Cowling and Alternator Blast Tube – 5 hrs

Saturday April 13, 2013

The cowl surfaces are rather porous in the stock condition so the plans recommend that the surfaces be sealed with a thin layer of epoxy. But because the surface has tiny holes that the epoxy will not flow into naturally the epoxy should be thinned with acetone at about a 1 to 1 ratio. This morning I sanded the interior surface of the upper cowl and brushed on a thin layer as prescribed. The acetone slows down the curing process so it will sit a few days before I sand off the gloss in preparation for a coat of primer.

The instructions for the Plane Power alternator call for a blast tube to direct cooling air into the back opening of the alternator case. I did some research on this in the forums to determine how to mount the blast tube. The alternator is near the front of the engine so the standard place for the tube is in the right inlet ramp just behind the cowl nostril. I saw various ways of attaching the blast tube to the alternator, none of which I really liked. I also read of cases of alternator failure in rain conditions which is more likely to occur with the blast tube so near the air inlet of the cowl. So I decided to try to come up with a better way of mounting the blast tube to the alternator and reduce the risk of water blowing into the alternator case. I don’t know for sure that the water is a problem for the alternator but it seems like it can’t be a good thing.

To attach the blast tube to the alternator I designed a duct that I will rivet directly alternator rear cover. I removed the cover and built a mockup using  file folder paper. Then I made it from two pieces of .025 aluminum sheet epoxied and riveted together, mounting directly over the air inlets of the alternator. Here it is clecoed to the alternator rear cover with a blast tube installed.

Here is a side view showing how it is mounted to the cover. Air comes through the blast tube, turns 90 degrees and enters the alternator cover, exhausting out through the vents further forward on the alternator.

From the inside of the cover you can see the two square holes where the air will enter. I will give the epoxy a couple of days to fully cure then I will prime the duct and rivet it to the cover.

I will show you how I plan to address the water problem when I install this to the alternator.