What are the thoughts on the amount of load transfered to the spars by the spar attach fittings (WFT-14 & 14 on the KFIII). The reason I ask is that the hole in the spar itself where the forward pin is inserted looks worn. The pin still fits tight in the fitting itself.
I remember reading about an insert that was available for this area but can find any information on it. Any one have ideas or suggestions.
Stainless steel does not bond well. It can be done reasonably well, if you first roughen the bonding surface. Coarse grit emery blasting works, sand does almost nothing. Coarse grit emery or aluminum oxide sandpaper is better than nothing, but leaves scratches, which are stress risers. I would suggest using 4130 steel tubing that has been roughened.
A rough surface is a rough surface, is a rough surface and introduces stress risers if you want to think of it that way... On the other hand, getting a good anchor profile for the hysol to bond to is very important.
In reality, do you honestly think that a few small scratches in the spar reinforcements for the wing attach points are going to see enough stress that it will be an issue?? Those are just there for wear points, not for any real added strength.
I don't know if this is the info you are looking for, but I am attaching a clip from a photo of our airplane after the rather violent emergency landing we made four years ago. What you will see is the right wing root and the attachment to the carry through. This was a Model IV built to the manual. Those 4130 strips, the eight SS rivets and the bit of 3M structural adhesive created a strong attachment. The break was not in the area of the bolt.
Lowell
It was the spar carry through that broke about six inches inboard on the passenger side. In answer to the tank question, you are looking at a 13 gallon tank. The rest of the carry through remained attached to the left wing, but separated from the rest of the cockpit area tubing. I reviewed the photos the other day and was quite impressed by the strength of the wing spar attachment to the fuselage. None of the four attachments separated from their immediate structural connection. Another thought; in the day, the skylight rib as well as the turtledeck attachment were attached to the respective carry through tubes with rivets rather than the Hysol only in the current plans. No separation there either, All remained attached to the carry throughs. The Aft carry through tube remained intact and attached to the fuselage structure. Why it broke where it did is anyone's guess. It broke sufficiently far from any weld, to eliminate welding embrittlement issues - who knows.
The main point in my original post with the image was to illustrate the strength of the engineered connection there between wing spar and carry through.
Lowell
Spar attach fittings
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