Editing Lockheed P-38 Lightning (section)

===High-speed compressibility problems===
[[File:Lockheed P-38G cockpit looking in from left wing 061019-F-1234P-004.jpg|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 [[Wing flutter|flutter]]. During high-speed flight approaching Mach&nbsp;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 [[Trim tab|elevator trim]].<ref name="Aviation Museum"/> Lockheed engineers were very concerned by this limitation, but first had to concentrate on filling the current order of aircraft. [[1941 in aviation#June|In late June 1941]], the Army Air Corps was renamed the [[U.S. Army Air Forces]] (USAAF), and 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, which freed up time 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 designed to aid the pilot when control yoke forces rose over {{convert|30|lb-f|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. Expert test pilot 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 {{convert|3500|ft|m|sigfig=1|abbr=on}} 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 USAAF personnel were sure it was flutter, and ordered Lockheed to look more closely at the tail.

In 1941, flutter was a familiar engineering problem related to a too-flexible tail, but the P-38's [[empennage]] was completely skinned in aluminum rather than fabric and was quite rigid. At no time did the P-38 suffer from true flutter.{{sfn|Bodie|2001|p=58}} 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, although 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 were 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 mass 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.{{sfn|Bodie|2001|p=57}}

[[File:Compressibility010.png|thumb|The P-38 pilot training manual compressibility chart shows speed limit vs. altitude.]]
Johnson said in his autobiography{{sfn|Johnson|Smith|1985|p=74}} that he pleaded with [[National Advisory Committee for Aeronautics]] to do model tests in its wind tunnel. They already had experience of models thrashing around violently at speeds approaching those requested and did not want to risk damaging their tunnel. Gen. Arnold, head of USAAF, ordered them to run the tests, which were done up to Mach 0.74.<ref>Erikson, Albert L. "Wind-Tunnel Investigation of Devices for Improving The Diving Characteristics of Airplanes." ''NACA MR No. 3F12, Summary''.</ref> The P-38's dive problem was revealed to be the [[center of pressure (fluid mechanics)|center of pressure]] moving back toward the tail when in high-speed airflow. The solution was to change the geometry of the wing's lower surface when diving 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 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.{{sfn|Bodie|2001|pp=174–175}}

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 friend or foe|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.{{sfn|Ethell|1984|p=14}}

Johnson later recalled:
{{Blockquote|I broke an ulcer over compressibility on the P-38 because we flew into a speed range where no one had ever been before, and we had difficulty convincing people that it wasn't the funny-looking airplane itself, but a fundamental physical problem. We found out what happened when the Lightning shed its tail and we worked during the whole war to get 15 more kn [28&nbsp;km/h] of speed out of the P-38. We saw compressibility as a brick wall for a long time. Then we learned how to get through it.<ref>Goebel, Greg. {{usurped|1=[https://web.archive.org/web/20020223121418/http://www.vectorsite.net/avp38.html "The Lockheed P-38 Lightning."]}} ''vectorsite.net'', Version 1.3. Retrieved: 21 January 2007.</ref>}}

[[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 [[fillet (mechanics)|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 {{convert|500|mph|km/h|abbr=on}} at {{convert|25000|ft|m|abbr=on}} 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.{{sfn|Bodie|2001|p=58}}

[[File:Lockheed P-38 fighter plane engines slnsw.jpg|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.{{sfn|Bodie|2001|p=210}}

The engines were unusually quiet because the exhausts were [[muffler|muffled]] by the [[General Electric]] turbosuperchargers on the twin Allison V-12s.{{sfn|Kaplan|Saunders|1991|p=56}} 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.{{citation needed|date=September 2023}}

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&nbsp;mm) machine guns (instead of the two .50 in/12.7&nbsp;mm and two .30 in/7.62&nbsp;mm of their predecessors) and a 37&nbsp;mm (1.46&nbsp;in) cannon. They also had armored glass, cockpit armor, and [[Fluorescence|fluorescent]] instrument lighting.<ref name="Baugher P-38">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38 "Lockheed P-38 Lightning."] {{Webarchive|url=https://web.archive.org/web/20120526012706/http://www.joebaugher.com/usaf_fighters/p38 |date=26 May 2012 }} ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref> One was completed with a pressurized cabin on an experimental basis and designated '''XP-38A'''.<ref name="Baugher XP-38A">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_1.html "Lockheed XP-38A Lightning."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref> 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 tank]]s 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.<ref name="Baugher P-38D">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_6.html "Lockheed P-38D Lightning."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref>

