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For those interested in a good discussion on spar shear loads, I'll point you here:
https://www.rcsoaringdigest.com/pdfs...SD-2005-04.pdf Dr. Mark Drela did a multi-part series on this that I referred to upthread. Note Dr. Drela does NOT agree that vertical or horizontal grain orientation materially affects shear web strength. Rather he argues convincingly that the difference between the two orientations is how the failure propagates after hitting the failure point. Note the math doesn't really start until part 3, the first two parts are diagrams with explanations. |
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Controlling the failure mechanism is extremely important.
Fred Weick really pioneered the concept, and as a result all our transportation systems are far safer today. Crush zones, roll cages, etc. are the everyday applications of this technology. Even bullet proof vests use the same concept. Andy |
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So when in doubt, make it stronger, make it beefier and it’s heavier by default. Your right though, everything that goes into an aircraft structure is a compromise, one thing to another are all trade offs. The heavier you build a structure, the heavier, the structure has to be in order to support that structure in flight! Bob |
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As far as I can remember I rarely every used an 'I' beam spars, even probably only if I was building to a plan that required using them. Never bothered with shear ribs, too messy. But then I don't believe all the maths actually always make for a better flying model (Cries of, Oh! horror)
My standard built usually used a top and bottom 1/4" sq strip, or a solid 1/4" sheet full depth spar with split ribs. With some of my tow and catapult gliders in the past I often felt seeing a wing flex and knowing you could be over stressing it was a good safety valve. And of course my 'kiss' build method. But if people like 'I' beam building, not a problem to me. Ray |
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Carbon fill spar failure is different to me:
Using carbon to build a spar and using balsa or something vertically the glue joint is critical. Whether the grain runs vertical or horizontal the glue joint is critical. The design of the spar requires a means to hold the spar together. Some wrap spar with fiberglass or kevlar tow. The failure of the wrap is the failure of the spar. Between the spar is really a filler. The bond to the carbon will fail but the wrap will keep the spar together until it fails. If the spar is wood then the discussion is for similar materials and how the fill is bonded and the grain runs controls the failure modes. Art |
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Bob, you beat me to that one. You can make a spar stronger by adding significantly more material everywhere with no attention paid to the load path or the wing loading....
OR, you can make a light spar very strong by using proper engineering principles and plotting the load path.... Both methods work, one just works better than the other. Scott Abc, Def, Ghi, Jkl.....Etc Had to put letters past my name to make myself more important. |
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Art, in the instructions for Mark Drela's Allegro Lite design there's some importance given to getting a good bond between the carbon strips to each other and to the vertical grain full width filler webbing. So yes, it's certainly important.
As for the binding I have always taken this to be the binding around the joiner wire or rod wing boxes. The "box" where the joiner slips in produces strong shear stress at the face rib end and at the internal end. The joiner blade, rod or wire acts like a pry bar inside the spar assembly after all. So it's important to bind the box spar setup at the ends of the joiner where those strong vertically outwards shear loads are highest. |
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Funny story about all the letters after names.
My brother worked for a very successful mechanical contractor. The guy owned a couple of homes, a Cessna 206 on floats, etc. Anyway, he was at a construction meeting with a bunch of State officials and inspectors, all who were passing around their business cards with PE, PHD, etc. trying to seem important. Well, he passed out his card with the letters HSG after his name. When asked, he explained that the letters stood for High School Graduate. What does this have to do with I beam spars? Not a thing. I personally build with I-beam spars as they're pretty easy to do, really. Even on a tapered thickness wing they're not difficult. Just cut the web sheet to fit between the ribs, then perch the web sheet on top of the bottom spar piece, and draw a pencil line along the bottom of the top spar piece, then cut the web to the line, and sand an edge for a nice fit and glue in place. It goes faster than you'd think. |
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Now that is hilarious. I do have "Letters after my name", but now that I am retired it doesn't matter. I may change to HSG. Hope you don't mind if I steal that one. Scott |
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Last edited by Pylonracr; Jul 31, 2021 at 03:44 PM.
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"BS" just doesn't have the right connotation for a college grad (although for many it may be accurate). "MS" would be confusing - "shouldn't that be in front of a name?"
I think just my name and e-mail is plenty. If I think you're more important, I'll give you my phone number. Andy |
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Scott |
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Andy, had to chuckle at this comment....I cut all my airframes on a CNC router and kerf offset must be correct.... I'd programmed all the parts to be cut with a 1/16" end mill...except I forgot to change the bit....I cut everything with an 1/8" endmill.....Well, guess what, nothing fit......the pieces were too small, the notches and slots were too big........complete mess........DOH !
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