Propulsion System Classification Questions and Answers

Answer: C

It measures how hard the engine works each kilogram of air it takes in. A turbojet has high specific thrust and a high bypass turbofan a low one, which is why the latter needs a much larger engine to produce the same total thrust.

Answer: D

Lower values mean a more economical engine, and it is the figure that dominates the operating cost of a transport aircraft. It falls as bypass ratio rises, which is the commercial reason engines grew steadily larger in diameter over the decades.

Answer: A

It rises as the jet velocity approaches the flight velocity, and would reach unity if they were equal, at which point the thrust would be zero. That tension between efficiency and thrust is what sets the optimum jet velocity for a given flight speed.

Answer: B

It depends chiefly on the pressure ratio and the turbine entry temperature, both of which have risen steadily with materials development. Overall efficiency is the product of thermal and propulsive efficiency, so both must be pursued together.

Answer: C

A high thermal efficiency wasted on a poorly matched jet velocity gives a disappointing result, and so does the reverse. The turbofan succeeded precisely because it improved propulsive efficiency without sacrificing the thermal efficiency of the core.

Answer: D

Thrust is proportional to mass flow, and mass flow follows density almost directly. Falling temperature partly offsets this by improving the cycle, which is why the thrust lapse is less severe than the density ratio alone would predict.

Answer: A

Tip speed combines rotational and forward velocity, so compressibility losses set in well before the aircraft itself is transonic. Swept and thin blade designs push the limit higher, which is what makes a modern turboprop competitive to around 400 knots.

Answer: B

It offers the lowest fuel consumption in the cruise speed band those aircraft occupy, together with acceptable noise. A turbojet would burn far more fuel and a ramjet would not work at all at those speeds, having no thrust at rest.

Answer: C

It is the only air breathing type that avoids decelerating the flow to subsonic speeds, which would produce temperatures no material could survive. Every other air breathing type has an upper speed limit far below the hypersonic regime.

Answer: D

Gas turbines scale badly to small sizes, where their efficiency falls and their cost stays high. The piston engine is heavier for a given power but far cheaper, which decides the matter for most aircraft below a few hundred horsepower.