum-pigmented dope. Allow to dry and sand with No. 280 paper or finer. Spray two or more coats of color. Water-sand smooth with No. 320 paper, wiping clean between coats. For natural gloss, mix last color coat 50-50 with the clear dope, reduced to spraying consistency with thinner. There will not be enough difference between cotton and linen to warrant a complete instruction. Therefore, we will discuss the prime differences as compared with cotton. II. LINEN. A heavier fabric than cotton, it is prone to hold wrinkles from shipping and handling. It will be necessary to REMOVE THESE WRINKLES BEFORE DOP-

ING, which can be done by a tight fit to the components and proper and careful adhering to frames. Linen will not stretch in several directions at once. It must be worked carefully to eliminate "draws" at corners or on radius edges. Thorough wetting by sponging will completely remove wrinkles. But as soon as the fabrics are wet with the first coat of thinned (or mildew proofer "Fool Proofer") dope, the fabric sags. It appears to have been ruined, but subsequent coats of thinned dope will bring it back to the original taut condition. Linen uses more dope than cotton because of the filling required by the coarse weave. Each full coat of dope should have a scuff sanding with No. 280 paper to insure complete smoothness in final finishing. Linen will probably require six to seven coats before proper filling is apparent. This will be evident with each sanding, when the amount further needed can be seen and felt. Finish with "silver" (aluminum) as in cotton, and then on to the color. A

New World Soaring Goal Record Claimed

A which a new world soaring goal record is being claimed. LVIN H. PARKER, who operates a soaring school at Odessa. Tex., made a flight on Aug. 27, 1963, for

He soared a Sisu 1A sailplane from Odessa, Tex., 490 miles to a predetermined destination at Great Bend, Kans. The flight exceeded the existing goal record of 443.672 miles set on June 18, 1960, by Mikhail Vierietiennikov of Russia in an A-15 sailplane, flying from Orechkovo (Kalouga) to Ravkoska, and another claim to the record of 456 miles made on Aug. 7, 1963, by Ben Greene of Elizabethtown, N.C., in a Standard Austria sailplane from Marfa, Tex.. to Boise City, Okla. Substantiating evidence and documentation must be approved by the National

Aeronautic Association and the Federation Aeronautique Internationale in Paris before the flight may be considered an official record. Parker's flight took a total of 8 hrs., including a short tow to 1,800 ft. by an airplane at the start. From then on he utilized thermal upcurrents under cumulus clouds in which to gain altitude by circling, such that straight glides of irom 90 to 110 mph could be made between

clouds. The Sisu 1A has a maximum gliding angle in still

air of 40 to 1.

A 25 mph tailwind worked in his favor and he arrived over his goal with 2,500 ft. of altitude. A SPORT

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PART II

Aircraft Coverings And Their Applications By Marty Haedtler 4920 Stone Circle, Oak Lawn, 111.

SYNTHETIC FABRICS AND THEIR METHODS OF COVERING AIRCRAFT PARTS DACRON HERE ARE two types of synthetic fibers. Their physical properties are the separating qualities. First, we will discuss the chemically man-made fibers which react to heat for their shrinking action. These are composed of one of the miracle fibers made by chemistry since World War II. Basically, the material is commonly known as "DACRON." We are indebted to Marv Hoppenworth, Cedar Rapids, Iowa Chapter 33 and Harry Cooper of Cooper Industries for this information. The same general procedures can be used on various brand names of Dacron, such as "EONITE" and "CECONITE." Some methods vary as to the bonding agents used to secure the doping materials to the Dacron. Dacron is a synthetic, which, for the same weight, has twice the pull strength as cotton and outlasts the aging of cotton by three to one. It is not subject to mildew, nor will it absorb moisture. It is chemically inert, except for strong acids, and will not be affected by gas and oil. Dacron covering costs are about the same as cotton, even though the cost of the fabric is higher. There is less labor and less dope required. The fabric is tautened by heat, thus saving at least four coats of dope. Since the finished product weighs the same as cotton, no rebalancing is necessary. Repairs can be made with CAM 18 as a guide, using synthetic, longer-lasting cord for rib stitching, etc. A. Covering and fastening methods are in two separate types: 1. The envelope or blanket covering is sewn with special synthetic threads, and slipped over the parts, then hand-sewn to completion. 2. Cementing to structure . . . here begins the variance with such natural fibers as cotton and linen. A "Super Cement" is available which, when used properly to attach the cloth to the members, precludes the stitching required by CAM 18 as to speed of aircraft (above 150 mph), thus eliminating another step. The cementing method is recommended where the width of the fabric is sufficient to reach the entire side of a surface. In cementing it to the wing panels, the fabric should run spanwise and be cemented on the leading and trailing edges as well as on the root end and the tip. The opposite side is applied using the "Super Cement" with at least a 4 in. overlap on the leading edge, and a 3 in. overlap on the trailing edge. At least a 1 in. lap at the tip and root ends should be used. Now there should be a surface tape of at least 6 in. wide applied over the lap of the leading edge, and at least a tape of 3 in. on the trailing and tip edges. Fuselage attachment can be accomplished by cementing the side panels first, and then the top and bottom applied with a 1 in. lap cemented to the previously cemented fabric at the longerons. Surface tape of at least 2 in. wide should be used

