Free Flight Duration - 2009/05

120 MODEL AVIATION
Which covering is right for your model project?
[[email protected]]
Free Flight Duration Louis Joyner
Also included in this column:
• Covering tips
Bud Romak covered his original Moffett with yellow Japanese tissue accented with darkblue
tissue. Yellow artist’s ink added to clear dope provides a brighter, fade-resistant finish.
Bob Mattes used Polyspan to cover his scaled-up Amazoom’s wing and stabilizer. Red
paint on the rudder, forward fuselage, and underside of tips aid in visibility. Power is a
Fox .35 he’s had since he was 16.
WHEN I STARTED in FF, there were only
a few covering-material choices. Japanese
tissue offered light weight and a wide range
of colors, but it was easy to puncture.
Silkspan, available in several weights,
was stronger than tissue and had enough wet
strength to allow wet covering. But the color
choices were limited, and it required more
dope to fill the pores. Silk was great for wet
covering but was expensive and took a lot of
dope to fill.
Wet covering used to be the best way to
deal with curved surfaces, elliptical wingtips,
or built-up fuselages. The process involved
wetting the silkspan or silk with water,
blotting off the surface water, and then
draping and pulling it over the model
structure. Dope was brushed through the
covering material, to adhere it to the balsa.
Pros could cover a polyhedral wing, top and
bottom, with one piece of silk.
Now, a half century later, we have almost
too many choices. Tissue, silkspan, and silk
can still be found. In addition, a wide variety
of plastic films offer resistance to moisture
and high-nitromethane fuels, as well as faster
covering.
Which covering material is best? The
answer is, “it depends.”
For lightweight Rubber designs, Japanese
tissue is still a good choice. In addition to the
wide range of colors, it provides a fair
amount of torsional strength when tight. This
can be important for models with simple
wing structures that aren’t inherently strong.
Most wings with straight ribs and a single
spar fit this category.
Tight tissue covering will dramatically
increase the stiffness of such a wing.
However, a damp morning or a light rain will
cause the tissue to loosen and sag, with an
almost total loss of strength.
Extra coats of clear dope help seal out the
moisture. For additional protection in the
rain, spray on a coat of 3M Scotch Guard.
This fabric protector will cause the raindrops
to bead, making them easier to wipe off with
a paper towel.
Another problem with tissue is fading.
Keeping the model out of the sun as much as
possible will help, but the best solution for
long-term protection is to add color to the
clear dope used to coat the tissue.
One of the most popular techniques is to
use Higgins fade-proof ink, which is
available at most art-supply stores. The
suggested mix is one part ink, 9-10 parts
thinner, and one part clear dope. Spraying
will give a more even finish than brushing.
The Higgins method works best if you use
the same color ink as tissue (such as a yellow
ink mixture over yellow tissue).
On his Moffett design, Bud Romak added
wide strips of dark-blue tissue to a mostly
yellow model. To save weight, the blue strips
were not applied over yellow tissue; instead,
Bud covered the model with yellow, omitting
tissue where he wanted blue strips.
Then he covered those wing bays with
blue tissue. He sprayed the entire wing with
the yellow ink-dope mixture, which had only
a slight effect on the color of the dark-blue
tissue. But little can be done to increase the
tissue’s resistance to punctures, except to fly
over the proverbial tall grass.
The plastic covering material that is
closest to traditional tissue and silkspan is
Polyspan. Available only in white, this
material resembles silkspan.
05sig4.QXD 3/24/09 12:28 PM Page 120
May 2009 121
Dick Hall used UltraCote on his 655-square-inch Jaysbird powered
with an O.S. Max .35. “It’s got a nice short nose,” says Dick. “I use
4 ounces of lead in the nose to get up to the 35-ounce weight.”
For Power models, heavier coverings such as UltraCote offer
strength and resistance to high-nitro fuels. An O.S. Max .25 ductedfan
engine powers Ronnie Thompson’s 528-square-inch Sunbird.
Paul Crowley’s Moffett Rubber model is covered with a few materials. Wing inner panels
are covered with metalized Mylar. Tips and stabilizer are covered with red-dyed clear
Mylar. The fuselage is covered with Polyspan and sprayed with clear dope that has red
dye added.
Polyspan is applied with dope, as is tissue,
and then tightened with an iron, as are plastic
film coverings. Then several coats of clear
dope or lacquer are applied to seal the pores.
It’s important to make sure that the smooth
side of the covering is on the outside and that
the covering runs spanwise as it comes off
the roll.
The finished weight for Polyspan is nearly
3.6 grams per square foot—approximately
double the weight of doped tissue. However,
Polyspan is much tougher than tissue.
Stubble that would puncture tissue usually
leaves only a slight depression in Polyspan,
and those are easy to remove with the tip of a
hot iron. Many Rubber fliers use Polyspan to
cover built-up fuselages for extra strength and
less tendency to absorb rubber lube.
For coloring Polyspan, you can use the
same ink-dope technique I mentioned or one
of the special dyes available from FAI Model
Supply. Another approach is to leave most of
the wing white but add areas of bright color
to the tips, to increase visibility. Design
Master florist’s spray, available at most craft
stores, is available in bright colors as well as
a surprisingly opaque black.
As in the old days of tissue or silkspan,
you should dope Polyspan with butyrate dope
when used on a fuel-burning model. There
are also other synthetic tissues, such as Lite
Span and Air Span.
Where weight is critical, 1/4 mil Mylar is
almost as light as you can go. Available in
both clear and metalized versions, this plastic
film is not precoated with adhesive. Instead,
the framework of the wing or tail must first
be coated with a heat-sensitive adhesive, such
as Balsaloc. After that’s dry, the Mylar is
attached using a trim iron and heat-shrunk.
