Mon Dec 1 2008

Fabricating F-796-B, C, & D

First thing on tap for tonight was to drill the F-902s to the longerons. That's what the instructions say to do anyway. I found the drawing of that showed an AN470AD4 rivet going through the lower longeron, but it took me abunch of time to find the callout for the upper longeron. I finally found this note that says to wait until the sub-panel structure is drilled. So I decided to just skip the drilling operation all together for now.

As far as I could tell, there are no dimensions called out for the hole location through the lower longeron and into the bottom flange of the F-902, so I just used the F-902 from the opposite side, lined it up with this one, traced the flange...

...and drilled right in the center. I think it came out quite well.

Next it was time to work on the F-796C and F-796D shims. The former are .063 thick while the latter are .032 thick. The idea here is that the .032 thickness of the F-902 flange plus one F-796C (.063) plus one F-796D (.032) will add up to the .125 thickness of the lower and auxiliary longerons.

If Vans included material specifically for these shims (they probably did), I couldn't find it so I just used some scrap I had laying around. The .063 stock is actually from one of the bellcrank halves that I mis-drilled a while back. After trimming it to 1/2 inch wide, I slid one of the pieces in behind the F-902 and marked at the top and bottom edges of the F-902 flange.

Some time later I had these. Note I didn't really follow the plans here. I figured it'd be better to have the ends angled to match the flange of the F-902.

Then it was on to working on the F-796B angle. I used my angle transfer tool to match the angle between the lower longeron and the F-902 bulkhead.

Looks like 2 degrees. Cool.

Same process repeated for the angle between the auxiliary longeron and the F-902. This time it's 10.5 degrees.

After some judicious trimming and filing, the part fit quite nicely.

I decided to mark through the prepunched rivet holes, but had to sharpen my Sharpie on the scotchbrite wheel so the tip would fit through the 3/32 diameter holes.

Well the 15/32 dimension looks good, but at the top edge, I've got 5/32 instead of the recommmended 7/32. I think I'll check with Van's tomorrow. I could probably file the hole somewhat that goes through the auxiliary longeron, but for a #10 bolt, the minimum edge distance is 3/8 inch which means it really needs to be drilled exactly on the centerline of the auxiliary longeron. I suspect the folks at Van's will say to keep the bolt on center of the auxiliary longeron and just live with this edge distance as that seems the more critical part.

Actually, having this hole at even 7/32 violates the minimum edge distance rule anyway. If the hole is on the centerline of the auxiliary longeron (3/8 from the edge) and the flange of the longeron is 1/8 thick, then that puts this edge distance at 1/4 inch (measured perpendicular to the edge). Since we're at a 10.5 degree angle, the dimension would be .250/cos(10.5)-15/32*tan(10.5)=.1674 This is just over 5/32, so I think my parts are right on the money.

Update: I called Van's and talked to Ken. His answer was predictable..."build on" that's exactly what I did.