CHAPTER SIXTEEN

Republic Thunderbolt

By mid 1940, it had become obvious to the US Army Air Corps that the current crop of American fighters were no match for corresponding types being developed by the warring nations in Europe. The USAAC therefore set about formulating a series of requirements that would close the gap. One of the firms contacted was the Republic Aviation Corporation of Farmingdale, Long Island, which had recently changed its name from the Seversky Aircraft Corporation.

Seversky had been formed in 1922 and had produced the stubby P-35 fighter of 1935. Designed by Alexander Kartveli, the P-35 was moderately successful (120 being ordered by the Swedish Air Force). However, the USAAC favoured the Curtiss P-36, which had better performance, although the Seversky design had superior manoeuvrability and range. In order to be able to compete with rival manufacturers more effectively, Kartveli’s thoughts turned to turbo-supercharging as a means of extracting greater speed at higher altitudes. This led to the YP-43, which had a top speed of 350 mph at 20,000 ft.

Although a turbo-supercharged radial engine offered great potential, the design trend in Europe was very much towards use of inline, water-cooled engines, which offered low frontal area and reduced drag. This theme had been perpetuated in the US with the Allison V-12, and Kartveli realised that to ignore this powerplant might prejudice the chance of Republic receiving orders. He thus proposed the Model AP-10 (ordered in prototype form as the XP-47 in November 1939), but by the following summer the Air Corps hierarchy had begun to recognise the limitations of the Allison engine and designers were being encouraged to look elsewhere. The most powerful engine available was the new Pratt & Whitney XR-2800 Double Wasp air-cooled radial offering around 2000 hp but the original XP-47 was too small to accept it. Following a complete redesign, the XP-47B Thunderbolt eventually emerged in early 1941, bearing a strong resemblance to the earlier P-43. It was first flown on 6 May 1941 by Lowery L. Brabham and had a gross weight of 11,600 lb, more than twice that of the XP-47.

By the standards of the day, the P-47 Thunderbolt was truly colossal for a fighter, its capacious body housing the ducting for the R-2800’s turbo-supercharger. The fuselage was of semi-monocoque, all-metal, stressed-skin construction and employed transverse bulkheads and longitudinal stringers. A General Electric turbo-supercharger was located in the lower rear fuselage and exhaust gases were passed through insulated pipes to the turbine, before being expelled through a waste gate on the aircraft’s underside. Air for the supercharger, in the meantime, was passed from an intake at the bottom of the NACA cowling. After passing through the supercharger, the air was fed to the engine via an intercooler.

To harness the power of the engine a four-blade Curtiss Electric propeller of 12 ft 2 in diameter had to be used, but this caused problems with ground clearance. As each wing was to house a battery of four 0.50-in machine-guns with 500 rounds per gun, the amount of space for the undercarriage was limited. Republic therefore came up with the novel solution of a telescopic landing gear that was 9 in shorter when retracted than when extended. The tailwheel was fully retractable and had its own doors.

The P-47B Thunderbolt entered USAAF service with the 4th, 78th and 56th Fighter Groups at Debden, Goxhill and Kings Cliffe respectively in early 1943, and was to be a stalwart of Eighth Air Force operations over northern Europe. Used mainly in the escort role, the Thunderbolt offered fighter protection over part of Germany, although even when fitted with overload fuel tanks it could still not provide cover for American B-17 and B-24 bombers on deep-penetration raids. The Thunderbolt was also flown in the Mediterranean theatre. It was here that a comparative trial was conducted by the USAAF involving a P-47D-4 and a captured Focke-Wulf Fw 190A. As one of the early variants, the D-4 had the original ‘high-back’ fuselage and hood, but its performance was enhanced by the inclusion of water-injection equipment, which increased the top speed for short periods.

The Thunderbolt was flown with a full war load and the Fw 190 also carried full ammunition for its two wing cannon and fuselage-mounted machine-guns. The Focke-Wulf was considered to be in exceptionally good condition for a captured aircraft and it easily developed 42 in Hg manifold pressure on take-off. Four flights were made, each of one-hour duration. The aircraft were compared in terms of acceleration, climb and dive performance and turning capability. During low-level acceleration tests up to 5000 ft, the Fw 190 was initially able to pull ahead of the Thunderbolt and gain about 200 yards, but this could not be sustained. If full power was maintained, the P-47 (using its water-injection) quickly caught up again and assumed the lead. A similar trial was carried out at 15,000 ft, with almost identical results.

