How to build a brushless motor test bench?

[nativepaul]

A prop differs in load according to how fast it is spun. If one motor pulls 50A on a 10x6 prop and another pulls 60A on the same 10x6 prop, it tells us nothing without a tacho telling us how fast it is spinning, the 60A one could be amazingly efficient but spinning it 3000rpm faster than the 50A one, or it could be the worst motor ever and spewing 50A out as heat and spinning it 5000rpm slower.

As well as knowing what RPM they are achieving the motors will have to be of a very similar KV to draw any physical conclusions, although assumptions could be calculated despite some differences, you should also take into account where the maximum efficiency of the motor lies, ((an extreme example) you could test a Lehner 3080 and a Feigao 540xl and find they spin the same prop at 50A at the same speed and assume they are equals, but the Feiago is operating at its peak efficiency current, and the Lehner is 200A short of its and running a lot less efficiently than it can do.)