As you suspect, it does need to cool as heat will build.  From another thread where I had corresponded with Lucien who helped develop the Cobra from Pulso,

"Lucien at Innov8tive responded to my temperature question - "When I do the testing on the motor, I do not like to see the core of the motor exceed 160˚ F.  That is the point where I call the “Max Current Point” on the Cobra motors during my prop tests.  The “Can Temperature” will typically be about 30˚ F cooler than the core temperature."  and "The magnets used in the motors are rated for 150˚ C (302˚ F) but you really never want to get them that hot.  Likewise, the insulation on the winding wire is rated for the same temperature, but you can get localized heating on the inside strands of the wire, closest to the stator core, that can be significantly hotter than the core itself."  "If a motor is getting that hot, then you either do not have enough cooling, or you are simply pushing the motor too hard for the application."


The time to cool will depend on air temperature and movement as you are sitting trying to cool.  Leaving the battery cover off can help, and some have developed a small battery fan to speed up cooling, but you are stuck waiting.  Then use an IR thermometer and shoot the motor. I try to shoot the nonmoving portion if able as it is the hottest. You can fairly quickly develop an estimate of how long to wait considering ambient temperature and the measurements.  Hope that helps.

Interesting thought Tim.  You could be correct and the motor could be at and stay at that temperature after say 4 minutes having equilibrated, then it wouldn't matter unless close to too hot.  Then time at temperature might be cumulative, having the rapid second flight at top temperature the whole flight, not the last 1.5 minutes.  I used to IR my cowled plane and the outer was about 120, and the Green/inner at about 160F.  I was fortunate not to have back to back on a hot day.

I have a fan on my Cobra 3520/10 and I have run back to back to back flights to deliberately test that theory that it would get too hot. Now I may be the exception here and I have no numbers to back myself up with, but my motor temperature has pretty much run not much higher than the outside air temperature. This is with a finger test only but I can assure you that it was barely warm and I could have held my finger on it indefinitely. It was no hotter after the third flight than after the first one. I have also tested this set up with both a turbo cool spinner and a standard spinner and was unable to feel any difference in temperature. Your temperatures may vary based on the the amount of cooling available, the weight of your plane, prop used and other factors.

Can you fly right away after changing the battery? How long do you let your motor cool? I never worried about it on a profile but now my motor's cowled in it runs kinda hot like can't touch it hot.


MM

I have flown multiple back to back flights with just a fan (and I am not sure that that is even required but I use one because it adds very little weight for peace of mind) and neither the motor or battery ever exceeded 115F including in 107F ambient air temp. You do not need to "Cool" a motor. You just need to take the heated air away. The motor will attain what ever temp it is going to reach and there is little you can do about it with "Cooling". I.E. You will not be able to reduce the temp if at all but you may be able to keep it from rising.

The battery on the other hand MUST be allowed to cool BEFORE recharging.

If your motor is getting really hot you have other problems. (prop pitch, dia, rpm, bearings, rubbing, airflow)

Kim.

In 2009 I had the same question so I put the temperature probe from my Eagle Tree data recorder into the windings with some thermal grease. You can see how the temperature goes up directly after the motor stops because there is no more airflow. Of course the right place for the probe would be into the metal core then you would not see that temperature climb after the motor stops. This is why Igor's heat-sink mount is such a good idea, because it removes heat from the metal core. It also makes for a really good motor mount.

Keith R