An auxiliary flap is movably arranged on a planar trailing edge of an aerodynamic element such as a wing, rudder, stabilizer, or flap. The auxiliary flap is rotatable and/or slidable relative to the aerodynamic element, to move selectively into three positions. In a first position, a free edge of the auxiliary flap protrudes into an airflow boundary layer on one side of the aerodynamic element, to decrease lift. In a second position, a free edge of the aerodynamic element protrudes into an airflow boundary layer on the other side of the aerodynamic element, to increase lift. In a third neutral position, the auxiliary element does not protrude into either boundary layer, so as not to influence lift. The auxiliary flap is simple and rapidly acting. The flap protrudes substantially perpendicularly into the boundary layer flow. The auxiliary flap has a planar plate shape.