From: earlb@ssd.intel.com (Earl Brabandt)
Newsgroups: rec.aviation.piloting
Subject: Re: climbing high with a fixed pitch prop
Date: 8 Oct 92 16:26:34 GMT
Article-I.D.: SSD.1992Oct8.162634.22414
Organization: Supercomputer Systems Division, Intel Corp.

In article <8Z4-r5A@engin.umich.edu> dlevy@viper.engin.umich.edu (David W Levy) writes:
>
>The basic equation for rate of climb, ROC, at constant true airspeed is:
>
>                      Pavail - Preq
>           ROC  =    ---------------
>                            W
>
>     Where: Pavail = Power Available = np * SHP
>            np     = Propeller efficiency
>            SHP    = Shaft Horsepower
>
>            Preq   = Power Required = Drag*True Airspeed
>            W      = Gross Weight
>
>So ROC is a maximum for a given weight when Pavail - Preq is a maximum.
>The main difference between the climb, cruise, and constant speed props
>is how efficiency varies with speed.  

The other important factor is the SHP vs. RPM curve for the engine.  Full
throttle (maximum manifold pressure) horsepower is a function of RPM 
which in turn is a function of engine load, which, in the case of a fixed 
pitch prop, is a function of airspeed.  Remember, a constant speed prop 
can keep an engine operating at its peak horsepower RPM regardless of the 
aircraft airspeed.  Take a look at the horespower curves for most aircraft 
engines and you'll see that they don't make anywhere near maximum rated 
power at the RPM typically found under static or takeoff roll conditions.
I'll check some SHP and prop efficiency curves but I think this is 
at least as great a factor during takeoff and climbout as prop efficiency 
in most cases.  

Yet another illustration would be a fixed climb prop pitched to keep keep 
an engine operating at peak power during climbout in order to maximize 
climb performance.  In fact, some of the "climb" props are pretty efficient 
in cruise and they cruise pretty fast except that you have to overspeed 
the engine past redline to do it!  The engine manifold pressure may 
still be low enough to be within "cruise" horsepower settings, but 
the engine speed is too high.  In order to keep the engine below redline,
you have to retard the throttle, reducing the manifold pressure, and 
limiting yourself to something less than a normal 75% (or 65% or whatever 
you use) cruise.  Many RV builders set up there homebuilts like this 
to get good takeoff roll and climb, but they accept cruise engine speeds
in the range of 2800 RPM.  The overspeed from the 2700 RPM redline of 
the Lyc O-320s and O-360s doesn't appear to be a problem.  The engine 
and prop may not be quite as efficient in this kind "overspeed" operation so
fuel consumption is expected to be somewhat higher than with a constant
speed setup flying at the same airspeed and producing the same horsepower.  

In reality, it is difficult to even measure such differences in fuel 
consumption in side-by-side comparison flights between otherwise virtually
identical airplanes.  Things like mixture leaning technique are probably 
more important.  This fact is a good indicator that "climb" props can
be quite efficient in cruise and even approach the efficiencies of a 
constant speed prop, but the cruise comes with a higher engine RPM.  
Two excellent articles were published in Kitplanes magazine on the
subject over the last year or two.  They contained real-life test flight
data, efficiencies, and horsepower calculations for several props popular
on RV homebuilts.  They are worthwhile reading for anyone interested.

Earl Brabandt, CFI, RV-6 builder
Xref: sparky rec.aviation.piloting:72 rec.aviation.ifr:41