Squealing engines...

[Nishko]

This question refers to a 747-400, but I think it is relevant for all turbofan engines...

When I watch these aircraft taxi up to the hangar at EGSC, the engines sound very much as you'd expect, but when the HP cocks are cut, the engines emit a very load high pitched squeal as they spool down. This sound is not audible during normal idle conditions, so why does it appear when the fuel is cut? Does it have anything to do with gear trains or something? The intensity of the noise seems higher at the front end of the engine also.

Any engine experts out there who can offer an explination, it's been bugging me for ages!

Nish

[Lu Zuckerman]

Most likely it is noise generated by the turbine. When the engine is under power, the flow of fan air into the engine muffles the sound. (I think)

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The Cat

[OverRun]

It is the contract pilots who are squealing because they are paid by the minute and the clock is linked to the engines.


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This is obviously a joke response, but it's too good a chance to let it go past

[Danish Pilot]

Nishko: It is pretty hard to give an excatly reason. Engine sounds are very different depending on manufactor (GE, PW, RR, CFM, Aero international...etc) and allso from where you observe the sound.

More info please...

[Nishko]

Danish Pilot,

Cheers, these engines are nearly always the Rolls Royce ones, fitted to the -400, and always BA aircraft.....

Does that help?

Nish.

[Mark 1]

Don't know if I can help specifically, but a few pointers on engine noise at low powers:

-535s especially, have a noticeable hoot at ground idle powers, put down to combustion instabilities. Combustion rumble is most audible at low powers.

IP and HP turbine (rotor/stator) blade interaction tones, which are normally at too high a frequency to be heard, will drop through the audible range when spooling down. The relative Mach no.s will still be high enough for this to be significant.

Handling bleeds may be open, jettisoning HP air into the by-pass duct, though this will be broadband noise usually.

Hope this helps.

[Nishko]

Mark 1

That's fantastic response - thanks.

Nish

[Danish Pilot]

Nishko, sorry, cant help you on this one, as we don´t see many 747-400 with RR here in Scaninavia. Mark 1 said it well. But I do know you are never in doubt when an RR RB-211 starts up.... :)

[This message has been edited by Danish Pilot (edited 25 November 2000).]