McDonnell XV-1 Convertiplane
Early helicopter engineers soon realized that retreating blade-stall would greatly limit the maximum forward speed of rotary-winged aircraft. If helicopters were to rival airplanes in utility, they would have to approach comparable airplane speeds. By the early 1950s the Army was greatly interested in increasing the mobility of its forces and Vertical Takeoff and Landing (VTOL) aircraft, which combined the best features of airplanes and helicopters, seemed to be the ideal solution.
In 1951 the Air Force, which was tasked with overseeing Army aircraft development, opened a competition for a convertiplane that could take off and land like a helicopter, yet operate at much higher speeds, utilizing airplane wings for lift. Three companies, Bell, McDonnell, and Sikorsky, responded with proposals, but Sikorsky was unable to follow through because of its commitment to Korean War helicopter production. The McDonnell and Bell models differed greatly in their approach to vertical flight. The Bell XV-3 proposal used the tilt-rotor concept consisting of two lateral rotors mounted on the wingtips. The rotors could rotate a full 90 degrees forward, and then act as propellers in cruise flight. They were driven through a complex transmission system by a single engine. The McDonnell XV-1 avoided the XV-3's complexities by using a single rotor for vertical lift, and a pusher propeller for thrust in forward flight.
Helicopters are limited in their maximum forward speed because of retreating blade stall, which occurs when the forward speed of the aircraft approaches the velocity of the blade moving towards the rear of the aircraft, resulting in a stalled condition and loss of lift on that rotor blade. The helicopter will then roll onto the side of the retreating blade, usually resulting in a catastrophic loss of control. The XV-1 avoided this danger by relying entirely on its fixed wings for lift at high speeds.
The challenge for the XV-1's engineering team was to enable a single power plant to power both the rotor and propeller without the use of two separate transmissions, with their added weight penalty. The solution was to use compressed air tip jets, which did not require a mechanical transmission or an anti-torque tail rotor. McDonnell benefited greatly from the expertise of Friedrich von Doblehoff, an Austrian who had flown the first successful tip-jet helicopter during World War Two. The XV-1 relied on an efficient method of tip propulsion. Two compressors geared to the engine forced high-pressure air through piping in the rotor blades to a combustion chamber on each of the three rotor tips, where a burner ignited fuel for increased thrust, which drove the rotors around and allowed the aircraft to fly like a conventional helicopter. The transition from rotary to fixed wing flight was made when the airspeed was higher than the fixed wing stalling speed. The pilot would then pull a lever in the cockpit, which released a brake on the pusher propeller causing the differential transmission to disengage the compressors, and provide power to the propeller. The cabin allowed for a pilot and co-pilot to sit in tandem or a pilot with three passengers.
It took twenty-two months to complete the XV-1. Project test pilot John R. Noll began hover tests on February 11, 1954. Noll was one of the most experienced helicopter pilots of the time, and had flown some of the first combat helicopter medical evacuation missions while under hostile fire during World War Two. During these tests, the aircraft was weighted down so that the aircraft could only marginally lift into ground effect. Free-flight tests were delayed until July 14, 1954 while problems with the tip-jet system were ironed out. In the meantime, McDonnell completed the second XV-1 prototype. The new aircraft incorporated several significant design changes. The rotor pylon was lowered to reduce interference drag and tiny tail rotors were added to the ends of the twin tail-booms to improve directional control while in helicopter mode. Noll had recommended that the spindly landing skid struts be replaced by beefier versions, but this was not done until one of the XV-1s suffered damage after one of the struts failed on landing.
Noll made the first transition from helicopter to airplane mode on April 29, 1955. The highlight of the XV-1's career came on October 10, 1956 when the second prototype became the first VTOL aircraft to exceed 200 miles per hour. By 1957, the changing priorities of the military resulted in a shift in funding and the cancellation of the convertiplane research programs. The Army retained the first XV-1 for its Army aviation museum and donated the record-setting second prototype to the Smithsonian Institution. McDonnell tried to make good use of the XV-1's tip-jet technology by employing it on the prototype Model 120 helicopter crane, but in spite of its potential, it failed to win any contracts.
The VTOL concept did not remain dead for long. Because convertiplanes could not operate as efficiently as either airplanes or helicopters could in their respective roles, they were clearly doomed as commercial aircraft. However, their speed advantage over helicopters, combined with the ability to operate from unprepared surfaces, encouraged the military services to pursue the technology. The convertiplane became less popular as thrust-vectoring systems, employing turbine engines without complex and heavy transmission systems, became practical. However, the convertiplane configuration is much more efficient than thrust-vectoring for heavier VTOL transport aircraft, and interest in the type reappeared when the helicopter's widespread use in the air assault role in Vietnam and Afghanistan revealed that its slow speed made the type vulnerable to antiaircraft fire and shoulder-launched missiles. The convertiplane finally began to appear as a practical production aircraft in the form of the Bell/Boeing V-22 Osprey, which was developed from the XV-3 tilt rotor and its successors. While the XV-1 was successful in demonstrating the potential of the convertiplane, its fixed rotor and dual power transmission systems were less efficient than the tilt rotor configuration, though the teething troubles of the V-22 illustrate the complexity of that approach.
Rotor Diameter:9.45m (31 ft)
Wingspan:7.92 m (26 ft)
Length:9.14 m (30 ft)
Height:3.28 m (10 ft 9 in)
Weight:Empty, 1940.0 kg (4277 lb)
Gross, 2497.1 kg (5505 lb)
Engine:Continental R-975-19, 550 hp
References and Further Reading
Francillon, René J. McDonnell Douglas Aircraft since 1920: Volume II. Annapolis, MD: Naval Institute Press, 1990.
Harding, Stephen. U.S. Army Aircraft Since 1947: An Illustrated Directory. Stillwater,
MN: Specialty Press, Inc., 1990.
XV-1 curatorial file, Aeronautics Division, National Air and Space Museum
R. D. Connor
