Republic P-47D Thunderbolt
Republic P-47 Thunderbolt
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The Republic P-47 Thunderbolt was a World War II era fighter aircraft produced by the United States from 1941 through 1945. Its primary armament was eight .50-caliber machine guns and in the fighter-bomber ground-attack role it could carry five-inch rockets or a bomb load of 2,500 pounds (1,103 kg). When fully loaded, the P-47 weighed up to eight tons, making it one of the heaviest fighters of the war. The P-47 was designed around the powerful Pratt & Whitney R-2800 Double Wasp engine, which was also used by two U.S. Navy/U.S. Marine Corps fighters, the Grumman F6F Hellcat and the Vought F4U Corsair. The Thunderbolt was effective as a short-to medium-range escort fighter in high-altitude air-to-air combat and ground attack in both the European and Pacific theaters.
The P-47 was one of the main United States Army Air Forces (USAAF) fighters of World War II, and also served with other Allied air forces, including those of France, the United Kingdom, and the Soviet Union. Mexican and Brazilian squadrons fighting alongside the USAAF also flew the P-47.
The armored cockpit was relatively roomy and comfortable and the bubble canopy introduced on the P-47D offered good visibility. A present-day U.S. ground-attack aircraft, the Fairchild Republic A-10 Thunderbolt II, takes its name from the P-47.
P-43 Lancer / XP-47B
The P-47 Thunderbolt was a design of Georgian immigrant Alexander Kartveli, and was to replace the Seversky P-35 that was developed earlier by Russian immigrant Alexander P. de Seversky. Both had fled from their homeland to escape the Bolsheviks. In 1939, Republic Aviation designed the AP-4 demonstrator powered by a Pratt & Whitney R-1830 radial engine with a belly-mounted turbocharger. A small number of Republic P-43 Lancers were built but Republic had been working on an improved P-44 Rocket with a more powerful engine, as well as on the AP-10 fighter design. The latter was a lightweight aircraft powered by the Allison V-1710 liquid-cooled V-12 engine and armed with eight .50 in (12.7 mm) M2 Browning machine guns. The United States Army Air Corps (USAAC) backed the project and gave it the designation XP-47.
In the spring of 1940, Republic and the USAAC concluded that the XP-44 and the XP-47 were inferior to Luftwaffe fighters. Republic tried to improve the design, proposing the XP-47A but this failed. Kartveli then designed a much larger fighter, which was offered to the USAAC in June 1940. The Air Corps ordered a prototype in September as the XP-47B. The XP-47A, which had little in common with the new design, was abandoned. The XP-47B was of all-metal construction (except for the fabric-covered tail control surfaces) with elliptical wings, with a straight leading edge that was slightly swept back. The air-conditioned cockpit was roomy and the pilot’s seat was comfortable—”like a lounge chair”, as one pilot later put it. The canopy doors hinged upward. Main and auxiliary self-sealing fuel tanks were placed under the cockpit, giving a total fuel capacity of 305 U.S. gal (1,155 L).
Power came from a Pratt & Whitney R-2800 Double Wasp two-row 18-cylinder radial engine producing 2,000 hp (1,500 kW)—the same engine that would power the prototype Vought XF4U-1 fighter to just over 400 mph (644 kph) in October 1940—with the Double Wasp on the XP-47B turning a four-bladed Curtiss Electric constant-speed propeller of 146 in (3.7 m) in diameter. The loss of the AP-4 prototype to an engine fire ended Kartveli’s experiments with tight-fitting cowlings, so the engine was placed in a broad cowling that opened at the front in a “horse collar”-shaped ellipse. The cowling admitted cooling air for the engine, left and right oil coolers, and the turbosupercharger intercooler system. The engine exhaust gases were routed into a pair of wastegate-equipped pipes that ran along each side of the cockpit to drive the turbosupercharger turbine at the bottom of the fuselage, about halfway between cockpit and tail. At full power, the pipes glowed red at their forward ends and the turbine spun at 21,300 rpm. The complicated turbosupercharger system with its ductwork gave the XP-47B a deep fuselage, and the wings had to be mounted in a relatively high position. This was difficult since long-legged main landing gear struts were needed to provide ground clearance for the enormous propeller. To reduce the size and weight of the undercarriage struts and so that wing-mounted machine guns could be fitted, each strut was fitted with a mechanism by which it telescoped out 9 in (23 cm) when extended.
The XP-47B was very heavy compared with contemporary single-engined fighters, with an empty weight of 9,900 lb (4,490 kg), or 65 per cent more than the YP-43. Kartveli said, “It will be a dinosaur, but it will be a dinosaur with good proportions”. The armament was eight .50 caliber (12.7 mm) “light-barrel” Browning AN/M2 machine guns, four in each wing. The guns were staggered to allow feeding from side-by-side ammunition boxes, each with 350 rounds. All eight guns gave the fighter a combined rate of fire of approximately 100 rounds per second.
