Avertical take-off and landing(VTOL)aircraftis one that can hover,take off and landvertically. This classification includesfixed-wing aircraftas well ashelicoptersand other aircraft with powered rotors, such ascyclogyros/cyclocoptersandtiltrotors.Some VTOL aircraft can operate in other modes as well, such asCTOL(conventional take-off and landing),STOL(short take-off and landing), and/or STOVL(short take-off and vertical landing).Others, such as some helicopters, can only operate by VTOL, due to the aircraft lacking landing gear that can handle horizontal motion. VTOL is a subset ofV/STOL(vertical and/or short take-off and landing).

In 1947,Ryan X-13 Vertijet, a tailsitter design, was ordered by the US Navy, who then further issued a proposal in 1948 for an aircraft capable of vertical takeoff and landing (VTOL) aboard platforms mounted on the afterdecks of conventional ships. BothConvairand Lockheedcompeted for the contract but in 1950, the requirement was revised, with a call for a research aircraft capable of eventually evolving into a VTOL ship-based convoy escort fighter.Another more influential early functional contribution to VTOL wasRolls-Royce'sThrust Measuring Rig("flying bedstead") of 1953. This led to the first VTOL engines as used in the first British VTOL aircraft, theShort SC.1(1957) which used 4 vertical lift engines with a horizontal one for forward thrust.

Although there are many variants of the Harrier family, the basic layout of the aircraft has not changed since 1957/58. This section outlines the common features of all aircraft in the Harrier family, from the original P.1127 through to today's Harrier II+.The P.1127 was originally designed not only as a research aircraft to explore V/STOL flight but also as a tactical strike fighter to be used in support of land forces. It was this objective, in addition to its novel take-off and landing method, that has shaped the design.The P.1127 was originally designed not only as a research aircraft to explore V/STOL flight but also as a tactical strike fighter to be used in support of land forces. It was this objective, in addition to its novel take-off and landing method, that has shaped the design.

As the main mission of the Harrier has usually been support of troops in combat, rather than long-range interdiction, it has always been acceptable to have only one crewmember. This has allowed weight to be limited, reduced the problem of maintaining the aircraft's centre of gravity and allowed a larger proportion of internal volume to be used for fuel and equipment. These points have been illustrated by the problems found in all these areas with the development of two-seat trainer Harriers; all of with have featured operational penalties. In the first generation of Harriers the lack of importance attached to rearward vision that resulted from the ground attack mission also enabled the drag to be minimised by adopting a canopy flush with the upper-fuselage lines. This has been reversed in the case of the Sea Harrier and Harrier II, which both feature a bubble canopy to both improve rearward vision and as a consequence of increasing cockpit volume.

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Because the main flight regime of the Harrier has always been the low-level, high speed one, and because the provision of vectored thrust for take-off and landing has reduced the importance of wing lift at these important points, the wing has always remained relatively small. This has again allowed a saving in weight and drag over the design of a larger wing aimed at providing high altitude manoeuvrability, or one equipped with leading edge devices for low-speed flight. Even when a larger wing was adopted for the Harrier II the size increase was a modest 15%, while the larger weight of stores and fuel carried by the Harrier II meant that wing loading remained high.

The wing has always featured a considerable degree of anhedral. This originated as a means of reducing the problems of Dutch roll encountered at high angles of attack on high-set swept wings such as that on the Harrier. It also helped in reducing the length of the outriggers on the wingtips.Structurally the wing has always been a single piece unit. It is used as one of the main fuel tanks for the aircraft as well as providing the main location for stores pylons. By removing the wing the engine can be lifted out of the aircraft - the wing being fixed to the fuselage by a number of bolts allowing it to be quickly removed.

The fuselage of the Harrier aft of the cockpit section is dominated by the need to house the Pegasus engine. This is the one area that has changed least over the life of the Harrier, although in detail it has been constantly refined. The most obvious example of refinement are the twin lateral air intakes. Mounted just aft of the cockpit, these have to provide air to the engine with minimum distortion, whether flying backwards at 50 knots or diving at supersonic speed. This has led to a constant process of re-design, although the basic features of large diameter, short length and semi-circular section have remained constant.Aft of the intakes the Pegasus engine takes up most of the centre fuselage. The four exhaust nozzles are attached to the engine via four circular cutouts in the fuselage side. The fuselage cross section is basically a boat-like U shape in this area, with a large opening above the engine where the wing and engine access panels are attached.

Fore and aft of the engine are the lower-fuselage mounting points for the forward and main undercarriage units, with a ventral stores pylon and mounting points for gun pods or strakes between them. Fuel is carried in the intake walls, between the front and rear nozzle cut outs and aft of the rear (hot) engine ducts. Forward this rear fuel tank is the demineralised water tank.The rear fuselage houses an avionics bay with two lateral access doors. Aft of this is a bay housing electrical and conditioning equipment, while the front hinged airbrake is mounted underneath the avionics bay. The fin and tailplane assemblies are mounted at the rear of the fuselage, with the former having an S shaped leading edge with an aft mounted rudder. The all-moving tailplane features sharp anhedral, being mounted at the same level as the wing.

