Monday, June 14, 2010

Searching for a mate

There are so many nerve-wracking parts of building an RV-7A, that it's hard to pick out the worst. I suppose it depends on what nerve-wracking portion of the project you're on at a moment.

At the moment, I'm revisitng setting the wings for their eternal life mated to the fuselage.

You may recall I did this a few years ago, but screwed up the right wing, by violating the requirement that there be at least 5/8" of edge distance between the center of the hole for the bolt that attaches the rear spar of the wings to a "fork" sticking out of the fuselage. I had to rebuild the entire rear spar and part of the fork.

The left wing at the time was installed correctly.

So this week, I'm reattaching the right wing and setting the "angle of incidence". But I'm limited because (a) the left wing is already drilled (b) One hole on the "fork" is already drilled so I have limited ability to move the wing up and down without -- again -- violating the edge distance and (c) I can't seem to get both sides of the fuselage (as measured along the side canopy skirts) to be level. Side-to-side, I'm good. But on the left side of the fuselage, my Smart Level says it's at .1 degrees out of level and on the right side, it says I'm about .3 degrees out of level.

If I were attaching both of these wings, I'd simply set the angle of incidence for both at .2 degrees (in the Van's kit, you simply place a level along the wing cord, raise the end of it three inches above the rivet line and place the Smart Level atop that.

Because the left wing incidence was already set -- and is not bolted for eternity -- I'm pretty much committed to what it says. Tonight, it said .2 degrees. Unfortunately, were I to raise the right wing incidence to .2 degrees (instead of the .3 to .4 range I'm getting now), I get uncomfortably close to the "no man's land" that I drew on the rear spar, now visible through the holes.

The rest of the wing is fine...there's absolutely no forward sweep, and there doesn't appear to be any twist in the wings, or -- if there is -- it appears to be the same on both. And triangulating the wings from a fixed point on the aft fuselage leads EXACTLY the same measurement to the tips of both wings. Great!

So, it seems clear that there's going to have to be a .1 degree difference between the left and right wing angle of incidence.

My question? What does that do to the handling of the airplane in the air?

Nerve-wracking, ain't it? I spent two hours tonight just measuring, measuring some more, comparing, measuring some more, and measuring some more before I gave up for the night.

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