Sun Oct 5 2008

Front fuselage and firewall

Today I decided to throw caution to the wind and try using a Unibit to drill through firewall for the lower port on the brake fluid reservoir. It worked great. Using low pressure and high speed, the hole came out nice and clean. I was even able to correct for a slightly off center pilot hole with a little side force on the unibit.

To clean up the chips, my dad recommended using a magnet. Whatever this alloy of stainless is, it is slightly magnetic. I took another suggestion of my dad's and used a tissue to cover the magnet and prevent the chips from sticking.

Next it was time to set the firewall in place. This looks too cool!

I had previously drawn a centerline along the F-601M bottom most firewall stiffening angle. This will be used to set the fore / aft position of the firewall relative to the F-772 forward bottom skin.

For the time being, I'm just using a couple squeeze clamps on each side to hold the firewall in place.

I marked another centerline on the F-601M bottom most firewall stiffening angle. This one is in the left / right orientation. This should make it easy to center the firewall and the F-772 forward bottom skin.

Here's the F-772 forward bottom skin. The second row of holes has an odd number of holes (33 to be exact), so I circled the center hole (number 17). When the firewall and this skin are aligned, I should see the crosshairs of my marks on F-601M through this hole.

Actually, even better than that, I decided to just drill a #40 hole at the intersection of my marks. Now I can just cleco at this position.

After setting the F-772 skin in position, I used some AN470AD4 rivets per the instructions to prevent the longerons from sagging.

However when I tried clamping the longeron to the weldment, I found that the radius of the longeron interfered with the outside corner of the weldment.

Since I haven't riveted the firewall together yet, I just removed the weldment to examine the problem in more detail. Here you can see that the weld at the forward end of the weldment kind of forms a chamfer which creates clearance for the longeron radius.

Further aft on the longeron, however, the ears of the weldment need to be pried apart somewhat to create a similar clearance. Even with the ears pried apart, there's still some slight interference. If you look at the plans, you can actually see this very same interference. Note that you might have to click on the full sized image to see this clearly. Apparently Van's thinks it's too slight to worry about.

I used the point of my scribe to pry the ears apart somewhat while clamping it all up. Note that my scribe has been sharpened, so the taper is very shallow compared to how it originally came to me. I think this made it easier to wedge it in position during clamping.

The end result is a slight gap between the ears of the firewall bracket. No more interference with the longeron radius.

Okay...this is really hard to see, but the angle which the swivel pad is clamping on...that's the longeron. The angle at the top of the picture (has 608 printed on it)...that's the F-601? top firewall stiffening angle. You can also probably see a slight gap between the longeron and the weldment ear. The c-clamp is tight, but I couldn't seem to get that gap closed up. That's because the upper leg of the longeron has a slight interference with the F-601? top firewall stiffening angle.

My solution was to remove the longeron and trim back the upper leg of the longeron by 3/16 inch. I also put in a radius to prevent any stress risers. If I had it to do over again, I think I'd have used a fillet radius instead of an undercut radius. Oh well.

After a bunch more clamping, measuring, clamping, and more measuring it was time to drill the firewall to the F-770 side skins. Because I had drilled the firewall to the F-601M bottom stiffening angle, I wasn't able to get the firewall to sit exactly 5/8 off the forward edge of the F-770 side skins. It's about 1/32 inch too far aft on the left side and about 3/64 inch too far aft on the right side.

After dinner, we clamped, measured, clamped, measured, then drilled the right side of the firewall to the F-770 side skin. Thanks, dad, for all the help. :-)