thinning down from 6061 to 2024

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

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  1. May 13, 2019 #21

    BBerson

    BBerson

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    The Ercoupe wing is really a triangle space truss. But it is tipped on it's side.
    The "base" of the triangle is the I beam main spar, the "sides" are diagonal sheetmetal ribs. The apex is the rear spar.
    And nose ribs are installed on the front.
     

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  2. May 13, 2019 #22

    Victor Bravo

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    If you have a flat spot on the top of the triangle where you put in rivet holes for the ribs, you are creating the very weak spot that is going to start the failure, regardless of whether the spar is the right overall shape.

    Regardless of that, although there are lots of people here with far higher levels of engineering education than me, I'm just about 100% convinced that the shape of your spar, basically a triangle, is a large part of the problem. The webs (diaphragms as you call them) are oriented at a significant angle to the primary flight loads on the wing. This means that the sheet aluminum is not oriented in a way to best resist the flight loads.

    Lifting the wingtips upward applies a strong force that wants to bend the angled spar webs in toward each other, which means they will buckle or collapse long before the same piece of metal woould collapse if it was at a right angle to the load.

    If you made a square or rectangular spar instead of a triangle, your wing would very likely be able to withstand a far higher load. I'm sure that you have heard this before, or at least you should have heard it before.
     
  3. May 13, 2019 #23

    Norm Langlois

    Norm Langlois

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    Dana No I did not
    Building an Ultra light with little money as many do. I cant affords an engineer, or that avenue of testing.
    The G load test shows enough .Doing a math analysis ,I am not capable
    Long ago I made it clear to all I am not an engineer . I use common sense logic. And advice from those who will give it or supply information .
    I did fine with my plane It flies well . It holds up to my abuse even if the hull is now broken the airplane is not. The above tests led to the use of 1/8th S/S flying wire .Logically by the tensile strength of 2 such wires the plane reaches over 4 G and the compression load is halved by its anchor point.

    This whole thread is about the anchor point and arm for cantilevering this type of spar
    The same point as in the above tests.

    I need to remind you Ultra lights do not need to be treated as GA. The forces of nature ,the airspeed make the approach to those planes much more a concern for fatigue. The light weight and the environmental use Ultralights were meant to be used in,make it more like a balsa plane kids throw around all day .
    Floppy wings as many super light approach to light weight and lots of fatigue does not occur in a solid limited movement structure. This spar is solid it moves very little and is only subject to the compression .
    Surely this would not be acceptable for GA and has limits going up in weight and speed.
    The Back Yard flyer has a mono triangle welded tubing spar Big tall and ridged. If it doesn't move and is likely over the requirements of the loading ,theirs appears to be. Mine I hope to make as the toy plane described. Ridged un moving low flex wing.
    If we are trying to make a aerobatic plane for high G loads I would not use this in a cantilevered plane. For UL not meant for windy or stormy. What is the point of stress analysis . Would you do a stress analysis if you wanted to replace a wood 2X2 with a steal I-beam.
     
  4. May 13, 2019 #24

    BBerson

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    A stress analysis can help the designer see where to make it lighter. More of a challenge for ultralight designers actually, than GA. GA are typically simple metal boxes. Ultralights can't afford metal skin usually and need other structure types.
    But I don't trust a stress analysis without testing because unexpected things happen. I always test load.
     
  5. May 13, 2019 #25

    Norm Langlois

    Norm Langlois

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    Actually I've heard only a very few disagree with the triangle. I have only 1 engineer declaring to be so speak to me about that spar . And he said to invert it. It would then most likely be able to cantilever.
    The spar in my plane is the same one as in the test . The diaphragms are large and weak. The strength come from the 3 apex not the sides.
    The new spar has much enhanced apexes. The 2 with cap side are far above the strength of the original. the flat side cap is the weakest ,If this become in tension it will also have greater distance to lighten holes with the new intended only a single row of holes can exist .
     
    Last edited: May 13, 2019
  6. May 13, 2019 #26

    BBerson

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    The triangle is lightest because it has three sides instead of four in a box beam.
    Only one web is needed for an I beam which is the lightest beam.
    That's why I think the Ercoupe with a single vertical I beam spar is lightest. The Ercoupe wing was designed for lightest possible using fabric. Useful for ultralights.
     
  7. May 13, 2019 #27

    Dana

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    The triangle is not a good shape for a spar.

    For sake of comparison, a 6" triangle weighs slightly more than a round tube of the same wall thickness but is about 2/3 as strong in bending as the round tube. A square tube weighs 15% more than the triangle but is 140% stronger. That's not taking into account the holes which severely weaken your spar.

    Ultralight or GA, the principles are the same and both can kill you dead if you get it wrong.
     
