With MCDR and MCRIT, what happens with sweepback?

• A. Both increase
• B. Both decrease
• C. One increases while the other decreases
• D. No change

Answer: (A)

Sweeping the wing reduces the flow’s effective normal Mach number. The important quantities here—critical Mach number and drag-divergence Mach number—are tied to when sonic effects and shock-related drag begin on the wing. With sweep, the flow to the wing sees M_n ≈ M cos(sweep). Because this normal component is smaller for a given free-stream Mach number, the onset of sonic flow and the associated drag rise are pushed to higher Mach numbers. In other words, the wing can reach higher speeds before waves develop and drag climbs rapidly, so both the critical Mach number and the drag-divergence Mach number increase with sweepback.