CHAPTER EIGHTEEN
Although the Wildcat remained in first-line service throughout the Second World War, its performance in comparison with its principal rival, the Mitsubishi Zero, was deficient in several respects, most notably in terms of speed, climb rate and manoeuvrability. The Hellcat came from the same Grumman stable and, together with the Vought Corsair, was to secure a level of superiority in the air war over the Pacific that proved vital in pushing Japanese forces back towards their homeland.
Grumman’s long association with the US Navy led to close links with those at the sharp end of naval aviation. Pilots’ experiences with the Wildcat were instrumental in the design philosophy that produced the Hellcat. Compared with its predecessor, the top speed was increased by 50 mph, its climb rate was significantly better and its service ceiling and range were much improved. The pilot had more armour protection and the use of a low-set wing allowed the undercarriage to be retracted into the centre section instead of the fuselage, thereby allowing a wider track for better ground handling. The cockpit was also placed as high as possible to aid lookout. This was also helped by a three degrees downwards inclination of the engine thrust line, which produced a tail-down attitude in flight.
A contract was awarded to Grumman on 30 June 1941 for two prototype XF6F-1s and the first flight took place on 26 June 1942. The engine to power the Hellcat was to have been the Wright Cyclone R-2600 14-cylinder radial of 1600 hp. However, this was dropped in favour of the 2000 hp Pratt & Whitney R-2800 Double Wasp 18-cylinder radial, which was installed in the second prototype, becoming the XF6F-3. The Hellcat entered service with US Navy Squadron VF-9 aboard the USS Essex in January 1943, and saw action for the first time in August 1943 with VF-5 during strikes on Marcus Island in the Pacific.
Shortly before its operational debut with the US Navy, the Hellcat was made available to Britain under Lend-Lease. It was known initially as the Gannet I. This name was soon dropped and aircraft in FAA service were known as the Hellcat I (equivalent to the US Navy F6F-3 with R-2800-10 engine) and Hellcat II, which was equivalent to the F6F-5 with the R-2800-10W engine featuring water-injection that increased emergency power to 2200 hp. A total of 1182 had been delivered to the FAA by the end of the war.
One of the first Hellcat Is to arrive in the UK was delivered in July 1943 to the Carrier Trials Unit at Crail in Fife. Although the family resemblance was obvious, the Wildcat appeared almost toy-like in comparison with its big brother which, with a loaded weight of 12,727 lb, was over twice that of its forebear. Unlike the cramped conditions to be found in most British cockpits, the Hellcat offered the commodious proportions to be expected of an American fighter. The positioning of the cockpit at the high point of the fuselage, together with the shorter nose afforded by the use of a radial engine, meant that the view forwards was better than many aircraft tested previously.
Taxying was straightforward thanks to the wide-track undercarriage, although it was necessary to lock the tailwheel in a crosswind to prevent the aircraft weather-cocking. The huge mass of the R-2800 Double Wasp meant that a nose-over was also a distinct possibility if the pilot did not hold the elevator full up, especially on soft ground. Engine torque produced a moderate swing to the left on take-off, but this could easily be corrected by rudder. The initial climb rate was 3000 ft/min, performance being adequate up to 20,000 ft. However, above this height it began to deteriorate. Although the service ceiling was quoted as being 37,800 ft, the Hellcat struggled much above 30,000 ft.
When flown in the clean configuration, there was very little warning of the stall, which could be accompanied by wing drop, although recovery was straightforward. The stalling speed with the gear and flaps down was a very low 67 mph IAS. One slightly disconcerting aspect of the Hellcat’s performance was a tendency towards auxiliary supercharger surging in high gear when weak cruise had been set. This caused rough running and a deep rumbling noise, but it could be quickly stopped by selecting low gear, or advancing the throttle and reducing rpm.
At higher speeds, the controls tended to become heavy on early aircraft, although this problem was addressed on the Hellcat II with the introduction of spring-tab ailerons. These produced a dramatic improvement in lateral control. The aircraft was stable about all three axes, but exhibited quite large changes of trim with changes of speed and throttle setting, which had to be carefully monitored.
