Which compressibility effect is associated with the critical Mach number, with the correct option as the last choice?

• A. Increased Thrust Demand
• B. Lateral Instability
• C. Higher Fuel Consumption Pattern
• D. Drag Increases

Answer: (D)

Transonic speeds bring compressibility effects into play: the critical Mach number is the point where some portion of the flow over the airfoil first reaches Mach 1, triggering shock waves. These shocks create wave drag, boosting the overall drag as speed climbs. So the compressibility effect tied to the critical Mach number is that drag increases. The other options describe consequences or phenomena not directly caused by the onset of compressibility at this speed—thrust demand, lateral stability changes, or fuel usage patterns may follow from higher drag, but the fundamental compressibility effect here is the rise in drag due to shock waves forming as the flow goes sonic.