speedracer1407
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How Flexible Is An Aircraft's Fuselage?

Thu Mar 09, 2006 4:36 pm

I've heard often on these forums that a keen eye can notice some fuse flex during turbulence, as long as that eye is seated towards the rear of the plane, thus looking down the majority of the tube. Seems sensible to me, though I've tried many times and failed to detect it. Thus, I wonder just how much things flex. I'm reminded of the automotive industry's recent (last 10 years or so) attention to publicizing dramatic increases in chassis stiffness, thus making it yet another well-known factor of proper driving dynamics. But the figures in this case are something like tens of thousands of newton-meters per inch of tortional deflection; nobody can really detect it, unless they drive a convertible on a rough road and whitness a tiny bit of cowl shake.
Naturally, a multi-hundred-ton airplane is a different story, at least as far as the visible amount of fuse flex. But I'm curious about the figures involved. Do manufacturers measure fuse flex the same way auto manufacturers do? What are the figures? Is there a range of flexibility that's optimal? Or does it vary dramatically from plane to plane based on manufacturer philosophy.

Also, please keep in mind that I'm not talking about wing flex, as I feel somewhat more comfortable with that concept.

O

[Edited 2006-03-09 08:39:10]
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
AmericanB763ER
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RE: How Flexible Is An Aircraft's Fuselage?

Thu Mar 09, 2006 7:28 pm

Fuselage flex is easily noticeable when you're standing at the gate with the aircraft parked in front of you - just look down the window row and you'll see where the fuselage is bending (the portion above the wing box is slightly higher than both ends of the fuselage due to gravity)

The longer the fuselage the more visible it is but it also depends a lot on the aircraft type - out of my experience it's most visible on Boeing 767-300's (also from the inside you'll notice the 'wobbling' while taxiing + during turbulence).



Marco
 
2enginesonly
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RE: How Flexible Is An Aircraft's Fuselage?

Thu Mar 09, 2006 9:23 pm

Fuselage flexing is extremely noticable during a high power testrun when your at the headset and standing in front of the aircraft.
I've seen Fokker 50 tails rotating the opposite direction of the wings ( torsion ) and this at a rate 60 times a minute ( lh wing down and lh stabilizer up and vice versa ).
This severe flexing is also very noticable on the B752 during a high power testrun.
You really have to see this to believe it  Smile

The flexibility of a wing is just the same: if you're standing at the wingtip ( in a cherrypicker or something ) it takes almost no effort to move the wing up and down....even if there's 15 tons of fuel in it ( Boeing 763ER ).

Arjan
 
A300605R
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RE: How Flexible Is An Aircraft's Fuselage?

Thu Mar 09, 2006 9:41 pm

I could clearly see fuse flex once while climbing out of DOH on a QR A300.
As I was sitting over the wings I could see that the overhead bins of the forward section seem to move. I guess the reasons were a steep departure together with turbulences.
But I can't quantify the movement anyway.
Was interesting to see, but was the first and the last time that I saw it on an aircraft as well.
 wave 
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Fly2HMO
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 9:47 am

I was sitting in the very rear of an MD88 a while ago and we passed rather fast over some pretty nasty bumps in the taxiway. I dare say it flexed almost a foot up and down at the center!!!  eyepopping 

I've noticed that its very easy to see this in the longer mad-dogs.
 
SFOMB67
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 1:42 pm

I was always told that if you were seated in the rear of a stretch 8 (-71,73), you can look up the isle during take-off, and see the flex, although I never looked for it. Also, it's evident during jacking on many aircraft. The 747 fuselage will develop wrinkles aft of the MLG at about 10:00 & 2:00 position. The control cables to the tail will also loose about half their tension on a 747 when jacked.
Not as easy as originally perceived
 
Buzz
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 2:09 pm

Hi Speedracer1407, Buzz here. Ever walk up and down the aisle of an empty airliner? When I walk aft on a 757 I feel the airplane bounce up and down a bit... maybe half an inch as 200+ lbs of mechanic (and tools) rhythmically step down the aisle.
When the airplane is pressurized it puts the structure in tension, which would stiffen it a bit.
And on the ground you see skin wrinkles in the 737 and 757's, I hear that when the skin stretches (about 1/16 inch when you rig a Boeing cabin door) the wrinkles disappear. Different set of wrinkles on a B-52, that part of the fusel-sausage isn't pressurized.
g'nite
 
pilotaydin
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 3:03 pm

I just saw a picture of a 737 fuselage that buckled and then fractured because th pushback tug pushed the a/c back with its parking break set....
The only time there is too much fuel onboard, is when you're on fire!
 
