Discussion
A manufacturer advertises 40 kilonewtons of thrust, measured on a test bed with the engine stationary. In cruise at 250 metres per second the aircraft experiences considerably less.
Why is the difference expected?
Answer: B
Correct answer: Ram drag grows with flight speed
The advertised figure was measured at rest, where the ram drag term is zero and the reading is the gross thrust. At 250 m/s the engine must first arrest the momentum of the air it swallows, and that deduction \( \dot{m}V_0 \) is substantial - on a large fan it can be half the gross thrust. The engine has not deteriorated and the fuel flow has not been cut; the difference is inherent in what the two figures measure.
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