It's too bad he couldn't have an simply automatic grid at low speeds and once going the grid pulls or rotates out of the way for better efficiency. Actually a

It's too bad he couldn't have an simply automatic grid at low speeds and once going the grid pulls or rotates out of the way for better efficiency. Actually a rotating grid may work using the air as its triggering force, once the air speeds up some sort of venturi force enables/disables a safety mesh across the opening.