Selecting the correct size of propeller is quite a technical process but through past experiences you can obtain acceptable results by following simple guidelines.
Propellers are described by two basic parameters. these are Diameter and Pitch. Diameter is the size of the propeller from tip to tip and the pitch is a theoretical measurement of the distance the propeller would travel through the air against zero resistance in one full revolution.
The diagram above demonstrates how Fine or Coarse pitch affects the progression of the propeller through the air. As with all practical situations, there are compromises to be made in order to work within the practical operating parameters of the engine or motor you are using.
N.B. Excess prop size + pitch will overload the motor/engine and substantially reduce performance. It can also lead to premature failure of the power system.
So, propeller selection is a trade-off. You select based on the aircraft’s desired performance (after selecting the appropriate power system to make the propeller turn.)
For fast acceleration and steep climbs you want low pitch and larger diameter. This is similar to low gear for an automotive transmission. Lots of pulling power, but low maximum speed.
For speed you use high pitch and smaller diameter. Again compared to a car transmission, this would be high gear. Good while at speed but if you are trying to tow a trailer uphill you’ll be in trouble.
For sport flying we generally choose a medium pitch and diameter. For “.40″ size glow planes the 10 x 6 is a commonly used propeller (10 inch diameter and 6 inch pitch) although it is not always the best choice.
Let us take as an example our typical 40 size trainer using a common .46 glow engine (the same principles apply to electric motors). A better prop is the 11 x 4 or 11 x 5. At the risk of getting a little complicated, let us look at the calculations we need to do to appreciate the difference between using a 10 x 6 and an 11 x 5 propeller.
First of all we need to work out our “Pitch speed” = 1.1 to 1.5 times “full throttle level flight speed”
The formula for calculating Pitch Speed is:-
Rotational Speed in Revolutions Per Minute (RPM)/1048 x Propeller Pitch.
Let us use 1000 to make the calculation easier. ( It also accounts for the engine speeding up in flight or “unloading” vs ground tachometer readings)
So if the engine is spinning a 10 x 6 at 13,000 rpm, pitch speed is approx. 13,000/1000 x 6 = 78 mph.
Our 40 size trainer typically has a max speed of appx 55 mph. 78 mph pitch speed/55 mph level flight speed = 1.418… (This is pretty high in the useable range.)
But spinning a 11 x 5 at 13500 rpm (lower pitch means slightly higher engine revs) would give more static thrust (better acceleration and climb) and 13,500/1000 x 5 = 67.5 mph pitch speed. So 67.5 mph pitch speed/55 mph level flight speed = 1.227
For most purposes appropriate to the plane, the 11 x 5 will give our trainer better performance.
Lower pitch also means the plane will slow down more at low throttle settings. Many trainers have a reputation for “floating” past the runway when using a 10 x 6 propeller. Idle speed for most glow engines is high enough that the 10 x 6 can keep the plane flying at idle. the 11 x 5 will reduce the idle pitch speed to where the plane lands a little slower and more easily. (This is not so critical with electric motors as the propeller can be stopped completely during the landing phase if necessary).
I hope this wasn’t too complicated for you to understand. Don’t worry if it was, just accept the fact that either a 10 x 6 or 11 x 5 size propeller will fly your trainer.