Lockheed P-38 Lightning

The Lockheed P-38 Lightning is an American single-seat, twin-engined fighter aircraft that was used during World War II. Developed for the United States Army Air Corps (USAAC) by the Lockheed Corporation, the P-38 incorporated a distinctive twin boom design with a central nacelle containing the cockpit and armament. Along with its use as a general fighter, the P-38 was used in various aerial warfare roles, including as a highly effective fighter-bomber, a night fighter, and a long-range escort fighter when equipped with drop tanks. The P-38 was also used as a bomber-pathfinder, guiding streams of medium and heavy bombers, or even other P-38s equipped with bombs, to their targets.

Some 1200 Lightnings, about one of every nine, were assigned to aerial reconnaissance, with cameras replacing weapons to become the F4 or F5 model; in this role, it was one of the most prolific reconnaissance airplanes in the war. Although it was not designated a heavy fighter or a bomber destroyer by the USAAC, the P-38 filled those roles and more; unlike German heavy fighters crewed by two or three airmen, the P-38, with its lone pilot, was nimble enough to compete with single-engined fighters.

The P-38 was used most successfully in the Asiatic-Pacific and China Burma India theatres of operation as the aircraft of America's top flying aces: Richard Bong (40 victories), Thomas McGuire (38 victories), and Charles H. MacDonald (27 victories). In the South West Pacific theatre, the P-38 was the primary long-range fighter of United States Army Air Forces until the introduction of large numbers of P-51D Mustangs toward the end of the war. Unusually for an early-war fighter design, both engines were supplemented by turbosuperchargers, making it one of the earliest Allied fighters capable of performing well at high altitudes. The turbosuperchargers also muffled the exhaust, making the P-38's operation relatively quiet. The Lightning was extremely forgiving in flight and could be mishandled without incident in many ways, but the initial rate of roll in early versions was low relative to other contemporary fighters; this was addressed in later variants with the introduction of hydraulically boosted ailerons. The smaller and more streamlined P-51 was significantly faster in a dive, which led to the P-51 replacing the P-38 in most European fighter groups by mid-1944. The P-38 was the only American fighter aircraft in large-scale production throughout American involvement in the war, from the Attack on Pearl Harbor to Victory over Japan Day.

Design and development

The Lockheed Corporation designed the P-38 in response to a February 1937 specification from the United States Army Air Corps (USAAC). Circular Proposal X-608 was a set of aircraft performance goals authored by First Lieutenants Benjamin S. Kelsey and Gordon P. Saville for a twin-engined, high-altitude "interceptor" having "the tactical mission of interception and attack of hostile aircraft at high altitude." Forty years later, Kelsey explained that Saville and he drew up the specification using the word "interceptor" as a way to bypass the inflexible Army Air Corps requirement for pursuit aircraft to carry no more than of armament including ammunition, and to bypass the USAAC restriction of single-seat aircraft to one engine. Kelsey was looking for a minimum of of armament. Kelsey and Saville aimed to get a more capable fighter, better at dog fighting and at high-altitude combat. Specifications called for a maximum airspeed of at least at altitude, and a climb to within six minutes, the toughest set of specifications USAAC had ever presented. The unbuilt Vultee XP1015 design was offered to fill this requirement, but was not advanced enough to merit further investigation. A similar proposal for a single-engined fighter was issued at the same time, Circular Proposal X-609, in response to which the Bell P-39 Airacobra was designed. Both proposals required liquid-cooled Allison V-1710 engines with turbosuperchargers and gave extra points for tricycle landing gear.

thumb|Preliminary Lockheed twin-engined P-38 fighter concepts

thumb|P-38 armament, concentrated in the nose of the aircraft

Lockheed formed a secretive engineering team to implement the project apart from the main factory; this approach later became known as Skunk Works. The Lockheed design team, under the direction of Hall Hibbard and Clarence "Kelly" Johnson, considered a range of twin-engined configurations, including both engines in a central fuselage with push–pull propellers.

The eventual configuration was rare in contemporary production fighter aircraft design, with the Dutch Fokker G.I heavy fighter, and the later Northrop P-61 Black Widow night fighter and Swedish SAAB 21 having a similar planform. The Lockheed team chose twin booms to accommodate the tail assembly, engines, and turbosuperchargers, with a central nacelle for the pilot and armament. The XP-38 gondola mockup was designed to mount two .50-caliber (12.7 mm) M2 Browning machine guns with 200 rounds per gun (rpg), two .30-caliber (7.62 mm) Brownings with 500 rpg, and a United States Army Ordnance Department prototype T1 23 mm (.90 in) autocannon with a rotary magazine as a substitute for the nonexistent 25 mm Hotchkiss aircraft autocannon specified by Kelsey and Saville. In the prototype YP-38s, an Army Ordnance Department T9 37 mm (1.46 in) autocannon (later designated as the M4 in production) with 15 rounds replaced the 23 mm T1. The 15 rounds were in three, five-round clips, an unsatisfactory arrangement according to Kelsey, and the T9/M4 did not perform reliably in flight. Further armament experiments from March to June 1941 resulted in the P-38E combat configuration of four M2 Browning machine guns, and one Hispano 20 mm (.79 in) autocannon with 150 rounds.

