by Colin Downes
During the Second World War the German Luftwaffe continually increased the weight of fire of their fighters in confronting the Allied bombers to increase the kill probability. The German fighter armament progressed from the early armament of 7 mm and 13 mm machine-guns to 20 mm and eventually 30 mm cannon. Luftwaffe studies showed that although twenty hits from a 20 mm cannon was sufficient to bring down a four-engine bomber, to achieve a 50 per cent kill probability required at least forty hits with the 30 mm shells fired from 500 yards range. With double this number of 30 mm strikes from the same range the probability kill factor rose against the defensive B-17 and B-24 USAF day bombers to 95 per cent. As experienced during the Korean War the 12.7 mm Browning although very accurate and effective at the lower level altitudes during air combat, was very ineffective at high altitudes as its weight of fire was insufficient to inflict the massive structural damage necessary to destroy a high flying and high speed bomber, or even a fighter. Unfortunately, in the early Hunter firing trials the muzzle blast from the four 30 mm cannon blanked the engine intakes at the wing root causing engine surges as the gun gases were drawn into the engine. This engine surging could occur at any altitude and speed, and it often resulted in an engine flame-out with the Avon engine. Rolls Royce modifications incorporated a fuel-dipping switch in the electrical firing circuit thereby reducing fuel flow to the engine. The recoil of the four cannon together with a loss of thrust from the engine resulted in a loss of speed and at the same time the aircraft experienced a marked nose-down pitch making gun sighting unsteady and erratic with a spray pattern from the cannon. Finally, the cannon shell links that were jettisoned in flight created the danger of the steel links being ingested by the engine. This state of affairs was hardly encouraging when considering that the firing range and the duration of fire was critical against the defensively armed Soviet bombers, or combat against the Mig-15 and the improved Mig-17. The nose-down pitch change of the Hunter while firing the cannon was countered by the installation of downward directed muzzle deflectors at the gun ports. The shell link hazard was corrected by the addition of two blisters on the nose of the aircraft to collect the expended links from the cannon. Both of these cannon firing modifications involved the penalty of a small increase in airframe drag.
The Hunter Mk 2 with the Armstrong Siddeley Sapphire engine did not suffer from the problem of compressibility stalling of the turbine as was the case with the Avon engine in the Hunter Mk 1. The superiority of the Sapphire compressor over the Avon compressor was due to its heritage from the Metrovick Beryl engine used in the Saunders-Roe flying-boat fighter, the first aircraft to fly with an axial-flow designed compressor based on the design of industrial gas turbines by Metropolitan-Vickers (Metrovick). The advantage of the axial-flow compressor over the centrifugal compressor as used in the earlier Rolls Royce Derwent and de Havilland Goblin and Ghost engines was a smaller overall diameter leading to more airframe design flexibility with the potential for greater power. As a result of the deficiencies in the design of the Avon compressor the Hunter production stalled. The Air Ministry with pressure from the government forced the cooperation of the two engine manufacturers in the national interest. Rolls Royce, with a little help from Armstrong Siddeley, was able to sort out their turbine design problems and produce the Avon Series 200 engine giving 10,150 lb. s.t. This engine entered service in the Hunter Mk 6 during 1956 and with this engine the Hunter came of age, becoming a rugged, reliable and much loved aircraft. With a newly designed engine giving a 30 per cent increase in thrust, the rate of climb improved dramatically over the Mk 1 and Mk 4 Avon engine Hunters reducing the time to 45,000 feet of 13 minutes down to 7 minutes, a comparative rate of climb to that of the Mk 2 and Mk 5 Sapphire engine Hunters. The maximum speed of the Hunter Mk 6 increased to Mach 0.95 and the service ceiling rose to 51,000 feet. An additional improved feature with the Series 200 Avon engine over the Series 100 engine was with the engine starting system. Initially, when starting the engine, the compressor was turned by an explosive gas charge supplied by one of three cartridges fired from the cockpit as the fuel cocks opened. This system had proved unreliable and was replaced by a quicker and more reliable liquid-fuel system of isopropyl nitrate called Avpin. The Hunter production programme continued with the Rolls Royce Avon, and an equivalent Armstrong Siddeley Sapphire engine producing 10,500 lb.s.t. was, unfortunately, never used in the Hunter. This engine became the production engine for the twin-engine Gloster Javelin All Weather interceptor fighter.
