[travis]

I am not an aeronautical engineer, but I do sit next to one. I also attended the AE interweb school of hard knocks when I decided that work well or not, propellers are going on the 2091 machine. Here’s what I found.

Your standard RC airplane propeller is designed to run at high RPMs AND at high forward velocity. Of course they work stalled, lest a plane wouldn’t take off, but their peak efficiencies are when they are moving. I don’t like the idea of gearing up a motor either, and the 5krpm type props were too big for what I was going for. They make propellers designed to run on motors (not engines) with typically higher, low-RPM torque requirements and lower total RPM sweet spots. I was poking around looking at those when I found the so called “slow fly” propellers. These are designed to fly at low RPM, and in stalled conditions. APC propellers makes FRP slow fly propellers in left and right handedness -- so you can have a counter rotating arrangement -- in lots of pitch and diameter combinations, surprisingly (to me) cheaply. The ones I settled on were 12x3.8 through dragan fly innovations. They are marketed for dual rotor RC helicopters in the 300W motor range, which looked perfect for cim motors. The slow fly propellers maximum RPM is about 65,000/D[=]in, so for 12” props it is ~ 5500, which I should never see in a loaded cim. This was one more piece of evidence that this was the right propeller/motor combination. A couple of suspect online propeller calculators indicated that each propeller would be in the neighborhood of half the motive force of what the wheels could punch out, which is testing enough for me. I’m bring’n the Heavy Metal (well, FRP) Noise to New Orleans (world capital of airboats), whether they work or not; 2024-T3, solid rivet construction enclosures by the way, before anyone jumps all over me.

Any real AEs see anything wrong with that?

Travis