Abstract:
A fluid dynamic device including a shroud disposed about a central axis and having an upstream fluid intake region and a downstream fluid exit region, the shroud directing fluid flow between the upstream fluid intake region and the downstream fluid exit region. The shroud is configured so that, at least during some operating conditions of the fluid dynamic device, at least a portion of fluid exiting the fluid exit region is directed towards the central axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Australian Application No.  2007902596 , entitled “Thrust Vectoring Shroud for Fluid Dynamic Device,” filed May 16, 2007, incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    This invention relates to a fluid dynamic device. More particularly, the present invention relates to the design of a thrust vectoring shroud for use in a fluid dynamic device. The invention is particularly useful for use in aerodynamic lifting devices using drum rotor type fans for thrust generation. 
         [0003]    The applicant has also developed a new type of airborne craft capable of hovering in a stationary position while airborne. 
         [0004]    Powered airborne craft, manned and unmanned, may be capable of hovering in a stationary position while airborne. Such aircraft may range from craft which operate close to the ground relying on a cushion of air to those capable of free flight and vertical takeoff and landing. Craft operating close to the ground may be designed for transportation and recreational use whereas the free flight craft may operate at generally low altitudes compared to commercial aircraft and may be considered for applications including airport-to-downtown shuttle, home-to-office commuting, search and rescue and surveillance operations. 
         [0005]    The most common craft that hovers close to the ground is the hovercraft which is generally a craft used for recreational and general transport and ferry duties. This craft has a number of disadvantages that have limited its penetration of markets for motorized recreational products and general transportation of personnel and goods. 
         [0006]    One important limitation of such craft is the inability to operate over terrain with obstacles of significant size such as waves, boulders, riverbanks and the like because close contact must be made with the ground to avoid the leakage of the air cushion. Any increase in the operating height of the hovercraft is accompanied by an unrealistic horsepower requirement. A further notable limitation is the inability to develop significant lateral thrust for acceleration, braking, climbing gradients and changing direction with realistic horsepower requirements despite the use of separate fans for developing this lateral thrust. In general, the “footprint” of the hovercraft is acceptably small for its lifting capacity because the entire area under the craft and an appropriate peripheral skirt encapsulates an air cushion which can operate at sufficient pressure with low power requirements provided that the clearance between the grounds and the skirt is small so as to minimize air leakage. In effect the air cushion of a hovercraft provides a low friction sliding surface over which the hovercraft may move. 
         [0007]    The most common and widely employed free flying vertical takeoff and landing (VTOL) craft that operates at higher altitudes is the helicopter. The success of this vehicle is due to the urgent need for this VTOL capability and the ability to achieve hovering flight with acceptable power consumption because of the very large amount of air that is contacted by the large diameter lightweight blade structure. The main undesirable characteristic of helicopters is the long rotating blades, which are a hazard to personnel and to the aircraft itself should they strike anything in the area and the very large footprint or minimum safe space requirements that these aircraft require, particularly during takeoff and landing. 
         [0008]    Further undesirable characteristics include the requirement for a remotely mounted propeller to counteract torque reaction of the airframe to the drive of the main rotor, complicated and relatively fragile rotor blade attack-angle controls, high maintenance requirements and rotor blades which must be long, thin, and relatively light and thus are flexible and subject to fatigue problems. Add to this incomplete list of limitations the fact that failure of any one of these components is likely to have catastrophic consequences for the aircraft and all on board and it is evident that an alternative design is desirable. 
         [0009]    In a craft free of ground effect, lift can be generated by the acceleration of a mass of air by a fan, propeller, wing, or other system. When a mass of air is changed from rest to a given velocity in a downward direction, an upwardly directed reaction force is produced. In general, the more air that is directed, the less power is required to produce a given lift. This defines the technical challenge which this invention attempts to address because increasing the volume of air generally involves an increase in the size of the craft as evidenced in the large diameter, high speed blades used in helicopters. 
