Abstract:
A propeller includes a plurality of propeller blades. Each blade has an adjustable drag coefficient. A plurality of actuators adjusts the drag coefficients of the propeller blades. A controlling unit controls the plurality of actuators such that the drag coefficients of each propeller blade is adjusted according to a pattern that is dependent upon the rotational angle of the particular propeller blade so that the drag of each propeller blade is maximized at the same point through a course of revolution and minimized throughout the remainder of that revolution.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to an asymmetrically changing rotating propeller and, more specifically, to an asymmetrically changing rotating blade shape propeller and its airplane and wind turbine applications. 
         [0003]    2. Description of the Related Art 
         [0004]    A propeller is a device which forces fluid passing through it and increases the fluid kinetic energy. Vehicles or vessels such as airplanes, ships and submarines may use propellers to propel though a fluid such as air or water. Alternatively, a propeller can be placed in the path of a moving fluid to absorb the fluid kinetic energy, such as a wind mill. A typical propeller is generally comprised of one or more twisted blades which are rotated around a central shaft. 
         [0005]    Conventional propellers have an identical geometric configuration that remains constant as the blades revolve around the central shaft. Both lift and drag force components are developed on the blade. Only the fluid momentum increase in the axial direction is utilized to produce desired axial thrust. 
         [0006]    While conventional propellers provide symmetric thrust with respect to each revolution cycle, examples of asymmetric propulsion techniques are known. For example, when rowing a rowboat, a boater sweeps an oar from front to back while its face is submerged in water and then, to complete the rowing cycle, the boater carries the oar to its original front position while removed from the water. By moving the oar from the more dense water to the less dense air on the return stroke, the boater is able to apply maximum energy during the front-to-back sweep and conserve energy on the return thereby maximizing propulsion efficiency. The angle of the oar may also be adjusted by the boater to further optimize propulsion efficiency 
         [0007]    Another example is when a swimmer performs the breast-stroke. Here a swimmer&#39;s arms are swept from front-to-back while under the water and are then returned to their original front position while removed from the water. The angle of the swimmer&#39;s hands may also be adjusted to optimize the propulsion efficiency. 
         [0008]    In this way, the boater and swimmer are able to adjust the coefficient of drag of the oar/arm so that the drag is higher where it is needed the most and lower where it is needed the least. However, conventional propellers are unable to adjust their coefficient of drag during the blade&#39;s cycle of rotation and thus propulsion cannot be optimized. 
         [0009]    Some propellers in the art have the ability to change blade pitch to improve propeller performance. For example, U.S. Pat. No. 6,991,426, to Pietricola, and U.S. Pat. No. 6,942,458, to McCallum et al., describe variable pitch propellers where blade pitch is adjusted according to the needs of the airplane. For example, the blades may have one pitch during takeoff and another pitch during high-altitude cruise. 
         [0010]    While such variable pitch propellers have the ability to alter blade lift and drag coefficients, lift and drag are not changed during the course of the propeller&#39;s rotational cycle. Another example involved the wing flapping of a bird or an insect. In the first half of a wing-flapping cycle, the wing pushes the air downward/backward of the bird. This increases downward/backward air momentum passing through the wing. The reaction to this air momentum increase is a force which pushes the bird upward/forward. Next, the wing must return to its starting cycle configuration in order to be ready for its next wing-flapping cycle. This is done during the second half of the wing-flapping cycle. Most wings are not designed to produce desired thrust in both half cycles. Therefore, the wing must conserve its energy to perform its second half cycle return journey. 
         [0011]    Accordingly, it is desired that a propeller have the ability to adjust the drag coefficient of each blade during the course of the propeller&#39;s rotational cycle. 
       SUMMARY 
       [0012]    A propeller includes a plurality of propeller blades. Each blade has an adjustable drag coefficient. A plurality of actuators adjusts the drag coefficients of the propeller blades. A controlling unit controls the plurality of actuators such that the drag coefficient of each propeller blade is adjusted according to a pattern that is dependent upon the rotational angle of the particular propeller blade. 
         [0013]    A method for propelling a vehicle or vessel though a fluid includes rotating a propeller having one or more blades. The drag of each of the one or more blades is adjusted according to a pattern that is dependent upon the rotational angle of the particular propeller blade. 
