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Faro
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Jet Engine Synchronisation

Fri Sep 07, 2007 3:05 am

One often notices regular, weak "highs" in airliner engine noise which recur at intervals of say 2-20 seconds. Sometimes they coincide between different engines and sometimes not, leading to a recurrent overlap effect. My questions are:

i) what are these local "peaks" in engine noise due to; and
ii) do flight crew actively strive to synchronise them between engines and if so why?

Faro
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pilotpip
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RE: Jet Engine Synchronisation

Fri Sep 07, 2007 3:14 am

The noise you're hearing is from one engine spinning slightly faster than the other. It's not as noticable in jets as it is with props. Most turboprops have some sort of prop sync as a result of this.

We like to keep the engines at the same fuel burn so we don't end up having an imbalance.
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Faro
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RE: Jet Engine Synchronisation

Fri Sep 07, 2007 3:50 am

Quoting Pilotpip (Reply 1):
The noise you're hearing is from one engine spinning slightly faster than the other. It's not as noticable in jets as it is with props.

So, in fact the noise is produced by the airframe itself "beating", ie humming at slightly different frequencies between one engine pylon and another? It's not actually produced by the engines?
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KELPkid
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RE: Jet Engine Synchronisation

Fri Sep 07, 2007 3:55 am

Ever sit between the engines on a DC-9? Jet engines can get quite out of synch, and it's about 10 times more annoying than it is on a piston bird...  hypnotized 
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Jetlagged
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RE: Jet Engine Synchronisation

Fri Sep 07, 2007 6:22 am

Quoting KELPkid (Reply 3):
Ever sit between the engines on a DC-9? Jet engines can get quite out of synch, and it's about 10 times more annoying than it is on a piston bird...

Certainly is noisy back there. Probably why they installed engine synchronisation to the MD-80.
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rwessel
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RE: Jet Engine Synchronisation

Fri Sep 07, 2007 8:32 am

Quoting Faro (Reply 2):
So, in fact the noise is produced by the airframe itself "beating", ie humming at slightly different frequencies between one engine pylon and another? It's not actually produced by the engines?

No, it' just the "beat" from the two engine hums going in and out of phase with one another. You can get the same effect with two musical instruments (pianos or guitars are easy cases) that are slightly out of tune with respect to one another. Hit the same note on both, and you'll get than same "WWWWAAAAHHaaaahhWWWWAAAAHHaaaahhWWWWAAAAHHaaaahh" effect.

It's more noticeable on propeller aircraft since the "hums" are lower frequency and louder.

Or were you misunderstanding the word "beat?"

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