T

over the lap on the longerons. These are cemented with the same cement. Surface tapes should be "pre-doped" cotton or fiberglass tape. Reinforcing tapes can be Dacron or cotton. 3. The tightness with which Dacron should be applied to the structure is less than cotton, as there will be a dramatic shrinking action when heat is applied. Dacron is shrunk by heat, and it shrinks in direct proportion to the amount of heat applied. 400 deg. F. will shrink unrestricted Dacron approximately 10 percent, or 5 in. in a 50 in. width. Coverings on gliders and light construction aircraft (most homebuilts) should initially provide at least 1 in. slack to prevent structure warpage. Dacron subjected to 240 deg. F. (household iron on "Wool" setting) shrinks in a satisfactory and controlled manner, as would be produced by five coats of dope on cotton. Heating should be discontinued when the envelope has the desired tautness. Tautening should be done in two or more steps as in doping. Damage to the Dacron, as well as the understructures of wood and the wiring, can take place if a heat of 248 deg. F. is reached. On large surfaces, such as wings and fuselages, two-stage shrinking is recommended. Remove the slack with the initial application of heat, then the second pass will complete to tauten and remove the slight remaining wrinkles. Heat lamps of under 500 watts usually do not put out sufficient heat, and are unsatisfactory and slow. Care must be taken not to heat the fabric in excess of 450 deg. F., as the fabric will melt, loosening the cloth-like weave until it resembles celluloid. Non-shrinking nitrate and butyrate dopes are available, and will produce no further shrinking or tightening action. Regular dopes will pull the fibers and strands together, and can damage light structures. Plywood can be covered in the usual manner with an ample application of dope under the fabric. After cementing and shrinking the fabric, the fabric is attached by applying thinner which attaches from below. B. Finishing the fabric: After the shrinking is accomplished, the rib-stitching takes place prior to doping. However, there should be at least two heavy coats of dope under the surface tapes and in the areas of reinforcing tapes and stitching to insure adhesion. Inspection rings and drain grommets should be applied. Surface tapes may be of pre-doped cotton, and reinforcing tapes may be Dacron, fiberglass or cotton. Step 1: Apply at least four, and preferably six brush coats, thinned with two parts dope to one part thinner, to all surfaces. Step 2: Sand lightly with No. 320 wet or dry sandpaper to remove the nap of cotton surface tapes or blemishes. (Continued on bottom of page 16) SPORT

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15

KEEPING THE A-40 GOING . . . (Continued from page 14)

signs of blow-by. There will be some when the engine is cold, but not enough to harm compression when it is hot. After anything from 100 to 200 hrs., the working of the

gasket metal can cause real leaks, and when that happens, it's time for new gaskets. A-40 gaskets are as scarce

as EAA members in Red China, but one place to try is Walt Hawkins, Inc., 3004 N. Hollywood Way, Burbank.

models. Later, steel-backed replaceable insert bearings were used, which allowed top rpm to be boosted to 2575 continuous, at which speed 40 hp was developed. As the later rods would fit series 2 and 3 motors, they were often used, and you are apt to find them in many an

older A-40 . . . crankpin nominal diameter, 1% in., desired fit .0018L, tolerances .0008L and .0028L. Watch that an A-40-2 camshaft does not get into an A-40-3, 4, or 5 engine when building one up from parts.

Timing is different:

Calif. Or make your own! Go easy on tightening the head nuts, as the aluminum head castings are relatively soft. An unusual feature is that the four-throw crankshaft has only two main bearings. Because of its shortness, it has adequate stiffness to get by with this mini-

A-40-2: Intake opens 10 deg. ETC, closes 55 deg. ABC. Exhaust opens 55 deg. BBC, closes 10 deg. ATC. Others: Intake opens 10 deg. BTC, closes 50 deg. ABC. Exhaust opens 60 deg. BBC, closes 15 deg. ATC.

an hour-glass shape, due to slight crankshaft bending. The more such wear there is, the more the shaft can whirl, bearings will wear progressively faster, allowing of more bending, and fatigue failure is not unknown. This

