Nozzles and Thrust Generation Questions and Answers

Answer: C

Thrust follows mass flow almost directly and mass flow follows density. Falling ambient temperature partly offsets this by improving the cycle, so the real lapse is somewhat gentler than density scaling alone would predict.

Answer: D

The engine must first cancel the momentum the incoming air already had before any of the exhaust momentum counts as thrust. That is why a high bypass engine, whose exhaust velocity is modest, loses thrust with speed more rapidly than a turbojet does.

Answer: A

Boundary layers reduce the effective flow area below the geometric one, so the actual mass flow falls a little short. Values above 0.98 are typical, and the shortfall is accounted for when the nozzle area is sized during development.

Answer: B

Friction and heat transfer prevent the expansion being perfectly isentropic, so the exit velocity falls short of the ideal. Because thrust depends directly on that velocity, a small shortfall here translates straight into lost thrust.

Answer: C

The doors block the jet pipe and redirect the gas forward at an angle, converting thrust into retarding force. It acts on the hot stream, which limits how far it can be used before ingestion of the reversed gas becomes a concern.

Answer: D

Most of the thrust on such an engine comes from the bypass stream, so reversing it alone gives most of the available effect. Leaving the hot core stream untouched also keeps the system lighter and avoids the worst of the ingestion problem.

Answer: A

Correct answer: Turbulent mixing between the jet and the surrounding air

Jet noise arises in the turbulent shear layer where the fast exhaust meets still air, the eddies there acting as an aerodynamic quadrupole source. Lighthill’s analysis gives the acoustic power as varying with the eighth power of the jet velocity, which is why lowering that velocity is so effective. Combustion, compressor and structural noise all exist but are separate sources treated by other means; the mixing region is the dominant one on a high-velocity jet.

Answer: B

The serrations generate streamwise vortices that break up the strong shear layer where most noise is produced. The penalty is a small thrust loss, which is accepted because noise limits often bind more tightly than performance does.

Answer: C

Thrust per unit mass flow is essentially the velocity increment the engine imparts. A turbojet produces a fast jet and high specific thrust, while a high bypass turbofan produces a slower jet and correspondingly low specific thrust.

Answer: D

A test bed measures the engine alone under controlled conditions, while an installed engine must also pay for spillage, nacelle drag and any air or power taken for the airframe. The difference is accounted for carefully in performance guarantees.