One of the most important aspects of building really good flying electric model aircraft is the elimination of unnecessary weight from the airplane wherever possible. If we're not careful, redundant weight can show up just about anywhere in our models. Improper wood selection, over-built structures, as well as poor selection of system components, can all lead to an overweight model. As electric modelers, it is especially important that we be on constant lookout for ways to eliminate weight throughout the entire building process.
A couple of years ago I designed and scratch-built two small electric models using the excellent Graupner Speed 400 motor. These were actually study models for my Hummingbird MkIII, a much larger sport/pattern ship designed around a direct-drive Astro 6turn FAI40. Both prototypes were quite small, the first spanning 30 inches with 180 squares, the second spanning 36 inches with 259.2 squares. Since weight control for models of this size is especially important, I decided to experiment with tissue covering to see if I could save an appreciable amount of weight over conventional iron-on films while maintaining structural integrity.
I constructed a set of tailfeathers for the first (smaller) Hummingbird prototype from very light 3/32" sheet balsa and Japanese tissue to see how much torsional stiffness would be added by the covering. I was quite pleased with the results. While the bare wood parts were extremely flexible, when the Japanese tissue was added and shrunk, the resulting increase in torsional stiffness was more than adequate for an R/C model of this size, and the weight gain was very little.
I decided to extend the tissue concept to the rest of the model. I built a super light fuselage out of 1/16" contest grade sheet balsa, the sides of which were skinned on the inside with bond paper. For the wing I built a very light frame using 1/8" x 1/4" balsa spars with 1/32" balsa ribs spaced 3 inches apart. This frame was then covered completely in 1/32" balsa sheet. The idea was that the additional strength provided by the wing sheeting, which was necessary as a base for tissue covering, would allow for the use of a smaller, lighter wing frame.
The results of my efforts paid off in a model with an all up flying weight of only 18 ounces and a wing-loading of 14.4 ounces per square foot; not too bad for a model of this size. But the question remained of whether or not the tissue covering would prove adequate.
For the test flight I removed the landing gear and hand-launched the model over soft grass. It became quickly apparent that the tissue covering posed no problems; the model zipped all over the sky with no hint of flutter. I was also pleasantly surprised to see how durable the tissue covering was. Doped tissue is very slick and my plane tolerated repeated belly landings on grass without so much as a scratch. The only problem that I encountered was that after repeated hand launching the fuselage started to get a little dirty. This was remedied on prototype number two by an additional coat of dope to better seal the tissue.
Well, if you've read this far you might be interested to know the details about covering with tissue paper. If you are an experienced free-flight modeler, the following techniques should be familiar. However, if R/C has been your main thing, read on. Covering with tissue is not difficult, but it is different. The following guidelines should insure success.
The way I cover model parts with tissue is actually very similar to covering with an iron-on film. However, there are a few additional steps since tissue has no adhesive backing.
Step 1 Begin by giving the entire part to be covered a few coats of 50/50 thinned nitrate dope. Allow each coat to thoroughly dry, sanding lightly with very fine sandpaper between coats. Two coats are usually sufficient, but three may be needed. You'll know you've got enough dope when the wood has a slight shine to it. These preliminary coats add strength to the wood and provide a base for the tissue to adhere to.
Step 2 After thinning a small amount of "Balsarite for Film" to the consistency of dope, brush on a single coat just around the edges of the part to be covered. Balsarite is a very good adhesive and a thin 1/8" line around the perimeter of the part is all that's needed. I use Balsarite for Film, although I'm sure that Fabric formula Balsarite would also work well. Of course, if you prefer, you can attach the tissue with dope thinner just like the free-flight guys do. If you go this route you can skip this step. However, I would consider using Balsarite at least for curvy areas since tissue will stretch around curves with heat much like, but not quite as good, as the expensive plastic stuff.
Step 3 Now iron the tissue around the perimeter of the part in the same manner as when covering with plastic film. Take care to smooth out wrinkles but don't worry about stretching the tissue tight. When finished, trim away the excess tissue.
Step 4 Now it's time to shrink the tissue tight. There are a couple of ways to do this, but they all involve carefully wetting the tissue. If you have an atomizer you can spray on a light mist of water, or it can be carefully applied with a damp paper towel. As for myself, I simply wet my fingers under the sink faucet and carefully wet the tissue with my bare hand. This allows me to feel when the tissue is evenly wet and I avoid leaving small traces of wet paper towel on the tissue. Obviously, a little extra care must be taken if you do it this way. However, I've covered many models this way and I've never once torn the tissue. After the tissue is thoroughly moist it will be sagging all over the place. But worry not; as the water dries the tissue will shrink tight. If, after the tissue is fully dry, you find a small wrinkle, it can easily be removed by touching it with a covering iron set to medium heat.
Step 5 After the tissue is nice and tight, brush on a few coats of 50/50 thinned nitrate to seal it. Actually, a single coat is sufficient, but two coats will provide better protection against dirty hands. If you want to add a little trim work, simply cut your design from another color tissue, lay flat on the surface, then bruch on a single coat of (you guessed it) thinned nitrate dope.
Step 6 Step back and admire your handywork, then proceed to cover the rest of your model in the same manner.
IMPORTANT NOTE ABOUT COVERING DELICATE PARTS WITH TISSUE
One of the greatest things about covering with tissue is that it allows the use of very light wood. This is because properly applied tissue and dope will add a considerable amount of strength to the finished model part. However, for very small light parts such as tailfeathers or ailerons, care must be taken to see that the parts do not become warped when the tissue is shrunk. The following technique will insure strong, super light parts which are also dead straight;
To prevent warping of delicate parts (such as an aileron carved from very light balsa), first take care to see that the tissue is applied in a similar manner when covering both sides. Smooth out the wrinkles but do not stretch the tissue while covering either side. Next, pin several scraps of equal thickness balsa onto your workboard in a slightly smaller outline of the part. If that last step seems vague, read on, hopefully it will become clearer. After wetting the tissue, carefully positioned the wet part over the scrap balsa perimeter and secure it in place with more scrap balsa. In essence, you want to jig the part in place in such a way as to allow air to the underside, thus allowing the tissue on both sides to shrink simultaneously while the part is held straight. After the tissue is thoroughly dry and tight, the part should be removed, quickly given a coat of 50/50 thinned nitrate dope on both sides and placed back on the jig to dry. The result will be a strong, torsionally resistent, yet feather light part. This is the technique that I used when covering all of the ailerons and tail parts for both Hummingbird prototypes, with gratifying results.
Following is a summary of the pros and cons of covering small R/C models with tissue paper.
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