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Last month, we discussed the advantages of building lightly when
designing or constructing a model plane. This month we look into
how one can adopt lightweight building practices when constructing that
slick new kit. The ideas presented here are from a series of articles
by Clay Ramskill.
There are two parts to building lightly: 1) knowing what to build
with, and 2) knowing how to put those pieces together to create
a lightweight but strong structure. Let's look at each piece of this
puzzle separately. First, building materials:
- WOOD: Choose your wood carefully!! When scratch building,
select your wood at the hobby shop. Get good, straight grained balsa
pieces and compare them - then buy the lightest. If you buy mail
order, get contest balsa (4-6#) or "light" (6-8#) wood and hope for
the best.
With a kit, you may have to throw out some wood and replace the
heaviest stuff with lighter stock. Plywood is heavy, but necessary in
some spots - such as firewalls, landing gear plates, and some
formers. But use it sparingly, substituting lite ply or even balsa ply
where you can. Even using the heavier ply, you can shave some
weight. For instance,
3/16" ply is enough for most 40-size firewalls and landing gear
plates. Throw out the 1/4" stuff. Further, use the strength of the
engine mount. If it's a one-piece mount, you don't need the wood
inside the bolt pattern. Cut it out, stuff the hole with balsa. The
same goes for a plate for mounting landing gear. [See the figures in
last month's newsletter]
- GLUE: Use light glue - CA has the highest strength/weight
ratio, aliphatics come next, and epoxies are the worst. Where you feel
you must use epoxy, such as on your firewall or landing gear blocks,
use as little as possible. Glopping it on will not make your joint
stronger, only heavier.
- COVERING: Not all coverings weigh the same! The
low-temperature mylars are lighter, and fabric is substantially
heavier. Even among the "standard" coverings, the weight
varies. Monocote is lighter than Ultracote, but heavier than 21st
Century Film for instance. Color also makes a slight difference - lighter
colors are heavier! It takes less pigment to color something black
than white! Transparent films are lighter than comparable opaque
ones. When covering, avoid making large overlapping areas on your
covering - that's just wasted weight! Trim each color so they only
overlap by 1/2'' when possible.
Once we've chosen our building materials, we now need to consider how
to put them together to make a better-flying, lighter plane. Here's
where building techniques come into play. If you're scratch-building,
it's only natural to come up with your own construction
design. Kit-builders may find that they can trim their plane by
modifying kit parts (lightening holes, etc) or by replacing certain
pieces altogether, replacing them with parts traced on lighter
wood. Here are some tips to try on your next plane:
- CUT AND CHOP: Air weighs less than wood! Lightening holes
can be cut in balsa fuselage sides, tops and bottoms. Ply formers can
usually be trimmed from the inside, ply fuselage sides can usually
stand larger lightening holes. Thinner balsa may often be used for top
and bottom cross-grain planking. You can hollow-out or even eliminate
large balsa blocks, such as are often supplied for wingtips.
- BUILD UP: Ailerons, rudders, and fins/stabs may be heavy
1/4" - 5/16" solid balsa. Building these up
from 1/16" balsa skins with trusswork interiors gives you a light,
stiff structure, perhaps even stronger
than the originals. Alternatively, use the original stock but cut
numerous lightening holes in them.
- WINGS: Consider that the actual bending loads on the wing
are greatest at the
center, ranging down to near zero at the tips. So strength can be
tapered off as we go out to the tips. Spars can be of thinner
stock near the tips. Gradually cut the thickness of the webbing to
1/32" out at the wingtip. For foam wings, skin and foam may be cut away
increasingly progressing outboard toward the wingtips; this is best
done behind the thick point on the wing. Note: The above is fine
for flight conditions - but if you "catch a wingtip" on landing,
that's another story altogether!
- SANDING: Can anything be simpler? The more you sand, the
less your plane will weigh! Obviously, this could be overdone - but
at least sand in curved corners when they are shown on the plans. That
sawdust may not seem like much weight, but it all adds up. I typically
remove 10% of my planes' structural weight in sanding.
- BALANCE: DO NOT ACCEPT adding any lead to a plane! The
exception is an ounce or two in the wingtip to balance laterally. Move
servos, battery, and even the engine to avoid adding lead to your
plane. Bolted or glued-in lead is unnecessary weight - and adds extra
loads to your airframe. Plus, adding weight near the extremities of
your plane adds a lot of inertia to your plane's movement; it will be
less responsive to flight controls. Scale builders may be stuck with
this - quite often scale planes tend to be heavy anyway; adding lead
becomes the "last straw" leading to the stall, snap, crash tendencies
of some of those planes.
- ELECTRICS: There are additional things that can be done
for planes converted to electric power. No fuel proofing is needed,
for example. Since there is no airframe vibration, you can lighten the
fuselage substantially. The only places you need anything stronger
than balsa are for doublers near the wing mounting, landing gear
mounting, and motor mounting. For example, the fuselage for my .20
size electric Skyvolt weighs about 4 oz uncovered, including rudder, stab,
and landing gear.
- EXOTICA: There are more exotic measures taken to save
weight - usually in larger, more complex aircraft. Modelers use
aluminum, carbon and boron fiber, expensive honeycombs, and kevlar to
increase the strength-to-weight ratio. All this stuff is expensive,
but worth it when performance is the prime consideration.
Most model designs are heavily overbuilt - but it's always going to
be up to you to decide what is overbuilt and what isn't. We're not
rocket scientists (and neither are the designers!), so don't be afraid
to ask an experienced modeler about modifications that you're not sure of.
And don't be reluctant to experiment! If a ply former seems heavy, try
making a duplicate of balsa ply, and compare. Try doing a built-up
stab (they're easy, really!) and compare weight and stiffness with the
solid version.
Keep weight in mind constantly as you build. Keep the
following adage in mind: build strong where needed, and
lightweight everywhere. Pay careful attention to the parts of your
plane where strength is most important (center of the wing, landing
gear attachments, engine mount, wing mounting, etc). Question the
design of the kit you're building: Would EZ
hinges be lighter than the conventional hinge-pin type? Can I use
shorter (lighter) bolts than the ones the kit supplied? Can I trim the
ends off the engine mount? Could I replace this lite ply doubler with
balsa?
An ounce here, a half-ounce there - it all adds up fast!
Here's an experiment: try adding 6 ounces to the CG of your favorite plane
sometime, then go fly. Not nearly as much fun, is it? Now consider
this: just about every glow kit out there could easily have 6 ounces
removed from the structural weight with NO measurable loss of
strength. It's worth thinking about...
Next: Until Next Month...
Up: Front Page
Previous: Club Instructors are:
Craig Kulesa
Fri Dec 4 18:51:34 MST 1998