Quote:
Originally Posted by Aeroplayin
JPF  Getting back to the efficiency discussion.... .

Aero,
thanks for the analysis. I think where we are differing in our approaches is that you are trying to make the data fit an assumed 'normal' kv x volts / RPM ratio of approx 0.82 (I don't like the term efficiency for this ratio) and then assuming the data must be incorrect if it doesn't fit. Also you assume that this RPM ratio is the same as the actual output/input efficiency of the motor when in fact it usually is not.
The relationship of actual RPM vs that calculated by kv x voltage is not a constant. It is a decreasing curve which starts at a ratio of 1.0 on no load and decreases in theory to 0 when the motor stalls (though it would burn out before you ever got there). Yes a good motor with a well selected prop may well end up with a ratio of around 0.8 but it's incorrect to assume all motors with any prop will be anywhere close to this figure, you could very easily be 10% out either way. Bear in mind that mechanical power is proportional to the cube of prop RPM so a 10% error in RPM produces a 33% error in power. Even as a rule of thumb these 'efficiency' assumptions are at best 'shaky'.
Attached is a plot from Motrolfly calc.
Note the following points:
 The curve for RPM/kv x voltage is shown initially in a pink dotted line tagged 'RPM/RPMO%' It starts off at 1 (100%) when the motor has no load it intersects the motor efficiency curve (the red line) at about 25A and descents to zero at about 45Amps (motor stalled).
 Over the motors realistic operating range 'RPM/RPMO' (the pink curve) is not the same curve as motor efficiency (the red curve). In the heart of a motors range the difference is very marked, about 10 percentage points at 10A in the attached example. Only when the motor is well past it's realistic operating range do the curves come together. The assumption that 'kv efficiency' is the same as actual motor mechanical output /electrical input efficiency is therefore fatally flawed.
Steve