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Faro
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Lighter, Arched Cabin Floors?

Fri Jun 05, 2009 8:32 am

Would a slightly up-arched passenger cabin floor be a practical way to reduce airframe weight?

I may be wrong but from what I recall, metals have greater resistance to compressive than to bending or tensile loads. Conceivably, if an arched cabin floor were to involve mainly compressive loads (or a greater proportion of compressive loads), one could make it lighter for a given floor load than a normal, level floor. The degree of convexity need not be significant and would not be too apparent to passengers, perhaps only resulting in seat sides which are slightly divergent instead of being perfectly parallel to each other.

If the floor's points of attachment to the fuselage can handle the lateral loads resulting from such an arrangement, could this be feasible?

Faro
The chalice not my son
 
phollingsworth
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RE: Lighter, Arched Cabin Floors?

Fri Jun 05, 2009 12:39 pm



Quoting Faro (Thread starter):
I may be wrong but from what I recall, metals have greater resistance to compressive than to bending or tensile loads. Conceivably, if an arched cabin floor were to involve mainly compressive loads (or a greater proportion of compressive loads), one could make it lighter for a given floor load than a normal, level floor.

Actually they do better with tensile loads. The ultimate tensile stress for most metals is significantly above the compressive yield stress (in most cases the tensile yield stress is also greater). Add to that the fact that when dealing with compresive loads you often get a buckling or crippling failure are stress levels below yield, you want to avoid tom much compression. Remember the upper spar or skin is usually the first part of the wing to fail. That being said, the reason you see a lot of "arched" members is because you are starting with what is effectively a negative stress in the member. In this case the upper beam cap would start in compression and the lower cap in tension, then as it is loaded the stress would move from tension to compression and vice versa. This is what pre-stressed concrete is all about. The thing about most floor beams is not the load from above, but the tensile load that is transmitted across them to maintain fuselage barrel integrity as a result of the cusp.
 
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Faro
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RE: Lighter, Arched Cabin Floors?

Wed Jun 10, 2009 12:01 pm



Quoting Phollingsworth (Reply 1):
Actually they do better with tensile loads. The ultimate tensile stress for most metals is significantly above the compressive yield stress (in most cases the tensile yield stress is also greater). Add to that the fact that when dealing with compresive loads you often get a buckling or crippling failure are stress levels below yield, you want to avoid tom much compression. Remember the upper spar or skin is usually the first part of the wing to fail.

Thanx for the feedback, learned something there.

Does the above also hold true for CFRP or this a more complex issue?

Faro
The chalice not my son
 
tdscanuck
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RE: Lighter, Arched Cabin Floors?

Thu Jun 11, 2009 4:42 am



Quoting Faro (Reply 2):
Does the above also hold true for CFRP or this a more complex issue?

It's true in general, but CFRP lets you tailor the properties along different axes, and also has the yield stress much closer to the ultimate stress, so it tends to perform better.

Tom.
 
phollingsworth
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RE: Lighter, Arched Cabin Floors?

Mon Jun 15, 2009 4:13 pm



Quoting Tdscanuck (Reply 3):
It's true in general, but CFRP lets you tailor the properties along different axes, and also has the yield stress much closer to the ultimate stress, so it tends to perform better.

Yes, the beauty of an-isotropic materials. Also because they typically have a higher stiffness many composites have higher buckling and crippling stresses.

Oh, it looks like I swapped my upper and lower beam caps in the previous statement. The upper cap should be in tension and the lower in compression prior to loading (for a floor or bridge beam).

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