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spar attach plates

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Posted

The holes in the spar around the forward attach plates (where the attach pin goes into the fuse. attach fitting) are worn on my KFIII. The steel re-enforcement plates (WFT-14) are still tight it's just the hole in the alum. spar. Any thoughts on how concerned I should be about about this ? Are the loads being transfered to the spar by the steel plates ? I cant see how a hole in the alum spar could be expected to hold up very long anyway. Any thoughts on how the structure works in this area?

thanks,

EG

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Posted

The holes in the spar around the forward attach plates (where the attach pin goes into the fuse. attach fitting) are worn on my KFIII. The steel re-enforcement plates (WFT-14) are still tight it's just the hole in the alum. spar. Any thoughts on how concerned I should be about about this ? Are the loads being transfered to the spar by the steel plates ? I cant see how a hole in the alum spar could be expected to hold up very long anyway. Any thoughts on how the structure works in this area?

thanks,

EG

If the aluminum is worn, I would think that would be a sign of the wing working back and forth. The only way for it to do this is if the rivets that hold the steel reinforcement straps in place are loose and alowing the steep straps to move around. I would swing the wing back and see if you can wiggle the straps in and out or side to side.

I think that Blue Sky Aviation used to make a spar insert that was basically a round steel tube that would slip inside the spar and had the "tangs" protruding from one end for the pin to slip through. This was a great fix for problems just like yours.

If your reinforcement straps are loose you will need to drill the rivets out, re-prep the area, put some structural adhesive on it and rivet it back in place. This will tighten it up and make your problem go away.

:BC:

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Posted

Thanks. I spoke with the folks at Blue Sky, they dont make the inserts anymore because there was a lack of demand. They were very nice and described the insert. It sounds like a good machine shop could turn some out. I like the idea of the inserts, they would increase the spar strength in this area. I'm not so warm and fuzzy about all the loads being transfered to the spar by four ss pop rivets although it obviously hasnt been a problem.

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Posted

Four rivets? I went out and checked the doublers that I removed when I redid the C model wings on my Avid,& on the large upper doubler, there are nine(9) rivets holding them in place along with the adhesive. The top doublers for the front spar are about 5"long and 1 1/2" wide at the outer end and 3/4" wide at the inner end. The lower doubler is 3 3/4" long, 1" wide at the outer end and 5/8" wide at the inner end. The lower doubler has 4 rivets. On a new doubler I have there are only four rivet holes pre-drilled on the inner end and none on the outer end. The ones that were removed have the same 4 drilled on the inner end, but six more on the outer wider end.

I wonder if on your wings, all the rivets were installed that were supposed to be installed? Are the doublers that are installed in your wings about the same size as the ones I mentioned above? If they're the same size and only have the four rivets installed, there could be a bunch of force being applied on them that isn't meant to be, or the design on the Kitfox is entirely different than the Avid.

I hope this helps,

Jack

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Posted

Jack (and EG)-

Here are the attach bracket drawings from my KF-III and Avid+ Manuals for comparison. 1050 vs. 1250lb. M.T.O.W. ratings respectively.

-Doug

post-53-13163506485476_thumb.jpg

post-53-13163506868343_thumb.jpg

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Posted

Thanks Doug.

Jack

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Posted

just another thought. I had one wing not quite where I wanted it so to relocate the sweep- very minutly probably not necessary but I'm being a bit anal, I filed out the hole oversized very carefully and sleeved it with some 4130 tubing it goes all the way through the aluminum spar and steel reinforcment brackets sticks out on both sides just a bit then with the wings in place and pined throught this new sleeve which was tapped in tightly I carefully applied some hysol with a tooth pick inside and all around the top til it tapered nicely into the spar, after it dried I applied more hysol inside made for a real nice fit and now the pin will have to wear on a 1/4" +- long steel sleeve and not dirrectly on the aluminum. I also have the 4 rivet reinforcment plates on my bird. good luck

 

-Robert-

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