Convertible plane

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Aerial refueling of an MV-22 of the USMC by a KC-130J over the Mediterranean
AgustaWestland AW609 hovering (2007 still Bell-Agusta BA-609)

Convertiplane ( English Convertiplane for convertible plane is) the name for a vertical takeoff and landing capable aircraft that is able to change the flight modes of the flight (flight modes) during. The flight conditions relevant for a convertible aircraft are hovering in helicopter mode, the subsequent transition to level flight and level flight as a fixed-wing aircraft .

Until 2012, however, no model had proven its economic competitiveness compared to conventional helicopters. The ability to take off and land vertically (VTOL) together with the greater speed and range of a convertible aircraft must offset the economic disadvantages of higher fuel consumption, higher acquisition costs and increased noise emissions. Currently, the Bell Boeing V-22 is mainly used as a combat zone transporter in the military sector and has proven itself in use.

Another military convertible aircraft is in development with the Bell V-280 . According to the conditions of the Future Vertical Lift (ATCO) , the first start of a prototype must take place in 2017 at the latest. This US Armed Forces program aims to determine how a number of older helicopter types can be replaced.

In the civil sector, the AgustaWestland AW609, originally designed by Bell Boeing and publicly presented in 1996, is in the test phase with three prototypes, a fourth is under construction (as of 2019). Series production is scheduled to start in 2020.

definition

According to the definition by Gersdorff & Knobling, convertible aircraft are characterized by the fact that they take off and land like helicopters and change their configuration to a fixed-wing aircraft when they transition to cruise flight. Thus, included in this category flipper - tiltrotor - tiltrotor - Einziehrotor- and stop rotor aircraft. The general aim is to “combine the favorable properties of the rotary wing aircraft with those of the fixed wing aircraft”. Polte also gives an identical definition.

The definition of convertiplanes in English-speaking countries is broader . Aircraft that also use a rotating rotor in a powered autorotation mode (unloaded rotor technology) in level flight are also classified here . An example of this is the McDonnell XV-1, which is also classified as a convertiplane in the US Army Category XV . In German-speaking countries, these aircraft are usually referred to as composite aircraft helicopters . In general, pure jet-powered VTOL aircraft are therefore not convertible aircraft.

Constructive design

In the case of convertible aircraft, the aim is in particular to increase the travel or marching speed compared to pure helicopters. Tilt rotor and similar designs avoid the negative influence of the aerodynamic drag of the rotor blades and the rotor hub. Since the parasitic resistance of the rotor blades increases with the third power of the speed, a rotor speed as low as possible is advantageous for designs with a rotor that cannot be tilted or stowed away (composite aircraft) for high-speed flight. If the speed becomes too low, the returning blade can no longer be adjusted sufficiently to generate enough lift. Even in high-speed flight mode, the rotor must therefore be driven to such an extent that there is no flow separation at the returning blade tips. However, if the rotor can be stowed in the fuselage for high-speed flight, a convertible aircraft is theoretically able to reach speeds like a fixed-wing aircraft. During the practical testing of different concepts, however, it was shown that the aerodynamic instabilities of the rotor that occur when the rotors tilt during the transition from hovering to horizontal forward flight can lead to critical flight phases.

If the narrower definition for convertible aircraft is used as a basis, without the inclusion of composite helicopters, the following constructive interpretations for convertible aircraft result.

Stop rotor / accumulator rotor

The stop protor technology was used by the Herrick HV-2 , one of the first convertible aircraft to become known, as early as the 1930s. Here, the upper wing of a biplane could rotate during take-off and landing, while it was fixed as the upper wing in horizontal flight. In the 1950s, Sikorsky submitted his design XV-2 as part of a tender by the US Army. The single-blade rotor should be stowed in the upper part of the body after it has stopped. However, after the evaluation phase, the project had to be abandoned. The Messerschmitt project Me 408 , which provided for two foldable accumulation rotors, did not get beyond the design phase at the end of the 1960s.

