From the newsletter of the Tampa Bay Line Flyers, Control Line Model
Airplane Club
Build for Better Performance
by Phil Bayly
Concept: We all know that a
lighter-weight airplane is easier for the motor to
pull through the air and will perform better,
especially with a stunt ship—right? “Lighter” also
means the airplane has a more favorable wing loading
(weight vs. wing area) and stunt maneuvers are done
more easily. The airspeed doesn’t sag off during
maneuvers, and this preserves the energy needed to
continue the flight smoothly without stalling. We
also know that we need to build in enough wood to
give the strength needed to withstand the forces of
flight, landings, and engine power, including
vibration. So, here comes the weight penalty.
Therefore, the real question is, how can we get the
best of both worlds? Obviously a light weight and
strong airplane is the ideal solution. But, reality
says we probably need to find a compromise between
the two.
With this accepted, the intent of
this article is to outline some of the tricks of the
trade that should help you lighten up your airplane
without losing strength and achieve better
performance. In fact, the first principle to
understand is that a lighter airplane has less
inertia. Therefore, less force is available to drive
an airplane to its destruction as easily as a
heavier one under similar conditions, e.g. crashes,
air loads, etc. The guiding theme then says that
what is really needed is just the right amount and
kind of wood in the right places, and no more. This
will give the optimum between the airplanes weight
and its required strength. That’s it! Now, let’s
examine some of the important details of
construction principles, techniques, and wood
selection that let us do this—the key to it all.
Bending Moments and Force Distribution: From
physics we find that something breaks when enough
force is applied to distort it beyond its elastic
limits. When this happens, one side gives in
compression and/or the other gives in tension. When
less force is applied, we only get minor bending or
distortion with a return to original form as the
force is reduced. We should visualize this principle
of breakage each time we select the wood (type,
size, and density) for every part of the airplane,
joint locations, and reinforcements. Try to imagine
what forces each part will actually experience and
choose the wood type, density, and size accordingly
without any excess anywhere. You should use as
little (light) as possible, but as much as necessary
in every location throughout the airplane. This
assessment includes the wood’s size, density, grain,
location, etc. in conjunction with the stress
expected. Most important, realize that extra weight
is simply unnecessary cargo that actually increases
the inertia and force that is extended to the weaker
places that break under stress.
The wing: So, where and how can we save this
extra weight? Logically, you must attack the
heaviest parts first to make the most difference
with less effect elsewhere. So, let’s start with the
wing since it is normally the heaviest part of the
airplane. In practice, diminishing the weight
towards the wing tips with proper limits will make
it stronger. Why stronger? Because the weight toward
the tips is the major leveraging force that finally
causes the wing to break at the usual spot, the
intersection of the fuselage or edge of the wing
capping, whichever is weaker. On nose impacts,
especially with profiles, you typically find the
wing’s trailing edge tears loose at the body as the
leading edge compresses, or the wing buckles up or
down from the vertical force during flight maneuvers
or when bellied in to the ground. With this
understood, you can and should taper spars, trailing
edges, leading edges, and capping to effectively
reduce the overall weight progressively towards the
wing tips without sacrificing the wing’s strength.
Other parts of the wing, including the tips, should
be made of very light weight density wood. But think
a minute. The outboard wing tip is usually weighted
for flight stability. Therefore, heavier and
stronger wood is always better than lead for tip
weight, except for the need for a small amount of
adjustable flight trim. Since the outboard wing
needs to be heavier, it accordingly needs a little
more strength throughout the outboard wing (higher
density in the main spar is probably enough, so
select the heavier one for the outboard).
Joints are the next consideration. Always be
careful how joints are designed and where they are
placed. Butt joints are the worst for strength!
Diagonally cut, well matched, and glued joints are
the best, especially with the reinforcement since
the stress is distributed over a large area.
Matching a diagonal joint is an easy fit if you
overlap the two pieces of wood and cut the diagonal
with a razor saw without letting them move.
Overlapping spars vs. diagonal
matching and reinforcement is a great technique for
strength and weight reduction since reinforcement is
unnecessary, but difficult to achieve except with
Free Flight wings. Since all joints become stiff and
strong when reinforced, the wing spar’s bending and
breakage usually begins at its edge or thereafter.
If not, you should reexamine your methods of
jointing, including the type glue you use. Clamping
joints while the glue dries is always best and can
double its otherwise holding strength. Clothes pins
work well too.
The wing’s spars’ distribution of
force, beyond the stiff center area, should be
diminishing toward the tip to optimize its overall
strength. This means you don’t want the forces to be
able to over-concentrate at one spot causing the
compression-tension relationship and breakage to
happen as discussed earlier. You also want to trade
off to have more wood (density and size) toward the
fuselage at the tip. Smoothly distributed
(non-visible) bending absorbs the force by spreading
the load throughout instead of applying most of it
at one place. Therefore, tapered spars,
reinforcements, gussets, and anything else that
helps the forces to be distributed smoothly
throughout the spar is what we are looking for as we
progressively have more wood approaching the
fuselage where it is needed to help counteract the
increasing leverage (breaking) force. This happens
because most of the forces will now be concentrated
there (as balsa spar enters a rigid reinforcement)
when leveraged from the tip or from the wing during
its high levels of flight loading (such as 90° or
120° turns).
Additionally, wood in the center
of a spar or a wing does less for its strength (and
stiffness) than the same amount at the surface.
