Nozzles and Thrust Generation Questions and Answers

Answer: D

Reheat raises the gas temperature and therefore its specific volume, so the same mass flow needs a larger throat. A fixed throat would choke and raise the back pressure on the turbine, rematching the whole engine unfavourably.

Answer: A

Correct answer: They overstate what is available at any flight speed

The gross thrust column is constant at 40 kN while the ram drag grows in proportion to flight speed, so the net thrust falls from 40 kN at rest to 16 kN at 300 m/s. The static figure is therefore an upper bound reached only when the aircraft is not moving - useful for comparing engines on a common basis, but never representative of cruise, where the deduction may exceed half the gross thrust.

Answer: B

Correct answer: Under-expansion loses less than severe over-expansion

A fixed nozzle cannot be right at every altitude and power setting, so the designer must choose which way to err. Under-expansion simply leaves some expansion to occur outside, a modest thrust loss. Severe over-expansion brings shocks inside the divergent section and, in the worst case, asymmetric separation with side loads on the structure. The asymmetry of the penalties dictates the choice; under-expansion does not give maximum thrust, and full expansion does.

Answer: C

Correct answer: Less noise but less specific thrust

Reading down the table, the jet velocity falls from 800 to 350 m/s, the noise power collapses by a factor of 250, and the specific thrust falls from 550 to 100 N per kg/s. The quiet engine must therefore pass five times the air for the same thrust, which means a very much larger fan, nacelle and installation. That is the trade high bypass design makes, and the fuel efficiency gained is what pays for it.

Answer: D

Correct answer: Most of the thrust comes from the bypass stream

On a high bypass engine roughly 80 per cent of the thrust is produced by the fan, so reversing that stream alone recovers the great majority of the available retarding force. Leaving the core untouched saves the weight and complexity of a hot stream reverser and avoids exposing the mechanism to exhaust temperatures. The core is not forbidden from being reversed - it simply is not worth the hardware for the last fifth.

Answer: A

Every item after the first subtracts from what the airframe actually receives, and together they can amount to several per cent. Stating which figure a guarantee refers to is therefore essential to any contract between engine and airframe.

Answer: B

Both statements are true and the second is the reason. The engine must cancel the momentum the incoming air already carried before any of its exhaust momentum counts, and that penalty grows steadily as the aircraft accelerates.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. A divergent duct accelerates only supersonic flow; on subsonic flow it acts as a diffuser and converts velocity back into pressure. Below the critical pressure ratio the throat never goes sonic, so the divergent section would slow the exhaust and reduce the thrust. It earns its weight and complexity only where the pressure ratio is high enough to keep the flow supersonic through it.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the asymmetry that decides the choice. Under-expansion costs a little thrust, the remaining expansion occurring harmlessly outside. Severe over-expansion brings shocks into the divergent section and can separate the flow asymmetrically, producing side loads on the structure as well as a large thrust loss. Faced with a fixed geometry that cannot match every condition, the designer errs towards the gentler failure.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is why the nozzle must vary. Reheat raises the exhaust temperature and hence its specific volume, so the same mass flow requires a substantially larger exit area. If the nozzle could not open, the back pressure on the turbine would rise, the whole engine match would shift, and the compressor could be driven towards surge. The nozzle therefore opens as reheat is selected and closes as it is cancelled.