Covering Balsa Models With Tissue
by Gene Norman.
By Gene Norman |
Aug 01, 1996, 01:00 AM
Covering Balsa Models With Tissue
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
COVERING WITH TISSUE
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.
- Very light finished weight
- Allows the use of very light wood
- Unique and attractive appearance
- Very easy to apply trim
- not quite as fast as covering with film
- will not handle sharp compound curves quite as well as Monokote
(but unlike film, wrinkles can be sanded of and the tissue touched
up with dope)