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Faro
Topic Author
Posts: 2020
Joined: Sun Aug 12, 2007 1:08 am

### Fuselage Flex

Just wondering how much flex is built into a modern fuselage (for example, in cm of deviation from nominal centerline). On the one hand, it would have to take the gravitational sagging of the tail end while on the ground and on the other the up-flex of the nose and tail ends at rotation and -to a lesser extent- in the cruise. For aircraft like the A346, this must be quite significant.

At what range of flex frequency would one get resonance (in clear air turbulence?) and does this get modelled during development or is it too remote a risk? Is fatigue due to such flex a long-term maintenance issue or not?

Faro

[Edited 2010-04-06 01:31:51]
The chalice not my son

oly720man
Posts: 5813
Joined: Fri May 21, 2004 7:13 am

### RE: Fuselage Flex

According to this

http://www.pprune.org/archive/index.php/t-329650.html

lateral bending seems to be more of a problem than vertical bending, for passenger comfort at least.

In the above link, it seems there was an interaction between wing bending and fuselage bending that was found in flight testing rather than through analysis, but was related to flying through turbulence.
wheat and dairy can screw up your brain

tdscanuck
Posts: 8573
Joined: Wed Jan 11, 2006 7:25 am

### RE: Fuselage Flex

 Quoting faro (Thread starter):Just wondering how much flex is built into a modern fuselage (for example, in cm of deviation from nominal centerline).

It depends, a lot, on dimensions (length and diameter) and material.

 Quoting faro (Thread starter):On the one hand, it would have to take the gravitational sagging of the tail end while on the ground and on the other the up-flex of the nose and tail ends at rotation and -to a lesser extent- in the cruise.

Why would the nose and tail "up-flex" at rotation? The fuselage is still being pulled down by gravity, held up by the wings. Other than the nose gear load (which is pretty small), the fuselage loading in flight is slight more "saggy" because of the download on the tail.

 Quoting faro (Thread starter):At what range of flex frequency would one get resonance (in clear air turbulence?)

A couple of Hz is typical, but it should be damped.

 Quoting faro (Thread starter):and does this get modelled during development or is it too remote a risk?

It's modelled. Your yaw damper won't work properly if you don't include the fuselage flexible mode (unless the mode is really small, like on little stubby fuselages).

 Quoting faro (Thread starter):Is fatigue due to such flex a long-term maintenance issue or not?

If you're just talking about the oscillation mode..compared to the stress of normal flight, the stress from fuselage wiggle is relatively small. If you're talking about the entire stress picture, then yes, it's extremely significant. Fuselage bending stress is *the* major reason that crown skins are so much more prone to fatigue cracking than side or belly skins.

Tom.

Faro
Topic Author
Posts: 2020
Joined: Sun Aug 12, 2007 1:08 am

### RE: Fuselage Flex

 Quoting tdscanuck (Reply 2):Why would the nose and tail "up-flex" at rotation? The fuselage is still being pulled down by gravity, held up by the wings. Other than the nose gear load (which is pretty small), the fuselage loading in flight is slight more "saggy" because of the download on the tail.

Pulled down by gravity and held up by the wings resulting in no wing flex once in stable climb, yes. But at rotation, the incident airstream has (with respect to the fuselage) a vertical component to it imparting an upward acceleration. Until the AoA is reduced and this acceleration subsides once in stable climb, the force of the upward acceleration should flex the fuselage up, or am I missing something here?

I see this in the same way as wing flex on rotation really: both the wings and the fuselage tips flex upwards on rotation with respect to an aircraft's center of gravity because both receive an upward acceleration due to AoA and both are less rigid than the structure surrounding that CG point.

Faro

[Edited 2010-04-07 01:05:50]
The chalice not my son

Starlionblue
Posts: 20557
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Fuselage Flex

 Quoting oly720man (Reply 1):lateral bending seems to be more of a problem than vertical bending, for passenger comfort at least.

Makes sense. Bendy roads tend to make people car sick, while driving over bumps is less nausea inducing. Also it is harder to hold your book steady when the plane is moving side to side than up and down.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

cpd
Posts: 6702
Joined: Sat Jun 28, 2008 4:46 am

### RE: Fuselage Flex

How about Concorde and fuselage flex! That's surely a good example of it.

