XV-1 (XL-25 / XH-35)
The McDonnell XV-1 was a tip-jet autogyro that exceeded contemporary rotor-wing speed records by hitting 200 mph on 0 October 1955. With conventional helicopters improving their cruise speeds, the program was canceled in 1957. During its development, the XV-1 was a classified program and the lack of details in the open literature reflects this status. Two aircraft were built and flown and now they reside-intact-in museums; one at Fort Rucker and one in the Air and Space Museum storage.
The basic airframe came from an early post-World War II commercial airplane program for a four-place airplane in the "Bonanza" and "Navion" class. The pitch/cone rotor was invented by Kurt Hohenemser in Germany during WW II. The tip jet propulsion system was invented by Fred Doblhoff in Austria during WW II. The XV-1 was powered by a single 550 hp Continental R-975-19 nine-cylinder radial piston engine that drove a pair of air compressors to power the 31 ft three-bladed rotor for vertical lift, and powered a 6 ft diameter two-bladed propeller mounted at the rear of the fuselage for forward flight. A small rotor at the end of each tail boom provided yaw control. First tether test was in 1954, with the first free flight on 11 February of that year. First transition to horizontal flight was on 29 April 1954. The second of the two aircraft was damaged in autorotation testing in December 1954.
The XV-1 had 3 operating modes. In the first mode, the helicopter mode, the aircraft flew on the pressure jet tip drive units and the propeller was declutched and stationary. The design rotor speed of the 31 foot diameter rotor was nominally 410 rpm in the helicopter mode and was controlled by the pilot. An autogyro mode was adopted that captured the transition between the helicopter mode and the "airplane" mode. In the autogyro mode, the propeller was clutched in, the 3 tip drive units were turned off and the rotor autorotated at a nominal 325 rpm with collective pitch set to 6 degrees. The pilot controlled rotor rpm in the autogyro mode. In the airplane mode (110 to 125 knots and higher), the rotor rpm was reduced to a nominal 180 rpm and collective pitch was further reduced to 0 degrees. In the airplane mode, rotor rpm was controlled through longitudinal hub plane angle of attack, which was controlled by a flyball governor. The full scale wind tunnel test investigated only the autogyro and airplane modes. Flight testing, of course, included all three modes.
The XV-1's stiff inplane, bearingless and damperless rotor system changed its configuration when transitioning from helicopter/autogyro to airplane flight (A-81, 82,125, 127, and 138 to 150). Each blade was attached to the hub by 2 flex strap bundles, which gave an equivalent flapping hinge offset of 0.062R. A large diameter torque tube controlled feathering and provided the air passage to the blade. This torque tube was centered between the fore and aft flex strap bundles. The hub itself was gimbaled to the rotor mast.11 The swashplate was mounted to a large diameter tube (called a "stem") and this "stem" was tilted for cyclic input. Blade cyclic feathering was introduced by directly controlling the swashplate plane relative to the aircraft, much as Cierva/Pitcairn/Kellett did on their direct control autogyros. In the helicopter and autogyro modes, the gimbal was free. The blades then had pitch-cone coupling of 65.5 degrees (i.e., 2.2o pitch down for 1o cone up) and pitch- flap coupling of 15 degrees. In airplane mode, the hub and gimbal were both locked to the "stem" and the collective pitch was reduced to zero degrees. In this locked mode for airplane flight, the pitch-cone and pitch-flap coupling both became 65.5 degrees. Thus, in the airplane mode, the rotor system became a stiff inplane, bearingless and damperless main rotor system.
While the XV-1 achieved a speed of 200 mph in 1955, it became apparent that the compound helicopter, in high speed flight, would still experience the type of severe oscillatory load conditions that limits the speed capability of the conventional helicopter. These vibrations are due to the edgewise movement of the rotor through the air during cruise flight. Helicopter rotors operating in the cruise mode are burdened with the tasks of producing the required thrust and lift while delivering the forces and moments to maintain a balanced, or trimmed, flight state. Because of the essentially edgewise motion of the rotor, the blades experience an aerodynamic acceleration and deceleration as they "advance" into and "retreat" from the airstream.
Although the compound helicopter uses a conventional fixed-wing to produce the required lift while in the cruise flight mode, thereby unloading the rotor from the burden of producing lift and trim moments, it still encounters the variations in rotor blade drag due to the advancing and retreating airloads during each rotation. In addition, the edgewise rotor limits maneuver capability at high speeds because of the extreme load oscillations that occur on the rotor. Also, the exposed rotor hub and control hardware contribute significantly to drag in the high speed cruise mode, further limiting maximum airspeed.
The compound helicopter also suffered the weight penalty of carrying the additional cruise mode propulsion system hardware. Collectively, these issues inhibit the performance potential of the compound helicopter. The compound helicopter was not the answer to the search for a viable low disc loading VTOL high performance aircraft.
A rotor system from one XV-1 was used on a mini-crane, the McDonnell Model 120 (A-176). Pitch-roll coupling was corrected and problems with the pressure jet tip drive system were corrected on this small aircraft. Then, with interest growing for a military heavy lift helicopter, a 75 foot diameter rotor with pressure jet system was built and whirl tested. Ultimately, military interest dissolved, little additional progress was made.
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