An aircraft has a critical Mach number (Mcrit) of 0.72 and a drag divergence Mach number of 0.76. As the aircraft accelerates from Mach 0.72 toward Mach 0.76, which statement correctly describes what is occurring aerodynamically?
- ALocal shock waves have formed and are producing a sharp, disproportionate rise in drag as speed approaches Mach 0.76
- BThe airplane exceeds Mmo before reaching the drag divergence Mach number
- CAirflow first becomes locally supersonic at Mach 0.76, not at Mach 0.72
- DDrag decreases in this range due to reduced induced drag at higher speed
Show answer & explanationAnswer & explanation
Correct answer: A. Local shock waves have formed and are producing a sharp, disproportionate rise in drag as speed approaches Mach 0.76
Mcrit (0.72) is the speed at which airflow over some point on the wing first reaches Mach 1.0 locally, but drag has not yet risen sharply. As speed increases further, local shocks strengthen and cause boundary layer separation; the drag divergence Mach number (0.76) marks the point of a sharp, disproportionate drag rise (often defined as where Cd rises by a set increment). Between these two speeds, shock-induced drag is building rapidly.
Why the other options are wrong
- B. Mmo is a separate operating limit and isn't necessarily tied to this drag divergence value.
- C. Incorrect — local sonic flow begins at Mcrit (0.72), which is by definition lower than drag divergence Mach.
- D. Drag increases sharply in this regime, it does not decrease.
Drag Divergence Mach Number
The Mach number, slightly above Mcrit, at which shock-induced flow separation causes a sharp, disproportionate rise in drag coefficient.
- Always higher than Mcrit
- Marks onset of significant shock-induced drag rise
- Distinct from Mmo, which is an operating limit
- Design features (sweep, area rule) increase Mdd
Memory trick: Mcrit opens the door to supersonic flow; Mdd is when drag crashes the party