It defeated me. I admit it. Total and unconditional surrender - how embarrassing!
What happened?
After a few days of letting the left longeron sit I came back to the shop tonight and I checked the fit of the Big Bend yet again. It still looked decent enough to get accepted by my Quality Control Department and so I moved on to the last step of removing some obsolete metal at the aft area and bending the last 6" of the longeron inboards by 4 degrees.
Finally I had to verify that the upper flange of the angle is still close to an even plane - it wasn't. So I adjusted the heavily downward pointing tail to get back to the plane it was meant to be in. This reduced the angle from 4 degrees to just around 2 degrees. To make this story a bit shorter, I went back and forth until I achieved the desired 4 degree inboard angle as well as the upper flange still being part of the plane.
This wasn't the only part of the upper flange that needed adjustment. So I went along and straightened the slightly warped plane along the upper flange. Once I was happy I went back and checked a final time if my template curve still matched my Big Bend...
It did not! In fact it was way out of the shape it was supposed to be in. It is obvious that the flanges of the angle are mechanically very closely coupled. Any bend that you introduce on one side, has an effect on the adjacent flange and vice versa. It is also coupled when you straighten one of the flanges, meaning that it will deform the adjacent side again.
This is getting ridiculous now as I am already working for many days on just one longeron and I am almost back to where I started when the angle was straight and untouched.
What am I going to do about this before I lose it and form it to look like a nice little knot of aluminum angle? I had just recently seen a post on the VAF that talked about longeron dies that someone made to help getting this bend done. They are CNC machined from aluminum and cost around $36 including shipping. These are supposed to help bending the longerons laterally while dampening the undesired response in the vertical plane. It appears that this helped some people and I am willing to wait for them to arrive before going back to this task.
So for this weekend I'll try to learn something else than smithing. It's time to learn plumbing as I will start section 28 which requires a lot of precise tube bending which I haven't done ever. At least it won't require a mallet, so I guess this is an improvement, right?
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The irritating thing is that these would be super easy for Van's to do at the factory, and given how critical the fit is, it seems like just the kind of thing they should put some effort into.
ReplyDeleteInstead of, for example, dimpling a row of holes that I could easily dimple myself, and insult to injury, a set of holes that need to be undimpled so I can break the edge correctly.
Sometimes I just don't understand their thought processes.
I totally agree with you, Dave. This just doesn't make a lot of sense. I know, dimpling the wing doubler when they shouldn't have and then leaving the longerons untouched. Almost stupid. If pre-bent longerons were at least an option for ELSA builders like the fuel tank is. I'd be happy to pay a little extra for the convenience of perfectly fitting longerons...
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