Propellers!

Propellers are neat things that you can attach to motors. When they turn, they create a pressure difference on the front/back of the blade. The front has high pressure, and the back has low pressure.

These are done in Sketchup using a technique that I am developing. Each blade is 120 degrees from the others. The profile is extruded up 1/16" and then the top edge is rotated on the center point. The new top edge is then extruded again 1/16" and rotated by the same number of degrees. There is a