How Fast is Mach 1?
- by Anton Skorucak and ScienceIQ.com
Mach 1 is the speed of sound. A Mach number is simply the ratio of an object's speed to the speed of sound in the air around it. An aircraft flying at Mach 1 is moving exactly as fast as sound travels; at Mach 2 it is going twice as fast, and at Mach 0.85 (a typical airliner) it is going 85 percent as fast.
So how fast is Mach 1 in everyday units? It depends on the air. Sound travels faster in warm air and slower in cold air, so Mach 1 is not one fixed speed:
| Conditions | mph | km/h | m/s |
|---|---|---|---|
| Sea level, 20 °C (68 °F) | 767 | 1,235 | 343 |
| Sea level, 15 °C (59 °F), the standard value | 761 | 1,225 | 340 |
| Sea level, 0 °C (32 °F) | 741 | 1,193 | 331 |
| 36,000 ft (11 km), −56 °C | 660 | 1,062 | 295 |
Altitude matters only because the air up high is colder. Air pressure on its own has almost no effect on the speed of sound; temperature is what counts. That is why pilots and engineers talk in Mach numbers rather than miles per hour: the air flowing around a wing behaves according to how close the plane is to the local speed of sound, whatever that happens to be.
Engineers divide flight into speed ranges: subsonic (below about Mach 0.8), transonic (about Mach 0.8 to 1.2, where shock waves begin to form over parts of the aircraft), supersonic (above Mach 1) and hypersonic (above about Mach 5). Once an aircraft passes Mach 1, the pressure waves it creates can no longer travel out ahead of it. They pile up into a shock wave, which people on the ground hear as a sonic boom.
Some milestones: Chuck Yeager made the first confirmed supersonic flight on October 14, 1947, in the rocket-powered Bell X-1. The Concorde airliner cruised at about Mach 2, the SR-71 Blackbird reconnaissance jet flew at more than Mach 3, and the X-15 rocket plane reached Mach 6.7 in 1967.
The Mach number is named after Ernst Mach (1838–1916), the Austrian physicist and philosopher who, in the 1880s, made the first photographs of the shock waves around bullets flying faster than sound. The term "Mach number" was introduced by the Swiss engineer Jakob Ackeret in 1929.
