Scott,
The P51 is an actual recording. I used an interior vantage point of the 51 and then created a loop based on the external sounds of a 51 flying overhead. I had to get creative and try to reverse the doppler effect that the external sound had. Then I mixed the 2 sounds. They aren't exactly the same sound that you would hear from flying on, but I believe that it adds a bit of what everyone "knows" audibly of a plane in flight. Plus it helps when the sounds are being used externally in the game. ie if you're in a vehicle, tower, hanger ect....
AH only has one sound source for both internal/external sounds - so I found a happy medium which seems to work well.