         [0010]    The rotor blades of a helicopter develop lift by accelerating air downward and parallel to the axis of its rotation (axially). The velocity of the tip of the rotor blade is typically set to a maximum that is close to sonic (Mach 1) conditions (being approximately 1250 km/hr (i.e., approximately 350 m/s) at sea level and normal temperatures) on the advancing blade when the helicopter is at maximum forward speed (typically helicopters are limited to forward speeds of about 320 km/hr, and blade tip speeds relative to the helicopter itself are of the order of 900 km/hr, i.e., 250 m/s). The remainder of the blade must operate at a lower velocity proportional to its distance from the axis of the rotor. Unfortunately this non-uniform velocity along the blade means that significant blade length is underutilized despite varying the angle of attack and changing the aerodynamic profile along the length of the rotor blade because lift is proportional to the velocity squared. To compound the problems of the rotor, because the highest lift is generated at the highest velocity region, at the tip, a very high bending moment is generated on this cantilevered structure. Further, to get the maximum lift from the rotor, the blade tip must operate at the highest permissible velocity close to sonic conditions, which means that considerable noise is generated. Correspondingly the rotor diameter cannot be reduced because to generate the same lift, the velocity would have to increase beyond sonic conditions or some part of the operating envelope would have to be compromised. 
         [0011]    Further, and within the class of airborne hovering craft capable of free flight, it would be a desirable object to achieve an increase in payload or lift at the same or reduced power in order to improve fuel efficiency and operating cost. 
         [0012]    It is a further object of this invention to provide an aerodynamic lifting device for airborne craft such that such craft deliver performance characteristics superior to helicopters by generating superior lift capability and/or a reduced horsepower requirement from a lifting device with a smaller footprint, or at least offer a useful alternative. 
         [0013]    The applicant has therefore developed a new aerodynamic lifting device which uses a drum fan type rotor in an airborne craft with a relatively small footprint. The fan may be described as a drum rotor or radial drum fan which may be defined as a fan with the blades advantageously occupying an annular region having a radial depth that is less than 25% of the radial pitch of the blades. By placing the rotor blades at a distance from the rotation axis of the fan, a central region within the rotor is conveniently provided for a payload, or in the case of a larger sized craft, a pilot and/or passengers. 
         [0014]    The use of such a drum rotor type fan also provides other benefits. One such benefit is that effectively the entire length of a blade is being fully utilized as an aerodynamic device (as compared to the tip of the helicopter blade, described above) since it is vertically disposed and the airflow is radial. Additionally, the design of the drum rotor allows for each blade to be supported at either end via upper and lower support rings (again, as opposed to the cantilevered design of the helicopter blade). Also, simple constant cross-section blade profiles may be used which offer manufacturing cost savings. 
         [0015]    As has been discussed above, whilst measures can be taken to balance the torque required to drive the fan (e.g. by using appropriate stator blades or utilising rudders in the outflow air stream (as discussed further below), the use of counter-rotating rotors may also be implemented. The present invention is equally applicable whether using a single or counter-rotating rotor, or stator blades on the inner or outer, or both, sides of the rotor. 
         [0016]    The drum fan type rotor generates air flow in a radially outward direction, from the rotor, this airflow incapable of generating sufficient lift to sustain flight. Therefore, in order to generate lifting thrust this air flow is re-directed, by means of a shroud that surrounds the rotor, from the radially outward direction as provided by the rotor to a generally downward direction to thus produce an upward thrust. 
         [0017]    In one form, the shroud comprises a flexible skirt attached to the area around the top part of the rotor. In this form the shroud may conveniently (and interchangeably) be referred to as a skirt. However, this is to be contrasted to the skirt of a conventional hovercraft which simply maintains a close contact with the ground rather than providing a means to deflect the airflow to provide thrust and directional control of the airborne craft. 
         [0018]    The flexible skirt may have a lower edge in the form of a rigid hoop. Alternatively, the shroud may be supported by a number of vertically disposed ribs. By movement of the hoop (or alternatively, the ribs) the exit of the flexible skirt can be manipulated and thereby a directional nozzle type effect can be used to control the thrust direction of the exiting air (i.e., the effective centre of action of the thrust) and/or change the centre of lift in relation to the centre of gravity of the craft and thereby apply a torque to the craft. The exit area of the skirt may be moved by translation in a plane, or may pivot so that one part of the exit area of the skirt rises whilst another part dips in relation to the rotor. By manipulating the exit area in this fashion, the thrust vector and/or centre of pressure can be altered. As yet a further alternative, all or at least a lower portion of the shroud may translate with respect to the craft thus moving the centre of pressure with respect to the centre of gravity of the craft. In this latter case the shroud may be rigid and move as a solid body in translation to effect the change in the centre of lift. Rotational motion of the skirt (in the roll or pitch direction) may be superimposed and independently controlled to the translational motion. Using a shroud in any of the above configurations has the benefit that all of the air that is used to generate thrust is also being acted upon by the shroud to control the craft attitude. 
