Kelly Johnson issued Report No. 2414, which stated that the tests on the balance weights mandated by the Army Air Corps did nothing. Lockheed re-skinned the elevator and stabilizer with a thicker aluminum sheet with a gage thickness of 67% greater than the standard covering. This also showed no improvements in the situation. Turbo exhaust hoods were modified to reduce intake and attempted to redirect the airflow pattern, but again there were no changes. Lockheed took the YP-38 back into the air tunnel to conduct further testing on the canopy fit, gondola skin roughness, and canopy shape. Johnson and other aerodynamic scientists concluded that Major Gilkey encountered compressibility for sure, but did not want to publicize it.
Johnson knew the main problem was compressibility, not tail flutter, but Colonel Wolfe at Wright-Patterson did not agree. Using the government’s leverage over the contract signed with Lockheed, Wolfe applied pressure to add external mass balances on the horizontal stabilizer. The genius of the P-38 design already called for balances that were built inside the structure, but Johnson’s arguments fell of deaf ears. Johnson would later say that the only thing the mass balances did were to kill a few pilots who had to bail out of their aircraft.
Johnson needed to know more about the problem with compressibility, and had Lockheed test pilots repeat Gilkey’s flight. Lockheed test pilots repeated the flight and encountered the problem exactly as had Gilkey. Johnson concluded that a high airflow velocity was obtained at the wing-fuselage junction was accelerated as much as 40% above the flight speed. If the P-38 was traveling at 500 mph, the airflow over the wing was approaching the speed of sound. Tests would prove this regularly. Engineers developed a special wing fillet that would enable the airflow to flow smoother over the wing. The X-15 wing fillet was incorporated to the design and along with a slotted leading edge, the tail buffeting problem was solved. However, the Air Corps still maintained the requirement for the mass balances on each P-38.
The compressibility problem would plague the P-38 well into the war. This problem created many rumors, especially in the ETO (where combat missions were normally above 20,000 ft., which is where compressibility is encountered). In the MTO, where the nature of the combat missions did not call for the aircraft to fly above 20,000 ft., therefore the P-38 excelled. General Kenney loved the P-38, and was always trying to obtain any he could get his hands on. He argued that it was the only aircraft able to perform in the MTO. On the other hand, in the ETO, opinions normally were different. Lt. Col. Mark Hubbard openly disliked the P-38. His top complaints about the aircraft was compressibility, propellers spinning wrong directions (in his opinion), and incorrect intercoolers. Lockheed designers cannot be held accountable for the prolonged compressibility problems. Well known scientists of this time could not say what the solution was. In addition, the NACA blocked progress for months by not allowing Lockheed to use the wind tunnel at high speeds for fear that the tunnel would be damaged. On the other hand, the intercooler problem was a result of engine power reaching power output unheard of only a few years previous. Kelly Johnson did not envision that engine hp would exceed 1060, and did not believe many aircraft would be ordered. By 1944, Allison engines were producing 60% more power than in 1938. Hubbard felt it was third best in the ETO (fifth if the ME-109 and FW-190 were counted), and fifth best in the PTO.
Lockheed gave the compressibility problem top priority. By September 1941, one YP-38 was designated for compressibility testing. Flight engineers wanted the test pilots to go past 300 mph above 30,000, which was not normally done. Mattern and Bircham declined to perform the tests because they thought the engineers were being too aggressive. Virdin was determined to go through the tests himself.
After some preparations, the YP-38 was ready to go, and the first test flight was scheduled for November 4, 1941. The YP-38 was fitted with new spring servo tabs at each end of the control surface. The tabs were designed to increase leverage in order to assist the pilot overcome loads when pulling out of a dive. The P-38 design held true to the design specifications. Johnson admired the Spitfire’s razor thin wings, but the P-38 was supposed to perform long range missions. To be able to perform this mission, the wing design was thicker than Johnson would like, and the wing thickness caused the airflow to "splash" over the wing, and at high speeds, it disturbed the airflow.
By the time for the flight, Lockheed engineers were in high spirits. The flutter problem was disproved, and tail buffeting was close to being resolved. Ralph Virdin took off in theYP-38 and began his testing. He was partially through his testing when a piece broke off the aircraft, and witnesses reported that the engines were making a different sound. The aircraft entered a flat-inverted spin and crashed into the ground. Virdin was not able to escape the YP-38 and was killed. Kelly Johnson also heard the crash and described it as, "I was back in my office when I heard Virdin’s plane screaming towards the plant. That most unusual sound probably resulted from the propellers striking the air at an angle abnormal to the line of flight."
Few pilots had experienced the compressibility phenomena, but this was the first occasion where it claimed the pilot as a result. Designers were pushing the limits of aerodynamic knowledge and material strength in their quest for performance. Often times, they exceeded the limit of their own ability to calculate solutions in advance. In other words, the test pilots had to experience many unknowns, and the designers would have to solve any problems in the shortest time possible. Unfortunately, this process was normally slow, and many occasions new problems were encountered faster than older ones could be solved. Johnson concluded that the spring tab operating link broke, causing full deflection, which explains witness reports saying that the aircraft rose sharply just before entering the spin. At an altitude of 3,000 ft., and moving at an airspeed of 300 mph, full deflection would exceed design criteria. Essentially, when the spring tab broke, Virdin did not have a chance. Further testing would be a requirement for future designs.
In a continuing battle against compressibility, Lockheed designers developed a special kind of flap that would be incorporated into later P-38 designs. Tony LeVier was selected to perform initial testing of this new type of flap. The flaps were supposed to be deployed prior to entering a dive. The flap were not designed to slow the aircraft down, such as traditional flaps, but were designed to redirect the airflow over the surface area of the wing, which would increase lift and hopefully correct the compressibility problem. Tests were promising and Ben Kelsey was also assisting in the dive testing. In one such occasion, Kelsey wanted to test the flaps when they were deployed after entering a dive. On April 9, 1943, Kelsey climbed the aircraft to a high altitude, and began the dive. After accelerating, he pulled the handle for the flaps. As Kelsey pulled the handle, it suddenly broke. Before assessing what happened, Kelsey heard a loud crack and entered a flat inverted spin. Kelsey was able to open the canopy and was thrown free from the aircraft. The P-38’s wing sheared off, but Kelsey was able to parachute to safety.
Though the compressibility problem was never fully resolved, Lockheed was able to work around it with the introduction of the dive flaps. The P-38 would be destined to encounter this problem because of the 1930's style of a thick wing to accommodate the amount of fuel needed. Johnson openly admired the Spitfire's wing design, but as good as it was as a fighter, it did not have the ranger for long-range escort duties like the P-38.
