[EricH]

Originally Posted by zrop

The propellers are perfectly safe as long as they don't exceed their maximum rpm, the rate at which we are spinning them is no where near their maximum rpm (the maximum is about 13k rpm). Having them spontaneously exploding is not a huge concern to us, having balls getting in the way of the props is a much bigger concern. (oh and its 12.25 in in diameter btw ;D)

12.25 inches????

Let's run the numbers. That's 374.15 feet per second if something breaks! If a prop breaks, it's going 7, yes 7, FRC fields per second.

As for perfectly safe if they don't exceed maximum RPM, that's all well and good, but R/C aircraft don't hit things. That's the facts of it--if they hit anything, they crash (or just crashed) and the prop is broken anyway. These props are going to be jostled through shock loads when they get hit. This could potentially weaken them. If they're weak, and they get one hit too many, somebody is going to get hit, hard. I haven't run the numbers on KE for, say, 1/3 of a prop, but that's far more than I want to have hitting anybody!

As for who's going to poke hands in, I can think of a few things--little kids, poles from a tipped trailer, unsuspecting students... I am pretty sure I could get my hand in enough to contact at least one prop.

Bottom line, don't worry about keeping balls out, worry about keeping the props in. If you do that (better than the cage shown will), the balls will take care of themselves.

Oh, and I just remembered--if you reverse those blades while they're going full bore, that's a pretty hefty acceleration force you put them through. This will contribute to propeller weakening. They aren't designed to run in reverse.