Front End

This site was last updated on February 15, 2008.

The front end attaches the firewall to the spar, effectively holding the engine and wing in alignment. It's a tricky shape to mold; we finally decided to mold it around a sacrificial piece of blue foam. Thus, there ended up being three significant steps:

Build the firewall,
Cut the sacrificial foam piece to shape, and
Mold the front end.

The foam parts are produced en masse in a short series of steps using a table saw, a router, and a hot-wire cutter.

The jig for the final hot-wire cut.

The foam part with the firewall set in place.

The complete mold is a little complicated. In the picture above, you can see an aluminum base plate with two wooden supports at each end. These provide clearance for the vacuum lines leading from the pump to two holes in the base. The yellow stuff is tacky tape that seals the bag to the base.

The little stacks of wooden blocks at the top and the bottom will support the bridge (see below). Note the holes drilled in the top of each for the the alignment pegs.

In the center, you can see the foam part with the firewall set into an aluminum section of the O-beam spar.

The spar mold was turned on a lathe with a taper attachment. We used 4 pieces of 3/8" aluminum, bolted together (too many bolts?) and held in a 4-jaw chuck, supported by a live center in the tail stock. This approach allowed us to take it apart and use half of it to effect the mold.

Buffing the spar mold, after lots of hand rubbing.

Cutting the CF cloth to shape, working from a template.

We weigh the CF cloth, then mix up a batch of epoxy that weighs the same amount (to ensure the proper balance between the CF and the epoxy). Placing the CF cloth between two pieces of mylar, we pour the epoxy over it and brush it around so that it's reasonably well distributed. Finally, we roll it out (using an Xacto knife handle as a rolling pin) to ensure even distribution.

In the picture above, the wetted carbon cloth has been installed on the mold, covered with teflon sheet, and the entire mess enclosed in the vacuum bag. The pump is on and the bag has been folded around the various corners, carefully leaving a smooth surface across the firewall. Notice the string wrapped loosely around the mold to serve as a breather.

In these pictures, you can see the bridge we install across the top to ensure that the firewall is properly aligned with respect to the spar. We begin by setting the bridge approximately in place. Then we insert 4 screws into holes in the bridge, piercing the vacuum bag, the teflon, and the wet carbon-fiber cloth. Tightening the screw pulls the firewall into alignment with the bridge.

Installing the special armor-piercing screws.

The screws (ordinary 4-40 socket-head bolts) are sharpened on a grinder and sprayed with a release agent.

Tightening the bridge. Notice the alignment pegs.

Now we place the entire assembly in our autoclave to cure (we use a packing box, with a light bulb to provide heat, and a cheap thermometer to let us monitor the temperature).

Our first part. The sides are wrinkled, but the firewall and spar interface are fine.

The aft view, with foam.

We scratched out the foam, but it's so light it might have been left in place. It could also be removed with gasoline or acetone.

After finishing the first prototype part, we remembered our plan to hang the shutoff and bellcrank from the front end. Oops. So, we added a new piece to the mold, allowing room for a slot with flanges on the inboard side of the front end.

The front-end mold, Mk II.

It's in the bag.

The finished part, with fingers for scale. Weight is about 22 grams.

Finally! Here's a picture showing how the front end fits into the wing. The wing halves are supported by foam shucks glued to a base and the front end is fixed by a bit of aluminum angle.