a in the arm 12 a ring 16 which by means of a bolt 17 and a locking nut 18 screwed thereupon can be locked against a hexagonal portion of the arm 12 is provided with an arm 19 situated next to the tubular portion 11 a by means of this arm 19 the arm 12 can be adjusted around its longitudinal axis and therefore the angle of the blade 13 attached thereto can be adjusted by means of a mechanism not represented in the figures and provided between the hub 11 and the reduction box on the shaft of this latter a bolt 21 which is screwed through this arm 19 and can be locked by means of a locking nut 20 allows for a fine adjustment of the aforementioned angle between the ring 16 and the tubular portion 11 a an o ring 22 is provided around arm 12 in order to prevent dirt from penetrating into the portion 11 a the portion of the arm 12 which is turned away from the hub 11 is hexagonal the connection 14 substantially consists of a channel or open box profile 23 which is attached to the upper side of the blade 13 for example welded thereto and a beam 24 fitting with a play into the open box profile 23 and being connected to the arm 12 by means of fixation means to wit two screws or bolts 25 these bolts 25 loosely extend.