Saturday, October 12, 2013

13-02, small parts prep, Tie Downs

Well, it certainly was fun jumping in and getting to work on the wings.  That is, until I found my first blunder.

Look at the photo below. You are seeing the L upper wing tie down (tie up?).  The photo shows the effect of not establishing a good bore-sight down the predrilled hole.  I was actually at a 6° angle and drilled through the surface of the aluminum extrusion.  Unfortunately, I was not looking at the extrusion itself, as I had turned the spar over to improve access.  (I don't know if watching it would have made much difference, but I probably would have caught it earlier and been able to re-align the drill.)  The tap has been re-inserted for this photo and is just barely visible at the lower end of the hole.


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The photo below shows the 6° up angle relative to the spar surface.  (iPhones make great general purpose tools:  the current iOS has an excellent inclinometer.)

So far, I have identified 3 paths to pursue.  Easiest (and therefore of greatest suspicion) is to ignore it and use the tie down as if nothing happened.  The threads are intact along the upper (as oriented in the photo) and lateral walls of the long axial hole, but probably non-existent on the lower edge of the hole directly under the unwanted opening.  As a total WAG, I guess that there's a 30° arc x 1/2" where there are no threads.  Is that lack of contact enough to compromise the strength of that bolt?  (What is the function of that bolt, anyway?  I'm further guessing that its to suspend the wing in maintenance actions.)

2nd option is to ignore it completely. That is to say mark the tie-down as InOp and never use it.

3rd option is to re-drill with a larger bit and re-tap with larger threads.  To ensure good strength, I would tap 2" to obtain a full inch (original spec) of completely engaged threads.  Will there be sufficient aluminum to accept the larger bolt?  Hmm.  Sounds like I need an engineer. 

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On the other hand, I did get some useful things done correctly.  As is typical for the first few pages of a chapter, it was a lot of separating and preparing small parts.  Fun, mindless, productive:  all in all a good day.  (Learning how to repair mistakes is part of the goal of this project, eh? :0)


Here are the various attach angles (W-1206V x20, W-1206T x2) and the assymetric rib doublers.

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Not easy to see, but there are the match drilled holes on the L spar.

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Here are the properly drilled & tapped tie downs :-)

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