For deck landings, the speed was reduced to 125 mph IAS before the landing gear and arrester hook were lowered. The other pre-landing checks to be made were: tailwheel unlocked, mixture to AUTO RICH, supercharger to NEUTRAL, propeller to fine pitch, booster pump ON, cowl gills CLOSED and flaps fully down. On final approach to land, the aircraft was flown at around 90 mph IAS, depending on the weight. It was important to leave the power on until the last second, to achieve the correct touchdown attitude. Throttling back slightly too early was likely to lead to a nose-down pitch. This could lead to problems as the mainwheels hit the deck, although in general the undercarriage was substantially built and was not as prone to bouncing as the Vought Corsair.
Intensive flying trials were carried out at Boscombe Down between July and August 1943 using FN331 and FN333. Both aircraft carried the standard armament of six 0.50-in machine-guns, three in each wing. The gun barrel fairings and muzzles, but not the ejector chutes, were sealed with fabric. An aerial mast was situated immediately behind the pilot’s hood, with an aerial running to a short mast on top of the fin, the aerial lead entering the fuselage on the starboard side. IFF aerials ran from the tailplane tips to the fuselage. The elevators were fabric-covered and balanced only by a set-back hinge with trim tabs fitted to both surfaces. The horn-balanced rudder and Frise-type ailerons were also fabric-covered. The rudder had a tab for trimming purposes and the ailerons (on the Hellcat I) had a ground-adjustable tab on the starboard surface and a movable tab on the port surface.
The supercharging system of the R-2800-10 Double Wasp was rather unusual. A normal mechanically-driven blower supercharged the mixture between the carburettor and the engine. When required, however, the air supply to the carburettor could be boosted by an auxiliary two-speed blower. Before reaching the carburettor, the air passed through an intercooler to lower the temperature and reduce the risk of detonation. The pilot was thus left with three supercharger options: a) main blower, b) main blower plus auxiliary low and c) main blower plus auxiliary high. When using the main stage, the intake air passed through an air cleaner, but that for the auxiliary stage was not cleaned. The pilot could, however, select air from the forward facing cold air intake or from warmed air taken from the engine bay. The exhaust system consisted of ten individual pipes of approximately 2-in diameter, eight of which carried the exhaust from two cylinders, the remaining two pipes being each connected to a single cylinder. The propeller was a metal Hamilton Standard Hydromatic of 13 ft 1 in diameter, with a pitch range of 26–65 degrees.
The cockpit could be entered from either side of the aircraft, but the hood could only be opened from the starboard side. The size of the cockpit was appreciated, but the seat was found to be rather uncomfortable after flying for some time and was liable to cause cramp in the pilot’s back. An American-type harness was fitted. Although it held the pilot down better than a Sutton harness, it was difficult to adjust and irritating across the shoulders. The Hellcat proved to be very noisy at about 2200 rpm, and it was necessary to keep the hood firmly shut. A tightly fitting helmet was also a priority or the radio became inaudible. There was also considerable vibration when the engine was operated in the range 1500–1900 rpm.
Like a number of other American aircraft, the control column of the Hellcat was too far away from the pilot and also too short. As a result, it could not be pushed right forward, even by a tall pilot, without undoing the harness. Difficulty was therefore experienced in keeping the nose up when inverted. The other controls were generally well positioned, except that several pilots commented adversely on the position of the aileron trimmer, which was too near the pilot’s side. The usual American blind flying panel was fitted and all the flying and engine instruments were clearly visible.
The view was reasonably good, except directly forward, where it was obscured by the engine cowling and gunsight. The windscreen pillars were very thick and there was annoying distortion at the curved parts of the screen. When the gills were opened, the forward field of vision was reduced further. This situation was made even worse when cruising in conditions of bad visibility at the recommended speed of 115 mph IAS, as the nose attitude was somewhat higher. There was no clear-view panel, which meant that the hood had to be opened when flying in conditions of bad visibility.