777wt
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 5:01 pm

Sit in the back of a A340-600 and look towards the front in flight, that's one lonnnngggg plane  Wink
 
speedracer1407
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 6:22 pm

Thanks for the replies. I guess I must be looking in the wrong places or something. I always figured bouncing around while boarding was due mostly to tire flex and minor landing gear strut compression. I guess I just need to suck it up and get a seat in back next time.

O
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
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Starlionblue
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RE: How Flexible Is An Aircraft's Fuselage?

Fri Mar 10, 2006 10:28 pm

I've seen 757s twist in turbulence, that is the front would roll one way and the rear another. Pretty cool.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Chema
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RE: How Flexible Is An Aircraft's Fuselage?

Sat Mar 11, 2006 12:33 am

In this video you can see the fuselage flexing like crazy. I read somewhere that the tail of the aircraft was replaced and it continued flying as a test plane, so I assume that the rest of the fuselage wasn´t damaged by the massive flexing.

http://www.youtube.com/watch?v=uYAVLfuUjPA&search=landing%20accident
 
lehpron
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RE: How Flexible Is An Aircraft's Fuselage?

Sat Mar 11, 2006 9:16 am

On of the reasons a canard surface was placed on preliminary SST studies from Boeing in the 60'd and NASA/Boeing concepts of the late 1990's because the long fuselage would bend too much. In some of those designs, the pilot sat almost 90 feet from the nose wheel! Even if it were a few inches, a 300-foot plane would sway up and down on landing and takeoff like a bridge in the wind.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
flashmeister
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RE: How Flexible Is An Aircraft's Fuselage?

Sat Mar 11, 2006 5:40 pm

Sit at the back of a 757-300 and fly through turbulence... you'll see the flex for sure.
 
emiratesa345
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RE: How Flexible Is An Aircraft's Fuselage?

Sat Mar 11, 2006 6:06 pm

When i flew on the Concorde I sat at the very back in the aisle seat. The whole flight you could see the cockpit door moving up and down.

Mark
You and I were meant to fly, Air Canada!
 
WickedPenguin
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RE: How Flexible Is An Aircraft's Fuselage?

Mon Mar 27, 2006 1:10 am

On our last trip to Europe, my wife and I were sitting in the last row of a Virgin A340-300.

On approach into Heathrow, we kept hearing these awful groans and creaks. I asked the flight attendant sitting next to us what those were (though I already had a pretty good idea).

When he confirmed that it was fuselage flexing, I made a mental note to avoid A340's if possible. I mean, the thing sounded like it was crying!  Silly

I've flown all my life (both my parents are in the airlines) and never heard anything quite like it on any other aircraft. I can only imagine what those sounds would do to someone who was afraid of flying.
 
2H4
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RE: How Flexible Is An Aircraft's Fuselage?

Mon Mar 27, 2006 1:26 am



A study in wrinkled fuselage skin:


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Photo © Michael Baldock
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Photo © Howard J Curtis




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Photo © Daniel Butcher - UK Airshow Review




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Photo © Roel Kroes
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Photo © DAI - Dutch Aviation Images






2H4


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HAWK21M
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RE: How Flexible Is An Aircraft's Fuselage?

Sun Apr 02, 2006 2:08 am

Quoting Pilotaydin (Reply 7):
I just saw a picture of a 737 fuselage that buckled and then fractured because th pushback tug pushed the a/c back with its parking break set....

What Section.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
ANother
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RE: How Flexible Is An Aircraft's Fuselage?

Mon Apr 03, 2006 6:52 pm

Quoting Sfomb67 (Reply 5):
I was always told that if you were seated in the rear of a stretch 8 (-71,73),

Flew DC8-63s (73s were just re-engined 63s) quite a few times. Through rough weather the flex was VERY obvious.

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