Aircraft Propulsion Questions and Answers

Answer: A

Carrying only fuel rather than fuel and oxidiser is an enormous mass saving, which is why every aircraft engine is air breathing. The penalty is that performance depends on the atmosphere, so thrust falls with altitude and the engine cannot work outside it at all.

Answer: B

Newton's third law gives an equal and opposite reaction to the momentum imparted to the working fluid. Propellers, fans, jets and rockets differ only in how much mass they accelerate and by how much, which is the single idea that unifies the whole subject.

Answer: C

All of the energy extracted from the gas appears as jet momentum rather than as shaft work. That gives a high exhaust velocity and suits high speed flight, but it is inefficient at low speed where the difference between jet and flight speed is largest.

Answer: D

Moving more air more slowly gives the same thrust at higher propulsive efficiency than a small fast jet does. That single change is why the turbofan displaced the turbojet on essentially every transport aircraft built since the nineteen sixties.

Answer: A

A high bypass ratio means most of the air never passes through the combustor at all. Raising it improves propulsive efficiency and cuts noise, at the cost of a larger, heavier nacelle with more drag, which is the trade every engine designer negotiates.

Answer: B

The turbine extracts nearly all the available energy to the shaft, leaving only a small residual jet thrust. It is the most efficient arrangement at low and moderate speeds, which is why regional and utility aircraft continue to use it.

Answer: C

Helicopters, auxiliary power units and marine and industrial installations all use the same gas generator with a different load. Having no propeller means there is no residual jet thrust contribution worth counting in the output.

Answer: D

Forward speed alone compresses the incoming air, which is why the engine produces no thrust at rest and must be boosted to operating speed. The absence of rotating machinery makes it mechanically simple and capable of speeds a turbojet cannot reach.

Answer: A

Decelerating hypersonic flow to subsonic speed would raise the temperature beyond what any material could survive, so the flow stays supersonic through the combustor. Mixing and burning fuel in milliseconds at those speeds is the central difficulty of the concept.

Answer: B

Every other type depends on atmospheric oxygen and stops working where there is none. Carrying the oxidiser costs a great deal of mass, so a rocket's endurance is short even though its thrust is independent of flight speed and altitude.