Also available are 1/2 mil and 1 mil
thicknesses that give a bit more strength.
Although these thin Mylar coverings work
well in the rain, they are prone to tears or
punctures. The thin Mylar provides little
torsional strength. The clear Mylar can be
colored with fabric dye before covering.
An interesting approach is to combine
Mylar and tissue. To do this, cover the model
with clear Mylar and then give it a coat of
05sig4.QXD 3/24/09 1:03 PM Page 121
clear dope. Then smooth tissue over the
Mylar. Thinner applied with a brush activates
the dried dope, attaching it to the Mylar. A
final coat or two of clear further seals the
tissue.
For larger models, the heavier, adhesivebacked
plastic coverings provide extra
strength and resistance to fuels. That’s why
many Power fliers have gone to coverings
such as Hangar 9 UltraCote.
“It’s like MonoKote, just better,” says
Ronnie Thompson. “Film is the only way to
go.”
On his larger AMA Gas models, Ronnie
uses transparent UltraCote for the flying
surfaces. But he also uses a traditional
covering material. The balsa D-box on his
Sunbird series of models is covered with
silkspan, and then the entire wing is covered
with UltraCote.
“The silkspan really adds to the strength,”
he says.
Tips from Dick Hall: Another Power flier
who really likes plastic coverings is Dick
Hall. I asked him a few questions about his
use of those materials, and he generously
replied with three pages of tips and
suggestions. Following are some of them.
“I’ve gone entirely to polyester film
covering. There are several reasons: The
plastic films are not susceptible to rain or
humidity; in our area test flying in the
evening in summer was useless with tissue
and dope because the covering would get so
loose. Flying in high humidity or any kind of
rain was also a problem. With plastic
covering I have flown in the rain many times.
“The many coats of dope required for a
large engine-powered model took a lot of
time to apply and even more time to cure.
The fumes permeated the whole house.
“I tried the polyester tissue coverings [i.e.,
Polyspan] and found them great for rubber
models but not for power models. I covered
an Old Timer with polyester tissue and used
what should have been plenty of coats of
dope, but the oil from the fuel soaked
through to the balsa beneath.
“I have been using UltraCote exclusively
for many years. Everyone I have talked to
has found it superior to anything else. I use
regular weight [roughly 5 grams per square
foot] on large models and the lightweight
version [approximately 3.4 grams per square
foot] on 1/2A and 1/4A models.
“I like to use red on the wing and stab for
visibility against the sky and white on the
fuselage for visibility down low with a tree
line background. The fluorescent red is a
more visible color than the darker normal
red, but it’s not available in the lighter weight
film.
“Since I like to have the wing and stab the
same color, I use the regular weight film for
all the surfaces. Ronnie Thompson
sometimes uses the lighter weight material
on the stab and the heavier one on the wing.
“The UltraCote lasts very long. I have not
seen any indication of color fading as with
tissue. Sometimes the covering will loosen
slightly over time; it is best to check and
retighten those areas.
“The loosening may actually have been
caused by the covering not being uniformly
tight to begin with. I use the maximum
temperature to try to get the tightness
uniform; the tighter the covering the better.
“I have a DC to AC converter that works
off the battery in my vehicle. With the plastic
materials I can adjust warps and do repairs in
the field and do not have to wait for dope to
dry. For small holes or tears I use clear
plastic package sealing tape, which can be
removed back at home with the covering
iron.
“The use of a plastic film without the
adhesive backing would probably provide
some weight savings, but I have not found it
worth the trouble. The brush-on adhesives I
tried were temperature-activated adhesives
and I couldn’t use a heat gun or a shrinking
iron close to the edges without the covering
coming loose.
“One tip is to tack the covering around
the periphery at many places before trying to
iron down all the way around the edges prior
to shrinking. This is particularly important
when covering sheeted areas.
“I use the plastic films on the fuselage
where it is convenient. I use Klass Kote
epoxy paint to foolproof areas that are
impractical to cover such as stab platforms,
engine mounts, and wing pegs.
“With UltraCote, I have not found it
necessary to seal the edges and I do use a lot
of nitromethane in my fuel. Using CA
[cyanoacrylate] to seal the edges is useless
since nitromethane is an excellent solvent for
CA adhesives. I have a bottle of solvent for
CA removal and it says on the back ‘100%
Nitromethane.’
“Perhaps the biggest problem with plastic
coverings is that they do not have a ‘grain’
like tissue and will sag between ribs. This is
not a problem with sheeted leading edges but
can be of major significance with some
Nostalgia designs that don’t have spars on
the upper surface. The Nostalgia rules allow
adding extra spars for strength and one
‘turbulator’ spar in the first third of the upper
surface.
“I add one additional ‘non-sag’ spar
instead of ‘turbulator’ spar between the
leading edge and main spar. I also locate the
main spar at the high point of the airfoil and
use an I-beam spar configuration with
vertical grain webbing and the top spar
thicker than the bottom one. I will not build
an airplane anymore without an I-beam spar;
I have folded too many wings with just a
bottom spar while returning on my chase
bike.” MA
Sources:
Hangar 9 (available through Horizon Hobby)
(800) 338-4639
www.hangar-9.com
Polyspan Lite Span, Air Span, Mylar films,
Japanese tissue, Polyspan (which can be
found in the Starline section of the Web site):
FAI Model Supply
(570) 882-9873
www.faimodelsupply.com
Polyspan and Mylar:
Model Research Labs
www.modelresearchlabs.com
Klass Kote
(612) 243-1234
www.klasskote.com
Louis Joyner
12404 Pleasant Lake Ct.
Richmond VA 23233