The XP-47B first flew on 6 May 1941 with Lowry P. Brabham at the controls. Although there were minor problems, such as some cockpit smoke that turned out to be due to an oil drip, the aircraft proved impressive in its early trials. It was lost in an accident on 8 August 1942 but before that mishap, the prototype had achieved a level speed of 412 mph (663 km/h) at 25,800 ft (7,864 m) altitude and had demonstrated a climb from sea level to 15,000 ft (4,600 m) in five minutes.
The XP-47B gave the newly reorganized United States Army Air Forces cause for both optimism and apprehension. While possessing good performance and firepower, the XP-47B had its share of teething problems:
- Its sheer size and limited ground-propeller clearance in a fuselage-level attitude made for challenging takeoffs which required long runways—the pilot had to hold the tail low until considerable speed was attained on the initial run.
- The sideways-opening canopy covers had a tendency to jam.
- The multiple-gun installation, with its tight fit and cramped ammunition belt tracks, experienced jamming problems, especially during and after hard maneuvering.
- Maneuverability was less than desired when compared with the Supermarine Spitfire and Messerschmitt Bf 109.
- The ignition system arced at high altitude.
- Access to the rear engine accessory pad was difficult due to the short engine mount used.
- At high altitude the ailerons “snatched and froze”.
- At high speeds the control loads were deemed excessive.
Republic addressed the problems by fitting a rearwards-sliding canopy that could be jettisoned in an emergency, a pressurized ignition system and all-metal control surfaces. The deficient maintenance access to the Double Wasp radial on the B-series subtypes had to wait until the P-47C introduced a new engine mount. While the engineers worked to get their “dinosaur” to fly right, the USAAF ordered 171 P-47Bs. An engineering prototype P-47B was delivered in December 1941, with a production prototype following in March 1942 and the first production model provided in May. Republic continued to improve the design as P-47Bs were produced and although all P-47Bs had the sliding canopy and the new General Electric turbosupercharger regulator for the R-2800-21 engine, features such as all-metal control surfaces were not standard at first. A modification on the P-47B, also required for the early marks of the U.S. Navy’s Grumman F4F Wildcat and Grumman F6F Hellcat was the radio mast behind the cockpit that was slanted forward to maintain the originally designed antenna wire length in spite of the new sliding canopy.
The P-47B led to a few “one-off” variants. In September 1942, the 171st and last P-47B (41-6065) was also used as a test platform as XP-47E to evaluate the R-2800-59 engine, a pressurized cockpit with a hinged canopy and eventually a new Hamilton Standard propeller. The plans for production were canceled after increased emphasis on low-level operations over Europe. Another P-47B was later fitted with a new laminar flow wing in search of higher performance and redesignated XP-47F. In 1942 an example of the potentially 3,000 hp Fairey P-24 Monarch engine along with its Fairey Battle test bed was shipped to Wright Field for testing with a view to possible installation in the P-47. After around 250 hours of test flying of the P-24 engined Battle at Wright Field, the idea to re-engine the P-47 with the P-24 was abandoned.
Production changes gradually addressed the problems with P-47B and the USAAF decided that the P-47 was worthwhile, quickly following the initial order for P-47Bs with another order for 602 more examples of an improved P-47C, with the first of this variant delivered in September 1942. The initial P-47Cs were very similar to the P-47B. Initial deliveries of the Thunderbolt to the USAAF were to the 56th Fighter Group, which was also on Long Island. The 56th served as an operational evaluation unit for the new fighter. Teething problems continued. A Republic test pilot was killed in the fifth production P-47B when it went out of control in a dive on 26 March 1942 and crashed, due to failure of the tail assembly, after fabric-covered tail surfaces ballooned and ruptured. Revised rudder and elevator balance systems and other changes corrected these problems.
Similar to the P-47B, the initial P-47C featured strengthened all-metal control surfaces, an upgraded GE turbosupercharger regulator and a short vertical radio mast. After the manufacture of a block of 57 P-47Cs, production moved to the P-47C-1, which had an 8 in (20 cm) fuselage extension forward of the cockpit at the firewall to correct center of gravity problems, ease engine maintenance and allow installation of a new engine mount. There were a number of other changes, such as revised exhausts for the oil coolers and fixes to brakes, undercarriage and electrical systems, as well as a redesigned rudder and elevator balance. The 55 P-47C-1s were followed by 128 P-47C-2s with a centerline hardpoint with under-fuselage shackles for either a 500 lb (227 kg) bomb or a 200 U.S. gal (758 l, 167 Imp gal) fuel tank that conformed to the underside of the fuselage. The main production P-47C sub-variant was the P-47C-5 which introduced a new whip antenna. With the use of pressurized drop tanks, the P-47C was able to extend its range on missions beginning 30 July 1943. By the end of 1942, most of the troubles with the P-47 had been worked out and P-47Cs were sent to England. The 56th FG was sent overseas to join the Eighth Air Force, whose 4th and 78th Fighter Groups would be equipped with the Thunderbolt as well.
