[Rick TYler]

Originally Posted by AdamHeard

I've seen similar sized objects with similar energy and similar speeds break up; it will take a serious, serious object to hold it in. A cage won't suffice, plates of metal or polycarb will be needed, and not thin stuff at that.

The propeller blades are pretty low mass. Put a curved Lexan duct around the fan and call it good. The nice thing is that you know the blades are going to fly straight out -- it's not like they're made of C4. Wrapping the perimeter with a 6-inch wide Lexan duct should do the trick. Even if the blade hits the Lexan and bounces back to penetrate the finger-grid, there certainly won't be enough energy left to kill someone standing around the field.

How often do RC plane propellers spontaneously explode? This is a really mature technology -- I would be surprised if they weren't overbuilt for safety. How about replacing those 2-blade props with 3- or 4-blade units? For the same power you can have shorter blades with much lower tip velocities. At least, that's how it works on real airplanes.