cedarjet
Posts: 8161
Joined: Mon May 24, 1999 1:12 am

"Engines In Sync" - What Does This Mean?

Sat Oct 13, 2001 9:29 am

Hey now. Heard many references to engines being 'out of sync' and indeed I'm very familiar with the sensation from sitting at the back of many MD80s and hearing that weird throbbing sound.

What does 'sync' mean, how is it achieved, any info or observations please. Including an answer to the question, "Can engines not being in sync cause damage to the airframe, at least in as much as pop a few weakened rivets?" Is it important to the operation of the aircraft, or is it just a crowd pleaser for the back rows?
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
flight152
Posts: 3251
Joined: Fri Nov 24, 2000 8:04 am

Engines In Sync- What Does This Mean?

Sat Oct 13, 2001 9:48 am

Does it have to deal with a boy band? Big grin Big grin Big grin
 
Purdue Arrow
Posts: 947
Joined: Tue May 25, 1999 1:49 pm

RE:Engines In Sync - What Does This Mean?

Sat Oct 13, 2001 2:56 pm

"Sync" is short for "syncronized". The engines are in sync when they are turning at the same rate. The pulsing sound you hear when the engines are not in sync comes from the different frequencies of the sound waves. I don't think it's damaging to the airframe (though I could be wrong), but it is not comfortable for the occupants and doesn't provide even thrust on both sides. It can be very fatiguing to flight crews to have that pulse in the background.
 
FBU 4EVER!
Posts: 980
Joined: Sat Jan 27, 2001 6:53 am

RE: RE:Engines In Sync - What Does This Mean?

Sat Oct 13, 2001 5:54 pm

You mentioned the MD-80,but the DC-9 is worse regarding this matter.
On the DC-9,the pilots themselves have to manually synchronize the engines.Usually,it is the N1 (low pressure) fans that are synchronized,but,engines being the individuals they are,N2 (high pressure) fan synchronization might occasionally be better.I know some airlines have auto synch. on their -9's,but here in SAS we don't.
On the MD-80 and 90,there is a switch on the overhead panel which allows the pilots to select either EPR (default position),N1 or N2 synchronization.The autothrottle system has to be engaged to allow auto synch. which operates by adjusting the engine fuel controls automatically.
"Luck and superstition wins all the time"!
 
cedarjet
Posts: 8161
Joined: Mon May 24, 1999 1:12 am

RE: "Engines In Sync" - What Does This Mean?

Sat Oct 13, 2001 6:03 pm

How close do the RPMs have to be to cancel each other out?

And is it, how can I put this, possible to achieve with different RPMS, ie they are in sync if the RPMs are the same, or if one is 2%/4%/8%/16% apart? Just curious cos it seems to be a kind of harmonic effect.

Thanks for the info so far, especially from Flight 152 (I was on a plane with N'Sync once, they threw peanuts).
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
FBU 4EVER!
Posts: 980
Joined: Sat Jan 27, 2001 6:53 am

RE: "Engines In Sync" - What Does This Mean?

Sat Oct 13, 2001 6:15 pm

Achieving 100% perfect synchro is almost impossible as there will almost invariably be a slight difference between the N1 and N2 spools.You can synchro N1 but there will be a difference between N2,if you synchro N2,there will be a difference between the N1 spools,so you try to use the mode that gives the least noise.The auto synch uses one engine as "master" and "slaves" the other to this.Not sure about tolerances,but the number 1,5% seems familiar from somewhere.My AOM doe not mention any values or limits.
"Luck and superstition wins all the time"!

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