A further operational limitation of the initial Hunter design was the lack of an efficient air brake. Combat between the F-86 and the Mig-15 in Korea showed that this was an essential requirement during jet fighter combat. Sidney Camm in designing the Hunter was resistant to any excrescence to detract from what he described as his ‘most beautiful aeroplane.’ Therefore, he assumed that the use of aircraft flap was sufficient to slow down the aircraft and this was true in so far as circuits and landings were concerned, but not while manoeuvring in aerial combat. Neville Duke, chief test pilot of Hawker Aircraft, was one of the RAF’s premier fighter aces during the war and as Hawker’s chief test pilot would become one of Britain’s premier test pilots. I knew him well from his time as CO of 615 RAuxAF Squadron at Biggin Hill, as well as the other test pilots at Hawker’s test site at Dunsfold who were all ex-RAF pilots. I had the greatest respect for Duke not only as an exceptional test pilot but also as an individual, and I knew he had flown the F-86 and studied Korean combat reports. I asked him while on a visit to Dunsfold when flying the Hunter at the Central Fighter Establishment, how it was that Hawker Aircraft finished up with such an afterthought of an air brake attached beneath the rear fuselage instead of it being incorporated within the airframe. The air brake modification not only spoiled the clean lines of the aircraft but resulted in a less efficient air brake with more airframe drag. Duke shrugged, commenting that Sir Sidney Camm was a great aircraft designer but not the easiest man in the world to work with where his aircraft were concerned. The use of the flaps on the Hunter other than for take-off and landing had no operational function due to airspeed limitations and excessive nose-down trim. The air brake modification although visually unsightly and incorporating some airframe drag, nevertheless, was fairly effective in the air although not wholly satisfactory. Furthermore, it had to be retracted for landing otherwise in a tail down attitude at touch-down the 67 degree deflection of the air brake could strike the ground.
In producing the Hunter Mk 4 with the Avon 107 and the Hunter Mk 5 with the Sapphire 101, in addition to the modifications resulting from the firing trials, the aircraft’s handling problems were partially improved but were not fully addressed until the arrival of the Hunter Mk 6. Although the Sapphire engine Hunter 5 was a better performing aircraft than the Hunter Mk 4, the Sapphire was not entirely without problems associated with the compressor, as there were instances of lost turbine blades during flight. Therefore, after the great expectations for the new RAF interceptor the realization for one who had flown the F-86 operationally and in combat was one of great disappointment, with some relief that we did not go to war with the aircraft. As it happened, it was the comparatively more effective Hunter Mk 5 that saw the only operational service of the early Hunters while escorting Canberra and Valiant bombers during the Suez fiasco in 1956, although without resorting to the use of their armament.