Climbs were attempted from 2000 ft commencing at 250 mph IAS, both aircraft being pulled up rapidly to the angle of maximum climb and held until an altitude of 8500 ft had been reached. The Fw 190 climbed faster than the Thunderbolt through the first 1500 ft, but, once again, it was quickly overtaken and was then out-climbed by around 500 ft per minute. When similar climbs were made from 10,000 ft the Fw 190 again had the initial advantage, but by the time that 15,000 ft had been reached, the Thunderbolt was 500 ft above its rival. Dives were carried out from 10,000 ft with an entry speed of 250 mph IAS, the dive being flown at a constant throttle setting and an angle of around 65 degrees. The Fw 190 pulled away rapidly in the early stages of the dive but the Thunderbolt passed it, flying at much greater speed as the two aircraft passed through 3000 ft. It also appeared to have a much better angle of pull-out than the Focke-Wulf.

At speeds in excess of 250 mph IAS, the two aircraft were turned on each other’s tail as tight as possible, alternating the turns to the left and right. The Thunderbolt easily out-turned the Fw 190 at 10,000 ft and had to throttle back to keep from overshooting, a level of superiority that increased with altitude. During turning manoeuvres, it was found that the Fw 190 had a nasty habit of blacking out its pilot. Below 250 mph IAS, however, the situation was rather different as the Fw 190 could be made to hang on its propeller. This made life extremely difficult for the pilot of the Thunderbolt and the Focke-Wulf’s ability to turn inside its opponent was very evident. The Fw 190’s superb rate of roll also allowed it to change direction very quickly. When this was used in conjunction with its excellent initial acceleration, the Thunderbolt was unable to follow and the Fw 190 was able to rapidly move to a more advantageous position.

As Britain did not have a requirement for another high-altitude fighter, the Thunderbolt was used for ground attack, carrying up to three 500-lb bombs. The RAF received a total of 830 P-47s, which were put to good use in the Far East against Japanese forces. Of these, 240 were Thunderbolt Is (equivalent to the P-47B) and 590 were Thunderbolt IIs with ‘low back’ fuselage and clear-view canopy, equivalent to the USAAF P-47D-25.

Handling trials were carried out at A&AEE Boscombe Down in August 1944 to clear the Thunderbolt for operations in the fighter-bomber role. Most of the test work was performed by FL849, although 274699 was also used when the former aircraft became unserviceable towards the end of the trial. Both aircraft were powered by an R-2800-59 of 2300 hp with water-injection (limited to a maximum of 51½ in Hg manifold pressure and 2700 rpm for take-off) and carried the normal armament of eight 0.50-in machine-guns. Trim tabs were fitted to each elevator and the port aileron, and a combined balance/trim tab was fitted to the rudder. The normal take-off weight (as a fighter) was 14,170 lb, but with two 500-lb bombs under the wings and one 500-lb bomb under the fuselage, this figure increased to 15,715 lb, or slightly more than twice that of a fully loaded Spitfire IX.

The test flying that was carried out involved take-offs, stalls, dives and landings at all loadings. The effect on handling characteristics of releasing bombs singly in the dive was tested, but due to its limited operational role with the RAF, the dives were restricted to low altitude. When carrying a single 500-lb bomb under the fuselage, a flap setting of 20 degrees was used for take-off, but even so the run was long. This was no great surprise, as a clean aircraft required around 600 yards to get airborne, which was longer than most other fighters of the period. The aircraft took off at around 110 mph IAS and although carrying the bomb did not affect handling, the climb away was noted as being poor.

The stall speed with the flaps and undercarriage up was 112 mph IAS. As speed was decreased, the aircraft became nose-heavy, about half elevator trim being required to keep the stick force down to a reasonable level. A decrease in lateral control became apparent as speed dropped below 120 mph IAS, and there was snatching of the ailerons. As speed was reduced further, lateral control deteriorated rapidly until at the stall the port wing dropped, followed by the nose. There was no stall warning apart from the aileron snatch mentioned above. With the flaps and undercarriage down, the stall came at 98 mph IAS. The nose-heaviness experienced in the ‘all-up’ case was even more marked, to the extent that full elevator trim was needed. Aileron snatching was again present, but in addition there was vibration of the whole aircraft before the port wing dropped and the nose went down. At the point of stall, the control column was about two-thirds back from its central position. In each of the above cases, the rate of sink prior to the stall was high and control forces were heavy, but recovery on centralising the controls was normal and there was no tendency to spin.

