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History
John Farley's Lecture

John Farley first flew the P.1127 in 1964 while a test pilot at the Royal Aircraft Establishment. He spent 19 years contributing to the development of the Harrier, retiring as Chief Test Pilot BAe Dunsfold. He then spent five years as Manager of Dunsfold and a further two as Special Operations Manager at BAe Kingston. In 1990 he became the first Western test pilot to fly the MiG-29 fighter. He is currently part of the Farnborough Aircraft team developing the the F1 air taxi.

This lecture was delivered to the Munich Branch of the Royal Aeronautical Society in May 2000. It is reproduced here by kind permission of John Farley.

The Harrier Development Story - by John Farley OBE AFC CEng

From the Hawker P1127 in 1960, the Harrier family of single engined vectored thrust military aircraft has developed to the point where, today, the aircraft is in service with six nations and seven air arms. Over thirty distinct versions have seen service during this 40-year period. The Harrier has allowed the Royal Air Force to land swept wing jet fighters vertically for 31 of its 82 years.  Over a third of its history.  Such freedom from the problems inherent in high-speed landings of tactically deployed aircraft represents a major achievement. I believe this long-term success has been due to three things:

  • The basic V/STOL concept employed was innovative and had no flaws.
  • The V/STOL elements have been continually improved by sound development.
  • The operational equipment fit has kept up to date and has often led the way.

I will take a few highlights from this last point first, because Harriers are built to fight, and not just to do V/STOL. Operational military competitiveness has been vital for the RAF, Royal Navy and USMC units who have fought wars with their Harriers.

When the RAF formed its first Harrier GR1 unit in April 1969 (fig 1), it was their first aircraft with:

  • A self contained inertial navigation system (INS).
  • A moving map display that showed the present position of the aircraft.
  • A head up display (HUD) of flight, navigation and weapon aiming information.
  • A totally self contained starter running off the main engine fuel system (GTS).

Fig 1 Harrier GR1

Fig 1 Harrier GR.1

Fig 2  Harrier GR3

Fig. 2 Harrier GR.3
In the mid seventies the RAF Harrier fleet was updated to GR3 standard (fig 2) which included a new nose having a laser range-finder and marked target seeker that greatly helped single pass attack capability and accuracy.

Jumping ahead to today, the Royal Navy single seat Sea Harrier FA2 (fig 3) was the first European aircraft capable of using the beyond visual range US AMRAAM missile. This capability at once made it the best interceptor with any European air arm.

Fig 3 Sea Harrier FA2

Fig 3 Sea Harrier F\A.2

Fig 4 Harrier GR7s with T10

Fig. 4 Harrier GR.7s and T.10
Today’s RAF single seat GR7 / T10 fleet (fig. 4) is operational at low level at night in the close air support role using the latest forward looking infra red HUD and night vision goggle equipment. This is a role that many said was impossible only a few years ago. Such military headlines are a topic in themselves and form a very important part of the Harrier story. But for this paper I will concentrate on the V/STOL technology incorporated in these aircraft.

The vectored thrust story

The vectored thrust story started in the mid 1950s when a Frenchman, Michel Wibault, proposed a single seat fighter that he called the Gyroptere. Wibault proposed to vector the thrust of four separate centrifugal blowers driven by a single 8000 HP Bristol Orion engine (fig 5).

He chose the Bristol engine because it was then the most powerful turbo-shaft engine in prospect. He was unable to interest the French authorities with this idea, but in 1956 he left a brochure with the US officer running the Paris office of the Mutual Weapons Development Programme Colonel Chapman USAF.

Fig 5 Origins of thrust vectoring

Fig 5 Origins of thrust vectoring

Chapman was working on the Orpheus engine with Bristol Aero-Engines' Technical Director Stanley Hooker, so he naturally turned to Hooker for advice on Wibault's brochure. At Chapman's request, Hooker directed a study on Wibault's idea that led to an all new Bristol engine, the Pegasus, having four rotating nozzles. By 1959 the Pegasus 1 was running on a Bristol test-bed and Hawkers were making the P1127 airframe.

