Required N1 speed for a windmilling engine start?

[tunnelcat 1]

On the 747, what should be a realistic minimum N1 fan speed required to get a windmilling engine to start while in flight? I was able to get an engine to light off with an N1 rotation of only 11% with continuous ignition turned on (at normal cruise speeds, no autostart). It took about 30 seconds to start, but it worked. This seems like it would invite a hot start and serious engine damage on the real aircraft.Kim

[michal 44]

Not sure I see a connection with hot start. I am not a turbine engine expert but common sense dictates that in flight you have such a high flow of air through the chamber (even if you managed to keep N1 at 0) that hot start is unlikely. There must be an important distinction between flow of air created by the N1 (compressor) when airplane is stationary and N1 caused by flow of air while windmilling. In my opinion these are two different N1s.Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

[rgamurot 1]

I don't know about N1 but I think you need at least 20% N2 to start. Also, I do believe that Michael is right in that a hot start is unlikely. A hot start is the result of either a higher than normal fuel flow, or if the combustion chamber is filled with too much fuel (i.e. you don't have autostart on and move the cutoff switch to run at the same time you pull the start switch). In this case however, you would probably have so much air moving through the engine, the excess fuel would be carried right out anyway. But yes, you usually don't want to take 30 seconds for the engine to light. Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

[tunnelcat 1]

I was experimenting around with the 747's flight handling with #4 engine out (I had shut it down in cruise flight earlier) and was curious to see if I could get it restarted without using autostart.Next time I fly the 747, I'll set up the same scenario and check and see what the N2% speed is when N1 is rotating at 11% in cruise flight and check how close it is to the fuel-on indication for an in-flight start. I should have been paying attention to the EICAS display that showed N2%, which is a requirement for proper engine start. I shouldn't get distracted from proper procedures, even if it is only a simulation.Kim :-doh