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broken wing spar advice

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Old 09-23-2008, 07:18 PM
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cubfloater
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Default broken wing spar advice

My loving son fell on my avistar breaking the wing spars. Is it worth repairing? It's about a third the way in from the tip. Several ribs broken too. I know I could sandwich two thin plywood piecies to put the spars back together but is that advisable? I know how to repair the ribs just just didn't know about repairing the spars. He did a number on that wing. Thanks for any suggestions.
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Old 09-23-2008, 07:57 PM
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Default RE: broken wing spar advice

The best way is to do a tapered joint, basically, you want the taped to be 8 times the thickness, so say your spar is 1/8" thick, make the taper 1" long. 1/4" would be 2" long, top to bottom or front to back, it really doesnt matter, as long as you have sufficent bonding surface for the glue to get a grip. Replacing sheer webs as normal will lock the whole assembly together, making it just as strong as the other side.
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Old 09-23-2008, 08:17 PM
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Default RE: broken wing spar advice

thanks planebuilder, I'll do it.
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Old 09-23-2008, 08:22 PM
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Default RE: broken wing spar advice

Whilst its true that the longer the better you will have trouble getting an accurate 2" taper on a 1/4 spar (unless your skills are way better than mine, which is quite possible).

Good news is that the load on the spar reduces as you move towards the wingtip, so your son did the right thing falling where he did.

IMO you could get away with a 45 degree taper if you backed it up with a shear web front and back. If you can find a good hard piece 1/16 would probably be thick enough for the webs.

Terry
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Old 09-23-2008, 09:03 PM
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Default RE: broken wing spar advice

I don't follow what you mean by a 45 degree taper. And by taper do you mean decrease in size the sister plywood as it gets farther away from the break?
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Old 09-24-2008, 01:45 AM
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Default RE: broken wing spar advice

Cubfloater,

45° Taper:

To visualize this: imagine a square; draw a dotted line from the upper left corner to the lower right. This dotted line will represent a slant of 45° from top to bottom. This will show what the glue joint will look like between the old spar stub and the new spar section that you are replacing.

Then the joint between this replacment piece and the (now tapered) end of the remaining original spar (running outboard toward the wing tip) will look just like the first joint that you made. It's probably best to locate these two joints in the middle of separate rib bays in order to be able to access them readily.

Shear Web:

As I understand it, a shear web would be a piece of sheet wood BRIDGING AN UPPER SPAR AND A LOWER ONE. This may be how your spars are configured, or maybe not. If there is no lower spar in this location, you'll need to DOUBLE the spar material both fore and aft across the entire rib bay. When all is done, check the lateral balance again, since the added material at the repair may cause a weight bias on that side.

Good luck,
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Old 09-24-2008, 08:21 PM
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Default RE: broken wing spar advice

Thanks Virg, I got it. Sometimes my little pea brain doesn't get entirely into gear. I was thinking about some lite ply as support for a glue joint and tapering those. Anyway, someone said a 1/4 inch spar needed a 2 inch taper so I'm wondering how I'll get to and cut those spars and of course I'll have to remove plenty of covering and neighbor ribs to do so and then put them back. Bad part is those three color stripe designs that I won't be able to put back on without buying three rolls of monocote to do so. Might as well have just bought a new wing. Oh, wait though my son could buy it for me couldn't he? I'm not holding my breath. Thanks again.
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Old 09-25-2008, 09:12 AM
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Default RE: broken wing spar advice

Have a mini razor plane?, taper the one in the wing with it, then glue a piece of sandpaper to a small piece of 1/8" ply, slightly wider than the spar and about the liength of a popsicle stick, sand the final shape and then do the same to the repair spar, grft them together with epoxy and rebuild the ribs as usual and add new sheer webs to lock it all together. Hope this helps explaining how to repair the wing.
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