I know it's been pretty much settled, but here's the list of mhz frequencies that aircraft/ground units use:
27mhz: Ground, shared with toys R us cars, 6 USA, more in other countries, shared with CB radios
40mhz: Aircraft, shared with toys R us cars (I think)
72mhz: Aircraft
75mhz: Ground, approx 30, maybe more, maybe less, I forget.
Each of the above have different frequencies in them. I know there's 6 on the USA 27mhz, and 30 (or so) on the 75mhz. I'm sure someone with aircraft knowledge can chime in on the others.
Couple that with AM, FM and PCM (which, oddly enough, some people think that AM, FM and PCM are seperate "bands". They are not. Same frequency, same channel, just different wavelengths. They can NOT share channels. AM 77 and FM 77 at the same time will result in interferrence on both.
Then you have the spectrum units, which are 2.4ghz (I believe). They are literally in their own little world.
Now, as for servos from Aircraft in cars... You can, but you might find that aircraft servos generally tend to be higher speed/lower torque. You would honestly be better off with a 'standard' servo.