Engine Acceleration

[Broadcast Control]

Today we had a discussion about acceleration times for different types of turbine engines. I claimed that an engine with a centrifugal compressor will accelerate slower than a comparable engine with an axial compressor due to the higher intertia of the centrifugal compressor. However, as several axial stages are required to obtain the same pressure increase as for a centrifugal compressor, I am in doubt about the net effect. Any views?

[Shawn Coyle]

Perhaps the question is best answered by looking at the deceleration rates - a centrifugal only compressor (250-C30 engine) will decelerate a lot more quickly than an axial/centrifugal compressor (250-C20B).
Acceleration rate is also dictated by the surge margin of the engine, which has something to do with the compressor design, but I'm not exactly sure what it has to do with it!

[212man]

Surely any inertial acceleration or deceleration characteristics will be masked by the fuel scheduling properties of the FCU/FADEC?

[ShyTorque]

Yes,

Surge / stall margins must also be considered. A turbine might need to have it's acceleration rate slowed to prevent surge. The Gnome engine (axial) was renowned for problems if the pilot tried to accelerate it too quickly from ground idle to flight idle.

[Graviman]

Acceleration rate is also dictated by the surge margin of the engine, which has something to do with the compressor design, but I'm not exactly sure what it has to do with it!

This is due to the pressure ratio / flowrate performance of the compressor, Shawn. As you put more fuel in the higher combustion chamber temp causes higher expansion, and thus high pressures. If increased slowly the engine speed increases mass flowrate, so pressure change is not a problem. I increased too quickly the pressure can cause reduced flow, until the compressor stalls. This sets a limit on dRPM/dt.

With FADEC controllers i imagine the main limitation on rate of increase of fuel mass flow rate is Turbine Entry Temperature.

Mart