So, the saga of the canopy fit continues. Yesterday I got in the special drill bits I’d ordered to open up the anchor block hole a bit. Good thing I ordered a couple sizes, because the amount the hole needed to be opened was more than I anticipated. But after a couple runs, the pin now fits in snugly, so all’s well with the anchor block.
Next up was continuing to consider the fitment interaction between the frame, anchor block, rail, and slide block. Taking care of the anchor block removed one variable from this scenario, but that still leaves two: the position of the hole in the slide block, and the position of the slide rail itself. Initially I was focusing on the rail itself. This piece basically has two “points” of contact with the fuselage, which in turn control its location. The first “point” is where the straight portion of the rail rests on top of the turtledeck. The second is where the bevel at the forward end contacts down inside the recess where the canopy drops in.
The goal I was going for here was to get the right gap between the canopy frame bracket and the slide rail – after all, in practice the slide block is what connects these two piece together, and it has to fit in that gap. To start with the gap definitely seemed too small, and the most obvious way to tweak this is to remove more material from the beveled area of the slide rail; doing this allows the rail to move further aft and increase the gap. So I filed some more material off the rail and fit it again, but couldn’t tell very well just how much the gap had changed. The reality is that it’s pretty hard to find an effective way to measure the gap in here.
So I decided instead to try something practical – I put the slide block on the rail, and figured I’d see if I could slide it in place between the rail and the frame. And while the gap looked on the small side, the block actually slid in place OK, though the fit was pretty snug. At this point, I decided to continue with this practical idea – if I could go ahead and drill the slide block, I’d remove another variable and be able to work just with the slide rail, and maybe even get everything finalized.
What I decided to do was to lay out the hold in the block such that the canopy frame sat as low as possible, so I measured the clearance between the pin hole in the frame and the edge of the bracket, and used that to work out the vertical positioning of the hole. For the horizontal positioning, I just put it on the centerline front-to-back. Then I went to the drill press and made the holes, before heading over to the fuselage to try a fit.
Unfortunately, I couldn’t get the pin to go through – the resulting hole in the block was a bit too low. There were a couple factors here – first, I probably should have given myself a little more margin for error when laying out the holes. But more importantly, when drilling the block, I should have put a wood block in the open center to help support things. With no support, the slide block wanted to deform a bit when I drilled, and I think that contributed to the holes wandering a bit too.
I didn’t quit at this point, though…I decided to see if I could work out a way to make this thing fit. It was probably already junked anyway, so I didn’t have a lot to lose. First idea was to open the hole in the block up slightly to #10 instead of #12…maybe that would give me just enough extra wiggle room to get the pin through. I reasoned that some slop here wasn’t a problem since this interface only locates the canopy frame when it’s open. Lo and behold, the slightly larger hole allowed me to get the pin through…but it took a good bit of encouragement. It was just occurring to me that this was still way too snug to work in the real world when I spotted another problem – with the hole in the block so low, the tip of the anchor pin was dragging on the slide rail. That took care of any brief thought I might have had to open the hole up even more – I’m not going to want that pin marking up the slide rail for the plane’s entire life.
So it’s time for another parts order from Van’s, I suppose. As per usual, the part itself is pretty cheap, it’s just the shipping cost and the waiting that’s mildly annoying. Then again, I’ve got other stuff to work on around the house this weekend so maybe it’s not that much of a loss.