P-47D / P-47G and XP-47K / XP-47L
Refinements of the Thunderbolt continued, leading to the P-47D, which was the most produced version with 12,558 built. The “D” model actually consisted of a series of evolving production blocks, the last of which were visibly different from the first.
The first P-47Ds were actually the same as P-47Cs. Republic could not produce Thunderbolts fast enough at its Farmingdale plant on Long Island, so a new plant was built at Evansville, Indiana. The Evansville plant first built a total of 110 P-47D-1-RAs, which were completely identical to P-47C-2s. Farmingdale aircraft were identified by the -RE suffix after the block number, while Evansville aircraft were given the -RA suffix.
The P-47D-1 through P-47D-6, the P-47D-10, and the P-47D-11 successively incorporated changes such as the addition of more engine cooling flaps around the back of the cowl to reduce the engine overheating problems that had been seen in the field. Engines and engine subsystems saw refinement, (the P-47D-10 introduced the R-2800-63, replacing the R-2800-21 seen in previous P-47s) as did the fuel, oil and hydraulic systems. Additional armor protection was also added for the pilot.
The P-47D-15 was produced in response to requests by combat units for increased range. “Wet” (equipped with fuel plumbing) underwing pylons were introduced to allow a bomb or drop tank pressurized by vented exhaust air to be carried under each wing, in addition to the belly tank. Seven different auxiliary tanks were fitted to the Thunderbolt during its career:
All the P-47s produced to this point had a “razorback” canopy configuration with a tall fuselage spine behind the pilot, which resulted in poor visibility to the rear. The British also had this problem with their fighter aircraft, and had devised the bulged “Malcolm hood” canopy for the Spitfire as an initial solution. This type of canopy was fitted in the field to many North American P-51 Mustangs, and to a handful of P-47Ds. However, the British then came up with a much better solution, devising an all-round vision “bubble canopy” for the Hawker Typhoon. USAAF officials liked the bubble canopy, and quickly adapted it to American fighters, including the P-51 and the Thunderbolt. The first P-47 with a bubble canopy was a modified P-47D-5 completed in the summer of 1943 and redesignated XP-47K. Another older P-47D was modified to provide an internal fuel capacity of 370 U.S. gal (1,402 l) and given the designation XP-47L. The bubble canopy and increased fuel capacity were then rolled into production together, resulting in the block 25 P-47D (rather than a new variant designation). First deliveries of the P-47D-25 to combat groups began in May 1944.
It was followed by similar bubble-top variants, including the P-47D-26, D-27, D-28 and D-30. Improvements added in this series included engine refinements and the addition of dive recovery flaps. Cutting down the rear fuselage to accommodate the bubble canopy produced yaw instability, and the P-47D-40 introduced a vertical stabilizer extension in the form of a fin running from the vertical stabilizer to just behind the radio aerial. The fin fillet was often retrofitted in the field to earlier P-47D bubble-top variants. The P-47D-40 also featured provisions for 10 “zero length” launchers for 5 in (127 mm) High velocity aircraft rockets (HVARs), as well as the new K-14 computing gunsight. This was a license-built copy of the British Ferranti GGS Mark IID computing gyroscopic sight which allowed the pilot to dial in target wingspan and range, and would then move the gunsight reticle to compensate for the required deflection.
The bubbletop P-47s were nicknamed “Superbolts” by combat pilots in the field.
Specifications (P-47D-30 Thunderbolt)
- Crew: 1
- Length: 36 ft 1 in (11.00 m)
- Wingspan: 40 ft 9 in (12.42 m)
- Height: 14 ft 8 in (4.47 m)
- Wing area: 300 ft2 (27.87 m2)
- Empty weight: 10,000 lb (4,535 kg)
- Loaded weight: 12,731 lb (5,774.48 kg)
- Max. takeoff weight: 17,500 lb (7,938 kg)
- Powerplant: 1 × Pratt & Whitney R-2800-59B twin-row radial engine with water injection fed by a General Electric C-23 Turbosupercharger, 2,600 hp (1,938 kW)
- Maximum speed: 433 mph at 29,000 ft (697 km/h at 8,839 m)
- Range: 800 mi combat, 1,800 mi ferry (1,290 km / 2,900 km)
- Service ceiling: 43,000 ft (13,100 m)
- Rate of climb: 3,180 ft/min (16.15 m/s)
- Wing loading: 42.43 lb/ft2 (207 kg/m2)
- Power/mass: 0.204 hp/lb (335 W/kg)
- 8 × .50 in (12.7 mm) M2 Browning machine guns (3400 rounds)
- Up to 2,500 lb (1,134 kg) of bombs
- 10 × 5 in (127 mm) unguided rockets