Nozzles and Thrust Generation Questions and Answers

Answer: D

Correct answer: A-1, B-2, C-3, D-4

The clamshell reverser uses doors within the jet pipe to block and turn the hot exhaust. The bucket reverser swings external buckets behind the nozzle to do the same job from outside. The cold stream cascade reverser blocks the bypass duct and turns that air forward through cascades exposed by a translating cowl, leaving the core untouched. A turboprop achieves the same end by taking the propeller into reverse pitch and turning the slipstream forward.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

The first three coefficients each compare an actual quantity with its ideal isentropic value: mass flow, exit velocity, and thrust respectively. The critical pressure ratio is a different kind of quantity altogether - not a measure of loss but the condition at which the throat reaches sonic velocity, about 1.85 for hot gas with \( \gamma = 1.33 \) and 1.89 for cold air.

Answer: B

Correct answer: Nacelle skin friction drag

Net thrust is the exit momentum flux less the ram drag, plus the nozzle exit pressure term. Nacelle skin friction is an external drag on the airframe and belongs in the aircraft drag build-up, not the engine thrust equation. It is real and it matters - it is one of the reasons installed thrust falls short of test bed thrust - but it is accounted on the other side of the ledger.

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.