         [0014]    A method for harvesting wind energy with a wind turbine includes configuring one or more propellers with one or more blades to be rotated by the force of wind. The drag of each of the one or more blades is adjusted according to a pattern that is dependent upon the rotational angle of the particular propeller blade, relative to the incoming air velocity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A more complete appreciation of the present disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a perspective view of an example propeller according to an embodiment of the present invention; 
           [0017]      FIG. 2A  is a graph showing an example of how the opening angle of the propeller blades may change as a function of the angle of rotation; 
           [0018]      FIG. 2B  is a graph showing an example of how the surface area (perpendicular to the incoming tangent velocity due to propeller rotation) of the propeller blades may change as a function of the angle of rotation; 
           [0019]      FIG. 2C  is a graph showing an example of how the drag coefficient of the propeller blades may change as a function of the angle of rotation; 
           [0020]      FIG. 3  is a plan view showing four connecting members which may connect four blade-pairs of a propeller according to the present invention; 
           [0021]      FIG. 4  is a side view of the four connecting members depicted in  FIG. 3 ; 
           [0022]      FIG. 5A  is a plan view of a first propeller blade of a blade-pair according to an embodiment of the present invention; 
           [0023]      FIG. 5B  is a plan view of a second propeller blade of a blade pair according to an embodiment of the present invention; 
           [0024]      FIG. 5C  is a side view of the propeller blade depicted in  FIG. 5A ; 
           [0025]      FIG. 5D  is a side view of the propeller blade depicted in  FIG. 5B ; 
           [0026]      FIG. 6A  is a plan view of a rotating disc with support tubes according to an embodiment of the present invention; 
           [0027]      FIG. 6B  is a side view of the rotating disc with support tubes depicted in  FIG. 6A ; 
           [0028]      FIG. 7A  is a plan view of a cover according to an embodiment of the present invention; 
           [0029]      FIG. 7B  is a side view of the cover depicted in  FIG. 7A ; 
           [0030]      FIG. 8A  is an exploded plan view of a control link and control mechanism according to an embodiment of the present invention; 
           [0031]      FIG. 8B  is a side view of the control rod depicted in  FIG. 8A ; 
           [0032]      FIG. 8C  is a rear view of the control rod depicted in  FIG. 8A ; 
           [0033]      FIG. 8D  is a top view of the control rod depicted in  FIG. 8A ; 
           [0034]      FIG. 9A  is a plan view of a ring base according to an embodiment of the present invention; 
           [0035]      FIG. 9B . is a side view of the ring base depicted in  FIG. 9A ; 
           [0036]      FIG. 9C . is another side view of the ring base depicted in  FIG. 9A ; 
           [0037]      FIG. 9D  is a diagram of the shape of the guide track throughout the course of a single revolution as a function of the angle of rotation; 
           [0038]      FIG. 10  is a partial side view of a propeller blade with actuating mechanism according to an embodiment of the present invention; 
           [0039]      FIG. 11A  is a plan view showing the arrangement of four propellers on an airplane, according to an embodiment of the present invention; 
           [0040]      FIG. 11B  is a side view of the airplane propeller arrangement depicted in  FIG. 11A ; 
           [0041]      FIG. 11C  is another side view of the airplane propeller arrangement depicted in  FIG. 11A ; 
           [0042]      FIG. 12A  is a front view of an airplane according to an embodiment of the present invention; 
           [0043]      FIG. 12B  is a top view of the airplane depicted in  FIG. 12A ; 
           [0044]      FIG. 12C  is a side view of the airplane depicted in  FIG. 12A ; and 
           [0045]      FIG. 13  is a cross-sectional view of a windmill according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    In describing the preferred embodiments of the present disclosure illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner. 
         [0047]    A propeller according to embodiments of the present invention may include a plurality of propeller blades arranged in pairs. Each propeller blade-pair may have a set of two hinged blades that are capable of opening and closing at specific intervals in each cycle of revolution. When the blade-pairs are fully closed, the blades may be nearly flat. As the blade-pairs open, the blade becomes non-flat and the surface area of the blade-pairs, which is perpendicular to the incoming wind velocity vector, increases. 
         [0048]    Propellers according to embodiments of the present invention may have multiple blades. While the propeller may have any number of blades, examples described herein have four blade-pairs. 
         [0049]    According to one embodiment, all of the blade-pairs may have hinged blades. 
         [0050]      FIG. 1  is a planar view of an example propeller according to an embodiment of the present invention. The propeller  10  has four blade-pairs  20 ,  21 ,  22 ,  23  that each consist of a first blade  1   a  hingedly connected along a radial axis to a second blade  1   b . The details of the structure and connection of the blades will be described in more detail herein with reference to  FIGS. 5A through 5D  and  FIG. 10 . The four blade-pairs  20 ,  21 ,  22 ,  23  are further arranged in pairs of two  20 ,  22  and  21 ,  23  with each blade-pair being disposed 180 degrees from the other blade-pair in the pair, and each pair being 90 degrees displaced from the other pair, for example. The blade-pairs are interconnected with connecting members  4   a  to form the propeller  10 . The propeller  10  rotates around a central axis  1   p , in a clockwise direction so that the opening of the blade-pairs faces the incoming tangential air velocity vector  1   v.    