22 deg. BTC. Use Champion C-26 spark plugs or equivalent.

mum support. But in time the main bearings can wear to

wear cannot be felt when the shaft is tried for play by

hand. The only sure way to detect it is with a good internal micrometer, taking enough measurements on the lengths of the bearings to detect hour-glass wear. These bronze bushings are removable, and a good machine shop should be able to make up a set for you. Authorized Continental repair shops in the old days had a special reamer for reaming front and rear bearings to size and true to

one another, in one operation. Today it would be necessary to make up some pilots to hold different-size reamers true, to cut one bearing and then the other, and have both bores in a straight line. Here are the figures needed for crankshaft work: Crankshaft in front bushing, nominal diameter 17s in., desired tit .002L, tolerances .001L and .003L. Crankshaft in rear bushing, nominal diameter 2Vs in., desired fit .0025L, tolerances .0015L and .0035L.

Crankshaft end play is important because too much, due to wear of the thrust face on the forward main bearing, can make connecting rod big ends ride cocked on their journals. There is a three-holed, %-circular shim on the rear main bearing support plate that controls end

play. Nominal distance between faces of the front and rear main bearings is 8% in. Desired end play is .015L, tolerances .010L and .020L. Connecting rod big ends are

slightly offset — do not get them in backward! Early A-40's had babbit connecting rod bearings which allowed them to be run at 2550 rpm top, at which speed 37 hp was produced. These were the A-40-2 and A-40-3 AIRCRAFT COVERINGS . . . (Continued from page 15)

Step 3: Apply three coats of aluminized synthetic aircraft enamel, or three coats of aluminized acrylic aircraft enamel, or three coats of aluminized nitrate or butyrate dope. Step 4: Sand very lightly with No. 320 wet or dry sandpaper. Step 5: Apply two coats of aircraft enamel, or two

color coats of acrylic enamel, or two coats or more of pigmented nitrate or butyrate dope.

IMPORTANT:

1. It is necessary to achieve mechanical adhesion with

Dacron, as it has no pores or fibers to absorb dope. Therefore, nitrate dope is preferred as it has better adhesive qualities than butyrate, and is preferred in Step 1, even if the final finish is butyrate. 2. In Step 3, use approximately 4 to 6 oz. aluminum per

gallon for clear enamel or dope.

16

JANUARY 1964

Valve clearance for timing is .020 in. exhaust, .025 in. intake. Running cold clearance is .015 in. for all valves. Ignition on the A-40-2 fires 27 deg. BTC, all other models NOTES ON THE CONTINENTAL A-40

One of our members who has had much experience with the Continental A-40 is Robert Thompson, EAA 5351, Box 107, Cherry Fork, Ohio. Bob's experience with them

goes back to 1932, when he flew Heath "Midwing" planes powered with them. In a letter to EAA's editors, he makes these interesting comments:

"I have been flying, servicing and rebuilding Continental A-40's ever since they came on the market, and the little A-40-4 and A-40-5 are particular pets of mine.

I have some information which may be helpful. "If any of the boys need piston rings, they can get genuine aircraft piston rings from the Perfect Circle Ring Co., at Hagerstown, Ind., and if the grooves in their pistons need resizing, Perfect Circle is equipped to do the work and furnish oversize rings for the new grooves. In

regard to connecting rod bearings, Federal Mogul has a

much improved steel back, copper-lead, replaceable bearing that can be precision bored for A-40 crankpins. As

of 1961, I was informed that Continental Motors Corp. Aircraft Engine Division, can furnish new piston pin bushings for the connecting rods as well as new pistons

and exhaust valves. "If all the fellows who are really interested in rebuilding A-40's would drop me a postcard saying so, and there turned out to be enough of them, I might be able

to work out some kind of quantity purchase of A-40 parts which would enable everyone interested to put his A-40

into top condition at a reasonable price."

A

3. Butyrate dope is more fire resistant than nitrate, which does not have the shrinking continuance of butyrate. However, when the use of butyrate is planned, care must be taken in the original shrinking, as the dope will continue to shrink and extreme tautness can cause structural failure. 4. Extreme tautness can be relieved . . . a. Where seam cement was used, the fabric can be loosened by applying MEK (Methyl-ethyl-ketone) at

the laps. It evaporates rapidly and can be worked with some ease. b. Clear lacquer or acrylic enamels combine with, and dilute, dope solids to a degree that relieves tautness. 0 (EDITOR'S NOTE — The material presented

on aircraft covering was prepared by amateur builders, and may not always reflect the professional approach to the subject. Any comments on these procedures by professional people would be welcome).

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

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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