Hi Jim;

Good that you pointed out that a propeller operates with some of the same characteristics as a (fixed) wing. The lift/drag ratio of a prop at zero airspeed will be very poor, and even worse at negative airspeed, such as during a tail slide.

It's been mentioned twice in this interesting discussion that pitch is related to airspeed, and it was suggested that pitch should be selected to match the desired slippage. Actually, pitch should be selected to produce the least slippage at cruising speed.

This explains why in some cases an increase in pitch will produce more thrust, and in another aircraft less pitch will produce more thrust. One originally had too little pitch for its airspeed and the other not enough.

Slippage is defined as the difference between the forward distance actually traveled by the airplane and the theoretical forward distance that the prop would achieve if were screwing through jello instead of air. Remembering that the prop is an airscrew, one can calculate the correct pitch for a given airspeed using the simple formula; Pitch = Airspeed/rpm. Units must be matched, of course. If you measure pitch in inches, you will need to convert airspeed into inches per minute. For example, if your airplane flys at 50 mph, what would be the correct pitch? 50 * 5280 * 12 /60 = 52,800 inches per minute. My 12-inch prop turns at 6,500 rpm, so the correct pitch for 50 mph airspeed would be 52,800 / 6,500, or 8.123.

A prop with the pitch matched to airspeed will have noticeably more thrust than one that's trying to bite too much air (or too little) for a given airspeed.