08 September, 2006
Today, I set out with a plan. I wanted to fabricate a firewall that, with the use of a Graupner spinner, would roughly maintain the front lines of the Spirit. I started out by picking a cut line and taking the balsa block off the nose. I then machined out a cavity in the remaining wood that would enable the motor to spin.
After this, I fabricated the firewall using a piece of 3/16 ply bonded to a piece of 3/32 ply. I used this to give me the right spacing off the existing first bulkhead of the Spirit. I drilled this for the motor mount and used 4-40 blind nuts for the anchors.
Once I got this done, I ground the first bulkhead down so that I could bond in the firewall. I kept the edge of the bulkhead to use as a positioning guide, but machined enough away so that the bolts would clear and I wouldn't have to try and cut any more once the assembly was done.
Then I could bond the firewall in place. I put tape over the back of the blind nuts so that I wouldn't fill the threads with epoxy. I wet down the edges of the fuse with 20 minute epoxy and then added more epoxy with microballoons once the bulkhead was in place. After the cure, it was a simple matter of bolting the engine on.
Finally, I had to cut quite a bit of the cockpit away under the canopy to provide some air space for cooling air to flow around the motor and through the battery compartment. When complete, this provided a functional, if ugly, method of ramming cooling air through the forward fuse. I tried to compensate for the ugliness by painting the canopy silver.
Of course, like all good tinkerers, I just couldn't wait to see the fruits of my labor. I strapped an APC 11x4 prop to the front of the motor, dropped a 3S LiPo on the ESC, and let it rip on the table of my bandsaw. Two things were learned from this. First, that we surely have enough thrust and second, that we have plenty of cooling. Just to validate this, I will do some static runs of several minutes with thermocouples in the battery bays. Should be fun!
Tomorrow's episode will cover my new hatch hold downs and my current approach to radio mixing.