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Design & Development
Specifications called for a maximum airspeed of at least 360 mph (580 km/h) at altitude, and a climb to 20,000 ft (6,100 m) within six minutes; the toughest set of specifications USAAC had presented to that date. The unbuilt Vultee XP1015 was designed to the same requirement, but was not advanced enough to merit further investigation. A similar single-engine proposal 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 turbo-superchargers and both gave extra points for tricycle landing gear.
The Lockheed design team, under the direction of Hall Hibbard and Clarence "Kelly" Johnson, considered a range of twin-engine configurations including both engines in a central fuselage with push-pull propellers.
Lockheed XP-38 prototype.
An armorer's assistant works on the installation of one of the
guns in the nose position of a new Lockheed P-38 Lightning.
[Source: Unknown]
The Lockheed design incorporated tricycle undercarriage and a bubble canopy, and featured two 1,000 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 the engines in the booms. Counter-rotation was achieved with the use of "handed" engines, which meant that the crankshaft of each engine turned in the opposite direction of its counterpart. The V-12 engines only required that the spark plug firing order be changed in order for the direction of the crank shaft to be reversed, according to the General Motors Allison V1710 Service School Handbook.
It 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 fighter to fly faster than 400 mph (640 km/h).
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 US$761,000. Construction began in July 1938 and the XP-38 first flew on 27 January 1939 at the hands of Ben Kelsey.
YP-38.
[Source: USAF Photo]
Manufacture of the YP-38s fell behind schedule, at least partly due to the need for mass-production suitability making them substantially different in construction than the prototype. Another factor was the sudden required facility expansion of Lockheed in Burbank, taking it from a specialized civilian firm dealing with small orders to a large government defense contractor making Venturas, Harpoons, Lodestars, Hudsons, and designing the Constellation airliner 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 Air Corps 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, 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 r ather than inwards as before. This improved the aircraft's stability as a gunnery platform.
High-speed compressibility problems
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, 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. Lockheed engineers were very concerned at this limitation, but first they had to concentrate on filling the current order of aircraft. In June 1941, the Army Air Corps was renamed the U.S. Army Air Forces (USAAF) and a total of 65 Lightnings were finished for the service by September 1941
with more on the way for the USAAF, the Royal Air Force (RAF) and the Free French Air Force operating from England.
By November 1941, many of the initial assembly line challenges had been met and there was some breathing room for the engineering team to tackle the problem of frozen controls in a dive. Lockheed had a few ideas for tests that would help them find an answer. The first solution tried was the fitting of spring-loaded servo tabs on the elevator trailing edge; tabs that were designed to aid the pilot when control yoke forces rose over 30 pounds-force (130 N), as would be expected in a high-speed dive. At that point, the tabs would begin to multiply the effort of the pilot's actions. The expert test pilot, 43-year-old Ralph Virden, was given a specific high-altitude test sequence to follow, and was told to restrict his speed and fast maneuvering in denser air at low altitudes since the new mechanism could exert tremendous leverage under those conditions. A note was taped to the instrument panel of the test craft, underscoring this instruction. On 4 November 1941, Virden climbed into YP -38 #1 and completed the test sequence successfully, but 15 minutes later was seen in a steep dive followed by a high-G pullout. The tail unit of the aircraft failed at about 3,000 ft (900 m) during the high-speed dive recovery; Virden was killed in the subsequent crash. The Lockheed design office was justifiably upset, but their design engineers could only conclude that servo tabs were not the solution for loss of control in a dive. Lockheed still had to find the problem; the Army Air Forces personnel were sure it was flutter, and ordered Lockheed to look more closely at the tail.
Although the P-38's empennage was completely skinned in aluminum rather than fabric and was quite rigid, in 1941 flutter was a familiar engineering problem related to a too-flexible tail. 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, but the increase in rigidity made no difference in vibration. Army Lieutenant 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 stabilizer. Various configurations of external mass balances were equipped and dangerously steep test flights flown to document their performance. Explaining to Wolfe 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 external ma ss balances did not help at all. Nonetheless, at Wolfe's insistence, the additional external balances were a feature of every P-38 built from then on.
After months of pushing NACA to provide Mach 0.75 wind tunnel speeds (and finally succeeding), the compressibility problem was revealed to be the center of lift moving back toward the tail when in high-speed airflow. The compressibility problem was solved by changing the geometry of the wing's underside when diving so as to keep lift within bounds of 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 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.
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. Unfortunately, these crucial flaps did not always reach their destination. In March 1944, 200 dive flap kits intended for European Theater of Operations (ETO) P-38Js were destroyed in a mistaken identification incident in which a RAF fighter shot down the Douglas C-54 Skymaster (mistaking it for an 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 tarmac 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 were effective in improving tactical maneuvers, a 14-month delay in production limited their implementation with only the final 50% of all Lightnings built having the dive flaps installed as an assembly-line sequence.
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. 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 chord/thickness ratio was highest. An airspeed of 500 mph (800 km/h) at 25,000 ft (7,600 m) 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.
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 engine 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-engine aircraft when losing an engine on takeoff would be to push the remaining engine to full throttle; 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 slam into the ground. 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 dead engine, and then increasing power gradually until the aircraft was in stable flight. Single-engine takeoffs were possible, tho ugh not with a full fuel and ammunition load. This exact design feature had also emerged on some multi-engined Luftwaffe aircraft of the war years - specifically the Henschel Hs 129 ground-attack aircraft, the Heinkel He 177A Greif heavy bomber and even the huge Messerschmitt Me 323 Gigant transport aircraft, with the Hs 129 having it from its earliest days, and the He 177 from its fourth prototype onwards.
The engines were unusually quiet because the exhausts were muffled by the General Electric turbo-superchargers on the twin Allison V12s. There were early problems with cockpit temperature regulation; 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.
P-38 Lightning outdoor production line, Burbank, CA.
[Source: Jack Cook Collection via the Warbird Information eXchange]
In March 1940, the French and the British ordered a total of 667 P-38s for US$100M, designated Model 322F for the French and Model 322B for the British. The aircraft would be 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, and thus ordered for the Model 322 twin right-handed engines instead of counter-rotating ones, and without turbo-superchargers. After the fall of France in June 1940, the British took over the entire order and christened the aircraft "Lightning".
Royal Air Force Lightning Mk. I
[Source: Unknown]
One positive result of the failed British/French order was to give the aircraft its name. Lockheed had originally dubbed the aircraft Atalanta from Greek mythology in the company tradition of naming planes after mythological and celestial figures, but the RAF name won out.
Sources:
Wikipedia
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