The large signs on the crane boom and balance arm should play the role of sails.

That's right. The billboard on the balance arm of tower crane should be in the state of automatic downwind when there is wind. Therefore, after hanging billboards, the front and rear arms of the tower crane shall not be affected to rotate with the wind, otherwise the tower crane will lose its automatic downwind protection function. Therefore, the large signs set on the crane boom and balance arm should play the role of navigation.