Nozzles and Thrust Generation Questions and Answers

Answer: A

Thrust depends on the difference between jet and flight velocity, and the turbofan's modest jet velocity shrinks that difference quickly. The same characteristic that gives it excellent cruise efficiency is what makes its thrust lapse steeper.

Answer: B

Correct answer: Yes, noise limits often bind more tightly than the small performance cost

Community noise limits are a hard constraint: an aircraft that fails them cannot operate at the airports its business case depends on, or pays substantial penalties and curfew restrictions. A fuel burn penalty of a fraction of a per cent is a cost that can be carried. Chevrons are fitted on exactly this reasoning, and where the penalty matters most, at cruise, some designs retract or morph them.

Answer: C

A test bed measures the bare engine under controlled conditions, while installed operation must also pay for the nacelle and for the air and power the aircraft takes. Performance guarantees account for that difference explicitly.

Answer: D

The engine must first cancel the momentum the air already carried in the flight direction. Ram drag is zero at rest and grows with speed, which is precisely why a static thrust figure overstates what the aircraft actually receives in flight.

Answer: A

Sonic conditions at the throat fix the mass flow whatever happens downstream. Any further pressure ratio then appears as exit pressure above ambient rather than as additional exit velocity, which is the under-expanded condition.

Answer: B

Ambient air pushes the jet inward, producing oblique shocks and possibly separating the flow from the divergent wall. Severe over-expansion loses more than under-expansion, which is why fixed nozzles are sized towards the latter.

Answer: C

That extreme sensitivity means halving the jet velocity cuts the noise power by a factor of over two hundred. It is why raising bypass ratio transformed aircraft noise more effectively than any acoustic treatment ever could.

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.