EVALUATE THE MODEL
;e first thing I do is figure out what kind of
model I have and what its mission is. Will it
fly fast or slow? Does it have a high or a low
wing loading? Is it a race plane, an aerobat,
or a trainer? You get the idea. Here’s an
example: A biplane with a moderate
wing loading will generate a lot of drag
and will generally fly more slowly than a
sleek, low-drag monoplane. ;e biplane is
happier with a large-diameter prop with a
low pitch, and the monoplane will perform
better with a smaller-diameter prop of
higher pitch. ;is is when I determine
whether the model follows a horizontal
or a vertical performance direction.
Biplanes generally excel in climb
rate because of the increased lift
produced by two sets of wings,
;rust and power-to-
weight ratio are two
DETERMINE ENGINE PERFORMANCE
Over the years, I have concluded that
engine-horsepower (hp) figures alone
are pretty much useless. You also have to
look at the rpm at which that horsepower
number is produced. Horsepower figures
should be expressed in terms such as
“2.5hp 7,500rpm.” But regardless of an
engine’s hp rating, it can do nothing for
us until we find a prop that e;ectively
converts engine power into the thrust
required to fly our model. ;is is where the
engine’s torque, or twisting force, comes
into play. ;e key is to find the correct
combination of hp and torque to turn a
prop of su;cient size to produce the thrust
required to overcome the model’s drag.
And it has to do this without overloading
and overheating the engine.
Most of the time, for general-use sport
models, maximum thrust will not be
produced with the prop that allows your
gasoline engine to turn the highest rpm.
;at sweet spot of rpm will be closer to its
maximum torque numbers. Don’t worry;
I am not going to get into the math here. I
just want you to have a clearer picture
of how to match your engine,
prop, and airframe.
Flown at Top Gun, this modified Top Flite
P-51D Mustang is piloted by Sean Curry,
who powered the warbird with a 50cc
gas engine. ;e 26-pound, 84-inch-
span warbird has excellent performance
with a 22 x 10 propeller.
Regardless of brand, always check for
proper balance and condition. Don’t try
to fix a slightly cracked propeller.
Dump it, and buy a new one!