Main title

P-47 Thunderbolt: Aviation Darwinism

Chapter Three:

When one studies the design philosophy of the P-47, one cannot help but realize that Major Seversky’s vision of a true high performance fighter had been fulfilled. Since 1939 Seversky had stated that any new fighter designs would need to be bigger, faster and higher flying. The USAAF, already unhappy with Seversky for late deliveries and using Air Corps money to fund racing versions of the P-35, not only ignored the Major, they spared no effort to discredit him. Yet, within a few months, circumstances had evolved, largely as a result of war in Europe. As a result, the lightweight fighter concept, liked very much by Kartveli, was a philosophy now bankrupt. Finally out from Seversky’s shadow, Kartveli had thrown himself and his engineering staff into the XP-47 and XP-47A designs. It must have been a terrible shock to find out that all their work was as good as for nothing. A more sobering realization for Kartveli was that the Major had been correct all along.

XP-47B, 5/2/41
The XP-47B on roll out day, May 4th, 1941.

Unlike the XP-44 design, no mock-up of the XP-47B was requested by the USAAC, nor was any money allocated for one. The new fighter could not hope to go efficiently from the drafting table to the factory floor. Therefore, a mock-up was built at company expense. In early production P-47’s the vast majority of the cockpit section was taken straight from the P-43. However, for some unknown reason, the mock-up, prototype and the first three production P-47B aircraft were built with an unusual and difficult to use fixed canopy equipped with a forward opening door not greatly unlike the P-39 and early Hawker Typhoon. Fortunately, whoever selected this oddball canopy design was eventually over-ruled and the P-43 arrangement was implemented after the fourth aircraft.

XP-47B, canopy
The unusual cockpit door of the XP-47B is clearly seen in the inset.
Note that the vent window is open.

Work on the first flyable XP-47B moved along at a brisk pace and the “eggheads” from Wright Field were invited to inspect the new fighter. Generally, they were happy with what they saw. One problem discovered was that measured fuel capacity was somewhat less than the specification had called for. Only 298 gallons could be squeezed into the tanks, 17 gallons less than the requirement. The weight of the plane was greater than the specification requirements as well. Republic’s design team had built immense strength into the design. This pushed the weight up to just over 12,500 lbs, or about 900 lbs over the required limit. The engineers from Wright Field indicated that these issues could be overlooked if the fighter performed to specification.

XP-47B tethered for high power run-up
Tied down and chocked, the prototype XP-47B is readied for
a maximum power run-up the day before its first flight.

Finally, on May 6th, 1941, the big fighter was ready for its first flight. With test pilot Lowery Brabham at the controls, the XP-47B roared off Republic’s wet sod field, getting airborne after a scant 2,500 feet of takeoff roll. Brabham was instantly pleased with the fighter's handling and power. Nonetheless, as he climbed and the ambient pressure dropped, smoke began to fill the cockpit. Unable to open the cockpit door in flight, Brabham opened a small vent window. That, however, was a mistake. The velocity of the air rushing past the vent served only to lower the relative pressure across the vent, resulting in even more smoke being drawn into the cockpit. Concerned, but not panicked, Brabham decided to get the ship down quickly. Thoughts of losing the prototype on its maiden flight were all the motivation he needed. Remembering how soft the wet sod had been at Farmingdale, Brabham headed for the paved runways of the nearby Air Corps facility at Mitchel Field.

XP-47B in color
The XP-47B was photographed in color several times during its early test flights.

The first landing of the XP-47B was uneventful. The flaps, brakes and landing gear worked as advertised and Brabham taxied in towards the Air Corps hangers. His arrival, however, was indeed an event. Army and Air Corp personnel poured out to greet the big fighter as it rolled to a stop with its huge propeller winding down. Nothing like the XP-47B had ever been seen before. Senior officers quickly cleared the field and the new fighter was quickly rolled into a hanger and the doors shut.

It took but a quick inspection to determine what had caused the smoke in the cockpit. Oil in the turbo-supercharger ducting was the culprit. Prior to taking off, Brabham had performed an extensive run-up on the concrete ramp. He checked, double checked and even triple checked every engine instrument. He performed several mag checks and made sure the engine was at optimal operating temperature. During this time, oil had been accumulating in the ducting leading to the turbo-supercharger installed behind the cockpit. The ducting ran just below the cockpit. The engine is fitted with a pair of wastegates that dump excess boost and thereby regulate manifold pressure. The wastegates are in turn, controlled by a governor. At low altitudes the governor monitors and is itself controlled by maximum manifold pressure. At altitude, the governor responds to turbine speed. As the XP-47B climbed out, the governor closed the wastegates. The oil in the ducts was rapidly heated and began to give off smoke.

Mitchel Field, 1931
Mitchel Field as it appeared in 1931. The photo has been colorized.

The XP-47B would remain at Mitchel Field for about a month as modifications were made to eliminate the possibility of oil smoke entering into the cockpit. Some additional, but minor changes were implemented and surprisingly, no national insignia was yet applied to the wings and fuselage. The XP-47B was never delivered to Wright Field, as had been the practice for all new designs for many years. It was tested in the skies over Long Island. Having been assigned to Republic in order to expedite any required changes, it would remain in the hands of its manufacturer until its inadvertent loss in 1942.

The First production P-47B
The first P-47B off of the line was flown to Wright Field for testing. The first production
aircraft was actually the fifth airframe completed, and the first with a sliding canopy.

In the meantime, The XP-47B revealed that it was everything that it was hoped it would be. It attained a corrected true airspeed of 412 mph at 25,800 ft. The big Pratt & Whitney R-2800-17 proved to be reliable and actually produced the horsepower that Pratt & Whitney had claimed it would. The Curtiss Electric propeller worked well at getting all 1,960 hp harnessed for thrust. There were some problems still to be worked out. The turbo-supercharger installation increased the risk of a fire (this is exactly what caused the loss of the prototype nearly a year later). The cockpit canopy needed to be rethought. Indeed, there would be a myriad of minor changes that were to be incorporated into the first production aircraft. But, the die was cast. Alexander de Seversky had been vindicated. His theories had passed the test of reality and the fate of many a German and Japanese airman had been sealed.

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