thinning down from 6061 to 2024

Discussion in 'Sheet Metal' started by Norm Langlois, May 12, 2019.

  1. May 14, 2019 at 1:08 AM #41

    BBerson

    BBerson

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    The triangle spar/torsion cell is a great plan. Just need to get the details correct as much as possible
     
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  2. May 14, 2019 at 1:09 AM #42

    Norm Langlois

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    Poormansairforc
    Are you suggesting, to just double or add to the thickness at the apex with no space ,no box.?
     
  3. May 14, 2019 at 3:13 AM #43

    poormansairforce

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    Yes, I am suggesting moving the doubler layer in your drawing all the way up into the apex to add thickness there. If at that point the webs buckle then making the entire spar from thicker material with lightening holes and/or adding stiffeners may be your answer.
     
  4. May 14, 2019 at 9:49 AM #44

    Victor Bravo

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    OK, since the OP and several other people here have the ability to "do the math", I would like to propose a test. A series of calculations to arrive at an answer to whether the triangulr spar is advantageous.

    Do the math for a 5 and a 6 inch diameter tubular aluminum spar, identical to what the Kolb ultralight aircraft use. The Kolb uses small steel inserts at the critical points in the spar... the root attach fitting and the strut attach. Other than that, it's a 6061-T6 aluminum extruded tube spar with (I believe) an .049" wall thickness.

    Use the same weight, span, G-loads, etc. as the triangular spar. Also, run the numbers for strut braced, cable braced, and cantilever.

    I am willing to bet that the round tube spar will have more buckling resistance and more G load capability than the triangular spar, no matter whether it is strut braced, cable braced, or cantilever. It will be easier to build, lighter, and have more torsional stiffness.
     
  5. May 14, 2019 at 11:28 AM #45

    Dana

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    Norm, it's obvious that bumblebees can fly; any tales of math saying otherwise are urban legends. It's also obvious that your plane can fly and that the spar didn't fail in flight... yet My point is, for how long? Your post about fatigue and stuff and GA planes being somehow different, though, is incorrect. An ultralight is lighter and flies more slowly, yes, but the laws of physics are exactly the same.

    You've shown that your design will fly and hold together under the conditions you've flown it in so far. 1G? Obviously no problem. But will it hold together in a steep turn, at 2G? In turbulence? In turbulence during a steep turn? Standard category airplanes are designed to a positive load factor of at least 3.8, with a 50% safety margin above that. Do you know what your safety margin is? It could last twenty years... or fold when you hit a bump on the next flight. Fatigue from cyclical loading is an whole other factor, and yes, it applies to ultralights too.

    Actual load testing to destruction has limited value. It can serve as a sanity check on design or build technique, if the inflight loads are exactly duplicated, but not as a replacement for a proper or even rudimentary stress analysis.

    I've flown ultralight aircraft with 2" tube spars, that had a long history of successful flight. I've flown a prototype ultralight that I did the calculations for. But I would decline to fly yours, absent one or the other. It may be perfectly safe... but you don't know.

    Instead of spending your time building and testing samples, spend some time learning the math for at least a simple beam bending analysis. It's not difficult at all, mostly high school level math. Then test your samples and see how they agree with the calculation.
     
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  6. May 14, 2019 at 2:54 PM #46

    Norm Langlois

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    In test No. 2, I had added in about 3 ft of 1X1 X 1/16th sq tube .Had I used a 10 ft or 12 ft. The result would have been much different. But weight was a big issue as building for UL always is.
     
  7. May 14, 2019 at 3:16 PM #47

    Norm Langlois

    Norm Langlois

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    Dana
    I expect many people feel that way about flying UN-certified . They would not fly my plane ether.

    The flying wire changes how the wing is built and what from it also changes the Direction of the load. Out board of the wire the arm being shorter less shear load . Inboard of the wire no shear. The G load is then transferred to the Wire or struts,depending on the point of attachment, whether it becomes a fulcrum then the root would be Compression and down on the root attachments. My plane,the inboard load be comes compression and less shear. This allows for simple hardware fasteners at the roots.
    Like all other strut supported aircraft my plane will never come apart unless the wire fails (strut failure)
    Speculating if the wire were to part the wing may or may not destruct it may only bend there by, becoming crippled but still flying. Any other type aircraft with a strut failure your done.

