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Two approaches to achieving short takeoff and vertical landing

21st February 2013


After 40 years in service, the Harrier is due to be replaced by a variant of the Lockheed Martin F-35 Lightning II joint strike fighter that is designed for short takeoffs and vertical landings. Alistair Rae looks at the two aircraft and the different technologies employed to achieve similar goals.

The Harrier Jump Jet is in service with air forces and navies around the world, including the UK's Royal Air Force (RAF) and Royal Navy which use the GR7/7A, T10 two-seater and GR9/9A types. As well as the aircraft's performance capability, you also have to admire its longevity; the Harrier can trace its roots back to 1957, though the first pre-production Harriers did not fly until 1967. Harriers first entered service a remarkable forty years ago in 1969.

At the heart of the Harrier is the 48 inch (1.22 m) diameter Rolls-Royce Pegasus turbofan engine with a unique thrust vectoring system that utilises four swivelling nozzles to give the Harrier lift and/or forward thrust. Current versions of the Pegasus engine have a thrust range up to 23800 pounds.

While the Harrier is probably the best know VTOL (vertical takeoff and landing) or STOVL (short takeoff and vertical landing) aircraft, there are others. The Yakovlev Yak-38 (known within NATO as the Forger) was the Soviet Naval Aviation's VTOL multi-role combat aircraft, entering service in 1976. Following that, the Yakovlev Yak-141 (known within NATO as the Freestyle) was a supersonic VTOL fighter aircraft. As with the Yak-38, the Yak-141 gained its VTOL capability through a combination of a lift and lift/cruise engines. Two lift jets were mounted behind the cockpit and contributed to takeoff only; once the aircraft was in horizontal flight, the lift engines were shut down. The main engine was installed in the rear of the fuselage, with a swivelling nozzle and an afterburner. For takeoff and hovering, the exhaust from the jet was vectored downwards through 90 degrees to work in conjunction with the forward lift jets. To obtain sufficient power for vertical takeoff, it was necessary to use the afterburner, which limited the types of runway surfaces that could be used. In August 1991 the programme was halted due to budgetary constraints.

Lightning II

Inevitably it has become necessary to replace the Harrier and the route being taken is to develop a variant of the Lockheed Martin F-35 Lightning II joint strike fighter (JSF). The conventional Lightning II, designated the F-35A, is a single-engined supersonic multirole aircraft that benefits from integrated stealth capability. Two other variants are also in development: the F-35C carrier variant (CV) for the US Navy; and the F-35B short takeoff/vertical landing (STOVL) variant for the US Marine Corps and the UK's Royal Air Force and Royal Navy. Flight testing of the F-35B began on 11th June 2008, initially using conventional takeoffs and landings. It is expected that the F-35B will be available from 2012 onwards. Primary customers for the F-35B will be the US Marine Corps, the UK's Royal Air Force and Royal Navy, and the Italian Navy.

Unlike the Harrier, the F-35B is supersonic, though it still shares the Harrier's ability to operate from small ships, roads and austere bases. Internal fuel capacity is seven tons, providing an unrefueled range of more than 900 miles (1450 km) without external tanks.

Despite being almost identical in appearance to the F-35A, the F-35B incorporates a counter-rotating lift fan that is positioned directly behind the cockpit. The lift fan is powered by a drive shaft from the F-35's single engine, which features a swivelling rear exhaust nozzle that vectors thrust downward during vertical flight.

The lift fan, engine and stabilising roll ducts beneath the F-35B's wings combine to produce 40 000 pounds of lifting force. Converting the F-35B from STOVL to conventional flight and vice-versa simply requires the pilot to push a button; otherwise the system operates fully automatically.

During the conversion from conventional flight to STOVL flight, doors associated with the STOVL propulsion system open, including the lift fan inlet and exhaust doors, the roll-nozzle doors, the auxiliary-inlet doors on top of the fuselage (which increases the efficiency of the main engine) and the aft fuselage three-bearing swivel duct doors. The three-bearing swivel duct begins vectoring engine thrust downward as well, taking just 2.5 seconds to rotate through 95 degrees.

Once all doors are open, the clutch engages and the lift fan begins turning. As the lift fan reaches full speed the clutch locks, providing a direct drive from the engine to the lift fan. The aircraft control algorithms then begin using the STOVL propulsion system to provide aircraft flight control.

While Rolls-Royce is a member of the Fighter Engine Team with GE on the F136 engine for the F-35, the company is also subcontracted to Pratt & Whitney on the F-35's alternative engine, the F135, to provide the lift system comprising the lift fan, clutch, drive shaft, roll posts and the three-bearing swivel module (3BSM).

Lockheed Martin developed the concept for a STOVL lift system that uses a vertically-orientated, shaft-driven lift fan (SDLF). A two-stage low-pressure turbine on the engine provides the power necessary to drive the lift fan. The lift fan generates a column of cool air that provides nearly 20000 pounds of vertical thrust using variable inlet guide vanes to modulate the airflow.

An equivalent amount of thrust is provided by the downward-vectored rear exhaust. Because the lift fan extracts power from the engine, exhaust temperatures are reduced.

In December 2008 Rolls-Royce delivered the first production variable-area vane box nozzle (VAVBN) to Pratt & Whitney for integration on the F-35B. The VAVBN is integral to the aircraft structure and is used to control the lift fan exit airflow when the F-35B operates in powered-lift mode.

Installed as one of the first items in the aircraft assembly sequence, the VAVBN is never removed throughout the service life (8000 flight hours). The MOD plans to operate up to 36 F-35s from each of its two new future aircraft carriers, namely HMS Queen Elizabeth, which is expected to enter service in 2014, and HMS Prince of Wales in 2016.







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