         [0019]    It is to be noted that the shroud need not be entirely flexible. Semi-rigid shrouds can be applied and/or the material used for the shroud may have orthotropic characteristics in that it is relatively stiff in one or more direction(s) of stress and relatively flexible in the other direction(s). For example, the shroud may be predominately, or at least in part, constructed of a flexible sheet or fabric material (for example, rip-stop nylon—as often used for hot-air balloons, kites or parachutes) which is relatively stiff in the plane of the material, but which is flexible in the direction orthogonal to this plane. For instance the top portion of the shroud may be rigid with only a lower portion being flexible. Alternatively, certain sections of the shroud along it circumferential extend may be rigid, with intermediate sections being flexible. In this later case the movable hoop would be adapted so that it only moved the flexible sections of the shroud. 
         [0020]    Pitch and roll can also be controlled by appropriate control of the skirt. One or more rudders disposed downstream of the rotor and within the downward directed air flow within the shroud can be adapted to provide yaw control. That is the rudder can deflect the airflow to one side or the other (in a circumferential sense) thus creating a force imbalance which in turn manifests as a torque about the central axis of the rotor and thus causing the craft to rotate about a vertical axis (i.e., yaw motion). Conveniently, two diametrically opposed rudders could be utilised to provide a symmetrical balance of forces about the rotor. Alternatively a plurality of downstream stator blades can be utilized, such stator blades. Conveniently such stator blades may be movable about their own axis to provide a degree of yaw control of the craft. A permanent off-set of the rudder(s) or downstream stator blades could be used to counteract the unbalanced drum rotor yaw torque produced by air flow through the rotor. It is to be noted that the use of such rudder(s) or stator blades does not, it itself, significantly change the distribution of lifting thrust generated by the rotor, but merely creates a torque in the yaw direction. Alternatively or additionally, permanent deflector surfaces could be incorporated into the upper duct (i.e., the upper surface of the shroud) that surrounds and deflects the radial airflow so as to create a counter-acting torque thus balancing the drive torque of the rotor. Whilst the use of such flexible skirts has provided a convenient thrust vectoring means, there are certain limitations to such a system. 
       SUMMARY 
       [0021]    The applicant has found that an alternate design of the shroud produces desirable benefits, including increased thrust per unit of power input at certain operating conditions, including those operating conditions that are out of ground effect (for example when the craft is more than two times the diameter of the craft away from the ground surface). 
         [0022]    Therefore, in one form of the invention, there is provided a fluid dynamic device comprising a shroud disposed about a central axis and having an upstream fluid intake region and a downstream fluid exit region, the shroud directing fluid flow between said upstream fluid intake region and said downstream fluid exit region wherein the shroud is configured so that, at least at during some operating conditions of the fluid dynamic device, at least a portion of fluid exiting the fluid exit region is directed towards the central axis. 
         [0023]    The applicant has found that in certain modes of operation of a fluid dynamic device, redirecting the air flow back onto itself, that is directing or focusing it towards a centralised region, produces increased thrust from the fluid flow. This is in contradistinction to a shroud that redirects the airflow away in a parallel fashion. 
         [0024]    Preferably, the fluid dynamic device is an airborne craft, with the fluid being air, and the shroud is used to redirect airflow from the fluid intake region in a generally downward direction to create a lifting thrust for the airborne craft. 
         [0025]    Preferably, the shroud is disposed about the central axis of the device, said central axis being in the same direction as the thrust being generated by the airborne craft. 
         [0026]    Preferably, the shroud has a neutral position, in which predominately all of the fluid, or air, exiting the fluid exit region of the shroud is directed towards the central axis. 
         [0027]    Preferably, the fluid intake region causes fluid to enter the shroud in a direction parallel to the central axis of the shroud. 
         [0028]    Alternatively, the fluid intake region causes fluid to enter the shroud in a radial direction. Preferably the inlet side of the shroud is in the form of an annular ring. 
