Can you use a folding prop on a reciprocating fuel engine? Depends on the engine and spinner/prop. The engine must be smooth and the prop/hardware must be very strong. It is not done very often at all. The equipment for folders is available for electric gliders. Electric motors are significantly less shock (pun intended) to the hardware than fuel engines so it's up to the user to judge and employ safely.
http://www.espritmodel.com/spinners-...prop-nuts.aspx