Lockheed P-38 Lightning YP-38

The YP-38 was the Lightning's pre-production batch: thirteen service-test aircraft that the US Army Air Corps (USAAC) ordered from Lockheed shortly after the XP-38 prototype crashed on 11 February 1939, at the end of its transcontinental record attempt. According to the French Wikipedia, about two and a half months after that flight Lockheed signed a contract with the Air Corps covering the thirteen pre-series aircraft plus an extra airframe for static testing, and its design office defined the new aircraft, the Model 122, in October 1939. Of the thirteen, twelve were kept for flight testing and one was used for destructive stress testing.

The YP-38 was not a copy of the hand-built XP-38. Lockheed redesigned it thoroughly for mass production, and those changes, together with the sudden expansion of the Burbank plant — which went from a specialised civilian firm handling small orders to a large defence contractor building Venturas, Harpoons, Lodestars and Hudsons while designing the Constellation for TWA — put deliveries behind schedule. The revised structure made it somewhat lighter than the prototype: 5,067 kg empty, 6,123 kg gross and 6,508 kg maximum take-off weight. It was powered by two Allison V-1710-27 and V-1710-29 engines (Allison models F2R and F2L), liquid-cooled V-12s now fitted with General Electric B-2 turbosuperchargers instead of the XP-38's B-1s; they gave 1,000 hp normal power at 2,600 rpm and 1,150 hp (858 kW) for take-off at 3,000 rpm, and drove three-bladed Curtiss Electric constant-speed propellers through a 2.00:1 reduction gear. The cowlings received a new air intake and the radiators were enlarged; according to the German Wikipedia, the wider cooling flaps made it slightly slower than the XP-38. It reached 651 km/h at 3,000 m and climbed to 6,000 m in six minutes, half a minute quicker than the prototype.

The most visible change was in the propellers. The engines were still handed — the -27 was a right-hand tractor and the -29 a left-hand one — but whereas the XP-38's blades turned inward at the top of their arc, on the YP-38 the rotation was reversed so that they spun outward, away from the cockpit. Lockheed did this because it improved the aircraft's stability as a gunnery platform, and the P-38 kept it. It came at a price: with outward rotation, if one engine failed on take-off and the pilot pushed the other to full throttle, as normal twin-engine training taught, engine torque and P-factor produced a sudden, uncontrollable yawing roll that flipped the aircraft over; a specific procedure had to be taught, beginning with reducing power on the good engine and feathering the failed one.

The YP-38 was also the first armed Lightning. The English Wikipedia states that the YP-38s were to carry an Army Ordnance Department T9 37 mm cannon, later designated M4; the French Wikipedia adds that it replaced the 20 mm gun, with fifteen rounds, and that two of the four 12.7 mm machine guns were swapped for 7.62 mm weapons. Two 12.7 mm guns, two 7.62 mm guns and the 37 mm cannon is the combination also given by the German sources.

The first YP-38, serial 39-689 (Lockheed serial 122-2202), left the factory in September 1940 and flew with test pilot Marshall Headle at the controls; most sources date that flight to 17 September, while This Day in Aviation gives the 16th. The thirteenth and last was delivered in June 1941. During the joint Lockheed and Air Corps test campaign the aircraft earned the nickname “Yippees”, from the American pronunciation of the letters Y and P.

It was with the YP-38s that the problem that would define the P-38's story appeared. Approaching Mach 0.68, especially in dives, the tail would begin to shake violently, the nose would tuck under and the dive would steepen; the controls stayed locked until the aircraft reached denser air. In May 1941 USAAC Major Signa Gilkey rode out one of these lockups in a YP-38 and recovered gradually using elevator trim; the first response was to train pilots to use trim and to set airspeed limits. At first the problem was thought to be tail flutter, but the aluminium-skinned empennage was very rigid: one elevator and its vertical stabilisers were skinned with metal 63% thicker than standard with no change in the vibration, and the external mass balances requested by Lieutenant Colonel Kenneth B. Wolfe did not help either, although they were fitted to every P-38 built from then on (Kelly Johnson set this out in his Report No. 2414). A second phenomenon, buffeting caused by airflow disturbances ahead of the tail, was hard to tell apart because pilots reported both as “tail shake”; it was cured with fillets between the wing, the central nacelle and the engine nacelles, developed on the YP-38s and then fitted at the factory and sent to squadrons as field kits.

On 4 November 1941 Ralph Virden, a Lockheed test pilot with more than 15,000 flying hours, took up YP-38 No. 1, 39-689, fitted with spring-loaded servo tabs on the elevator trailing edge. He completed the planned test sequence, but fifteen minutes later he was seen in a steep dive followed by a high-g pull-out; the tail unit failed at about 1,000 m — at 860 km/h according to the French source — and the aircraft crashed into a house in Glendale, California. Virden was killed, and Lockheed concluded that servo tabs were not the answer. After the accident Lockheed, the Army and NACA launched an extensive test programme: the second YP-38, 39-690, was sent to Langley, where it was photographed in February 1942 and tested in the full-scale wind tunnel in December 1944. The cure, quick-acting dive flaps, would not reach the production line until the P-38J.

The batch suffered heavy attrition. According to the French Wikipedia, two more YP-38s were lost to engine failures on 23 June and 20 July 1941, two further accidents followed on 6 November and 9 December 1941, and the remaining aircraft were struck off; only one survived the war, ending up at Park Air College as an instructional airframe with 234 flying hours. Before the war, designer Harley Earl arranged for several of his staff to view a YP-38, and its tail fins directly inspired those of the 1948–1949 Cadillacs.