"Peter Dohm" wrote in message
...
Some of us are mostly interested in airplanes that really need a redrive
to
get good propeller efficiency from a 40 HP VW. Others are interested in
slippery airplanes that cruise at 150 to 200 kts. My interest is in the
faster type of airplane, and the only reason the specification isn't for
something even faster is a desire to keep the simplicity of a fixed pitch
prop. Therefore, if I want to use the old formula of 0.2G static thrust
for
good takeoff performance on a 150 kt airplane, I only need to divide the
expected gross weight of the airplane by 10 to arrive at a reasonable
horsepower figure. (Since I want a static thrust of one fifth of the
gross
weight, and also since each horsepower results in 2 pounds of thrust at
the
150 kt speed--or would if efficiency was 100%) I really DON'T care about
efficiency, because I only intend to operate at low speed and high power
for
less than a minute per flight. Propeller efficiency will always be zero,
by
mathematical definition, at the beginning of the take off roll; and my
numbers work just fine with 40% efficiency during the initial climb to
clear
the obstacles. On the other hand, if your plan is to cruise at 60 kts,
with
a proportionately slower initial climb speed, then you probably need a
larger diameter prop than I do, even with a much lighter and less powerful
airplane.

We really need to look at what is workable, reliable, and affordable for
each specific application. I admit to being a long time advocate of
automotive conversions, and the various GM and D-C all aluminum 60 degree
V6s from 3.0 to 3.7 liters really do look promising; but I really would
have
to think long and hard before I trying to adapt one to an airplane that
has
already been designed around a standard airplane engine. Just making the
cooling system work reliably, with reasonable drag, would probably cause
insomnia!

Peter



I recommend Fred Weick's book on Propellor Design. I think you will find
that the thrust per horsepower is not a constant but rather decays
proportionately to the log of the RPM. The pounds of thrust per horsepower
gets pretty punk past about 2500 RPM of the prop. At 1000 RPM you get great
thrust out of 25 or 30 horsepower! At 2500 RPM you can get the same thrust
from 100 HP with a good prop! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )