canadiantree
Topic Author
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Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 6:17 pm

Hi all,

I'm a member of a diecast aircraft forum and I posted a thread asking about the 757 fuselage. The thread kind of got out of hand with people agreeing and disagreeing so I figured this would be the place to get my answer:

IS the 757 fuselage the same in front of the wings and behind the wings (past the wing/body fairing).

I'm not super technically inclined in aviation but every photo I've seen on A.net shows that the aft portion of the 757 has a thicker vertical cross section... such as this photo:

http://www.airliners.net/photo/Condo...d=de0d8a77758fd1fad0c9778276edb2e5

People have blamed it on photo distortion, plane angle and so on so forth, here is the link to the thread, I am not sure if it's allowed to share thread links on here, if it is not mods please feel free to remove it:

http://www.diecastaircraftforum.com/...atter-behind-wings.html#post776278

Thanks for any help and clarification any of you might bring  
 
avroarrow
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 7:06 pm

Check out the 7th page of this document from Boeing.
http://www.boeing.com/commercial/airports/acaps/753sec2.pdf
It lists the external width dimension as 12' 4" and the external height as 13' 2" it does not show a separate measurement at the rear which leads me to assume that it is the same for the entire length of the tube and the percieved extra girth behind the wings is an optical illusion.
Dimensions also here:
http://www.boeing.com/commercial/757family/pf/pf_200tech.html
Give me a mile of road and I can take you a mile. Give me a mile of runway and I can show you the world.
 
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Laddie
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 8:17 pm

Quoting avroarrow (Reply 1):
It lists the external width dimension as 12' 4" and the external height as 13' 2" it does not show a separate measurement at the rear which leads me to assume that it is the same for the entire length of the tube and the percieved extra girth behind the wings is an optical illusion.

That is correct. The 757's fuselage is a constant cross-section. The drawing in Canadiantree's post must be in error.
 
canadiantree
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 8:26 pm

Thanks guys for the response. Man that is one heck of an optical illusion though isn't it? I mean to me it was like black and white that the back was thicker.

I even have 9 1:200 scale die-cast models of the 757 and they all seem to be fatter in the back too. Now I realize a model is not like the real thing but dang, I'm baffled that the dimensions are not different.
 
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keesje
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 9:18 pm

I think the 757 inherited the lower aft fuselage from the 727.
"Never mistake motion for action." Ernest Hemingway
 
zanl188
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 10:24 pm

Quoting keesje (Reply 4):
I think the 757 inherited the lower aft fuselage from the 727.

  

The aft pit is deeper than the front, no optical illusion.

See page 16 of this document:

http://www.boeing.com/commercial/airports/acaps/753sec2.pdf
Legal considerations provided by: Dewey, Cheatum, and Howe
 
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Starlionblue
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 11:22 pm

So the width is the same but the height is different?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Viscount724
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RE: Question About The Boeing 757 Fuselage

Thu Feb 25, 2010 11:48 pm

Quoting Starlionblue (Reply 6):
So the width is the same but the height is different?

I think both dimensions are the same, at least until the rear fuselaage begins to taper upwards towards the tail.
 
zanl188
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 12:25 am

Quoting Starlionblue (Reply 6):
So the width is the same but the height is different?

Exactly...

Also per the document in reply 5 the aft cargo door is taller then the forward cargo door.....
Legal considerations provided by: Dewey, Cheatum, and Howe
 
zanl188
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 12:34 am

FWIW: The aft fuselage of the 727 is also deeper than the forward fuselage. See the cargo pit heights on page 13 of this document:

http://www.boeing.com/commercial/airports/acaps/727sec2.pdf
Legal considerations provided by: Dewey, Cheatum, and Howe
 
boeingfixer
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 12:45 am

As with the 727 the aft fuselage on the 757 is 10" deeper aft of the wing.

Cheers,

John
Cheers, John YYC
 
canadiantree
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 9:53 am

OK now that it has been confirmed that the aft pit behind the wings is closer to the ground, can anyone explain why? I mean most jetliners have the same vertical cross section all along the fuselage, why not in this case? Is it an aerodynamics or weight management issue?

Is there a reason why the fuselage section in front of the wings did not benefit from the same deeper fuselage? I mean if it were, I'm assuming the 757 would've been able to carry more cargo which is why I am wondering.
 
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Starlionblue
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 10:51 am

The fact that airliners have the same cross section is mostly to do with manufacturing ease. Aerodymanics would dictate a Whitcombe body, meaning it has to taper at both ends, sort of like the Connie.

I'm going to guess that it has something to do with area ruling.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
OldAeroGuy
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 4:47 pm

Quoting ZANL188 (Reply 5):
Quoting keesje (Reply 4):
I think the 757 inherited the lower aft fuselage from the 727.


The aft pit is deeper than the front, no optical illusion.

Correct.

Quoting Starlionblue (Reply 6):
So the width is the same but the height is different?

Yes

Quoting canadiantree (Reply 11):
OK now that it has been confirmed that the aft pit behind the wings is closer to the ground, can anyone explain why?
Quoting Starlionblue (Reply 12):
I'm going to guess that it has something to do with area ruling.

The 757 entered design using the 727 fuselage as baseline as it was originally a 727 derivative, so it had the 727 dimensions for fore and aft lower lobes. It was later decided to evaluate a deeper forebody lower lobe, ala the 737. The forward fuselage cross section decision was made to meet a manufacturing commitment before all wind tunnel testing was complete. Early indications were that if the deeper lower lobe was used for the forebody, directional stability would have been reduced to an undesireable level. When all testing was finished, it was shown that a deeper forward lobe would have been OK. Unfortunately, it was too late to revise the manufacturing plans.

I was a member of the wind tunnel testing team at the time the 757 was being developed and was aware of the situation. After the 757 was certified, I asked the 757 Chief Engineer what design decisions he would like to change with the benefit of hindsight. The forward lower lobe decision was the first on his list.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
lotsamiles
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RE: Question About The Boeing 757 Fuselage

Fri Feb 26, 2010 7:19 pm

The bottom of the fuselage forward of the wings is at WL 148.50
The bottom of the fuselage aft of the wings is at WL 138.50, 10 inches lower (before the taper).

The fwd cargo floor is at WL 156.80
The aft cargo floor varies from WL 144.00 from STA 1200 to 1300 up to WL 145.4 at STA 1480
 
HaveBlue
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RE: Question About The Boeing 757 Fuselage

Sat Feb 27, 2010 1:38 am

Quoting OldAeroGuy (Reply 13):
I was a member of the wind tunnel testing team at the time the 757 was being developed and was aware of the situation. After the 757 was certified, I asked the 757 Chief Engineer what design decisions he would like to change with the benefit of hindsight. The forward lower lobe decision was the first on his list.

Very interesting OldAeroGuy, thank you for sharing! Another reason I love these forums... input from people that were actually there.  
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HAWK21M
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RE: Question About The Boeing 757 Fuselage

Wed Sep 22, 2010 8:59 am

Quoting OldAeroGuy (Reply 13):
I was a member of the wind tunnel testing team at the time the 757 was being developed and was aware of the situation. After the 757 was certified, I asked the 757 Chief Engineer what design decisions he would like to change with the benefit of hindsight. The forward lower lobe decision was the first on his list.

This type of Information is so Important & can never be found in any Technical Books available to us AME/AMTs.

regds
MEL.
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