The starting point

Fig. 6 Powerplant installation

Fig. 6 Powerplant installation
In October 1960 Hawkers were ready to fly the first P1127. It was decided to look at the unknowns of hovering before flying the aircraft conventionally. The engine, by now the Pegasus 2 of 11,000lb thrust, was installed as shown in fig.6. The main characteristics were a large bifurcated intake and four swivelling exhaust nozzles interconnected together which enabled the thrust vector to be moved, as required, from aft to about 18 degrees forward of the vertical.

The LP fan (fig.7) had two stages and was fitted with inlet guide vanes. It supplied the front nozzle efflux and the eight stage HP compressor. A most important feature was that the HP and LP components of the engine contra rotated to remove gyroscopic couples.

A motor under the engine, (fig.8) driven by compressed air from the final HP stage, was used to rotate the four exhaust nozzles through a simple and reliable mechanical system of shaft and chain drives.

This air motor was signalled mechanically by a slim lever mounted alongside the outboard throttle (fig.9). The throttle was a conventional fighter type, push forward for more thrust. The sense of the nozzle lever was similar - push forward for more speed. So lever forward selected the nozzles aft and lever back rotated them down.

 

Fig 7 Engine gas flow

Fig 7 Engine gas flow

Fig 8 Nozzle actuation system

Fig. 8 Nozzle actuation system

Fig 9 Nozzle selector lever

Fig. 9 Nozzle selector lever

Fig 10 Initial reaction control system

Fig. 10 Initial reaction control system

Attitude control in the hover was by a jet reaction control system (fig.10). With the nozzles deflected, high-pressure air was continually bled from the engine at the rate of 12lb/sec to supply roll and pitch nozzles at the nose, tail and wing tips.

Pitch control was obtained by increasing the aperture of one of the pitch valves and reducing the size of the other - the total lift force on the aircraft remaining constant. Roll control used differential control of the roll jets. Yaw control came from swinging the pitch jets laterally. The control valves were operated by the stick and rudder pedals in the conventional sense. A 20% authority autostabiliser was fitted, giving artificial damping and stiffness in pitch and roll. The ailerons and tailplane were powered the rudder was not.

The airframe was clearly representative of a typical fighter aircraft (fig.11). It had been designed as such from the start and not as a pure research vehicle. The undercarriage, however, was unusual. The layout was determined by the presence of the four engine exhausts under the high wing, especially the two hot rear nozzles. It was considered that although a wide track tricycle could have been employed it would have incurred a greater structural weight penalty than the bicycle with the outriggers. You are warned that it took from 1960 to 1967 to completely eliminate the adverse handling and performance of this undercarriage during take-off and landing.

Fig 11 P1127

Fig 11 P1127

First hovers

For the initial hovering trials the aircraft was fitted with tethers and was operated from a 40 x 50 ft grid platform fitted flush with the ground surface above a pit used to duct away exhaust gases. The tethers had a certain nominal length. When this was used up an ever-increasing number of metal disc weights was lifted up, so smoothly increasing the restraining force on the aircraft. The first lift was carried out with 1 ft. tethers and the aircraft was restrained by the tethers most of the time. For the next lift the tethers were increased to 4 ft.

Fig 12 P1127 on grid platform

Fig. 12 P1127 on grid platform

Immediately the first handling problem due to the undercarriage came to light. As the aircraft lifted, the main undercarriage leg extended more than the outriggers until lateral freedom was available, (fig.12), then the aircraft banked and the resulting horizontal component of lift force moved the aircraft sideways and swung it round in yaw, about either the nose or main wheel.

Clearly this loss of control in roll and yaw could have been fixed with greater control powers, but bleed air was at a premium as its provision reduced engine performance. So it was essential to use the absolute minimum possible. This is a basic fact of life with jet V/STOL and must be continually borne in mind. Additionally, the horizontal component of a slightly inclined vertical force gets significant very quickly if that force is equal to the weight of the aircraft. Only six degrees of tilt is needed to achieve 0.1g horizontal acceleration for example.

After a second attempt with 4 ft tethers, during which the tethers broke a few yards from where the design executives were standing, the tethers were rapidly doubled in strength, halved in length and the outriggers were propped up on platforms. The thing could then only tilt a little, but it was still helpless after the first landing since it then sat wing low again and could not be levelled by the pilot.

