RC BASICS: What is KV? 

My uncle is machine engineer

Kv is the reciprocal of the back-emf constant. A 500 Kv motor will generate a 1 V back-emf when the motor is rotating at 500 RPM. At 3700 RPM that motor will generate 7.4 V. Applying it the other way around ("7.4V will make the motor rotate at 3700 RPM") is only an approximation and technically wrong. This video is actually full of misinformation because RCModelReviews relies on the latter understanding of Kv which is wrong. The motor RPM is NOT calculated by V*Kv. The actual RPM = (V - R*Io) * Kv = (Voltage - Motor Resistance * No Load Current) * Kv. So given the above motor with R=0.5, Io=1, we get 3450 RPM at 7.4V. Also, Kv tells you nothing about how powerful or efficient the motor is. That's another point where RCModelReviews is wrong. High Kv does not mean the motor has low power. You can get high Kv motors with higher power than low Kv motors....

In the movie Boondock Saints a huge man-hating feminist lesbian pointed out that saying meant back in the medieval times or earlier a man was legally able to beat his wife with a stick no bigger than his thumb. One of the Irish twin brothers who are the lead characters who was training the huge woman with on-the-job training in some meat warehouse then jokingly replies( knowing the woman hates men by the "Never touched by man" tattoo under her chin) , "Rule of thumb? Well what's that gonna do? Perhaps it shouldve been rule of wrist?". Which of course led to a fight where the boys convinced the woman to keep her comments to herself with a right hook to her chin. Damn good movie.

+DrBleck35 They should have used "KRV" ... then again, as you say you used to work with high voltage stuff, I'm guessing it was something to do with power distribution? Then you'd probably confuse "KRV" with "kvar" XD

Nope about power distribution. Capillary Electrophoresis. Also, "KRV" would always remind of the time I met Stevie Ray Vaughn's illegitimate (and fictitious) brother Kevin.

I think then the motor would not be working at it's optimum. If you mean a smaller motor with a smaller propeller, I think that would not give you enough thrust. If you mean a smaller motor with big propeller and running the motor at slower speed , then the motor would not take it, as smaller motors has less momentum force. Torque. When you choose motor and propeller for your plane, you start with the propeller. Choose a big enough propeller for your plane. Then choose a motor that would spin that prop at the speed required. And a motor with enough torque. It usually doesn't say how much torque a motor has, but you can sort of guess when you consider the KV rating and how many Watts the motor uses and the lenght vs diameter of the motor, like Bruce said. A small diameter 250Watt motor with 2800KV = less torque. A larger diameter 250Watt motor with 1000KV = more torque.

pauljs75 pretty late but KV is a rating without any resistance, so it will not vary with motor load

Out/In runner only means which part is moving, the outer shell or the inner shaft.

If you can get it set up to be sensored, you can make it rotate as slow as you want. Without a sensor it will very dependant on the size of the prop, the amount of voltage being sent to the motor, the amount of poles in the motor, the size of the can on a road going RC motor.

That's because they aren't AC motors and calling them that is wrong. They are still voltage controlled. The difference to a standard DC motor is that the commutation (switching polarity of the coils) is done externally via electronics instead of brushes. The ESC doesn't create the frequency, the motor has to follow. Only two of the three coils in a set is energized at any given time. The third coil is used by the ESC to sense when the motor has turned enough (angle is depending on the number of coil sets) to switch to the next two coils. How fast the motor is moving there is only dependent on the voltage just like in any DC motor.