[[File:P-38 Lightning at sunset.jpg|thumb|P-38 rear view]]
In March 1940, the French and British, through the [[British Purchasing Commission|Anglo-French Purchasing Committee]], ordered 667 P-38s for US$100M,{{sfn|Bodie|2001|p=46}} 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 Warhawk|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.{{sfn|Bodie|2001|pp=45, 47}}{{refn|Turbosuperchargers were not secret nor restricted by the United States government. Related designs were known from French and Swiss firms. France and the UK did not want turbosuperchargers; they had never employed them and they knew the American ones were in short supply and did not want delivery delayed<ref name="Baugher Lightning I"/>|group=Note}} Performance was supposed to be {{convert|400|mph|km/h|abbr=on}} at {{convert|16900|ft|m|abbr=on}}.<ref name="Baugher Lightning I">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_7.html "Lightning I for RAF."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 2 December 2002. Retrieved: 29 January 2007.</ref> After the [[fall of France]] in June 1940, the British took over the entire order and gave the aircraft the [[British military aircraft designation systems|service name]] "Lightning". By June 1941, the War Ministry had cause to reconsider their earlier aircraft specifications based on experience gathered in the [[Battle of Britain]] and [[the Blitz]].{{sfn|Bodie|2001|p=60}} British displeasure with the Lockheed order came to the fore in July, and on 5 August 1941, they modified the contract such that 143 aircraft would be delivered as previously ordered, to be known as "Lightning (Mark) I", and 524 would be upgraded to US-standard P-38E specifications with a top speed of {{convert|415|mph|km/h|abbr=on}} at {{convert|20000|ft|m|abbr=on}} guaranteed, to be called "Lightning II", for British service.{{sfn|Bodie|2001|p=60}} Later that summer, an RAF test pilot reported back from Burbank with a poor assessment of the "tail flutter" situation, and the British cancelled all but three of the 143 Lightning Is.{{sfn|Bodie|2001|p=60}} As a loss around US$15M was involved, Lockheed reviewed their contracts and decided to hold the British to the original order. Negotiations grew bitter and stalled.{{sfn|Bodie|2001|p=60}} Everything changed after Japanese [[attack on Pearl Harbor]] on 7 December 1941, after which the United States government seized some 40 of the Model 322s for [[West Coast of the United States|West Coast]] defense;{{sfn|Bodie|2001|p=63}} subsequently, all British Lightnings were delivered to the USAAF starting in January 1942. The USAAF lent the RAF three of the aircraft, which were delivered by sea in March 1942{{sfn|Bodie|2001|p=61}} and were test flown no earlier than May{{sfn|Bodie|2001|p=64}} at [[Cunliffe-Owen Aircraft]] Swaythling, the [[Aeroplane and Armament Experimental Establishment]] (A&AEE) and the [[Royal Aircraft Establishment]].{{sfn|Bodie|2001|p=60}} The A&AEE example was unarmed, lacked turbochargers, and restricted to {{convert|300|mph|km/h|abbr=on}}, though the undercarriage was praised and flight on one engine described as comfortable.{{sfn|Mason|2010|pp=204–205}} These three were subsequently returned to the USAAF, one in December 1942 and the others in July 1943.{{sfn|Bodie|2001|p=61}} Of the remaining 140 Lightning Is, 19 were not modified and were designated by the USAAF as '''RP-322-I''' ('R' for 'Restricted', because noncounter-rotating propellers were considered more dangerous on takeoff), while 121 were converted to counter-rotating V-1710F-2 engines without turbosuperchargers and designated '''P-322-II'''. All 121 were used as [[Trainer (aircraft)|advanced trainers]]; a few were still serving that role in 1945.{{sfn|Bodie|2001|p=64}} A few RP-322s were later used as test-modification platforms such as for smoke-laying canisters. The RP-322 was a fairly fast aircraft below {{convert|16000|ft|m|abbr=on}} and well-behaved as a trainer.{{sfn|Bodie|2001|p=64}}{{refn|Some of the fastest postwar racing P-38s were virtually identical in layout to the P-322-II.|group=Note}}

Many of the British order of 524 Lightning IIs were fitted with stronger F-10 Allison engines as they became available, and all were given wing pylons for fuel tanks or bombs. The upgraded aircraft were deployed to the Pacific as USAAC F-5A reconnaissance or P-38G fighter models, the latter used with great effect in the [[Operation Vengeance|operation that shot down Admiral Yamamoto]] in April 1943. Robert Petit's G model named ''Miss Virginia'' was on that mission, borrowed by [[Rex T. Barber|Rex Barber]], who was later credited with the kill. Petit had already used ''Miss Virginia'' to defeat two [[Nakajima A6M2-N]] "Rufe" floatplanes in February and to heavily damage a [[No.1-class auxiliary submarine chaser|Japanese submarine chaser]] in March, which he mistakenly claimed as a destroyer sunk. Murray "Jim" Shubin used a less-powerful F model he named ''Oriole'' to down five confirmed and possibly six Zeros over Guadalcanal in June 1943 to become ace in a day.{{sfn|Bodie|2001|pp=111–116}}

The British name was retained over Lockheed's original name ''[[Atalanta]]'', the swift-running Greek goddess, following the company tradition of using mythological and celestial figures.{{sfn|Yenne|1987|p=60}}