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 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. Lockheed engineers were very concerned by this limitation, but first had to concentrate on filling the current order of aircraft. 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 , 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.