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Date: Wed, 22 Dec 2004 09:09:32 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: Re: [LML] airspeed vs. power
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<<In a message dated 12/21/2004 10:30:38 P.M. Central Standard Time,
glcasey@adelphia.net writes:
1.  Using the MAP as-is the airspeed should have changed 1.7%
2.  Using MAP - 4 inches the airspeed should have changed 2.2%
3.  Using fuel flow directly the airspeed should have change 1.9%

Gary,

Where might I find more about these calculations.  I'd like to compute a %
power chart even though 100% in HP is not known.

Scott Krueger AKA Grayhawk
N92EX IO320 Aurora, IL (KARR)>>

There's not much to find.  Using either fuel flow or manifold pressure as a
surrogate for engine power is an approximation.  A problem with either
method is that the engine requires significant power just to operate itself
("friction" horsepower), and what you are really looking for is "brake"
horsepower or the power left over after the engine takes its share of the
"indicated", or total power.  From the fuel flow number, assuming a constant
air/fuel ratio, or manifold pressure you need to subtract the fuel required
for engine friction.  For manifold pressure I somewhat arbitrarily -
SWAGGED - subtracted 4 inches from the manifold pressure in line 2 above.  I
don't know of any published friction horsepower number for these engines,
but from comparisons I have made with automotive engine friction numbers I
can say that they are quite efficient.  The question is what to subtract
from the MAP or fuel flow number to use for this correction?  Since the
engine is "efficient" and the BSFC is relatively flat when running LOP there
have been people that say just to multiply fuel flow times a constant to get
horsepower.  That is probably accurate enough for our purposes.  Maybe
someone has some accurate friction horsepower numbers for these engines.

Gary Casey