A Cat Chasing his Tail (Wermacht not withstanding)
A power plant has a snowball effect in both directions. Using a
powerful engine requires a more robust structure to handle it. Now we
not only have to beef up the structure to handle the engine but we have to
beef up the structure to handle the beefing.
All of the sudden the landing gear is heavier, the landing gear mounting
system is heavier, the wing spars are heavier, the wing mounting system is
heavier, the fuselage needs to be more robust, the tail surfaces need to be
stronger, heavier servos are needed, a bigger onboard battery is needed,
etc. The list goes on.
If we go the opposite route and use a lighter, less powerful engine, then we
can lighten all the above. This is why some guys can build monster
airplanes that fly on a .40 and other guys build the same size airplane and
stuff a chainsaw engine in it. I'd rather fly the plane with the .40
A lighter aircraft
has less mass which means there is less inertia which means
the structure is more likely to survive impacts. The adage,
"Build to fly, not to crash" is absolutely true. Learn to live by
A lighter airplane can maintain flight at lower airspeeds, will climb
better, will have better stall characteristics and will perform better
A light airplane will begin and end maneuvers much more crisply
than a heavy airplane. Swing a baseball bat and try to stop it.
Now strap a couple bricks on it and do the same thing. Which way was
it easier to start and stop the swing? Physics is physics —
airplanes behave in exactly the same way.
The "Right" Material in Every Circumstance
Many designers get stuck using the same material in an application
regardless of the model size, intended power plant or flight envelope.
For example, a designer may use 1/8" light ply fuselage sides for any
model from .20 to .90. If the sides can handle a .90, then they are
certainly overkill for a .20 size model.
Why is it that 1/4 x 20 nylon bolts are used to retain a wing on a .40
size model if they are also used to hold the wing on a 1.20 size model?
If a standard servo can handle a .40 size model, then why would you use it
in a .15 model? Take the time to think things like this through.
Check the specs of equipment and use nothing larger and heavier than
necessary. Don't install heavy duty
unless they're needed. So called "standard servos" are the most
useless servos made. They were good when they first came out, but they
have become obsolete.
There are now mini servos that have more power,
more speed and sometimes include a ball-bearing. They're also half the
size and weight of a standard servo. If a standard servo will work,
then so will a Hitec HS-85 and there are plenty of others.
Don't use a larger onboard
pack than necessary. A small 3-channel model can get several
flights from a 250 - 350 mAh pack. Having a field charger will allow
you to top off the battery between flights.
Landing gear can add a lot of weight in a hurry. Landing gear
weights vary greatly. You don't have to buy expensive carbon fiber gear, but
if you do you'll save more weight. Practice learning how to fly
your airplane to a landing and you can get away with a lighter landing
A way to save a lot of weight
inexpensively is carefully selecting wheels for the model. Take a
postage scale to the hobby shop to weigh different brands and styles.
spinners are pretty, but heavy. If you need the nose weight to
balance the plane then fine, but if it's nose heavy, don't put lead in the
tail before you switch to a plastic spinner. Aluminum
mounts are also heavy. For most engines, a fiberglass-filled nylon
mount is strong enough and much lighter than aluminum.
Always choose the lightest radio gear, hardware,
systems and other installed equipment that will do the job and be