The Lockheed P-38 Lightning
By Chuck Hawks
The P-38 shot down more Japanese aircraft than any other USAAF fighter in WW II. It was flown by both of the top American aces of the war. Its incredible range became legendary and its twin engines particularly suited it for long over water flights. This last point is striking, for the P-38 was not designed for long range missions. It was originally designed as a high performance, short range interceptor!
Equally contradictory, while the P-38 was produced in large numbers during World War II, it was not designed for mass production. On the contrary, it was designed only for limited production, almost to be hand made.
The story of the design of the P-38 is a fascinating one, perhaps the strangest of any of the famous fighters of WW II. I do not have the space here to go into it in great detail, but I will try to touch on a couple of the high points.
It started in January of 1937, when the Army Air Corps circulated to aircraft manufacturers a specification for a new pursuit plane for the "interception and attack of hostile aircraft at high altitude". They wanted a max. speed of 360 m.p.h. at 20,000 ft., and climb from takeoff to 20,000 ft. in 6 minutes. There were other details, but the point is that the demand was for a high performance interceptor. The government anticipated an order for a maximum of 50 planes, so suitability for mass production was not a consideration. Lockheed was one of the companies that entered the competition to design and build the new fighter.
H. L. Hibard and Clarence "Kelly" Johnson were assigned the job of primary design. Johnson realized that no existing engine could provide enough power to meet the government specification and began a series of single seat, twin engine fighter designs. The new Allison V-1710 had developed 1000 hp. in tests and was chosen by the Lockheed design team for the new fighter.
The final layout of the new twin engine fighter (called the Model 22 by Lockheed) had counter rotating props that eliminated the torque related control problems that plagued many contemporary fighters (such as the German ME 109), twin tail booms and a central fuselage for the pilot. It was powerful, heavy and had a wing loading far in excess of any contemporary fighter, but maneuverability was not deemed particularly necessary for an interceptor. It also had a tricycle landing gear at a time when almost all other fighters were tail-draggers and a control wheel (later yoke) instead of a stick.
In order to meet the high altitude requirements, turbo superchargers were adopted. The nose of the central fuselage provided an ideal place to mount the very effective armament of 1-20mm cannon and 4-.50 cal. MG. There was no need for an interrupter gear to shoot through a propeller and no need to "converge" wing guns. The armament fired straight ahead for a full 1000 yards. Compare that to the widely separated wing guns of the British Spitfire, usually set to converge at 300 yards.
In June 1937, the Army notified Lockheed that their design had won the competition, and authorized Lockheed to build one prototype airplane, designated the XP-38. In late December 1938 the prototype was ready to fly. It was the most streamlined plane ever seen, built with flush riveted external panels butted together. Stainless steel was used extensively in its construction.
That first XP-38 proved to be capable of a level speed of 413 m.p.h., and had a terrific climb rate. In fact, throughout the war, the P-38 remained one of the fastest climbing American fighters. Unfortunately, the first prototype lasted only 16 days. The testing program had barely begun when the Army decided to use it in a record setting cross-country flight that ended with a landing short of the runway, which wrote off the prototype. Tony LeVier (Lockheed Chief Test Pilot) later estimated that disaster set the program back nearly two years. It also probably cost many brave American aviators their lives when their inferior and obsolescent planes came up against advanced Axis fighters like the Zero and ME 109.
Had the original prototype not been lost, those men could and should have been flying high performance P-38s. As it was, Lockheed had to start from scratch, build another prototype and run a whole new test program.
Jump ahead to April 1939. The Air Corps ordered 13 YP-38 airplanes for testing. In September 1939, the Army ordered 66 more for service. In August 1940, the Army ordered over 600 more P-38s. The war was on in Europe and China and the P-38 was the only high performance fighter available. Except, of course, ordering almost 700 fighters was not the same thing as delivering the airplanes. At that time, Lockheed had not even delivered the first YP-38!
As alluded to earlier, the P-38 was not designed for mass production. In fact, it was intended to virtually build each of the 50 originally anticipated aircraft by hand. Many, many production problems had to be solved before the Lightning could be produced in quantity. As well as some serious engineering problems.
The P-38 was one of the first airplanes fast enough to encounter "compressibility" (more properly called shock stall) problems in high altitude, high speed dives. The basic problem was that in a sustained dive from high altitude, speed quickly built to the point that the airflow over parts of the airplane (such as the upper surface of the wing) reached supersonic speeds. Not that the airplane itself was breaking the sound barrier, but the airflow in certain places was. A shock wave forms. This destroys the lift over that part of the wing. It also caused the air flowing off the wing to affect the tail in an unusual manner: it increased lift at the tail (Which is normally negative--an airplane is balanced by the weight in front of its wings, a down force; the lift of its wings, an up force; the negative lift of its tail, a down force--imagine a teeter/totter).
