Motor efficiency, anyone?

[billpa]

We've gotten a few requests to calculate/approximate "Motor Efficiency", which is presumably POut/Pin. I believe we can add this, but I have heard a few definitions for this term and the best way to calculate it. Is this parameter (or parameters) something people are interested in, and if so, what's your definition of the term?

I suspect we'd need to do some model-specific approximations as part of this.

Regards,

Bill, for Eagle Tree

[Cobra]

It's rather hard to measure power out without some specialized equipment. We've had some discussion about this for helicopters, but no easy ways to measure the power produced.

You can do comparison measurements for various power systems, given you can match input voltage (for example) and mechanical setup for output loading. But that isn't efficiency. It can give an idea of how efficient two systems are relative to each other.

[Socomon]

It sure would be nice to have a way to compare the relative efficiency of different motors. I am not technical enough to have an opinion as to how this should be accomplished. I sent Dr. Kiwi a link to this thread. I hope he might chime in here with the best way to accomplish this. He seems to know a lot about motor physics / engineering.

Steve

[pmackenzie]

Motor efficiency is by definition 100*Pout/Pin.
Anything else like thrust/power is a "figure of merit". Useful perhaps, but not efficiency.
The only way to measure efficiency is to measure rpm and torque.
The Hyperion Emeter (http://aircraft-world.com/prod_datasheets/hp/emeter/emeter.htm) uses the characteristics of "known" props to estimate power output, but it is only an estimate.
Unless you are doing it the "right" way ( torque and rpm) it is not worth the bother IMO. Just muddying the waters. Stick with the facts.

Pat Mackenzie

[Dr Kiwi]

Pat is of course quite correct. I find that a useful way of comparing motors is to use "thrust" relative to "power in" - as measured in g/w - but of course, to be truly useful there needs to be a third component ("pitch speed") in the equation too. If one is comparing two motors, at the same voltage, one can use the same prop(s) and thus ignore either thrust or pitch speed, depending one which is most useful for one's application.... then g/w or rpm/w are perhaps useful measures to determine what one is getting out in relation to what one is putting in (a very crude measure of, in a sense, "efficiency").

[Socomon]

Thank you Dr, Kiwi for weighing in. I hope Bill can make heads or tails of that - it is waaaaaay over my head.

Steve