Tue Mar 23 2010

Drilling horizontal stabilizer to fuselage

After reading the instructions about how to position and drill the horizontal stabilizer to the fuselage, I was still unsure of the correct hole location. The instructions say to center the hole 5/16 inch from the edge (not vertex) of the longeron. However, since this will be drilled for an AN3 bolt, shouldn't the edge distance be 3/8 inch? I called Van's about it and spoke with Gus. He indicated that the reason for deviating from the standard 2X diameter edge distance is because the washer would interfere with the radius of the longeron. I decided to drill a piece of scrap to see how it worked out. Here's the scrap angle with two holes. One is 3/8 inch from the edge (i.e. centered on the angle) and the other is 5/16 from the edge.

With a bolt temporarily installed in the 3/8 edge distance hole, you can see that the washer doesn't sit down tightly and the bolt is tipped somewhat.

However, with the bolt installed in the 5/16 edge distance hole, everything seats down nicely. After thinking about this, it occurred to me that in the fuselage this washer won't actually be seated against the longeron. There's a doubler on the top edge of the F-710 bulkhead which rests against the underside of the horizontal leg of the longeron angle. Thus the radius won't be a problem. That said, if I use the 3/8 spacing on the longeron, the edge distance on the F-710 angle will be only 1/4 inch. So I still think it's better to use the 5/16 edge distance for the longeron. I'll end up with 5/16 edge distance on the F-710 angle as well.

It took a bit of searching to locate these spacers which go under the forward spar of the horizontal stabilizer.

After a lot of thought about how to locate the hole which will go through the lower angle of the horizontal stabilizer forward spar, the spacers, the aft deck, the longeron, and the F-710 angle I decided I needed to make a drilling jig. The idea is to have this jig fit through one of the lightening holes in the aft deck and reference off the edge of the longeron. Then it will extend above the aft deck and over the lower angle of the horizontal stabilizer forward spar where there will be a pilot hole. Confused? You'll see. Here are a couple parts. The bottom plate is 2 inch x 3.5 inch. Since I didn't have any 2 inch wide bar stock, I hacked off one flange of some 6061-T651 square corner angle I had laying around leaving the other 2 inch wide flange.

I also fabricated a 0.063 inch thick football shaped spacer to transition the drilling jig from below the aft deck to above the aft deck.

Here's the (nearly) completed drilling jig. The football shaped piece is sandwiched between the 2 inch wide plate and the 1 inch wide bar where the three rivets are. Next step is to accurately locate the pilot holes.

I started by drilling a hole in a piece of scrap angle exactly 5/16 inch from the vertex. For transferring the first hole, it only took me two tries to get the position of this hole correctly located from the vertex. For the second transfer, It took about six tries. A milling machine could have made this much easier.

...then clamped the angle to the edge of the 2 inch wide plate and also to the face of the cross bar and drilled through the pilot hole. Now the hole in the cross bar is exactly 5/16 inch from the edge of the 2 inch wide plate. The process was repeated for the other side.

Here's the completed jig. You can see the two #30 pilot holes in the cross bar.

...and here's how it works. Just reach through the lightening hole and bias the edge of the 2 inch wide plate against the edge of the longeron. The pilot hole is now positoned correctly ready for drilling. I think I'll wait until tomorrow to actually drill this though. I've got to position the horizontal stabilizer and clamp it in place beforehand anyway.

By the way, if anyone would like to use this drilling jig, you're welcome to it. Just pay for shipping both directions.