 Quoting Starlionblue (Reply 4):Also it is harder to hold your book steady when the plane is moving side to side than up and down.

Like being on a speeding express train on a twisting section of track trying to type on a laptop computer - it doesn't work well, same with trying to write. Very difficult and uncomfortable.

That's my everyday commute to work pretty well described.

474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

### RE: Fuselage Flex

 Quoting cpd (Reply 5):Like being on a speeding express train on a twisting section of track trying to type on a laptop computer - it doesn't work well, same with trying to write. Very difficult and uncomfortable

Thats because the train shifts outboard on the track as it rounds a curve. If it didn't shift it could not turn. The outboard wheel has to travel a greater distance than the inboard wheel. So the wheels are beveled to make there diameter larger or smaller compensating for the distance traveled.

tdscanuck
Posts: 8573
Joined: Wed Jan 11, 2006 7:25 am

### RE: Fuselage Flex

 Quoting faro (Reply 3):But at rotation, the incident airstream has (with respect to the fuselage) a vertical component to it imparting an upward acceleration.

It imparts an upward force. There's no upward acceleration (except during the very brief period when the pitch-rate is changing) because the upward force from the wind is countered by the downward force of gravity.

 Quoting faro (Reply 3):ntil the AoA is reduced and this acceleration subsides once in stable climb, the force of the upward acceleration should flex the fuselage up, or am I missing something here?

You're missing that the fuselage doesn't generate anywhere close to enough lift to hold itself up (that's why you need a wing). So, even though you do get a small upwards force on the fuselage due to the AoA at rotation, it's not enough to overcome the fuselage's weight so the net load is still downward. Also, the downward force on the tail to rotate is larger than the upward force on the fuselage, so the portion aft of the wing has a higher down load than when just sitting static. And the upward force on the front of the fuselage is partly countered by the loss of the upward force from the nose gear.

Tom.

seattle
Posts: 55
Joined: Sat Oct 10, 2009 3:26 pm

### RE: Fuselage Flex

 Quoting 474218 (Reply 6):So the wheels are beveled to make there diameter larger or smaller compensating for the distance traveled.

Geeze... I work for the railroad and i didn't even know that. Learn new things about everything on here.

DocLightning
Posts: 22102
Joined: Wed Nov 16, 2005 8:51 am

### RE: Fuselage Flex

I could swear that when I see the 77W's on approach into SFO, it seems as if the fuselage is very slightly bent. I'm not shocked; no structure that long could be perfectly rigid and yet maintain a reasonable weight.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

Viscount724
Posts: 19316
Joined: Thu Oct 12, 2006 7:32 pm

### RE: Fuselage Flex

 Quoting DocLightning (Reply 9):I could swear that when I see the 77W's on approach into SFO, it seems as if the fuselage is very slightly bent.

Optical illusion.

aer lingus
Posts: 367
Joined: Sun Mar 25, 2001 6:40 pm

### RE: Fuselage Flex

 Quoting DocLightning (Reply 9):I could swear that when I see the 77W's on approach into SFO, it seems as if the fuselage is very slightly bent.

I seem to notice this too. Does it look like that the fuselage behind the wing is bent downwards?
As Viscount724 says, it could be an optical illusion. There could also be a possibility that the aircraft fuselage section on behind the wing is lower than the section in front of the wing like the 757. There was a thread about this not too long ago about the 757 fuselage.

Split Scimitar or Sharklets?

FlyingColours
Posts: 2355
Joined: Fri Dec 26, 2003 3:13 am

### RE: Fuselage Flex

 Quoting Aer Lingus (Reply 11):There could also be a possibility that the aircraft fuselage section on behind the wing is lower than the section in front of the wing like the 757.

Speaking of the 757 has anyone seen the fuselage twisting and flexing during turbulence?
Its a scary sight for the passengers and does really get the attention of new crew. Its very noticeable if you are in the rear several rows looking forward.

On the 757 I've had a few very hard landings sat at the back and it's felt like we were bending to take the strain. That reminds me of the video of the MD80 doing landing stress tests or something and one flexes excellently and the other snaps in two...

Phil
FlyingColours
Lifes a train racing towards you, now you can either run away or grab a chair & a beer and watch it come - Phil

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