PDF | FULL TEXT

You will need Adobe Acrobat to view this document.
Get a copy here

From the Central Indiana Aeromodelers newsletter, New Palestine, Indiana

Covering Model Structures with Tissue

by Robert Natch, SAM Chapter 8, Pacific Northwest

There are a variety of tissue products available for covering model aircraft. They generally fall into one of two categories: domestic or Japanese. Excellent results can be obtained utilizing domestic tissue and it is available in a rainbow of colors. However, it is usually made with more coarsely ground pulp and is heavier than oriental tissue. Perhaps the best known of these is Esaki, a standard for many years. It’s currently available in basic colors and they don’t run when wet. It’s light and relatively strong and available from a number of different sources one may identify as “Esaki Tissue” on the Internet. Esaki is suitable for any size and type of aircraft from Peanut Scale to “B” size gas models when properly applied and finished.

There are even lighter materials available that are used by individuals specializing in indoor and small rubber-powered Scale models. Gampi tissue and condenser paper are two such products, both available from Internet sources. Gampi is handled much like Esaki. The advantage with condenser paper is that it does not pass air, (unlike the author of this article), and therefore does not require any weighty coating.

The following tools are essential:

1. New double-edge razor blades. (These become dull quickly when trimming tissue.)

2. A good, sharp pair of scissors.

3. A sanding block with 220-grit aluminum oxide paper attached.

4. Various grades of aluminum oxide paper—220 grit and finer.

5. A hobby knife with #11 blade.

6. A round, pointed, synthetic artist’s brush. About size eight.

7. A flat, synthetic artist’s brush about one inch in size.

8. A spray bottle that delivers a fine mist of water.

Materials Required:

1. Tissue

2. Rubbing alcohol and/or water.

3. Non-tautening dope and compatible thinner, preferably nitrate.

4. Fuel proofer if using glow engines.

No covering job is better than the preparation of the airframe underneath. Once the basic structure is complete, considerable time should be spent with sandpaper and sanding blocks to contour, remove pumps, sawyer’ s marks, rough edges, excess glue, fuzzies, and the like. If the airframe has a round fuselage, sand scallops between the stringers. It’s surprising how covering will accentuate any defects underneath.

Once the structure is smooth, it’s time to apply the adhesive. On smaller aircraft the dope can best be applied with the round brush. Use both of them on larger frames. It is important that every part of the structure that comes in contact with the covering be doped. If not, those balsa areas not sealed will absorb the dope applied over the tissue and will cause an unwanted blotchy effect.

Modelers use two kinds of dope: butyrate and nitrate. Up until the time Ray Arden introduced the glow plug we all used nitrate. However, it did not stand up to glow fuels and butyrate was introduced to cure the problem. It did, to a certain extent, but some of the stuff the circle burners were putting in their mixes went right through it. A variety of final finishes sold as “fuel proofers” have appeared since. The best are two-part epoxies that seem to withstand any fuel mixture.

Butyrate does not have a good reputation within the stick and tissue fraternity. To quote Mr. AA Lindberg, butyrate has, “no redeeming qualities, poor adhesion, not really fuel proof, and clouds tissue colors.” Most of us use nitrate that usually arrives in the shop undiluted. When preparing dope, the first additive should be a plasticizer, especially when the dope will be applied to lightweight buildups. For really lightweight structures, 20 drops of TCP per ounce of raw dope should be added. Use 10 drops per ounce for larger models. (TCP stands for tricresyl phosphate and is also called tritolyl phosphate). This is not found in your local hobby shop, but in a chemical supply house. The last I bought came from Spectrum Chemical Mfg. Corp. located in Gardenia, California.

Once the plasticizer has been added, cut the dope to a 50/50 mixture with nitrate thinner. Or you can purchase non-tautening nitrate dope from either Sig or Randolph. Sig is more expensive by unit volume, but Randolph is only available locally in gallons. Suggestion: buy Randolph and divide with friends.

Apply four coats of the prepared dope to every surface on the framework that will come in contact with tissue. Carefully sand off the fuzzies with fine sandpaper after each coat. When you are absolutely sure the structure is smooth, it’s time for covering.

Although tissue is a wonderful material for our use, it does have one shortcoming: it does not like compound curves. There are modelers who can work with wet tissue; it’s very difficult and requires tissue that has good “wet strength.” I find that I waste a lot of material in attempts to cover compound surfaces and have become a “dry technique” builder.

Good tissue, like Esaki, has a grain that is easy to find by tearing a corner. The tissue should always be applied with the grain parallel to the long dimension of the part being covered. The four coats of dope you previously applied to the structure is the adhesive that holds the tissue. If the airplane is square, Powerhouse, I like to start with the top of the fuselage. Cut a piece of tissue and lay it on the structure. Use the pointed brush and thinner to adhere the tissue. Start forward and then tack it at the back. Work back along both sides evenly, gently pulling the tissue smooth as you go. If you get a wrinkle, go back with the thinner; soften the dope and smooth things out. When you have it right, trim the excess with a sharp razor blade, then apply a coat of dope all around the perimeter and work the edges down tight. It sometimes helps to use a little water or saliva on a finger to make the tissue conform to the round edge you sanded into that upper longeron.

I forgot to tell you that Esaki, and some other tissues, have two distinct sides. (Well maybe not too distinct.) If you examine a sheet carefully, you will find there is a shiny side and a dull side. The shiny side should be on the exterior surface.

