How Much Do The Holes Weigh ?

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DaveK

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Notice those are flanged holes. The flanges stiffen the part. The parts may have been designed to rely on the stiffening effect and a flat sheet without the flanged holes may very well buckle below required demand. Adding a hole to what is specified as a solid sheet could cause the same issue, all depends on the loads it must take and how the piece was designed. Now the door frame mentioned above, doesn’t take flight loads, so putting holes in it isn’t a big deal, but a spar or other critical component I’d stick to what the designer specified.
 

TFF

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I want to say, someone weighed the holes of a Bearhawk build, probably the guy with build manual. Lots of holes in that wing. Just with all the metal circles stacked together, you could see a difference. I believe the holes in the Legal Eagle spar web is to make UL weight or add some margin. If the design needs a 100, you probably need to do them. 3, probably not so much.
 

Marc Bourget

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I also witnessed John Thorp giving his opinion on "weakening vs. lightening." I add an additional detail John's preference to beading that NACA made a study on detailing how lightening holes could be installed with additional beading steps mentioned by WT and the structure could maintain its original strength properties.

For airframes of EAB size, the special tooling and process to achieve "same strength" makes me wonder if lightening holes, except for routing wires, tubing, cables and push-rods, is a "romance" issue, like putting chrome valve covers on the engine and boasting they increase speed.
 

challenger_II

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Ok, I'll bite at the bait...

So, a "lightening" hole, cut and flanged, will weaken the rib significantly, but a riveted aluminum "space frame" rib is not significantly weakened as designed?
 

Angusnofangus

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It's been my understanding that a flanged hole, in a rib or elsewhere, is stronger than the solid article. And double flanged holes stronger yet.
 
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Marc Bourget

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Angus, IIRC, No, not exactly. single flanged is weaker. Double flanged, depending on how it's done, may be equal (per a NACA report but I'm "missing" my folder with that information)
 
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I see a lot of references online for flanged holes in sheet aluminum parts, but what if the material to be "lightened" is a polyethylene foam sandwiched between two layers of fiberglass or carbon fiber fabric. Is there then any point in trying to add material, such as a section of carbon fiber tube that is just a bit wider than the hole, and filleting with a carbon fiber or fiberglass "peanut butter"?
 

rv7charlie

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Tangential to the question... in many cases, the holes have additional uses. For instance, RV-x wings, and I'm sure a lot of other aluminum designs, can't be built without the holes in the ribs. There'd be no way to access many of the rivet shop heads to buck them.
 

PMD

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Angus, IIRC, No, not exactly. single flanged is weaker. Double flanged, depending on how it's done, may be equal (per a NACA report but I'm "missing" my folder with that information)
Now I am intrigued! I have always viewed flanging of a lightening hole as a 90 degree affair with nice radius, ending at the 90 degree point. Anything you could relate about "double flanges" would be most interesting indeed - especially the NACA report.

BTW: for those looking to push things to the limit: IMHO (have not done any calcs, but intuitive design consideration (from doing a lot of boat hull design - where lightening holes have another downside) is that making a nice hole with flange could stiffen a shear web enough to drop down a size in thickness (making a greater weight savings). If someone has done a wing rib calc with this in mind, it would be most revealing.
 

TFF

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By double flanges, does that mean a bead around the hole, not like a double flare auto brake line.
 

TFF

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I guess I never heard it called anything in aviation. It just was. My term is from general fabrication.
 
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