However, nearly all model helicopters have symetrical blades for the same reason that most aerobatic airplanes have symetrical wings, so they can fly upside down just as well as they can right side up and inverted autorotations are a pretty standard stunt for the RC heli guys.
Even if they did have cambered blades, they could most likey do an inverted autorotation. Trainers with their flat bottom wings can fly upside down after all. The guy who trained me how to fly would fly a Gentle Lady glider inverted in slope lift and nothing has a more flat bottomed wing than a Gentle Lady.
Sure, but this misses the point. It's
lift, not
drag, that keeps a helicopter from falling in autorotation, so the fact that helicopters can autorotate doesn't "prove" that a rotating propeller generates more drag than a fixed prop.