Taxying was relatively easy and could be assisted considerably in crosswinds by locking the tailwheel. The brakes were toe-operated and performed well. Take-offs were often performed with the flaps up and the tendency to swing to the left could be overcome by selecting one division of right rudder trim. Ideally, it was best not to lower full flap for take-off, due to deterioration in lateral control. The slotted flaps operated by means of an electro-hydraulic system, with manual control being available in an emergency. Any intermediate position of the flaps (up to the maximum of 50 degrees) could be selected as desired, the flap position being shown by an indicator connected to the port wing flap. There was no mechanical inter-link between the port and starboard wing flaps and it was possible for the flap on one side to be at a different angle to that on the other side, particularly when an intermediate position had been selected. The flaps were spring-loaded so that speed had to be reduced to about 195 mph IAS before partial flap could be used, and they could not be lowered fully until speed had been reduced to around 107 mph IAS.
At a take-off weight of 11,420 lb (CG 27.3 in aft of datum – normal forward) the Hellcat was very stable in the climb. The best climb speed was about 150 mph IAS, and at this speed full right rudder trim was necessary. At lower speeds, there was not enough rudder trim to give zero foot load. There was no tendency for the engine to overheat at 150 mph IAS with the gills closed, and the oil temperature was easily kept within limits by opening the oil cooler shutters, which produced a slight nose-down change of trim. Generally, it was necessary to use the fuel booster pump at heights above 15,000 ft to maintain fuel pressure, and to throttle back slightly when changing supercharger gear to avoid over-boosting. Some trouble was experienced with the trim tabs freezing up during sustained flight above 20,000 ft. The aircraft was stable in level flight and could be trimmed to fly ‘hands and feet off’ at all speeds above 130 mph IAS. Right rudder trim was required up to about 215 mph IAS, with left rudder trim being needed above this speed.
The Hellcat was found to be very manoeuvrable, although the controls, in particular the ailerons, became heavy at high speed. This defect was accentuated by the awkward position of the control column. During a quick turn it was necessary to wind the elevator trimmer back to reduce the load on the stick, and coarse rudder was needed to come out of a turn quickly. In the dive, the aircraft became tail-heavy, and it was necessary to trim into the dive and apply some left rudder to keep straight. There was also considerable vibration, which was felt mainly through the rudder pedals and was most marked at high power settings. At lower engine speeds, the amount of vibration was reduced, but not entirely eliminated.
The approach was made at about 150 mph IAS, which was the recommended speed for lowering the flaps. The speed could be reduced very quickly by throttling back gradually and easing the control column back. With full flap set, lateral control was much less precise and the undercarriage could be lowered at speeds below 125 mph IAS. The final approach was made at 105 mph IAS. If baulked, no difficulty was experienced in going around, the undercarriage coming up quickly and the change of trim being easily held on the control column. On the whole, the Hellcat was well liked and it was thought that it would make a good fighter for naval operations.
Handling trials were carried out using FN322 and FN323, with FN360 joining the programme later. This aircraft featured a strengthened tailplane with extra internal stiffening, but this did not appear to reduce the amount of vibration experienced at high speed. It also had modified trimmer control circuits, but these proved as difficult to operate at high altitudes as the previous design, and it was not until a special lubricating oil was used (DTD.539) that the tabs were not subject to ‘freezing up’.
The handling trials were carried out at a take-off weight of 12,140 lb. Take-offs could be made with 15 degrees of flap without upsetting lateral control too much, and the normal technique was to keep the tail low. This method was found to be more comfortable and also had the benefit of protecting the propeller, which had very little ground clearance in the take-off attitude. Acceleration was high and the lift-off speed was around 80–85 mph IAS.
With the flaps and undercarriage up; the aircraft was trimmed to glide at 140 mph IAS, the speed gradually being reduced until a ‘hooting’ noise commenced at about 110 mph IAS, the note changing as speed was reduced further. At about 90 mph IAS there was slight shuddering, rudder buffet and instrument vibration. The stall occurred at 86 mph IAS and was characterised by either wing dropping gently, followed by the nose. With the flaps and undercarriage down, there was a tendency for the port wing to drop during glides below 100 mph IAS, but this could easily be corrected by aileron. The stall warning was similar to the previous case and the stall came at 72 mph IAS, the nose dropping about 30 degrees and the port wing about 20 degrees.
