Lockheed P-38 Lightning

Lockheed P-38 Lightning — The Lockheed P-38 Lightning was a World War II American fighter aircraft built by Lockheed. Developed to a United States Army Air Corps requirement, the P-38 had distinctive twin booms and a single, central nacelle containing the cockpit and armament. Named “fork-tailed devil” by the Luftwaffe and “two planes, one pilot” by the Japanese, the P-38 was used in a number of roles, including dive bombing, level bombing, ground attack, photo reconnaissance missions, and extensively as a long-range escort fighter when equipped with drop tanks under its wings.

The P-38 was used most successfully in the Pacific Theater of Operations and the China-Burma-India Theater of Operations as the mount of America’s top aces, Richard Bong (40 victories) and Thomas McGuire (38 victories). In the South West Pacific theater, the P-38 was the primary long-range fighter of United States Army Air Forces until the appearance of large numbers of P-51D Mustangs toward the end of the war. The P-38 was unusually quiet for a fighter, the exhaust muffled by the turbo-superchargers. It was extremely forgiving, and could be mishandled in many ways, but the rate of roll was too slow for it to excel as a dogfighter. The P-38 was the only American fighter aircraft in production throughout American involvement in the war, from Pearl Harbor to Victory over Japan Day.

Design and Development

Lockheed YP-38 (1943)Lockheed designed the P-38 in response to a 1937 United States Army Air Corps request for a high-altitude interceptor aircraft having “the tactical mission of interception and attack of hostile aircraft at high alitude”. Specifications called for an maximum airspeed of at least 360 miles per hour (580 km/h) at altitude, and a climb to 20,000 feet (6100m) within 6 minutes. The Bell P-39 Airacobra and the Curtiss P-40 Warhawk were designed to the same requirement.

The Lockheed design team, under the direction of Hall Hibbard and “Kelly” Johnson, considered a range of configurations. All options considered by Lockheed were twin engined, as it was judged that no single available engine was powerful enough to be able to meet the USAAC’s requirements (Engine development during World War II subsequently saw an approximate doubling of fighter engine horsepower, allowing many single engine designs to achieve 400+ mph). The eventual design was somewhat unique in comparison to existing fighter aircraft. The Lockheed team chose twin booms to accommodate the tail assembly, engines and turbo-superchargers, with a central nacelle for the pilot and armament. The nose was designed to carry two Browning. 50” (12.7mm) machine guns with 200 rounds per gun, two. 30” (7.62mm) Brownings with 500 rounds per gun, and an Oldsmobile 37mm cannon with 15 rounds. Clustering all the armament in the nose was unlike most other US aircraft, which used wing-mounted guns, where the trajectories were set up to criss-cross at several points in a “convergence zone”. The nose-mounted guns did not suffer from having their useful ranges limited by pattern convergence, meaning good pilots could shoot much farther. A Lightning could reliably hit targets at any range up to 1,000 yards, whereas other fighters had to pick a single convergence range between 100 and 250 yards. The clustered weapons had a “buzz-saw” effect on the receiving end, making the aircraft effective for strafing as well. It is therefore ironic that, Dick Bong, the United States’ highest-scoring World War II air-ace (40 kills solely in P38s), would fly directly at his targets to make sure he hit them (as he himself acknowledged his poor shooting ability), in some cases flying through the debris of his target (and on one occaison colliding with an enemy aircraft which was claimed as a “probable” kill).

The design was the first fighter to utilize tricycle undercarriage, and featured two 1000 hp (746 kW) turbo-supercharged 12-cylinder Allison V-1710 engines fitted with counter-rotating propellers to eliminate the effect of engine torque, with the superchargers positioned behind them in the booms.P-38J flying over California.Lockheed won the competition on 23 June 1937 with its Model 22, and was contracted to build a prototype XP-38. Construction began in July 1938 and the XP-38 first flew on 27 January 1939. The 11 February 1939 flight to relocate the aircraft for testing at Wright Field was extended by General Henry “Hap” Arnold, commander of the USAAC, to demonstrate the performance of the aircraft. It set a cross-continent speed record by flying from California to New York in seven hours and two minutes, but landed short of the Mitchel Field runway in Hempstead, New York, and was wrecked. However, on the basis of the record flight, the Air Corps ordered 13 YP-38s on 27 April 1939.

Manufacture of the YP-38s fell behind schedule – the first aircraft was not completed until September 1940, first flying on 16 September 1940,the last delivered to the Air Corps in June 1941. They were substantially redesigned and differed greatly in detail from the hand-built XP-38. They were lighter, included changes in engine fit, and the propeller rotation was reversed, with the blades rotating outwards (away) from the cockpit at the top of their arc rather than inwards as before. This improved the aircraft’s stability as a gunnery platform.

