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Roger Mazengarb

How come jet-engine shut-downs start with higher RPM's

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I've noticed from the first time I landed and shut-down the Hawker that when pulling the fuel controls back to cutoff from idle (in order to turn off the engines), the sound of the engines unrealistically "jumps" to a higher pitch before scrolling down to shutdown. I noted the same effect with the PMDG 777. Am I doing something wrong in shut-down? For example, are engines supposed to be throttled up before fuel cutoff, and so what I'm hearing is my own inaccurate shut-down from idle?-DK----Fly! II v230Dell 8200 P4/1.8G, 512MB RAM, Nvidia GF4 Ti4600 v28.83, WinXP Home Edition.CH Pro Pedals and Yoke USB.

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DK-No- you are hearing it correctly....Certain types of engines have a stored compressed air charge that is released into the engine at the time the fuel cutoff switch is pulled. The purpose of the charge is to ensure that excess fuel is cleared from the engine and does not cook slowly in the combustion chamber, causing fumes, some smoke and an "environmental pollution hazard." (I put that in quotes because I'm not sure what they mean by that...) ;-)The surge is more noticable in some engines than in others....Robert S. RandazzoPrecision Manuals Development Group http://www.precisionmanuals.com/images/coolcap.gifwww.precisionmanuals.com

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The "transition" from idle to the higher pitcher does not appear realistic sounding, though. Its instantaneous (at least on my PC). I would imaging that in real life, the engines would spool up and then slow to a stop. There would be a brief transition as the idle-to-higher RPM's increase prior to decreasing - wouldn't you think?-DK----Fly! II v230Dell 8200 P4/1.8G, 512MB RAM, Nvidia GF4 Ti4600 v28.83, WinXP Home Edition.CH Pro Pedals and Yoke USB.

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Anyone else on this?-DK----David KohlFly! II v230Dell 8200 P4/1.8G, 512MB RAM, Nvidia GF4 Ti4600 v28.83, WinXP Home Edition.CH Pro Pedals and Yoke USB.

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David,Take a listen to the link attached, This is an actual recording at ramp level of the 757 PW2037 on shutdown. Its a 2.0 meg file so be ready. The shutdown covers the last 20 seconds or so. See if is what you are hearing and let me know.http://www.isd.net/pgollnic/images/IDLE.WAVThe loud Air Noise is the Compressor Bleed opening and unloading the High Stage Conpressor for shutdown. Every turbine engine I have worked has had some form of a compressor bleed to unload it on start up and shut-down. Hope this HelpsRegardsPaul:-cool

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Wow - I am amazed that this is how it actually sounds. I fly frequently and never noticed this high-pressure immediate change in pitch of the engines during shut-down. I guess from inside the plane, it doesn't sound like this.Well - I stand totally corrected. Fly!2 engine shut-down is obviously modelled accurately. Thanks very much for the WAV file. -DK----David KohlFly! II v230Dell 8200 P4/1.8G, 512MB RAM, Nvidia GF4 Ti4600 v28.83, WinXP Home Edition.CH Pro Pedals and Yoke USB.

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Showoff!!! ;-)Nice job Paul! Your participation is really appreciated (on the beta as well).George

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Paul, Wow, that's amazing! I never realised that this sound was so prominent either. :-)Matthew.

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Poor programing I'm afraid. If I understand your question correctly and you are at a stabilized idle before shuting off the fuel.Yes some engines do discharge an air accumulator (small) to purge the fuel manifold but the check for correct operation is to check that a small momentary rise in EGT occurs, not rpm.Compressor surge bleed valves should already be wide open at idle so they should not affect shutdown from idle.Just my 2C worthCheers,Roger @YSSY

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