Nozzles and Thrust Generation Questions and Answers

Answer: C

Correct answer: False, they coincide only at rest

Net thrust is gross thrust less the ram drag \( \dot{m}V_0 \), which is zero only when the flight speed is zero. At every other speed the two differ, and the gap widens as speed rises - in cruise the ram drag may exceed half the gross thrust. Net thrust is therefore always less than gross thrust in flight, never more, and choking the nozzle has no bearing on the relationship.

Answer: D

Correct answer: It is fully expanded

This is the false statement. A convergent nozzle can accelerate the flow only to sonic conditions at its exit, so once choked, any further available pressure ratio cannot be converted into velocity there; the gas leaves above ambient pressure and expands the rest of the way outside. The other three are all true of a choked convergent nozzle: sonic throat, mass flow fixed by throat area and upstream conditions, and exit pressure above ambient.

Answer: A

Correct answer: No, below critical it decelerates

A divergent passage accelerates supersonic flow but decelerates subsonic flow. Below the critical pressure ratio the throat never goes sonic, so the divergent section acts as a diffuser, trading velocity back for pressure and reducing the thrust below what a simple convergent nozzle would give. The extra weight is a secondary objection; the aerodynamic one is decisive, and it is why convergent-divergent nozzles appear only on supersonic and rocket applications.

Answer: B

Correct answer: An increase in flight speed at constant exhaust velocity

Net thrust depends on the increment \( \dot{m}(V_e - V_0) \), so raising the flight speed while the exhaust velocity stays put narrows that increment and reduces the thrust. This is the thrust lapse with speed. The other three all increase it: more mass flow, a higher exhaust velocity, or better expansion at the nozzle exit recovering the pressure term.

Answer: C

Under-expansion leaves some energy unconverted but keeps the flow attached and steady. Severe over-expansion produces shocks and possible separation inside the nozzle, costing more thrust and imposing unsteady side loads.

Answer: D

Correct answer: The engine runs at higher speed

Installation does not make the engine run faster; if anything a distorted intake flow makes it run less well. The genuine deductions are the other three: nacelle external drag, intake spillage drag when the capture area does not match the demand, and the bleed air and shaft power taken for the aircraft’s systems. Together they typically account for several per cent, which must be allowed for rather than discovered in flight test.

Answer: A

Noise is generated by turbulent mixing in the shear layer and rises extraordinarily steeply with velocity. That relationship is why the industry's most effective noise measure was raising bypass ratio to lower the jet velocity.

Answer: B

Correct answer: Degree of reaction

Degree of reaction is a real and useful quantity, but it belongs to turbine and compressor stage design, describing how the pressure drop or rise is divided between the rotor and the stator. Nozzle performance is described by the coefficients of discharge, velocity and thrust, each comparing an actual quantity with its ideal isentropic value.

Answer: C

Correct answer: False, reheat needs a larger throat area

Reheat raises the exhaust temperature and hence the specific volume, so the same mass flow requires a substantially larger exit area. With a fixed nozzle the back pressure on the turbine would rise when reheat was lit, shifting the engine match and driving the compressor towards surge. The nozzle must therefore open as reheat is selected, whatever the duration and however the turbine is sized; thermal expansion of the structure is a separate and much smaller matter.

Answer: D

Correct answer: An increase in specific thrust

Raising bypass ratio spreads the same core power over more air, so each kilogram receives a smaller velocity increment and the specific thrust falls - from about 800 N per kg/s on a turbojet to nearer 200 on a high bypass engine. The other three do follow: the jet velocity falls, the noise falls steeply with the eighth power of that velocity, and the thrust lapses more sharply with flight speed because the increment is smaller to begin with.