         [0051]    The changing area of the propeller blades can best be understood with reference to the plane of rotation defined by the x- and y-axes depicted in  FIG. 1 . The angle of rotation  1   e  of any given blade-pair  20 ,  21 ,  22 ,  23  is measured with respect to a predefined start point (0°), in this case chosen to coincide with the x-axis. The angle of the opening if between the blades  1   a ,  1   b  of each blade-pair  20 ,  21 ,  22 ,  23  changes throughout one cycle of rotation (360°) from a maximum at angel of rotation of 0° (i.e.  1   e= 0°) to a minimum (i.e.,  1   f= 0°) when the blade-pair is outside the range −80°&lt; 1   e&lt;+ 80°. The angle of opening if may range from 0° to a maximum of, for example, 90°. The angle of opening if may be dependent upon the angle of rotation  1   e  which describes the blade&#39;s position in the rotational cycle. 
         [0052]    Because the opening angle  1   f  is a function of the angle of rotation  1   e  and there are four blade-pairs spaced 90° from each other, each blade-pair closes and opens according to the same formula, but at 90° out of phase from the nearest blade-pairs, and 180 degrees out of phase from the opposite radii blade-pair. Accordingly, the drag of the propeller is asymmetric as at any given time, as all blades may not share the same closing angle. 
         [0053]      FIGS. 2A through 2C  show different parameters of the blades as a function of the angle of rotation  1   e . It is important to note that the diagrams of  FIGS. 2A through 2C  show different parameters as a function of angle of rotation, and that, if the propeller is rotating clockwise as anticipated in one embodiment of the present invention, the cycle of rotation is from right to left in each of the diagrams, i.e. from +180° to −180°, as depicted by the arrows R. According to one embodiment, as shown in  FIG. 2A , the opening angle if remains at a minimum (0°), i.e. fully closed, until the angle of rotation  1   e=+ 80° at which point the opening angle if gradually increases until the angle of rotation  1   e  reaches the starting point (0°) at which point the opening angle if reaches it maximum, 90° in this embodiment. Thereafter, the opening angle if gradually decreases to 0° as the angle of rotation  1   e  approaches −80°. Asymmetric variation of the opening angle  1   f  will cause asymmetric variations of the surface area S of each blade-pair and of the drag coefficients of each blade-pair throughout a cycle of rotation. 
         [0054]      FIG. 2B  shows changes in the surface area S of each blade-pair throughout a cycle of rotation. Again, the surface area of each blade-pair reaches a maximum when the angle of rotation  1   e  is at the starting point (0°). 
         [0055]      FIG. 2C  shows changes to the drag coefficient C D  for each blade-pair throughout a cycle of rotation. The blade-pairs have a maximum drag coefficient C D  of 2.0 at the point where the opening angle  1   f  is at a maximum, i.e. at an angle of rotation of 0°. The drag coefficient for a wing is computed using wing area that is defined as the wingspan times the wind-cord. However, for a blunt object, the area used to calculate the drag coefficient is the area perpendicular to the incoming air stream. Since an opened blade-pair is essentially perpendicular to the incoming tangential air velocity  1   v , such blade-pair can be considered a blunt object, so that the blade surface area S is used for all blade drag force calculations. 
         [0056]    The asymmetric changes in surface area and drag coefficients lead to an asymmetric increase of air momentum as the air is pushed away by the blade-pairs tangentially in the rotational plane. 
         [0057]    The aerodynamic force on a typical blade-pair at constant rotation speed  0 ) can be calculated using formulas described below. Only a net drag force component is developed on each blade-pair; a lift force component is not developed. This drag force vector is in the direction of the incoming tangential air velocity  1   v . A useful calculation can decompose this drag force vector into its x-axis, D x (θ), and y-axis, D y (θ), components, where θ is the angle of rotation  1   e . The x-axis and y-axis coordinate system is shown in  FIG. 1  for clockwise rotating blade-pairs. According to the present invention, the drag force components are derived as follows: 
         [0000]    
       
         
           
             
               
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       Where, 
       [0058]    ρ is the air density, 
         [0059]    ω is the rotation speed of the propeller, 
         [0060]    C D (θ) is the drag coefficient, 
         [0061]    h 0  is the diameter of the blade pair hinge channel  5   g,    
         [0062]    b is the blade-pair cord  5   q , which is the width of the leading edge  5   a  or  5   b  of a blade  1   a  or  1   b , as measured from the center of the hinge channel  5   g  to the edge, 
         [0063]    φ is the blade-pair opening angle  1   f,    
         [0064]    R 2  is the distance from the central axis  1   p  to the furthest end of the blade-pair, 
         [0065]    R 1  is the distance from the central axis  1   p  to the nearest end of the blade-pair. A typical four blade-pair propeller will have its drag force components in the y-axis and x-axis expressed as the sum of four calculations for D x (θ) and D y (θ) taken at 90-degree intervals along the cycle of rotation, e.g. at an angle of rotation  1   e=− 180°, −90°, 0°, +90° and +180°. These drag force equations can be useful to make preliminary estimates of the performance of the propeller blade systems described herein. 