Temporary extensions were therefore fixed to the outriggers so that they supported the aircraft level until just after lift-off. This action greatly improved the handling and two successful hovers, one with 2 ft and one with 4 ft tethers, followed.

A second pilot then joined the programme. Investigation of autostabiliser behaviour at this point showed that it was saturated most of the time so some lifts were made with the autostabilisers off, the results being rather inconclusive within the tethers.

Initial taxying trials were then started. Directional control was by nosewheel steering and only the main wheels had brakes.

The undercarriage, with by now well established tradition, fought back hard when taxying, this time through the medium of the nosewheel steering mechanism. This proved to have a large dead-band around centre and was harsh in operation. Once the dead-band was overcome, the sensitivity was then far too great. Thus a generally unsatisfactory weaving progress was made between A and B.

Following some small changes to the autostabiliser, intended to correct a drift in the stiffness term, the aircraft was returned to hovering. The final hovers in this series included a tethered lift over a solid surface, some more tethered lifts over the grid and then finally two free lifts over the grid. This programme took five weeks and included 21 hovering sorties. The engine had many limitations, in particular RPM bands where steady running was prohibited and a total hover life of 35 minutes. It was marginal on thrust, resulting in the ability to carry only about three minutes fuel and a need to strip from the airframe about 700 lb of electrical equipment, cabin conditioning, instruments and undercarriage components in order to get it into the hover. However it was a start.

First conventional flight

The aircraft was then prepared for more taxying tests and its initial conventional flying. The nosewheel steering was still unsatisfactory, a brake induced torsional oscillation developed and broke a main leg. The castoring outriggers showed a tendency to shimmy and finally had to be locked for flight. The aircraft banked badly when taxying especially in a cross wind. The first conventional flight took place in March 1961 and was followed by ten more flights.

These took the aircraft (fig 13) to 400 kt, 0.8M, 30,000ft and 4.7 g. It was concluded that longitudinally the aircraft was unstable and suffered from pitch up. Laterally it was good and directionally was satisfactory, except with the undercarriage down when there was slight instability. The flap and airbrake trim changes were too great. There was a large nose down trim change in ground effect so full back stick was needed to land.

 

Fig 13 P1127 3-view

Fig 13 P1127 3-view
The engine operated satisfactorily but only with many handling restrictions necessary to avoid fan blade resonance behind the very short intakes. Strip examination after the trials showed that an HP turbine blade had been shed at some time. The fuel and hydraulic systems worked well, the cabin conditioning discharged water at times. The nosewheel steering system was barely useable, the brake judder was severe, the anti-skid often failed and the pilot could not feel that the wheels were skidding at any time. Several tyres were burst. Thus encouraged the team prepared the aircraft for further taxying and hovering trials.

Hovering changes

For the next hovering trials the aircraft was fitted with the 12,000 lb thrust Pegasus 3 engine and the reaction controls were modified to give a variable bleed of 9 - 15 lb/sec depending on control deflection. This was in an attempt to achieve greater max control powers and at the same time to reduce engine thrust losses due to control demands. 51 hovering sorties were carried out for a total of 61 minutes using two engines between mid May and mid June 1961.

The aircraft was hovered at heights up to 100 ft and translated forward by changing its attitude at speeds up to 50 kt. Numerous landings were made on concrete and tarmac, although roll control was inadequate for dealing with jet induced disturbances close to the ground. Previous operations over the grid did not allow ground effects to be sampled as the exhaust gases passed through the ground plane and did not rebound off it.

A major characteristic of the P1127/Kestrel/Harrier family was established at this stage. This was that at low speeds the aircraft was directionally unstable. This instability resulted from the high intake momentum drag of the large engine acting ahead of the CG, the fin of course had only a weak stabilising effect at low speeds.

Due to the change to partially variable bleed the yaw reaction control power was reduced since it was obtained by swivelling the pitch jets, but they were now worth only 7 - 10 lb/sec. This yaw control power was insufficient to overpower the directional instability in cross winds of 10 kt. (Or if you prefer, the aircraft turned backwards uncontrollably if it was translated sideways in the hover at 10 kt)

Following an increase in the roll reaction control gearing it was found possible to dispense with the outrigger extensions although untidy lateral behaviour persisted. Satisfactory un-autostabilised hovering was also demonstrated. Free from tethers, height control could be looked at for the first time and it was found that a linkage giving low sensitivity at high RPM was best removed and the pilots were happier with a more sensitive throttle. A hand rest was incorporated outboard of the throttle to enable more precise hand movements. The typical thrust weight ratios flown were 1.07 to 1.

