Engine Performance and Matching Questions and Answers

Answer: A

Every component is sized for that one set of conditions, and everywhere else the engine runs off design. Most of an engine's life is spent away from the design point, which is why off design behaviour receives at least as much attention.

Answer: B

Continuity fixes the flow, the work balance ties turbine output to compressor demand, and a common shaft ties their speeds. Those three conditions together determine where the engine actually runs, regardless of where each component would prefer to be.

Answer: C

It is where the matching conditions place the compressor at each power setting, not where the designer might wish it were. Its distance from the surge line is the surge margin, which must remain adequate through transients and deterioration.

Answer: D

The choked turbine nozzle fixes the flow the compressor must deliver at a given pressure ratio. Adjusting that area by a few per cent moves the working line a long way, which makes it the standard remedy for an inadequate surge margin.

Answer: A

Correct answer: They are choked, fixing the corrected flow

At almost every operating condition the first stage nozzle guide vanes run choked, so the corrected flow through them is set by their throat area alone and cannot change with what happens downstream. The compressor must therefore deliver exactly that flow at each pressure ratio, which is what fixes the working line. It is a fixed geometry doing the controlling, not a variable one.

Answer: B

Dividing by the appropriate functions of inlet temperature and pressure collapses all conditions onto a single chart. Without correction a separate map would be needed for every altitude and ambient temperature the engine might meet.

Answer: C

The relation follows from the way blade Mach numbers depend on temperature. Cold air gives a higher corrected speed for the same physical speed, which is why an engine behaves differently on a cold day at the same shaft rpm.

Answer: D

Correct answer: Find their own speeds independently

There is no mechanical connection between the two shafts - each carries its own compressor and turbine and settles at whatever speed balances its own work and satisfies flow continuity. They are coupled only aerodynamically, through the gas passing from one to the other. This freedom is the whole point of the twin spool arrangement, since it lets each spool run near its best speed over a wide range.

Answer: A

Fuel is added faster than the shaft can accelerate, so the pressure ratio rises before the flow has risen to match. The fuel control schedules acceleration precisely to keep that transient excursion within the available surge margin.

Answer: B

Cutting fuel faster than the shaft decelerates leaves the chamber below its weak extinction limit. The fuel control schedules deceleration for the same reason it schedules acceleration, protecting the engine at both ends of the range.