The force to hold the aircraft in a tight turn at 5 g was rather large for a fighter, but there was no tendency to tighten up. Over the speed range 150–300 mph IAS the elevators were light and effective, but the ailerons became heavy above 240 mph IAS, and full aileron could only be applied below 290 mph IAS to the left and 260 mph IAS to the right. The ailerons of FN323 tended to overbalance slightly when full deflection was applied at 180–200 mph IAS. Very brief handling checks were made on FN322 at 32,000 ft, the aircraft being put into a gradually tightening turn at 200 mph IAS (Mach 0.52). When the acceleration had increased to an estimated 2½ g a violent vibration commenced, which appeared to originate in the tailplane. This vibration ceased immediately the force on the control column was released.
A number of out-of-trim dives were made at full and one-third throttle, up to the limiting speed of 460 mph IAS. The longitudinal control characteristics were satisfactory, although the push force to hold the aircraft in the dive was large, and the pull on recovery was of a similar magnitude. At speeds above 300 mph IAS, the ailerons were considered to be too heavy for a fighter; and by the time that 350 mph IAS was reached, they were almost immovable. The rudder was fairly heavy, but was effective in producing yaw up to the limiting speed.
The Hellcat was looped with the engine operating at climbing power (2550 rpm, 44 in Hg), starting at an initial speed of 350 mph IAS and applying an estimated 4 g. At the top of the loop, the speed had reduced to around 120 mph IAS, before it increased again to 240 mph IAS in the recovery. The stick forces involved were not excessive. Rolling performance was best at speeds between 180–230 mph IAS, being approximately 60 degrees per second. At higher speeds, the ailerons were too heavy for good manoeuvrability. The initial force required to apply full aileron was large, but once the roll had been started the force needed to maintain full deflection reduced.
Owing to engine unserviceability with FN323, the handling trials had to be continued with FN322 and JV224. The latter was a Hellcat II that differed from the earlier aircraft principally in having a Double Wasp R-2800-10W engine with water-injection and spring-tab ailerons. Tests with JV224 showed there was no noticeable difference between the longitudinal handling characteristics at the recommended aft and normal forward CG positions. The stick forces in out-of-trim dives changed from a 4 lb push to a 3 lb pull at 460 mph IAS, and the stick force per ‘g’ was around 14 lb at accelerometer readings of 4-5 g. The effect of fitting spring tabs to the ailerons was to lighten the control considerably at high speeds, although this tended to affect harmonisation, with the elevator becoming the heaviest control.
Compressibility dives were carried out from 25,000 ft, and a speed of 460 mph IAS was reached by the time that the aircraft passed through 17,000 ft. At this speed, the nose became very heavy and speed increased rapidly. This sudden change of trim was attributed to compressibility effects, commencing at a Mach number of 0.77. The pull force to recover at 480 mph IAS was initially around 70 lb, but when speed had reduced to 460 mph IAS, the stick force reduced rapidly. During high-speed recoveries, pilots noticed sheets of vapour forming over the wing roots.
Performance trials were carried out at the same time as the handling assessment and involved FN322. Readings could only be made up to 28,000 ft, due to internal sparking in the magnetos when the aircraft was flown at higher altitudes. The maximum rate of climb in main supercharger gear was 2260 ft/min at 5400 ft and 20,000 ft was reached in ten minutes. The recommended height to change supercharger gear was 9200 ft. The full results were as follows (* and ** denote full throttle heights).
Level-speed performance was measured for ‘main blower’, which was maintained up to 6000 ft, and for ‘main blower plus auxiliary low’, the setting used above this height. All the speed runs were made with the cooling gills and flaps closed and the maximum achieved was 371 mph TAS at 18,700 ft.
Several aircraft were tested with a variety of under-wing and fuselage stores, including FN344, which was fitted with four rocket projectiles under each wing, just outboard of the undercarriage. With RP in place, there was little effect on handling characteristics, although considerable vibration was experienced in the dive. It was found that this emanated from the front of the blast plate. By careful modification of the nose fairing, the intensity of the vibration could be much reduced. During the tests, it was also established that end fairings fitted to the rocket installation had no noticeable effect on handling or the level of vibration. The Hellcat was eventually cleared for service use at all typical loadings with the revised nose fairings and no end fairings. FN360 was tested with a 125-US gallon drop tank on a rack under the fuselage. Once again, there was no noticeable effect on handling in any condition of flight. The ground clearance was minimal however, so care had to be taken when taxying on rough ground.
