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XV-3 / XH-33 / Bell Model 200

Bell produced a series of vertical takeoff and landing (VTOL) aircraft, building two using jet lift and one with tilting ducted fans, the X-22. However, it was the Model 200 (XV-3) of 1955 which was to prove of greatest future significance because it pioneered the tilt-rotor concept that was to be followed by the XV-15 of 1977 and the V-22 Osprey which flew in 1989.

Following World War II the Transcendental Company, a small American firm, built their Model 1-G tilt rotor which flew 100 flights for a total of 23 hours. The Model 1-G was very small (1,750 pounds), was never fully converted to forward flight, and crashed in 1954 to end the project. It proved the concept, but it did not end debate over which vertical takeoff and landing configuration minimized the weight penalty-that is, minimized the need for more powerful engines and stronger shafting that made propulsive lift aircraft heavier than regular airplanes.

The Bell XV-3 began under a joint Army-Air Force program in 1951. Bell had started working on tilt rotors in 1944, and accelerated their research by hiring Robert Lichten, an engineer for Transcendental. For the next two decades, Lichten would be the dominant player in American tilt rotor development. The XV-3 that Lichten and Bell designed for the U.S. Army was a small aircraft, only 5,000 pounds gross weight. It used the 450 hp Pratt & Whitney R-985 radial engine mated to a two-speed manual gearbox. The single engine mounted in the center turned a complex gear box that powered large rotors at the tips of the wings.

The Bell XV-3 convertiplane was extensively tested over several years. The XV-3 first flew in 1955, and every flight was nerve racking. The cockpit vibrated up and down whenever it hovered. To compensate for an engine simply too underpowered, Bell built the airframe too light. It made its first flight as a helicopter in August 1955, but crashed two months later [25 October 1955] after experiencing severe in-flight rotor instability, but before completing a full conversion.

In a hover flight, in 1956, a rotor pylon coupling failed catastrophically and the pilot was severely injured. Bell strengthened the structure, thus restricting it to ground-tethered flights while they searched for solutions. Following this crash, Ames engineers entered the picture in 1957, and started with some tests in the 40 by 80 foot wind tunnel. An investigation was conducted in the Ames 40- by 80-foot tunnel to study the effectiveness of a number of modifications to correct the wing-pylon oscillation which was evident on the initial flights of the airplane. This investigation showed that the airplane could be flown through transition and gearshifted to low prop-rotor rotational speed in airplane flight without serious airplane or rotor stability problems. A limited flight evaluation was performed by the Air Force Flight Test Center. The flight evaluation explored the flight characteristics of the airplane from near hover to about 155 knots. Since the completion of the Air Force tests, the airplane was been flight-tested by the National Aeronautics and Space Administration at the Ames Research Center to explore further some of the problem areas noted in previous tests and to study general handling-qualities requirements for V/STOL aircraft. Much of the NASA flight testing of the XV-3 has centered around the cruise configuration of the airplane in order to study the effect of the large flapping rotors on the handling qualities at cruising speed and above.

Rotor instability concerns led to a change from 23 ft three-bladed full-articulated rotors to 24 ft two-bladed semi-rigid rotors. The XV-3 flew again in 1958, with NASA pilot Fred Drinkwater at the controls to define the conversion envelope between vertical and horizontal flight. The second XV-3 made its first flight on 12 December 1958, with a conversion only 6 days later. Full conversion from helicopter mode to conventional forward flight was flown in August 1959, and the entire XV-3 test program proved a major advance in understanding the transition from ground to air. Conversions over the full 90 could be conducted in 10 seconds. Inadequate power and high weight growth precluded the XV-3 from hovering out of ground effect.

The XV-3 made 110 full conversions and over 250 flights before it was damaged in a wind tunnel test in 1965 when a rotor housing separated from the aircraft. The XV-3 program ended in 1965 after a rotor pylon tore loose from the XV-3 while it was inside the 40 by 80 foot tunnel. For a few months, Ames and Bell engineers did a radical redesign of the remaining pylon to test ways to improve pylon stability-a major weak link in tilt rotor design. In 1966, Ames finally mothballed the XV-3.

The low-disk-loading flapping rotors of the XV-3 are of the semirigid teetering or seesaw type construction and are driven by a 450-horsepower reciprocating engine in the fuselage. A gear shift is incorporated which permits the operation of the prop-rotors at either of two prop-rotor rotational speeds while maintaining maximumengine rotational speed. Blade flapping is designed into the rotor system to relieve the unbalanced momentsacross the rotor disk and to provide a meansof controlling the aircraft longitudinally and directionally in hover and low-speed flight. The flapping rotor also provides damping about all axes at low airspeed and in hover. When the prop-rotor is converted into a propeller the control provisions of the rotor are washed out and the blade-flapping function is to relieve the blade stresses that occur once per revolution.

In evaluation flights of the airplane at high airspeeds pilots have reported a condition in which the airplane oscillated about all axes simultaneously. An analysis of the time histories taken during this maneuver has shown that it consisted of longitudinal and lateraldirectional oscillations that were very lightly damped. The longitudinal and lateral-directional oscillations are not directly coupled. They are at different frequencies and oscillations can be performed in either mode without exciting the other, but with such low damping it is easy to excite both modes at the same time. These damping ratios are much lower than are considered acceptable by any of the criteria for airplanes in cruise. The damping ratios are not only low but also change appreciably over a relatively small airspeed range, approaching zero at the higher speeds.

While the XV-3 convertiplane was the first VTOL aircraft to be plagued with these blade-flapping problems, they have been encountered in the past on a prototype STOL fighter equipped with flapping propellers mounted on the wing tips. These problems are largely attributable to the compromise required to maintain good hovering efficiency as well as high propeller efficiency in cruise which dictates the use of flapping proprotors at high blade angles in cruising and high-speed flight regardless of the disk loading. Solutions to these problems can be approached in several ways on future VTOL airplanes. Higher dynamic stability can be provided in the design of the airplane itself.

Using lessons learned from the XV-3, NASA's Ames Research Center, Moffett Field, Calif., in partnership with the U.S. Army, developed design specifications for a new aircraft to demonstrate the viability of the tilt rotor concept. After extensive ground, wind tunnel and simulator tests at Ames, the first of two XV-15s, built by Bell Helicopter Textron, took its maiden flight on May 3, 1977.



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