Powered lift

V-22 Osprey

AV-8B Harrier II

Powered lift or powered-lift refers to a type of aircraft that can take off and land vertically and functions differently from a rotorcraft in horizontal flight.

The term is particularly used by the United States Federal Aviation Administration for classification purposes. Powered-lift is one of the seven categories of aircraft designated by the Federal Aviation Administration; the other six being Airplane, Rotorcraft, Glider, Lighter-Than-Air, Powered parachute, and Weight-shift control.

Powered-lift means a heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.

— FAA^[1]

The first powered-lift ratings to be issued by the FAA on a civilian pilot certificate were on 21 August 1997, to pilots of Bell Helicopter and Boeing, and of the United States Marine Corps.^[2]

Convertiplane

A convertiplane is an aircraft which uses rotor power for vertical takeoff and landing (VTOL) and converts to fixed-wing lift in normal flight.

In tiltrotor and tiltwing designs such as the Bell Boeing V-22 Osprey, the rotor swings forward to act as a propeller in forward flight. Some designs have a ducted rotor design, in which the rotor is surrounded by a large ring-shaped duct to reduce tip losses.

Tiltrotor

An USAF CV-22 in flight

Powered lift and thrust forces of various aircraft

The powered rotors of a tiltrotor (sometimes called proprotor) are mounted on rotating shafts or nacelles at the end of a fixed wing, and used for both lift and propulsion. For vertical flight, the rotors are angled to provide thrust upwards, lifting the way a helicopter rotor does. As the aircraft gains speed, the rotors progressively rotate or tilt forward, with the rotors eventually becoming perpendicular to the fuselage of the aircraft, similar to a propeller. In this mode, the wing provides the lift and the rotor provides thrust. The wing's greater efficiency helps the tiltrotor achieve higher speeds than helicopters.

The V-22 Osprey by Bell Helicopter/Boeing, a twin-engine tiltrotor design that has two turbine-powered engines driving three-bladed rotors. The rotors function similar to a helicopter in vertical flight, and similar to an airplane in forward flight. The aircraft first flew on 19 March 1989.

The Bell/Agusta BA609 tiltrotor is the evolution of Bell Helicopter's V-22 Osprey into a civilian aircraft. The aircraft can take off and land vertically with 2 crew and 9 passengers, and within 20 seconds, transition to forward flight (by tilting its rotor blades into a fully forward position, much like the V-22 Osprey). In forward flight it can cruise at speeds of up to 275 knots (509 km/h), with a range exceeding 1,000 nautical miles (with long-range fuel tanks). It is rated to fly above FL210 (21,000 ft), has a maximum payload capacity of over 5,500 pounds, thanks to two Pratt and Whitney PT6C-67A turbines rated at 1,940 shp, each driving a 26-foot (8 m) diameter 3-bladed rotor blade. The aircraft is not yet in full production; however, over 80 orders have been taken.

Tiltwing

The tiltwing is similar to the tiltrotor, except that the rotor mountings are fixed to the wing and the whole assembly tilts between vertical and horizontal positions.

The Vertol VZ-2 was a research aircraft developed in the late 1950s. Unlike other tiltwing aircraft, Vertol designed the VZ-2 using rotors in place of propellers.^[3] On 23 July 1958, the aircraft made its first full transition from vertical flight to horizontal flight. By the time the aircraft was retired in 1965, the VZ-2 had accomplished 450 flights, including 34 full transitions.

Rotor wing

The Boeing X-50 Dragonfly had a two-bladed rotor driven by the engine for takeoff. In horizontal flight the rotor stopped to act like a wing. Fixed canard and tail surfaces provided lift during transition, and also stability and control in forward flight. Both examples ended their lives in crashes.

The Sikorsky X-Wing had a four-bladed rotor utilizing compressed air to control lift over the surfaces while operating as a helicopter. At higher forward speeds, the rotor would be stopped to continue providing lift as tandem wings in an X configuration. The program was canceled before the aircraft had attempted any flights with the rotor system.

Tail-sitter

A Tail-sitter is an aircraft that rests on the ground pointing vertically upwards, so that it rests on its tail and takes off and lands vertically. The whole aircraft then tilts forward horizontally for normal flight. No type has ever gone into production, although a number of experimental variants have been flown, using both proprotor and jet thrust. Some have achieved successful transition between flight modes.

The coleopter type has an annular wing forming a duct around a lift rotor. The transition to forward flight has never been achieved, although the SNECMA Coléoptère took off, hovered and landed vertically.

The German Focke-Wulf Fw Triebflügel was a design studied during the Second World War. It used pulse jets to power a rotor that rotated about the fuselage axis behind the cockpit. Similar to a coleopter fixed-wing aircraft, the Triebflügel was intended to take off and land on its tail, rotating on the pitch axis after takeoff and acceleration for forward flight. The design was never been built beyond model wind tunnel testing.