The aircraft was easiest to hover well out of ground effects at some 50 - 100 ft. Below 20 ft ground effects increased rapidly until below about 5 ft control was not really adequate and full control was often used.

By now a great deal had been learned about the hovering business. The pilots were asking for a better longitudinal feel with less friction and force feedback from the front reaction control that was connected directly to the stick. They also wanted the datum shift trimming system deleted and full authority over tailplane travel to be available to the stick at all times regardless of the trim setting.

The vital nose wheel steering system was proving unreliable as well as deficient in handling. The aircraft still needed 500 lb of its full compliment of bits removed for these trials. Importantly for the future, experience had been obtained of the strong lateral out of trim that could be obtained with sideslip at even modest forward speeds. When combined with the directional instability this was to become a very significant handling characteristic of the P1127/Kestrel/Harrier family.

Second aircraft

In July 1961 a second aircraft joined the programme. It expanded the conventional envelope to 40,000 ft, 538 kt, 1.02 M and 6 g. It crashed on its 35th flight when one of the Pegasus cold nozzles fell off in conventional flight. Prior to this the aircraft had started the speed reduction programme from conventional speeds and reached a minimum speed of 95 kt by deflecting its nozzles and flying partially jetborne. The speed reduction programme was stopped at this speed, as by then the first aircraft had reached the same IAS, starting from the hover and was thus cleared to continue with full transitions.

Flight to high mach was temporarily held up at 0.89 M to 0.90 M due to a bad left wing drop, which happened very suddenly. The fix for this was a row of 13 vortex generators on the wing. However despite these VG's the improvement was limited and the following list of major conventional flight problems was noted:

  • Transonic wing drop, allied with poor useable lift coefficient characteristics.
  • Longitudinal manoeuvring instability under most flight conditions, coupled with pitch up at high AOA.
  • Unacceptable directional behaviour with the undercarriage down.
  • Engine restricted running conditions as a result of intake conditions.

If these problems are examined it will be seen that they all resulted from the V/STOL capability of the design.

The wing had been designed as a delta to give the maximum structural strength for the vital minimum weight. It undoubtedly needed a swept trailing edge. The longitudinal instability was largely due to having to keep the tailplane above the jets, thus exposing it to the wing wake. The engine needed very large intakes, which had to be horribly short by normal standards if excessive volume was not to be occupied in a small aircraft. The reasons for the undercarriage layout have already been mentioned. A moment’s reflection on fig. 13 shows the constraints inevitable with mounting a Pegasus close to the CG. But the eventual success of the programme shows how right the designers were to face these problems and solve them.

Intake problems

The first aircraft had flown in the hover with a metal intake having a bulbous shape for low speed efficiency, but for the conventional flights this was replaced by one with a slimmer lip. The second aircraft had this slimmer version. Since the sharp leading edged shape produced an unacceptable thrust loss at low speed and the other too much drag at high speed, it was intended that the aircraft would be fitted with variable geometry intakes. These took the form of rubber bags on the intakes, inflated for low speed and sucked down for high speed  (fig.14).

One flight was made to 335 kt with them sucked down, but at this speed they started to lift and flap. This behaviour persisted even after speed was reduced to 250 kt and so the search for a higher level of suck was on.

Fig 14 Inflatable air intake

Fig 14 Inflatable air intake

Undercarriage problems

The nose wheel steering system on the second aircraft incorporated a gear change system so that full rudder bar gave either +/- 3 deg of nosewheel steer, or +/- 30 deg. Like this it was better but still poor. The wheel brakes again suffered from V/STOL. They had a lower share than usual of the aircraft weight acting on their wheels, they were light in the interests of weight saving and yet had to cope with high levels of idle thrust from the big Pegasus engine.

