Engine Performance and Matching Questions and Answers

Answer: C

Correct answer: 65 kg/s

Invert the correction to recover the physical flow: \[ \dot m = \dot m_{corr} \frac{\delta}{\sqrt{\theta}} = 200 \times \frac{0.30}{0.922} = 65\ \text{kg/s.} \] The engine is doing the same aerodynamic work as at sea level static, but the air is thin, so it swallows less than a third of the mass. This is precisely why thrust falls so steeply with altitude.

Answer: D

Correct answer: 17 per cent

The surge line sits 25 per cent above the steady working line and the transient excursion uses 8 of that, leaving \[ 25 - 8 = 17\ \text{per cent.} \] The acceleration schedule exists to keep that excursion bounded: fuel added faster than the shaft can accelerate raises the pressure ratio without a matching rise in flow, and the working line climbs.

Answer: A

Correct answer: 35 K, about 4.5 per cent

The rise is \( 815 - 780 = 35 \) degrees, and a difference in Celsius is the same as a difference in kelvin. As a fraction of the earlier reading, \[ \frac{35}{780} = 0.045 = 4.5\ \text{per cent.} \] The engine is producing the same thrust for more heat, which is the classic signature of deterioration and the quantity the margin tracks.

Answer: B

Continuity must be satisfied, so the operating point moves until the compressor delivers exactly what the turbine will pass. That adjustment is the essence of matching and is what places the working line where it sits.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason for the first. The design point is a single condition chosen for component sizing, but the engine idles, taxis, takes off, climbs and cruises at part power, and spends almost none of its life at the design point. Off design behaviour is therefore what determines whether the engine is usable.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Because the nozzle is choked, its throat area alone determines the corrected flow the compressor must supply at each pressure ratio - so changing the area by a few per cent changes that requirement by the same few per cent, which shifts the entire working line. It is the most powerful single lever available for adjusting surge margin.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second gives the mechanism. Dividing speed by the square root of the inlet temperature ratio and flow by the appropriate combination of temperature and pressure ratios removes the dependence on inlet conditions, so measurements taken on a hot day at sea level and a cold one at altitude fall on the same curve. One map then serves the whole flight envelope.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason. Fuel added faster than the shaft can accelerate raises turbine entry temperature and pressure ratio while the flow has not yet risen to match, so the working line climbs towards surge. The acceleration schedule limits the fuel flow to what the engine can absorb, trading a slightly slower response for a guaranteed margin.

Answer: C

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

The second statement is true and it is why the first is false. The shafts are mechanically independent, so each finds the speed at which its own turbine work matches its own compressor demand while flow continuity is satisfied. There is no reason for the two percentages to coincide, and in practice they differ noticeably across the operating range.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and they describe one continuous practice. The margin is computed from recorded data on every flight and trended, because it shrinks steadily as the engine deteriorates. When the trend line approaches zero the engine can no longer make its rated thrust within the temperature limit, and the removal is planned in advance rather than forced.