Another project was the Canard Rotor Wing (CRW) program of DARPA , announced in 1998 , which featured a large duck wing, double tail booms and two rotors powered by a turbofan. The vertical take-off took place with the rotors via a blade tip drive fed by the jet engine; After the transition, the rotor should be fixed and serve as an additional wing. In the same year, Boeing also presented its similarly designed ship-based Dragonfly design for an unmanned aerial vehicle. The hull is only 5.19 m long.

Tilt rotor

One of the first designs was a one -and-a-half-decker designed by Henry Berliner from the early 1920s, which was equipped with tilt rotors on the lower protruding wing. The hybrid aircraft is said to have reached a speed of around 65 km / h during flight tests. The rotor axes could not be tilted completely into the horizontal position, but the design by Berliner was one of the first applications of this design for convertible aircraft, which is still common today.

Another design with this design was the "Lehberger Flying Machine" patented by George Lehberger in September 1930. However, there was no practical implementation.

In the late 1930s a patent for the British Baynes Heliplane , also a tilt rotor design, was granted, but could not be realized due to a lack of financial support from the inventor. The 1940s Pearse Convertiplane was a single-engine tilt rotor aircraft, but it never flown.

One of the first German drafts that can in principle be seen as feasible was the Focke-Achgelis Fa 269 , which was developed from 1941 on behalf of the RLM. Here, pusher propellers were provided, which, however, in contrast to the previous designs, should be tilted down for the hover flight.

In the USA, Haviland Platt and Wynn LePage developed a tilting protor construction based on the Platt-LePage XR-1 helicopter in 1950, which should have an all-up weight of 26 t, but also did not get beyond the drawing board stage. The design of the Transcendental Aircraft Corporation (TAC) Model 1-G also dates from the same time (1947). The design engineer was Bob Lichten, who brought his experience with the tilting protector concept to TAC at Platt LePage. The 1-G completed more than 100 flights, with partial transitions reaching up to 10 ° to the horizontal flight mode, before the machine crashed on July 20, 1955 during flight attempts in the Chesapeake Bay. However, the pilot was uninjured because the crash occurred in the shallow bank area. The 1-G is considered the first convertible aircraft that was able to successfully demonstrate the transition between the different flight modes. The tests with the enlarged successor TAC Model 2 were also discontinued in 1957 due to a lack of financial resources.

In the 1950s, the US Army and the US Air Force began a research program to investigate the feasibility of a high-speed VTOL aircraft to complement existing helicopters. Three concepts were promoted, of which the Bell XV-3 was the most successful design. With the XV-3, extensive investigations were carried out for the first time into the difficult to control aeroelastic instabilities of the combination of rotor, rotor nacelles and wings. This even included tests with a specimen in the large wind tunnel of the Ames Research Center.

Bell had initial successes with the XV-15 . In 1981 the USMC tendered an order for an advanced VTOL combat zone transporter under the designation "VHXM". However, this was abandoned and replaced by a joint tender for all branches of the armed forces (JVX: “Joint services, Vertical lift, Experimental”). It was planned to purchase 1,086 machines in 1982. This concept eventually resulted in the Bell V-22 .

The designs with tiltable ducted propellers can be viewed as a separate subgroup of tilt rotor aircraft . These include the Doak VZ-4 , which carried out its first hover flight in February 1958, the Nord 500 , which was first flown in 1968, and the Bell X-22, which was tested from 1966 to 1980 .

Tilt rotor types that are still being tested are the AgustaWestland AW609 (first flight in 2003) and a variant of the Vahana (first flight in 2018).

Tilt vane

A special advantage of the tilt vane design can be seen in the optimal flow path over the control surfaces during the transition phase. The downwash is also not hindered, since the raised wing does not significantly shield the rotor flow downwards and thus loss of lift is avoided. The greatest disadvantage is the wind sensitivity of a vertical wing when hovering, which acts like a "barn door". The tilt mechanism, which has to transmit large forces, increases the overall weight of the aircraft more than with the tilt protor concept.

