Repairing broken wing spars

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sgilkey
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Repairing broken wing spars

Post by sgilkey »

This topic came up on our "Upcoming Events" thread and I thought it was worth posting here;

I prefer to repair planes rather than build new. I used to throw out wings with broken spars, but I have been trying a new technique which I believe was taught to me by Long Haul Loescher.

First, some history. My first attempt at spar splices was to cut out a broken section and splice in a new one, using brass tubing as a coupler sleeve and epoxy glue. Works fine in compression, but in tension, the glue joint shears and pulls out of the tube. Then I tried PU gluing stub spars to the broken spar piece- the PU glue is way too weak in shear and the joint failed quickly. Then I tried epoxying three stubs to the broken spar, around it like a bundle, this seems to work OK but is a mess and takes a lot of work to dig out the hole in the wing, slop on the glue etc.

The new technique that so far seems to work well is to use a dremel router (I got a Kawasaki knock-off at Meijer for $9) to cut grooves in the foam about 1/8" away from the broken spar, and two about 8 inch long stub spars on either side of the main spar, glued in with PU. Very fast and easy to make the repair, and so far seems to be holding pretty well.....
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Big Dog
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Post by Big Dog »

Scott I do something similar with CF flat stock. I have gotten away with only doing one side but both would be better. and flat stock holds better in shear because you slip it in vertical.
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boiler
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Post by boiler »

I just fixed one a couple of hours ago. The spar that Brian broke at the NATS on my virgin wing needed a fix. I got a foot long scrap piece of spar and put it into a new grove next to the broken spar. I then used PU to put it in place and attach it to the old spar. We will see how well it holds up this weekend in case Brian wants another crack at it.[:)][;)]
sgilkey
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Post by sgilkey »

Craig, I like your idea, much batter grab on the foam....like LongHaul, I've been using f/g rod scraps

We too had many virgin wings centerpunched at Nats, must have been something in the water. We have wings that have been bashed and repaired many times but never get "totalled." A virgin wing always flies better than the repaired ones. Then when we put up a "virgin" it seems to get centerpunched and destroyed and we're back to the beaters! We had four virgins with broken spars at Nats, two of them were a minute into their first combat heat! ARRRRGH. one of them looks like it was hit with an RPG and has both spars broken, not sure if that one will be repairable. that's a winter project. the rest I TRIED to repair and they, too, are targetd for use this weekend! EN GARDE
sgilkey
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Post by sgilkey »

Well, I have an actual contest under my belt with some of these "stub spar" repairs and the results are mixed. Much to my surprise a VERY BATTERED wing can be straightened and fly GREAT after repairs. I've repaired four such wings so far and been amazed by the good flying qualities. On the other hand, the repair is not sufficiently durable. A relatively minor midair will break the stub spar loose from the foam. This is not in a case where the original spar was broken by a prop or point impact- these are cases where antire chunks of foam and spar were broken out of a wing. I repaired with two stub spars each top and bottom, with about four inches overlap to the original spar on each end, and a block of foam PUd in and later carved to shape. This kind of repair leaves too many voids, and the PU glue is not strong enough in shear, resulting in the repair breaking loose of the foam core.

I think Craig's suggestion of flat carbon strips would help here. Also, I plan to use much longer stub spars, to try to get at least 8-10 inches of overlap to the original spar, and make sure they are anchored in good, void-free foam.

Yes this does add a bit of weight but I have seen no noticeable degradation in flight performance.
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Air Scharnell
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Post by Air Scharnell »

Nothing fly's as well as a NEW wing... and doesnt quite fly the same after repairs....[:(!]

My question for you guys with your repairs is ?
Which brand of poly glue are you using ?
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boiler
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Post by boiler »

I've been using Elmer's since I can find it cheaper than Tightbond which was my source a couple of years ago.
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lightning
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Post by lightning »

I stocked up with a few 16oz bottles of the Elmers "Ultimate" P/U glue before they changed the shape of the bottle, cut the volume in half but left the price almost the same!!
I splice spars with a 12" length of rod alongside the break. I always scuff the piece with sandpaper before installation. (I chuck it in a drill and hold a piece of sandpaper around it while running the drill motor). That seems to make a huge difference to the glue adhesion.
I then put the wing back in some shucks and weigh it down with bricks or paint cans or other heavy items to hold it straight while setting up.
Most wings are OK but if it ain't straight when it comes out, it goes in the trash!
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boiler
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Post by boiler »

I usually use scrap used spars to splice with. That way I don't need to scuff them up. I only scuff virgin rods when I run out of recycled stuff.
Fred420
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Post by Fred420 »

"We too had many virgin wings centerpunched at Nats, must have been something in the water. We have wings that have been bashed and repaired many times but never get "totalled." We had four virgins with broken spars at Nats, two of them were a minute into their first combat heat! ARRRRGH. one of them looks like it was hit with an RPG and has both spars broken, not sure if that one will be repairable."

Hey Scott ........... I was guilty of 2 [8D]

[:D] I was that RPG, but I got the cut [;)]
I also turned my fusalge into a "L" shape. A complete right angle! [B)]
sgilkey
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Post by sgilkey »

"Sparky" RPG!!! THAT wing was actually subsequently repaired and was PILE DRIVEN not 30 seconds into the first heat at our (RCCD) meet, Brian caught it on video and if I ever figure out how to post it I will, it's cool! I then repaired it AGAIN and it suffered the flaccid wing syndrome described above- stub spar breaking loose of foam. I guess that wing is JINXED!!!!!!!
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