Nozzles and Thrust Generation Questions and Answers

Answer: C

Correct answer: Only three

Clamshell and bucket reversers both act on the hot exhaust, and the cold stream cascade reverser acts on the bypass stream - three correct pairs. The fourth contradicts the third: the cold stream reverser leaves the core untouched, which is exactly what makes it suitable for a high bypass engine, where the bypass stream carries some 80 per cent of the thrust and reversing it alone is sufficient.

Answer: D

Correct answer: Only three

Raising bypass ratio, chevrons and acoustic lining are all correctly described. The fourth reverses the physics: raising the jet velocity increases the noise power, and steeply, since it goes as the eighth power. Every effective measure against jet noise works by reducing that velocity or by making the mixing with the surrounding air more gradual.

Answer: A

Correct answer: Nacelle skin friction drag

The thrust equation contains the exit momentum flux \( \dot{m}V_e \), the ram drag \( \dot{m}V_0 \), and the pressure term \( (p_e - p_a)A_e \). Nacelle skin friction is an external drag on the airframe, accounted in the aircraft’s drag build-up rather than in the engine’s thrust. It matters greatly to installed performance - which is why installed thrust is less than bare engine thrust - but it is a bookkeeping item on the other side of the ledger.

Answer: B

Correct answer: Mass flow increases as back pressure falls

This is what a choked nozzle does not do. Once the throat is sonic, information cannot travel upstream against the flow, so lowering the downstream pressure has no effect on the mass flow, which is fixed by the upstream stagnation conditions and the throat area. The other three all describe choking correctly: sonic conditions at the throat, a pressure ratio at or above critical, and mass flow independent of what happens downstream.

Answer: C

Correct answer: Coefficient of reaction

There is no such nozzle coefficient. The three in use are the coefficient of discharge, comparing actual with ideal mass flow; the coefficient of velocity, comparing actual with ideal exit velocity; and the coefficient of thrust, comparing actual with ideal thrust. Degree of reaction is a real quantity, but it belongs to turbine and compressor stage design, describing how the pressure drop is shared between rotor and stator.

Answer: D

Correct answer: Cascade compressor reverser

No such device exists - a reverser acts on the exhaust or bypass streams leaving the engine, never on the compressor. The three genuine types are the clamshell and bucket reversers, which turn the hot exhaust, and the cold stream cascade reverser, which turns the bypass air through fixed vanes exposed by a translating cowl. Turboprops achieve the same end with reverse propeller pitch.

Answer: A

Correct answer: Increasing the exhaust velocity

Raising the exhaust velocity increases jet noise, and steeply, because acoustic power varies as the eighth power of that velocity. The other three all reduce noise: raising bypass ratio lowers the jet velocity directly, chevrons promote a gentler mixing between the streams, and acoustic lining absorbs fan and turbine noise propagating in the ducts - though lining cannot reach the jet noise generated outside the nacelle.

Answer: B

Correct answer: Ram drag grows with flight speed

The advertised figure was measured at rest, where the ram drag term is zero and the reading is the gross thrust. At 250 m/s the engine must first arrest the momentum of the air it swallows, and that deduction \( \dot{m}V_0 \) is substantial - on a large fan it can be half the gross thrust. The engine has not deteriorated and the fuel flow has not been cut; the difference is inherent in what the two figures measure.

Answer: C

A diverging passage accelerates supersonic flow but decelerates subsonic flow, and below the critical ratio the flow never reaches sonic conditions. The geometry must therefore suit the pressure ratio, which is why variable nozzles exist.

Answer: D

Reheat raises the gas temperature and therefore its specific volume, so the same mass flow needs a larger throat. With the throat fixed the back pressure rises, the turbine expands less and the whole engine rematches unfavourably.