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Cox Engine of The Month
November-2019
jmcalata's

"Pee Wee .020 throttled combinated"



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Post  batjac on Sun Aug 25, 2019 1:03 pm

I finally had some time yesterday to set up my test stand and run some engines. Knowing I'd have the time, I decided, for some reason, to pull out some Sky Copter .020 engines I've gotten over time from eBay and other sources. I'd been waiting for the right time to pull them out and modify them into "Product" .020 airplane engines. I had previously done three, and had six in a box awaiting modification. So I sat down and pulled out five to modify. The sixth was too grungy and will need more than a cursory cleaning.

I do the conversions because a PeeWee just has too short a run, and drilling a hole in the tank for a fuel tube to an external tank is sacrilege. Besides, the sky copter engines are fairly common on eBay. I need to weigh the finished engine to compare to a PeeWee engine, and the only product type backplate .020 I have, an engine/firewall from a Pitts Special. After the mod, the total length from backplate to driveplate is only a 1/16" difference between the modded engine and a PeeWee.

After doing the mods, I took one engine and mounted it on the nice aluminum test stand that Bernie sells to run it. I'll slowly cycle through the engines to make sure they're all running fine. When I fired up the engine, it would only get up to 16K RPMs, and not very steady at that. I checked everything for tightness and changed out the glow head, but still only around 16K. Very disappointing. I was hoping for better. Just for reference, I changed out the yellow 2 bladed prop that the engine came with for the Cox three bladed prop for .020 engines. Wow! A steady 20,500 RPM. Much more like what I was wanting. I'm surprised that changing to the three bladed prop added 4.5K in speed and a steadier run. I'll have to try a couple of other props to see what I get.

The Pleased Mark
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Post  balogh on Sun Aug 25, 2019 11:02 pm

Yes my TD020 swings the 3bladed COX prop near 24k while it is robbed of power if propped with a 4 inch 2 blader..apart from power absorbed by the prop, the shaft power varying with rpm also goes into the equation...both the 020 PeeWee and TD engines are happier at higher rpm-s and will max out their output way higher than the 16k your original prop pushed down the PeeWee to.
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