A side benefit of foam parkflyers as opposed to balsa is that a foam plane will do a lot less damage than a balsa. If you compare a 2lb. foam plane to a 2lb. balsa plane, the inertia is the same for the two planes, but the "crumple" zone is vastly different, therefore the energy transfer is also vastly different. Keep in mind that on a balsa plane, the frame members that are carrying the primary loads, like the motor mount, are actually plywood and not balsa. Take a piece of plywood and a piece of foam and try to bend/break each one. The plywood remains rigid and requires a considerable amount of force to deform it, while foam deforms at the slightest force. If you are flying a foam parkflyer that is 2lbs. or less, don't worry about the occassional person walking near the flying field. Even if you lost control of the plane, its highly unlikely that it would crash exactly where the person is, and if it did, it would not cause serious injury. When I was learning to fly, at one point I actually flew a 1lb. plane into my chest as I was trying to land it and got the control reversal thing all screwed up. Very embarrasing, but absolutely no injury.