In reading Neville Duke’s interesting biography ‘Test Pilot’, nowhere is there mention of the many problems associated with the early development of the Hunter. This is disappointing from the pilot best qualified to comment, and it may be a deliberate omission by the author and his collaborator. The impression given in the book is that the Hunter was a superb interceptor and combat fighter from its inception, but this was not so until the arrival in service of the Hunter Mk 6 in 1956. The Hunter Mk 6 had much improved flying controls and the ineffective longitudinal control at high altitude and high Mach number was corrected by fully powered elevators and ailerons with a
utomatic reversion to manual control in the event of power failure. Most importantly, a variable incidence tailplane followed the movement of the elevator. Although not as effective as the full flying tail on the F-86, it was a great improvement both in feel and control. Modified gearing in the standby manual control greatly improved the feel and control on the ailerons and reduced the very heavy loads when flying in manual control. The automatic reversion to manual relieved the need to ensure that the aileron controls were neutral before selecting manual otherwise the locking jacks could jam on the control rods with disastrous results. The rudder control remained manual with an electric trim tab and auto stabilizer. The air brake operated hydraulically either in the fully extended or retracted position. The undercarriage, flaps and wheel brakes were hydraulically controlled with a pneumatic system for the emergency selection of wheels and flaps, while the anti-skid Dunlop Maxaret brakes were supplied by two oil accumulators. A leading edge ‘saw tooth’ extension at the wing tips improved the handling at altitude and corrected the pitch-up of the aircraft under G conditions. The aircraft’s air-conditioning system was greatly improved with no more misting and icing up of the canopy that had restricted visibility at altitude. The short duration of flight of the Hunter was addressed in the Hunter Mk 6 by increasing the internal fuel capacity to 414 gallons by additional fuel tanks in the leading edge of the mainplane. In addition, two inboard 100 gallon drop tanks increased the total fuel capacity to 614 gallons giving flight duration of close to one hour. There was one interesting Hunter aircraft during the Hunter development that involved a racing version designated the Mk 3. This was a stripped down Mk 1 aircraft with a specially adapted windscreen and pointed nose, powered by an RR Avon 107 engine with reheat. During the summer of 1953 this aircraft, flown by Neville Duke, broke the airspeed record of 715 mph set by an F-86D with an average speed of 727 mph over the 3 km course, as well as the 100 km closed-circuit record. The sole version of the Hunter Mk 3, the only Hunter to be fitted with engine reheat, can be seen at the Tangmere Military Aviation Museum.
With the arrival of the Hunter Mk 6 during 1956 the aircraft finally came of age and realised its operational potential. It became a rugged, reliable, versatile and much liked interceptor fighter, and later a fine tactical reconnaissance fighter. It was a very pleasant and easy transonic aircraft to fly. The handling was now predictable with no particular vices at low speed manoeuvring where any mishandling resulting in a stalled condition was easily corrected. In traditional British fashion the cockpit could only be described as a close or intimate fit. Without the complications of an American designed fighter cockpit it unfortunately did not have their more logical layout. Although everything fell easily to hand when flying the aircraft there was still an impression of an absence of ergonomics in design. With a satisfying take-off and initial rate of climb of over 10,000 feet per minute aerobatics in the Hunter Mk 6 was a delight, as was exemplified by the many RAF formation aerobatics teams. To exceed Mach 1 required the Hunter to be dived at not less than a 40 degree dive above an altitude of 30,000 feet. Any dive of less than 30 degrees regardless of altitude and the Hunter Mk 6 could not be persuaded to exceed Mach 0.96. In going transonic there was little trim change as the aircraft attained a maximum speed of Mach 1.01. There were plans for further developments of the Hunter line by sweeping the 40 degree wing back to 50 degree with a thinner wing to further delay drag-rise generated by shock-waves while approaching Mach I, and by using a Rolls Royce Avon 200 engine with reheat. The ‘thin wing’ Hunter was designated the P.1083 project and we looked forward to its acceptance as it would have given the Hunter a true supersonic capability of Mach 1.2 in level flight, in line with the F-100 Super Sabre development from the F-86. Unfortunately, the project was cancelled after the end of the Korean War in favour of the English Electric supersonic Lightning to meet the Soviet bomber threat. With the advent of the Lightning, the RAF’s first supersonic fighter, in the early 1960s the Hunter followed the progression of RAF interceptors into the ground attack and fighter reconnaissance roles. The Hunter Mk 6 became the Hunter FGA Mk 9 weighing some 600 lb more than the Hunter Mk 6, and the installation of a braking parachute in the tail resulted in improved braking distances on wet runways. The use of additional outboard 100 gallon drop tanks gave an increased fuel capacity of 814 gallons. For long range ferry flights 230 gallon tanks were fitted during which there were G limitations until the tanks emptied. The Hunter continued for many years into the supersonic fighter era to perform well in the ground attack and reconnaissance roles. The side-by-side two-seat trainer version of the Hunter, the Mk 7, followed the Hunter Mk 6 two years later in 1958. It can be said of the Hunter Mk 6 that it not only looked right, it flew right. It was certainly a pilot’s aircraft and it became one of the most satisfying fighters to fly during my flying career, joining the Spitfire, the P-51 Mustang and the F-86 Sabre as my favourite aircraft. The Hunter finally equipped thirty-eight RAF squadrons and twenty other air forces around the world for a total production figure of 1,975 aircraft.
