Although the Bell X-1 was commissioned by the Air Force and flown by Air Force test pilot Chuck Yeager, when it exceeded Mach 1 NACA was officially in charge of the testing and development of the aircraft. NACA ran the experiments and data collection, and the bulk of the research used to develop the aircraft came from NACA engineer John Stack, the head of NACA's compressibility division. Compressibility is a major issue as aircraft approach Mach 1, and research into solving the problem drew heavily on information collected during previous NACA wind tunnel testing to assist Lockheed with the P-38 Lightning.