         [0029]    Preferably the fluid exit region causes fluid, such as air, to exit the exit region towards a common centralised region, most preferably, towards the central axis of the shroud. For the sake of clarity, an exit angle of 0 degrees will be taken to mean that the fluid flow is parallel to the central axis and away from the device and an angle of 90 degrees means that the fluid flow is orthogonal to and towards the central axis of the shroud (conversely and for the sake of clarity, an angle of negative 90 degrees, or positive 270 degrees, means that the air flow is orthogonal to, and away from, the central axis), and an angle of 135 degrees means that the fluid flow is directed upwards towards the central axis and back towards the direction of the inlet region of the shroud. 
         [0030]    Preferably, the angle of the airflow exiting the fluid exit region of the shroud—in the case of an aerodynamic device such as an airborne craft—is between 45 and 135 degrees to the central axis of the shroud. In tests conducted by the applicant it has been found that directing the airflow at 90 degrees to the exit of the shroud produces a desirable thrust characteristic for an airborne craft particularly when in free flight and away from any ground effect. 
         [0031]    Preferably, the fluid dynamic device or airborne craft comprises at least one drum rotor fan having a rotor for generating radial air flow from an inner region of the rotor to an outer region of the rotor. Conveniently, the airflow emanating from rotor enters the fluid intake region of the shroud, is redirected by the shroud, and exits the shroud at the fluid exit region. 
         [0032]    Preferably, redirection of the airflow is at a total angle of airflow from the fluid intake region of the shroud to the fluid exit region of the shroud greater than 90 degrees. 
         [0033]    When a drum rotor fan is used to generate airflow for thrust of an airborne craft, the torque required to drive the rotor causes an equal and opposite torque on the chassis of the device—referred to as a yaw torque. A yaw torque is also produced on the chassis each time the rotor speed is increased or decreased which cause a yaw motion of the chassis. This yaw torque may be counteracted by the use of stator blades which act upon the fluid flow so as to counter the yaw torque produced by the airflow through the rotor. Furthermore, by moving some or all of the stator blades, the yaw torque produced by them may be increased or decreased thereby providing a means to control the orientation of the device in the yaw direction. 
         [0034]    By using a shroud that wraps around, or forms an envelope about, the drum rotor fan and exits below the fan, the envelope includes a region below the drum rotor fan that can be conveniently utilised to place stator blades. 
         [0035]    Therefore, in an airborne craft utilising a drum rotor fan for generating airflow for thrust production and having a shroud which re-directs airflow from a fluid intake area disposed at the top of the shroud and proximate the fluid exit region of the rotor to an exit area below the rotor and wherein the air flow at the fluid exit region of the craft is directed towards the axis of the shroud, there is provided a plurality of stator blades proximate the exit region of the shroud and below the rotor. When the air flow is at 90 degrees to the central axis of the shroud, the stator blades are conveniently positioned directly below the rotor and are disposed in an annular region about the axis of the rotor. 
         [0036]    Preferably, at least some of such stator blades are actuated or movable so as to provide varying degrees of yaw torque. At least one of the stator blades may be controllably movable, for example by being pivotably mountable, so as to effect the degree of yaw torque produced by the at least one stator blade. 
         [0037]    As mentioned briefly above, during acceleration or deceleration of the drum rotor fan, a reaction torque acts on the chassis of the airborne craft in a yaw direction. A counter rotating mass may be provided to counter this reaction torque. The counter rotating mass may be a second counter-rotating drum rotor fan. Most advantageously, this counter rotating mass may also be selected so as provide an equal and opposite gyroscopic effect to the primary rotor. 
         [0038]    Whilst such counter rotating mass may be in any one of a number of forms, or in a combination of different forms, the use of a shroud that re-directs airflow back towards the central axis of the device provides a convenient location to provide a second counter rotating drum rotor fan in the fluid exit region of the shroud. 
         [0039]    Thus, there is provided a fluid dynamic device, such as an airborne craft comprising a first drum rotor fan having a first rotational direction for generating a radially outward fluid flow and a shroud for redirecting the radially outward fluid flow from said first drum rotor fan to a region below said first drum rotor fan, and a second drum rotor fan disposed in said region below the first drum rotor fan, said second drum rotor having a second rotational direction. 
         [0040]    Preferably, the second drum rotor fan receives radially inwardly flowing fluid from the shroud and discharges such fluid radially inwardly towards the rotational axis of the second drum rotor fan. 
