Bell XV-15

The Bell XV-15 is an American tiltrotor VTOL aircraft. It was the second successful experimental tiltrotor aircraft and the first to demonstrate the concept's high speed performance relative to conventional helicopters.

The idea of building VTOL aircraft using helicopter-like rotors at the wingtips originated in the 1930s. The first design resembling modern tiltrotors was patented by George Lehberger in May 1930, but he did not develop the concept further. In World War II, a German prototype called the Focke-Achgelis Fa 269 was developed starting in 1942, but it never flew.

Two prototypes that made it to flight were the one-seat Transcendental Model 1-G and two-seat Transcendental Model 2, both powered by single reciprocating engines. Development started on the Model 1-G in 1947, and it flew in 1954. The Model 1-G flew until a crash in Chesapeake Bay on 20 July 1955, destroying the prototype aircraft but not seriously injuring the pilot. The Transcendental 1-G was the first tiltrotor aircraft to have flown, and it accomplished most of a helicopter-to-aircraft transition in flight to within ten degrees of true horizontal aircraft flight. The Model 2 was developed and flew shortly afterward, but it did not fly much beyond hover tests. The United States Air Force withdrew funding in favor of the Bell XV-3.

The Bell XV-3 was first flown in 1955. Like its predecessors, the XV-3 had the engines in the fuselage and driveshafts transferring power out to tilting wingtip rotor assemblies.

Other rotor concepts, such as slowed rotors, stopped rotors, folding rotors, and variable-diameter rotors, were investigated for subsequent designs, but Bell engineers Kenneth Wernicke and Bob Lichten deemed them unfeasible compared to experience with the XV-3 project.

One of the major problems with the early tiltrotor aircraft designs was that the driveshafts carrying power from the fuselage out to the wingtip rotors, along with the gearbox and tilting mechanisms at the wingtips, had substantial loads placed upon them and were heavy. They were transferring large amounts of power and torque long distances for an aircraft power transmission system.

The XV-15 experimental aircraft introduced a major design concept advance: instead of engines in the fuselage, the XV-15 moved the engines out to the rotating wingtip pods, directly coupled to the rotors. The normal path for power was directly from the engine into a speed-reduction gearbox and into the rotor/propeller without any long shafts involved. There was still a driveshaft along the wings for emergency use to transfer power to the opposite rotor in case of engine failure, but that shaft did not normally carry any power loads, allowing it to be lighter.

The tilting engine concept introduced complexities in the design of the engines and engine pods to be able to shift from operating horizontally to operating vertically. Those problems were addressed fairly early in the XV-15 program.