The "Converta-Wing" by DH Kaplan from the 1950s is considered to be one of the first developed concepts. In this joint project by the Navy and Air Force, two Boeing gas turbines were supposed to provide the drive for the rotors at the wing ends. The rotor blades were relatively short compared to later designs and should be operated at high speed. In the late 1950s, NASA investigated, including in the large wind tunnel in Langley, a sample with six also short-bladed rotors that were supposed to be driven by a 1000-hp electric motor. Double-slit flaps, which reached over 60% of the depth, were used to increase lift.

Other completed tilt vane projects were:

  • Vertol VZ-2 , the first tilt-wing aircraft that was able to demonstrate the full usefulness of this technology as a demonstrator. The first flight took place in April 1957 and the first complete transition (vertical take-off - level flight - vertical landing) in July 1958. The VZ-2 carried out 450 flights and 34 complete transitions.
  • Hiller X-18 ; the only one built on the fuselage of a Chase XC-122 and flew for the first time in November 1959. In addition to the two Allison T40 turboprop engines, the X-18 also had a Westinghouse J34 for steering around the pitch axis .
  • Kaman K-16B , a US Navy project from the early 1960s for a tilt-wing flying boat based on a Grumman JRF Goose. The wing could only be swiveled up to a 50 ° position. Two General Electric T58 each propelled a propeller with a diameter of 4.58 m. However, for unexplained reasons, only wind tunnel investigations and no flight tests took place.
  • Ling-Temco-Vought XC-142 ; this program was intended to demonstrate the operational capability of a VTOL transporter under field conditions and was the first VTOL draft jointly supported by all US armed forces. The first transition was carried out in November 1964.
  • Canadair CL-84 ; the first of four built prototypes carried out the first successful transition in January 1966.

More convertible planes

literature

  • Steve Markman, Bill Holder: Straight Up. A History of Vertical Flight . A Schiffer Military History Book, 2000, ISBN 0-7643-1204-9 .
  • Franklin D. Harris: An Overview of Autogyros and The McDonnell XV-1 Convertiplane . NASA / CR-2003-212799 (PDF; 14.2 MB) .
  • Martin D. Maisel, Demo J. Giulianetti, Daniel C. Duagn: The history of the XV-15 Tilt Rotor Research Aircraft. From concept to flight . Monographs in Aerospace History # 17, The NASA History Series (PDF; 2.5 MB) .
  • Kyrill von Gersdorff, Kurt Knobling: helicopter and gyrocopter . German aviation . Vol. 3. Bernard & Graefe Verlag Munich, 1982, ISBN 3-7637-5273-0 .
  • Hans-Joachim Polte: helicopter. History, technology, commitment . Publishers ES Mittler, ISBN 978-3-8132-0924-2 .
  • Breakthrough for the Tilt Rotor . In: Air International , September 1982, p. 146.

Individual evidence

  1. ^ Www.lexingtoninstitute.org: “With thousands of hours of operational employment in Iraq and Afghanistan the V-22 has left behind any doubts about its performance and effectiveness. In the future, the Osprey could well become one of this country's most important strategic platforms. "
  2. ↑ Leonardo (AgustaWestland) AW609 TiltRotor , accessed January 16, 2020
  3. ↑ von Gersdorff, Knobling: p. 255.
  4. ↑ Hans-Joachim Polte: p. 13.
  5. ↑ Rotorcraft Speed ​​Limitations
  6. ^ Photo of the Berlin Convertiplane in Maisel, Giulianetti, Dugan, p. 6
  7. ^ Patent drawings and text on the Lehberger Flying Machine
  8. ↑ Drawings and data for the Baynes Heliplane , accessed January 16, 2020
  9. ^ Pioneer Pearse (Talkback), in AIR International, July 1978, p. 32 f.
  10. ↑ von Gersdorff, Knobling: p. 47f.
  11. ↑ Photo and history of the Transcendental Aircraft Corporation 1-G at www.aviastar.org
  12. ↑ http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080022367_2008020986.pdf , p. 13 ff.
  13. ^ Markman & Holder, p. 22
  14. ^ Markman & Holder, p. 22
  15. ^ Aerospatiale X-910 (Airdata File), in AIR International, June 1977, p. 307