Why do windmills with tail fans sometimes still have tie beams in the cap?
As I experienced first-hand with my model windmill project, the extra timbers at the rear of the cap help to counter-balance the weight of the sails and wind shaft; without them, it could be easier for the wind to blow the cap off of the tower in a tail wind. Another answer, however, is that the mill may have, at one time, been operating with a tail pole prior to the installation of the tail fan (the Peotone Windmill is an example of this).
