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poorjohn
28th Mar 2011, 18:42
The past couple of rides on AA TATL 777's I've observed periods of very noticeable low frequency vibration that I'd guess would be the beat frequency between the two engines. Sometimes it's constant for minutes, other times varies in amplitude with a short period (a few seconds), other times it's not noticeable at all. At its strongest it can be felt easily through well-padded seats.

I suppose it's been happening for years and I've just noticed, but given the search for "shareholder value" at many corporations, I wonder if tolerance on some maintenance step has been increased. (Still well within limits, of course.) And of course the underlying question would be what all that energy does for the airframe longevity.

pj

BOAC
28th Mar 2011, 18:45
I don't know about 777s but I spent many fruitless minutes trying to eliminate a 'beat' on 737s - the problem is that, unlike a piston aircraft, you have more than one rotating body running at a different speed in each engine and altering one affects the other.

411A
29th Mar 2011, 18:58
Three-shaft Rollers are easy to 'sync', two shaft models from other manufacturers, less so.

The late XV105
29th Mar 2011, 19:33
Noticed exactly this on am EK 773 DXB-BHX a few days ago (along with the usual invasive start-up vibration from those massive donks).

Dan Winterland
30th Mar 2011, 03:58
Syncronising engines in modern aircraft such as the B777 is usually a function of the FADEC (Full Authority Digital Engine Control) and the benefits are less vibration and consequently less fatigue on the structure and a quieter environment for the passengers.

However, the systems needs the engines to be reasonably matched and if one engine is changed, the parameters can be very different with the new engine being much more efficient. This is when the engines can can get out of phase with the FADEC having a hard time balancing the mismatched engines.

Older types without digital engine control need manual synchronisation. One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1. Turboprops have a bigger problem with the propellers being larger rotaing masses. Analogue sycnronising systems were common from an ealier age, but again these were unreliable and often needed manual intervention.

Start up vibration is common. this is because the large fan baledes are slightly loose in their sockets. They rely on the enormous centripetal force when the fan is up to speed to hold them in position and give the fan it's rigidity. Before the fan is up to speed during start, one or more blade may be slightly out of position leading to the vibration.

lomapaseo
30th Mar 2011, 14:52
Syncronising engines in modern aircraft such as the B777 is usually a function of the FADEC (Full Authority Digital Engine Control) and the benefits are less vibration and consequently less fatigue on the structure and a quieter environment for the passengers

Does anybody have access to any words from Boeing or Airbus confirming this ?

Beeline
30th Mar 2011, 16:57
Throttle stagger or lack of it is a function of FADEC equipped engines, never heard of a synchronous function but would be interested to find out.

I thought any damage critical vibration prevention is better than cure e.g vibration trim margins lower, and resonance flutter margins/ranges kept lower, V2500 bus drivers will understand, the engine races through the flutter range when going through the quadrant up to max climb.

Got the heart thumping the first time I did a run on one!!

GarageYears
30th Mar 2011, 17:15
Found the following:

The 717 autoflight system is adapted from the MD-11 system. It consists of two flight control computers (FCC) and a glareshield control panel (GCP). The FCCs provide autopilot, autothrottle, flight directors, stall warning, wind-shear detection and guidance, engine synchronization, and various other functions
And from Airliners.net regarding 737-700/800:

Below are some extracts from the SDS.

Display Electronics Units (DEU) The A/T computer sends mode data to the DEUs to show A/T modes of operation on the FMA on the CDS.
The FMC calculates engine N1 limits and N1 targets during each flight phase and sends the data to the DEUs. The DEUs show N1 limits on the engine display. The DEUs send N1 targets to the EECs.

