Propulsion Integration Questions and Answers

Answer: C

Correct answer: Increasing the engine's thrust rating

The moment is thrust times arm, so raising the thrust per engine raises the moment directly. It is the one change listed that makes the problem worse.

The other three reduce it: rear-fuselage mounting shortens the arm to two or three metres, moving the engines inboard shortens it likewise, and spreading the same total thrust over more engines reduces the thrust lost when any one fails - which is why a four-engine aircraft's asymmetry is milder than a twin's despite its longer arm.

Answer: D

A longer pylon reduces the interference but never removes it, and beyond a point it costs weight and wetted area of its own. The remaining statements are all correct descriptions of how the installation is actually shaped.

Answer: A

Correct answer: A reduction in engine core size

Cores have indeed grown smaller relative to the fan, but that is a consequence of rising bypass ratio rather than a cause of larger nacelles - and a smaller core would, on its own, allow a smaller nacelle.

The other three are the real drivers, and they are one story told three ways: the pursuit of lower specific fuel consumption led to higher bypass ratios, higher bypass ratios demand larger fans, and a larger fan requires a larger nacelle around it.

Answer: B

The requirement assumes brakes alone, and fully worn at that, because the reverser is not a guaranteed system. Its real value lies instead in the brake wear and heat it saves on every normal landing the aircraft makes.

Answer: C

Correct answer: A loss costs rather more than itself in thrust

Thrust is more than proportionally sensitive to pressure recovery: a loss of five percentage points typically costs six or seven per cent of thrust.

The reason is that lower total pressure reduces both the mass flow the engine can pass and the pressure ratio available across the nozzle, so the two effects compound. It matters at every speed, not only supersonically - which is why even a simple subsonic pitot inlet is shaped with care to reach recoveries above 0.98.

Answer: D

Correct answer: Raising the tyre inflation pressure

Higher tyre pressure reduces deflection under load by a small amount, but the gain is a centimetre or two at best - nowhere near the fifteen centimetres a larger fan typically costs - and it worsens the ride and the runway loading.

The other three are the measures actually used: lengthening the undercarriage raises the whole aircraft, flattening the nacelle bottom removes depth where the clearance is critical, and repositioning the accessory gearbox from beneath the core to the side allows a more circular, shallower nacelle.

Answer: A

An engine change on a buried installation is a major operation, which is the principal reason the arrangement was abandoned. The other three statements are all true of buried installations and were their original attraction.

Answer: B

With equal thrust the moment follows the arm, so the outboard engine sizes the fin. Propeller aircraft add a further asymmetry from slipstream and P-factor effects, which can make one side distinctly worse than the other.

Answer: C

Chevrons cost a small amount of thrust in cruise, which is exactly the penalty accepted for the noise benefit. The other three describe chevrons correctly, and the trade of a little thrust for quieter takeoff is one designers accept readily.

Answer: D

Correct answer: It must keep the pod-to-wing channel unchoked

The gap between the nacelle and the wing lower surface forms a channel, and the flow accelerates through it. If the pylon is too short or the pod too close, that channel can choke, producing shocks, separation and a sharp drag rise at cruise Mach number.

The pylon therefore has an aerodynamic job as well as a structural one. It carries thrust, inertia and gyroscopic loads as well as weight; its length has direct aerodynamic consequences; and it is generally shorter on a wing installation than on a rear-fuselage one.