Discussion
The cruise constraint on a constraint diagram is derived from
Answer: B
Correct answer: Matching thrust to drag at the cruise point
The cruise constraint comes from the condition for steady level flight: thrust must equal drag at the cruise altitude and Mach number. Expressed in the diagram's variables, the required \( T/W \) is the reciprocal of the lift-to-drag ratio achieved at that wing loading, corrected for the thrust lapse with altitude.
Because both induced and parasite drag vary with wing loading - one falling and the other rising - the constraint plots as a curve with a minimum, and the wing loading at that minimum is the aerodynamically most efficient cruise point. Field lengths and stalling speed give other constraints entirely.
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