A tiltrotor is a rotorcraft that generates lift and propulsion by way of one or more powered rotors (sometimes called proprotors) mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a transverse rotor design, with a few exceptions that use other multirotor layouts. Tiltrotor design combines the VTOL capability of a helicopter with the speed and range of a conventional fixed-wing aircraft. For vertical flight, the rotors are angled so the plane of rotation is horizontal, generating lift the way a normal helicopter rotor does. As the aircraft gains speed, the rotors are progressively tilted forward, with the plane of rotation eventually becoming vertical. In this mode the rotors provide thrust as a propeller, and the airfoil of the fixed wings takes over providing the lift via the forward motion of the entire aircraft. Since the rotors can be configured to be more efficient for propulsion (e.g. with high root-tip twist) and it avoids a helicopter's issues of retreating blade stall, tiltrotors can achieve higher cruise speeds and takeoff weights than helicopters. A tiltrotor aircraft differs from a tiltwing in that only the rotors pivot rather than the entire wing. This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.
History
The first work in the direction of a tilt-rotor (French "Convertible") seems to have originated ca. 1902 by the French-Swiss brothers Henri and Armand Dufaux, for which they got a patent in February 1904, and made their work public in April 1905. Concrete ideas of constructing vertical take-off and landing (VTOL) aircraft using helicopter-like rotors were pushed further in the 1930s. The first design resembling modern tiltrotors was patented by George Lehberger in May 1930, but he did not further develop the concept. In World War II, Weserflug in Germany came up with the concept of their P.1003/1 around 1938, which was tilting to the top with part of the wings but not the full wings, so it may be in between tilt-rotor and tilt-planes. Shortly after a German prototype, the Focke-Achgelis Fa 269, was developed starting in 1942, which was tilting to the ground, but never flew. Platt and LePage patented the PL-16, the first American tiltrotor aircraft. However, the company shut down in August 1946 due to lack of capital. Two prototypes which made it to flight were the one-seat Transcendental Model 1-G and two seat Transcendental Model 2, each powered by a single reciprocating engine. Development started on the Model 1-G in 1947, though it did not fly until 1954. The Model 1-G flew for about a year until a crash in Chesapeake Bay on July 20, 1955, destroying the prototype aircraft but not seriously injuring the pilot. The Model 2 was developed and flew shortly afterwards, but the US Air Force withdrew funding in favor of the Bell XV-3 and it did not fly much beyond hover tests. The Transcendental 1-G is the first tiltrotor aircraft to have flown and accomplished most of a helicopter to aircraft transition in flight (to within 10 degrees of true horizontal aircraft flight). Built in 1953, the experimental Bell XV-3 flew until 1966, proving the fundamental soundness of the tiltrotor concept and gathering data about technical improvements needed for future designs. A related technology development is the tiltwing. Although two designs, the Canadair CL-84 Dynavert and the LTV XC-142, were technical successes, neither entered production due to other issues. Tiltrotors generally have better hover efficiency than tiltwings, but less than helicopters. In 1968, Westland Aircraft displayed their own designs—a small experimental craft (We 01C) and a 68-seater transport We 028—at the SBAC Farnborough Airshow. In 1972, with funding from NASA and the U.S. Army, Bell Helicopter Textron started development of the XV-15, a twin-engine tiltrotor research aircraft. Two aircraft were built to prove the tiltrotor design and explore the operational flight envelope for military and civil applications. In 1981, using experience gained from the XV-3 and XV-15, Bell and Boeing Helicopters began developing the V-22 Osprey, a twin-turboshaft military tiltrotor aircraft for the U.S. Air Force and the U.S. Marine Corps. Bell teamed with Boeing in developing a commercial tiltrotor, but Boeing went out in 1998 and Agusta came in for the Bell/Agusta BA609. This aircraft was redesignated as the AW609 following the transfer of full ownership to AgustaWestland in 2011. Bell has also developed a tiltrotor unmanned aerial vehicle (UAV), the TR918 Eagle Eye. Russia has had a few tiltrotor projects, mostly unmanned such as the Mil Mi-30, and has started another in 2015. Around 2005–2010, Bell and Boeing teamed up again to perform a conceptual study of a larger Quad TiltRotor (QTR) for the US Army's Joint Heavy Lift (JHL) program. The QTR is a larger, four rotor version of the V-22 with two tandem wings sets of fixed wings and four tilting rotors. In January 2013, the FAA defined US tiltrotor noise rules to comply with ICAO rules. A noise certification will cost $588,000, same as for a large helicopter. AgustaWestland stated they have free-flown a manned electric tiltrotor in 2013 called Project Zero, with its rotors inside the wingspan. In 2013, Bell Helicopter CEO John Garrison responded to Boeing's taking a different airframe partner for the US Army's future lift requirements by indicating that Bell would take the lead itself in developing the Bell V-280 Valor, with Lockheed Martin. In 2014, the Clean Sky 2 program (by the European Union and industry) awarded AgustaWestland and its partners $328 million to develop a "next-generation civil tiltrotor" design for the offshore market, with Critical Design Review near the end of 2016. The goals are tilting wing sections, 11 metric tons Maximum takeoff weight, seating for 19 to 22 passengers, first flight in 2021, a cruise speed of 300 knots, a top speed of 330 knots, a ceiling of 25,000 feet, and a range of 500 nautical miles.
Technical considerations
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