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Faro
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Airliners' Resonant Frequency

Thu Sep 13, 2007 7:45 pm

i) What typically is the natural resonant frequency (RF) of an airliner, ie the frequency at which the airframe oscillates at maximum amplitude?;
ii) Presumably, this would mainly be the fuselage RF or is there an overall RF taking into account the wings and empennage?;
iii) Can RF resonance be triggered by inflight conditions, turbulence, gusts, CAT, etc?;
iv) Is RF resonance potentially harmful/destructive?; and
v) same question as i), iii) & iv) above but applied to wings/stabilisers/fin individually.

Faro
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roseflyer
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RE: Airliners' Resonant Frequency

Thu Sep 13, 2007 9:40 pm

Well there is not a specific frequency for the whole aircraft. Specific components certainly have them. The wings have a resonant frequency. This case was well documented with the L-188 inflight failures due to the engine vibrating at this frequency. Resonance can be destructive. All they did in the case of the Lockheed L-188 is add an extra piece of metal to the engine which changed the frequency and solved the problem of wings breaking off in flight.

It's pretty much impossible to know what the frequency is for an airliner. It's not a single degree of freedom so the square root of K/M doesn't work. Certain components do have problems with resonance though. It can cause wear on specific components.

The most noticeable form of natural frequency observed in aviation is on propellers. This is how noise cancelling systems on ATRs and Q400s work. They cancel out that frequency.
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Starlionblue
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RE: Airliners' Resonant Frequency

Thu Sep 13, 2007 10:07 pm

AFAIK the 345 and 346 automatically dampen pitch oscillations partly to avoid resonating. This could cause instability. The other reason is simply to alleviate bending stresses on the fuse.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: Airliners' Resonant Frequency

Fri Sep 14, 2007 1:21 am

Quoting Faro (Thread starter):
i) What typically is the natural resonant frequency (RF) of an airliner, ie the frequency at which the airframe oscillates at maximum amplitude?;

Every component has its own RF. As a result, the composite aircraft has an enormous number of potential resonances.

Quoting Faro (Thread starter):
ii) Presumably, this would mainly be the fuselage RF or is there an overall RF taking into account the wings and empennage?;

The will be a different set of RF's for each part (fuselage, wings, horizontal stab, vertical stab, etc.) and for each resonance mode.

Quoting Faro (Thread starter):
iii) Can RF resonance be triggered by inflight conditions, turbulence, gusts, CAT, etc?;

Yes, although you try to avoid that through good design.

Quoting Faro (Thread starter):
iv) Is RF resonance potentially harmful/destructive?; and

Yes, it is one of the few things that can destroy an aircraft in flight with little/no warning.

Quoting Faro (Thread starter):
v) same question as i), iii) & iv) above but applied to wings/stabilisers/fin individually.

All applies, just with different RF's.

Quoting Starlionblue (Reply 2):
AFAIK the 345 and 346 automatically dampen pitch oscillations partly to avoid resonating. This could cause instability. The other reason is simply to alleviate bending stresses on the fuse.

That's a little bit different situation. In the case you're discussing, the whole airframe is going through a phugoid oscillation, which is pretty normal, but it's not a structural resonance in the sense that I think the OP meant it.

Tom.
 
BAE146QT
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RE: Airliners' Resonant Frequency

Fri Sep 14, 2007 1:45 am

You just know that somewhere, the US government has a list of resonant frequencies of various aircraft hulls or critical components, just waiting for the day when they have a sonic generator powerful enough to take an aircraft out at a distance by vibrating it to pieces.





Oh - hang on - there's someone at the door.

Be right back...
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roseflyer
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RE: Airliners' Resonant Frequency

Fri Sep 14, 2007 2:50 am

Quoting Faro (Thread starter):
iii) Can RF resonance be triggered by inflight conditions, turbulence, gusts, CAT, etc?;

Resonance is more of a phenomenon observed at steady state, so a gust or turbulence will not cause it. It's possible I guess, but very unlikely.

One more factor is that with damping, resonance can be eliminated. There aren't models out there for a whole plane that I've heard of, but if the damping coefficient is greater than about .9 in any single degree of freedom, then resonance will not be observed at all. So many components will never experience it since there is something "absorbing" the energy.
If you have never designed an airplane part before, let the real designers do the work!
 
WingedMigrator
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RE: Airliners' Resonant Frequency

Fri Sep 14, 2007 1:44 pm

Quoting Tdscanuck (Reply 3):
In the case you're discussing, the whole airframe is going through a phugoid oscillation

I don't think so-- for the A346 it is indeed the first bending mode of the fuselage (think banana), which is at a much higher frequency than the phugoid mode.

Quoting Tdscanuck (Reply 3):
As a result, the composite aircraft has an enormous number of potential resonances.

Additionally, the aircraft has a somewhat different frequency response when standing still versus when it is in flight, because aero effects modify the structural response.
 
FredT
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RE: Airliners' Resonant Frequency

Sat Sep 15, 2007 5:59 am

Quoting RoseFlyer (Reply 1):
It's pretty much impossible to know what the frequency is for an airliner. It's not a single degree of freedom so the square root of K/M doesn't work. Certain components do have problems with resonance though. It can cause wear on specific components.

When the resonance is fed by aero loads it is known as flutter. It can cause significant components, such as wings and control surfaces, to break off in next to no time.

For this reason, a lot of work is putting into finding out just which frequencies and vibration modes the airframe has. This is done analytically, through calculations and complex computer models, and empirically through mounting an entire aircraft on elastic mounts, adding accelerometers all over the airframe and exciting the aircraft at various frequencies.

While it has never been my primary area of work I've been involved and I've had some fun hours putting the accelerometers and associated wiring in place...  Smile

Quoting RoseFlyer (Reply 5):
There aren't models out there for a whole plane that I've heard of,

Now you have heard of them.  Wink

They're simplified, of course, but detailed enough to accurately predict the major (read: dangerous) resonant modes and frequencies.

It can also be a PITA with components. I've been through having the weight saving from a component replacement being consumed - and then some - by the structures people adding reinforcements to get the replacement components inherent frequency responce out of a known vibration frequency in the aircraft.  Sad
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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