lever 17 the twist grip throttle 16 is connected to the engine 6 and is used to control the acceleration of the engine in known manner the control lever 17 is connected to the control vane 18 for the fan 2 by a rod 22 as shown in greater detail in fig8 the lever 17 is pivoted to the outer end of the arm rest 14 and the end of the lever 17 below the arm rest 14 is connected to one end of the rod 22 the other end of which is connected adjacent the lower end of the control vane 18 the control lever 19 is connected to the arm rest 15 and the control vane for the fan 3 in the same manner the control arrangement for the control vanes is a simple lever control in the case of fan 2 movement of the control lever 17 in the direction of arrow x moves the control vane in the direction of arrow b and movement of the lever 17 in the direction of arrow y moves the control vane in the direction of arrow a alternatively a cable could be used in place of each rod another possibility is to use a non mechanical control system by substituting an electric actuator for controlling each control vane each control vane is mounted diametrically across the lower edge 9 a 10 a of the corresponding duct 9 10 and is arranged to pivot about an axis substantially perpendicular to the axis of rotation of the corresponding fan and hub each vane has an aerofoil cross section each vane projects partly below the lower edge of the corresponding duct.