Question

If an aircraft is flying at its Drag Divergence Mach Number and the pilot increases speed slightly, what happens to the fuel consumption?

• A. It stays the same because the aircraft is still subsonic
• B. It decreases because the wing becomes more efficient
• C. It increases sharply due to the rapid onset of wave drag and flow separation
• D. It increases linearly at the same rate as in subsonic flight

Hint:

Beyond the Drag Divergence Mach Number, \[ \boxed{ \text{Wave Drag} \uparrow \Longrightarrow \text{Fuel Consumption} \uparrow } \] because additional engine thrust is required to overcome the rapid increase in drag.

Answer 1

The Correct Option is C

Step 1: Recall the Drag Divergence Mach Number. The Drag Divergence Mach Number (\(M_{DD}\)) is the free-stream Mach number at which the drag coefficient begins to increase rapidly due to compressibility effects.

Step 2: Determine the effect of increasing speed beyond \(M_{DD}\). When the aircraft speed increases beyond the drag divergence Mach number,
• shock waves begin to form,
• wave drag increases rapidly,
• flow separation may occur,
• engine thrust requirement increases. Consequently, fuel consumption rises sharply. Therefore, \[ \boxed{\text{It increases sharply due to the rapid onset of wave drag and flow separation}.} \] Thus, \[ \boxed{(C)} \] is the correct answer.