If you get a scale sound, make sure you get the correct sound for the airplane you are building. The reason is that airplanes sound different. For example if I was standing at an airport and a T-28, P-51, AT-6 and F4U took off and I could not see the airplane in question I could identify the airplane just by the sound. The AT-6 has very loud prop noise that is does not sound like a F4U or the T-28.

And then how to you prove the sound is correct?

Do you get a typical radial engine sound and under what power setting, cruise, full power for takeoff or idle.

another question, if the airplane is 200 feet up how big of speaker would you need to create the effect of the engine sound from that distance? it is easy to get a scale factor for speed, distance and areas (although scale speed is another can of worms), but I have never heard a good explanation on how scale sound. but most engineering explanations for sound are based upon distance squared.

Good luck