Well, it's time for another major event! Ever since the top fuselage skin was riveted in place, less than a week ago, I've been on the edge of my seat to get the rear window installed. This is the last major component of the body of the fuselage, at least until the cowling gets installed at some point. So today I began the final planning and prep work for this window. I'll be using SikaFlex, of course, as I did for the canopy. There will be no holes drilled in this piece of plexiglass either. Today is kind of a dry run, if you will, of the methods that will be used to install the window. I want to make sure that my ideas will work before actually applying product to the plexi and the skin.
The challenge with the rear window, is that with no hardware or holes drilled, you have to figure out how to apply pressure in the right places, in the right direction, to get a nice tight fit while the Sika cures. The forward edge of the window lies across the roll bar, and obviously needs to be pulled down onto it. So I devised this ratcheting strap that goes all the way around the airplane, to pull down on the forward edge of the window after the Sika is applied. The stack of white papers are there just to protect the fuse from any scratches from the ratchet hardware.
But the rest of the window needs to be "pushed" in the exact opposite direction. How are you going to do that? Well, once again, I wish I could take credit for this idea. But I shamelessly stole it from some other builders. In fact, two of my friends generously donated these "tools" to me. My friend Bruce Hill, down in Ramona, California, gave me these flexible sticks, complete with cushy foam tips taped on. Thank you, Bruce! And my local good friend, Trevor Conroy, gave me this special-fitted piece of plywood after finishing his. It has been carefully trimmed to match the shape of the longerons, and it rests on top of them. Later on, in the actual installation, I will use some clamps to hold it in position, and clamp it to the longerons. It has a slight tendency to want to slip forward, and that will keep it in place. Anyway, you can see how I screwed these small blocks in place to hold the flexible "pushers" in the right position. It doesn't take much, but these push the window up tight against the skin. It's perfect!
Here's a shot of the right side. I wanted to carefully position the window and check to make sure I didn't need to do any more final trimming or anything else. I think it's good to go!
On the outside of the window, on the top, I installed these duct tape "handles" and tested them a bit. My idea is, I will have some small spacers between the rear edge of the tipup canopy, and the window, to give me just the right gap between them. Then I can pull the window up against the spacers to get the correct final gap. Not shown here, but very important, is to carefully mark the rear edge of the window where it comes in contact with the skin. This is to know where the Sika will go, and where it won't go. I'll be scuffing up the plexi and I don't want any of that scuffing showing! You don't want to go too far forward. So I put a masking piece of electrical tape on the plexi right along the skin, leaving a very small gap for the tiny fillet I want to have when it's finished. I'm leaving hardly any of the fillet showing, probably about 1/32".
With the dry run complete, it's time to take the window out and start some prep work. I'm beginning by masking off the window everywhere that I DON'T want any Sika. Then I'll scuff up the plexi on the contact areas. You have to think carefully here. The window gets Sika on the top surface of the plexi all around the inside where it will contact the skin. But all across the top of the roll bar, the Sika is on the bottom surface of the plexi. I'm using ordinary black electrical tape for masking. I've found it to be an excellent product to use with Sika.
The lower, or inside surface of the plexi inside the cabin, will not have any Sika on it at all (except around the roll bar) and I'm going to be smoothing out a fillet to the skin, so I really taped up the window so no Sika stays on it in those areas. I'll be able to smooth a fillet and then pull the tape for a nice, clean installation.
Next, it's time to prep the roll bar. The top surface, that will have the Sika applied, has been sanded with rough sandpaper, and cleaned with acetone or MEK. You can see, it's bare aluminum. Then some masking with electrical tape has been applied. I will be smoothing out a fillet all along here, too. The tape will keep the roll bar clean, and I can pull it as soon as the fillet is smoothed out.
After the roll bar is prepped, it's time to final-prep the skin. Much of this work was done before the skin was riveted on, but at this point I'm applying electrical tape, and the skin has been scuffed up, all paint removed down to bare aluminum, and cleaned with acetone or MEK. This is now ready to go, too!
I've been thinking about some spacers, to give me the correct gap between the window and the tipup canopy's rear edge. Then it occurred to me, I'm using electrical tape that's 7 mils thick. Some simple math gives me the correct number of layers of tape to build up, to give me the 1/16 inch gap that I want. Use more or less layers for your own preference. I'll put several of these spacers around the aft edge of the tipup canopy, and just pull the window up to these points for the correct positioning. You can also see that I taped up the aft edge of the canopy, so I don't get any Sika on it.
Here's what they look like from below. Easily removable spacers that will stay put until I pull them off.
I've spent quite a bit of time on all this prep work. There's no way I'm starting on the Sika application in this session, so I'm calling it a night.