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Choosing Your Propeller
by FH Huber
How do you know which propeller to use
when? Here's some tips. 
First is matching the propeller to the engine. Each engine has
a range of propellers appropriate for it. Some the range is
narrow due to specialized purpose of the engine, but most have
a fairly wide selection of props.
A .40 Nelson racing engine. you basically use one specific size
propeller to get the maximum speed from the airplane. This propeller
would be inappropriate for any common .40 to .46 powered sport-aerobatic
airplane.
A typical .40 engine such as the O.S. LA.40 can take from a
8X8 to a 12X3. You note that the pitch decreased and the diameter
increased. that keeps the propeller load relatively constant.
The 8X8 is something you would never use as a beginner. It is
for SPEED. The high pitch and short diameter
prop would try to pull the airplane very fast. This would sacrifice
"static thrust" (the pull when the plane is stationary
on the runway) for thrust at high airspeed. Takeoffs would be
very difficult if you tried to use this on your trainer. The
plane would have a hard time slowing down to land when the engine
was idling too.
The most commonly used prop for a .40 is a 10X6. This is a compromise
propeller. Having decent static thrust, and reasonable speed
capability. At 10,000 rpm it wants to pull the plane close to
80 mph. The drag of the plane resists that, and the trainer
typically cruises closer to 50 mph. Actually, this propeller
is more appropriate for use with the .40 on a plane such as
a Sig Kougar... a relatively speedy airplane. when the engine
is running at maximum rpm on the trainer, you are "spinning
your wheels"... the plane just has too much drag to go
anywhere near as fast as the propeller is trying to pull and
if it did go that fast, you would easily rip the wings off doing
a loop!
The 11X4 or 12X3 are propellers which give great static thrust,
at the cost of top speed of the airplane. On a trainer this
is NOT a sacrifice! The better static thrust will improve take-off
performance. The top speed of the plane will actually increase
because of the the propeller working more efficiently. Aerobatics
become easier, and its easier to slow the airplane down for
a smooth landing. But put one of these propeller on the Sig
Kougar and you will lose a significant amount of top airspeed
vs the 10X6. (you'd still gain some improvement in some aerobatics
maneuvers)
The basics outlined here can be applied to any propeller range
appropriate for any given engine. If the plane is designed to
fly slowly, use a long, low pitch propeller to see the best
possible performance. If the plane is designed for speed, use
a shorter, higher pitch propeller, but realize you are sacrificing
some low speed performance (and will lose some of the pull needed
to get through those long vertical legs of the square loop.)
Match the prop to the engine AND the airplane.
(and you will find that the designer was correct about the engine
to put on the airplane.) |
Developed by Geistware of Indiana© ., 1999.
Updated March 6, 2004
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