Still on the undercarriage, directional handling on the runway in crosswinds was very poor due to the continual leaning tendency. The wing AOA in the normal ground attitude was about 8 deg - deliberately so in order to generate significant lift for short take-off's, as rotation cannot easily be achieved with a bicycle layout. Thus, when leaning during a landing run, the not inconsiderable lift force present was tilted, producing a side force which was only too willing to pull the aircraft straight off the runway should the pilot be imprudent enough to add a crosswind of 10 kt or so to his task.

On top of this when leaning heavily on one outrigger, only one main wheel tyre was properly on the ground so the other usually burst - they were not locked to the axle until later. Never mind, a measure of reverse thrust was available instead of relying on the brakes, so that was used instead.

Given that the nozzles only moved 18 degrees forward of the vertical, powered nozzle braking as it was termed, only worked at high RPM. This high RPM increased the total lift on the aircraft, so it rose up the oleos, obtained greater lateral freedom, developed more side force and left the runway even quicker. About this time the number of "See me about the undercarriage" notes sent by the pilots to the designers increased!

Successful escape

Finally, as mentioned earlier, this aircraft was lost when the left cold nozzle dropped off in wingborne flight and the aircraft crashed in the final stages of an emergency approach to land, after lateral control was abruptly lost at about 170 kt and 300 ft. The pilot ejected and was unhurt. The front nozzle material was then changed from fibreglass to steel regardless of the weight penalty.

First transitions

Fig 15 Modified reaction control system

Fig. 15 Modified reaction control system

Before the initial transitions were carried out, in September 1961, a further important change was made to the reaction control system, with the fitting of a fully variable bleed system (fig 15).

With controls central, all valves were now shut, reducing bleed demands on the engine, lowering jet pipe temperature, saving engine life and increasing installed thrust. One effect of this was that it was no longer possible to tilt the pitch jets in the nose and tail to yaw the aircraft - since they were no longer always open - thus separate yaw jets were fitted either side of the tail as shown. This improved yaw control raised the limit of control over the directional instability to lateral speeds in excess of 20 kt.

Transitions were a major milestone although in the event quite straightforward.

Following a vertical lift-off, the nozzle lever was slowly pushed forward until the nozzles were aft and the aircraft fully wingborne. If the nozzles were moved too fast the aircraft sank, if the nozzles were moved too slowly the aircraft climbed unnecessarily high. What could have been easier? There was the question of directional control and minimising sideslip but with the improved yaw control this was easier than, say, keeping a tail wheel propeller aircraft straight during its initial take-off ground roll.

Decelerating transitions were equally no problem although in this case it was found easier to lower the nozzles to 40 deg on the downwind leg and fly the aircraft in this configuration, using conventional techniques, until about 1000 yards from the intended point of hover. Here at 130 - 150 kt the nozzles were selected to the hovering position. With all horizontal thrust removed the speed quickly reduced due to drag and all that was required of the pilot was to open the throttle to hold height as the wing lift reduced.

With both types of transition it was found that AOA could be used as a height control at the higher speeds and there was great latitude in how the pilot handled the combination of wing lift and jet lift. The use of an AOA gauge was necessary to avoid any risk of stalling the wing. There was also tunnel evidence that above about 15 - 18 deg AOA, nozzles down, the aircraft would pitch up. Two double transitions were made without autostabilisation and it started to become clear that given adequate controls the pilot could be expected to cope unaided.

STOL flying

The final initial V/STOL step occurred before the end of 1961. This was the development of STOL techniques at weights in excess of hover weight. For a short take-off the aircraft accelerated, nozzles aft, to a speed calculated from its weight such that the wing lift would provide the deficit of thrust weight ratio, plus a margin to compensate for the fact the nozzles could not be lowered all the way to the vertical in order to keep enough horizontal component to maintain the speed already reached. At this speed, termed the nozzle down speed, the nozzles were selected down to the intermediate angle and the aircraft left the ground under the combined wing and jet lift. The pilot then had to complete the latter part of an accelerating transition. This STO technique proved entirely straightforward.

Slow landings were carried out by gradually deflecting the nozzles down and increasing the power until the point where there was only a small power reserve left and the AOA in use had reached the maximum safe AOA below the stall. The speed at this point was the minimum possible for the particular weight and thrust available. The nozzle lever was then left and the approach controlled on the throttle. Slow approaches tended to be a bit vague and imprecise. There were two possible controls of lift (wing and engine) and two of speed (RPM and nozzle angle) and only much later did the optimum handling technique become clear.