The undercarriage of the Harrier has always been one of its unique design features. Despite early attempts at a more conventional undercarriage for the P.1127, the only practicable method was the 'zero-track tricycle' (i.e. bicycle) with wing-mounted outriggers adopted. The geometry of the main units was dictated by the need to avoid interaction with the engine exhaust during jet-borne flight and to provide good ground handling. The location of the outriggers was originally intended to minimise the weight penalty they incurred, although on the Harrier II they have been moved inboard to reduce the width of the aircrafts track, easing ground and ship-based manoeuvring. From the P.1127 to the initial mark of Harrier the undercarriage underwent considerable refinement to make its handling qualities acceptable.

The single-wheel nose undercarriage unit not only supports a significant proportion of the aircraft's weight but also provides steering over a range of 45 degrees to port or starboard. It is free to castor through 179 degrees in either direction for towing. On retraction the unit's leg shortens to minimise stowage volume. The main undercarriage unit has twin wheels and is fitted with powerful brakes, retracting aft when the aircraft is airborne. The main unit leg has considerable 'give' on contact with the ground, such as to allow both outrigger wheels to achieve positive contact, although the greater part of the aircraft's weight is borne by the main unit.Each outrigger has a castoring wheel (although these were locked in the early 1980's after the loss of the tyres on several occasions), which is left exposed after the unit has retracted.

The Harrier has two integrated flying control systems - one for wing-borne flight and one for jet-borne flight- with only one conventional set of cockpit controls.For wing-borne flight the Harrier uses conventional aerodynamic control surfaces, with the ailerons on the outer wings and the all-moving slab tailplane being driven by hydraulic jacks. The rudder is manual in first generation aircraft and powered in the Harrier II. The surfaces are linked to the pilot's control stick and rudder pedals by a system of rods and cables - the latter being used to reduce weight. As the rudder was unpowered on earlier generation aircraft simple auto-stabilisation was provided for pitch and roll only. The Harrier II features a comprehensive automatic flight control system, with stability augmentation active in both jet-borne and conventional flight.

To cater for jet-borne flight, where the aerodynamic forces on the conventional surfaces are reduced or eliminated, a system of air jet reaction control valves are utilised. These are placed in the extreme nose, tail and at the wingtips to provide pitch, roll and yaw control. The system uses air bled from the high-pressure compressor of the engine and the valves are opened using pilot commands from his normal controls. Indeed, the valves at the wingtips and in the tail are directly linked to the aileron, tailplane and rudder so that when each of these surfaces moves its corresponding valve also opens. This occurs during both wing and jet-borne flight, but as the engine bleed is only operative when the main engine nozzles are vectored below 20 degrees no jet reaction force is produced unless the aircraft is partially jet-borne. The interlinking of the aerodynamic and reaction controls, allied to the progressive increase in the amount of air bled from the engine with increasing nozzle vectoring above 20 degrees, ensures that the aircraft is fully controllable at all airspeeds and during transition.

The key to the Harrier's unique abilities lies in its Pegasus engine. Like the airframe, this has developed considerably since it first ran in 1959, but the fundamentals have remained unchanged.Air enters the engine via the two intakes and first passes through the low-pressure compressor. Upon exiting the LP compressor around 58% percent of the airflow enters a plenum chamber. On either side of this chamber are the two forward vectoring nozzles through which this cold (100 C) air is expelled to provide thrust. The remaining 42% of the airflow passes from the LP compressor to the high-pressure compressor.

On leaving the HP compressor it enters the combustion chambers, is heated by burning fuel in the air stream and then passes over the HP and LP turbines, which drive their respective compressors. Once the heated air leaves the turbines it passes into a bifurcated duct which has a further pair of lateral vectoring nozzles. These nozzles allow the hot (650 C) air to exit the aircraft and balance the thrust from the forward nozzles, the two sets of nozzles being set about the aircraft's centre of gravity. In order to eliminate gyroscopic precessional effects when manoeuvering in the hover, the LP and HP spools of the engine contra-rotate, their respective gyroscopic forces cancelling each other out.From this brief description it can be seen that the Pegasus is essentially a conventional turbofan engine. The only exceptions are the four nozzles that are required to vector the engine's thrust. In fact it is the control of these nozzles that represents the Harrier's only marked departure from a conventional aircraft.

In the cockpit, next to the throttle, the pilot is provided with an additional lever that controls the angle of the nozzles and therefore the amount of jet lift imparted. By the judicious selection of throttle and nozzle angle it is possible to fly the aircraft from 50 knots backwards to 600+ knots forward, including many low speeds where the aircraft is supported on a mixture of jet and wing lift.It is important that all four nozzles move at the same time to ensure the stability of the aircraft. To this end they are linked by a system of shafts and chains that are driven by an air motor using air bled from the engine. The engine also provides power for the electrical, hydraulic and conditioning systems via a number of generators, pumps and air bleeds.

Their main advantage is their V/STOL (Vertical and/or Short Take-Off and Landing) capability which allows the Harrier to fly from short rough-field strips near to the front-line to provide rapid close air support (CAS) and hence the ability to operate away from conventional paved runways and airfields which may be attacked preventing conventional non V/STOL aircraft from operating.Other advantages of it's V/STOL capabilities are the ability when in flight to vector the engine nozzles to perform movements without changes in orientation or profile of the aircraft which can confuse an opponent during ACM (air combat manoeuvring). This also allows the pilot to reverse the engines causing a rapid deceleration causing a pursuing aircraft to overshoot the Harrier, giving the Harrier pilot the chance to take the initiative during ACM.

Highly skilled pilots are required to pilot the Harrier. There is a high maintenance requirement. They are relatively slow by fast jet standards and have a short range and payload.