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An engineer proposes reducing an engine's jet velocity until it equals the aircraft's flight velocity, arguing this will make the propulsion perfectly efficient.

What is wrong with the proposal?

  • A.The thermal efficiency would collapse
  • B.The engine would exceed its temperature limit
  • C.Thrust depends on the velocity increment and would fall to zero
  • D.Propulsive efficiency would fall rather than rise

Answer: C

Propulsive efficiency would indeed approach unity, but an efficiency of one applied to zero thrust is useless. The practical cycle settles on a jet velocity somewhat above flight speed, trading some efficiency for the thrust the aircraft actually needs.

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