Technically a model would fly if the blades were allowed to flap up-down quite a bit.... since once they autorotate and in flight they will seek their own level. But, it is not necessary for them to flap down below zero/level and thus strike the model, so they are generally limited to zero down. They need to be allowed/restricted to flex/hinge upward maybe a max of about 20-30 degrees simply to make it easier to obtain autorotation for launch... otherwise they may initially flap virtually straight up and make it impossible to spin up. I have also seen a model lose its blade "upper" limiter in flight and a gust of wind actually folded the blade vertical and loss of control occured.

So to answer the question: zero down and 30 degrees up is a good guide.