The International Cessna^® 170 Association

All other things being equal, air speed is directly proportional to horsepower. Horsepower is determined by three things, displacement, BMEP, and RPM, only.(BMEP=Brake Mean Effective Pressure, The average working pressure on the piston head during the power stroke as measured at the propeller shaft.)
Normal power charts are based on standard day conditions. BMEP will be primarily controlled on a fixed pitch installation by manifold pressure which in turn will be dependent on ambient atmospheric pressure. With a WOT at any given pressure, horsepower will be directionally proportional to RPM. Pitch variation will therefore allow horsepower to vary with RPM. A thin prop will therefore produce more power at WOT than will a thick one as long as ambient pressure remains constant.
However, at less than WOT, constant RPM will require modulation of BMEP by throttle adjustment therefore altering horsepower. A fat pitch will require more BMEP to maintain the desired RPM. The increasesd BMEP at a constant RPM increases the power. Increased power uses more fuel and results in more speed.
A thin pitch would require less BMEP and less power to maintain an eqvivalent constant RPM and would therefore burn less fuel at a lower speed.
Maintaining a constant air speed with various different prop pitches would require different RPMs and BMEPs for each prop but fuel burn should be very near the same, the differences being friction losses due to the different RPMs.

blueldr wrote:All other things being equal, air speed is directly proportional to horsepower....

Oh if only that were literally true! "Directly proportional" would mean that if I want to go twice as fast, all I would need to do is install twice the horsepower. Actually power required goes up as the cube of the speed, ie. doubling the speed requres 8 times the horsepower, IF you can do it without increasing weight. What you probably meant to say, Dick, is that a given airframe will travel at a given speed when pulled along at a given horsepower, regardless of how that horsepower gets generated.

Climb is a little different story. A little extra power goes a fairly long way. Every extra horsepower coming OUT of the prop will increase a 2200 lb airplane's climb rate by 15 feet per minute (assuming no change in speed).

Miles

Miles,
You're absolutely correct. I was not taking into consideration large speed changes. Only the very small changes in the normal cruise range. My use of the term "Directionally Proportional" was used to mean that an increase in power resulted in an increase in speed rather than being "Inversely Proportional" which would result in a loss of speed.

This is a picture of My 170A with a Kenmore STC's McCauley 8042 prop installed with the original gear legs and 8:00 X 6 tires. It works fine with 6:00 X 6 tires. I can get 2650 rpm on take off. This picture was taken at West Fork Montana just before I left the state west bound. In level flight I have to watch the RPM that I do no go over Red Line RPM. I can take off with full fuel from West Fork and full bagage.Dimensions: 2600 x 65 ft. / 792 x 20 m
FAA Identifier: 4U7
Lat/Long: 45-51-38.1500N / 114-13-10.9610W
45-51.635833N / 114-13.182683W
45.8605972 / -114.2197114
(estimated)
Elevation: 4247 ft. / 1294 m (estimated)

Thanks Metal Master... looks good!