Sound Barrier -
The Rocky Road to MACH 1.0+
As the speed of early aircraft gradually increased, there eventually became awareness during the 1940s, that strange things were occurring at around 500 mph. Many later WWII fighter aircraft were reported to become dangerously uncontrollable in high-speed power dives.
Pilots and aircraft designers were begin-ning to encounter the sound barrier. We now realize it to be a phenomenon that occurs when the speed of sound is reached and air compressibility demands additional power to break through it.