Instruction in RC has traditionally been free, usually with the instructor and the student becoming good friends. When someone decides to charge for instruction, he has put it on a business basis and, as such, I would have an agreement, in writing, that for that $40.00/hr., the instructor would be responsible for any damage sustained to the plane. This would be regardless of whether on a buddy cord or just standing alongside verbally coaching. It's a business, remember?.. And that sounds like a good business practice to me.