sqa380fan
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753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 5:30 am

Hi,
This is my first time posting in the forums but I've been meaning to ask about this particular picture ever since I saw it a year or so ago:


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Photo © Daan Krans - AirTeamImages



One can clearly see that the window line along the left side is severely warped, and droops toward the front and rear. I'm assuming that this is due in part to the length of the 753's fuselage, but it still seems pretty crazy to me... so my question is, how often does this happen? And why? Could this possibly be some photographic illusion? It seems to me that the plane is pretty clearly 'bent'...

Thanks!
 
amccann
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 5:53 am

Quoting sqa380fan (Thread starter):
I'm assuming that this is due in part to the length of the 753's fuselage, but it still seems pretty crazy to me... so my question is, how often does this happen? And why?

I would have to say you are correct, the fuselage is indeed bent. You can imagine the fuselage of an aircraft as beam, supported only in the middle. The horizontal tail produces a down force on the rear of the fuselage, and the weight of the fuselage itself also produces a down force on both the front and rear of the fuselage. Again, using the beam analogy, a fuselage can be imagined as a beam supported in the middle with loads applied on the ends.

Additionally because the cross sectional area of the 757-300 is relatively low, when compared to other aircraft of long fuselage, the bending stiffness is lower and therefore the bending deflection is higher. The DC-8-60 series experienced a similar bending phenomenon.
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tb727
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:11 am

If it didn't bend it would break. You'd be surprised at how much stuff moves!
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sqa380fan
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:26 am

Thanks for the replies! I do realize that wings are designed this way for obvious reasons, guess it just had never occurred to me that the same could be true for the fuselage. I suppose the amount exhibited in the picture just really caught my attention... had no idea it could bend *that* much.
 
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:27 am

When sitting in the back of the 753 and looking forward you can see the fuselage wiggle in turbulence. It's quite the sight!
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HAWK21M
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:32 am

Aluminium alloy stretches & bends.......
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tb727
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:33 am

The other day I was in the back during flight(a freighter) and we were going through a little light chop so I tried to look down the side to see if the fuselage was flexing or twisting any but I don't think it was rough enough. Of course if it gets too rough, I don't think I'll be in the back to see it, I'd be buckled in up front.
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Starlionblue
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 6:48 am

Not only does it bend but it also twists. This is quite noticeable in a 757. If you sit in the back during turbulence you can really see it.
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Max Q
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 9:01 am

Quoting tb727 (Reply 6):


The other day I was in the back during flight(a freighter) and we were going through a little light chop so I tried to look down the side to see if the fuselage was flexing or twisting any but I don't think it was rough enough. Of course if it gets too rough, I don't think I'll be in the back to see it, I'd be buckled in up front.

I don't think you will see that in the B727, the fuselage is built like a tank, the wings take all the load and they flex significantly, that's why she has such a smooth ride.


The 753 fuselage is very long and limber, it also flexes significantly, that is why it has a much better ride than a -200.


It is also a much nicer flying Aircraft, although I seem to be in the minority on that opinion !
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747classic
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 12:08 pm

I experienced the same fuselage bending and twisting on a extremely bumpy crew positioning flight with a KLM DC8-63 (the flying pencil) during the seventies.
Sadly I never operated that aircraft type myself.

[Edited 2012-02-06 04:08:41]
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rfields5421
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 2:19 pm

Quoting amccann (Reply 1):
The DC-8-60 series experienced a similar bending phenomenon.

Back in the summer of 1972 - on a long MAC charter flight from Travis AFB to Southeast Asia - I was amazed to see how much the DC "Stretch" 8 flexed. I was seated on the next to last row of the one-class aircraft in the aisle seat. At times the aisle looked like it belonged in a carnival fun-house.
 
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 8:01 pm

Quoting tb727 (Reply 2):
If it didn't bend it would break. You'd be surprised at how much stuff moves!

I'm no aircraft engineer, but that is a definitive answer from a structural point of view...up to a point.

Not trying to resist flex or tortional force can make a structure more durable. It's a basic principle in skyscrapers. But allowing harmonic rhythm can make things worse - which is what happened at the Tacoma Narrows.

So the fact that an airliner bends, flexes, swells and contracts in flight - and that this is planned for - is no surpise. It's *really* obvious if you sit at the back of a descending 747.
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vikkyvik
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 8:28 pm

Quoting sqa380fan (Thread starter):
Could this possibly be some photographic illusion? It seems to me that the plane is pretty clearly 'bent'...

Just to add a bit - although the effect is very real, the angle of the photograph makes it appear more significant that it probably is.

It looks like the max deflection is in the vicinity of 1 window-height. I'll assume a window is 18" tall, though I actually have no idea. I'll also assume that the first and last windows are about 140' apart (so I'll use 70' for half of the windows). That gives me a deflection angle of about 1.2 degrees. Noticeable, but small.
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seabosdca
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RE: 753 Fuselage Warp/Torsion?

Mon Feb 06, 2012 11:42 pm

Quoting vikkyvik (Reply 12):
Just to add a bit - although the effect is very real, the angle of the photograph makes it appear more significant that it probably is.

Very true. The OP's photo really doesn't give any hint of the massive length of the 753. Try to juxtapose it with this one in your mind's eye...


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Max Q
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RE: 753 Fuselage Warp/Torsion?

Tue Feb 07, 2012 5:33 am

The Md80 bends like Gumby as well !
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