FMC The FMC calculates thrust N1 limits and N1 targets for each flight phase. The data goes to the DEUs. The DEUs show the N1 limits on the engine display. The DEUs send the N1 targets to the EECs which calculate equivalent TRA targets to send to the A/T to set thrust. The FMC also sends N1 targets directly to the A/T. During takeoff and max thrust go-around, the A/T uses EEC TRA targets and FMC N1 targets to set thrust. During takeoff, climb, and max thrust go-around, the FMC N1 targets are the same as the N1 limits. During reduced thrust climb and cruise operations, the FMC N1 targets are less than the N1 limits.
EEC The DEUs send FMC N1 targets to the EECs. The EECs use the data to calculate equivalent TRA targets. The A/T uses the EEC TRA targets to set thrust during takeoff, climb, and max thrust go-around. For takeoff and max thrust go-around, the A/T initially uses EEC TRA targets to advance the T/Ls. As the T/Ls get to within 4 to 6 degrees of the FMC N1 limit, the A/T computer then uses FMC N1 targets to make final T/L adjustments to the FMC N1 limit.
Flight Management Computer (FMC) The FMCs send this data to the A/T computer: N1 targets Gross weight Minimum speed FMC altitude Static air temperature FMC modes GMT/Date.
BITE test information. The autothrottle computer converts the target N1 values from the FMC to an equivalent TRA target. The target N1 rating is dependent on the FMC engaged mode. Gross weight is used in the go around control logic and approach control logic. Minimum airspeed is the lowest airspeed that is acceptable during VNAV operation. FMC altitude from the FMC is used for anticipation of altitude acquire during VNAV operation. SAT is used to calculate a backup TRA limit value. The FMC mode discretes are used to determine control gains and limits.
Electronic Engine Control (EEC) Each EEC channel sends this data to the A/T computer: Thrust resolver angle (TRA) N1 command indicated TRA for max forward idle Estimated corrected thrust TRA for actual N1 TRA for N1 target TRA for N1 max TRA for 5 degree/sec response. Thrust resolver angle is used by the autothrottle computer to calculate an N1 command. N1 command indicated is used to set a throttle position using the error between the target N1 (from the FMC) and the commanded N1 from the EEC.
TRA for maximum forward flat is the throttle angle below which the engine is in the idle range. Estimated corrected thrust is used in the reduced go around control logic. TRA for actual N1 and TRA for N1 target are used in the N1 mode control logic. TRA for N1 maximum is used as a reversion limit in the event that the airplane is dispatched without an operative FMC and as a protection from excessive throttle angles. TRA for 5 degree/sec response is used in the retard control logic.
So with autothrottle engaged N1 should be very closely matched. However the beat frequency heard will be a function of the tolerance allowed by the control system (there will necessarily be hysteresis in the system I assume or the RPM will be in a constant chase mode).

This may be more significant with a larger engine (like the 777 donk) I'm guessing without thinking too hard.

- GY :cool:

misd-agin
30th Mar 2011, 18:35
727 - pilot sync (good luck with three engines, some days it was a bear)
S80 - pilot sync unless N1 SYNC switch engaged
A300 - pilot sync
757/767 - pilot sync
737-800 - electrons. Automatic, does an excellent job.
777 - ? Electrons or pilot?

I've always enjoyed the heavy vibration cycle that comes with power reduction for descent. If the pilot allows the N1's to get out of sync it's very noticeable in the mid 60's% N1.

Shaggy Sheep Driver
30th Mar 2011, 22:08
Older types without digital engine control need manual synchronisation. One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1.

Nimrod MR2 has this. Did the Comet 4 on which that aeroplane is based have it? The Comet has Avons while the Nimrod has Speys (which are said to be turbofans, but to me they look like hi-bypass turbojets).

NSEU
1st Apr 2011, 07:04
How do you sync the N1s on engines which use EPR as the primary reference?

The EPR's may be equalised on FADEC aircraft with EPR as the primary parameter, but this does not mean the N1's are running at the same speed.

Rgds
NSEU

Capt Claret
1st Apr 2011, 07:13
On the Douglas/Boeing 717 the EEC synchronises the Left EPR to the right during takeoff, and during climb, cruise & descent, the N1s.