         [0066]    A more detailed understanding of the structure of one embodiment of each blade-pair  20 ,  21 ,  22 ,  23  can be had by reference to  FIGS. 5A through 5D . As more particularly shown in  FIGS. 5A and 5C , an upper blade  1   a  may consist of a leading portion  5   a , a hinge portion  5   m , and a trailing portion  5   c . As shown in  FIGS. 5B and 5D , the lower blade  1   b  may consist of a leading portion  5   b , a hinge portion  5   n , and a trailing portion  5   d . To assemble the blade pair, the leading portions  5   a ,  5   b  and hinge portions  5   m ,  5   n  of the upper and lower blades  1   a ,  1   b , are fit together like a door hinge with the connecting member  4   a  inserted through a channel  5   h  formed by the hinge portions  5   m ,  5   n  to hold the leading blade portions  5   a ,  5   b  together. The hinge channel  5   h  has a diameter  5   g  as shown in  FIGS. 5C and 5D . Next, the trailing portions  5   c ,  5   d  are welded onto the respective hinge portions  5   m ,  5   n  for weight balance. As can be further seen in  FIGS. 5A through 5D , depending fins  5   e ,  5   f  are attached the undersides of the lower leading portion  5   b  and the upper trailing portion  5   c , respectively, for connecting the blade-pair to the actuating mechanism, as will be described more fully hereafter. 
         [0067]    As discussed above, a propeller may consist of four blade-pairs  20 ,  21 ,  22 ,  23 , arranged at 90 degree intervals. As shown in  FIG. 3 , four rigid connecting members  4   a  may be arranged in a cross-shape around a hub  4   b . One blade pair is mounted on the end of each connecting member  4   a , by inserting the connecting member  4   a  through the hinge channel  5   h  and capping the end with a removable endcap  4   d , as shown in  FIG. 4 . The hub  4   b  of the connecting members  4   a  is attached to the shaft of a motor  4   c  for rotating the propeller  10  about the center axis of rotation  1   p . The rotation may be clockwise. 
         [0068]    While there are many ways to implement the opening and closing of the blade-pairs, an actuating mechanism  80  which enables the blade-pairs  20 ,  21 ,  22 ,  23  to open and close according to one embodiment of the present invention will now be described. There is one actuating mechanism  80  for each blade-pair, and all actuating mechanisms are the same. As shown in  FIG. 8A , the actuating mechanism  80  includes a control rod  8   a  which is vertically arranged as will be described herein. The control rod  8   a  includes an upper control wheel  8   m  rotatably mounted on the control rod  8   a  via shaft  8   x , and a lower control wheel  8   n  rotatably mounted on the control rod  8   a  via shaft  8   y , as shown in  FIGS. 8A through 8C . The control wheels  8   m  and  8   n  rotate freely and independently of one another on respective bearings  8   p ,  8   q , and are vertically aligned. A connection extension  8   d  extends from the top of each control rod  8   a  to connect the control rod to the control links  8   b ,  8   c  which connect to the blade-pairs. Thus, a hole  8   f  in the lower end of the leading control link  8   b  is aligned with a hole in the connection extension  8   d  of the control rod  8   a  and a hole  8   h  in the lower end of the trailing control link  8   c , as shown in  FIG. 10 . The connection between the three pieces (the control links and the control rod) is such that they are rotatably connected, by, for example, a bolt inserted through the aligned holes. The holes  8   e ,  8   g  in the upper ends of the control links  8   b ,  8   c  are attached to the depending fins  5   e ,  5   f  of the lower leading portion  5   b  and the upper trailing portion  5   c , respectively, by, for example, a bolt which permits relative rotation of the joined pieces, as shown in  FIG. 10 . 
         [0069]    According to this construction, when the control rod  8   a  moves down, the lower leading portion  5   b  and the upper trailing portion  5   c  of the blade-pair also move down through action of the control links  8   b ,  8   c . Since the upper trailing portion  5   c  is rigidly connected to the upper leading portion  5   a , when the upper trailing portion  5   c  moves downward, the upper leading portion  5   a  moves upward, thereby causing the blade-pair to open. Conversely, when the control rod  8   a  moves up, the lower leading portion  5   b  and the upper trailing portion  5   c  of the blade-pair also move up through action of the control links  8   b ,  8   c . Since the upper trailing portion  5   c  is rigidly connected to the upper leading portion  5   a , when the upper trailing portion  5   c  moves upward, the upper leading portion  5   a  moves downward, thereby causing the blade-pair to close. The vertical movement of the control rod  8   a  is imparted by vertical movement of the control wheels  8   m ,  8   n  along a circular guide track, as will be described more fully herein. 