Therefore, for the maximum strength for its weight,
intelligently laminated spars and V- or U-shaped and
tapered reinforcements add the (least) wood at the
right points where there is little compression and
tension and the most wood near the surface where the
stress is greater. You may recognize this as an
“I-beam” concept for the spar with its veneer
capping on a wing. Light weight sheet balsa on the
surface adds much greater strength (and prevents
distortion) than the same wood will do near the
center of the wing. Its curvature to the airfoil
also improves its rigidity. The ideal structure for
weight vs. strength is tubular for stress to be
applied from any direction; whereas, an I-beam wins
for vertical stresses alone. Again, because this
puts most of the mass of the material at the point
of compression and tension where breakage begins or
is countered for flight stresses. Additionally, you
are always tasked to consider where some wood’s
weight would be better removed for use somewhere
else or not at all.
Finally, you should inspect all
spars and stringers for minor nicks. Forces can
concentrate here too and cause easy breakage under
stress. You are much better served to sand out all
of the nicks to help the distortion under stress to
be uniform instead of concentrated at a flawed
point. Don’t leave it “rough cut” or as is. Strange
enough, sanding the spars is more for strength than
saving weight, unless you significantly change the
dimension of the wood.
The fuselage: A proper combination of woods,
good design, and craftsmanship is essential here.
The engine must be mounted on hardwood beams with a
plywood firewall and gear mount. The sides must be
hard and strong balsa reinforced internally to
solidly support the power, vibration, and G loads of
the motor while the sides continue to support the
tail section’s air loads. The top and bottom blocks
are the final elements that require good wood
selection for lightness and strength, whereas a
removable cowling contributes no structural strength
and can be ultra light. In flight, leverage stresses
are amplified at the wing’s leading and trailing
edges and are enormous for stunt airplanes with long
moments. Ultimately, cracking occurring at these
high stress points is normal, even through the top
and bottom blocks.
Don’t discontinue internal beefing there unless you
expect a short life airplane. Strange as it seems,
thin plywood will provide the required beef-up
strength at less weight than more volume of balsa,
since it does not tear or compress easily, e.g. 1/64
inch. All of the same rules apply. Internally trim
away all of the wood that does not contribute to the
strength of the airplane while filling (non load
bearing) holes such as cowlings with light wood. The
tail portion of the fuselage may progressively get
lighter (thinner) as your proceed rearward from the
stabilizer’s leading edge, but leave enough to
support the tail wheel stresses. They are high
stress during a hard landing, so a ply mount is best
here.
The Stabilizer, Elevator, and Rudder: The
previously described considerations for the wing’s
construction and stresses apply equally to the
entire tail section except that the shorter linear
dimensions do not have as much leverage to cause
breakage. Therefore, lighter materials and designed
construction should be used accordingly. Equally
important, the tail section is critically important
to the airplane’s horizontal (nose to tail) center
of gravity and must be kept as light as possible to
prevent addition of nose weight for balance and
performance degradation. Most tail sections are
overbuilt (with heavier and too much wood) well
beyond what is needed. The stabilizer and elevator
intersecting spars must endure the continuing air
loads and control system forces and care must be
taken to select strong wood for them. Proper wood
selection is even more difficult for solid wood
stabilizer-elevator construction to achieve light
weight and the required strength. After that, you
may go very light, including the entire rudder and
fin. Examination of many crashed airplanes seldom
finds damage in the tail section! So, judge
accordingly.
Wood Selection: Good wood selection is also
an art and a science. The serious modeler will never
rush down to the hobby shop to buy all the wood he
needs to build the airplane he is ready to build.
It’s too late. The right selection of wood will
likely not be there. The right approach is to always
look over the wood every time you go to the hobby
shop and buy the good stuff when you find it! This
way, you will have it available when you are ready
to build. Your inventory of wood on hand is a quick
measure of how light you will be able to build your
airplanes. Kits are typically terrible for wood
selection (and fit). Therefore, don’t hesitate to
replace the heavy parts accordingly. In fact, it is
best to look the wood over before buying any kit to
be sure you are getting what you expect. Otherwise,
you may have only bought a set of plans. Your first
indication of the weight your airplane will be is
the “as is” weight of the kit in the box, right off
the shelf. Too heavy will always be too heavy unless
you plan to change out the kit’s bad wood.
Wood grains or “cuts” is an
article of its own, therefore, it won’t be covered
further here except to say that all woods of the
same weight are not equal for all applications. The
is A, B, and C grain with correct and incorrect use
for each that goes well beyond its weight
considerations alone, e.g. do not use C grain for
spars or linear strength. Its strength is
undirectional and doesn’t like to bend. For
additional information, SIG provides an excellent
information brochure on balsa grains and correct
uses. Also, remember the earlier comments suggesting
you visualize the stress each part will experience
as you select its type, size, density, and grain of
the wood for them.
Covering and finish: The covering and finish
are great contributors to an airplane’s weight and
strength. The primary job of the finish is to
provide the protection needed to prevent weakening
from fuel penetration. To most, it significantly
adds to the overall strength of the airplane,
especially since they are at the surface where the
maximum (tension and compression) stresses occur. If
you are planning to go light on the covering and
finish, additional strength will be required in the
wood construction to survive. And, if you experience
a tear in the wing’s covering near the fuselage,
without repair you may easily buckle the wing during
a subsequent flight. A complete article on good
covering and finishing techniques is in order for
this complex subject. Maybe next time.
Conclusions: No airplane is crash proof.
Still, the better airplanes incorporate the building
techniques discussed herein so they will last
longer, fly, and look better. If you still crash a
lot from inexperience, this article can improve your
survival rate and guide you toward building a better
flying airplane. But just as important, examine
every crash (not just your own) for the evidence of
what broke and use your new knowledge to improve the
weak spot(s) on the next airplane you build. Our
progress only comes from doing it better the next
time. Q
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