Republic P-47 Thunderbolt
By Joe Baugher
The P-47 originated on the drawing board of Alexander Kartveli of Seversky Aircraft Corp (later Republic Aviation) and was consistently rated as one of the three outstanding USAAF fighters of World War 2right up there along with the P-38 Lightning and P-51 Mustang. It was built in greater numbers than any other American fighter, 15,683 of them, before production finally ended.
At one time during the heady days of 1944 there were no less than 31 front-line fighter groups flying Thunderbolts on all fronts, including Alaska. Approximately two-thirds of all Thunderbolts built actually reached operational units overseas, and in combat from Mar 1943 to Aug 1945 they flew more than a half-million combat missions, destroying more than 12,000 enemy aircraft in the air and on the ground, as against a total of 5,222 Thunderbolts lost, only 824 of them in aerial combat. That corresponded to 54% of the Thunderbolts overseas being eventually lost to enemy action or accidents, a fairly typical attrition rate for a wartime fighter. Losses on operational missions were 0.7 percent of those dispatched, an exceptionally low figure. By war's end Thunderbolts had established an over-all ratio of aerial combat victories of 4.6 to 1, and had dropped 132,482 tons of bombs, fired 59,567 rockets, and expended 135 million belts of ammunition.
In the relatively short time from D-Day to V-E Day Thunderbolts destroyed 86,000 railway cars, 9,000 locomotives, 6,000 armored vehicles and tanks, 68,000 trucks, 2,752 enemy aircraft in the air and 3,315 on the ground.
The P-47 as originally conceived was quite different from what would ultimately emerge from Republic's factories. On Aug 1, 1939 Kartveli, in response to an official requirement, proposed a lightweight high-altitude interceptor with a company designation of AP-10, to be powered by an 1150hp Allison V-1710-39 liquid-cooled inline engine. That was a radical change in design philosophy for Kartveli, as he had preferred air-cooled radials for fighters because of their greater simplicity and ability to absorb more battle damage. Estimated gross weight was 4900# and top speed 415 mph. Armament would be two .50 machine guns in the top engine housing.
The USAAC was impressed; however, they stressed that more armament would be required, even at the cost of performance. Kartveli increased the size of his AP-10 design somewhat and added four wing-mounted .30 machine guns. In this guise, the AAC ordered one prototype in Nov 1939 as XP-47 [40-3051]. Then on Jan 17, 1940 they ordered a stripped version of the same basic design as XP-47A [40-3052], devoid of armament, radio, and other tactical equipment so it could be tested before the fully-equipped XP-47.
Meanwhile, reports from Europe were changing everyone's ideas about air combat. More firepower, more armament and armor-plate, and self-sealing fuel tanks were likely to be critical in future air battles. Both XP-47 and XP-47A had insufficient power to bear the additional weight required by such features, and the AAC concluded that they would likely fall far short of future air combat requirements. The Army complained that XP-47 was too lightly armed, that it had too high a wing loading and was too slow in comparison with the Curtiss XP-46. Anticipating that the Army would ultimately reject his XP-47 design, Kartveli went back to the drawing board.
In order to accommodate heavy firepower, armor, and self-sealing fuel tanks and still provide a performance capable of meeting enemy aircraft on equal terms, a lot of horsepower would be needed. Kartveli's design was based around an 18-cylinder, air-cooled, turbo-supercharged P&W R-2800 Twin Wasp radial, one of the most powerful aircraft engines at the time. Armament was to be eight .50 machine guns in the wings, following the RAF's trend toward eight-gun fighters. It would be the heaviest-armed fighter considered by the AAC up to that time. Total weight was a massive 11,500#, unprecedented for a single-seat fighter. Maximum speed of 400 mph at 25,000 feet and 340 mph at 5000 feet was projected, as was a climb rate of five minutes to 15,000'.
On June 12, 1940 Kartveli submitted his ideas to the AAC, who were sufficiently impressed that on Sept 6, 1940 a prototype was ordered as XP-47Bit was somwhat unusual at the time to use the same P-number for what was in effect a totally new design. All work on the XP-47 the XP-47A was cancelled, and the serial number of the abortive XP-47's s/n was transferred to XP-47B.
A week later, on September 13, 773 production examples of the new fighter were ordered, 171 to be delivered as P-47Bs and 602 as P-47Cs. At the same time, a 1939 contract for 80 P-44 Rockets was cancelled and replaced with an order for a similar quantity of P-43 Lancers, which would keep Farmingdale production lines occupied pending introduction of their new fighter.
