HighPlains is right on the money. Prop pitch is comparable to the gear ratio in your car. You adjust the gearing in your car to get the desired RPM at a specific speed. When you need plenty of RPM at zero or very low speed (to get the car moving) you use low gear. At slow speed or needing to pull a moderately steep hill, use second gear. At cruising speed, use high gear for best performance on flat ground. Same engine, same car, just adjusting for a different condition. Fixed pitch prop selection is similar except that you make do with a single blade angle and take advantage of the fluid coupling to make up for lack of pitch change capability. You can select an optimum prop pitch for any speed within the operating envelope of your airplane. As an example, a prop that gives lots of static thrust is excellent for takeoff acceleration and for hovering, but won't be worth a darn in high speed flight.

In an airplane that is optimized for cruise, prop pitch and diameter are set in the following fashion:

Step 1. Determine the cruising speed and cruising RPM of your airplane based on the airframe's horsepower required and the engine power available. From the speed and RPM, determine the propellor pitch.

Step 2. Select a propellor diameter which loads the engine to the target RPM at the target Airspeed.


As you can see from this approach, it doesn't make any difference in pitch whether you are flying a 19 sized airplane or a 120 size, providing the airspeed and RPM are the same. You just have a bigger diameter prop on the 120 engine.



Hal Debolt had an interesting rule of thumb for prop pitch which works surprisingly well. This isn't strictly scientific, but it gets you close:

Prop pitch (in inches) equals Airspeed (in MPH) divided by RPM (in thousands).


Dick