Would you like to vastly enhance a future model's
performance? Are you interested in competition technology
and performance? Are you lazy and want to build a
strong wing in 3-4 hours work? Read On!
The wing that is being made
in this how-to is for a 400 racer that most of you
know, the Sliver 400. The methods I show you here
are the real deal when it comes to what is done by
the fastest racers in the country. The current national
record fast time for 400 pylon was set in 2000 by
Pete Bergstrom using a plane I built and set up. The
methods I show you here are exactly the same
as I used to make his wing with one exception: his
wing didn't have glassed skin hinges or servo-mount
reinforcement. This costs .2-.3oz and I wanted to
have all the help I could give him.
With this in mind, if you follow
the instructions I give here you can have wings that
are stronger and lighter than any that can be bought
commercially. In fact, wings made with Balsa and Foam
are half the weight of the best hollow molded wings
on the market and just as strong.
When people hear "composite
wing" these days, they immediately think of fiberglass
cloth over blue or pink insulation foam or hollow
molded wings. A bagged Balsa wing is still a composite
wing, just not as glamorous because Grampa used to
make his airplanes from the stuff.
Balsa has several benefits:
The first is that they
stay looking nice longer than a glass/foam wing and
are repairable with scrap balsa when you break them.
The second is weight.
While the glass over foam wings can be built light,
they are very easy to dent and ding when you are being
weight concious. The hollow molded wings are really
strong and beautiful, but are very heavy. This
is because they have to be cosmetically "pretty"
in order to sell and because they are making them
as fast as they can to fill orders. To prove the point,
one of the commercial 400 racers out there has only
75sq. in. of area and the wing itself weighs 1.8oz.
before adding torque rods and hardware. Compare this
with the 163sq. in. wing built in this article that
weighs 1.7oz. After covering and torquerods, the bigger
wing will only be .1-.2oz heavier. Balsa may be low
tech, but it is light!
All you need for this project
is a set of foam cores, glue, a Vac-Bagger, and modeling
odds and ends that you probably already have in your
workshop. Once you make a wing this way, you'll never
want to build up a ribbed wing again. Bagging a wing
is so fast and easy that it feels like cheating!
Major items that you'll need:
1) A set of cores!
These can be from a commercial kit
or ones that were cut from your own templates. There
are many cheap ways to make a foam bow, and even exotic
machines such as Tekoa's FeatherCut. One other option
is to have a core cutting company make them for you.
Don't forget to account for sheeting thickness when
drawing your core templates!
The cores you need for bagging the
wood wing I show here are made from 1lb. Virgin white
foam. Do not buy 'regrind' foam, it wire cuts like
a hot knife through gravel. For glass composite wings,
2lb blue or pink foam is used, but here the balsa
skins are a lot of the wing's strength so 1lb is more
than sufficient. The spar is only a 9" long by
1/32" thick piece of plywood. The foam's main
job is just to maintain the airfoil shape, that's
it. On your wing, make sure that the spar extends
3" past the root of your aileron-this is where
wings like to fold. For those that worry, this wing
will take up to a medium size brushless motor, i.e.
250-300w. To exceed this, you'll need a more substantial
spar and glass/carbon capping it on top and bottom.
2) Epoxy Glue!
You'll need a quality laminating epoxy
for this job, the 5, 15, and 45min. hobby shop slop
won't work here! I've been using West Systems 105
resin and 206 hardener with good results and recommend
them because they are inexpensive, easy to find, sold
in smaller quantities, and there are pre-set ratio
pumps available for them. Don't buy a lot of glue
as it has a shelf life! You are going to use it an
ounce at a time and be throwing most of that away
3) A Vacuum Bagger!
There are ready made systems available
in the U.S. from Aerospace Composites and Composite
Structures Technologies. I bought ACP's bagging machine
on advice of a friend and am very pleased with the
results I have seen with it. Up until then, I had
been using his home-made bagger, but didn't want to
try to source the components and get bogged down with
troubleshooting my own. The first wing I made with
the ACP system turned out perfect, best one I had
made up to then. My first wing was in the bag less
than 10 minutes after the bagger was pulled out of
the box. They really do provide a turn key bagger!
There are a lot of details that make
this the Cadillac of baggers-the wide range of vacuum
adjustment is a plus. Probably the most advantageous
part of the ACP setup is the large plenum(tank). This
is the large cylindrical object in the pic. It allows
the air pump to run only briefly and less often to
maintain vacuum. This is a good thing if you have
sleeping children or make airplanes in your apartment
bedroom's walk in closet like me!
As mentioned above, the wood is the
primary source of strength in this wing. If this wing
were to be used strictly for competition, then Ultra-Light
contest balsa could be used. The only problem with
this is that you can't use a high heat iron-on covering
or you'll transfer too much heat through the wing,
melting the cores underneath. Silkspan and dope, LightSpan,
or UltraCote Lite are good choices if you use lighter
For more strength and ding resistance,
you should select medium weight balsa skins. Medium
weight skins were used in this wing and it was still
very, very light. In the case of the 400 racer, there
is always plenty of this to be had at the local hobby
shop because it's all that's left after the Free-Flight
guys picked through the lightest 1/32" sheet
This is only a luxury.. You'll be
placing strips of light (1-2oz) cloth at the ailerons
and the center section. None of this is for strength-the
piece in the center goes under the top sheeting so
that you have a reinforced area to mount the aileron
servo when buried in the wing. For a seriously stressed
wing, fiberglass and unidirection Carbon Fiber should
be sandwiched in strips and placed on the top and
bottom of the spar. This creates an "I"
beam underneath the skins making the wing extremely
strong, yet still very light. A typical application
would be in a long, thin wing such as those used in
F5D and B. Remember that the thickness and aspect
ratio of your wing will affect the strength it needs
when completed. If you are bagging a Sp400 wing that
will only be .3" thick when finished, it would
be a good idea to add light glass spar caps.
6) A sanding
Meet you new best friend, the extruded
aluminum sanding bar. You'll be using this to prep
your skins and sand the wing to shape once out of
the bag. This tool is what gives the commercial results
that you are looking for. You'll be able to perfectly
radius the leading edge and sand the trailing edge
to a razor sharp point on your first try. Very handy,
you'll wonder what you ever did without one before.
7) A 36" metal ruler, masking tape, squeegee,
pens and markers, Exacto blades, wax paper, and paper