Nozzles and Thrust Generation Questions and Answers

Answer: A

Gross thrust is what the nozzle produces, and subtracting the ram drag of the incoming air gives the net thrust the aircraft actually feels. The two are equal only at rest, which is why a static thrust figure overstates what is available in flight.

Answer: B

The engine must accelerate air that already had momentum in the flight direction, and only the increase counts as thrust. Ram drag grows with flight speed, which is a large part of why net thrust falls as an aircraft accelerates.

Answer: C

It is the exit area multiplied by the difference between exit and ambient pressure, and it vanishes when the nozzle is perfectly expanded. A choked convergent nozzle always has a positive pressure term, which is part of why it remains useful despite not expanding fully.

Answer: D

Beyond the critical pressure ratio the throat is sonic and the mass flow will not increase whatever happens downstream. Additional pressure ratio then appears as a pressure difference at the exit rather than as further acceleration.

Answer: A

For a specific heat ratio near 1.33 the ratio of total to static pressure at sonic conditions is about 1.85. The reciprocal, near 0.53, is the same condition expressed as the static pressure fraction, which is why both numbers appear in textbooks.

Answer: B

Only a diverging section can accelerate an already sonic flow to supersonic speed. Below the critical ratio the divergent portion would act as a diffuser and slow the flow, which is why the geometry must suit the pressure ratio available.

Answer: C

The flow continues to expand in the free jet, so some of the available energy never acts on the nozzle wall as thrust. A choked convergent nozzle at high pressure ratio is always under-expanded, and the resulting pressure term partly compensates.

Answer: D

The ambient air pushes the jet back inward, producing oblique shocks and possibly separation inside the divergent section. Severe over-expansion loses more than under-expansion would, which is why a fixed nozzle is sized towards the under-expanded side.

Answer: A

All the available pressure energy has been converted into exit velocity, which gives the maximum thrust for the given conditions. A fixed nozzle achieves this at only one flight condition, so variable geometry exists to hold it over a range.

Answer: B

Adding heat downstream of the turbine raises the gas temperature and hence its specific volume, so a fixed throat would choke and force the turbine to work against a raised back pressure. Opening the nozzle preserves the match while reheat is lit.