Flying with the Fighter Leader Squadron was the nearest thing to operational combat flying and while the previous zero limitations were raised slightly to avoid the danger of repeated bird strikes and a repetition of the Marham incident, it was still the most exhilarating flying and when the DFLS became the first squadron to receive the Hunter Mk 6, the flying was certainly the best available on RAF fighter squadrons.
The Day Fighter Leader course covered all types of fighter operations from high level fighter sweeps and interceptions to low level strikes. During all these exercises simulated combat resulted as directing staff pilots attacked the formations. The results of these missions were assessed by the directing staff flying in the formations and analysis of the camera gun films. Particularly stimulating were the strike missions carried out at low level and high speed. The navigation on such sorties required precise use of compass and watch, with accurate map reading while flying close to the ground to evade radar detection. At that time the sophisticated navigation systems currently available such as inertial navigation, moving map display and ground positioning from satellites to guide the aircraft and pinpoint its position were not in service. To assist the formation leader in his D/R navigation the sorties were flown at speeds in increments of 60 knots, so that a speed of 420 knots gave a still air ground distance covered of 7 nm per minute, and 480 knots a distance of 8 nm per minute. Low flying below 200 feet at over 400 knots while avoiding obstacles, scanning for hostile aircraft and map reading, can certainly concentrate the mind acutely. Course corrections required quick mental calculation to arrive accurately at the initial point for a timed run to the pull-up over the target. It was at this time that after a simulated attack on the target the lurking ‘hostile’ directing staff pilots attacked the formation. The course always terminated with a squadron sweep to Germany to be intercepted by the 2nd ATAF fighter squadrons, and on the following day the squadron returned to West Raynham to be intercepted by the squadrons of Fighter Command.
In addition to these visits to Germany the DFLS staff toured overseas to visit RAF air staff and fighter squadrons in the Middle East and Far East, when we flew on the several types of aircraft with the fighter squadrons and evaluated their operational proficiency. During these tours in the Middle East I flew with Meteor squadrons in Malta, Venom squadrons in Cyprus during the Suez campaign, and also with the Meteor FR squadron and the Venom fighter-bomber squadron operating from Aden and attacking the insurgents in the Radfan. By the time of the introduction of the Hunter to re-equip the Meteor squadrons in the UK, the Venom squadrons in Germany were re-equipped with the F-86. The Venom squadrons operated only in the Middle East and Far East Commands during their withdrawal from front-line service by 1962. Further developments of the Venom by de Havilland produced a two-seat night-fighter, and in 1959 the all-weather Sea Vixen for the Royal Navy. The Sea Vixen was powered by two RR Avon turbojets and carried four DH Firestreak missiles. In the Far East I flew with
Venom squadrons in Malaya and Singapore attacking communist encampments in Malaya. The Venom fighter-bomber was developed from the Vampire with modifications to the wing and tail, but the main difference was a change of engine from the DH Goblin of 3,350 lb.s.t. to the DH Ghost of 5,150 lb.s.t. Although these changes increased the weight of the aircraft, the considerable increase in power nearly doubled the initial rate of climb. This only resulted in a marginal increase in maximum speed to just over Mach 0.8. The big disappointment of the aircraft was that whereas the Vampire was one of the nicest handling of all jet aircraft to fly, the Venom could only be described as unpleasant in comparison. The main reason for this was the modifications that incorporated geared tabs to the ailerons and elevators. As an interceptor fighter the Venom was obsolescent when introduced into service and as a fighter-bomber the Venom had many failings in that, although it carried a useful armament of four 20 MM cannon, for rocket attacks it relied on Second World War 3-inch rockets on rails that had a considerable circular error of probability (CEP) factor. In bombing attacks the Venom became restricted to shallow dive attacks, and consequently the aiming errors were large and generally unacceptable.