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 root-tip twist) and it avoids a helicopter's issues of retreating blade stall, the tiltrotor can achieve higher cruise speeds and takeoff weights than helicopters.
A tiltrotor aircraft differs from a tiltwing in that only the rotor pivots rather than the entire wing. This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.
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.[1][unreliable source?]
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 Germanprototype, the Focke-Achgelis Fa 269, was developed starting in 1942, which was tilting to the ground, but never flew.[2][3][4]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.[5]
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.
VTOL disc loading lift efficiency
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.[6]
In 1968, Westland Aircraft displayed their own designs—a small experimental craft (We 01C) and a 68-seater transport We 028—at the SBACFarnborough Airshow.[7]
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.[8][9]
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.[8]
Bell teamed with Boeing in developing a commercial tiltrotor, but Boeing went out in 1998 and Agusta came in for the Bell/Agusta BA609.[9][10] This aircraft was redesignated as the AW609 following the transfer of full ownership to AgustaWestland in 2011.[11] 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.[12]
Around 2005[13]–2010,[14] 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.[15][16]
AgustaWestland says they have free-flown a manned electric tiltrotor in 2013 called Project Zero, with its rotors inside the wingspan.[17][18][19]
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,[20] 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"[21][22][23] 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,[24] a top speed of 330 knots, a ceiling of 25,000 feet, and a range of 500 nautical miles.[10][25][26]
Technical considerations
Controls
In vertical flight, the tiltrotor uses controls very similar to a twin or tandem-rotor helicopter. Yaw is controlled by tilting its rotors in opposite directions. Roll is provided through differential power or thrust. Pitch is provided through rotor blades cyclic-, or nacelle, tilt. Vertical motion is controlled with conventional rotor blade pitch and either a conventional helicopter collective control lever (as in the Bell/Agusta BA609) or a unique control similar to a fixed-wing engine control called a thrust control lever (TCL) (as in the Bell-Boeing V-22 Osprey).[27]
Speed and payload issues
The tiltrotor's advantage is significantly greater speed than a helicopter. In a helicopter the maximum forward speed is defined by the turn speed of the rotor; at some point the helicopter will be moving forward at the same speed as the spinning of the backwards-moving side of the rotor, so that side of the rotor sees zero or negative airspeed, and begins to stall. This limits modern helicopters to cruise speeds of about 150 knots / 277km/h. However, with the tiltrotor this problem is avoided, because the proprotors are perpendicular to the motion in the high-speed portions of the flight regime (and thus not subject to this reverse flow condition), so the tiltrotor has relatively high maximum speed—over 300knots / 560km/h has been demonstrated in the two types of tiltrotors flown so far, and cruise speeds of 250knots / 460km/h are achieved.[27]
This speed is achieved somewhat at the expense of payload. As a result of this reduced payload, some[who?] estimate that a tiltrotor does not exceed the transport efficiency (speed times payload) of a helicopter,[28] while others conclude the opposite.[10] Additionally, the tiltrotor propulsion system is more complex than a conventional helicopter due to the large, articulated nacelles and the added wing; however, the improved cruise efficiency and speed improvement over helicopters is significant in certain uses. Speed and, more importantly, the benefit to overall response time is the principal virtue sought by the military forces that are using the tiltrotor. Tiltrotors are inherently less noisy in forward flight (airplane mode) than helicopters.[citation needed] This, combined with their increased speed, is expected to improve their utility in populated areas for commercial uses and reduce the threat of detection for military uses. Tiltrotors, however, are typically as loud as equally sized helicopters in hovering flight. Noise simulations for a 90-passenger tiltrotor indicate lower cruise noise inside the cabin than a Bombardier Dash 8 airplane, although low-frequency vibrations may be higher.[29]
Tiltrotors also provide substantially greater cruise altitude capability than helicopters. Tiltrotors can easily reach 6,000m / 20,000ft or more whereas helicopters typically do not exceed 3,000m / 10,000ft altitude. This feature will mean that some uses that have been commonly considered only for fixed-wing aircraft can now be supported with tiltrotors without need of a runway. A drawback however is that a tiltrotor suffers considerably reduced payload when taking off from high altitude.
