We often see figures quoted for "power". Almost always these relate to the power input to the motor,  measured under static conditions.

This value is easily obtained. An instrument such as the Astro Whattmeter gives a direct reading. Otherwise, measure EMF and current, and apply equation 4 [below].

The widely-used Kopski power-to-weight recommendations [eg: "level flight; 30 watts per pound"] relate to motor input power.

More important than input power is output power, which is the power delivered to the propeller by the motor shaft. The classic way to evaluate output power is to measure shaft speed and torque, and to apply equation 1. Few flyers have the equipment to measure torque, although it is not difficult to make a torque balance.

There is an easy way to evaluate static output power---but only if you have a particular bit of information about the propeller in use, namely, the propeller power characteristic [B in the equations below]. From equation 2 you will see that all you need to do is to measure the propeller speed and make one quick calculation.

For my own use, I have evaluated B values for a number of propellers [shown below]. There is no way I could measure B values for all of the large and ever-growing range of available propellers. Ideally, the propeller manufacturers would supply this data.

Power values quoted for IC motors are invariably output power.

B values for some propellers

Gunther 125 x 110..................0.0165

Master 6 x 4.........................0.018

Grish 6 x 4..........................0.022

Robbe 9 x 5 folder.................0.100

Graupner 9 x 5 slim................0.120

Master 9 x 6........................0.140

Master 10 x 6.......................0.228

Master 12 x 8 folder...............0.960

Update.........Sept 06

Lots of prop characteristics are listed in....                                                                       www.aircraft-world.com/prod_datasheets/hp/emeter/hp-propconstants.htm

These are provided for the benefit of users of the Hyperion Emeter. But you don't need an Emeter to use them--a pocket calculator will do--if you understand elementary mathematics.