Kartveli decided to design the XP-47B fuselage around the large turbo-supercharger from the start, rather than to add it later. In order to preserve a streamlined fuselage with a small cross-section, the large turbo-supercharger was placed in the rear fuselage, fed by an air duct beneath the engine. Engine exhaust gases were directed to the rear fuselage in separate pipes to the turbine and were expelled through an exhaust under the tail. Ducted air was fed to a centrifugal impeller and was returned to the engine under pressure via an intercooler.
Another problem was that the aircraft required a very large 12' four-blade propeller in order to take full advantage of the R-2800's high power output, which in turn required a long and stalky undercarriage to give it adequate ground clearance during take-off and landing. If a conventional retractable gear were used for the P-47, its suspension needed to be placed very far outboard on the wings, leaving insufficient space for the eight wing guns and their ammunition. The solution was a telescoping landing gear that was nine inches shorter when retracted than when extended. Somewhat surprisingly, the complex landing gear seems to have caused but few problems in the field.
Like the earlier P-35 and P-43, the P-47 was a cantilever low-wing monoplane, the wing being elliptical with ailerons on the outer trailing edge and flaps on the inner trailing edge. Its semi-monocoque fuselage was all metal, but initially the control surfaces were fabric covered. Tailwheel was steerable and fully retractable. All fuel tanks were inside the fuselage and were self-sealing. The cockpit was protected by armor and was unpressurized.
The name *Thunderbolt* for the P-47B was thought up by C Hart Miller, Republic's director of military contracts. The company approved his choice and the name stuck. The XP-47B prototype [40-3051] flew for the first time on May 6, 1941, piloted by Lowry L Brabham, only eight months after the order had been placedon that flight, Brabham had to make an emergency landing because of exhaust fumes into the cockpit. The XP-47B was the largest single-engine fighter built up to that time. At a loaded weight of 12,086 pounds, the XP-47B dwarfed all previous fighters, being almost twice as heavy as most of its contemporaries.
Its 18-cylinder XR-2800-21 radial provided 1960hp at 25,800' and gave it a maximum speed of 412, which was 12 mph faster than Kartveli had projected. Empty and normal gross weights were 9189# and 12,086#.
The first production P-47B [41-5895] was really a specially-built second prototype. Delivered to the Army on Dec 21, 1941, it was dispatched to Wright Field for testing; XP-47B remained with Republic. The next four production P-47Bs [41-5896/5899] were delivered in mid-March 1942 for an extensive test program by various agencies.
Numerous problems soon surfaced in testing. [41-5899] crashed when part of the tail assembly broke off in flight on Mar 26, 1942 killing Republic test pilot George Burrrell, which resulted in restrictions being placed on P-47B flight while the cause was under investigation. At altitudes above 30,000' ailerons tended to freeze, the cockpit canopy stuck shut, control forces were excessive, and the fabric of the elevators often ruptured after high-speed flight when aerodynamic pressures caused it to balloon and burst. Those problems caused further P-47 acceptances to be delayed until May 1942.
The problems of freezing ailerons and ruptured elevators was solved by having those control surfaces metal-clad on subsequent P-47Bs. Most earlier P-47Bs were eventually modified thusly and the earlier restrictions removed. Also the ailerons were revised in shape and fitted with blunt noses, which largely alleviated the excessive control force problem; balanced trim-tabs were adopted to reduce rudder pedal loads.
The canopy problem was solved by replacing the original hinged canopy by a rearward-sliding hood. That required the dorsal radio antenna to be redesigned and moved further aft, an innovation believed to have been applied from P-47B [41-5896] onward. Windshield defroster was introduced with P-47B [41-5951]. Beginning with [41-5974] major changes were made in control surface movement limitations and tailplane incidence, new landing gear tires were introduced at [41-5974], and modified link ejector chutes were added to guns on [41-6016] and subsequent aircraft.
Production P-47Bs had the 2000hp R-2800-21 and a 12'2" Curtiss Electric C542S-A6 propeller. An increase of internal equipment raised the empty, normal loaded, and maximum loaded weights to 9346, 12,245, and 13,360 pounds respectively. Consequently, the climb to 15,000 feet took 6.7 minutes rather than the promised five; however, the production-ready engine provided an increase in level speed to 429 mph at 27,000'.
At one time it was hoped that it would be possible for the RAF to test the Thunderbolt in combat in the Middle East, but production difficulties caused the Air Ministry to be informed in Sep 1941 that it was probably not a good idea until all the bugs had been removed from the design.
