fig1 is a schematic plan view of an aircraft 1 comprising a wing body 2 divided into a port wing body 2 a and starboard wing body 2 b , with a tangential flow rotor 3 which is not shown in detail but which is divided into a port rotor 3 a and a starboard rotor 3 b , driven by a common drive means . the aircraft has a fuselage comprising a pod 4 and a tail boom 5 which supports a tailplane 6 and a fin 7 . the underside of the wing body 2 is extended forwardly as a shroud 2 ′ to enclose the cylindrical space 7 occupied by the rotor 3 . at the leading edge of the shroud 6 is a pivotable lip - defining flap 8 which is shown in fig2 as intruding on the cylindrical space 7 to its maximum extent . the flap 8 is mounted for pivoting about a pivot defining a pivot axis 8 ′. in rotation of the rotor 3 , in the anticlockwise sense as viewed in fig2 the tangential flow in the rotor results in the air expelled from the exposed periphery of the rotor at the front and the top of the wing becoming divided by the nose 9 at the top of the wing - like body 2 such that the expelled air moves downwardly and rearwardly over the upper surface of the wing - like body 2 while the rest of the air moves in the anticlockwise sense within the cylindrical space 7 until it reaches the lip defined by the flap 8 in the fig2 position . as a result an anticlockwise vortex is generated within the rotor and greatly improves the lift force , as described in my wo 98 / 07622 . it has been found that in the high lift configuration shown in fig2 the noise generated by the wing and rotor combination is increased as compared with that which would normally be experienced in the cruising configuration shown in fig3 where the flap 8 is in the position 8 a substantially flush with the rest of the shroud 2 ′. in this fig3 position less lift is generated and also less thrust is generated but , provided the flap is as close as possible to the periphery of the fan rotor , the efficiency of the wing is at a maximum . the induced air flow through the rotor in the fig3 configuration is illustrated by the arrows moving along curved streamlines from right to left in the drawing . in the fig2 ( high lift ) position the flap 8 is even closer to the passing rotor blades than in the fig2 position but must not interfere mechanically with them . the difference in lift and thrust can be used in the following manner in order to provide optimum flight conditions . for running up the rotor as rapidly as possible on the ground , the rotor drive motor is accelerated while the flap is in the position 8 a shown in fig3 thereby minimising resistance to rotation of the rotor . when the rotor has been run up to a desired speed the flap may be moved into its position 8 shown in fig2 and this results in an increase in lift , allowing the aircraft to take off . when the aircraft is stationary the vortex - controlling flap 8 is at its most effective . the aircraft may already have been accelerating forwardly while in the fig3 configuration but generating much less lift , possibly only 20 % of the available lift , than it does in the fig2 position . in flight , the flap is returned to the position 8 a shown in fig3 or to a position close to it , in order to minimize the rotational drag on the rotor , but also to take advantage of the induced lift resulting from the air flow through the rotor and the air flow over the aerofoil wing - like body 2 . in order to control the angle of bank and / or the angle of yaw it is possible to vary the position of the flap to a position somewhere between the two extreme positions shown in fig2 and fig3 . in order to provide for optimum manoeuverability the aircraft may be flown during climb after take off or during approach to landing with the two flaps set midway between the extreme positions shown in fig2 and 3 , such that when bank is needed it is possible to increase the lift from the steady state by increasing the degree of intrusion of the flap on the side where higher lift is required and decreasing the lift on the other wing by similarly reducing the degree of intrusion into the space 7 . for take off and landing it is advantageous for both wings simultaneously to be in the fig2 configuration , or in a configuration close enough to it to generate high lift and high thrust but still leave some degree of differential adjustability for controlling bank and yaw on landing . from the above it will be understood that the rotor 3 may initially tend to slow down somewhat during the take off phase , until the forward speed is such that the air flow through the rotor again assists in reducing the resistance to rotation of the rotor . thus in the take off phase it is possible to some extent to build up excess rotor speed with the flap in the fig3 position and then to consume some of that energy in the take off phase when the flap moves towards the fig2 configuration and the resistance to rotation of the fan rotor 3 builds up . the principle illustrated in fig2 and 3 can be readily applied to any of the wing cross - sections shown in wo 98 / 07622 , and indeed to any other variations of the basic configuration of the rotor in the leading edge of the wing body . if desired and inboard section of each wing ( 2 a , 2 b ) may have its vortex - generating flap adjustable independently of that as the outboard section so that the inboard section may remain in the high lift configuration during take off and the landing approach , while the outboard section may be adjusted for manoeuvring . the embodiment shown in fig4 a and 4 b differs from that shown in fig2 and 3 in that there are two limbs 18 a and 18 b to the flap , pivotable about a single pivot axis 18 ′. the two limbs 18 a and 18 b are at a fixed angle of inclination relative to one another . in the fig4 a position the flap limb 18 b is substantially a continuation of the shroud 12 ′, while the flap limb 18 a is directed inwardly towards the rotor in the position giving maximum lift and thrust ( similar to the fig2 configuration ). in the fig4 b configuration this two part flap has pivoted clockwise about the pivot 18 ′ so as ( i ) to open a gap in the ‘ shroud ’ where the flap limb 18 b has moved inwardly from it flush position , and ( ii ) to decrease the extent to which the upper flap limb 18 a extends inwardly until it defines the front of the shroud 2 ′. in this configuration the high speed peripheral air moving forwardly along the inner surface of the shroud will spill out forwardly to reduce both lift and thrust still further as compared with the fig3 configuration , thereby giving more control of differential thrust and lift to improve manoeuvrability of the aircraft . in the embodiment of fig5 a , 5 b , and 5 c the control of this better manoeuvrability embodiment of fig4 a and 4 b is still further improved in that an additional facility is provided by virtue of the intermediate configuration of fig5 b . in the embodiment of fig5 a , 5 b and 5 c the two limbs 28 a and 28 b of the flap are to a degree pivotable independently of one another . if desired , complete independence may be provided . for movement from the fig5 a configuration to the fig5 b configuration the upper flap limb 28 a pivots clockwise while the lower flap limb 28 b is stationary , until the two limbs 28 a and 28 b are substantially mutually parallel in the fig5 b configuration and substantially flush with the shroud 2 ′. from then on the two limbs move in unison to reach the fig5 c configuration where the two limbs together define the spill passage to allow forward spilling out of the forwardly moving air between the fan rotor and the shroud .