February 26, 2010

Cabin Frame continued - 6.0 hrs.

I had an opportunity today to put in some good time and get a lot done. Now that the cabin frame is fitted in place, the brackets that will hold it down need to be drilled to the seatback bulkhead. Then, there are other bolts that need to be drilled and installed. This can be a troublesome area if you don't take the time and care to get it right.

I scratched my head and looked at other builders sites online, trying to figure out the best way to do this. Vans instructions here aren't very detailed at all. The idea is to line up the roll bar with the side edges of the top forward skins, as seen above. So I'm using a scrap piece of channel aluminum laid across the fuselage to line it all up (you could use an aluminum angle, or whatever else you have for a straightedge). Then, the angle brackets can be clamped down to the F-705 bulkhead in the position they need to be in. The problem is, how do you drill it to the bulkhead? The roll bar needs to be removed in order to drill because it's in the way. But to remove the roll bar, the brackets need to be unclamped and the brackets will move out of position. How do you locate the brackets and clamp them down for drilling and then get the roll bar out of the way so you can drill, without moving them? That's the problem. I think I came up with a pretty good method.

First, I took it all apart. Then I put down a wide piece of masking tape on top of the bulkhead. Then I put the brackets and roll bar back on, used the aluminum channel to line it all up, and clamped the brackets down to the bulkhead and to the roll bar. When everything was positioned right where I wanted it, I took a pen and marked on the masking tape an outline of where the bracket sits. This locates its exact position, and allowed me to remove the roll bar and then re-clamp the bracket down exactly in the same position for drilling. If you look closely above, you can see how I marked on the masking tape the position of the bracket. This is the left side. Below is the right side.

Then I unclamped everything, took the roll bar off, and clamped the brackets back down for drilling.

In this picture you can see the bracket positioned where I had marked it on the tape, and drilled it to the bulkhead. Here, it's drilled to #30. I started with #40, then drilled to #30, then gradually enlarged the holes to 1/4" for the bolts that hold it in place.

Here's another shot with the bracket removed after drilling it to #30, showing how the masking tape marking method worked. It worked very well, and I would highly recommend doing it this way. One thing you might notice from the lines on the tape and the picture above this one, is that when I put the keeper rivets in and riveted these two brackets together, I didn't get the edges of them exactly aligned with each other. Later, I filed them down a bit to smooth the edges and make them look nicer, and then painted them again. But the important thing for now was to get these holes drilled accurately.

Next, the outboard face of this bracket overhangs the side of the fuselage a bit, and it needs to be filed down to fit the curvature of the longeron and the side skin of the fuselage. It's important to make it fit because the skin covers this up and it needs to be flush. It took quite a while and a lot of filing to get both sides done, but here it is.

Then, it's time to bolt the bracket back in place so you can drill the rest of the holes and finish the rest of the installation. (NOTE: A friend of mine wrote me concerned that I put the bolts in upside down in the picture above. Standard aircraft procedure is to put bolts like this in place pointed down, with washers and nuts on the bottom. The idea is, if it vibrates loose and the nut falls off, at least the bolt will still be in place to hold things together until you get it fixed. I knew that. However, just this time, since it's temporary, I did it this way. It all has to come on and off a few times before it's installed permanently. And it's just so much easier to work with when the nuts and washers are on top where you can see them. You will also note that I used several washers on the bolt under the bulkhead and two on top, so the bolt could be tightened temporarily without getting into the nylock part of the nut (the red part). I've done this throughout the project where I have a part like this that will go on and come off several times before being installed permanently. Again, standard procedure is to have two or three threads showing beyond the end of the nut. I'm obviously way short here. I'm just trying to save some hardware and some time while I work on these parts.) I just wanted to clarify all that, so I'm not a bad example to anyone. Believe me, my final installation will have these bolts pointed down, with 2 or 3 threads extending beyond the nut, and everything properly torqued.

With the brackets bolted in place, the roll bar is put back on so that these 4 holes can be drilled to the roll bar. You have to study the drawings carefully here, since they're not all the same size. The two aft holes have countersunk bolts held with nuts, that will be covered up by the side skin and not seen at all. But the two forward holes are tapped and threaded in place through the side skin. There are no nuts inside.

After making some HUGE countersinks and drilling to the proper size hole, the two forward holes are tapped. The skin that covers this needs to be drilled out to #19 and dimpled for a #8 screw for these two holes.

Finally, on the inboard side of the roll bar, there are these two holes that need to be drilled out for bolts, nuts and washers. The angle on the bracket inside the roll bar has two pilot holes drilled when they were fabricated. I used my right-angle drill with a short bit to reach in and drill through the brackets and the roll bar, using those holes as a drill guide, and then enlarge the holes from this side to the right size for the bolts that will be installed here. In the upper right corner of this picture, you can see the big dimples in the side skin for the #8 screws that will be installed when this skin is riveted on for good. I checked it all for fit, and I'm very pleased with how everything turned out.

Once everything was taken apart, cleaned up, deburred, and re-painted, I bolted the brackets down to the bulkhead for good (properly pointed down) and torqued them in place. I won't install the roll bar permanently at this time, but I see no reason not to put the brackets on for good.

Then, I put everything back together. It's just clecoed on for now, but I wanted to see how it looks.

One more thing was on my mind. I pulled out my seat frames and installed them. I have read too many stories of interference, with the seat backs hitting on the frame. So I put mine in and checked it out. NO PROBLEMS! No clearance issues at all. I'm really happy!

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Contact me: swayze "at" europa.com (replace "at" with the @ sign... no spaces... you know the deal)