Wing Construction
All but one of the primary HLG's used at the contest
were of vacuum bag construction. The built up ships were
designed and flown by Jerry Krainock, whose wing contained ribs and had a great deal
of composites used in it's construction.
Vacuum Bagged Wings:
Core Materials:
Nearly all of the wings were hot wired using blue or pink Dow
foam. One designer used Spyder foam.
Epoxy:
Most designers used MGS epoxy for the wings and fuselages. MGS
epoxy is much thinner than West Systems epoxy and is much stiffer when
cured. The MGS epoxy can be removed from a lay-up after wetting
out the glass. Another epoxy used was from FiberLay.
West Systems cures more flexible, and has been used on fuselages.
Colors:
Most planes did not have color in the wings. Any transparent
color was added using powdered additions available from CST.
Some designers painted the cores prior to bagging with a foam safe
spray paint. Other designers painted the mylars prior to laying
up the wings. Bill Watson painted the cores of his Sidewinder
prior to bagging. Tom Hoops and Blake Nielsen painted the mylars
prior to bagging.
Logos:
Some designers added logos to their wings. This was very cool,
but how did they do it? To add the logo, the designer sprayed a
very light coat of 3M77 spray adhesive onto a pieces of light white tissue paper,
and placed the tissue onto a piece of wax paper. The design was
then printed in reverse onto the tissue using an inkjet printer. After printing and during
wing lay-up the printed tissue (without the wax paper) is placed on the wet fiberglass wing skin making sure
the image. When adding the logo make sure the logo reads
correctly when viewed. Examples of this
technique can be seen on the UpLink and Watson Sidewinder. These
designers also added their names and addresses to the lower surfaces
just in case of a HLG fly-away.
Coverings:
Fiberglass Skins:
Fiberglass skins varied with the design. Most
fiberglass planes used 0.75 oz fiberglass. The UpLink, designed
by Dick Barker used 1.4 oz fiberglass. The Raptors used 3 layers
of 0.75 oz fiberglass oriented at 45 degrees to the span. This
is a requirement for the composite wings as it reduces wing and
aileron flutter. (I know, I didn't. The ailerons fluttered
a nice tune, permitting Joe and others to sing out!) Extra
fiberglass layers are added at the tips where the wing pegs are added,
and around the polyhedral joints. At the center section, an extra
layer of glass is added, cut at an angle to spread out the wing
bending loads. Other fiberglass ships used 2 layers of
fiberglass on the wing, with added layers in critical areas. The
UpLink used 1 layer of fiberglass and an extra layer as a cap of the
shear web instead of carbon fiber tow. For the plane built with
spyder foam, one layer of 0.75 oz glass was used on the wing, with
extra layers of glass along the entire length of the forward portion
of the wing. Extra layers of fiberglass were used where the pilot
gripped the wing for the discus launch. Other planes used 2
layers of 0.75 oz fiberglass as wingskins.
To achieve the best strength to weight ratios, it is
better to use 2 layers of 0.75 oz fiberglass over 1 layer of 1.5 oz
fiberglass. The reason for this is that the skin holds less
resin in the cloth, the resin is easier to apply thin, and when the
skin is placed under the vacuum, the resin flows better to fill in the
voids of the weave.
Kevlar Skins:
The composite ships made by Maple Leaf Designs, the Feather and the
Logic used kevlar skins. The kevlar was oriented at a 45 degree
angle with respect to the span. The Encores and Logics used the
kevlar skins as lining hinges. The foam was removed from
the bottom of the wing just under the moving hinge area, approx.
1/4" wide. The Encores used 1 oz kevlar cloth. The
Logic by Phil Barnes used a composite cloth of kevlar and
carbon. Other kevlar skinned ships used 1.5-2 oz kevlar
cloth. The higher weight kevlar cloths were generally of a
higher density weave rather than of greater individual tow
sizes.
Spars:
Most of the bagged wings used carbon fiber tow as
spars and capstrips. At the wing center, the tows had varying widths
of approximately 2 inches, and tapered outwards toward the tips.
Many wings did not have any shear webs, relying upon
the foam to act as the basic shear web element. Some ships used
0.060" carbon rods embedded underneath the carbon tow. this
provides
some spar strength. Some ships used a prepreg thin carbon spars
placed vertically in the wing. These spars provided additional strength
to address the bending loads. For the Encore, Phil Pearson actually
cuts apart the wing section and places a full depth Rohacell web in
place, and rejoins the wing section. The wing skins are
then applied.
The main structural failures of the wing were due to
delamination or foam compression failure. Generally, compression failures can be attributed to two
causes. The first
is the normal cause where excess stress is placed near the polyhedral
joints, especially during launch. The second reason has to do with the
pre-launch position of the plane, wing and pilot. When the pilot
is waiting to discus launch, the wing opposite the peg often rests on
the ground. The wings are not designed to support the plane just
between the tips, and this causes a compression failure somewhere along the span.
Failure is near the outboard panel of the wingtip touching the ground.
Delamination of the wingskin is often due to the strength of the
epoxy-foam bond. to increase adhesion of the skins, builders
have tried many solutions. When the foam core is cut, the
cutting melts the foam and leaves a slightly skinned surface.
The builder can carefully sand cores to remove the skin, or they often
used a "woodpecker" tool which has pointed blades which
pierce the surface. In these cases the epoxy bonds to the sanded
surface, or flows into the holes pierced into the surface. Since
most builders are trying to reduce weight in the wing, they reduce the
amount of epoxy in the lay-up and this reduces the amount of epoxy which
flows into the holes. One designer, Bill Watson, adds span-wise
grooves under the carbon fiber tow. A brush of epoxy into the
groves creates a better bond to the carbon tow and wing skin, and at
the same time it creates a small shear web . In effect a
"T" shape is produced under the capstrip. The slit id
formed by using a ruler and razor blade. For a more aggressive
bond, a razor saw can be used to cut the grooves. If the groove
is too wide then the wing will add some weight, however with careful
application a stronger bond to the core under the capstrip can be had
with little added epoxy and weight.
The UpLink by Dick Barker uses an extra tapered 1.4 oz fiberglass cap
strip over a fiberglass laminated full depth shear web.
Built Up Composite Wings:
The Photon by Jerry Krainock takes advantage of composite materials in
it's built-up structure. The wing has the D-tube forward portion
of thing as a molded kevlar skin. Each rib is made of balsa
capped with prepreg carbon tow. The spars are carbon with balsa
shear webbing between the spars. The entire spar is wrapped with
kevlar thread to keep the spars form delaminating from the shear
webs. The trailing edge is of prepreg carbon tow.
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