Between Two Worlds

In Uncategorized on May 17, 2016 by hillermuseum

Developing Tilt-Rotor Aircraft

By Jon WelteMV-22 at Hiller

During the first half of the twentieth century aircraft development evolved into a tree with two branches. Fixed-wing aircraft were fast and efficient, carrying people and cargoes at speed or across long distances. Rotary-winged aircraft were more maneuverable and capable of vertical takeoff and landing, but at a cost of being more complex and having a slower maximum speed. For half a century, the choice of lifting device—a wing or a rotor—determined many of the characteristics of an aircraft. Entering the second half of the twentieth century, serious efforts were made to develop a hybrid aircraft featuring the best features of both.

Early on, the concept of tilting rotors in flight was explored as an avenue to hybrid flight. The first to fly in 1954 was the Transcendental 1-G. This small single-engine airplane foreshadowed later developments by featuring propeller pods at each wingtip that could be adjusted through 90 degrees of arc by small electric motors.

The Bell Aircraft Company soon produced a tilt-rotor of its own. Designated XV-3, this aircraft had the cockpit and landing gear of a helicopter but the fuselage and wings of an airplane. Like the 1-G it was powered by a single engine mounted in the fuselage, with tilting wingtip rotors linked to a single drive shaft. The XV-3 first flew in August 1955, but suffered its first of many crashes only a week into flight testing. Two XV-3s were built (and frequently rebuilt following mishaps). They provided Bell with a wealth of data regarding inherent instabilities in the tilt-rotor design.

The XV-3 was tested at the NASA Ames Research Center, only a few miles from the Menlo Park factory of the Hiller Aircraft Company. Founded by Stanley Hiller, Jr., the company often designed innovation solutions to vertical flight. In 1955 Hiller took on the challenge of a hybrid aircraft, but using a subtly different approach. Rather than tilting the rotors alone, the entire wing of the aircraft would rotate.

Designated X-18, the new Hiller aircraft first flew in 1959. Unlike the earlier tilt-rotors, the X-18 had two engines, with no provision to transfer power from one to another in the event of an engine failure. This arrangement virtually guaranteed a crash in the event of an engine failure. The large, tilting wing acted like a sail in low speed hovering flight, making landing difficult. However, the X-18 demonstrated high speed horizontal flight with vertical takeoff capability.

The follow-on Tri-Service Assault Transport program was launched in 1959, with Hiller Aircraft joining a team to build the XC-142. The XC-142 was larger than the X-18 and powered by four turboprop engines, each linked to all the others to ensure that, in the event of an engine failure, power would be supplied to all four rotors. Five XC-142s were built and flown, with maximum speeds of over 400 knots—far faster than any helicopter could fly. The XC-142 experienced many stability problems but nonetheless provided a dramatic example of what hybrid flight might achieve.

The XC-142 flew for the last time in 1970. In 1971 NASA took on research into hybrid aircraft in the form of the Bell XV-15. Managed through the Ames Research Center, this tilt-rotor first flew in 1977. The XV-15 was by far the most successful of the hybrid testbeds. Two aircraft were built, with research flights extending over more than two decades. The second XV-15 prototype spent much of its career developing technology to support the Bell Boeing V-22, the world’s first operational tilt-rotor aircraft.

The requirements that led to the V-22 had their roots in the failure of Operation Eagle Claw, an ill-fated 1980 attempt to rescue hostages held at the US Embassy in Tehran, Iran. The range and speed limitations of the helicopters used that night made the operation more complex and success less likely. Development of what became the V-22 was launched just one year later, with the V-22 achieving first flight in 1989.

Named “Osprey” after a sea-going raptor able to hover in flight, the V-22 entered a protracted and torturous development program. Although based on the successful XV-15, the V-22 was five times heavier and intended for the rigors of operational use. The test program experiences several fatal crashes, and the MV-22 version did not become operational with the United States Marine Corps until nearly two decades after its first flight.

The Ospreys in use today bear little resemblance to their forebears. Able to carry 10 tons of cargo nearly 900 nautical miles at top speeds approaching 300 knots, the V-22 is vastly more capable than the conventional helicopters it replaced and rivals the abilities of some fixed-wing transports. With the aerodynamics of tilt-rotor flight now well established and training procedures fully developed, the MV-22 has become one of the safest aircraft in military service, with a mishap rate below that of any conventional helicopter operated by the US Marines.

In addition to its mission for the Marines, the CV-22 version of the Osprey is operated by the United States Air Force’s Special Operations squadrons. By the end of the decade the United States Navy will take delivery of its CMV-22 version for at-sea supply delivery. After a full half century of development, the capabilities of the hybrid aircraft have at last become a reality.

On Saturday, June 4th, the Hiller Aviation Museum hosts HeliFest, a celebration of vertical flight. Among the aircraft planning to participate this year are V-22 Ospreys operated by both the Marines and Air Force, along with a bewildering range of more traditional high performance rotorcraft. Make your plans to join us for this event and experience some of the world’s pre-eminent vertical takeoff aircraft firsthand.

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