When partially jet borne the airflow round the wing and tail was much influenced by the four high velocity engine exhausts and the longitudinal handling suffered from instability. However, with the reaction controls bleed air supply main ON/OFF butterfly valve being linked to the initial nozzle down travel, the pilot had sufficient total control power, aerodynamic and reaction for adequate control.

Grass operation was started, with both VTOL and STOL operation being satisfactorily accomplished from a good quality grass surface. So at the end of 1961 the P1127 worked. It had its problems and there was a great deal to be improved but it only required engineering and not black magic to progress to the point where the vectored thrust aircraft could be seen to have some style.

P1127 to Kestrel

Six P1127 aircraft were built and took part in a programme that led to the Kestrel aircraft. The engine went from the Pegasus 3 at 13,500 lb thrust to the Pegasus 5 at 15,000 lb. From the Pegasus 3 onwards a third fan stage was used. Snubber blades were developed for the fan to further reduce fan blade vibrations with their attendant high levels of alternating stress. With this modification and a change from light alloy to titanium as the first stage fan blade material, the Pegasus 5 was the first version without restricted RPM / IAS bands.

 

Changes to the reaction controls continued in the quest for optimum sensitivity, and just before the Kestrel went into service the roll control power was given a further boost by the provision of up-blowing valves. With this reaction control system (fig.16) the need for the pitch and roll autostabiliser faded and the Kestrel was not fitted with one.

Slow approach handling was aided by the development of the fixed throttle technique instead of the fixed nozzle technique (or as that had usually turned out the fixed nothing technique) This principle was very simple. At the start of the approach, power was increased to about 5% below full throttle and the nozzles lowered to reduce speed. As speed and wing lift reduced, the flight path was maintained by using an ever-increasing AOA until the optimum AOA was reached. At this point the speed was noted and then maintained for the rest of the approach using the nozzle lever as the speed control.

Fig 16 Final reaction control system

Fig 16 Final reaction control system

This technique had the advantage that there was a known reserve of thrust (5% RPM) for use in the event of a late go-around being needed. This use of the nozzle lever as a speed control was excellent, as there was no lag since the engine did not need to accelerate.

But, for a single seat combat pilot, the outstanding virtue of this slow landing technique was that the pilot did not need to know his aircraft weight or the ambient pressure and temperature to decide his slowest possible approach speed. He just set 95% of max RPM, slowed down until optimum AOA and then held that speed.

The only other V/STOL matter that needed to be resolved before the Tripartite Kestrel Squadron standard of aircraft was reached was the intake. After a very great struggle the rubber bags were kept sucked down up to 0.75 M but only after a conventional take-off. It was not possible to inflate them, do a VTO, accelerate and suck them down adequately. At a very late stage a compromise metal lip intake was produced that did not suffer from a significant thrust loss at the hover and at the same time was satisfactory for conventional flight needs. With this the V/STOL Kestrel had arrived.

In parallel with this V/STOL programme conventional flight was progressing. Satisfactory transonic handling in the end needed a completely new wing. This had a different section, leading and trailing edge shapes and also different tips and vortex generators.

Fig 17 Kestrel 3-view

Fig. 17 Kestrel 3-view

The aircraft now looked like fig. 17. It now had a swept trailing edge. Longitudinal stability required the tailplane tips to be extended. The fuselage had to be made longer and the CG changed. It will be appreciated that this was a major redesign. The fin had 2 sq ft added to its tip to correct the undercarriage down instability in conventional flight.

The wings were given two hard points so that it was possible to carry 2 x 100 gallon fuel tanks or rocket pods although in the event the rockets were never fired. A small gun sight was fitted to allow the Tripartite Squadron to evaluate tracking. The nose wheel steering was re-engineered to remove backlash and a selector button was put on the stick which gave +/- 5 degrees of nosewheel at full rudder pedal with a second button on the throttle which gave +/- 35 degrees. The brakes were improved and changed from hand to toe operation.