         [0041]    Preferably, the second rotational direction is opposite to the first rotational direction. 
         [0042]    Preferably, the device or craft is provided with a control means to alter the relative rotational speeds of said first and second drum rotor fans. By changing the relative rotational speeds of the two rotors, the yaw orientation of the device or craft can be controlled. 
         [0043]    This counter-intuitive use of a drum rotor type fan to generate a radially inward fluid flow assists in additional airflow and thrust generation as well as providing a yaw torque cancellation function. 
         [0044]    In a further aspect, the present invention provides an airborne craft comprising a shroud disposed about a central axis of the device and having an upstream airflow intake region and a downstream airflow exit region, the shroud directing airflow between said upstream airflow intake region and said downstream airflow fluid exit region wherein the shroud is configured so that, at least during some operating conditions of the airborne craft, substantially all of the airflow exiting the airflow exit region is directed towards the central axis of the shroud. 
         [0045]    The fluid dynamic device and airborne craft of the invention may be more fully understood from the following description of preferred embodiments thereof made with reference to the accompanying drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIG. 1  is a schematic drawing of an airborne craft which uses a flexible shroud for thrust vectoring. 
           [0047]      FIG. 2  is a sectional view of the airborne craft similar to the one shown in  FIG. 1 . 
           [0048]      FIG. 3  shows the craft depicted in  FIG. 2  with the shroud shown in the neutral and in the deflected position. 
           [0049]      FIG. 4  is a sectional view of an airborne craft according to a first embodiment of the invention. 
           [0050]      FIG. 5  is a shaded drawing of the craft shown in  FIG. 4  in order to show additional detail. 
           [0051]      FIG. 6  is a sectional isometric view of the craft shown in  FIG. 4 . 
           [0052]      FIG. 7  is an isometric view, looking from below the craft, of the craft shown in  FIG. 4 . 
           [0053]      FIG. 8  is an isometric sectional view, of a further embodiment of the invention which uses a counter rotor. 
           [0054]      FIG. 9  is an isometric view of the craft shown in  FIG. 8  with the shroud removed. 
           [0055]      FIG. 10  is a sectional view, of the drive detail for the rotor and counter rotor of the craft shown in  FIGS. 8 and 9 . 
       
    
    
     DETAILED DESCRIPTION  
       [0056]    Referring to  FIG. 1  there is shown a fluid dynamic device in the form of an airborne craft  100 , which can be used in a wide variety of applications. The craft  100  comprises a central load carrying space  110  that provides a cockpit operating area  111  for an operator (not shown) while maximizing the area available for airflow into the drum rotor fan  120 . The operator may be seated in the cockpit  111  facing forward (as indicated by arrow  112 ). The air flow to the drum rotor fan  120  flows via the central area of the craft (that is, through the central load carrying space  110 ) into the drum rotor fan  120  and is expelled radially by the rotor fan  120 . The radial airflow is re-directed downwards by a shroud  130  to generate lifting thrust for the airborne craft  100 . The shroud  130  may, in part, be of a flexible material having orthotropic characteristics in that it is relatively stiff in one or more direction(s) of stress and relatively flexible in the other direction(s) of stress. The lower part of the shroud  130  comprises a rigid outer rim  140 . This rim  140  is movable by the operator so that the shroud  130  can be deflected to thereby change the flow area of one part of the shroud as compared to the flow area of another part of the shroud  130  to thereby change the location and/or orientation of the thrust acting on the craft. Such deflection provides directional control over the airborne craft  100  by producing a horizontal thrust vector and/or a torque about the centre of gravity of the craft (as further discussed with reference to  FIG. 3  below). The flexible shroud  130  is shown in a forward deflected position as may be used to effect a braking or reversing maneuver. 
         [0057]    Referring to  FIG. 2 , there is an airborne craft  100  of generally the same nature as that shown in  FIG. 1 . Similar items between the craft shown in  FIG. 1  and  FIG. 2  use the same reference number. In contradistinction to the craft  100  shown in  FIG. 1 , the craft  100  shown in  FIG. 2  is adapted for autonomous operation (generally referred to as an Unmanned Aerial Vehicle or UAV) and the cockpit area  111  of the craft shown in  FIG. 1  is replaced by a payload area  211 . 
