Helicopter rotor noise question
I was standing outside the office the other day (open countryside) and a small helicopter flew across my field of vision about a mile away. I could mostly hear the engine noise (piston engine) and just a faint rotor beat. As I watched it, for about two seconds the sound changed to a heavy rotor beat, and then went back to normal. The aircraft didn't seem to make any change in course, attitude or speed corresponding with the change in noise.
I was just curious to see if anybody has an idea what would cause the change in noise?
Thanks very much.