Bell, having considerable experience in rotary-wing and tilt-rotor aircraft, was selected in 1973 as a prime contractor for a research program to prove the operational practicality of the tilt-rotor concept. The program was launched by the US Army's Air Mobility Research and Development Laboratory and NASA. Drawing on their experience with the Bell Model 200, the Model 301 was developed. In May 1973, the U.S. Army and NASA requested an MDS designation for the forthcoming Bell Model 301 tilt-rotor demonstrator aircraft. On 30 May 1973, the designation XV-14 was allocated. However, on 7 August 1973, a change of this designation to XV-15 was requested to avoid confusion with the Bell X-14B VTOL aircraft. The V-13 designation was not used for superstitious reasons.
The XV-15, a NASA-inspired, Bell Helicopter-produced experimental tiltrotor aircraft, is the precursor to the V-22 Osprey. The success of the XV-15's program and Boeing advances in composite structures and rotors, electronic flight controls and aerodynamics demonstrated the maturity of the technology and provided the necessary confidence that led to the most extensive tiltrotor program supported by the United States government to date, the V-22.
The development of the XV-15 Tiltrotor research aircraft was initiated in 1973 with joint Army/NASA funding as a "proof of concept", or "technology demonstrator" program, with two aircraft being built by Bell Helicopter Textron (BHT) in 1977. Ship number 1 was given NASA number 702, and ship #2 was 703. Aircraft development, airworthiness testing, and the basic "proof of concept" testing were completed in September 1979. NASA Ames Research Center, where most of the NASA research is conducted, continues to be in charge of the joint NASA/Army/Bell program.
The aircraft are powered by twin Lycoming T-53 turboshaft engines that are connected by a cross-shaft and drive three-bladed, 25 ft diameter metal rotors (the size extensively tested in a wind tunnel). The engines and main transmissions are located in wingtip nacelles to minimize the operational loads on the cross-shaft system and, with the rotors, tilt as a single unit.
For takeoff, the proprotors and their engines are used in the straight-up position where the thrust is directed downward. The XV-15 then climbs vertically into the air like a helicopter. In this VTOL mode, the vehicle can lift off and hover for approximately one hour.
Once off the ground, the XV-15 has the ability to fly in one of two different modes. It can fly as a helicopter, in the partially converted airplane mode. The XV-15 can also then convert from the helicopter mode to the airplane mode. This is accomplished by continuous rotation of the proprotors from the helicopter rotor position to the conventional airplane propeller position. During the ten to fifteen second conversion period, the aircraft speed increases and lift is transferred from the rotors to the wing.
Operating as a conventional airplane, the XV-15 can cruise for more than two hours. To land, the proprotors are rotated up to the helicopter rotor position and flown as a helicopter to a vertical landing.
The tiltrotor concept has many advantages. The ease with which the aircraft can be converted from one flight mode to another enhances its maneuverability and permits the aircraft to be configured to meet mission requirements. Operating as a VTOL aircraft, it can take off like a helicopter and deliver payloads on half the amount of fuel consumes by a helicopter when traveling distances greater than 185 kilometers. Takeoff and landing terminals can be small, making tiltrotor aircraft ideal for intercity commuter travel. In the STOL mode, tiltrotor aircraft are ideal for long distance transport of heavy cargos into remote areas, where only short runways are available. The XV-15 has been the primary influence for Bell's V-22, the first production tilt rotor.
Full scale testing using the XV-15 research aircraft addressed significant elements of Tiltrotor hover performance, acoustic signature and ground wash effects (including isolated rotor performance, rotor/rotor interaction effects, rotor/wing/fuselage interaction effects, and the impact of advanced blade tip designs) to enable the development of devices that which improve Tiltrotor hover performance, reduce noise, and provide a tool for assessing Tiltrotor ground wash characteristics. To accomplish the research objectives, powered semi-span and full-span Tiltrotor models and advanced computational fluid dynamic CFD analyses were also used to develop an understanding of the hover flowfield, aerodynamic interactions, acoustics, advanced tip effects, and groundwash characteristics.
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