Tue Mar 20 2007

Re-leveling wings in jigs

After much tweaking, this end of the spar looks quite level.

I discovered that in spite of having a support under the center of the rear spar, I still had sag in the main spar. Sure the middle was supported, but from the middle towards either end of the spar, there was a definite U-shape. I found a nice straight piece of aluminum extrusion and laid it on the spar to illustrate the problem. Here you can see that the extrusion is touching the spar at the outboard end.

...also touching at the midpoint of the spar (right over the center support)...

But in between the center support and the outboard end, the spar is sagging about 3/64 inch. I think I'm going to add two more supports. That way I'll have them evenly spaced at the 25%, 50%, and 75% positions along the length of the spar. That should drastically cut down the deflection. If I remember some of the simple beam formulas, deflection goes down with the cube of the length of the beam. Sooo...cut the beam length in half and get 1/8 as much deflection.

I decided to make little opposing oak wedges to put under this end of the spar. The wedges worked out great and made it a lot easier to get the fine adjustment for getting everything nice and level.

Let's set the tank back on the spar and see what we've got.

...looking good so far...

WooHoo! That's about as freakin' perfect as it's gonna get. I'm soooooo happy! :-)

Here, you can see the extra supports under the rear spar. I'm not sure whether this was the key or re-leveling everything up was, but I'm gonna give the other wing jig the royal treatment like this one tomorrow. I'm so geeked! Crazy thing is, now that everything is set up perfectly, I've got to tear it all down. All the parts on this wing are ready to disassemble for deburring, dimpling, priming, etc in preparation for final assembly.