P-47Bs were first issued in mid-1942 to the 56th Fighter Group (FG), chosen as the first recipient because it was based near NYC, close to the Farmingdale plant where Republic engineers could be easily called upon to solve problems as they arose. P-47Bs of the 56th were used largely for stateside testing and operational training, very few went overseas, and the Group found working up to its new mounts rather difficult13 pilots and 41 aircraft were lost in accidents. By the end of June half of their aircraft were damaged or destroyed. Many crashes were the result of pilot inexperience, but a significant number were caused by loss of control during high-speed dives. When the rudder was ripped from a P-47B in flight, an order was issued on Aug 1, 1942 restricting the speeds to 300 mph or less, forbidding violent maneuvers, and stipulating that fuel be carried in the rear tank.
The omega P-47B [41-6065] was first redesignated XP-47E, modified with a hinged canopy and pressurized cockpit, then as XP-47F when it flew on Sep 17, 1942 to test a new larger-area wing with a laminar-flow aerofoil; no production of either followed. Prototype XP-47B was destroyed in an accident on Aug 8, 1942. The last P-47B was delivered in Sep 1942. Active P-47Bs in 1944 were redesignated RP-47B as "Restricted," not to be used for combat.
POP: 1 XP-47B [40-3051].
POP: 170 P-47B [41-5895/6065].
P-47Cs began to appear on the production lines in late 1942, externally similar to the P-47B, but strengthened and revised fin with a metal-covered rudder to eliminate a tail flutter. That resulted in an increase in over-all length of about an inch. The oxygen system was upgraded four cylinders, one of them in the leading edge of the port wing. A new radio set (SCR-274-N and SCR-515-A) was installed and the forward-slanted radio antenna of the P-47B replaced by a shorter upright mast.
The first P-47C [41-6066] was completed on Sep 14, 1942. Even with strengthened tail surfaces, it still had problems in recovering from high-speed dives. Beyond 500 mph, recovery from power dives was extremely hazardous, with unresponsive elevators because of compressibility forces. It was prototype for the modifications that would becom P-47-C1.
The P-47C-1-RE production block differed by having an extra eight-inch section added to the fuselage forward of the firewall, giving improved flight characteristics through movement of the center of gravity. There were also some detail changes to the landing gear and brakes, some in the tail wheel, and its steering was eliminated. There were modifications of the supercharger air ducting, bob-weights were installed in the elevator control system to help overcome the compressibility problems. Latches for linking the engine throttle, propeller, and turbo-supercharger were added, making correlated operation possible by moving a single lever.
On Nov 13, 1942 Lts Harold Comstock and Roger Dyar claimed indicated airspeeds of 725 mph during high-speed dives in their P-47Cs, which was beyond the speed of sound and which, if accurate, would make them first to break the sound barrier. However, airspeed readings probably were wildly inaccurate since the design terminal velocity of the P-47 was about 600 mph. True airspeeds reached were more likely in the 500 mph range.
Engineers at Wright Field did an extensive evaluation of the P-47C-1 and found that it had the best aileron-roll rate of any US fighter, but that that pilots' view over the nose was restricted, making deflection shooting extremely difficult. The complex ducting that ran along the bottom of the fuselage connecting engine and turbo-supercharger also had an unintended benefit in serving as a crushable buffer in belly landings, protecting the pilot's legs and preventing the aircraft from disintegrating on impact.
The first Thunderbolt to be considered truly combat-ready was the P-47C-2. Perhaps the most notable change in the production block was the provision for shackles and a release mechanism for a bomb or a fuel tank on the belly. With a 200-gallon belly tank, range was extended to 1,250 miles at 10,000' and cruising speed was 231 mph. P-47C-5 introduced revised radio and antenna, instruments, and cockpit heater. The improved P-47D replaced it on the production line in Feb 1943.
POP: 58 P-47C [41-6066/6123]
POP: 54 P-47C-1 [41-6124/6177]
POP: 128 P-47C-2 [41-6178/6305]
POP: 362 P-47C-5 [41-6306/6667]
Next up: P-47D Thunderbolt.
-- American Combat Planes, Third Enlarged Edition, Ray Wagner (Doubleday 1982).
-- The American Fighter, Enzo Angelucci & Peter Bowers, Orion, 1987.
-- Republic P-47D Thunderbolt, Aircraft in Profile, Edward Shacklady, Doubleday, 1969.
-- Thunderbolt!, Robert S Johnson & Martin Caidin, Ballantine Books, 1958.
-- Thunderbolt: A Documentary History of the P-47, Roger Freeman, Motorbooks, 1992.
-- United States Military Aircraft Since 1909, Swanborough & Bowers (Smithsonian 1989)
-- War Planes of the Second World War, Fighters, Volume 4, William Green, Doubleday 1964.