In August 1944 JV127, a Hellcat I, was used for brief handling trials with a 1000-lb bomb. This was an ANM.65 with a British tail, which was 48 in long and had a diameter of 15 in. Compressibility effects came into play during dives, as the blunt shape of the bomb caused excessive airframe shuddering at speeds in excess of 400 mph IAS. The maximum speed attained was 440 mph IAS, which at the height tested was equivalent to Mach 0.675. In view of its characteristics at high speed, it was recommended that the Hellcat be limited to a maximum dive speed of 400 mph IAS when carrying a 1000-lb bomb.
JV127 was also used to test the Hellcat’s suitability as a dive-bomber. However, problems were experienced with insufficient braking effect with the undercarriage retracted. Dives were then attempted with the undercarriage locked down, but owing to excessive shuddering at moderate to high speeds, these was not particularly successful. As a compromise, the aircraft was flown with the undercarriage trailing (down, but not locked), with the engine operating at one-third throttle, at speeds up to 350 mph IAS. Even with the undercarriage trailing, there was hardly sufficient braking effect, and there was also considerable airframe disturbance. The stick forces in out-of-trim dives, as well as the stick force per ‘g’ on recovery, were considered to be excessive, although the sight could easily be held on the target and releasing the bomb had no unusual effects on handling. Although the previous 400 mph IAS limit was retained, it was recommended that the maximum speed in the dive in this condition be kept below 350 mph IAS wherever possible, as the push force to hold the aircraft at higher speeds became too large and the ailerons were excessively heavy to the detriment of lateral manoeuvrability. At lower speeds, the recovery could also be commenced at a lower altitude, with better accuracy, and the disturbance due to the trailing undercarriage was not so severe.
In December 1944, JV109 was tested with a balloon hood, which considerably improved the vision to the rear as a result of the elimination of the vertical and horizontal stiffening members of the original hood. Handling was not affected, so the modified canopy was a worthwhile improvement. At the same time JX822, a Hellcat II, was being flown with two 1000-lb bombs at a take-off weight of 14,600 lb. No particular problems were experienced and the aircraft was considered acceptable for service use at the same limiting speed as the Hellcat I. In early 1945, JX901 was tested with six 60-lb RP on zero-length launchers with little effect on handling. It later flew with the US Mk. 5 rocket launcher.
The arrival of the Hellcat II with its R-2800-10W water-injected engine led to further performance testing with JV224. The rate of climb was assessed in auxiliary low gear, with level-speed performance being measured in auxiliary high gear. The use of water-injection allowed a higher manifold pressure of 60 in Hg to be used, instead of the previous take-off and combat limit of 54 in Hg. The rate of climb was increased by 650 ft/min (from 2570 to 3160 ft/min) at all heights up to 11,200 ft, which was the full throttle height. The top speed was also increased by about 20 mph TAS, up to the full throttle height of 18,600 ft. At this height, the speed was measured at 377 mph TAS, which compared with 367 mph TAS at 21,800 ft, the full throttle height for the maximum manifold pressure without water-injection.
The Hellcat provided a welcome boost to the operational capability of the FAA. It was introduced to service by No. 800 Squadron, which converted from Sea Hurricanes in July 1943. The Hellcat’s operational debut came in December 1943 during anti-shipping strikes off the Norwegian coast from the escort-carrier HMS Emperor. Its major work, however, was carried out in the Far East and Hellcats of Nos. 1839 and 1844 Squadrons flying from HMS Indomitable comprised the escort for strikes against oil refineries in Sumatra in January 1945. By March, operations had moved on to the Sakashima Islands and two months later Hellcats were involved in the fighting around Formosa. The Hellcat was also used by the FAA as a night-fighter with 892 Squadron forming at Eglinton in April 1945, followed by No. 891 Squadron in June 1945. Like all Lend-Lease aircraft, the Hellcat was quickly withdrawn from active service at the end of the war. The last operational aircraft were retired when No. 888 Squadron was disbanded in August 1946. One Hellcat (KE209) continued to fly at RNAS Lossiemouth until 1953 and this aircraft is now preserved at the Fleet Air Arm Museum at Yeovilton.