"In World War II, in combat in P-51s, during dogfights with 109s and 190s, for the first time we became exposed to the effects of the speed of sound on our airplanes. A Mustang, a P-47, or any of the other fighters that we were using in World War II, the fastest they would go was about 80 percent the speed of sound. They had very thick wings and canopies. That additional distance that the air had to travel to go around that wing that's going at about 80 percent of the speed of sound, brought its relative velocity to the skin of the wing up to the speed of sound."