         [0070]    As described above, a propeller  10  may consist of four blade-pairs, and, thus, four control rods  8   a . Each control rod  8   a  is mounted in a cylindrical support tube  60 , shown in  FIG. 6B . Each support tube  60  has a vertical slot  6   c  to guide vertical movement of the shafts  8   x ,  8   y  of the actuating mechanism  80 . The four support tubes  60  may be arranged at 90 degree intervals, or however the blade-pairs are arranged, along the circumference of a rotating disc  70 . The support tubes  60  are arranged so that the vertical slots  6   c  face outward. The rotating disc  70  may be made of an upper rotating disc  6   d , and a lower rotating disc  6   e , fastened together by screws (not shown) inserted in holes  6   g . The holes  6   g  further allow the upper rotating disc  6   d  and the lower rotating disc  6   e  to be aligned. Each support tube is comprised of an upper support tube  6   a , and a lower support tube  6   b . The upper support tube  6   a  is mounted on the upper rotating disc  6   d , by, for example, welding, and the lower support tube  6   b  is mounted on the lower rotating disc  6   e , by, for example, welding. When fully assembled, the upper and lower rotating discs  6   d ,  6   e  and the upper and lower support tubes  6   a ,  6   b , all rotate in unison along with upper and lower rotating hubs  6   p ,  6   q.    
         [0071]    For rotation of the blade-pairs, power is transmitted from the motor shaft  4   c  to the blade-pairs via the connecting members  4   a . The blade-pairs in turn impart rotational movement to the control rods  8   a , and, thus, to the rotating disc  70  about the axis of rotation  1   p.    
         [0072]    The vertical movement of the control rods  8   a , on the other hand, is dictated by the guide track  90  depicted in  FIGS. 9A through 9D . According to one embodiment of the present invention, the propeller  10  may include a circular ring base  9   f , which has a predetermined wall thickness  9   g  and a predetermined height  9   h , as shown in  FIGS. 9A and 9B . The circular ring base  9   f  may be made of metal, a polymeric material, or any other suitable material. The circular ring base  9   f  has an inner surface  9   j  and an outer surface  9   k . A guide track  90  may be carved into the inner surface  9   j  of the circular ring base  9   f . As shown in  FIG. 9A , the depth of the guide track  90  may be, for example, approximately half the wall thickness  9   g  of the circular ring base  9   f . The guide track  90  has an upper surface  9   a  and a lower surface  9   b  which together define the width  9   c  of the guide track  90 . The width  9   c  of the guide track  90  may remain constant throughout its length. The control wheels  8   m ,  8   n  travel in the guide track  90 , such that the upper control wheel  8   m  abuts the upper surface  9   a  of the guide track  90 , and the lower control wheel  8   n  abuts the lower surface  9   b  of the guide track  90 . 
         [0073]    According to one feature of the present invention, when the blade-pair is located at an angle of rotation in the range −90°&gt; 1   e&gt;+ 90°, i.e. when the angle of rotation  1   e  is not between points C and B in  FIG. 9D , the upper surface  9   a  and the lower surface  9   b  of the guide track  90  are located at fixed distances from the upper and lower edges  9   p ,  9   q  of the circular ring base  9   f  such that the guide track  90  does not exhibit any vertical displacement. Thus, through half a cycle of rotation, i.e. from −90° to +90° when the propeller  10  is rotated in a clockwise direction, the guide track  90  is at a constant vertical distance and, accordingly, the control wheels  8   m ,  8   n  do not move up or down, and the blade-pair remains closed. In  FIG. 9D , point A represents  1   e= 0°; point B represents  1   e=+ 90°; point C represents  1   e=− 90°; and point D represents  1   e= 180°. 
         [0074]    During the range of rotation where the angle of rotation  1   e  is between +90° and −90° (points B and C in  FIG. 9D ), the guide track  90  may exhibit a dip wherein the upper surface  9   a  and the lower surface  9   b  of the guide track  90  become downwardly vertically displaced in unison to the point where the angle of rotation  1   e  is 0° (point A in  FIG. 9D ), after which point the upper surface  9   a  and the lower surface  9   b  of the guide track  90  become upwardly vertically displaced in unison until the point where the angle of rotation  1   e  is −90° (point C in  FIG. 9D ). An example of the profile of the dip described above is shown in  FIG. 9B . When the guide track  90  dips, the control wheels  8   m ,  8   n  are vertically downwardly displaced, which causes the blade-pair to open. Thus, during the course of the dip, the blade-pair gradually opens as the control rod  8   a  descends and then closes as the control rod  8   a  ascends. At point A (i.e.  1   e= 0°), the guide track  90  exhibits its maximum dip  9   m . At this point, the blade-pair is open to its maximum angle of opening  1   f.    
