Sun May 30 2010

Fitting canopy frame to fuselage

In preparation for fitting the canopy frame to the fuselage, I drilled out the eight rivets attaching the F-774? top skin to the F-706 bulkhead.

...and riveted the spacer plate to the canopy frame support channel.

Here's the canopy frame assembly resting in place. It looks cool!

Once I centered the canopy frame and marked the position of the brackets, it didn't take long to drill through the pilot holes into the F-706 bulkhead with a #30 bit. Those #30 holes were then drilled up to 1/4 inch independently (bracket drilled with drill press).

Here's how the right bracket assembly looks with the bolts in position. Note that the outboard bracket isn't bumping into the skin as shown on the plans. This is because I'm planning on using Sikaflex adhesive rather than screws to attach the canopy to the canopy frame. To use this adhesive you need a 1/8 inch gap (minimum) between the plexiglass and the structure which means you need to make the canopy frame at least 1/4 inch narrower than the plans call for. Despite careful measurements, my frame ended up being 3/8 inch narrower. After doing a lot of reading, it seems to be quite common for the frame to get wider or narrower during the riveting process. Oh well.

Since the brackets are somewhat inboard of the plans dimension, I needed to fabricate some spacers to take up the gap between the outboard surface of the bracket and the skin. I made these two spacers from some of the leftover 3/16 thick angle.

Here's the left spacer resting just forward of the gap it's going to fill. Notice however that the skin isn't parallel with the bracket in either the forward / aft direction or the up / down direction? basically means this spacer needs thinned out in two dimensions. I carefully measured the gap between the skin and the bracket at all four courners using feeler gauges and marked these dimensions on the back side of the spacer (which needs to remain flat).

I thought it was going to be a tremendous amount of work to thin these out correctly, but it wasn't too bad at all. I just used my disc sander and some sticks to apply pressure near the various corners of the spacer. This preferentially removed material where the pressure was being applied.

Let me tell ya...those little spacers got hot! Here's one which I'm dipping in a cup of cooling water. You might notice the water boiling around the part and steam rolling off. Although these spacers started as 6061-T6, I'm pretty sure they're 6061-O, now. :-)

Here's one of the spacers after numerous cycles of sanding, cooling, and measuring. I went through two sanding discs to make these spacers.

...and this is how they fit. I couldn't be happier with the way they turned out.

With the spacer in position, I drilled through the pilot holes in the skin through the spacer, the bracket, and the canopy brace. Note that I'm holding the drill perpendicular to the skin so the AN509 flat head screws will be nice and flush when installed.

...both holes pilot drilled to #40. These were subsequently drilled up to #29 in preparation for being tapped with an 8-32 thread.

While the canopy brace was in position, I drilled through the pilot holes into the F-706 bulkhead.

Likewise, I drilled through the side brackets on the aft side of the canopy brace into the sides of the support channel. I used my 12" long drill bit for these holes

Upon disassembly I could see that the lower of the screw holes isn't going to hold on to much of the canopy brace.

After removal of the canopy brace assembly, I drilled the holes in the skin up to #19, deburred them, and dimpled for the AN5098R8 screws.

...the spacers needed drilled to #19 and countersunk to accept the dimples.

I then temporarily placed the brackets back in position and tapped them with an 8-32 thread. Why did I reinstall them before tapping the holes, you ask? It's because I wanted to make sure I held the tap perpendicular to the surface of the skin, and with the brackets mounted in the plane, I could use the reflection of the tap in the skin to ensure I was tapping straight.