I have no experience with 'lectric stunt (it doesn't smell right, and it's too quiet!).

From electronics design I know that a little bit of airflow goes a long way -- heat doesn't go away by itself, it either needs to radiate, or it needs to be carried away.  In a world full of air, the usual best way is air flow.

From working with servomechanisms and electric drive systems I know that two motors with the same Kv rating will pretty much act the same EXCEPT that a motor that's proportionally bigger in every way will have a lower winding resistance and so generate less heat internally, it'll have more thermal mass and so get less hot for the same heat, and it'll have more surface area and so be able to shed heat into whatever air is around it (assuming the air is of lower temperature than the motor).  So going to a bigger motor fixes a whole lot of problems -- but not necessarily for the right reasons, if you're already nose-heavy.

I would love to see someone do some experiments with airflow vs motor heat, with various combinations of airflow from fans and ram airflow.  But it's a heck of a lot of work for no material gain, and there seems to be a lot of successful electric stunters out there.

how hot is really hot and dangerous after landing

I can speak for AXI, what I know from maker and experience (it depends on components)

60 degrees celsia already makes life time of bearing shorter, but it is value for R/C flyers, if you fly C/L stunt and especially with 2 blade prop, it will kill bearing anyway, because of other reasons, especially if orriginal grease is dissolved by additional liquid oiling

80 degrees is still acceptable for the motor, but that is temperature of STATOR (stator makes the heat) so it must be measured on stator side, not the rotor, rotor contains magnets which will fail earlier then winding in stator, so rotor must be cooler

I remember 100 degrees C in really hot air in WCh Bulgaria. I also remember burned AXI of my friend in the same day, same prop and the same setup, so that is probably really highes acceptable extreme tempearture. Magnets will fail little over 100deg C, so if the stator has 100, it will soak to magnets, damage them and it will burn in next flight.

Tim, Igor,
The experience has taught me that when I start digging deep enough the "things" usually complicate instead of getting simpler but it is ok with me.

Igor,
 
You wrote:"So also Cobra pumps air". How? If I take my Cobra 2826/12 and run it without propeller at safe static RPM., say 5,000... Will this motor pump the air? Will I be able to feel the directional air flow through the motor along the shaft axis? Please note directional. The rotating parts will disturb the air inside the motor and there will be some air movement around the motor due to thermal convection but I doubt this movement will be directional along the shaft axis. I can lit a cigarette (I do not smoke but will this time), blow the smoke into the plastic bag, let it cool to ambient temperature to avoid the additional convection and release it slowly around the running motor to see what is happening.

Copper losses v/s iron losses and efficiency? I have to read more about it as this is extremely interesting.

I have received the photo of your heat sink for AXI in the rear mount configuration from Keith R. and immediately asked myself the question: will this heat sink work efficiently when the motor is front mounted? The answer is rather not.

Because I am going to front mount all the motors in my future planes, I am looking for the best cooling solution for this type of mount.

Tim,
You wrote: "I would love to see someone do some experiments with airflow vs motor heat, with various combinations of airflow from fans and ram airflow.  But it's a heck of a lot of work for no material gain, and there seems to be a lot of successful electric stunters out there."

If I manage to do this what I am planning to do, I will publish on this forum the results of my tests. I will do it only if the results are numerical and will make all the details available for the C/L community. Peter German (SUI) does always the same and I truly appreciate his "community spirit".

"Learning never stops and when it stops this means you are dead".

Going flying with my eParrot now. By the way: I made a little inlet for the air to cool the ESC and the ESC heat sink temperature immediately after 5 min. pattern flight in 20 deg. C ambient temperature dropped from 45 to 29 deg.C.

Regards,
Matt


 

Copper losses v/s iron losses and efficiency? I have to read more about it as this is extremely interesting.

Really short answers:

When current flows through a wire, it creates a voltage drop, and that voltage drop times the current is a power loss.  The lost power gets burned up as heat.  It's proportional to current squared times resistance, so you'll see it called "I²R" losses.  Wires are copper, so it's often called "copper loss" by motor and transformer people.

When a piece of steel is immersed in a magnetic field (like, the stator sections in a brushless motor), it takes energy to magnetize the steel.  Then when the magnetic field reverses, it takes energy to magnetize the steel in the other direction.  The energy just gets dissipated as heat, and is lost.  These losses are called "iron losses" by motor and transformer people.

The efficiency of a motor is the ratio of the power you get out of it vs. the power you put in -- and the power you get out is the power in minus any losses.  There are some significant tradeoffs involved in designing a motor that's small, light, affordable, durable, and efficient.  Usually something has to go, and as Igor mentioned, we tend to sacrifice some efficiency for a lighter motor.