Intruder
1st Apr 2011, 07:54
On the 744 the Autothrottles do the synch function. If the autothrottles are off (unusual, except on approach), there is no synch.

On the 747 Classic, the pilot or FE does it manually by ear/feel.

FullWings
1st Apr 2011, 09:27
Flying both GE & RR powered 777s, I've noticed that the GE engines appear to match N1 (rpm) but the RR does it on EPR which often leads to "phasing" vibration as the RR N1s don't correspond to an exact EPR value for individual engines, as NSEU points out...

Chris Scott
1st Apr 2011, 11:04
I also think that NSEU has hit the nail on the head. The primary requirement must be to match the thrust, not the RPM.

However, in the case of what is sometimes loosely referred to as an "N1 engine", such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s). The A/THR will choose the N1 it thinks appropriate, and order the FADEC of each engine to deliver it. So the N1s should be fairly well synchronised.

In the case of "EPR engines", such as RR, IAE and (previously) P&W, the A/THR must order all the FADECs to give the same EPR. If one of the engines is more worn than its neighbour(s), it will need a higher N1 (fan) RPM to achieve the ordered EPR.

Perhaps poorjohn will confirm that he is talking about RR engines?


Quote from Dan Winterland:
One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1.

We had those on the HS114 (De Havilland Heron), to synchronise the four Gypsy Queen engines (each 250HP). Some years later, I was amused to find the identical instrument on the VC10, for the N2 spool of its 21000lb-thrust Conways. Being merely the copilot, the grizzled FEs didn't like me fiddling with the throttles in the climb and cruise (they had their own set), and were not impressed when I told them why I knew what I was doing.

Max Angle
1st Apr 2011, 11:07
How do you sync the N1s on engines which use EPR as the primary reference?

The answer is you don't. Its quite common on the V2500 Airbus to see both engines showing the same EPR but different N1s. Its not really a problem unless one engine is much older than the other meaning it has to go round faster (and burn more fuel, higher EGT) to produce the same EPR as the other one. This is what produces the out of sync sound. The only solution, if you can be bothered, is to disengage the autothrust, sync the engines and just except the very slight power difference

GarageYears
1st Apr 2011, 13:48
Excellent thread gents! :D

For some reason EPR synch had not occurred to me. That would indeed explain the difference.

I'm not sure I understand exactly how "the fan of one of the engines is more worn than the other(s)", thinking that it would be more likely that the combustion/power turbine stage would be the point where "wear" would introduce power loss, and hence imbalance the RPMs as a function of equalizing EPR.

- GY :ok:

Chris Scott
1st Apr 2011, 15:35
Quote from GarageYears; my inference added in square brackets:
"...it would be more likely that the combustion/power turbine stage [rather than the fan] would be the point where "wear" would introduce power loss, and hence imbalance the RPMs as a function of equalizing EPR."

If you are referring to the core of the engine, then I think you are right to point out that the higher pressure used for the engine pressure ratio (EPR) is sensed after the last LP turbine (I think!); not in the bypass duct or C-duct. Perhaps someone can confirm that?

In the line of my post you quoted, however, I was not talking about trying to equalise EPRs; I was talking about engines that use N1 as the primary indication of thrust:
"...in the case of what is sometimes loosely referred to as an 'N1 engine', such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s)."

In the case of engines that use EPR as the prime indicator, I argued that a worn engine would need a higher N1 than its brand-new neighbour to achieve the same EPR. On reflection, I think that both (or all three) shafts would be faster, as they interact with each other.

But the "low frequency vibration" and/or "beat frequency" that poorjohn notices is presumably from the N1 shafts.

poorjohn
1st Apr 2011, 19:42
Scott, afaik AA uses Trent 800s.

The question behind my question was whether during the certification process engineers assumed a higher degree of syncronization and thus less vibration than the industry is achieving in the present economic climate.

