Bolt Hole Minimum Distance

[Oregonfox]

Happy Holidays!

Is there a standard bolt hole distance from the edge on steel for the KitFox. When installing the left horizontal tail strut the lower end is too long for the tab on the fuselage. We’ve filed away the powder coat on the fuselage to see if that helped, but the strut tab is still too long. I have the strut with the proper side up and streamlined. Before I start filing the strut tab to fit I thought I’d ask. From searching the web I find minimums of 2x to as small as 1.5x the nominal bolt hole diameter distance between center of the bolt hole to the plate edge. The right strut fits well.

Apologies as the photo is sideways.

Thanks ahead,

René
Oregon Fox
SS7

[Dave S]

Rene,

Just a few thoughts

2X seems to be the industry standard for edge distance, also reflected in FAA documents, usually with respect to rivets. I think that can be agreed on, although there are some lesser amounts that have been used as you indicated.

One point to consider is the over-reach does not affect the lower tab, just the upper tab.

Another consideration is that normally where three edges (through a 180 degree arc) are considered, there is little reason to have any of the three edges wider than the least of the three - like the sides compared to the end of the tab - the closest edge is the weakest link anyway. If the sides are less than 2X, simply radiusing the tab to agree might be an answer.

Others with a more extensive engineering background and kitfox experience might be able to provide a stress analysis and determine if the part would perform ok with a little addittional material removed, beyond the 2X factor.

Any doubts, I would call up Kitfox, I have always found they are the experts on design and tolerances of the aircraft.

[Oregonfox]

Thank you for the information. The part came from the factory already drilled at slightly less than 2x and the right strut that does fit has the bolt hole closer to the edge. I’m hoping to find out if there is a minimum that I may file the upper tab to. KitFox is closed for a well deserved rest for the holidays.

Rene,

Just a few thoughts

2X seems to be the industry standard for edge distance, also reflected in FAA documents, usually with respect to rivets. I think that can be agreed on, although there are some lesser amounts that have been used as you indicated.

One point to consider is the over-reach does not affect the lower tab, just the upper tab.

Another consideration is that normally where three edges (through a 180 degree arc) are considered, there is little reason to have any of the three edges wider than the least of the three - like the sides compared to the end of the tab - the closest edge is the weakest link anyway. If the sides are less than 2X, simply radiusing the tab to agree might be an answer.

Others with a more extensive engineering background and kitfox experience might be able to provide a stress analysis and determine if the part would perform ok with a little addittional material removed, beyond the 2X factor.

Any doubts, I would call up Kitfox, I have always found they are the experts on design and tolerances of the aircraft.

[109JB]

Can you bend the tab slightly to conform to the mating piece. Then you maintain the edge distance. That said, I believe the original builder of my model 4 did similar on some parts and about 800 hours later it is none the worse for wear.

[MFleming]

There must be someone who had difficulty with the stabilizer lower brace tabs not fitting!?

[Delta Whisky]

I'm don't remember exactly where but in more than one place during the build I ran into a problem just like this one. In my case I ground off just enough material to eliminate the interference. In the case of a horizontal tail strut, in most of its life it will be loaded in compression. Break out or bearing loads in the tensile condition should be relatively small. Running even a really conservative calculation (left to the reader) the result should support the minimum condition of 1.5 times the diameter of the hole to the edge of the part (around the entire periphery). If in any particular case there isn't a 1.5 diameter available, check out: https://www.machinedesign.com/fasten...tearout-stress for the formulas of interest to see if the particular thicknesses of a lift strut and its assumed load will be sufficient. Check out https://www.azom.com/article.aspx?ArticleID=6742 for the values commonly used in design calculations for 4130 material.

[Pushboy]

I had a similar problem as the OP.
I had already filed mine when I spoke with the factory about a few this and a few other issues. They told me that a small amount of filing or grinding the tab on the support was ok provided the edge distance didn’t get too small, but they didn’t say what too small was.

In my case, I found the least interference by switching the struts left to right, then I measured the smallest hole edge distance of all 4 holes and didn’t grind smaller than that. That gave my situation a good fit, but… every plane is different.



After touching up the grinds with epoxy primer.

Good luck,

Dave.


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