Conquering the Sky!

Air Classics, Apr 2005 by O'Leary, Michael

In July 1938, Lockheed began XP-38 subassembly construction in a partitioned area of a Burbank hangar. Workers had to overcome problems fitting and flush-riveting the aluminum skin; this was truly a state-of-the-art machine with an attendant learning curve. As Johnson moved up the company ladder, Jim Gerschler took over as project engineer and quickly made some changes - including the addition of Allison V-1710-C7/C9s with counter-rotating propellers. The star-board engine would rotate in a clockwise direction to counter the torque forces of the anti-clockwise-turning port propeller, eliminating the tremendous torque of two big engines turning in the same direction.

The XP-38 was like no other fighter that had flown in the USA. First of all, it was huge. Weighing in at more than 15,000-lbs, and with a wingspan of 52-ft, it looked more like a bomber. Lockheed had given the twin Allisons extremely snug cowlings. The flush riveting was first-rate, the polished metal unmarred by protrusions to impair streamlining.

Large Fowler flaps, used with great success on some of Lockheed's transport aircraft, were added to both of the XP-38's wing sections to improve low-speed handling. The long, slim booms that replaced a "traditional" fuselage housed General Electric Type F turbosuperchargers and the main gear, and supported the graceful twin fins that had become a Lockheed trademark.

The nacelle for the pilot was mounted on the wing center section between the two engines. The design team liked the idea of centering the armament package (at this point, still undecided) for several reasons: It simplified maintenance, a more concentrated cone of firepower could be achieved by a battery of guns, and the center of gravity could be more accurately maintained.

As construction neared completion the XP-38's armament was standardized as one 23mm Madsen cannon and four Browning .50-cal M2 air-cooled machine guns with 200-rpg, although armament was never fitted to the prototype.

Lockheed, Allison, and General Electric worked closely to ensure that the operation of the engines at high altitude would be as efficient as possible. The network of tubing and ducting connected each engine to its individual turbosupercharger in the boom. The turbine wheel was located in a semi-flush housing atop each boom over the trailing edge of the wing. As the aircraft gained altitude, the turbosupercharger would begin to function, taking exhaust gases from the engine to the turbine via the tubing. Spinning at tens of thousands of rpm, the turbine would compress the exhaust gases and pass them through an intercooler in the wing leading edge. The compressed and cooled gas was then channeled back down the engine's throat to maintain manifold pressure at altitude.

The designers knew the engines would have to be closely cowled for maximum streamlining but location of the oil coolers and radiators posed a problem. Openings would have to be as large as possible to enable cooling air to do its job but, at the same time, the opening had to be small to minimize drag. The XP-38 presented particular difficulties since it had two of everything. The radiators were located in the middle of the booms, in large blisters with openings in the front and control shutters at the rear. The oil coolers were immediately behind the spinners. The cooling fluid (Prestone) had to be pumped from the radiator to the engine via tubing and a series of pumps.