atlturbine
Posts: 79
Joined: Sat Aug 23, 2008 11:03 pm

Other than space advantage what is the substantial differences structurally or otherwise?

[Edited 2008-09-30 19:11:17]
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474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

 Quoting Atlturbine (Thread starter):Other than space advantage what is the substantial differences structurally or otherwise?

One word "weight".

atlturbine
Posts: 79
Joined: Sat Aug 23, 2008 11:03 pm

Wow..thanks..that seems simple enough. Thanks for the info.
To the World you might be One Person but to One Person you might be the World

atlturbine
Posts: 79
Joined: Sat Aug 23, 2008 11:03 pm

So why ever use a pressure dome to begin with I wonder?
To the World you might be One Person but to One Person you might be the World

Starlionblue
Posts: 17212
Joined: Fri Feb 27, 2004 9:54 pm

As I see it, a dome can be made less heavy for the same strength given its shape.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

acNDTTech
Posts: 173
Joined: Fri Jul 25, 2008 7:15 pm

This is just a guess. Maybe because of the strength of a sphere vs. the strength of a flat panel, a dome may be stronger than a flat bulkhead?

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

 Quoting AcNDTTech (Reply 5):a dome may be stronger than a flat bulkhead?

Given the same weight of material yes.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting Atlturbine (Thread starter):Other than space advantage what is the substantial differences structurally or otherwise?

Different loading. A dome is in pure tension (away from the rim), a flat bulkhead is in bending. That's the main reason the flat bulkhead is heavier.

Tom.

MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

Also, there is generally no space advantage involved with a domed pressure bulkhead. The space inside the bulkhead is normally wasted and empty.
A flat pressure bulkhead is also easier to manufacture, since it doesn't contain 3d curves.

Jan
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atlturbine
Posts: 79
Joined: Sat Aug 23, 2008 11:03 pm

 Quoting MD11Engineer (Reply 8):Also, there is generally no space advantage involved with a domed pressure bulkhead. The space inside the bulkhead is normally wasted and empty.

It seems that some of the more nightmarish jobs in my aviation career have involved working aft of flat pressure bulkheads (DC-9's, MD-88's & 727's specifically) Those jobs could be the subject of a new thread I am certain!

Mitch
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roseflyer
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 Quoting MD11Engineer (Reply 8):Also, there is generally no space advantage involved with a domed pressure bulkhead. The space inside the bulkhead is normally wasted and empty.

A flat bulkhead has a space advantage. It is 2-4 galley carts on a 737. That's enough to allow relocation of the lavatories and up to 6 more seats. That's worth a weight penalty.
If you have never designed an airplane part before, let the real designers do the work!

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting MD11Engineer (Reply 8):Also, there is generally no space advantage involved with a domed pressure bulkhead. The space inside the bulkhead is normally wasted and empty.

There is no space advantage to the curved bulkhead (a flat one provides more cabin room) but it's not true that the space inside the bulkhead is wasted and empty...the rear galley on many aircraft is curved at the back to fit partially into the dome.

Tom.

pmk
Posts: 609
Joined: Wed May 26, 1999 10:07 am

Not to impugn the others but there is another important reason.

Pressure containers, of any kind, operate best when the metal or frankly any other material does not have a 90 degree angle; this creates a fatigue point is generally to be avoided. Hence the reason that propane tanks, champagne bottles, soda pop cans, submarine pressure hulls, spacecraft, and other pressure vessels are ideally round or have hemispherical ends.

There are formuli that explain this in depth but suffice it to say rounder is better.

PMK

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

 Quoting Pmk (Reply 12):Pressure containers, of any kind, operate best when the metal or frankly any other material does not have a 90 degree angle; this creates a fatigue point is generally to be avoided. Hence the reason that propane tanks, champagne bottles, soda pop cans, submarine pressure hulls, spacecraft, and other pressure vessels are ideally round or have hemispherical ends.

All this is true and that is why the flat bulkhead have to made from thicker material with lots of stiffing and thus are much heaver.

jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

 Quoting Tdscanuck (Reply 7):Different loading. A dome is in pure tension (away from the rim), a flat bulkhead is in bending. That's the main reason the flat bulkhead is heavier.

I think the bending issue is the main reason flat bulkheads are far more robust structures compared with a pressure dome. Applying bending loads means you have to deal with compressive loads as well as tensile loads.

Tensile loads are a stable configuration, and are relatively easy to design for. The problem with bending is the compressive part of the loading. You have the additional scenario of buckling and instability to deal with, which is trickier to design for, and often requires additional material mass to ensure stability.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

atlturbine
Posts: 79
Joined: Sat Aug 23, 2008 11:03 pm

Thanks for all the great input! Adding to this..we are all aware of some infamous failures of P domes in the past (JAL 123 for example). Is anyone aware of a "flat pressure bulkhead" inflight failure?
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Starlionblue
Posts: 17212
Joined: Fri Feb 27, 2004 9:54 pm

 Quoting Atlturbine (Reply 15):Is anyone aware of a "flat pressure bulkhead" inflight failure?

Nope. There is one famous one for a classic bulkhead, the JAL 747 accident.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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