Test flights revealed a problem that initially looked like tail flutter. During high-speed flight approaching Mach 0.68, especially during dives, the airplane’s tail would begin to shake violently and the nose would drop. Once caught in this dive the plane would enter a compressibility stall and the controls would lock up, leaving the pilot no option but to bail out (if he could) or stick with the plane until it got down to denser air where he might have a chance to pull out. During one flight on 4 November 1940, the tail structure fell apart during a high-speed dive, killing YP-38 test pilot Ralph Virden. On another, USAAC Major Signa Gilkey managed to stay with a YP-38 in a compressibility lockup, riding it out until he reached denser air, where he recovered using elevator trim. In 1940, flutter was a familiar engineering problem related to a too-flexible tail, though the P-38’s empennage was completely skinned in aluminum (not fabric) and was quite rigid. At no time did the P-38 suffer from true flutter. To prove a point, one elevator and its vertical stabilizers were skinned with metal 63% thicker than standard; the increase in rigidity made no difference in vibration. Army Lt. Colonel Kenneth B. Wolfe (head of Army Production Engineering) asked Lockheed to try external mass balances above and below the elevator, though the P-38 already had large mass balances elegantly placed within each vertical stablizer. Various configurations of external mass balances were equipped and test flights flown to document their performance and in Report No. 2414, Kelly Johnson wrote “…The violence of the vibration was unchanged and the diving tendency was naturally the same for all conditions.” The tests were a bust. Nonetheless, at Wolfe’s insistence, the additional external balances were a feature of every P-38 built from then on.

The compressibility problem was ultimately sidestepped by changing the geometry of the P-38’s wing when diving. In 1943, quick-acting dive flaps were incorporated into the production line. The flaps were installed outboard of the engine nacelles and in action they extended downward 35° in 1½ seconds. The flaps did not act as a speed brake, they affected the center of pressure distribution so that the wing would not lose its lift.

Buffeting was another early aerodynamic problem, difficult to sort out from compressibility as both were reported by test pilots as ’tail shake’. Buffeting came about from airflow disturbances ahead of the tail; the airplane would shake at high speed. Leading edge wing slots were tried as were combinations of filleting between the wing, cockpit and engine nacelles. Air tunnel test number 15 solved the buffeting completely and its fillet solution was fitted to every subsequent P-38 airframe. The problem was traced to a 40% increase in air speed at the wing-fuselage junction where the chord/thickness ratio was highest. An airspeed of 500 mph at 25,000 feet could push airflow at the wing-fuselage junction close to the speed of sound. Filleting forever solved the buffeting problem for the P-38E and later models.

Mechanized P-38 conveyor lines.Another issue with the P-38 was that both engines were “critical” engines — losing one on takeoff, which often occurred, created “critical torque”, rolling the plane towards the live engine’s wingtip, rather than the dead engine’s. Normal training in flying twin-engine aircraft when losing an engine on takeoff, would be to push the remaining engine to full throttle; in the P-38, the resulting critical torque produced such an uncontrollable asymmetric roll the aircraft would flip over and slam into the ground. Eventually, procedures were devised to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the dead engine, and then increasing power gradually until the aircraft was in stable flight.

The engine sounds were a unique, rather quiet “whuffle,” because the exhausts were muffled by the General Electric turbosuperchargers on the twin Allison V12s. There were early problems with cockpit temperature regulation; pilots were often too hot in the tropics as the canopy could not be opened without severe buffeting, and were often too cold in northern Europe, as the distance of the engines from the cockpit prevented effective heating. Later variants received modifications to solve these problems.

P-38 at sunset.On 20 September 1939, before the YP-38s had been built and flight tested, the USAAF ordered 66 initial production P-38 Lightnings, 30 of which were delivered to the USAAF in mid-1941, but not all these aircraft were armed. The unarmed aircraft were subsequently fitted with four. 50s (instead of the two. 50 and two. 30 of their predecessors) and a 37 mm cannon. They also had armor glass, cockpit armor and fluorescent cockpit controls. One was completed with a pressurized cabin on an experimental basis and designated XP-38A. Due to reports the USAAF was receiving from Europe, the remaining 36 in the batch were upgraded with small improvements such as self-sealing fuel tanks and enhanced armor protection to make them combat-capable. The USAAF specified that these 36 aircraft were to be designated P-38D. As a result, there never were any P-38Bs or P-38Cs. The P-38D’s main role was to work out bugs and give the USAAF experience with handling the type.

In March 1940, the French and the British ordered a total of 667 P-38s, designated Model 322F for the French and Model 322B for the British. The aircraft would be a variant of the P-38E, without turbo-supercharging (due to a U.S. government export prohibition), and twin right-handed engines instead of counter-rotating, for commonality with the large numbers of Curtiss Tomahawks both nations had on order. After the fall of France in June 1940, the British took over the entire order and re-christened the plane Lightning I. Three were delivered in March 1942 and, after discovering, without their superchargers and when using lower-octane British aircraft fuel, they had a maximum speed of 300 miles per hour (480 km/h) and poor handling characteristics, the entire order was canceled. The remaining 140 Lightning Is were completed for the USAAF with counter-rotating engines but still minus turbo-superchargers. They were relegated to United States Army Air Forces training units under the designation RP-322. These aircraft helped the USAAF train new pilots to fly a powerful and complex new fighter. The RP-322 was a fairly fast aircraft (some of the fastest post-war racing P-38s were virtually identical in layout to the P-322-II) at low altitude and well suited as a trainer. The other positive result of this fiasco was to give the aircraft the name “Lightning”. Lockheed originally dubbed the aircraft Atalanta in the company tradition of naming their planes after mythological and celestial figures, but the RAF name won out.