tcarrico wrote:

I would think the Constant speed, or variable

pitch prop would have the

benefit of acting like several diferent fixed

pitch props.  You could

have a fixed pitch prop that performs well for

climb, but not for cruise

a Constant speed works better in both cases.  If

you took a constant speed

prop and set it up for best performance cruise, I

would think that the

fixed pitch prop optimized for cruise would give

basically the same

numbers... at cruise.

In the table Vance provided I would expect the

fixed pitch prop to be close

or better in one part of the envelope, but the

variable pitch prop to be

better overall.  Am I missing something?
tc

Vance Jaqua wrote:

The constant speed prop is better in thrust overall,
and my program did not penalize the constant speed
prop for twist areas so it matches the fixed pitch
prop at design point (top speed) top speed
optimization is a good match also for cruise -
especially high alt full
rpm cruise which is a valid cruise point. More thrust
is NOT automatically better efficiency, it takes more
than 30 percent more horsepower to get 30 percent more
thrust at climb - ie the efficiency is down -prop
efficiency  is not an easy concept - you cannot say a
given prop is 80 percent efficient, unless you say at
what rpm, speed, and power. it will have some other
efficiency everywhere else - including zero eff. at
static - Vance

Vance do we agree that HP can be converted to a thrust at any given speed?

Here is a little quiz. If we come up with the same answers we will agree
on the definition of prop efficiency.

What is the HP out of the prop for an airplane going a steady 200 feet 
per second with a drag of 200 pounds?

The engine in this airplane is generating 100 HP which we know from measuring
the torque into the prop and the engine RPM.

What is our prop efficiency under these circumstances?

BTW I am going to get around to doing a chart showing how the angle of
attack of a fixed pitch prop changes as RPM changes from 2100 RPM 
and zero MPH up to 180 MPH and 2700 RPM. With this chart one 
could look up the L/D and C sub L and C sub d as functions of the angle of 
attack. I'll do this any day now :-)

Paul Lamar
 
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