What is the critical Mach number?

• A. The maximum permissible landing speed.
• B. The lowest Mach number at which any part of the aircraft reaches Mach 1, causing the onset of compressibility effects and a drag rise.
• C. The speed at which stall occurs.
• D. The speed for optimum fuel efficiency.

Answer: (B)

Local sonic flow and drag rise define the critical Mach number. As speed increases, different parts of the aircraft’s surface reach Mach 1 at different times. The critical Mach number is the free-stream Mach number at which the first part of the aircraft’s surface actually hits Mach 1, causing the formation of a sonic shock and a noticeable rise in drag due to compressibility effects. This makes it the lowest speed at which those effects begin, signaling the onset of significant wave drag in subsonic flight. That’s why this option is the best choice: it precisely describes when compressibility effects first appear on the aircraft surface. It’s not about landing or stall speeds, and it isn’t a measure of optimum fuel efficiency. Designers try to raise the critical Mach number (for example, with wing sweep) to delay the onset of these effects to higher speeds.