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  1. #1

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    Foam core wing vs. built-up

    I'm building a wing from plans that were drawn for a balsa built-up wing. I'm going to cut a foam core and sheet it. I need to know how to build the foam core wing to match the strength of the built-up wing.

    On the plans, each wing half is comprised of two panels totaling 45". The plans call for 1/2x1/4 spruce spars, top and bottom, 1/16 sheer web, 1/4 sq spruce LE spar and two 1/4 sq spruce TE spars. LE is shaped from 1/2" balsa.

    I can sheet the wing with 1/64 ply, 1/16 or 3/32 balsa, or even obechi. Some options for spars include just adding everything just like the original at the expense of weight, or building a balsa/carbon spar and skipping the LE TE spars.

    I'm not a math wiz so I can't figure out the stress loads myself. Without doing a bunch of math, can someone suggest a strong and light structure?

    Any help is appreciated.

    -Ben

  2. #2

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    Foam core wing vs. built-up

    depends on what kind of plane it is and what weight and span, but even for most jets a foam wing with 1/16 balsa skin and a layer of 1-2.5 oz cloth is fine. You can use epoxy to bond the skin, and under the skin surface, the side that gets bonded to the foam, run a strip of carbon tow on the CG, or MAC, this way you wont have a bump if you wet out the tow well and spread it out a little on the balsa.

  3. #3

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    Foam core wing vs. built-up

    This is going on a 1/4 scale L-19 Birddog. There is no wing tube as this plane has functional struts. I expect the final weight to be around 25 lbs.

  4. #4

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    Bird Dog

    Ben,

    I think you have a good application for the foam core wing. Will this wing be one piece? 90" inches is a lot to handle in one piece!
    IMHO, if you use foam core, 1.5 oz. fiberglass between the foam and 1/16" balsa skin you won't need a spar. Reason being is the lift struts. Which will need special attention to the hard points you install for strut attachment. The fiberglass in the sandwitch makes the wing strong and stiff. I've used it on Pylon racers and never had a problem. As I recall a 50" wing (500sq. in) would come out of the press at about 11 oz. and finish up with servo at about 16 oz. Don't sell this construction short on strength and it will be light..... How you going to handle the flaps. ailerons? Good luck..... Jeff.......

  5. #5

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    Foam core wing vs. built-up

    Jeff,

    It's a two piece wing. Each half attaches to the fuse side with bolts and blind nuts in the root rib.

    I was thinking of bonding the balsa directly to the foam and covering with Ultracote. For strength, I might put a 1/2x1/4 spruce spar on top and a thin carbon strip on the bottom.

    What do you think?

    -Ben

  6. #6

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    Foam core wing vs. built-up

    Ben,

    IMHO, if I were going to use a spar, I'd make the spar out of end grain balsa sandwiched between two 1/64" ply sides. The spar is set in the wing on it's 1/4" side and is as tall as the foam core. You would only need to run the spar out about 75% of the panel length. Place the spar on the C.G. or on the high point of the airfoil. Foam core wings built in this manner are incredibly stiff.

    I have made several thin wings about 1/2" thick and 38" long. It's really to build a wing this thin with the proper stiffness and strength. Using the same spar as above (only1/8" thick) these wings come out very stiff. I'd encourage you to strongly consider putting the 1 1/2 oz. fiberglass cloth between the foam core and the skin. You only need to have the glass out to about 80% of the panel length. You will not believe how strong wings built like this will be, AAANNND, LIGHT. Rgds, Jeff.....

  7. #7

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    Foam core wing vs. built-up

    Jeff- Only glass the top, right? Structurally, is there any difference if I glass on top of the balsa?

    -Ben

  8. #8

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    Foam core wing vs. built-up

    Ben,

    Yes, there is a difference. When you add the fiberglass between the balsa and the core, it only takes slightly more epoxy, or polyurethane to wet out the glass and put the sandwich together. If you're adding the glass to the top of the balsa you have to add more resin plus more weight. Now, I don't claim any engineering skills, but in my experience, I's say that the glass in the sandwich is stronger because the glass is inside, has components on both sides, as opposed to having one open side as it would be as the outside layer. Make up a test sample and see for yourself. Were only talking 1.5 oz. cloth here so, your not talking about adding a lot of weight. It's also a lot easier than glassing the outside skin.......Jeff......

  9. #9

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    Foam core wing vs. built-up

    Good points. Thanks Jeff.


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