Abstract:
A rotary sound transducer having an improved output at higher frequencies. The invention includes stiff vanes that are preferably rigidly attached to a hub. A torsional actuator is provided in each vane. The torsional actuator selectively twists the tip portion of each vane. The torsional actuator for each vane is activated by an input energy source corresponding to the sound waves that are desired. The input force may also be electromechanical energy, purely mechanical energy, or some other form of energy.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       MICROFICHE APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention relates to the field of sound generation and modification. More specifically, the invention comprises a rotary transducer where the pitch of rotating vanes is used to create or modify pressure waves. The invention includes additional features to increase the output of the transducer in the upper portion of its frequency response range. 
         [0006]    2. Description of the Related Art 
         [0007]    Rotary sound transducers convert non-acoustic input energy into acoustic output energy by varying the pitch of rotating vanes. The vanes typically rotate in a fixed arc around a hub. The pitch of the vanes is varied as they rotate in order to create the acoustic output energy. One example of such a device is disclosed in U.S. Pat. No. 2,304,022 to Sanders (1942) (hereinafter “Sanders”). Sanders discloses a sound producing apparatus that resembles an electric fan. Cyclical electrical energy is fed into an electromagnet in the invention&#39;s hub. The input energy cyclically varies the pitch of the vanes—thereby producing sound waves at a desired frequency. 
         [0008]    Another type of rotary transducer is disclosed in my own prior patent application (U.S. patent application Ser. No. 10/442,852). My prior application uses a swash plate to vary the pitch of the rotating vanes in a manner reminiscent of the mechanism used to vary the pitch of a helicopter&#39;s main rotor.  FIGS. 1-3  refer to my prior design. 
         [0009]      FIG. 1  shows the main components of the prior art rotary transducer. A pair of vanes is driven in the arc shown by shaft  30 . A motor within housing  26  spins shaft  30 . Swash plate  36  actuates two linkages  40  connected to a pitch-actuating mechanism on each vane. The reader will observe that moving the linkages will cause the deflection of the two vanes  34  so that the angle of attack of each vane (relative to the air flowing over its leading edge) is increased or decreased. The angle of attack of the two vanes is changed in unison. 
         [0010]    Swash plate  36  translates in a direction that is parallel to the central axis of shaft  30 . The swash plate is urged toward the vanes or away front the vanes by the motion of voice coil  20 . Voice coil  20  is an electromagnetic device such as used in a common audio speaker. The voice coil is suspended in a neutral position by suspension spider  22  (which is also commonly used in audio speakers) or held in the neutral place by the influence of the air load on the leading and trailing edges of the vanes. Wire bundle  42  includes the wires used to provide electrical power to the motor that rotates the vanes and other wires used to provide the input for the motion of the voice coil. 
         [0011]      FIG. 2  shows a sectional view through the prior art device with the vanes in the “neutral” position. In this position the vanes have a zero angle of attack and no pressure waves are produced (other than a small cyclical output caused by the flat vanes “cutting” through the air). Motor  28  provides the driving torque for shaft  30 . Voice coil assembly  12  includes the components that move the swash plate. Magnet  18  is held between back plate  14  and front plate  16 . Voice coil  20  moves linearly (left and right in the orientation of the view) as a magnetic field is applied by center pole assembly  24  under the influence of magnet  18 . 
         [0012]    Conventional rotary hearings  32  support the rotating shaft. Bearing assembly  38  is a thrust-type bearing. It allows swash plate  36  to rotate with respect to voice coil  20  while also transmitting a linear force. Although the mechanism shown is reminiscent of that used in a helicopter&#39;s main rotor, the reader will note that the pitch of the two vanes is not varied independently but always in unison. Thus, using helicopter terminology, the simple swash plate is able to vary the “collective” pitch but is unable to create cyclical variations customarily produced by tilting the swash plate in a helicopter. 
         [0013]      FIG. 3  shows the same mechanism with voice coil  20  pushed away from the neutral position by the application of electromagnetic force. The voice coil and swash plate have been urged to the left in the orientation of  FIG. 3 . Thus, the two linkages  40  have also been pushed away from the neutral position and the two vanes  34  have been pitched as shown (in opposite directions). The result is that the vanes are given a substantial angle of attack. 
         [0014]    A transducer such as shown in  FIGS. 1-3  is able to produce low frequency sound waves without requiring a large and heavy conventional transducer. The shaft rotates the assembly at a speed which is often much higher than the frequency of the desired sound waves. As an example, the shaft might be rotated between about 600 and 1000 RPM, while the transducer might he used to generate sound waves in the range of 20 Hz to 100 Hz. Sound in this range may be effectively produced using a rotary transducer having an overall diameter of about 8 inches. In contrast, a 15 inch to 18 inch cone speaker will often be needed to produce a 20 Hz output. An enclosure for such a speaker will add considerable bulk and mass as well. 
         [0015]    By virtue of the rotational speed of the blades and the swept air of the blades for each cycle at very low frequencies the rotary transducer offers a significant impedance match advantage with air or fluids in comparison to a moving cone or piston. 
         [0016]    The rotary design can be used in an enclosure or box where the back wave pressure is captured and the transducer becomes a monopole. Because the rotary design has a significantly improved impedance match with the air, it can also he used as a dipole for low frequency sound reproduction. 
         [0017]    Of course, when operated as a dipole, air within the positive pressure generated on one side of the plane of rotation has an easy path of travel to the negative pressure on the opposite side of the plane of rotation. This forms a sort of “short circuit” for dipole operations. The effect of the “short circuit” in dipole operation varies with frequency. The transducer is generally rotated at a relatively constant speed. Thus, the “swept area” of the vanes is constant. For low frequency inputs, the output amplitude is good. A significant amplitude “roll off” is experienced for higher frequencies, however. 
