Nozzles and Thrust Generation Questions and Answers

Answer: A

Correct answer: Statement I is incorrect but Statement II is correct

Statement II is correct and is precisely why statement I is false. Reheat raises the gas temperature and so the specific volume, and the same mass flow then needs a much larger exit area. A fixed nozzle would raise the back pressure on the turbine when reheat was lit, disturbing the whole engine match and risking surge. Every afterburning engine therefore has a variable area nozzle that opens as reheat is selected.

Answer: B

Both are true and the second is the reason. Performance guarantees account for the difference explicitly, since an engine that meets its figures on a test bed may not deliver them once wrapped in a nacelle and supplying an aircraft.

Answer: C

Each step removes something the previous figure had included optimistically. The final step is often overlooked, yet spillage, nacelle drag and airframe offtakes can account for several per cent of what the test bed measured.

Answer: D

Once the throat is sonic the mass flow is fixed and further pressure ratio appears as exit pressure above ambient. Adding a divergent section converts part of that residual pressure into velocity and recovers the thrust it represents.

Answer: A

Correct answer: 1, 2, 3, 4, 5

The sequence follows the mechanism. The translating cowl slides rearward, which both opens the aperture and, through its linkage, closes the blocker doors across the bypass duct; the cascades previously hidden under the cowl are exposed; the bypass air, now unable to continue aft, is turned forward through the cascade vanes; and the reversed momentum produces the retarding force. Turning the air before exposing the cascades, as one distractor has it, is not possible.

Answer: B

Because noise power varies with roughly the eighth power of jet velocity, lowering that velocity dominates everything else. Lining and chevrons are worthwhile refinements but neither approaches the effect of the bypass ratio increase.

Answer: C

Correct answer: 1, 2, 3, 4, 5

As speed rises the ram drag \( \dot{m}V_0 \) increases directly with it, while the gross thrust rises much more slowly, if at all. Net thrust, their difference, therefore falls; the excess of thrust over drag available for acceleration diminishes; and the aircraft settles where thrust and drag balance. The order matters because the third and fourth steps are consequences of the second, not causes of it.

Answer: D

Correct answer: Only three

The first three definitions are exact: gross thrust is what the nozzle produces without regard to the incoming momentum, ram drag is that incoming momentum \( \dot{m}V_0 \), and net thrust is their difference. The fourth is wrong - gross thrust is not "thrust measured in flight" but a term in the equation, and it happens to be what a static test bed measures, since ram drag is then zero.

Answer: A

Correct answer: Only three

The first three are the standard definitions, keyed to the exit pressure relative to ambient. The fourth is wrong: shocks forming inside the divergent section are a feature of over-expansion, where ambient pressure compresses the flow. In under-expansion the flow leaves at higher than ambient pressure and continues expanding outside through a system of expansion fans in the free jet.

Answer: B

Correct answer: Only three

The first three coefficients are correctly paired with what they compare. The fourth confuses two of them: the coefficient of velocity compares actual with ideal velocity, while the mass flow comparison is the coefficient of discharge. The three are related, the thrust coefficient being approximately the product of the other two, and each isolates a different departure from ideal isentropic behaviour.