The GATE Grind

GATE 2021 AE – Question 17

Propulsion · Engine performance: ramjet, turbojet, turbofan, turboprop, turboshaft and after-burners · 1 mark · Multiple choice

At fixed flight Mach number and altitude, increasing compressor pressure ratio of a turbofan engine causes propulsive efficiency to

  1. Remains constant
  2. Increases monotonically
  3. Decreases monotonically
  4. First decreases and then increases

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Show answer and explanation

Correct answer: (D) First decreases and then increases

Explanation

The official trend is first decreases and then increases (D). Propulsive efficiency depends on jet velocity relative to flight velocity. Compressor pressure ratio also changes compressor work and the energy left for the exhaust, so a monotonic increase in exhaust speed cannot be assumed over the whole cycle range. The former monotonic-decrease argument omitted this turning behaviour.

Official GATE 2021 answer key: https://gate2026.iitg.ac.in/doc/download/Answer_keys2021/ae_2021.pdf