Propulsion System Classification Questions and Answers

Answer: A

Producing the same thrust from a third of the specific thrust means taking in more than three times the air. That is precisely why high bypass engines dominate the visual proportions of a modern airliner while a combat aircraft's engine is slender.

Answer: B

Thrust follows mass flow and therefore density, but the colder air raises the achievable pressure ratio and cycle efficiency. The two effects act in opposite directions, so the real lapse is gentler than density alone would suggest.

Answer: C

At 300 knots the propeller accelerates a very large mass of air gently, which is exactly the condition for high propulsive efficiency. The turbofan's advantage only appears at higher cruise speeds where propeller tip compressibility becomes limiting.

Answer: D

Carrying only fuel rather than fuel and oxidiser is an enormous mass saving, which is why every aircraft engine works this way. The penalty is that thrust falls with altitude and vanishes entirely outside the atmosphere, where only a rocket will function.

Answer: A

Raising it improves propulsive efficiency and reduces jet noise because more air is accelerated more gently. The cost is a larger and heavier nacelle with more drag, which is the balance every engine designer has to settle for a given aircraft.

Answer: B

Because the two multiply, a weakness in either one caps the result rather than merely reducing it a little. That is exactly why the turbofan pursued propulsive efficiency without giving up the thermal efficiency of the core it was built around.

Answer: C

The absence of rotating machinery makes it mechanically simple and capable of speeds a turbojet cannot reach. The price is that it produces no thrust at rest and must be accelerated to operating speed by a booster or another engine first.

Answer: D

It takes in no air at all, so there is no flight speed term in its thrust equation and no density dependence either. The cost is carrying the oxidiser, which limits endurance severely however large the thrust it can produce.

Answer: A

Accelerating more air more gently gives less thrust per kilogram of air but better propulsive efficiency. That is exactly why a high bypass engine must be physically much larger to produce the same total thrust as a turbojet would.

Answer: B

Thrust depends on the velocity increment, which vanishes when jet and flight speed are equal. A practical cycle therefore settles somewhere in the middle of this table, accepting some efficiency loss in exchange for useful thrust.