 |
|
| A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|
Mach number
 |
The actual speed of sound varies depending on the altitude above sea level because sound travels at slightly different speeds at different temperatures, and the temperature varies according to altitude. At sea level, the speed of sound, known as Mach 1.0, is about 761 miles per hour (1,225 km/h). At 20,000 ft (6,096 m), the speed of sound is 660 mph (1,062 km/h).
If an aircraft is traveling at half the speed of sound, it is said to be traveling at Mach 0.5. A speed of Mach 2.0 is twice the speed of sound. Because the speed of sound varies, a particular speed at sea level expressed as a Mach number would be faster than the same speed at 30,000 ft (9,144 m), which would be faster than the same speed at 40,000 ft (12,192 m). In other words, Mach 2 at sea level is a greater number of miles per hour (or kilometers per hour) than Mach 2 at 30,000 ft, which is a greater number of miles per hour than Mach 2 at 40,000 ft. When an aircraft reaches Mach 1, it is said to "break the sound barrier."
The following ranges of Mach numbers have been generally accepted to classify speeds:
| M < 0.8 | subsonic |
| 0.8 < M < 1.2 | transonic |
| 1.2 < M < 5.0 | supersonic |
| M > 5.0 | hypersonic |
A "critical Mach number" is the speed of an aircraft (below Mach 1) when the air flowing over some area of the airfoil has reached the speed of sound. For instance, if the air flowing over a wing reaches Mach 1 when the wing is only moving at Mach 0.8, then the wing's critical Mach number is 0.8.
Related category
• AERODYNAMICS AND AERONAUTICS
Source: U.S. Centennial of Flight Commission
Also on this site:
Encyclopedia of Alternative Energy & Sustainable Living
Encyclopedia of History
Transport Concepts & Designs (partner site)