Propulsion Integration Questions and Answers

Answer: A

Correct answer: The shock system is unstable and mismatched

Buzz is an unsteady oscillation of the shock system, and it occurs when the inlet is running away from its matched condition - typically capturing more air than the engine will swallow, so the terminal shock is expelled and drawn back repeatedly.

That it appears from Mach 1.6 on an aircraft designed for Mach 2 says the geometry is correct only near the design point and the schedule is wrong below it. A diffuser that is too long, or an oversized diverter, would degrade recovery steadily rather than produce an instability.

Answer: B

Correct answer: The twin, since losing one engine removes half the total thrust

With equal total thrust \( 4T \), the twin's engines produce \( 2T \) each and the four-engine aircraft's produce \( T \). Losing one leaves an asymmetry of \( 2T \) on the twin against \( T \) on the four.

Even though the four-engine outboard sits on a longer arm, the doubled thrust usually dominates: at 8 m and 12 m the moments are \( 16T \) against \( 12T \). Losing one of two is a far greater proportional loss than losing one of four, and that is what sizes the fin.

Answer: C

Correct answer: Hot gas and debris being drawn back into the inlet

Below the stowing speed the aircraft is no longer outrunning its own reversed efflux, so the hot gas curls forward and is drawn back into the inlet. Re-ingestion raises the inlet temperature, distorts the flow and can stall the compressor; the same flow field also picks up loose debris from the runway.

The stowing speed exists precisely to prevent this. Directional control, nose gear loading and brake temperature are all real considerations elsewhere, but none is the hazard this limit guards against.

Answer: D

Correct answer: pylon

The pylon is the structure that positions the engine pod ahead of and below the wing and carries its loads - weight, thrust, inertia and gyroscopic - into the wing box. It also routes fuel, bleed air, electrical and control services.

The nacelle is the aerodynamic fairing enclosing the engine, and the cowls are its openable panels. The diverter is an inlet feature on fuselage-side installations. The pylon's length and shape matter aerodynamically as well as structurally, since it must keep the channel between pod and wing from choking.

Answer: A

Correct answer: pressure recovery

Pressure recovery is the ratio of total pressure delivered to the compressor face to the total pressure in the free stream. A good subsonic pitot inlet achieves better than 0.98; a poorly matched supersonic inlet may fall far below.

It matters because thrust is more than proportionally sensitive to it - a loss of five points in recovery costs rather more than five per cent of thrust. Capture ratio compares the captured streamtube with the inlet area, and bypass ratio is a property of the engine rather than the inlet.

Answer: B

Correct answer: spillage

Spillage drag arises when the engine demands less air than the inlet is capturing. The excess spills around the outside of the cowl lip, and the momentum change involved appears as drag.

It is unavoidable in practice because the mass flow an engine demands varies enormously between takeoff and cruise, while a fixed inlet has one capture area. Sizing the inlet is therefore a compromise across the flight envelope, and variable geometry exists partly to reduce this loss on aircraft where it would otherwise be severe.

Answer: C

Correct answer: chevrons

Chevrons are the triangular serrations at the trailing edge of a nacelle or nozzle. They shed streamwise vortices that promote finer-scale mixing between the jet and the surrounding air.

Breaking up the large turbulent structures at the shear layer shifts the acoustic energy to higher frequencies, which the atmosphere absorbs far more rapidly, and reduces the low-frequency content that carries to the ground. A small thrust penalty is accepted for the noise benefit, which is significant on takeoff.

Answer: D

Correct answer: fuse

Fuse pins are deliberately weakened attachments in the engine mount, sized to shear at a load far above anything normal operation produces but below that which would fail the wing structure.

The reasoning is that an engine departing cleanly is survivable, whereas a pylon that stays attached and twists can tear open the wing box and its fuel tanks. In ordinary service the pins are simply structure, carrying the steady thrust and inertia loads with no intended flexibility at all.

Answer: A

Correct answer: Underwing outboard

The moment column settles it: 1150 kN m at the outboard station against 600 inboard, 320 at the wing root and 260 at the rear fuselage. The fin must be sized for the worst case, so it is the outboard failure that governs.

The figures follow directly from moment being thrust times arm, and they show the range at stake - the outboard installation demands more than four times the fin authority of a rear-fuselage one. This is a large part of why four-engine aircraft carry proportionally big vertical tails.

Answer: B

The two are indistinguishable subsonically, where the simple duct is obviously preferable, but the pitot collapses once the normal shock strengthens. The crossover is where compression inlets start to be worth their weight and complexity.