designing root fittings for carbon spar caps

Discussion in 'Composites' started by dbonner, Jun 3, 2012.

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  1. Jun 3, 2012 #1

    dbonner

    dbonner

    dbonner

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    Hello, I'm designing a 6m span wing for a 120 kilo takeoff weight. I'm designing to 6G positive 3G negative. Sketches are on electric car project (don't mind the name of the link, you can click through to the aircraft project). It's a high wing, strut braced halfway out, but the struts work only in tension, so the highest spar cap stresses are at the wing root in the 3G negative condition. The wing is in two halves that join above the pilot's head. At the wing root the idea is for the top & bottom spar caps from left & right to attach to each other at the root so that the spar cap tension & compression is fed directly from one side to the other. The spar caps are square rods 60mm2 (10 mm wide by 6 mm thick). I am thinking of making small 4130 steel fitments to do this. The 4130 piece would be gloves enveloping the carbon on all sides, screwed together on top of glue (screws to either side, not piercing the carbon). The steel fitment would then attach to the other fitment opposite, and also to the fuselage.

    As is typical in these cases I am not taking shear into account for a first estimate, since it tends to be so much lower than tension/compression in a spar cap.

    Does anyone know how to calculate the minimum safe length of a steel glove around a carbon spar cap so that it can transmit the full force in tension or compression of the carbon rod? ALL the carbon fibers will be interrupted at the root, so that we are talking about all of the stress getting dispersed to the surface of the rod by shear into the steel glove fitment. Can I just use the figure for shear strength of the resin matrix, over the entire contact surface, with some appropriate safety factor?

    With thanks in advance, David Bonner
     
  2. Jun 3, 2012 #2

    wsimpso1

    wsimpso1

    wsimpso1

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    First, why are you even talking about breaking a carbon beam at its max moment location and trying to make a steel glove pickup those bending moments? For a wing of this span, why not just make one continuous span and spar? It will be a lighter and more robust. If the spar just has to be two pieces, you could overlap the spars, and pin them together in two places....

    Next, your struts do not carry zero in compression. Look up how to use Euler's rule on columns, and then check out what you might need in terms of section and modulus to carry your negative g limit. A bit more strut might allow the spar ends to be simply pinned at the root, and save some more weight. One other thing about struts is that a jury strut can be rigged between the middle of the strut and the wing, and that quadruples the buckling strength of any strut you pick...

    Last issue, are you really willing to run your fuselage structure and control runs (for your elevator and rudder) through the hub of your propellor? Among other things, you have to dismount your empennage to service the prop. Then the prop hub has to be pretty big and you still will have the tail be pretty flexible to get it inside the hub. Let's not even think about a prop hub bearing seizing up and causing the tail to leave the airplane...

    I urge you to very carefully think through your wing setup and your prop hub to fuselage arrangement.

    Billski
     
  3. Jun 3, 2012 #3

    autoreply

    autoreply

    autoreply

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    Actual cap tension divided by the shear strength of the glue/joint multiplied by a safety factor?
    Think of thermal loads and the uncertainty in metal gluing (massive).

    Is it possible to overlap the two spars partially? Even a 25 cm overlap would make it far easier, simply insert two shear pins (like on a sailplane) and both analysis, design and construction gets far simpler.
     
  4. Jun 4, 2012 #4

    sachaknoop

    sachaknoop

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    I can not advise you here, but I like all of the planes you draw. they look different yet functional and all have a clear line of thinking behind them. They look like fun to sit on/in!

    Sacha
     
  5. Jun 5, 2012 #5

    wsimpso1

    wsimpso1

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    Problems with attaching the glove to the spars is that you either have to drill holes through the caps or rely on adhesives. Think hard about this - your carbon caps are essentially a bundle of tiny sticks, each with incredible strength, and then a glue fills the space between them. Drill a hole, and put a screw through, and the screw only has to overload the glue along two planes (and it only needs to do it a little at a time) to remove the sticks along that plane. To take loads in composites, you really have to figure out how to spread out loads. 50:1 on up to 100:1 feather is common in wing skin repair. Now think of a 0.2" thick cap - 10" to 20" scarf to pick up the load...

    Billski
     

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