I have no expertise but can imagine valid reasons for engine life being extended, thus setting up for bigger differences between a 'new' engine and one nearing end of (a longer) service period.

It's just as likely that nothing has changed and I suddenly noticed it. Probably no big deal if the airframe doesn't mind.

pj

lomapaseo
1st Apr 2011, 20:56
The so called beat frequency is sensed more by the human ear than the airframe.

This whole phenomena is about perceptions of the human ear and while likely associated with something spinning in the innards of the airframe it is very unlikely to be associated with causing any kind of damage to a critical aircraft structure.

I tend to think of the explanation of two disimilar drum heads sitting in the middle of the aisle of a plane. One begins to hum while the other doesn't and then when it stops then its neighbor starts and so on until they both become quiet. Bothersome to the ear but nothing else to be concerned about.

Landroger
1st Apr 2011, 23:16
The so called beat frequency is sensed more by the human ear than the airframe.

This whole phenomena is about perceptions of the human ear and while likely associated with something spinning in the innards of the airframe it is very unlikely to be associated with causing any kind of damage to a critical aircraft structure.


Loth that I am to wander - as a non aero engineer - into the halls of the knowledgeable, to say nothing of the experienced, but I'm not sure you can go quite that far sir.

It is true that the human ear is incredibly sensitive to sound and in particular to changes in sound. Even with my somewhat degraded and abused hearing, I can and have heard a change of less than a half Herz in fifty and, I suspect, could probably hear a change of a few tens of rpm in a couple of thousand.

I think where I have a problem is with the sheer mass of either rotating parts or, more probably, the air being moved by these huge donks. A friend of mine supplies quality sound equipment for professional 'gigs' and I watched him 'running in' a couple of speakers each about the size of a Mini. He had placed them 'face to face', up tight, in his living room! He was feeding an out of phase 'beat' frequency into them, at pretty much the full output of his amplifiers which, I can assure you, can move your internal organs.:uhoh: However, virtually no sound could be heard. Move one of them, at an angle, an inch away from the other and the furniture started to move. :eek:

If a beat can be heard in an aeroplane, it means that the vibrations created by one engine are not being cancelled by the other and that energy may be being absorbed by the airframe. Could this not produce stresses in the airframe, particularly if the perceived beat frequency hits a resonance?

As an engineer, it just seems rather ........ improper in some way, not to synchronize engines if it is possible. Untidy if you will?

Roger.

lomapaseo
2nd Apr 2011, 00:55
Landroger

I think where I have a problem is with the sheer mass of either rotating parts or, more probably, the air being moved by these huge donks. A friend of mine supplies quality sound equipment for professional 'gigs' and I watched him 'running in' a couple of speakers each about the size of a Mini. He had placed them 'face to face', up tight, in his living room! He was feeding an out of phase 'beat' frequency into them, at pretty much the full output of his amplifiers which, I can assure you, can move your internal organs.:uhoh: However, virtually no sound could be heard. Move one of them, at an angle, an inch away from the other and the furniture started to move. :eek:

If a beat can be heard in an aeroplane, it means that the vibrations created by one engine are not being cancelled by the other and that energy may be being absorbed by the airframe. Could this not produce stresses in the airframe, particularly if the perceived beat frequency hits a resonance?


Of course you are correct, for even I too have run such home experiments and broken the crockery.

But aircraft structures are not not that sensitive to what we percieve in our ear. They are incredibly well dampened to vibrations by virtue of their many joints and bonds let alone their myriad of frequency responses that cancel each other much like the clothing that we wear.

It's the single structure (without joints or bonds) that amplifies this noise to our ears in an airframe. Such things as interior wall surfaces (nomex?) and in some cases the skin between ribs (drumhead effects)

Fortunately we don't have aircraft falling apart due this effect in the data base (not withstanding the unimportant stuff in the cabins :) )

poorjohn
2nd Apr 2011, 17:03
In these cases the vibration was felt by body parts rather than heard. One time I commented to an FA (from my comfy semi-supine position) who (standing) agreed that she also 'felt' a vibration unusual enough that she communicated it to the flight deck.