The engine development continued with two mechanical engine failures in conventional flight. Both aircraft were force landed but one was written off with a fire. The other was rebuilt. The first engine failure was due to high 'g' distorting the casing and allowing the 8 th stage rotors to touch the stators. This problem was engineered out but the second case was more difficult. Here the engine surged at high mach (about 1.2 M) due to flow distortion from intake shocks. The surge was sufficiently violent to again tangle up stators and rotors. Improved clearances in the engine were arranged to prevent mechanical damage recurring but the only way of preventing engine surge at supersonic speeds was to limit RPM to 80% in transonic dives. The Kestrel went into service with this limitation. It did not, however, have any airframe IAS or Mach limits but could be dived vertically to its terminal velocity. To avoid high N / ? surges the engine had to be throttled back at altitude. A surge limit gauge was fitted indicating the maximum allowable fan RPM.

The Kestrel Tripartite Squadron (US, West German and UK ground and air crews) was equipped with nine aircraft and commenced operations in October 1964.

Kestrel to Harrier

 

Then in the spring of 1965 a new contract was obtained, for six development batch aircraft, of a type that was to be known as Harrier. This step, to the Harrier was many times greater than the change from P1127 to Kestrel. It was literally a new aircraft from an engineering point of view retaining only the same basic configuration, but that was fixed by the Pegasus engine (fig. 18).

The Harrier aircraft went into service as an operational type four years later in April 1969. Those four years saw much development before the airframe and engine design teams achieved their objectives. It will be appreciated from just the subject headings that follow that a great deal was involved.

Fig 18 Harrier GR1 3-view

Fig 18 Harrier GR1 3-view

Engine progress

Fig 19 Auxilary air intake doors

Fig. 19 Auxilary air intake doors

The intake design was changed twice in the pursuit of better specific air range in the cruise allied with good high altitude handling, without prejudice to the hovering performance. First 12 and then 16 auxiliary intake doors (fig.19) were fitted round the intake lip and also various boundary bleed door configurations were flown. The reaction controls were refined and to add a bit of polish to the V/STOL handling a new low authority pitch and roll autostabiliser was developed.

Undercarriage improvements

Then came the breakthrough - the undercarriage was finally sorted out. The nosewheel steering was freshly engineered, and this time by means of a cunning slotted link and slipping clutch mechanism a vernier range of travel about the centre rudder bar position gave excellent fine steering, without spoiling the minimum turning radius capability of full rudder. The main wheels were locked to the axle and the anti-skid system improved. The nosewheel tyre was changed improving its floatation on boggy ground.

Then finally the most important fix of all, the main leg was given a two-stage oleo. On touchdown this "self-shortening leg” gives no rebound from the first seven inches of travel. This means that at all weights the aircraft rests firmly on its outriggers and does not gain outrigger freedom until lift exceeds weight. Cross wind handling on the ground with this leg became the same as into wind handling with the aircraft running dead straight and wings level.

Wing development

Conventional flight problems, other than the engine ones already mentioned, centred on the wing development. It was originally deficient in useable lift coefficient at high mach number. The specification required 6 g at 16,800 lb, 400 kt IAS and 10,000 ft. This represented a lifting capability from the available wing area that was greater than any previous comparable military aircraft. Initially the aircraft was short of 1 g. The use of a combat flap position gave an extra 1/2 g. The other 1/2 g could be achieved by deflecting the thrust but the firm's pilots were against this expedient since, although it demonstrated the requirement, they were only too well aware that as well as getting the increment of 'g', one also got the effect of the world's finest airbrake as the rearwards thrust was lost. In the end a very considerable wind tunnel and flight programme of different types of leading edge and vortex generator modifications resulted in the 1/2 'g' required, allied to good handling and without the need to vector the thrust.

Kestrel longitudinal stability was satisfactory with a clean aircraft, but deteriorated with under wing tanks. Since the Harrier would need to always carry a wide variety of external weapons, further improvements were necessary to counter de-stabilising stores, so the Harrier CG was moved yet further forward. Handling was then satisfactory with the full range of stores.

The foregoing has been an outline account of the nine years that turned the P1127 flying machine into the first service version of the Harrier. It ignores the vast effort that was going on at the same time regarding the weapons system which was itself a very advanced programme for those days. Since then the later and much improved members of the Harrier family have shown what more can be done if you start with the right concept.