Fan-in-wing

Fan-in-wing is a Vertical takeoff and landing (VTOL) aircraft configuration in which lifting fans are located in large holes in an otherwise conventional fixed wing.^[4]

The aircraft takes off using the fans to provide lift, then transitions to fixed-wing lift in forward flight. Several experimental craft have been flown, but none has ever entered production.

History

A number of experimental fan-in-wing aircraft were evaluated in the USA during the late 1950s and throughout much of the 1960s.

The Avro Canada Avrocar, commissioned by the USA, was intended to be a technology demonstrator for a supersonic VTOL aircraft. It featured a single central fan in a circular flying wing, with engine thrust directed rearwards for forward flight. It underwent trials between 1958 and 1961 but, due to its unstable "flying saucer" aerodynamics and lower than expected thrust, never flew out of ground effect. The Verticraft verticar of 1961 was a similar single-fan, directed-thrust, all-wing (or lifting body) aircraft, of conventional but very low-aspect-ratio wing planform. It failed to fly. A tandem-fan version was proposed but never built.^[5] By contrast the Ryan XV-5 Vertifan of 1964 was an otherwise conventional delta-wing jet. It had a large fan in each wing and a third, smaller fan in the nose to provide balance in pitch. It was more successful, with one of the two prototypes flying until 1971.

The Vanguard C2 and C2D Omniplane had two fans side by side, with one in each wing. It underwent tethered trials between 1959 and 1962 but never flew untethered. In the late 1960s NASA conducted wind tunnel experiments on a series of larger aircraft designs using different numbers of fans in the wings.^[6]

Beginning with a one-sixth scale model in 1962, Paul Moller has been experimenting with the flying saucer configuration ever since. All have had a central cockpit but have varied between a single lift fan surrounding the cockpit, twin fans behind the cockpit or, more recently, eight individual fans distributed around it. Some examples such as the XM-2 of 1965 have been able to hover within ground effect. The artificially stabilised M200X was re-engined in 1989 and can fly out of ground effect, but has not entered production.^[7]

Several projects were announced in 2010 and 2011. The Ray passenger aircraft had four fans arranged approximately in a square, similarly to a quadrotor, with a small fan in front of a larger one behind it in each wing.^[8][9][10] The Lockheed Martin VARIOUS was a multi-role military UAV with twin fans side by side, one in each wing.^[11][12] The Northrop MUVR ship-to-shore resupply aircraft similarly had a fan in each wing.^[13] Unlike these the Anglo-Italian AgustaWestland Project Zero hybrid tiltrotor/fan-in-wing unmanned technology demonstrator was actually built and flew successfully in 2011. It has side by side twin fans which can rotate into a vertical plane to act as propellers in forward flight, while still located within the wing structure.

List of fan-in-wing aircraft

Vectored thrust

The Harrier Jump Jet covers a series of a military VSTOL jet aircraft. It is capable of vertical/short takeoff and landing (V/STOL) and is the only truly successful design of this type from the many that arose in the 1960s. These aircraft are capable of operating from small spaces, such as fields, roads, and aviation-capable ships. The F-35 Lightning II version B is proposed as the next military VSTOL in order to replace the Harrier.

Lift jets

A Lift jet is a lightweight jet engine used to provide vertical thrust for VTOL operation, and is then shut down in forward flight. Some VTOL designs have used both vectored thrust from the main engine together with auxiliary lift jets.

Helicopter-airship compound

Piasecki Helicopter developed the Piasecki PA-97 Helistat using the rotor systems from four obsolete helicopters and a surplus Navy blimp, in order to provide a capability to lift heavier loads than a single helicopter could provide. The aircraft suffered a fatal accident during a test flight. In 2008, Boeing and SkyHook International resurrected the concept and announced a proposed design of the SkyHook JHL-40.

Examples

• Harrier Jump Jet
• V-22 Osprey
• Canadair CL-84
• F-35B Lightning II
• Sikorsky S-72
• Vertol VZ-2
• Bell XV-3
• LTV XC-142
• Bell XV-15
• Bell/Agusta BA609
• Bell Eagle Eye

See also

• Pilot certification in the United States
• Convertiplane
• Ducted fan
• FanWing
• Quadcopter

Notes

References

• "Addressing the future challenges of the operation of powered lift category/tiltrotor class aircraft in international air navigation", report by "Assembly – 35 Session Executive Committee", www.faa.gov
• "Vertical flight technology in the national airspace system", Testimony of William Wallace, the National Resource Specialist for Rotorcraft Operations of the Federal Aviation Administration, before the Subcommittee on Space and Aeronautics, 9 May 2001, http://testimony.ost.dot.gov