         [0075]    Optionally, the blade-pair assembly may also include a cover  7   a , which is cylindrical in shape and adapted to be placed on the upper rotating disc  6   d  of the rotating disc  70 . The cover  7   a  may include a plurality of notches  7   c  (in this embodiment, four), which may be disposed at 90° intervals which coincide with the upper support tubes  6   a  used to support the control rods  8   a . A hub  7   b  is formed in the center of the cover  7   a  and is fastened to the power shaft  4   c . Thus, rotational power may be directly transferred to the cover  7   a , which will further assist in the rotation of the blades. 
         [0076]    According to the present invention, each rotating propeller blade is made to change its area S, perpendicular to the incoming tangential rotational velocity vector  1   v , in each revolution. This is accomplished by opening and closing each blade-pair  20 ,  21 ,  22 ,  23  at specified intervals of each rotation. Each blade area S will reach its designed maximum area at a given location in the rotation cycle. This maximum blade area location (at  1   e= 0°) is common to all blade-pairs  20 ,  21 ,  22 ,  23  in a multiple-blade propeller  10 . After each blade-pair reaches its maximum surface area S, it will quickly reduce its blade area S to a minimum for the remaining revolution cycle. Each blade-pair pushes the air away, which increase the air mass tangential and radial velocity components asymmetrically in each revolution. This action increases the air mass momentum, which exerts opposite drag force components on the blades  1   a ,  1   b . Since the blade-pairs&#39; rotating shaft  4   c  may be attached to the aircraft structure frame (as described below), these asymmetrical drag forces on the blade-pairs push on the aircraft. 
         [0077]    Propellers according to the present invention may be used individually or in combination. According to some embodiments of the present invention, the propellers may be used in counter-rotating pairs sharing an axis of rotation, or as counter-rotating pairs with parallel axes of rotation. 
         [0078]    Propellers according to embodiments of the present invention may be used on an airplane to provide asymmetric drag which may simultaneously propel an airplane in forward flight, improve airplane wing lifting capabilities and prevent or delay wing stall during very low speed high angle-of attack landing/take-off maneuvers. 
         [0079]    Propellers according to embodiments of the present invention may, more generally, be used to propel any vehicle or vessel, such as an airplane, boat, submarine or the like, through any fluid, such as air or water. 
         [0080]    According to one embodiment of the present invention, propellers  10  are used on an airplane  200  as shown in  FIGS. 11A through 11C , and  FIGS. 12A through 12C .  FIG. 11A  shows a top view of an airplane  200  employing two counter-rotating parallel-axes asymmetrically changing-shape propellers  100   a ,  100   b  and two tilted-constant-pitch regular propellers  11   a ,  11   b . A single power source  11   p  drives all four propellers through power transmission mechanisms  11   m ,  11   n  (shown generally). The details of the power transmission mechanisms  11   m ,  11   n  are well-known in the art and are not described in detail. A first tilted-fixed-pitch regular propeller  11   a  is disposed on the right side of the top wing surface  11   c  of the airplane  200 , and a second tilted-fixed-pitch regular propeller  11   b  is disposed on the left side of the top wing surface  11   c  of the airplane. The first tilted-fixed-pitch regular propeller  11   a  and the second tilted-fixed-pitch regular propeller  11   b  rotate in opposite directions about non-parallel tilted axes. The two asymmetrically changing-shape propellers  100   a ,  100   b  according to the present invention, also rotate in opposite directions about axes parallel to each other. A first asymmetrically changing-shape propeller  100   a  is disposed on the right side of the top wing surface  11   c  of the airplane  200 , and a second asymmetrically changing-shape propeller  100   b  is disposed on the left side of the top wing surface  11   c  of the airplane  200 . 
         [0081]    A protection shield  11   v  is provided on the top wing surface  11   c  of the airplane to shield a portion of each rotating propeller  100   a ,  100   b  as shown in  FIG. 11B . The protection shield  11   v  may be made of canvas or any other suitable material. The purpose of the protective shield  11   v  is to reduce drag during forward flight. 