A mono tiltrotor aircraft uses a tiltable rotating propeller, or coaxial proprotor, for lift and propulsion. For vertical flight the proprotor is angled to direct its thrust downwards, providing lift. In this mode of operation the craft is essentially identical to a helicopter. As the craft gains speed, the coaxial proprotor is slowly tilted forward, with the blades eventually becoming perpendicular to the ground. In this mode the wing provides the lift, and the wing's greater efficiency helps the tiltrotor achieve its high speed. In this mode, the craft is essentially a turboprop aircraft.
A mono tiltrotor aircraft is different from a conventional tiltrotor in which the proprotors are mounted to the wing tips, in that the coaxial proprotor is mounted to the aircraft's fuselage. As a result of this structural efficiency, a mono tiltrotor exceeds the transport efficiency (speed times payload) of both a helicopter and a conventional tiltrotor. One design study concluded that if the mono tiltrotor could be technically realized, it would be half the size, one-third the weight, and nearly twice as fast as a helicopter.[30]
In vertical flight, the mono tiltrotor uses controls very similar to a coaxial helicopter, such as the Kamov Ka-50. Yaw is controlled for instance by increasing the lift on the upper proprotor while decreasing the lift on the lower proprotor. Roll and pitch are provided through rotor cyclic. Vertical motion is controlled with conventional rotor blade blade pitch.[31]
A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-wing aircraft and other hybrid aircraft with powered rotors such as cyclogyros/cyclocopters and gyrodynes.
Bell Textron Inc. is an American aerospace manufacturer headquartered in Fort Worth, Texas. A subsidiary of Textron, Bell manufactures military rotorcraft at facilities in Fort Worth, and Amarillo, Texas, USA as well as commercial helicopters in Mirabel, Quebec, Canada.
The Bell XV-15 is an American tiltrotor VTOL aircraft. It was the second successful experimental tiltrotor aircraft and the first to demonstrate the concept's high speed performance relative to conventional helicopters.
A tiltwing aircraft features a wing that is horizontal for conventional forward flight and rotates up for vertical takeoff and landing. It is similar to the tiltrotor design where only the propeller and engine rotate. Tiltwing aircraft are typically fully capable of VTOL operations.
The AgustaWestlandAW609, formerly the Bell/Agusta BA609, is a twin-engined tiltrotor VTOL aircraft with a configuration similar to that of the Bell Boeing V-22 Osprey. It is capable of landing vertically like a helicopter while having a range and speed in excess of conventional rotorcraft. The AW609 is aimed at the civil aviation market, in particular VIP customers and offshore oil and gas operators.
The Curtiss-Wright X-19, company designation Model 200, was an American experimental tiltrotor aircraft of the early 1960s. It was noteworthy for being the last aircraft of any kind manufactured by Curtiss-Wright.
A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing only, and includes one or more conventional propeller or jet engines to provide forward thrust during cruising flight. During forward flight the rotor is unpowered and free-spinning, like an autogyro, and lift is provided by a combination of the rotor and conventional wings. The gyrodyne is one of a number of similar concepts which attempt to combine helicopter-like low-speed performance with conventional fixed-wing high-speeds, including tiltrotors and tiltwings.
The Bell XV-3 is an American tiltrotor aircraft developed by Bell Helicopter for a joint research program between the United States Air Force and the United States Army in order to explore convertiplane technologies. The XV-3 featured an engine mounted in the fuselage with driveshafts transferring power to two-bladed rotor assemblies mounted on the wingtips. The wingtip rotor assemblies were mounted to tilt 90 degrees from vertical to horizontal, designed to allow the XV-3 to take off and land like a helicopter but fly at faster airspeeds, similar to a conventional fixed-wing aircraft.
A convertiplane is defined by the Fédération Aéronautique Internationale as an aircraft which uses rotor power for vertical takeoff and landing (VTOL) and converts to fixed-wing lift in normal flight. In the US it is further classified as a sub-type of powered lift. In popular usage it sometimes includes any aircraft that converts in flight to change its method of obtaining lift.