         [0058]    In  FIG. 2  the arrows  201  indicate the direction of airflow into the drum rotor fan  120 . The airflow is then deflected by the shroud  130  to a generally downward direction as indicated by arrows  202 , thus generating a lifting thrust for the airborne craft  100 . 
         [0059]    As can be best seen in  FIG. 2 , the shroud  130  of the airborne craft  100  in  FIGS. 1 and 2  creates a generally downward facing air exit area in the shape of a flat annular ring  200 . The airflow exiting the shroud  130  is therefore in a direction generally parallel to the axis  210  of the craft. 
         [0060]    Referring to  FIG. 3 , there is shown a neutral position of the shroud  130  and an actuated deflected position  131  (in dotted lines). A resultant force  218  acts through the centre of gravity  118  when shroud  130  is in the neutral position. Actuation of the shroud  130  to the deflected position  131  results in a change in the position of the centre of action of the lifting forces due to movement in location of the resultant force to  219  (although shown as a vertical arrow, the resultant force may also be inclined to the axis of the craft as a result of the vectoring of the airflow out of the shroud). This results in a torque being produced about the centre of gravity and the craft rotating in this direction. The resulting tilt of the craft thus vectors the thrust such as to propel the craft laterally. Once the desired level of tilt has been achieved, the shroud  130  is actuated to maintain this desired level of tilt. Although not shown in this FIG., movement of the shroud to the deflected condition  131  may also result in a change of direction of the resultant force vector as well its location. 
         [0061]    Referring to  FIG. 4  there is shown a schematic cross-section of an airborne craft  100 , according to a first embodiment of the invention. 
         [0062]    The airborne craft  100  is generally of a similar nature to that shown in the previous FIGS. and similar items use the same references as in those earlier figures. 
         [0063]    The airborne craft in  FIG. 4  is configured to provide an autonomous airborne vehicle capable of free flight as well as vertical take-off and landing, hovering and perch and stare capability. 
         [0064]    A rotor  120  rotates about a central rotational axis  210  to generate a radially outward airflow. Air flow, as generally indicated by arrows  201 , flows into a central region of the craft  100 , through a stator  215  and into the rotor  120 . Airflow is also drawn in from above the stator cap  216  and on the radially outer side of the stator  215 —as indicated by arrow  217 . The rotor  120  is driven by three electric motors (not shown in  FIG. 4 , but indicated at item  242  in  FIG. 6 ) mounted to a chassis  241 . The rotor  120  is provided with a lower peripheral rim (not shown) which is in frictional driving engagement with the motor. As a result of the drive torque produced by the motors to drive the rotor,  120 , a yaw torque is produced (the yaw axis being understood to be the vertical axis  210  of the craft  100 ). 
         [0065]    The outlet of the rotor  120  is surrounded by a shroud in the form of a flexible skirt or shroud  130 , having central axis  210 , which redirects the air from a fluid (air) intake region  220 , through approximately 180 degrees to fluid (air) exit region  222  (as indicated by arrows  223 ). 
         [0066]    The airflow emanating from the air exit region  222  is at 90 degrees to the central axis  210  of the shroud  130 . Because the air flow is radially inward towards the central axis  210  of the shroud  130 , it is deflected downwardly as generally indicated by the arrows  202  to produce a thrust. 
         [0067]    The thrust may be vectored by movement of the annular rim  140  of shroud  130  which is attached to the skirt at the lower part of the fluid exit region  222 . 
         [0068]    Stator blades  240  are provided at the fluid exit region  222 . These stator blades  240  provide a counter acting yaw torque to help to neutralize the yaw torque produced by the rotor  120 . 
         [0069]      FIG. 6  shows the airborne craft of  FIG. 4 . A rotor  120  rotates about an axis  210  to generate a radially outward airflow. Air flows into a central region of the craft  100 , through a stator  215  and into the rotor  120 . 
         [0070]    The outlet of the rotor  120  is surrounded by a shroud in the form of a flexible skirt or shroud  130  which redirects the air from an fluid (air) intake region  220 , through approximately 180 degrees to fluid (air) exit region  222  (as indicated by arrows  223 ). 
         [0071]    The airflow emanating from the air exit region  222  is at 90 degrees to the central axis  210  of the shroud  130 . Because the air flow is radially inward towards the central axis  210  of the shroud  130 , it is deflected downwardly as generally indicated by the arrows  202  to produce a thrust. 