         [0082]    As shown in  FIG. 11B , the rotating propellers  100   a ,  100   b  are disposed above the top wing surface  11   c  of the plane but below the level of the canvas shield  11   v . The two tilted-constant-pitch regular propellers  11   a ,  11   b  are mounted so that throughout a half cycle of rotation for each of the propellers  11   a ,  11   b , the blades are below the top wing surface  11   c  of the airplane, as shown in  FIG. 11C . 
         [0083]      FIGS. 12A through 12C  show the propellers  100   a ,  100   b ,  11   a ,  11   b  as mounted on the airplane  200 . In this case, the protective shield  11   v  is split into a right shield  12   n  and a left shield  12   m . As shown, the left propellers  10   b ,  11   b  are mounted on the left wing  12   e , and the right propellers  100   a ,  1   a  are mounted on the right wing  12   f.    
         [0084]    According to this embodiment, the rotational planes of the two parallel-axis counter-rotating propellers  100   a ,  100   b  are slightly above the upper surface  11   c  of each wing  12   e ,  12   f . These counter-rotating propellers  100   a ,  100   b  generate thrust forces which push the airplane  200  in forward flight. At the same time they increase the air velocity, thereby decreasing the air pressure above the wing. This action improves the wing lift capability. During high angle-of-attack takeoffs and landings, these counter-rotating propellers  100   a ,  100   b  force airflow over the wing top surface  11   c , which prevents or delays wing stall. In addition, the thrust vectors generated by these propellers  100   a ,  100   b  are also tilted upward to assist aircraft lift. The net effect will allow this aircraft to takeoff and land at shorter or possibly no runways, thereby enabling spot take-off and landing. 
         [0085]    Propellers according to embodiments of the present invention, for example a pair of propellers, may be arranged in a horizontal, co-axial counter-rotating configuration within a wind turbine to exploit wind energy in accordance with known wind turbine systems. According to this embodiment, the blade-pairs absorb wind kinetic energy, as more fully illustrated in  FIG. 13 . 
         [0086]      FIG. 13  illustrates an embodiment of the propeller blades according to the present invention as applied to a windmill.  FIG. 13  is a cross-sectional view of a portion of a windmill, which includes co-axial, counter rotating propellers which rotate about co-axial propeller shafts  13   a ,  13   b . Each propeller may contain four equally spaced blade-pairs, as shown in  FIG. 1 . Each blade-pair consists of an upper blade  1   a  and a lower blade  1   b , as described above. This windmill can be located on top of a water tower  13   k , as shown in  FIG. 13 , or on top of a building or any other structure. 
         [0087]    According to one embodiment of the present invention, the coaxial propeller assembly of the windmill  130  includes a circular base structure  13   e  located within the outer housing  13   r  of the water tower  13   k . The circular base structure  13   e  includes wheels  13   p  disposed along a perimeter thereof, so as to be downwardly protruding. According to a preferred embodiment, the base structure  13   e  includes four, equally-spaced wheels  13   p . The wheels  13   p  move in a circular track (not shown) formed in a platform  13   t  of the water tower  13   k , which supports the coaxial blade-pair structure. An electric motor  13   m  is mounted on the platform  13   t , and has a gear  13   u  attached thereto, which engages a gear  13   n  attached to the base structure  13   e . According to this arrangement, when the motor  13   m  operates, the gear  13   u  rotates and engages the gear  13   n , so as to rotate the base structure  13   e . The base structure  13   e , however, does not continuously rotate, but only rotates when necessary as described below. 
         [0088]    An inner propeller shaft  13   a  is rotatably mounted on the base structure  13   e , perpendicular to the plane of the base structure  13   e , as described below, so as to rotate with respect thereto. Similar to  FIG. 4 , in this embodiment, rigid connecting members  4   a  are attached to an upper end of the inner propeller shaft  13   a . Preferably, four rigid connecting members  4   a  may be arranged in a T-shape around the inner propeller shaft  13   a . Each rigid connecting member  4   a  contains a blade pair consisting of an upper blade  1   a  and a lower blade  1   b , as described in more detail above. In the windmill embodiment, however, the first blade  1   a  includes a leading portion  5   a , as shown in  FIG. 10 , but the trailing portion of  5   c  may be much reduced and exists only to provide a connection to the depending fin  5   e  for connection to the actuating mechanism. Also, second blade  1   b  includes a leading portion  5   b , as shown in  FIG. 10 , but does not include any portion of the trailing portion  5   d . Thus, the blade pair includes a first blade  1   a  and a second blade  1   b  which pivot about the connecting member  4   a , but which essentially do not include trailing portions  5   c ,  5   d . This is because during the first half of each rotation cycle, the blade pair will be closed ( 1   f= 0) to keep air resistance to a minimum, as the blade pair rotates into the wind. Here, the blade pair hinge axis  4   a  is leading the closed blade pair  1   a  and  1   b  in rotation into the wind. This applies to clockwise or counter-clockwise rotation. 