    Each single cable is rated for 2000 lbs the GV of the plane is 520 with fuel and a 170 pilot. What does the math tell you ?
    I claimed a 4G its more like 8
    I used total GV, the wing is a neutral self supporting weight and should not be added to the equation thus 430
    4000/430
    Excuse me for not knowing how to write with a key board. I don't write code ether.
    This is my plane not the proposed Spar.

    I asked about thinning with an alloy change not a long debate over triangular spars. I expect that 2024 would have a better resistance to the compression load at the apex but thinning would not be an option . Would 2024 make the grade for a cantilevered wing with the arm load so high. Maybe not.
     
    Last edited: May 14, 2019 at 3:57 PM
  8. May 14, 2019 at 4:31 PM #48

    BBerson

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    To answer your question it would have to be determined why it was under strength. It could fail two ways:
    1) the metal is too soft and yields like putty.
    2) the metal is very hard but is too thin and just collapses
    Certainly the harder 2024 will improve the strength, but not if it is too thin.
     
  9. May 14, 2019 at 4:44 PM #49

    BoKu

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    Hi Norm,

    You're definitely on to something with your trapezoid cross-section spar. It can have enough bending stiffness against both lift and thrust/drag, and enough torsional stiffness, that you need relatively little structure in the rest of your wing. It's a pretty elegant solution to a lot of the problems of ultralight construction.

    However, I can tell that you are struggling conceptually to figure out how it reacts the stresses of flight, and I worry that you won't know when you're in over your head until it's too late.

    The answers you seek are out there, and a little bit of math and some engineering would reveal it relatively easily. As a relapsed liberal arts dropout who learned beam theory in their 40s, I heartily endorse suggestions earlier in this thread to get some help to apply some actual engineering to the problems at hand.

    As a start, I'd suggest going to have a good look at a Vans RV-series wing spar or something similar. What you'll see is a relatively light shear web in the middle (what you are calling a "diaphragm") with big fat bars of aluminum top and bottom. The overall effect is to replicate the sort of I-beam that engineers and ironworkers have known since the 17th century is the lightest way to create a beam that resists bending. Concentrating material at the top and bottom, and having as little as you can get away with in the middle, maximizes the strength and stiffness per unit weight.

    As a counter-proposal to your proposed design revision, I'd suggest something like the attached. As you can see, it's the sort of trapezoid you're familiar with. But it also features strips of aluminum riveted on at the three vertexes. These bars would react the accumulated tensile and compressive stresses that result from bending.

    trap spar.png

    As to how big those bars need to be, well, that'd take a bit of engineering, and I'm not qualified to do that without some help. I might take a whack at some estimates, but I can't promise anything.

    As for your question about 2024 versus 6061 for the webs, the answer is not clear on the surface and is probably irrelevant. 2024 is stronger, of course, but it has the same stiffness as 6061, and stiffness is what is important for buckling resistance. And what you've been seeing so far is buckling failures. But 2024 is harder to bend and harder to work with, and switching to it might drive fabrication compromises that negate the greater strength.

    --Bob K.
     

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  10. May 14, 2019 at 5:44 PM #50

    Dana

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    The math tells me the cable probably isn't the weakest link. But this is why I encourage you to learn about basic stress analysis concepts. A strut braced wing does indeed have shear stresses both inboard and outboard of the strut attach point... though I'm not sure, what you're calling "shear" may be bending moments?

    Outboard of the strut, the wing has exactly the same shear and bending loads as it would if it were fully cantilevered (the outboard portion is, in fact, cantilevered). Inboard, it's reduced, but not zero or even close.

    One other thing about the cable, remember the tension in the cable at an angle is larger than the actual vertical load it's supporting. At a 30° angle, typical of wing struts or flying wires, a 2000# cable can only support 1000# of vertical load... and at that load it would be adding a 1600# compression load to the wing spar.
     
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  11. May 14, 2019 at 6:06 PM #51

    BBerson

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    Norm, if you are still considering different orientations, here is an alternative with possible advantages.
    It puts the structure forward of the CG, which helps prevent flutter.
     

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  12. May 14, 2019 at 6:37 PM #52

    poormansairforce

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    Yes, this is where I have been going lately.
     
  13. May 14, 2019 at 6:58 PM #53

    Victor Bravo

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    This is the equivalent of the guy who was certain he could live among the bears in Alaska. He proved everyone wrong for a couple of years.
     
