Propulsion Integration Questions and Answers

Answer: C

Correct answer: 180 kN m

The moment is the force times its perpendicular offset from the attachment:

\[ M = 100 \times 1.8 = 180\ \mathrm{kN\,m} \]

Because the thrust line lies below the wing, the pylon converts a pure forward force into both a force and a substantial nose-up pitching moment at the attachment. The pylon and the wing box local structure must carry it, and it also means that a thrust change produces a pitch change the pilot or autopilot must trim out.

Answer: D

Correct answer: 6.5 per cent

The recovery falls by five percentage points, and the thrust penalty is roughly 1.3 times that:

\[ (0.98 - 0.93) \times 100 \times 1.3 = 5.0 \times 1.3 = 6.5\ \mathrm{per\ cent} \]

The multiplier above one is the important point: thrust is more sensitive to recovery than proportionally, because a loss of total pressure reduces both the mass flow the engine can pass and the pressure ratio available across the nozzle. Small inlet losses are therefore expensive.

Answer: A

Correct answer: 749 kN m

Both the live engine's thrust and the failed engine's drag act at the same arm and in the same rotational sense, so they add:

\[ N = (95 + 12) \times 7.0 = 107 \times 7.0 = 749\ \mathrm{kN\,m} \]

The windmilling drag is easy to overlook and adds nearly 13 per cent here. It is largest when the failed engine is windmilling with a high bypass fan turning in the flow, which is precisely the case on the aircraft where the arms are longest.

Answer: B

Correct answer: 37 per cent as large

The moment is proportional to the arm, so the ratio is the ratio of the arms:

\[ \frac{2.6}{7.0} = 0.371 \]

The rear-mounted engine produces only 37 per cent of the wing-mounted moment for the same thrust. That is the central argument for rear-fuselage installation: a much smaller fin and rudder, and a lower minimum control speed. It is paid for with an aft centre of gravity, a heavier rear fuselage and the loss of wing bending relief.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. In flight the wing is bent upward by the lift it produces, and the engine's mass hanging beneath it acts downward at a long moment arm, opposing that bending.

The relief is substantial - a six-tonne engine at nine metres provides some 530 kN m at 1 g - and it scales with load factor, so it is greatest at the design manoeuvre and gust cases where the structure is critical. The result is a lighter wing than the same aircraft would need with rear-mounted engines.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second is the reason for the first. The engine-out yawing moment is thrust times the distance from the centreline, so moving the engines from a seven-metre wing station to a two or three metre rear-fuselage station cuts the moment to about a third.

The fin and rudder exist chiefly to balance that moment at low speed, so a smaller moment permits a smaller fin - which saves weight and drag, and lowers the minimum control speed on which the takeoff field length partly depends.

Answer: A

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

Statement I is incorrect: a pitot inlet is unsuitable at Mach 2.5.

Statement II is correct and explains why. A pitot inlet decelerates the flow through a single normal shock, and the total pressure loss across a normal shock grows rapidly with Mach number - at Mach 2.5 the recovery would be around 0.5, which is unusable. Supersonic inlets therefore use a system of oblique shocks, each weaker than a normal shock, with variable geometry to keep them correctly positioned.

Answer: B

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

Statement I is incorrect: raising bypass ratio makes underwing installation harder, not simpler.

Statement II is correct and is precisely why. A higher bypass ratio requires a larger fan, and the nacelle grows with it - frontal area rising with the square of the diameter. Under a low wing that runs directly into the ground clearance limit, and it has driven flattened nacelles, longer landing gear and engines moved forward and up on the pylon.

Answer: C

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

Statement I is incorrect: thrust reversers are not credited in the certified rejected takeoff or landing distance on most aircraft.

Statement II is correct and is the reason. A reverser may fail to deploy, and the certified distance must be achievable in the configuration that is guaranteed, so the calculation assumes wheel braking and spoilers alone. The reversers are therefore a real operational margin, valuable on a contaminated runway, but they buy no reduction in the published field length.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second is the reasoning behind the design. Fuse pins are deliberately weakened links that shear at a load far above normal operation but below that which would fail the wing structure.

An engine that departs cleanly takes its mass and its fire risk with it and leaves the wing intact. A pylon that holds and twists can rupture the wing box and the fuel tanks it contains. The design accepts the certain loss of an engine to avoid a far worse outcome.