Trimmed dives were made between 16,000 ft and 7000 ft with the engine set to 40 in Hg manifold pressure and 2550 rpm. The rate of increase of speed was low, unless the dive was entered very steeply. A general ‘roughness’ of the whole airframe was noted as speed increased to 400 mph IAS, but this did not get any worse with further increases in speed. Changes in directional trim with increases in speed were marked, a heavy left foot load being required to keep the aircraft straight. When this was trimmed out, the aircraft became left wing low and lateral re-trimming was necessary. Only a slight change in longitudinal trim occurred, the aircraft becoming slightly tail-heavy above 400 mph IAS. Recoveries from trimmed dives were made at 400 and 450 mph IAS. In each case, the recovery felt sluggish and a fairly heavy backward pressure was needed to initiate the pull-out and to maintain it.

Landings were carried out with the bomb still in place. No difficulties were encountered, provided that about 70 gallons of fuel had been used to reduce the landing weight to permissible limits. The rate of sink on the approach was high and the best speed was found to be 115 mph IAS.

With two bombs in place under the wings, the take-off run was noticeably longer and the stall speeds with the flaps and undercarriage up and down were 120 and 102 mph IAS respectively. The handling characteristics were similar to the previous test, except that in each case the effect was slightly worse and in some cases significantly worse. The loss of lateral control was almost complete, and the aircraft entered a right- or left-hand spiral, the direction depending on wing drop. Recovery was normal, but the height loss was considerable. In dives from 13,000 ft, the left wing low tendency was more marked and a considerable amount of aileron was required to keep the wings level at speeds above 420 mph IAS. This had to be counteracted by use of the controls, as there was insufficient time to re-trim laterally.

A further loss in take-off performance occurred when carrying bombs on all three stations, with lift-off taking place at 115 mph IAS. Acceleration and initial climb also showed a further slight deterioration. Some differences were apparent at the stall, and with the flaps and undercarriage up, vibration could be felt at 135 mph IAS, with aileron snatching from 125 mph IAS. If no attempt was made to control this, and the control column was left about one-third back from neutral, an indefinite stall occurred at 120 mph IAS, the aircraft becoming right wing heavy and the nose dropping very gradually. The snatching could, however, be controlled and by applying a considerable force to pull the control column hard back, the aircraft stalled properly at 110 mph IAS. In this case, there was no tendency for a wing to drop and the nose dropped quite sharply at the full stall. The characteristics were similar with the flaps and undercarriage down, occurring at 10–15 mph IAS less in each case.

The wing-mounted bombs were also dropped asymmetrically to check for any handling problems. The left wing bomb was released in a dive at 450 mph IAS and there was a very quick lateral rocking motion, which damped out immediately. Releasing the right wing bomb had no noticeable effect on handling; there was no ‘kick’ as it departed.

Trials were also carried out at Boscombe Down from November 1944 to clear various sizes and combinations of overload fuel tanks for carriage by the Thunderbolt. The aircraft used was FL844 and the tanks ranged in size from 108 to 165 US gallons. Handling was confirmed in all cases, the 165 US gallon tanks being cleared to 320 mph IAS and the smaller 150 US gallon tanks to 400 mph IAS. Jettison tests were satisfactory, except for the 165-US gallon tank, which caused damage to the wing flaps when dropped at 200 mph IAS

Although the Thunderbolt was never used as anything other than a fighter-bomber in RAF service, a comparative assessment was carried out by AFDS in early 1945 to determine its capabilities against the latest offering from Hawker, the Centaurus-powered Tempest II. As the Tempest had been designed as a short-range, low-medium altitude fighter, it naturally lost out in terms of endurance and the Thunderbolt had a greater radius of action at all engine settings and heights. The Thunderbolt allotted for the trial was not fitted with water-injection and so allowance had to be made for this in the calculation of its maximum speed. Even so, it was found that the Tempest II was some 80 mph faster at low levels up to 2000 ft, this advantage reducing to 40 mph by the time that 20,000 ft had been reached. Parity occurred at around 28,000 ft, and the Thunderbolt began to come into its own above this height, being 20 mph faster at 31,000 ft. The Tempest II had the advantage in acceleration in straight and level flight at all heights. Even at high altitude, where the Thunderbolt was faster, the Tempest was able to pull away initially.

