>>1855423
<convertible plane-helicopter things
are called Ospreys. (Generically they are tilt rotors.) The Marines use them. They are very unusual aircraft with a sound that is similar to a helicopter only much, much louder and heavier.
One time I was in a rural place far from civilization when a pair of Ospreys flew over. It was on the very same day that the Marines had a just announced that all Ospreys were grounded worldwide while a mechanical problem was investigated. Hmmm. When you witness something that's not supposed to be there, your mind starts to consider various things… Never forgot that sound.
"The Bell Boeing V-22 Osprey is an American multi-mission, tiltrotor military aircraft with both vertical takeoff and landing (VTOL), and short takeoff and landing (STOL) capabilities. It is designed to combine the functionality of a conventional helicopter with the long-range, high-speed cruise performance of a turboprop aircraft."