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. ne 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, making it 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.

The XV-15s were a standard demonstration in the annual summer airshow at the co-located Moffett Field Naval Air Station for several years during the 1980s. Both XV-15s were flown actively throughout the 1980s testing aerodynamics and tiltrotor applications for civilian and military aircraft types that might follow, including the V-22 and AW609 program.he first XV-15 prototype aircraft, N702NA, was transferred back to Bell for company development and demonstration use. On 20 August 1992, the aircraft crashed while being flown by a guest test pilot. He was lifting off for a final hover when a bolt slipped out of the collective control system on one pylon, causing that rotor to go to full pitch. The aircraft rolled upside down out of control and crashed inverted. While significantly damaged, the aircraft was largely structurally intact and both the pilot and copilot escaped with only minor injuries from the crash. The cockpit of the aircraft was salvaged and converted for use as a flight simulator.

The second XV-15 prototype, N703NA, was used for tests to support the V-22 Osprey military tiltrotor program and Bell/Agusta BA609 civilian medium tiltrotor transport aircraft. It continued in primarily NASA test operations until September 2003. The shortest takeoff distance was achieved with the nacelles at 75 degrees angle.[8]

After N703NA was retired from test operations, it was donated to the Smithsonian National Air and Space Museum in Washington, D.C. The XV-15 was flown cross-country from Fort Worth, Texas to the museum before being decommissioned for display. It is now on display at the Steven F. Udvar-Hazy Center at Washington Dulles International Airport.