I did some work on the Radian. I *finally* got the adapter off, with a good amount of cursing and a decently aggressive bastage file. Ohhh, yeah. Nothing eats up aluminum like a piece of roughened steel.
Anyways! Just as I suspected, there was a groove cut all around the motor shaft. In fact, there were two grooves cut. Something about this bugs me. The little tiny set screw did not have a flat bottom. It was deeply concave, and was basically just a hole punch. This, I think, contributed a little to the problem. The screw was originally set in the middle of the flat on the motor shaft, but when the prop spins up, the screw can move just a little before it digs in and holds on. (normal) This puts the screw on one edge of the flat. Then the screw gets tightened, and a permanent ring is cut into the flat. That ring makes the start of a nice ramp, and as the screw loosens with vibration and time, it takes that ramp one day, and leaves the prop spinning freely.
What I did about it was, after forcibly removing the adapter, I smoothed the shaft all around, removed the aluminum spalling that built up, and cleaned up the flat. I then dug in a little, and deepened the flat a smidge, only about .010", and tried to remove some of the ring. That didn't work as well as I wanted, and then I had a thought. I put the set screw on the end of the (thankfully very magnetic) screwdriver and I filed the end flat, so that it had more surface contact with the shaft and wasn't sharp, and wouldn't dig into the shaft any more.This also gives it more "foot" against the flat, and more leverage.
Once I, uh, buy a new adapter, I'll give it a whirl *ahem* and see how it holds. If I'm right, it should hold nicely.
I might give it a few more licks with the file though. Maybe widen the flat by the .020" required to get the better file to fit, and give it a few more gos.