Some things inside an engine cowl don’t react well to heat—they need a little cooling to keep the electronics happy. For our F1 Rocket project, that means we needed blast tubes (which feed relatively cool outside air at ram pressure) to keep our P-Mag electronic ignitions and the regulator on our alternator cool.
In the past, I’ve used the simple corrugated tubing you can find at Aircraft Spruce and drilled holes in the baffles to the “minor diameter” of the corrugation. You then squeeze them into place, add a little RTV to keep them secure, and route the other end to the device to be cooled.
For the Rocket, I decided to go a little fancier—I pulled out a piece of 1.5-inch diameter aluminum bar stock and chucked it up in the lathe. Since I had some 7/8-inch SCAT tubing in a box, I turned down all but a flange, then bored the interior out so that I had a very lightweight tubing mount. Some would say I could have done the same thing with a piece of 7/8-inch tube, some 3/16-inch aluminum, and a welder—which is true—but I don’t weld aluminum, and then I’d have a bead that would prevent the flange from lying flat on the baffle.
Installation was simple: Punch a 7/8-inch hole in the baffle, pull out a little Pro-Seal, glue it in place, and then come back the next day to attach the SCAT tube. I made little rings to fit inside the other end of the SCAT tube (also on the lathe) to stiffen it up, then attached it to the alternator with some safety wire, pointed directly at the regulator electronics. The P-Mags are a little trickier, requiring a more elegant solution—you wrap safety wire around the round “neck” of the P-Mag, twist about a foot of it, then feed that up through the SCAT tube. Pull that out through the flange, make a little short rod with a groove in the middle, twist the safety wire around it, and let that pull up tight against the flange.
Overkill? Probably—but it was either this project or getting to work on the fiberglass tail-tip fairings, and I’ll do anything to put off doing fiberglass!
Here is a nice solution for the magneto end –
https://avbrief.com/3d-printed-p-mag-shroud/
That’s a very nice solution – I saw it over on VAF the other day…just after I installed my usual safety wire trick. From what I read, not sure it fits on the six-cylinder … still to be determined… but I might use these on some of my other engines with four-cylinder P-Mags!
The highest heat exposure occurs after landing taxiing downwind (or even uphill) on a hot day. When taxiing downwind, the thrust from the propeller is not enough to push against the rising heat from the engine. An equilibrium can exist that provides no cooling at all. These vents do nothing for your Pmags in this downdraft condition.
For this and other reasons, power electronics will never be reliable when heat soaked by the engine.
I just reinstalled my scat tube to the left pmag last night. I wrapped the safety wire up and around the hose clamp, but your method is much cleaner! I’ll do that next time. Thanks for the tip, Paul!