Bell XV-15 "Tiltrotor"
Bell XV-15 Tiltrotor at the National Air and Space Museum Steven F. Udvar-Hazy Center, Chantilly, Virginia (Photo by Jim Hough)
The United States Bell XV-15 was the second successful experimental Tiltrotor VTOL aircraft and the first to demonstrate the concept's high speed performance relative to conventional helicopters.
Design and Development
Pre-XV-15 VTOL Rotor Aircraft
The idea of building Vertical Take-Off and Landing aircraft using helicopter-like rotors at the wing tips originated in the 1930s. The first design resembling modern tilt rotors was patented by George Lehberger in May 1930, but he did not further develop the concept. In World War II, a German prototype, called the Focke-Achgelis FA-269 was developed starting in 1942, but never flew.
Two prototypes which 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, though it did not fly until 1954. The Model 1-G flew for about a year until a crash in Chesapeake Bay on July 20, 1955, destroying the prototype aircraft but not seriously injuring the pilot. The Model 2 was developed and flew shortly afterwards, but the US Air Force withdrew funding in favor of the Bell XV-3 and it did not fly much beyond hover tests. The Transcendental 1-G is the first Tiltrotor aircraft to have flown and accomplished most of a helicopter to aircraft transition in flight (to within 10 degrees of true horizontal aircraft flight).
The Bell XV-3 was first flown in 1955. Like its predecessors, the XV-3 had the engines in the fuselage and drive shafts transferring power out to tilting wingtip rotor assemblies. It was fitted with ejection seats; they were never needed but would have fired downward.
The Tilt-Engine-Pod Concept
One of the major problems with the early tiltrotor aircraft designs was that the drive shafts carrying power from the fuselage out to the wingtip rotors, and the gearbox and tilting mechanisms at the wing tips, 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, so it was lighter.
The tilting engine concept introduced complexities in the design of the engine pods themselves, and the design of the engines to be able to shift from operating horizontally to operating vertically. Those problems were addressed fairly early in the XV-15 program, but would come back to haunt the successor V-22 Osprey aircraft.
In the late 1960s and early 1970s, NASA and other researchers worked extensively on theoretical and wind tunnel tests of various rotor pods. Two companies were involved in the research and proposing designs: Bell Helicopter and Boeing-Vertol. The focus was on tilt rotor pods and integration of the tilting rotors with the wings and fuselage of the aircraft, and studying the airflow as the rotors tilted. Tilt rotors with fixed rotors and with folding rotors were investigated.
The XV-15 Project History
What was to become the XV-15 program was launched in 1971 at NASA Ames Research Center. After preliminary work, a competition was held to award two $0.5 million research and development contracts for prototype designs. Companies that responded included Sikorsky Aircraft Corporation, Grumman Aircraft, Boeing-Vertol, and Bell Helicopter. R&D contracts were issued to Bell Helicopter and Boeing-Vertol on October 20, 1972. The two companies' design proposals were delivered on January 22, 1973.
Boeing proposed a design, Model 222 (not to be confused with the later Bell 222 conventional helicopter) in which the engines were in fixed pods at the end of each wing, and a small, rotating pod with the rotor was slightly closer to the fuselage on the wing. This design simplified the engine design by keeping it horizontal at all times, without having very long drive shafts to the tilting rotors.
The Bell design, Bell Model 301, in which the whole wingtip pod rotated between horizontal and vertical, with the engine and rotor assembly fixed together within the pod. This simplified the power transmission but had more complicated requirements for the engine design, and was probably slightly heavier than the Boeing proposal.
Bell Proposal Wins
After a review of both proposals, NASA selected the Bell 301 for further development, and a contract for further R&D was issued on July 31, 1973.
XV-15 Development and Flight
Extensive engineering and testing took the next four years to complete the development of the aircraft. The first of two Bell XV-15's, tail number N702NA, first flew on May 3, 1977. After minimal flight tests at the Bell test facility, the aircraft was moved to NASA Ames Research Center in Mountain View, California where it was then mounted in the large Ames wind tunnel and tested extensively in various simulated flight environments. The aircraft was then moved to NASA Dryden, which is at Edwards AFB in the California High Desert. The XV-15 flight testing continued expanding its flight envelope at that point. It was able to successfully operate in both helicopter and normal aircraft flight modes and smoothly transition between the two. Once the aircraft was considered sufficiently tested, it was returned to Ames for further testing.
The XV-15's were sufficiently tested by 1981 that one of the aircraft was taken to that year's Paris Air Show for demonstration flights. The XV-15's were a standard demonstration in the annual summer airshow at the co-located Moffett Field Naval Air Station for a number of years in the 1980s. Both XV-15's were flown actively throughout the 1980s testing aerodynamics and Tiltrotor applications for civilian and military aircraft types that might follow, including the V-22 program.
The first XV-15 prototype aircraft, N702NA, was transferred back to Bell for company development and demonstration use. On Aug 20, 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 continued in primarily NASA test operations until September 2003. N703NA was used for tests to support the V-22 Osprey military Tiltrotor program and Bell/Agusta BA609 civilian medium Tiltrotor transport aircraft. After N703NA was retired from test operations in September 2003, it was donated to the Smithsonian National Air and Space Museum in Washington, DC. N703NA was flown cross-country from Fort Worth, Texas to the NASM before being decommissioned for display. It is now on display at the Steven F. Udvar-Hazy Center at Washington Dulles International Airport.