         [0089]    During the second half cycle of rotation, hinge axis  4   a  will lead the blade pair  1   a ,  1   b  in the general direction with the wind. The angle if between the blade pair  1   a ,  1   b  starts to open at the beginning of this half cycle and reach its maximum value halfway through this half cycle. Finally, the blade pair  1   a ,  1   b  will be completely closed ( 1   f= 0) at the end of this half cycle. 
         [0090]    As the blade pair  1   a ,  1   b  move in the general direction with the wind (with  1   f&gt; 0), the wind will push into the opened blade pair cavity, thus forcing the blade pair  1   a ,  1   b  to rotate. Removing  5   d  and most of  5   c  will produce less air resistance and improve the wind energy absorption during this half cycle. 
         [0091]    The inner propeller shaft  13   a  is surrounded by the coaxial outer propeller shaft  13   b . Bearings  13   i  support the inner propeller shaft  13   a  within the outer propeller shaft  13   b.    
         [0092]    The cam track and control mechanism, as described in conjunction with  FIGS. 9 and 10 , similarly controls the opening and closing of the blade pairs in the windmill embodiment. As shown in  FIG. 13 , the propeller assembly includes a first control mechanism  14  for controlling the upper propeller, and a second control mechanism  16  for controlling the lower propeller. The first and second control mechanisms  14 ,  16  have guide tracks  90  formed on inner surfaces thereof (not shown) for guiding wheels  8   m ,  8   n  (not shown) which cause the control rods  8   a  to move up and down, so as to open and close the blade pairs; according to the mechanical mechanism heretofore illustrated in  FIG. 10 , for example. 
         [0093]    The first control mechanism  14  includes a shaft  14   b  which is fixedly mounted to the base structure  13   e . The inner propeller shaft  13   a  surrounds the shaft  14   b , and bearings  15  support the inner propeller shaft  13   a  around the shaft  14   b . According to this arrangement, the inner propeller shaft  13   a  rotates with respect to the base structure  13   e , and the first control mechanism  14  only rotates in conjunction with rotational adjustments of the base structure  13   e  as described below. 
         [0094]    The second control mechanism  16  is also fixedly mounted to the base structure  13   e  for rotation therewith so as to control the opening and closing of the lower propeller blade-pairs. Similar to the operation of the first control mechanism  14 , the second control mechanism  16  generally remains stationary and only rotates in conjunction with rotational adjustments of the base structure  13   e  due to change in wind direction as described below. Bearings  13   j  support the outer propeller shaft  13   b  in the second control mechanism  16 . Power is transferred from the blade pairs  1   a ,  1   b  through the shafts  13   a ,  13   b , to the bevel gears  13   d , to the horizontal bevel gear support shaft  13   q , and to the gear box  13   f , which transfers the power to the electric generator  13   g , to generate electricity. 
         [0095]    As discussed above, the base structure  13   e  remains stationary while the propeller shafts  13   a ,  13   b  rotate. The base structure  13   e  is only rotated in response to a change in wind direction. 
         [0096]    The end of each rigid connecting member  4   a  contains a pitot tube (or any other type of air pressure sensor) having an intake end  13   c  which is open and faces the incoming air velocity vector which is the rector sum of wind vector and the blade rotational velocity vector. A rear end  13   s  of the pitot tube is closed and opposite the intake end  13   c . The pitot tube is used to measure dynamic pressure on the rotating blade pair, according to formulas and methods well-known to one of ordinary skill in the art. As the wind changes direction, the dynamic pressure as measured by the pitot tubes changes. When this change in the direction of all air velocity vectors  1   v  are stabilized, the electric motor  13   m  rotates the base structure  13   e , which controls the opening and closing location of the blade pairs, so as to open the blade pair  132 ,  134  to a maximum opening angle  1   f  at the point where the blade pairs are directly perpendicular to the incoming stabilized wind air velocity vector, as described above. 
         [0097]    The propellers of the present invention push the air using the drag force developed on the blade-pair propeller. It is often easier to produce large drag force on a moving object than to produce large lift force on the same object. Generally, the maximum drag coefficient can be as high as 2.0 while the corresponding maximum lift coefficient is about 0.6 before wing stall and reduced lift on the moving surface. Another advantage of the propellers of the present invention is that they can be made to rotate closely above and/or below a wing surface. This will improve the wing lift capability especially at high angle of attack maneuvers, and, with the flaps down during takeoffs and landings, there will be less of a chance of wing stall. In addition, the propellers of the present invention have a small clean profile when placed on top of a building to absorb wind energy. 
         [0098]    The above specific embodiments are illustrative, and many variations can be introduced on these embodiments without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.