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ngr
Topic Author
Posts: 172
Joined: Fri Apr 13, 2001 6:38 am

Question About Jet Engine Sounds

Mon Jun 21, 2004 5:12 am

Hello everybody! I have noticed that anytime a jet engine is brought out of idle thrust at a quick rate, that jet engines make a sound almost like the roar of a large fire, and after about 2-3 seconds, the jet whine takes over and the combustion sound is over. What is it that causes that roar?

Thanks,

ngr
 
dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

RE: Question About Jet Engine Sounds

Mon Jun 21, 2004 5:18 am

Ngr

Have you noticed this perhaps mainly on the 757? There is a 14th stage bleed valve on the engine that allows excess air, that is forced into the engine by the slow to decelerate fan, to escape.

There is an excellent thread on this already.

https://www.airliners.net/discussions/general_aviation/read.main/1579396/4/

Dl757md
757 Most beautiful airliner in the sky!
 
ngr
Topic Author
Posts: 172
Joined: Fri Apr 13, 2001 6:38 am

RE: Question About Jet Engine Sounds

Mon Jun 21, 2004 7:12 am

I have noticed this sound on almost every jet aircraft I have flown in, but the bleed valve does not appear to be what I am hearing since the sound comes when the throttles are being increased.
 
dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

RE: Question About Jet Engine Sounds

Mon Jun 21, 2004 7:20 am

Sorry , I misunderstood your original post.

That sounds like it may be harmonic balance. The engines as they spool up don't do so quite evenly and you get this whooshing noise.

Try this thread for help.

https://www.airliners.net/discussions/tech_ops/read.main/91287/

Dl757md
757 Most beautiful airliner in the sky!
 
pw4460
Posts: 16
Joined: Thu May 13, 2004 4:48 am

RE: Question About Jet Engine Sounds

Mon Jun 21, 2004 2:42 pm

While you're on the subject of 757s and those noisy bleed valves, I take it that's a condition only common with the PW 2000s. I remember last year on one particular DL 757 (I believe it had a reg.# ending in the letters "AT"), that wooshing noise was continuous through the entire takeoff, which isn't typical. Obviously there can't be any harm done to the engine, right?
 
dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

RE: Question About Jet Engine Sounds

Mon Jun 21, 2004 3:06 pm

Pw4460

While you're on the subject of 757s and those noisy bleed valves, I take it that's a condition only common with the PW 2000s.

Yes, although I'm not familiar with the RR powered 757 I've been told they don't make the same noise.

I remember last year on one particular DL 757 (I believe it had a reg.# ending in the letters "AT"), that wooshing noise was continuous through the entire takeoff, which isn't typical. . Obviously there can't be any harm done to the engine, right?

DL 757s with AT at the end of their reg # are the four PW2037 powered 757s DL acquired from ATA.

If the 14th stage bleed valve was open during T/O the engine would show a higher EGT than normal. As long as EGT stayed below the TO limit full power would be available, it would just take more fuel than normal. Normally of course the 14th stage bleed valve is closed during TO and the fact that it is open is an indication of a problem. This probably wouldn't be noticed by the pilots unless it overtemped the engine. ACARS would however automatically downlink this info to the engine analysts on the ground. If an overtemp did occur they would either abort TO or do an air turnback and the engine would have to be boroscoped.

Dl757md
757 Most beautiful airliner in the sky!

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