I am always on the lookout for cheap toys that might give me project ideas. In a Hobby Lobby craft store I spotted this rubber powered helicopter for only $2.49 that had a foam body, actually a flat profile of the body of the helicopter. Previously I had a similiar helicopter that was made with a wood body profile that seemed too heavy to fly very high. The foam should be lighter and I know on anything that flies weight makes a big difference.
Assembly of the helicopter only takes a few minutes, everything slips together and a clear rubberband holds the foam body profile to the wood stick. The rubber strip that is provided is thin and long so it has to be folded into three loops. Propeller blades go into plastic sockets and twist into a locking arrangement so the angle is set.
To fly the helicopter just wind the propeller up about 75 turns, make sure it is pointed completely vertical and release. It should gain altitude to at least 40 feet and start to descend. The body of the helicopter still rotates slowly because of the torque reaction of the propeller. Shortly after the descent the rubber is unwound and then the helicopter starts fluttering towards the ground in more of a horizontal position. In the dozen flights I tried there was no damage even landing on hard surfaces.
It would be fun to try to build this as a group project, the challenge would be to build an effiencent propeller. If you can not find this locally this is the website link. Guillow's Copter Toys