Clustering all the armament in the nose was unusual in U.S. aircraft, which typically used wing-mounted guns with trajectories set up to crisscross at one or more points in a convergence zone. The P-38 cannon used heavier 20 mm rounds, creating a different trajectory, so it was inclined upward slightly more than the four machine guns such that the trajectories of the cannon rounds and .50-caliber bullets came together between 350 and 400 yards/meters. Nose-mounted guns did not suffer as much from having their useful ranges limited by pattern convergence, meaning that good pilots could shoot much farther. A Lightning could reliably hit targets at any range up to , whereas the wing guns of other fighters were optimized for a specific range. The rate of fire was about 650 rounds per minute for the 20×110 mm cannon round (130-gram shell) at a muzzle velocity of about , and for the .50-caliber machine guns (43-gram rounds), about 850 rpm at velocity. Combined rate of fire was over 4,000 rpm with roughly every sixth projectile a 20 mm shell. The duration of sustained firing for the 20 mm cannon was about 14 seconds, while the .50-caliber machine guns worked for 35 seconds if each magazine were  fully loaded with 500 rounds, or for 21 seconds if 300 rounds were loaded to save weight for long-distance flying.

The Lockheed design incorporated tricycle undercarriage and a bubble canopy, and featured two turbosupercharged 12-cylinder Allison V-1710 engines fitted with counter-rotating propellers to eliminate the effect of engine torque, with the turbochargers positioned behind the engines, the exhaust side of the units exposed along the dorsal surfaces of the booms.

The P-38 was the first American fighter to make extensive use of stainless steel and smooth, flush-riveted, butt-jointed aluminum skin panels. It was also the first military airplane to fly faster than in level flight.

XP-38 and YP-38 prototypes

Lockheed won the competition on 23 June 1937 with its Model 22, and was contracted to build a prototype XP-38 for US$163,000, though Lockheed's own costs on the prototype would add up to $761,000. Construction began in July 1938 in an old bourbon distillery purchased by Lockheed to house expanding operations. This secure and remote site was later identified by Johnson as the first of five Lockheed Skunk Works locations. The XP-38 first flew on 27 January 1939 at the hands of Ben Kelsey.

thumb|One of 13 YP-38s constructed

Kelsey then proposed a speed dash to Wright Field on 11 February 1939 to relocate the aircraft for further testing. General Henry "Hap" Arnold, commander of the USAAC, approved of the record attempt and recommended a cross-country flight to New York. The flight set a speed record by flying from California to New York in seven hours and two minutes, not counting two refueling stops. Kelsey flew conservatively for most of the way, working the engines gently, even throttling back during descent to remove the associated speed advantage. Bundled up against the cold, Arnold congratulated Kelsey at Wright Field during his final refueling stop, and said, "don't spare the horses" on the next leg. After climbing out of Wright Field and reaching altitude, Kelsey pushed the XP-38 to . Nearing his destination, Kelsey was ordered by Mitchel Field tower (Hempstead, New York) into a slow landing pattern behind other aircraft. Carburetor icing caused it to be brought down short of the Mitchel runway, and it was wrecked. On the basis of the record flight, though, the USAAC ordered 13 YP-38s on 27 April 1939 for US$134,284 (~$ in ) each. (The "Y" in "YP" was the USAAC's designation for service test, i.e. small numbers of early production aircraft, while the "X" in "XP" was for experimental.) Lockheed's chief test pilot, Tony LeVier, angrily characterized the accident as an unnecessary publicity stunt, but according to Kelsey, the loss of the prototype, rather than hampering the program, sped the process by cutting short the initial test series. The success of the aircraft design contributed to Kelsey's promotion to captain in May 1939.

thumb|Mechanized P-38 assembly lines in [[Burbank, California]]

Manufacture of YP-38s fell behind schedule, at least partly because of changes to meet the need for mass production, making them substantially different in construction from the prototype. Another factor was the sudden required expansion of Lockheed's facility in Burbank, taking it from a specialized civilian firm dealing with small orders to a large government defense contractor making Venturas, Harpoons, Lodestars, and Hudsons, and designing the Constellation for TWA. The first YP-38 was not completed until September 1940, with its maiden flight on 17 September. The 13th and final YP-38 was delivered to the USAAC in June 1941; 12 aircraft were retained for flight testing and one for destructive stress testing. The YPs were substantially redesigned and differed greatly in detail from the hand-built XP-38. They were lighter and included changes in engine fit. The propeller rotation was reversed, with the blades spinning outward (away from the cockpit) at the top of their arc, rather than inward as before. This improved the aircraft's stability as a gunnery platform.