  8. May 13, 2019 #28

    pictsidhe

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    Adding rectangular spar caps might be the way forward if the basic triangle will take the shear and torsion stresses.
    My best book for thin shells is "stresses in aircraft and shell structures" by kuhn. There is s pdf online. I bought a print of it from india for around $20
     
  9. May 13, 2019 #29

    Norm Langlois

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    The holes have not shown to have any contribution to the failure. The material failed by compression at the apex . Your claim the spars triangular shape is responsable over all to be a poor choice does not show evidence here
     
  10. May 13, 2019 #30

    Norm Langlois

    Norm Langlois

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    proposed new layering spar layers.jpg
     
  11. May 13, 2019 #31

    Norm Langlois

    Norm Langlois

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    The lighten holes are not represented there.
    Though round tube has torsional value over a triangle . this type of spar ,It is not a bare 3 sided triangle and can not be represented by straight comparison dia. VS ETC.
     
  12. May 13, 2019 #32

    Victor Bravo

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    Norm, I am very sorry to be the bearer of bad news. You are possibly poking the Grim Reaper in the eye here. This is supposed to all be fun, but you're betting your safety on something you don't have the knowledge to do. I can walk down a dark alley in the wrong part of the world, and not listen to the locals who are telling me that it's not a good idea. That's what you're doing here.
     
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  13. May 13, 2019 #33

    Norm Langlois

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    The spar I made for my UL and the rest of the make up of the wings, I made were and still are fine for a UL . IT could go on flying long after I am gone. And The Dana's of the world will keep saying bumble bees cant fly because the math says they cant.
    Sorry you are missing something. It is the total make up of the wing not just the spar shape. Being able to lighten it with holes while staying below its failings . Is what its all about.
    I am looking to enhance that compression zone. Any new spar will have to be load tested as well.
     
  14. May 13, 2019 #34

    Norm Langlois

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    Victor are you aware that I fly the wing with that spar. That my avatar is the plane and it still flies. If one has a negative view point put his way but has as many positives or more from some that have the knowledge.Who am I to believe, only the ones that say they think its a bad shape?
    I have had more positive than negative. That plane is 13 yrs old this is not a new subject .
     
    Last edited: May 13, 2019
  15. May 13, 2019 #35

    poormansairforce

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    Wait, I thought we were discussing a cantilever spar option i.e. not a current spar with a flying wire??? If so, then why the harshness with Dana and VB?


    If it is in fact a cantilever spar then you'll want more material at the apex and down around the bend rather than down inside.
     
  16. May 13, 2019 #36

    BBerson

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    Norm, if you make a three foot sample of your 60° corner section and test it in compression, it will not hold anywhere near the load of a 2" tube like on the Back Yard Flyer. The truss has material concentrated in the three corner tubes and is efficient.
    On post 30 you have a sort of tube on the bottom. Need that on all three corners.
     
    Last edited: May 13, 2019
  17. May 14, 2019 #37

    Norm Langlois

    Norm Langlois

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    Bill exactly my point its the corners not the diaphragms . Not the triangle . Its the compression load at the corners . My first test and the second show that. I was not defending the original. I was point at a new option as shown above. the old spar did not exhibit a cantilever able use but it did hint at it. A test will be done the numbers will tell the truth.

    On the mention of the Back yard Flyer . You may like the larger tubing making it compression acceptable. I am a welder and I wont fly anything welded that depends on the weld to fly. aluminum cracks to easy unless welded in an inert gas environment it can not be trusted. Do you think they x-ray all those welds for flaws.
     
  18. May 14, 2019 #38

    BBerson

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    I agree welded aluminum can be problematic. Also not homebuilder friendly.
    But it looks like you have access to a 12' press brake. Not many other homebuilders could follow up with your factory methods.
    My other point was the "diaphragms" don't need to be .050" if the caps are thicker. Since the spar is hand fabricated (instead of extruded) it could be made lighter with thick skin at the fuselage and thinner skin toward the tip.
    I just riveted an I beam spar with .020 2024-t3 web. Have not load tested it yet. Got the flu this week.
     
  19. May 14, 2019 #39

    Norm Langlois

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    Poormansairforc
    Yes we are talking about a new concept based on the old evidence in comparison.
    The old spar failed at the apex under loads applied as a cantilevered wing.
    Up grading to 2024 T3 would make a change.keeping at .050 was the original point. Adding some component in further change. Then adding a top diaphragm as draw . The old had only a flat sheet of .050 inboard and out board they are only .020 to prevent spreading and limit torque .Torque control was poor. Each wing rib then and in the new wing did and now will isolate deforming of Diaphragms. The old were large 11 inch,the new only 6 inches.
    The ribs both then and new are more structural than most UL solutions . You don't get much structure from an arched single 3/8th tubing.

    Comparing my solution to a ladder wing compression brace and drag braces and 3/8th tube airfoil made from round tubing.Requiring dual struts to make it usable. or even the tried and true wood wings . They also require struts none could ever be used cantilevered.

    I believe the spar made from 2024 T3 .050 and caped as drawn. Plus inverted has a chance of being cantilevered none of you has to believe it .
     
  20. May 14, 2019 #40

    Norm Langlois

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    Bill
    No I only have access to a 10 ft hand brake. That's Why I have a splice in the old spar.
    I want a 13 ft, spliced out board, at the tip. The intent is to use the first foot into a socket. This first 2 ft may need 360 degrees of compression limiter of an unknown solution to prevent focused point pressure. The trailing edge of a wing will have a short tube spar. My wing has this to prevent torque on the main.
     

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