Which design change can increase the critical Mach number of an aircraft?

• A. Increasing fuselage length
• B. Sweepback of the wings
• C. Increasing wing thickness
• D. Adding more winglets

Answer: (B)

Critical Mach number is the speed at which the airflow over part of the wing first reaches Mach 1, causing a shock and a rise in drag. Sweeping the wings reduces the component of the flight speed that is normal to the leading edge. Because the flow effectively meets a swept wing at a smaller normal velocity, the wing can stay subsonic over a higher freestream speed. In other words, the same airplane can cruise faster before sonic flow appears on the wing, so the critical Mach number is increased.

Other changes don’t have the same effect. Extending the fuselage changes drag and stability more than the local speed at which the wing reaches sonic flow. Making the wing thicker tends to promote earlier shock formation, lowering the critical Mach number. Adding winglets mainly reduces induced drag and improves efficiency, but doesn’t significantly delay the onset of sonic flow on the wing.