Originally Posted by
galaxy flyer
tdracer,
Would it more accurate to say it’s the compression ratio that matters?
It matters. but that's only part of it. Of course, the whole in-flight start thing is fairly complex (the PW4000/94", RB211-524, and CF6-80C2 had similar overall compression ratios, but the CF6 windmill start was significantly better, while the 3 spool RB211-524 was
way worse). Subtle things - like the profile of the splitter (that separates the core and fan airflow) can make a big difference. Other things, like the way the fuel is injected into the combustion chamber, and the increasing complex burner/combustion chamber designs also play into it. The GEnx was a SOB to start - even on the ground. GE had to play around with the fuel scheduling
a lot to get it to the point where we could get reliable light-off characteristics - and even they we had an unpleasant discovery late in the flight test program when they took a 747-8 to Alaska to do very cold temp ECS and other system testing (-30 to -40 degrees C) - only to discover that they couldn't get the engines to light off when it came time to come home. We ended up doing a panic trip to Iqaluit, Canada (it was late March and it was the only place we could find where they were expecting sub -30 degree C temps before winter ended). GE had to try a few different starting fuel schedules before we found one that would reliably light-off so we could get the engines started when it was super cold...
But for windmill starting, none of the rest maters if windmill N2 (N3) is too low to open the fuel shutoff valve and - depending on the circumstances (like an all-engine out) - to power the FADEC.