Propulsion Integration Questions and Answers

Answer: A

The engine mass outboard opposes the wing's upward bending in flight, so the structure can be lighter, and a pod at knee height is easy to work on and to change. The penalties are gear length and a large engine-out yawing moment.

Answer: B

Correct answer: A small engine-out yawing moment

A rear-fuselage engine sits close to the aircraft's centreline - typically two or three metres rather than the seven or more of a wing pod - so the yawing moment when it fails is far smaller.

The consequence is a smaller fin and rudder, a lower minimum control speed and a shorter takeoff run. The penalties are real too: an aft centre of gravity, a heavier rear fuselage to carry the loads, and no wing bending relief. Cabin noise is actually worse at the rear, and the pylon is short only incidentally.

Answer: C

Cutting a large hole in the primary structure at its most heavily loaded station is expensive, and an engine change becomes a major operation. A pod hung on a pylon, by contrast, can be removed and replaced in a matter of hours.

Answer: D

Getting the nacelle far enough forward and low enough keeps the channel between it and the wing from choking, which is where interference drag comes from. Structurally it also carries thrust and engine loads into the wing.

Answer: A

Correct answer: Subsonic flight, giving high recovery simply

A pitot inlet is simply an open duct facing forward. At subsonic speeds it achieves pressure recovery above 0.98 with no moving parts, no shock system to manage and almost no weight penalty, which is why every subsonic transport uses one.

It fails at high supersonic speed because the flow must pass through a single normal shock, and a normal shock at Mach 2.5 destroys a large fraction of the total pressure. Recovery is emphatically not independent of Mach number, which is the whole reason supersonic inlets are more elaborate.

Answer: B

It measures how much of the flow's total pressure survives the journey to the engine, and it maps almost directly into thrust. A one per cent loss in recovery can cost appreciably more than one per cent of the thrust available.

Answer: C

Correct answer: Match the shocks to Mach number

A supersonic inlet decelerates the flow through a system of oblique shocks, and the angles at which those shocks stand depend on the flight Mach number. A fixed geometry can therefore be correct at only one speed.

Variable ramps or a translating centrebody move the shock system so that it remains properly positioned across the flight envelope, keeping recovery high and avoiding the unstable shock oscillation known as buzz. The mechanism adds weight and complexity rather than saving it, and a diffuser is still required.

Answer: D

Correct answer: An unsteady shock oscillation at the inlet

Buzz is a violent oscillation of the shock system at a supersonic inlet, occurring when the inlet is operating away from its matched condition - typically when the engine demands less air than the inlet is capturing.

The shock moves rapidly in and out of the throat, producing large pressure fluctuations that can damage the structure and stall the compressor. It is a flow instability, not a structural resonance, fan noise or a combustion problem, and it is one of the conditions variable geometry exists to avoid.

Answer: A

Correct answer: Keep boundary layer out of the inlet

Air flowing along the fuselage ahead of a side-mounted inlet forms a boundary layer that is thick, low in energy and turbulent. Ingesting it lowers the pressure recovery and, worse, delivers badly distorted flow to the compressor face, which can provoke surge.

A diverter is a wedge or a gap that holds the inlet clear of the fuselage surface so the boundary layer passes outside it. The cost is extra frontal area and drag, which is exactly why the arrangement is avoided where a podded installation is possible.

Answer: B

The fan stream carries most of the mass flow, so turning it is where the retarding force is. Cascade or blocker-door arrangements are the norm, and the core stream is usually left alone entirely on a modern high-bypass installation.