Ernst Mach is born in what’s now the Czech Republic. His most memorable work in aerodynamics will be the understanding of supersonic speeds, leading to the measure that bears his name.
Mach explained his understandings of supersonic flow in a paper published in 1887. The paper included the first photograph showing the shock waves that form when an object moves at supersonic speeds.
Mach didn’t become a measure until 1929, thirteen years after the physicist’s death. A Mach number is the ratio of the speed of an object to the speed of sound in the surrounding medium. Mach 1.0 means that the speed of the object is equal to the speed of sound in that medium. Pilots who fly near the speed of sound use the Mach number because it gives the aerodynamic condition of the aircraft independent of changes in air density due to altitude, temperature, or humidity. Primarily because air density decreases as altitude increases, an aircraft “feels” less air the higher it flies. So the aerodynamic forces acting on an aircraft traveling 700 miles an hour at sea level are very different from the forces acting on an aircraft traveling the same speed at 50,000 feet. The airplane at sea level is traveling slower than the speed of sound, at Mach 0.92. The aircraft flying at 50,000 feet will be flying at Mach 1.06, faster than the speed of sound. High-speed pilots and engineers describe velocity by Mach number because airliner performance largely depends on Mach, not on miles-per-hour speed. An airliner at Mach 0.85 at 10,000 feet will behave much the same as at Mach 0.85 at 30,000 feet. Likewise, a fighter jet’s behavior at Mach 1.4 will be essentially the same whether the craft is at 10,000 feet or 30,000 feet.
The first time an aircraft flew faster than Mach 1.0 was October 14, 1947, when Captain Chuck Yeager flew a Bell X-1 to Mach 1.07.—JP