Although it passed through a large number of variants, the P-38 saw relatively few major changes. Most of those changes came with the P-38J, which saw major changes to the cooling system, flight controls and dive controls, with the result that the P-38J was able to hold its own against the best German and Japanese fighters.
The P-38H had seen the Lightning given Allison V-1710-89/-91 engines, capable of producing 1,450 hp. However, the intercoolers on the P-38, located on the leading edge of the wings, did not provide sufficient cooling, and so these engines were restricted to only 1,240 hp each.
In the P-38J the intercoolers were moved from the wing leading edge to the engine nacelles. This required a redesign of the nacelles, and the addition of a much larger three-part air scoop on the chin. Although this increased the drag caused by the engines, it allowed the use of all 1,450hp of the Allison engines, increasing the top speed of the P-38 to 414 mph. Lockheed used the space freed up by the removal of the intercoolers from the wings to fit extra fuel tanks.
In 1943 the RAE carried out a series of tests comparing how the P-38, P-47, P-51 and Spitfire behaved in a high speed dive. These tests revealed that the P-38 suffered from compressibility (loss of control at high speed) at a much lower speed than the other aircraft, failing at Mach 0.70 (the Spitfire was best, remaining controllable up to Mach 0.88).
German pilots soon learnt to take advantage of this problem, diving away from the P-38 until the American aircraft reached the critical speed. At that point the German pilot would pull out of the dive, leaving the P-38 in an uncontrollable dive.
This problem was largely solved during the production run of the P-38J by the addition of an extra set of flaps, under the outer wing panel. These compressibility flaps altered the air flow over the wings, restoring lift at high speeds and making it easier for the P-38 pilot to pull out of high speed dives.
The final major change made to the P-38J was the addition of hydraulically powered ailerons, dramatically reducing the effort needed to manoeuvre the P-38, increasing its roll rate and allowing the P-38J to stay with the more manoeuvrable German fighters.
The P-38J also features a redesigned windscreen, replacing the round windscreen used in earlier versions with a flat three-piece windscreen made from armoured glass.
Lockheed produced a total of 2,970 P-38Js. The type was also the basis of the F-5B, F-5E-2 and F-5E-3 photo reconnaissance aircraft. The P-38J became the standard version of the Lightning from early 1944.
Engine: Allison V-1710-89/-91
Max Speed: 414 mph
Ceiling: 44,000 feet
Range: 2,600 miles (with 300 gallon drop tanks)
Span: 52 feet
Length: 37 feet 10 inches