multi-path
Not an a/c man (did you guess ?) but is the prop metal ? Reason I ask is that if the spacing between the blades is right then it might be giving you multipath which would modulate the signal. Far enough from the tower and the fraction of the received signal intercepted by the blades would decrease so the effect would vanish. The rpm dependency would have to be explained by agc timeconstant or something. The directionality of a hand-held (unless you want to risk life and limb would prevent that happening to it.