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.

Let me start by summing up this evening’s work: that was kind of frightening but ultimately successful.

I shared my thoughts from the last session on the forums and got some good information. Most notably, my suspicion about the anchor pin/block nesting was confirmed: the pin should be going all the way into the block, not just a little bit. That should settle my large gap issues up front…well, once I get the hole enlarged. The hole is currently 1/4”, and the next largest drill bit I have is 9/32. Checking the different drill bit diameters with my calipers, the larger one seemed like it’d be a looser fit on the pin than I liked. To verify this (well, to convince myself not to just drill the block with what I had), I drilled a 9/32” hole in a piece of hardwood I had lying around – which I figured was about the closest delrin analog I had – and test-fit it to the anchor pin. Sure enough, it had a bit of slop that I didn’t really like. On the one hand, it’s probably not super important that this be a really snug fit, but on the other hand, it’ll never be as snug as it is when I first fit things, so I’d rather go for a precise fit out of the gate. So I guess I’ll be hunting up some weird drill bit sizes. Since the pin already goes in, just really snugly, I probably only need to remove a really tiny amount of material.

Anyway, the other item of discussion was the frame misalignment. Here, no one said anything good or bad about my intent to bend the frame, which is pretty much tacit approval on the Internet. That is to say, people are really good about pointing out problems with ideas like this. So my main goal tonight (and what I actually did first, before the block stuff discussed above) was to do the bending.

I’d been thinking offhand about how to do this for a day or two, and had a pretty decent idea how to proceed. Basically, I needed to lock the forward part of the frame and then pull sideways on the back end. So I made my big table into a jig of sorts. A piece of scrap 2×8 screwed to one end of the table served as my clamp point for the forward end, with another piece of scrap used to actually clamp the frame. I drew reference marks to center the frame on the table, then drew a centerline back where the anchor pin was. I’d hoped to find that the pin was obviously off-center, which would make me feel a little better about the whole thing, but that wasn’t the case…it was pretty much dead-on, definitely not out by the ~1/2” I’d seen while fitting on the fuselage.

Here’s a look at the whole setup on the table. I’d originally intended to hand a plumb bob from the anchor pin, but I couldn’t find mine. I decided that a combination square would work just as well, so that’s what’s seen here. It’s also worth noting that, in the end, I added more clamps to restrain the front end, because it took a lot more force than I expected for this operation:

Back to the whole “not off centerline” thing: that really made me stop and think, just to make sure I wasn’t doing something horribly wrong. Eventually I reasoned that the practical fitment on the fuselage was more important than the centerline. I’d checked the thing on the fuselage multiple times, it definitely wasn’t right, and in the end it needed to fit the plane, not some marks on a table. So I drew a second line 1/2” from my centerline, to use as a reference, and got to work:

At first, I tried just doing the bending just by pulling in the frame. After all, the manual talks about correcting the bow with “carefully controlled muscle power,” and in any case starting small is a good plan. But it was clear before long that this wasn’t good enough – I got the pin to move a tiny bit, but nowhere near what was needed. OK, time to get some mechanical advantage involved, in the form of a ratchet strap. The fun part was figuring out how to rig this. I ended up finding a large eyebolt lying around, which I installed on the bottom shelf of the table, so I could take the strap over the edge and anchor it down there.

The other consideration was some way to keep tabs on how much I was moving the frame – it was highly unlikely I’d get this right on the first pull, so I needed some way to repeatably pull to where I’d stopped on a previous attempt, and then go a bit further. To handle this, I just laid a piece of thin scrap wood along the left side of the frame. As the frame was pulled sideways, it’d move the scrap wood, and that’d serve as a sort of max-deflection indicator.

Finally, it was time to start pulling. I put what seemed like a decent amount of force on the frame, then released…and the pin had barely moved. The deflection indicator came in handy here, since I could just increment my pulling by a couple teeth on the ratchet each time. All in all, it took probably five or six iterations to creep up on the deflection I needed.

The scary part was that I was putting a lot of force into the system in the end. At first this concerned me because I was sort of necessarily standing right in line with the strap – if it broke I’d probably get a nice welt, but if that eye bolt came loos somehow I’d probably be in a world of hurt. So I tried standing off to the side as much as I could, but then on the last pull I noticed how putting all that force on a single point was noticeably changing the curve of the right-side frame bow. Oh boy, if I ruin this thing it’s gonna be really expensive to get a replacement. After releasing that pull, I sighted down from the back of the frame…whew, seems to have sprung back OK.

