Why does putting Eng Anti-Icing on (increase air bleed use) help control engine rpm

[tdracer]

Originally Posted by Le Flaneur

The C-17 engine is derivative of the PW 2000 used on the B757. In the C-17 they are flat rated to 40,400 lbsf to 30 C. I am sure there are other differences, but the biggest obvious difference is the C-17 has a core thrust reserver in addition to the fan reserver that the B757 version has.

I've always thought that applying a bleed load like the engine anti-ice reduced the compressor load (meaning that the compressor wasn't producing the same compression ratio). However, it would seem that the engine control would compensate to keep the engine operating at the same RPM.

Would be nice to hear from a propulsion engineer. Thank you

Sorry, I'd missed when Hueymeister posted the engine in question (I've been traveling a lot lately).
Never worked the C-17, but I was responsible for the PW2000 on the 757 for several years - the differences the F117 and the PW2000 are fairly minor.
As noted, bleeding air from the engine unloads the compressor and generally improves the stall margin. While the FADEC will work to keep the rotor speed constant, there are response times involved - the lag between when the FADEC senses a speed change, the fuel flow responds, and the resultant change in rotor torque can not only allow the rotor speed to change significantly, it can actually result in an oscillatory speed response.
Educated guess here, but I'm thinking that, under the circumstances in question, the HP compressor is on the edge of stall - and the variations in flow separation in the compressor can cause the rotor speed to fluctuate more rapidly than the FADEC can compensate for - possibly even resulting in an oscillatory response as noted above. Turning on anti-ice bleed unloads the compressor sufficiently, moving the compressor away from stall and helping the FADEC keep the rotor speed constant.