Sun Mar 15 2009

Fitting F-695 gussets

Today I continued working on fitting the F-695 gussets. I noted that the flange of the gusset does not lay flat along the length of the longeron. That makes sense due to the fact that the longeron is twisted in this area. I went to work whacking the flange in various spots with a mallet to get this flange to lay flat, and the main portion of the gusset to be parallel with the firewall angle.

While working on the flange, I noticed that the longeron was being held away from the weldment at the forward end by a slight interference with the vertical firewall stiffener. So I unclecoed the firewall stiffener and filed a chamfer on the corner of the stiffener.

Much better. Now the longeron fits tight against the weldment flange.

I also noted that the gusset wasn't cut at quite the correct angle. Notice the gap between the firewall and the gusset on the left side?

Not anymore! :-) A little sanding and filing took care of that issue.

I've still got this problem with the main part of the gusset not being parallel to the horizontal firewall stiffener. Actually there's another problem. Besides the non-parallelism, I've got a gap between these two components. It looks to be roughly 3/64 inch at the forward edge and opens to about 1/8 inch at the aft edge of the stiffener.

After quite a bit of work, I was able to file down a spacer that filled in the gap quite nicely.

Next I clamped the gusset in position and marked lines at some of the critical interfaces. Specifically, I wanted to ensure I had adequate edge distance at the forward end of the longeron, the aft end of the weldment, and the aft end of the gusset. I also wanted to ensure I didn't have issues squeezing or bucking rivets that attach to the firewall stiffener. A portion of this gusset is covered by the horizontal flange of the weldment.

As I type this, I think I need to check for adequate spacing of the forward most rivet in relation to the flange of the weldment. It has a bend at this location and I don't want to be drilling into that bend.

The hole pattern I ended up with is 3/8 inch spacing along the forward edge as indicated in the plans. The side uses 1/2 inch spacing for the forward 10 rivets, but the aft 4 rivets use roughly 27/64 inch spacing.

After clamping the gusset in position and match drilling the first hole through the longeron and the weldment, I decided to check how it came out before drilling the rest. I'm glad I did because the edge distance is about 1/32 inch too small. Crap. Ok...let's see why this is...

The plans indicated to drill the holes 7/16 from the edge of the gusset. This means they will be 7/16 (.438) from the vertex of the longeron. Subtracting 1/8 for the thickness of the longeron and .073 for the thickness of the weldment flange (I measured it with calipers) gives an edge distance of .240 which would be .010 under the minimum allowable edge distance already. I had spread the flanges by probably .015 to address this interference, so that just made it worse. Gonna have to call Van's tomorrow to discuss.