Time to do the real check, with the frame back on the fuselage. Before removing it, I clamped blocks on the table to ensure I could put it back in the exact same place if I needed to bend more. Then it was time for another deep breath as I carried it over to the fuselage, slid the rollers back in, and closed everything up…

…

…and it fits beautifully. Not quite perfect – with the anchor pin centered, the right-side gap is still maybe 1/32” of an inch, but I’m very sure I can live with that.

That was enough fun for one night. I suppose next I’ll worry about the anchor block, then refine the center slide rail, and then see what other bending I need to do while I still have this jig setup in place. I know I need to bend the aft end of the frame down a decent amount, and I may want to spread the forward bow a bit to match the fuselage sides.

whew

So the title sort of sums up tonight’s short work session, though I feel like I’ve got an idea of how to move forward with the fitting. I’m still going to let that idea marinate a bit before I try doing anything about it (plus I’m gonna run it by the Van’s forum people). Even better, tomorrow I’m flying a Young Eagles event and I see that one of the other pilots is bringing the RV-8 he built. Hopefully he shows up so I can pin him in a corner and pepper him with questions. You know, in a friendly sort of way…

Anyway, out of the assorted issues I outlined before, I felt like the first one to really consider was the large gap between the canopy and windscreen frames. As mentioned before (I think), the limiting factor on forward movement of the frame is an anchor pin at the aft end, which fits into a plastic block that rests against the back of the bulkhead just behind the back seat. One thought I had was to remove some material from that block to allow the frame to go further forward, but it seemed like I’d need to remove a ton of material. To help visualize how things might need to change, I decided to get the anchor block out of the picture entirely and see what kind of gap I could get with the anchor pin actually contacting the bulkhead – that is, as far forward as it could physically go if I went completely bananas on shaving down the block.

This seemed promising at first – looking at the gap on the left side (from where I was working), it was almost nonexistent. Using drill bits as crude feels gauges, the gap was about 1/16”, probably tighter than I’d want in real life. But then I noticed the gap on the right side was a good bit larger. I double-checked at the aft end to see if the anchor pin was centered, and it was. Hmm, seems like this frame is a little crooked. I was eventually able to get an equal gap up front, but only by shifting the aft anchor point as far right as it could go in the socket:

So it seems clear that I’ll need to bend the frame a bit to get both an even gap up front and a centered slide out back. That’s not too awfully surprising, it’s known that these frames almost always need some tweaking to get a final fit, though I wasn’t expecting a sort of twist like this.

But back to that gap up front – with the anchor pin touching the bulkhead and shifted as seen above, the gap was a nice consistent 3/16” all around, which seems like a reasonable number…until you remember that this is without the anchor block in place. At this point I started to get a twinge in the back of my mind, as usually happens when I’m getting subconsciously close to some sort of epiphany. The epiphany completed when I took another look at the anchor block and noticed that the hole for the pin went all the way through it – this wasn’t obvious until I removed the part sticker on the front side. All this time I’d been assuming that the intent was for the pin to barely engage the block, but now I think that’s not the case.

The pin itself looks to just be some rod stock that’s tapered to a point:

When engaging it with the block as provided, it slides into the block essentially until it gets to the end of the tapered area and stops:

Notably, at this point the engagement isn’t very positive. I didn’t think much about that before, but I was assuming the whole time that the hole in the block was similarly tapered to match the end of the pin. But it turns out that the pin can be inserted further, though it’s a pretty tight fit. It can, in fact, be inserted until it’s nearly sticking out of the front end of the block:

So that all homes in on a path forward to handle the gap issue: if the pin is able to freely drop that far into the block, the whole frame goes further forward and I no longer have the massive gap up front. And instead of shaving material off that block while trying to keep it square, I’d just need to open up that hole in the block a bit. Currently it’s a nice slip fit with a 1/4” drill bit, so I imagine the anchor pin is also 1/4” OD rod, and would slip in there nicely if not for the additional thickness of the powder coating. Probably I can just get in there with the next size up of numeric drill bit and remove a tiny bit of material, maybe still leaving the origin ID towards the front end to keep the tip of the pin from contacting the bulkhead and doing damage over time.

This, again, is an idea I’m going to run by the forums, and maybe by that -8 builder if I see him tomorrow morning.

As a final note, there’s going to be more frame bending to do in addition to trying to square the thing up. While looking over the manual section that talks about “massaging” the frame, it also mentioned “possibly” needing to bend the aft most portion of the frame for proper clearance from the slide rail. A glance at the plans says the appropriate gap is 3/4 – 5/8”. Yeah, I’m going to need to do some bending there, as my gap is currently way larger than that:

And that’s without even getting to possibly needing to expand the lower forward part of the frame for proper alignment with the fuselage sides. The alignment there doesn’t look too bad offhand, but I’m going to want to add some aluminum strips as per the manual to really properly evaluate things. Bottom line, I see a lot of trial-and-error application of force in my future.