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  14. May 14, 2019 at 7:34 PM #54

    Norm Langlois

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    Though Dana is correct about the reduced cable capacity at an angle. It is still greater than the 4 G load
    Stop with the BS
    I do not wish to defend MY AIRPLANE. No one has to fly it. I do not wish for anyone to accept it. The weakest point on the cable is the attachment to the spar top The S/S tang that it bolts to through the aluminum and that is reinforced.
    The wing final was tested with 40 bags at 50 lbs each 20 bags per side.

    The wing test I provided were to show one thing. That was to show the .050 6061 T4 does not resist the bending moment /Shear . That it folds over on itself.
     
  15. May 14, 2019 at 7:54 PM #55

    BBerson

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    The upper cap failure in a beam (spar) is almost all pure compression. Called column buckling or crippling because columns are pure compression. The upper cap is like a small column. Buckle resistant bars are needed instead of thin sheet for high bending moment areas at the root. The shear at the root is resisted by the web and much less than the cap compression. Understanding all this can help design a lighter optimal spar.
    Dana and others can help. Difficult, but try to ignore unhelpful posts.
     
    Last edited: May 14, 2019 at 8:01 PM
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  16. May 14, 2019 at 8:13 PM #56

    Norm Langlois

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    Bill
    The concept drawing show layers and contour the capped side this reinforces the flanges by way of layers lighter ,not much different than adding a flat bar. This is why I say invert the spar and make the pair , in compression and the single in tension. and 2024 would serve tension side better than 6061.
    Adding to the apexes is the whole concept. To change the otherwise weaker simple triangle to round comparison .

    Dana wants me to engineer first, then make something and test.
    I am just making and testing ,concept. its far cheaper to buy a sheet of aluminum than to pay an engineer .
    I never told you I was going to fly it, or sell it to anyone.
    If Dana could help he's not offering, He does not like triangles.
    If a spar tube of 6 inch dia. .050 wall had 3 , 1X1 square tubes riveted to it in thirds the way around it. would it be a triangle or a round spar ?
     
    Last edited: May 14, 2019 at 8:18 PM
  17. May 14, 2019 at 8:28 PM #57

    Victor Bravo

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    It would not need the square tubes.

    The Kolb 5 inch tube spar and strut system handles a 6G load on an 850 pound airplane (Kolb Firestar model 2 SS).

    The Kolb 6 inch tube spar and strut system handles a 6G load on a 1000 pound airplane (Kolb Mark 3).

    The lighter version of the Kolb 5 inch tube spar and strut system is used on the Kolb Firefly 103, which easily meets Part 103 ultralight rules under 254 pounds, and still provides a 5G load carrying capability.

    Importantly, their tube spar wing design uses built-up aluminum truss ribs that are very similar to the ribs on your flying boat. This spar design has 1/5 or 1/8 of the parts count, fasteners, etc. of the triangular spar. It is also far superior in terms of torsional stiffness. Since there are no flat faces (diaphragms) on the tube spar, and all of it is curved (round), it is a lot stiffer in any direction, and the resistance to buckling is far far superior to a triangle (or any other polygon) shaped spar.

    Of course you can build and test whatever you want to, this is homebuilding and experimenting. You have a fundamental right do try anything.

    We also have the same right to try and help, even if it is "tough love".
     
  18. May 14, 2019 at 8:29 PM #58

    Norm Langlois

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    I have been using geometry for solutions because I can . The tubing design serve the home builder. They have limits.
    I am just search out side the box where many can not go. I can.
    The aircraft engineers of this world have no interest in Ultralights there is no money in it for them.
    You know something else most can not do what I can do. They hire people like me to build there designs.
     
  19. May 14, 2019 at 8:38 PM #59

    Norm Langlois

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    The kolb Design is not a comparison to the concept presented here. its only an argument that the kolb is better than my plane as built.
    It also can not even have a chance to be used for cantilevered.
    If you have no love for the triangle concept that is your choice. Do you think its right to keep pissing on me?

    Your not helping . I asked a question about alloy choice now look where we are.
     
  20. May 14, 2019 at 8:39 PM #60

    BBerson

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    Norm, I don't know any professional ultralight engineers. That's why we are all here to get ideas from other amateur designers. The more we amateur designer learn, the less scrapped parts. I have plenty of trashed scrap also.
     

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