The later years

During the early 1970's a package was developed to help the pilot deal with the issue of sideslip during mid transition where intake momentum drag makes the aircraft directionally unstable. An autostabiliser working through the yaw reaction controls was produced to reduce this instability. Additionally, an indication was provided in the head up display (HUD) of a safe limit of sideforce, allowing the pilot to limit any lateral out of trim that might otherwise result in a loss of lateral control. Finally, in case the pilot was distracted from noticing the HUD warning, rudder pedal shakers were fitted which operated as the HUD sideforce limit was reached. The system shook the pedal that the pilot needed to use to reduce the sideforce, not only reminding him to use his feet, but also indicating which foot.

 

In 1977 ski jump trials were carried out, initially from a land-based ramp that was adjustable from a 6 to 20 degree exit angle (fig.20). This showed great improvements were possible with regard to performance, handing, safety and ship pitch motion limits when compared to flat deck ship STO's. It is interesting that the quite remarkable advantages of a ski jump deck have recently allowed the Russian Navy 's first conventional aircraft carrier to be built without catapults.

Fig 20 RAE Bedford adjustable ramp

Fig 20 RAE Bedford adjustable ramp

Fig 21 Sea Harrier FRS Mk1

Fig. 21 Sea Harrier FRS Mk1

In 1978 the first Sea Harrier (fig.21) was flown. It had a radar-based weapons system and a new raised cockpit for better air combat visibility. Many other changes were made to this aircraft to optimise it for shipboard operation.

The hover and VL handling was improved by yet another lateral control sensitivity increase to aid recovery to ships in rough water. The throttle box also included an ability to adjust the nozzle angle, +/- 10 degrees about the position set by the nozzle lever, using the airbrake selector. This enabled fore and aft position adjustments alongside a ship to be made without taking the left hand away from the throttle height control or changing the aircraft’s pitch attitude.

In 1979 the first of the Harrier II prototypes were tested. This branch of the Harrier tree uses a graphite epoxy McDonnell Douglas designed wing of 230 sq ft area compared to the metal Harrier I wing of 200 sq ft. This new wing effectively doubled the payload radius of action from the same length of STO run.

Harrier II VTO performance was also improved by developing larger under fuselage strakes and adding a cross dam behind the nose leg to contain the positive ground cushion that results during VTO from ground reflected jet impingement on the bottom of the centre fuselage. Harrier II handling was improved by a new three axes autostabilisation system that operated throughout the flight envelope. Harrier II plus is the designation of the latest radar based version now in service with the Spanish Navy.

Various uprated engines are used by Harrier II operators, all using zero scarf front nozzles some having a thrust as high as 24,000 lb.

Important past choices

In 1964, when I started flying the first P1127 prototype I was asked by the RAE scientists to compare it with the more complex lift and cruise concept of the Short SC1 research aircraft that I also flew. The SC1 was a small delta powered by five Rolls-Royce RB108 engines. Four were used for lift and one for propulsion.

This aircraft was easier to fly in the hover than the P1127 because the intake momentum drag of the lift engines acted downwards through the centre of gravity and so produced no tendency for the aircraft to yaw out of wind. So the fin had no destabilising force to overcome and was able to keep the aircraft into wind.

However although easier to fly in the hover the SC1 was a nightmare to operate.

It was literally a five engine aircraft being operated by a crew of one. In those days there was no automation so the pilot had five of most engine controls and instruments. On top of this was the need to shut the lift engines down after take-off and even more importantly restart each one before landing. Because of the high fuel consumption with the lift engines at flight idle, this complicated light up had to be left until the aircraft was on the final approach a mile or two from touchdown.

So, despite the cruise implications of having an engine and intake compromised by the need for high thrust at low speed, the P1127 simplicity and inherent reliability was preferred to the multiple lift engine concept.

The future

But what of the future? Given the march of technology, the availability of reliable automation and fly by wire, together with the benefits of not having your exhaust coming out around the centre of the aircraft, the argument for simplicity at all costs may become less overwhelming. For some roles, especially supersonic ones, the advantages of more complex multi - mode powerplants or lift and lift cruise concepts may well provide a better option.

But with the benefit of hindsight the Pegasus/Harrier combination was clearly the correct way to launch jet V/STOL. Of that I have no doubt.

  J F Farley    2 May 2000

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