Inlets and Diffusers Questions and Answers

Answer: A

The compressor needs air delivered at a modest and uniform axial Mach number, typically around 0.4 to 0.5, whatever the flight speed happens to be. Converting velocity into static pressure ahead of the first stage is free compression the engine does not have to pay for.

Answer: B

The faster the aircraft flies the more compression the intake performs before the engine does any work at all. At high supersonic speeds this contribution dominates, which is precisely why a ramjet can dispense with a compressor altogether.

Answer: C

It is the single figure that expresses how much of the available energy the intake has preserved. A loss of one per cent in recovery typically costs more than one per cent in thrust, which is why intake design receives such attention on high speed aircraft.

Answer: D

Its simplicity and low weight suit subsonic and low supersonic aircraft where shock losses are small or absent. The rounded lip matters because at static conditions the flow must turn sharply into the intake and a sharp lip would separate immediately.

Answer: A

The engine draws air from a wide region ahead of it and accelerates it into the duct. That is why the lip must be well rounded and why intake performance on the ground differs so sharply from its behaviour in high speed flight.

Answer: B

The flow is already moving faster than the compressor requires, so deceleration begins outside the intake in a diverging streamtube. The intake is sized so that this external diffusion is modest, since too much of it invites separation at the lip.

Answer: C

Air that approaches the intake but flows around it must be turned, and the pressure forces of that turning appear as drag on the airframe. It is worst when the engine is throttled back at high speed, since the demand falls while the capture area stays fixed.

Answer: D

A supersonic stream slows in a converging passage and accelerates in a diverging one, the reverse of subsonic behaviour, so smooth deceleration to subsonic speed is impossible. The shock provides the discontinuous transition, at the cost of a total pressure loss.

Answer: A

Loss across a normal shock rises steeply with the upstream Mach number, so a single shock wastes an unacceptable fraction of the available pressure. Splitting the deceleration across several weaker oblique shocks recovers most of that loss.

Answer: B

Ramps or a centre body generate oblique shocks outside the intake, and a weak normal shock completes the transition at the throat. The arrangement is robust but the turned external flow carries a cowl drag penalty that grows with Mach number.