So…if I were to go by the construction manual verbatim, the next thing I’d be doing would be to locate the center slide rail on the fuselage, drill the mount holes, and bolt it in place. However, I’m hesitant to do that until I get a good sense for how the entire system works together. Of particular note here is how the center slide rail bends down at the forward end, which causes the back end of the frame to drop in place when closed and put an anchor pin into a locating block. This provides a much more precise location of the frame as compared to the somewhat loose-fitting slide block. What this all means is that, to my way of thinking, it might be necessary to work on the fore-aft location of the slide rail, to ensure that everything nests in place properly when the canopy closes.

With that in mind, today I started out by extracting the canopy frame from the finish kit crate in order to do some test fitting. That, in turn, required a bit of excavation, since the frame is at the bottom of the crate. Next, I gathered up the slide hardware, which took some time because I couldn’t find the bag with some specific screws for a while. The aft hardware is pretty simple – there’s the hard plastic slide block, which gets attached to the frame with a clevis pin. The only mild complication is that the block isn’t predrilled for the clevis pin, so that’s yet another variable in the fitment back here that I’ll need to work out. At the forward end, there are rollers that nest inside the slide channels, which then attach to mount points with screws, nuts, bushings, and spacers. Those mount pieces then nest into tubing on the frame itself, and will eventually be bolted in place there.

With all that stuff assembled, I got the canopy frame in place and slide closed. For now the slide channels are just resting in place on the cockpit rails; refining their position will be part of the larger fitting job. Similarly, the center rail is just sitting on top of the fuselage.

So far, this operation has posed more questions than it’s answered. For one thing, with all this stuff in place, I don’t feel like the canopy frame is able to go far enough forward. I don’t think it’s supposed to actually contact the windscreens support, but it currently sits about 3/4” apart when closed. Later on in the construction manual, when discussing laying out the cut to separate the provided canopy bubble into the windscreen and sliding canopy sections, it suggests trapping a sheet of cardboard between these two pieces to provide a straight cut line. That implies to me that these pieces should sit a lot closer together…but the only way to make that happen would be to modify either the anchor pin on the frame or the anchor block it fits into.

The other question concerns drilling that hole in the slide block. I haven’t been able to find any sort of guidance on where or how to locate that hole. This seems important too, since if it locates the frame too far from the rail, the anchor pin won’t be able to engage in the block. And of course for this whole operation, the actual position of the slide rail figures in.

So I called it a night on that somewhat unsatisfying note. At the moment, based on some fiddling, I think  I want to be able to move the slide rail aft a bit from where it’s naturally resting right now. This should be achievable with some more filing on the relief on the forward end, but I’m definitely going to be thinking things over before I do anything of the sort.

Picked right back up on the rail this evening. I got to redo all the fun layout work I did the first time around – ok, it really wasn’t all that bad, but it’s still something like 50 holes to mark, punch, and drill on centerline, while keeping stuff aligned. Drilling the holes with the drill press was definitely slower than doing that same work by hand, but the results were much better. After getting all the deburring and countersinking done, I finished by riveting the cap and rail to make the finished product.

Next I get to mark a centerline on the fuselage and get the mounting holes drilled so this can be screwed in. Then I guess I get to start having fun with the canopy frame itself. For now, I just laid the rail on the fuselage for a photo op, with the slider block in place for good measure:

A week after getting back from vacation and I’m still trying to get back int he swing of things. I did have my firewall forward kit come in while I was gone, and a neighbor was nice enough to grab the box and hold it until we got back in town. Yesterday I cracked open the box and admired the contents a bit. I probably ought to do an actual inventory but I wasn’t really feeling like counting rivets and stuff at the time.

I also got in my replacement parts for the canopy rail I bother – the day we left. So they’ve just been sitting around waiting for me. Today I got them out and started getting to it, though I didn’t actually achieve much. I marked off and made the relief at the forward end of the rail, then went inside to cool off a bit (it got up to near 90° today…yay Houston) before starting to bend the sheet strip that forms the rail cap.

In the meantime, it came to pass that we finally were ready to fly the Helio Courier I’ve been working with for the past few months, so I ended up going over and acting as copilot for that whole evolution. So no more rail work, but I got to fly in a cool old plane at sunset, which seems kinda like a worthwhile tradeoff.