On that particular flight the a/c had emitted a god-awful screeching sound for perhaps 30 seconds during climb-out so one could imagine a loose panel involved.

Again, no expertise claimed....

pj

Landroger
2nd Apr 2011, 17:10
Ah, I see what you mean. There isn't a single structure that could resonate as such, just many, many connected ones which tend to damp each other out? That makes sense.

But do you see my point about leaving engines not synchronised? :) It just seems the right thing to do, if it is possible. :ok: I don't know if it has been mentioned, but during WWII, RAF crews always synchronised their engines - by ear I imagine. The Germans, on the other hand, deliberately left their engines to 'throb' in a effort to terrify the civilians they were about to bomb.

From what my Nana and Grandad used to tell me, all it actually meant was the average civilian, who knew nothing of beat frequencies and synchronisation, knew for sure when it was 'Gerry' and when it was 'one of ours' overhead. :D

Roger.

NSEU
3rd Apr 2011, 00:07
Also something to consider...

During idle descent, even on aircraft with N1 as the primary reference, the primary reference at idle becomes the N2 (N3 on RB211) or minimum compressor outlet pressure in some situations (on some engine types). The N1's can be highly mismatched in this situation.

I'll let the experts decide if the reverberation is less at lower rpms ;)

Rgds
NSEU

ZQA297/30
4th Apr 2011, 13:37
There are other things that can cause "beats". Air cycle machines, hydraulic pumps, and inverters all have characteristic whines, hums, and whistles. Even the outflow valves can produce rumbles and hums.
Some of the items are way off the basic engine frequency, but their harmonics can cause odd effects. There are so many permutations possible, many of them transient, that you can be quite entertained trying to pin them down.
Good luck

CJ Driver
4th Apr 2011, 22:36
For interest, on our two shaft engines we have an automatic synchroniser, and we actually have a choice of synchronising N1 or synchronising N2, depending on the phase of flight, and perhaps, depending on which you prefer the sound of.

To be honest, we invariably use N1...

lomapaseo
4th Apr 2011, 22:59
To be honest, we invariably use N1...

most of the accesories drive off N2 so if the sound is still present you might have a hint

Landroger
4th Apr 2011, 23:32
ZQA297/30's post reminds me of my first question on PPRuNe, to which I received plenty of interesting suggestions, but never a definitive answer.

There are other things that can cause "beats". Air cycle machines, hydraulic pumps, and inverters all have characteristic whines, hums, and whistles. Even the outflow valves can produce rumbles and hums.
Some of the items are way off the basic engine frequency, but their harmonics can cause odd effects. There are so many permutations possible, many of them transient, that you can be quite entertained trying to pin them down.
Good luck

I live in SW London, pretty much under the flight path to acquire the GS for 27L and have been watching and listening to aeroplanes since I was a kid. Concorde was delightful treat every evening. Mostly, of course, it is a succession of B7x7 and A3xx. What has always puzzled me - my 767 driver nephew and my 73NG driver friend as well - is a brief noise, occasionally heard, from some aircraft when overhead my house.

It seems to occur when throttles are pulled back, or perhaps on some change in configuration and takes the form of a single, quite loud groan, very like the noise a wooden door might make scraping across the floor. It only happens once, I cannot narrow it down to a single type or even a single manufacturer and it doesn't always happen. In fact I haven't heard it recently, but then I haven't spent a lot of time outside, so that might be why. :rolleyes:

Being a Scout Leader, I have also heard this noise when camping at Broadstone Warren on the A22 south of Forest Row in Sussex - approach to LGW at a guess. Any ideas anyone?

Roger.

misd-agin
5th Apr 2011, 01:29
It takes a minor throttle nudge to quiet the N1 rumble.

Minor - very small, little.

