The Sikorsky S-57 (military designation XV-2) was a planned experimental stoppable rotor aircraft, designated as a convertiplane, developed for a joint research program between the United States Air Force and the United States Army. It utilized a single-rotor design; a counterweight provided stability to the rotor system, while a tip-jet arrangement powered the rotor, which retracted into the upper fuselage when stopped, the XV-2 then flying like a conventional aircraft on delta wings. A single jet engine was provided for forward flight. The XV-2 prototype was assigned the serial number 54-4403, but the project was cancelled before construction could begin.
54-4403 