forecast for Bulgaria says hot hot hot hot   ... so I decided to make cooling for my AXI ... after first flights success :- )))) ... I hope my AXI will survive those conditions :- ))

all done from piece of alu "L" profile and mounted between firewall and motor stator ... "quick" solution but I like it anyway

You are a genius Igor.

Good idea for conductive cooling from the stator.  I wouldn't use it instead of air through the motor, though -- only as a supplement.

If you have some white silicone heat sink goo, be sure to use it.

Yesterday we had very hot day here, 35 degrees C and motor had aproximately same temperature than days before at 27 degrees without cooler, so for me sucessful fix :- ))

I use 6s battery and charge back ~2000mAh after 5 min. flight so motor has feeding 24A x 22.2V = 530W ... that is more than healthy for such small AXI :-))) 

Igor,

Maybe you can anodize the heat sink black to make it even more efficient!! 

Any PHDs in the crowd.  My question is: does the motor have to dissipate 530Ws of heat?  I thought the efficiency number determined the heating.  A 90% efficient motor would dissipate 53W of heat at 530W input.  Is this true or not?

True.  You don't need a PhD for that -- you just need to know that energy in equals energy out plus energy stored, and that for the purposes of this discussion a motor doesn't store any energy.

Of course 53W is still a bunch.

Yup.

BTW do not believe that such motor in PWM mode can run at 90% efficiency, I think also 80 is to optimistic. Especially with active regulation.

I'm not sure that an outrunner can achieve 90% efficiency at all, much less at useful power levels with a PWM drive.  But I'd be happy to be wrong.

I lamented on this forum at one point that none of the magazines that I take currently publish motor efficiency test results -- someone came back and named a magazine that does.  The name totally escapes me, but you could look.

IMO much less efficient.
A thermal engine (converting heat to work) is bound by Carnotīs thermodynamics law. It is in the order of 40% at the most. Diesels are more efficient on account of higher compression ratio.
Electric motors are converting work to work, i.e. in a sense the stored energy in the battery is "condensed" work.
Luiz

I think that since you're moving through the air anyway, that properly ducted ram air would be better than fans.