Go in peace(and quiet) my child. :D

STBYRUD
5th Apr 2011, 06:36
Each large thrust change can mean a brief phase of considerable noise, I am suspecting that the two engines spool down or up always slightly differently, additionally in my 737NG I know that a power-on descent (when its necessary) is best done outside of 55-65 N1 - seems to be the high vibration regime...

Chris Scott
5th Apr 2011, 09:58
Quote from Landroger:
What has always puzzled me - my 767 driver nephew and my 73NG driver friend as well - is a brief noise, occasionally heard, from some aircraft when overhead my house... I have also heard this noise when camping at Broadstone Warren on the A22 south of Forest Row in Sussex - approach to LGW at a guess. Any ideas anyone?

Hi, I'm afraid that, as you would expect, I'm not going to provide "a definitive answer" to this one! I've wondered about these noises for about 45 years, typically from One-Elevens flying over Ashdown Forest (as you say) on base-leg to LGW Rwy 27 (sorry, 26). In that case, it was a fairly high-pitched scream as the flaps ran out. Although I flew the One-Eleven for 7 years, I was never sure if it was the flap motors (hydraulic), the transmission shafts, or simply aerodynamic noise.

The brief grunts that are more typical of current types on the approach are probably also connected with flaps and/or slats selections; possibly the release of the associated brake mechanisms.

The characteristic whine of an A320-series (whether CFM or IAE-powered) when thrust is low or at idle, may well be from the two air-conditioning packs. It's similar to the loud noise you hear when they are running on the ramp with the engines shut down (using bleed air from the APU, of course). By the way, the series of loud grunts that follows the shutdown of the first engine of an A320 on arrival is from the hydraulic PTU (power transfer unit - the Americans neatly call them reversible motor-pumps), which is taking over the hydraulic system of the engine that has been shut down. When the second engine stops, the PTU loses its own hydraulic pressure and stops.

Now there's some thread-drift!

GarageYears
5th Apr 2011, 13:46
It seems to occur when throttles are pulled back, or perhaps on some change in configuration and takes the form of a single, quite loud groanI live on the approach to IAD and have heard that same noise many times and wondered what it is/was?

To be honest the sound from the ground does sound like an extended "groan" with the pitch decreasing through the duration of the sound.

Clues:

- Always occurs on approach
- Only heard this with twin engine aircraft (wing mounted, A320/737...?)
- Not heard with smaller RJs (at least I don't believe so)

My house is 8.75 miles to 19C, so that distance may give a clue

- GY

sb_sfo
5th Apr 2011, 15:34
Think you are hearing a bleed shift, when an engine switches between high-stage and low-stage bleed regimes.

CecilRooseveltHooks
5th Apr 2011, 18:01
Can it be Avro RJ flaps moving from or to zero, but that seems more likely for your case if we're talking LCY.

Live myself under approach to BMA with Malmö Aviation flying Avros in all day long with steady groaning as the flaps are usually dropped right above our front yard. Very loud when passenger as well.

CRH

Landroger
5th Apr 2011, 22:29
Think you are hearing a bleed shift, when an engine switches between high-stage and low-stage bleed regimes.

Now I know that there are various 'bleeds' from different points along the compressor stages, for air conditioning and even starting 'the other' engine plus, I imagine, other services, but your answer both confuses and intrigues me. :ok:

I am fairly certain the noise cannot be control surfaces being moved - their cycle time is too long for one thing - but changes in relative pressures being vented to atmosphere sounds promising - so to speak. :uhoh:

Would you elaborate for this non aero engineer please?:)

Roger.

brak
5th Apr 2011, 22:46
I live on the approach to IAD and have heard that same noise many times and wondered what it is/was?

To be honest the sound from the ground does sound like an extended "groan" with the pitch decreasing through the duration of the sound.


Not sure about the sound, but pitch change is almost certainly a result of Doppler effect. That is to say - the "real" sound is probably monotonic.