N210843 - Engine bleed

[mchristlieb]

The question asks what happens when you bleed air from the HP compressor while maintaining thrust.

Ignoring the answers I would say:

EGT goes up (more fuel)
N1 should stay constant (constant thrust)
N2 goes up (more fuel and you're bleeding air so need to speed up the HP compressor to get bleed air and combustion air demand.
Fuel flow goes up (can't make energy for nothing)

Two answers seem right:

What increases

EGT and fuel flow
N2 and fuel flow

Why the first and not the second?

[mchristlieb]

Which actually raises a second question: The two spools of a twin spool are not directly connected. But if one increases RPM can the other one remain unchanged? Or do they both move at the same time just not by the same amount?

[John CTKI]

I like your thinking, it shows that you are developing a good understanding.

To answer your first post, you can maintain the rpm by increasing fuel flow, this expands the gases more. The penalty is that temperature also goes up.

As to your second question, N1 & N2 accelerate & decelerate in the same direction but at differing rates.

[mchristlieb]

Right - so the first answer is more correct because EGT and fuel are the primary variables. RPM is a product of fuel flow. You control rpm through fuel not the other way round.