Which option correctly defines Mach number and the subsonic, transonic, and supersonic regimes?

Which option correctly defines Mach number and the subsonic, transonic, and supersonic regimes?

• A. Mach number is the ratio of velocity to the acceleration due to gravity; subsonic <1, transonic roughly 0.8–1.2, supersonic >1.
• B. Mach number is the ratio of lift to weight; subsonic <1, transonic roughly 0.8–1.2, supersonic >1.
• C. Mach number is velocity divided by the speed of light; subsonic <1, transonic roughly 0.8–1.2, supersonic >1.
• D. Mach number is velocity divided by the local speed of sound; subsonic <1, transonic roughly 0.8–1.2, supersonic >1.

Answer: (D)

Mach number is velocity divided by the local speed of sound. This ratio tells you how fast an object is moving relative to how quickly pressure disturbances travel through the surrounding air, and that depends on air temperature (speed of sound varies with temperature and gas properties). Subsonic means the Mach number is less than 1, so the object’s flow disturbances propagate ahead of and around it faster than the object itself. Transonic covers roughly 0.8 to 1.2, a regime where both subsonic and sonic effects are present and compressibility becomes important. Supersonic means the Mach number is greater than 1, where disturbances can no longer keep up and shock waves form. The other definitions aren’t correct because they mix in unrelated quantities: gravity, lift-to-weight, or the speed of light. The speed of light is a universal constant unrelated to airspeed; lift-to-weight is about flight performance, not a velocity scale; gravity isn’t the appropriate reference velocity for compressible flow.

Mach number is velocity divided by the local speed of sound. This ratio tells you how fast an object is moving relative to how quickly pressure disturbances travel through the surrounding air, and that depends on air temperature (speed of sound varies with temperature and gas properties).

Subsonic means the Mach number is less than 1, so the object’s flow disturbances propagate ahead of and around it faster than the object itself. Transonic covers roughly 0.8 to 1.2, a regime where both subsonic and sonic effects are present and compressibility becomes important. Supersonic means the Mach number is greater than 1, where disturbances can no longer keep up and shock waves form.

The other definitions aren’t correct because they mix in unrelated quantities: gravity, lift-to-weight, or the speed of light. The speed of light is a universal constant unrelated to airspeed; lift-to-weight is about flight performance, not a velocity scale; gravity isn’t the appropriate reference velocity for compressible flow.