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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 most critical area of concern has always been
weight. Despite the continuous growth in the thrust of the Pegasus
engine, no member of the Harrier family has enjoyed an excess
of thrust. It is a happy coincidence that many of the design requirements
of a ground attack fighter have mirrored the need for weight minimisation,
such as the small wing and single-seat cockpit.
Click on the image below, or one of the links
at right, to learn more about that area of the Harrier.
Cockpit
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.
However, these gains have come at the cost of greater weight and
drag in both cases.
Wing
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. The modest aspect
ratio of the wing was a result of aiming to optimise the wing
for low-level cruise, where aspect ratio is much less relevant
compared to high altitude - the increase on the Harrier II partly
reflected the medium level missions flown by the US Marines.
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.
Fuselage
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 cutouts 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 outer sections of the tailplane interact with
the local air flow to provide positive longitudinal stability.
The inner sections are washed by airflow strongly influenced by
the engine exhaust, so their angle of attack varies little with
air speed and aircraft angle of attack.
Undercarriage
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.
All the undercarriage wheels have low-pressure
tyres to facilitate dispersed site operations from grass and other
surfaces. With the undercarriage locked down the main doors are
closed to reduce the risk of foreign objects entering the undercarriage
bays.
Flying Controls
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.
Propulsion
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.
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