There were also significant variations in climb performance, which reflected the widely differing roles for which the aircraft had been designed. At sea level the Tempest II was totally dominant, having a climb rate 2000 ft/min better then the Thunderbolt, but this advantage gradually diminished until the two aircraft were equal at 21,000 ft. Thereafter, the Thunderbolt was superior and by the time that 28,000 ft had been reached, it had a climb rate 500 ft/min better than the Tempest II. At low altitude and with equal power settings, the Thunderbolt was slightly superior during zoom climbs, but at full power and at high altitude the Tempest II was the better. The Tempest II could always out-dive the Thunderbolt. This advantage was particularly noticeable at full throttle.

In the lateral plane, at speeds up to 300 mph IAS there was little advantage either way as regards rate of roll. The Tempest II had a slight advantage to the right and the Thunderbolt to the left, due to the different direction of propeller rotation, the former being a left-hand and the latter a right-hand tractor. Above 300 mph IAS, however, the Tempest II became increasingly superior. The Tempest II could also out-turn the Thunderbolt at any height, due to its considerably lower wing loading of 38 lb/sq.ft, compared with 49 lb/sq.ft for the American fighter.

In many ways, the AFDS trial was rather meaningless as the Tempest II had been designed as a relatively low-altitude fighter-bomber, whereas the Thunderbolt had been conceived with a very different role in mind, that of a high-altitude, air superiority fighter. There was also little likelihood of the two aircraft ever having to confront each other in combat. As a tactical exercise, however, it was of considerable interest. It showed that the Tempest II was definitely superior up to 21,000 ft (except in range and endurance), but that the Thunderbolt began to assert ever greater authority above this height.

The RAF was the second largest operator of the P-47 Thunderbolt. Nearly all were despatched directly from the USA to India, where they were reassembled for use by sixteen squadrons operating in Burma with South-East Asia Command. Duties included operations in support of the Army, often involving ‘Cab-Rank’ patrols and dive-bombing attacks when requested by a forward controller, escort to C-46 and C-47 transports and medium and heavy bombers, and long-range interdiction sorties attacking Japanese airfields and communications.

The Thunderbolt was popular in RAF service. Pilots were appreciative of the roomy well laid out cockpit and wide-track undercarriage, which was of particular benefit when operating from hastily improvised airstrips. Less well liked was the poor view when taxying (due to the P-47’s massive nose), its poor take-off performance and a somewhat less than sprightly climb rate when airborne. Because of its weight, it also had a much higher landing speed than the Spitfires that most pilots had been used to. Once in the air, however, it came into its own. Unlike a number of other fighters of the period, its handling characteristics were largely unaltered at altitude, and its stability in the dive was one of the prime reasons for it achieving levels of accuracy previously unseen during dive-bombing sorties. The Thunderbolt also possessed excellent range; sorties of up to three hours were commonplace, with some trips extending to five hours when carrying drop tanks and flying at economical power settings. A poor rate of turn was not too much of an embarrassment, as by this stage of the war, contacts with Japanese fighters were fairly infrequent.

The rugged nature of the P-47 was also ideally suited to the tropical environment, in particular its Pratt & Whitney engine, which gave far less trouble than the water-cooled Merlins of the Spitfires that the Thunderbolt replaced. Problems occured with the later Mark IIs, however, when Curtiss Electric paddle-blade propellers replaced the Hamilton Hydromatic propellers used on earlier aircraft. The humid conditions played havoc with the various electrical connections. Both the airframe and engine were able to take considerable battle damage and still get home, and Thunderbolt units were able to claim a very low fatality rate among pilots. Sadly, some aircraft were lost, including a number that blew up when recovering from dive-bombing attacks. It transpired that excessive ‘g’ during the pull-out from dives was causing the internal fuel tanks to rupture, and spilt fuel was coming into contact with the hot supercharger ducting to produce an explosion. Tactics were amended when the cause of the accidents was known, and dive-bombing attacks were subsequently flown at a shallower angle of about 30 degrees.

The Thunderbolt did not remain in RAF service for very long after VJ-Day, with many fighter-bomber squadrons being disbanded. Those that did form part of the post-war RAF were quick to re-equip with British-built aircraft, Nos 5 and 30 Squadrons at Bhopal receiving Tempest IIs and No. 60 Squadron at Surabaya the Spitfire FR.XVIII.

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