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.
*
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.