PW1100G in depth start-up sequence

[BarryMG]

Hi all
So the PW1100G has this "cooling" period during startup. During this phase, N2 is kept below 10% and the fan is not moving at all. I've been wondering the following:
- How is the N2 maintained? Is the starter valve modulated, not a simple on/off valve?
- Why does the fan remain stationary? Is it just a matter of internal friction vs low N2 not producing enough force to spin it, or is there some mechanism in place to hold it?

[tdracer]

I don't know about the PW1000G, but the LEAP uses "pulse width modulation" to control N2 - i.e. the starter air valve is rapidly opened and closed to maintain the desired N2 speed.
I'd be very surprised is there any physical mechanism to prevent N1 rotation - first off the failure modes are outright nasty, and there is no need to keep the fan from rotating.

[V2533]

Why does the fan remain stationary? Is it just a matter of internal friction vs low N2 not producing enough force to spin it, or is there some mechanism in place to hold it?

It is indeed too much friction for not enough air going through the core engine. The PW1100G N1 Rotor System is quite hard to turn (you`ll need both Hands to turn it) while the N1 Systems of the CFM56 or V2500 for example could easily be turned by hand (or only 2 fingers).

[Check Airman]

Interestingly, the LEAP engine (CFM NEO) is a lot easier to turn by hand than the PW NEO.

[FlyingStone]

Lack of gearbox would be the best bet as to why.

[Check Airman]

I should look that up. I thought all these current generation, oversized engines were geared.

[tdracer]

Originally Posted by Check Airman

I should look that up. I thought all these current generation, oversized engines were geared.

Nope, the PW1100G is one of the few (and by far the largest) turbofan engine to use a geared fan. All the GE and RR Trent engines use a straight shaft to tie the fan to the LP turbine. Those GE and Rolls engines can easily be turned by hand (it takes a bit to get them moving due to the inertia, but once moving you can spin them with one finger). They also routinely slowly spin with a gentle breeze.

Back during the GE90 development, one of the GE engineers showed me an analysis he'd done of how much oil cooling would be required for a gearbox capable of driving the fan on a 100,000 lbs. thrust class engine. Suffice to say the oil cooler would have had to be massive .

[Check Airman]

Thanks tdracer!