Bell Agusta Aerospace Company (BAAC) was a joint venture formed in 1998 by Bell Helicopter and Agusta, who collaborated on a variety of products dating back to 1952. The joint venture was dissolved in 2011, when AgustaWestland took full ownership of the project, renaming it as the AgustaWestland Tilt-Rotor Company (AWTRC).
The Bell Boeing Quad TiltRotor (QTR) is a proposed four-rotor derivative of the Bell Boeing V-22 Osprey developed jointly by Bell Helicopter and Boeing. The concept is a contender in the U.S. Army's Joint Heavy Lift program. It would have a cargo capacity roughly equivalent to the C-130 Hercules, cruise at 250 knots, and land at unimproved sites vertically like a helicopter.
A powered lift aircraft takes off and lands vertically under engine power but uses a fixed wing for horizontal flight. Like helicopters, these aircraft do not need a long runway to take off and land, but they have a speed and performance similar to standard fixed-wing aircraft in combat or other situations.
A proprotor is a spinning airfoil that function as both an airplane-style propeller and a helicopter-style rotor. Several proprotor-equipped convertiplanes, such as the Bell Boeing V-22 Osprey tiltrotor, are capable of switching back and forth between flying akin to both helicopters and fixed-wing aircraft. Accordingly this type of airfoil has been predominantly applied to vertical takeoff and landing (VTOL) aircraft.
A transverse-rotor aircraft is an aircraft with two large horizontal rotor assemblies mounted side by side.
The Baldwin Mono Tiltrotor project is a research effort into a tiltrotor aircraft that uses only one rotor. Like other tiltrotor configurations, the mono tiltrotor combines the vertical lift capability and structural efficiency of a helicopter with the speed and range of a fixed-wing aircraft.
The Bell V-280 Valor is a tiltrotor aircraft being developed by Bell Helicopter for the United States Army's Future Vertical Lift (FVL) program. The aircraft was officially unveiled at the 2013 Army Aviation Association of America's (AAAA) Annual Professional Forum and Exposition in Fort Worth, Texas. The V-280 made its first flight on 18 December 2017 in Amarillo, Texas.
The Vertical Take-Off and Landing Experimental Aircraft program is an American research project sponsored by the Defense Advanced Research Projects Agency (DARPA). The goal of the program is to demonstrate a VTOL aircraft design that can take off vertically and efficiently hover, while flying faster than conventional rotorcraft. There have been many previous attempts, most of them unsuccessful.
The Leonardo Next-Generation Civil Tiltrotor is a tiltrotor aircraft demonstrator designed and developed by the Italian aerospace company Leonardo S.p.A. Studies for a two times larger tiltrotor than the AgustaWestland AW609 started in 2000. Since 2014, its development is sponsored by the European Union's Clean Sky 2 program. By May 2021, major components were under production By 2023, the maiden flight had been pushed back to 2024, from a 2020 initial plan.
↑ Article title Leishman, J. G., Preator, R., Baldwin, G. D.,Conceptual Design Studies of a Mono Tiltrotor (MTR) Architecture, U.S. Navy Contract Number: N00014-03-C-0531, 2004.
↑ "Archived copy"(PDF). Archived from the original(PDF) on October 7, 2008. Retrieved June 5, 2009.{{cite web}}: CS1 maint: archived copy as title (link) Baldwin, G. D., 'Preliminary Design Studies of a Mono Tiltrotor (MTR) with Demonstrations of Aerodynamic Wing Deployment', AHS International Specialists Meeting, Chandler, Arizona, January 23–25, 2007.
Note 1: A tiltwing or tiltrotor aircraft functions as an aeroplane during normal (horizontal) flight and as a helicopter during low-speed flight.
Note 2: For full-size aircraft with powered rotors the rotor is normally tilted to achieve thrust (e.g. in a helicopter). Some toys (e.g. balloon helicopter) do have a powered rotor with no means to tilt the rotor to produce horizontal thrust.
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