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Lightening holes?? 2

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aderhy

aderhy

Mechanical
Oct 22, 2009
2
How do you decide where you would place the lightening holes on the ribs and spars of a wing box???
 
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1) (being a bit of a smart arsa) where nothing inside will leak out (fuel), and where nothing outside will leak in (birds)
2) generally where the shear is fairly low, ie not on heavily loaded ribs.
3) that being said, you can probably design lightening hole reinforcement that minimises their effect on panel strength.
 
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I would place lightening holes in locations that have the least impact on structure strength and where no/little impact on form, fit, or function will occur.
 
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Aren't you kind of doing this backwards? Usually lightening occurs as an integral part of the design. You design specific ribs, trusses, etc., that take their respective loads without being completely solid.

Ribs and spars are usually already designed for minimal weight. Further weight reduction would most likely incur a loss in strength.

TTFN

FAQ731-376
 
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aderhy,

Besides the many excellent guidelines regarding lightening holes in structures provided above, I would propose one more. A carefully configured and located opening in a structural member can sometimes be beneficial in altering the stress flow in a part, such that it reduces stress concentrations at the attachment interfaces.

Good luck.
Terry
 
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A good example of that is where a weld joins a relatively flexible part to a stiff part such as a tube. The stresses local to the weld can break it if the joint is too stiff, so a hole near the weld softens the joint and allows the stresses to build up more gradually.

Ignorance of this detail delayed a product launch by 2 years.

In general post-design lightening holes are a fairly depressing way of lightening a structure - you need to make a lot of them to make an appreciable difference, and every one is a risk or needs analysing.

Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Hi all.
When asked why the T-18 doesn't have more lightening holes, John Thorp's reply typically was "Because they weaken the stucture faster than they lighten it!!

Agree?? John Q.
 
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If formed, Lightening holes have a flange which has a significant stiffening role. The usual shallow flanges at about 45° to the plane of the web result in a holed panel that has less shear strength (for buckling) than the solid web. However, some severely formed flanges, pushed right out to 90° to the web, can result in a web that is more resistant to buckling than the unholed solid web. However, forming these is quite hard.
 
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Several FEA loading configurations and shape optimization.

Rob Stupplebeen
 
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How do you lighten a hole when there is nothing there in the first place?

Mike McCann
MMC Engineering
 
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Holes in semiconductors not only have mass, they also have mobility...

TTFN

FAQ731-376
 
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According to tradition the holes are there for the load to fall out of...
 
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Very, very early in my military career, upon arriving at my first duty assignment my shift chief was walking me around the first B-52 I had ever seen. Fresh out of electronics technical school, when he pointed out the "lightening" holes to me on the partially disassembled bird that was in the hangar, I thought he was saying "lightning" holes.

To my freshly electronically trained mind I was envsioning that these holes were an escape path for lightning strikes to prevent the lightning from doing serious damage elsewhere. Too overwhelmed to even consider I might be misunderstanding anything I was being told, it was several months later when another more experienced technician explained my misundstanding to me...after he had to wipe the tears from his eyes from laughing so hard at me!
 
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debodine,
I thought the same thing when I was fresh out of school. Fond memories there.


Rob Stupplebeen
 
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yeah, it's an important "e" in "lightening" ... and surprisingly often (or not, depending on your jadedness) overlooked by spell-checker.

like the 1st report i had to review ... you guy had written "asses" for "assess" ... "but spell-check didn't pick it up ..."
 
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The program was probably written in China...

Mike McCann
MMC Engineering
 
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Nui (Airframe Structural Design 2nd Ed. Section 6.2) says that lightening holes are either found a) in lightly loaded or shallow beams where the web has unnecessary stiffness/strength due to additional thickness required for the flange joints. The addition of lightening holes weakens the structure but the strength isn't required. or- b) where access holes are required for construction or pass-throughs of equipment. Usually you would add flange stiffeners between these holes or additional reinforcement.

That basically matches what rb1957 and other posters have said.

The remainder of the section has some further discussion on optimal arrangements, etc. You can also find more of this sort of information in the usual places (ESDUs, OEM design manuals, NACA reports, etc.).
 
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