Lomapaseo
 

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.

Landroger

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 :) )

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

Lompaseo
 

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.

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

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

You have a choice
 

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...

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

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.

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.

It takes a minor throttle nudge to quiet the N1 rumble.

Minor - very small, little.

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

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...

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!

Landroger: Wonder about that too!
 

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.

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

Landroger/GarageYears
 

Think you are hearing a bleed shift, when an engine switches between high-stage and low-stage bleed regimes.

groan
 

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

sb sfo
 

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.

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.