This loss of lift from the wings, coupled with increased lift from its tail, causes the nose of the airplane to go down. The increased dive angle causes the speed to increase farther. And so on, in a vicious and often fatal circle. The natural response of the pilot is to pull back on the yoke, which normally causes the elevators at the tail to increase the down force at the tail and brings the nose up to pull out of the dive. However, something terrifying happens. As the pilot tries to pull the stick back, the up force on the tail increases. No matter how hard he pulls, the aerodynamic force on the tail pushes harder. The controls have been described as feeling as if they were set in concrete. At this point the airplane is totally out of the pilot's control; there is literally nothing he can do.
The P-38 was not the only airplane to encounter this effect in dives from very high altitudes (where the air is thin), the P-47 and F4U both suffered the same problem. However, the P-38 was different. The big radial engine fighters would dive uncontrollably toward the earth until they reached the thicker air at lower altitudes. There two things happened: 1. The speed of sound increases as an inverse function of altitude (that is, the speed of sound goes up as the altitude gets lower); 2. The increased drag of the thick air on their large frontal surfaces would tend to limit further speed increases.
The result was that when the speed of sound went up as the airplane got lower, the shock waves started to dissipate (the airflow over the wings began to fall back below the increased speed of sound), and as the increased drag started to affect the airplane, the speed of the airflow also decreased, and the shock waves dissipated more. Finally the pilot would begin to get some control back, and still pulling back as hard as he could on the stick, would wind up in a screaming zoom climb (unless he was unfortunate enough to have begun the process over mountains high enough to intrude before he reached the thicker air of lower altitudes).
The way in which the P-38 differed was in its extremely "clean" (streamlined) design. Its drag was so low that the thicker lower air often (not always, some pilots did survive compressibility dives in P-38's) did not have enough effect for the pilot to regain control in time: the P-38 just dove straight into the ground like an arrow. The problem was magnified by a "flutter" (increasing amplitude vibration) set up in the tail by these excessive speeds, which often caused the tail to come off.
Lockheed and the Air Corps lost a number of test pilots and aircraft trying to understand and solve these problems. The P-38 had taken them into flight regimes unknown (or at best poorly understood) at that time.
A harrowing series of test dives, at progressively steeper angles, was required to plot the boundaries of these effects. The eventual solution included counter balancing and raising the tail of the airplane some 30 inches, and developing high speed dive flaps to control the rate of descent.
My father was at this time a young aeronautical engineer in the AAF, and he was the flight test engineer on some of those test dives. Years later he told me that he fully expected to be killed in that program; many of his friends were.
Lockheed produced dive flap kits to retro-fit to planes in the field, but it was not until they began producing the P-38J-25-LO model that dive flaps were incorporated in the new aircraft coming off the assembly line.
An interesting tidbit of Lightning lore is that during the war Charles Lindbergh traveled to the Pacific theater to teach pilots there some fuel conservation tricks for long range flights. He took the opportunity to fly a few P-38 combat missions (without authorization), and scored at least one aerial victory. The War Department was horrified (he was still a civilian, and far too famous to risk in combat), and whisked him home. A brief chronology of the major P-38 combat models follows.
The P-38D appeared in August of 1941. This was the first model to benefit from changing the angle of the tail, and re-balancing the elevator, which largely eliminated tail flutter. The "D" also introduced self sealing gas tanks.
The P-38F went into production in March 1942, and into combat in the Pacific in December, where they were to reverse the fortunes of AAF fighter pilots facing the previously unbeatable Zero. The "F" had an up rated 1,325 hp. Allison engine. Top speed was 395 m.p.h. at 25,000 ft.
P-38G models had strengthened Fowler flaps which could be used at combat speeds up to 250 m.p.h. to tighten the turning radius. In Europe, pilots of the big Lightnings now found that they could turn inside of the smaller German fighters, particularly at low altitudes. They also had more powerful engines (a 100 hp increase). Production began in August 1942. The "H" model was similar.
The P-38J began production in mid-1943. It incorporated many improvements, including more powerful engines, improved superchargers, relocation of the intercoolers from the leading edge of the wings to beneath the nose of the engines, a bulletproof windscreen, and, at the J-25-LO model, the factory installed dive flaps. Speed was up to 426 m.p.h., and best climb to 3,900 ft./min. It would climb to 20,000 ft. in 5.9 minutes.
The "K" was a special high altitude model, and the subsequent P-38L of 1944 was the final and best Lightning. It incorporated many of the improvements of the "J" and "K" models, and provided the greatest margin of combat superiority over Axis fighters of any Lightning model. Specifications of the P-38L-5-LO follow.
The P-38J and L models provided a margin of superiority over enemy fighters that must have been comforting to P-38 pilots. I have read that modern WW II aviation gamers often call the P-38L a "Super Fighter." An astonishing accolade from computer "pilots" that were not even born when the P-38 was dominating the skies over Europe and the Pacific.
Copyright 2003, 2008 by Chuck Hawks. All rights reserved.