Mach Number
Ratio of speed to the local speed of sound.
Formula
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Example
Mach 1 = sound speed
How it works
Mach number compares an object's speed to the speed at which small pressure disturbances travel through the surrounding fluid, usually air. Because sound speed changes with temperature and composition of the air, the same aircraft speed can correspond to different Mach numbers at different altitudes. A value below 1 means subsonic flow, 1 is sonic, and above 1 indicates supersonic motion.
When to use it
Use this formula when analyzing aircraft performance, compressibility effects, or airflow around high-speed vehicles such as jets, missiles, and rockets. It is especially useful in flight operations and aerodynamics because shock waves and drag rise sharply near and above Mach 1.
Worked example, step by step
An aircraft is flying at 255 m/s, and the local speed of sound is 340 m/s. M = v / a M = 255 / 340 M = 0.75 Final answer: the aircraft is flying at Mach 0.75.
Common mistakes to avoid
- Using a fixed speed of sound instead of the local value for the current altitude and temperature
- Mixing up Mach number with true airspeed in m/s or knots
- Forgetting that Mach is a ratio, so it has no units
Did You Know?
The sound barrier was first broken in 1947 by Chuck Yeager, reaching Mach 1 in the Bell X-1.
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