         [0072]    The thrust may be vectored by movement of the annular rim  140  of shroud  130  which is attached to the skirt at the lower part of the fluid exit region  222 . 
         [0073]    Stator blades  240  are provided at the fluid exit region  222 . These stator blades  240  provide a counter acting yaw torque to help to neutralize the yaw torque produced by the rotor  120 . 
         [0074]      FIGS. 7  shows a complete isometric view of the craft of  FIG. 6  and also shows the electric motors  242  that drive the rotor  120  (only two of the three motors are seen in  FIG. 7 ). 
         [0075]    In an alternate embodiment, as shown in  FIG. 8 , a counter rotating drum rotor fan  720  is be positioned in the area occupied by, and replaces, the stator blades  222  in  FIG. 4 to 7 . This counter rotating drum rotor fan  720  pumps air radially inwardly to assist in generating additional thrust in the airflow  202 . Drum rotor fan  720  also counteracts the yaw torques and gyroscopic forces generated by the primary rotor  120 . 
         [0076]      FIG. 8  shows the airborne craft of  FIG. 6 . A rotor  120  rotates about an axis  210  to generate a radially outward airflow. 
         [0077]    The outlet of the rotor  120  is surrounded by a shroud in the form of a flexible skirt or shroud  130  which redirects the air from an fluid (air) intake region  220 , through approximately 180 degrees to fluid (air) exit region  222  (as indicated by arrows  223 ). 
         [0078]    The second drum rotor fan  720 , is mounted for rotation about the common axis  210 , in the air exit region  222  of the shroud  130 . Because the air flow is radially inward towards the central axis  210  of the shroud  130 , it is deflected downwardly as generally indicated by the arrows  202  to produce a thrust. As looking from the top of craft  100 , primary rotor  120  spins in the clockwise direction, and secondary rotor  720  spins in the counterclockwise direction. Conveniently, both rotor  120  and rotor  720  may be powered and driven by the same electric motors  242  (as will be further described with respect to  FIG. 10 ). 
         [0079]      FIG. 9  shows an isometric view of the craft  100  shown in  FIG. 8  and with the shroud  130  removed so as to show further detail of arrangement of the primary rotor  120  (spinning in the clockwise direction as looking down onto the craft) and the secondary, counter-rotating rotor (spinning in the anticlockwise direction). A diffuser  226  is located intermediate the rotors and assists in redirection the airflow from rotor  120  to rotor  720 . 
         [0080]      FIG. 10 . shows a close up detail of the left hand side of the craft of  FIG. 8 . An electric drive motor  242  drives through a set of drive sheaves comprising an upper drive sheave  281  and a lower drive sheave  282 . These drive sheaves are in frictional drive engagement with a downward depending rim of the rotor  120  (seen to the left of the drive sheaves  281 , 282 ). The other side of this downward depending rim of rotor  120  is in frictional driving engagement with an idler  283  which is frictionally drivingly engaged with an upwardly disposed rim of rotor  720  to thereby provide drive to the rotor  720 . By this arrangement the rotors  120 , and  720 , are also axially located within the chassis of the craft  100 . 
         [0081]    Although a number of embodiments have been described, it will be appreciated that the invention is not only applicable to aerodynamic lifting devices and is not necessarily limited to a circular shroud as has been exemplified in the description. 
         [0082]    Furthermore, the invention contemplates an embodiment wherein the exit angle of the air from the shroud  130  may be controlled from a generally downward facing direction as seen in  FIGS. 1 and 2 , to a generally inward facing direction as seen in  FIGS. 3 and 4 . It further contemplates the air being redirected by more than 180 degrees from a radially outward direction at the rotor fan exit, so as to exit the shroud in a direction back towards the underside of the craft. 
         [0083]    The Applicant has found that a fluid dynamic device, or airborne craft, as above described produces desirable benefits, including increased thrust per unit of power input at certain operating conditions, including those operating conditions that are out of ground effect (for example when the craft is more than two times the diameter of the craft away from the ground surface). 
         [0084]    Other modifications and variations to the fluid dynamic device and airborne craft of the invention may be apparent to skilled readers of this disclosure. Such modifications and variations are deemed within the scope of the present invention.