With the top covered, proceed to the bottom of the fuselage and repeat the process. With that completed the next step is to be sure there is enough dope, (adhesive), around the edges of the sides to hold the tissue. Use a small brush to apply two more coats of dope to the upper and lower longerons. Cover the sides, trim the excess tissue, and carefully seal it down using the same techniques you used earlier.

Round fuselages are an entirely different matter. The time honored method is to cut lengthwise strips of tissue—called “gores”— and cover the area between each pair of stringers individually. Once a strip has been applied and it’s edges trimmed, two coats of dope are applied on the edges of the gore so the next one will adhere properly.

All this requires careful fitting and trimming, especially on the last strips top and bottom. These cannot be trimmed in place. It’s time consuming and tedious. So, the next time you see a really nicely covered Grumman Wildcat, offer the builder congratulations.

Prior to covering an under-cambered wing, it’s best to put an extra two coats of dope on the bottom of each rib. Cover the lower surface first. Cut a piece of tissue about ½ inch oversize and lay it on the wing. Use thinner to tack down the four corners. Then, starting at the center section, move evenly along the leading and trailing edges toward the tip, attaching the tissue with thinner and gently pulling out the wrinkles as you go.

Again, you may have to use thinner in the trouble spots to loosen the dope and pull out the puckers. With the tissue in place and reasonably smooth, use the pointed brush to run a line of thinner down the length of each rib. Follow up with finger pressure, forcing the tissue down on the rib. Repeat the process for each rib. Then turn the wing over, and using the same pointed brush, carefully run a seam of dope chordwise along the joint between the rib and the tissue on both sides of the rib. I have never had a lower surface detach from the ribs using this method.

The top surface is not as tedious, except possibly the tip. If it’s round, cover out to the last full-size rib and trim. Add a couple of extra coats of dope to the top of that last rib and then cover each remaining bay individually. Sometimes two can be done at once. Two extra coats of dope are required around the perimeter of each open bay for good adhesion.

Most fins and stabilizers are relatively flat and are not difficult. Under-cambered stabilizers are handled in the same way as similar wing surfaces. Ditto for elliptical tail areas.

Once the airframe is covered, it should be checked for any tissue-to-structure joints that are not well attached. The dope-finger-water method will fix most irregularities. Trim any excess tissue. A sanding block lightly applied at 45° to straight sections such as trailing edges will remove any excess tissue very nicely. Finally, seal all edges with dope, and sand away any fuzzies with very fine sandpaper.

It’s time to shrink the tissue. Either water or rubbing alcohol will do the job. Water is the most aggressive; alcohol is less so and dries much more rapidly. Look for a spray bottle that puts out the finest mist and as few big droplets as possible. The best ones I’ve found are hairspray applicators. Hold the surface to be sprayed vertically and the sprayer about a foot away. That way, the bigger droplets don’t get to the tissue. Do not force the drying with either heat or forced air. Allow the tissue to shrink at room temperature to minimize warping.

Very delicate structures require special treatment to avoid warps. Pre-shrinking tissue is perhaps the most important. Build an 18-inch x 24-inch frame out of hardwood stock. Apply four coats of dope. Then cover with tissue, spray with water, and allow to dry. Trim the tissue from the frame. Construct a frame from ¼-inch balsa that supports the leading and trailing edges of a wing or tail, and fasten it to a flat surface.

Cover the wing/tall with the preshrunk tissue, spray it with alcohol (not water), and quickly pin it down to the ¼-inch frame. Let this dry at least overnight; longer is better. Remove the wing tail from the frame. If dope is required, use one coat of thin, plasticized material on those areas that will not come in contact with the ¼-inch frame and pin it back down to dry.

Now, if everything has gone well, it’s time for finishing. The dope has been plasticized and thinned. Use the flat brush and apply a very light coat to all surfaces. At this point be very careful not to slosh on a heavy application. The tissue is quite porous at this stage and dope will pass through and puddle on the inside of the surface, which would make it heavy and unsightly.

On the first coat, try to load the brush very lightly and scrub the dope into the tissue a small area at a time.

How many coats? That depends entirely on the model and how it will be used. I find that peanut-scale airplanes do not require more than two coats of thin dope. Medium size, (36-inch span), rubber, and electric models will take roughly four coats, and bigger airplanes will require more. Gas models should probably have at least five or six coats. The same for glow-powered models so long as there is no nitromethane in the fuel. If you are flying glow power with nitro, you should overcoat the dope with a two-part epoxy. Omit this step and you will be the proud owner of a soggy mess.

Registration numbers and decorations can be cut from tissue and attached to a doped surface with a careful application of thinner. Be very careful with the alignment prior to the thinner application. This should be done after the first coat of dope has been applied to the airplane. If applied late in the painting process they won’t have the same sheen as the rest of the surface and will stick out like a sore thumb.

A tissue-covered airplane that has been properly finished will last a long time. I have an 020 Replica Playboy that went 20 years before requiring a complete recover. The principle enemies of tissue covering are light (UV), moisture, and, in some cases, fuel. Rubber lube also helps hasten deterioration. So, if you want your models to last a long time, build a box for them. Keep them cool and dry and be sure you have a dependable DT that is lit, or wound, and running when you launch.Q

Home       Archives       About       Contact      AMA       Publications       Subscribe       Unsubscribe