High-speed compressibility problems

thumb|left|upright|The P-38 was flown with a yoke, rather than the more-usual stick.

Test flights revealed problems initially believed to be tail flutter. During high-speed flight approaching Mach 0.68, especially during dives, the aircraft's tail would begin to shake violently, and the nose would tuck under (see Mach tuck), steepening the dive. Once caught in this dive, the fighter would enter a high-speed compressibility stall and the controls would lock up, leaving the pilot no option but to bail out (if possible) or remain with the aircraft until it got down to denser air, where he might have a chance to pull out. During a test flight in May 1941, USAAC Major Signa Gilkey managed to stay with a YP-38 in a compressibility lockup, riding it out until he recovered gradually using elevator trim. The P-38's dive problem was revealed to be the center of pressure moving back toward the tail when in high-speed flight. The solution was to change the geometry of the wing's lower surface when diving to keep the lift within bounds on the top of the wing. In February 1943, quick-acting dive flaps were tried and proven by Lockheed test pilots. The dive flaps were installed outboard of the engine nacelles, and in action, they extended downward 35° in 1.5 seconds. The flaps did not act as a speed brake; they affected the pressure distribution in a way that retained the wing's lift.

Late in 1943, a few hundred dive flap field-modification kits were assembled to give North African, European, and Pacific P-38s a chance to withstand compressibility and expand their combat tactics. The kits did not always reach their destination. In March 1944, 200 dive flap kits intended for the European Theater of Operations (ETO) P-38Js were destroyed in a mistaken identification incident in which an RAF fighter shot down the Douglas C-54 Skymaster (mistaken for a German Focke-Wulf Fw 200) taking the shipment to England. Back in Burbank, P-38Js coming off the assembly line in spring 1944 were towed out to the ramp and modified in the open air. The flaps were finally incorporated into the production line in June 1944 on the last 210 P-38Js. Despite testing having proved the dive flaps effective in improving tactical maneuvers, a 14-month delay in production limited their implementation, with only the final half of all Lightnings built having the dive flaps installed as an assembly-line sequence.

Johnson later recalled:

Buffeting was another early aerodynamic problem. Distinguishing it from compressibility was difficult, 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. Fillet kits were sent out to every squadron flying Lightnings. The problem was traced to a 40% increase in air speed at the wing-fuselage junction where the thickness/chord ratio was highest. An airspeed of at could push airflow at the wing-fuselage junction close to the speed of sound. Filleting solved the buffeting problem for the P-38E and later models.

thumb|upright|Airfield crew working on Lockheed P-38 fighter plane engines, circa 1944

Another issue with the P-38 arose from its unique design feature of outwardly rotating (at the "tops" of the propeller arcs) counter-rotating propellers. Losing one of two engines in any twin-engined, non-centerline thrust aircraft on takeoff creates sudden drag, yawing the nose toward the dead engine and rolling the wingtip down on the side of the dead engine. Normal training in flying twin-engined aircraft when losing an engine on takeoff is to push the remaining engine to full throttle to maintain airspeed; if a pilot did that in the P-38, regardless of which engine had failed, the resulting engine torque and p-factor force produced a sudden, uncontrollable yawing roll, and the aircraft would flip over and crash. Eventually, procedures were taught to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the failed engine, and then increasing power gradually until the aircraft was in stable flight. Single-engined takeoffs were possible, though not with a full fuel and ammunition load.

The engines were unusually quiet because the exhausts were muffled by the General Electric turbosuperchargers on the twin Allison V-12s. Early problems with cockpit temperature regulation occurred; pilots were often too hot in the tropical sun, as the canopy could not be fully opened without severe buffeting, and were often too cold in Northern Europe and at high altitude, as the distance of the engines from the cockpit prevented easy heat transfer. Later variants received modifications (such as electrically heated flight suits) to solve these problems.

On 20 September 1939, before the YP-38s had been built and flight tested, the USAAC ordered 66 initial-production P-38 Lightnings, 30 of which were delivered to the (renamed) USAAF in mid-1941, but not all these aircraft were armed. The unarmed aircraft were subsequently fitted with four .50 in (12.7 mm) machine guns (instead of the two .50 in/12.7 mm and two .30 in/7.62 mm of their predecessors) and a 37 mm (1.46 in) cannon. They also had armored glass, cockpit armor, and fluorescent instrument lighting. 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, no P-38Bs or P-38Cs were designated. The P-38D's main role was to work out bugs and give the USAAF experience with handling the type.

thumb|P-38 rear view

In March 1940, the French and British, through the Anglo-French Purchasing Committee, ordered 667 P-38s for US$100M, designated Model 322F for the French and Model 322B for the British. The aircraft was a variant of the P-38E. The overseas Allies wished for complete commonality of Allison engines with the large numbers of Curtiss P-40 Tomahawks both nations had on order, so they ordered the aircraft fitted with two right-handed engines (not counter-rotating) without turbosuperchargers.