Hi Dan,

The RPM (for a given prop) describes the load you are placing on the motor, power as measured by a wattmeter (amps x volts) is the input power, so it does not describe anything about efficiency.

Example

Motor A - input power 2500W, 21x14 prop turning 6000 RPM

Motor B - input power 2200W, 21x14 prop turning 6000 RPM

In this case, motor A appears more impressive due to the higher input watts, however knowing the load shows us that its actually less efficient at producing the same output power as motor B.

Knowing the RPM is vital to knowing how one motor/esc/battery setup compares to another, inputs watts only tell 1/2 the story. At the end of the day its power to the prop that we really care about!

To pick a prop is a little trickier on electric I think. Essentially an electric motor is trying to turn at a constant RPM regardless of load (kV x input voltage), so changing the prop results in usually small changes in RPM. The kV is measured under no load situation, in real life at load you will never achieve that RPM due to losses. My experience with the Pletty is the RPM range is about 500, from 6000 on the heaviest loads, to 6500 on the lightest. What changes are the amps. Knowing this you need to find a balance between pitch and diameter, that gives you the performance you need. You might change pitch of a prop and notice very little RPM change, but the input amps go way up, input power goes up and performance (usually) goes up.

Mostly in pattern the props are 20-21" diameter and 14-15" pitch. For the 10s setups we use those are the sizes that generally give the best all around performance. There are props that are outside that range if you are looking for even more speed, or more pull but they are not as commonly used.