         [0018]    At extremely low frequencies one can achieve one or more full revolutions of the drive shaft per pitch cycle of the vanes. As the input frequency is reduced, the impedance match with the air improves due to the increase in swept area. Conversely, at higher frequencies the swept area is reduced in comparison to the rotational velocity and each pitch cycle or oscillation may only consume a small portion of a full revolution of the drive shaft. This reduction in effective area and shorter wavelengths result in a 12 dB per octave decrease in output amplitude for increasing input frequency with the prior art construction. The “roll off” with increasing frequency is exactly the opposite of what occurs with a conventional cone-type loudspeaker. Such speakers are driven by a linear actuator (a voice coil) connected to a cone or “piston”. As the input frequency to the voice coil increases, the wavelengths decrease relative to the physical dimensions of the piston and the impedance match with the air becomes more favorable. Since the wavelength of sound decreases with increasing frequency and the net radiating area of the piston is constant, the impedance match of the cone with the air is improved. 
         [0019]    Two factors dictate the “roll off” a rotary vane transducer experiences with increasing input frequency. The first factor is loss of the impedance match with the air as the frequency is increased. The second factor is the inertia of the actuating mechanism and the vanes themselves which requires more force from the actuator to maintain the same acoustic output. It is therefore desirable to produce a rotary transducer that retains the ability to produce low frequency sound while reducing the “roll off” phenomenon inherent in the prior art devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0020]    The present invention comprises a rotary sound transducer having an improved output at higher frequencies. The invention includes stiff vanes that are preferably rigidly attached to a hub. A torsional actuator is provided in each vane. The torsional actuator selectively twists the outer portion of each vane. The torsional actuator for each vane is activated by an input energy source corresponding to the sound waves that are desired. For example, the input energy source may be hydraulic pressure varied at 100 Hz. The input force may also be electromechanical energy, purely mechanical energy, or some other form of energy. 
         [0021]    The torsional actuator tends to vary the pitch of the outer portion of each vane significantly more than the root portion. The outer portion travels through a greater arc length per revolution of the transducer than the root portion. Thus, the angular deflection is provided where the swept area and velocity is greatest. This fact increases the transducer&#39;s output. In addition, since only a portion of the vane is being twisted, inertial effects are minimized and a torsional natural frequency results. This allows the restoring force (primarily vane stiffness but possibly other restoring forces as well) to rapidly restore the untwisted state and the blade becomes easier to pitch at frequencies near the torsional natural frequency. This fact means that higher input frequencies may be converted to sound by the transducer without losing significant amplitude. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL. VIEWS OF THE DRAWINGS 
         [0022]      FIG. 1  is a perspective view, showing a prior art rotary transducer. 
           [0023]      FIG. 2  is a sectional elevation view, showing some internal details of the prior art rotary transducer. 
           [0024]      FIG. 3  is a sectional elevation view, showing the transducer of  FIG. 2  with the vane pitch mechanism activated. 
           [0025]      FIG. 4  is a perspective view, showing a vane used in the present invention. 
           [0026]      FIG. 5  is an exploded perspective view, showing the components of a torsional actuator as used in the present invention. 
           [0027]      FIG. 6  is a perspective view, showing the torsional actuator in an assembled state. 
           [0028]      FIG. 7  is a detail view, showing the tip portion of the torsional actuator. 
           [0029]      FIG. 8  is a perspective view, showing the position of a torsional actuator in a vane. 
           [0030]      FIG. 9  is a perspective view, showing some details of a vane. 
           [0031]      FIG. 10  is a sectional view, showing internal details of a vane. 
           [0032]      FIG. 11  is a sectional view, showing internal details of a vane. 
           [0033]      FIG. 12  is a sectional detail view, showing portions of the torsional actuator. 
           [0034]      FIG. 13  is an exploded perspective view, showing an assembly of three vanes and a hub. 
           [0035]      FIG. 14  is a plan view, showing three vanes attached to a hub. 
           [0036]      FIG. 15  is a perspective view, showing the hub of  FIG. 14 . 
           [0037]      FIG. 16  is a perspective view, showing the hub of  FIG. 14 . 
           [0038]      FIG. 17  is a sectional elevation view, showing a representative actuating mechanism. 
           [0039]      FIG. 18  is a sectional elevation view, showing a representative actuating mechanism. 
           [0040]      FIG. 19  is another embodiment using more than three vanes. 
       
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
       [0000]    
       
           10  rotary transducer 
           12  voice coil assembly 
           14  back plate 
           16  front plate 
           18  magnet 
           20  voice coil 
           22  suspension spider 
           24  center pole assembly 
           26  housing 
           28  motor 
           30  shaft 
           32  bearing 
           24  vane 
           36  swash plate 
           38  bearing assembly 
           40  linkage 
           42  wire bundle 
           44  tip 
           46  root 
           48  leading edge 
           50  trailing edge 
           52  hub interface 
           54  mounting hole 
           56  torsion rod 
           58  sleeve 
           60  hub bearing 
           62  pitch arm 
           64  rod receiver 
           66  rod tip 
           68  variable pitch region 
           70  middle region 
           72  hub assembly 
           74  drive shaft 
           76  center disk 
           78  bolt 
           80  actuator pin 
           82  center bore 
           84  actuator plate 
           86  actuator shaft 
           88  magnet 
           90  electromechanical actuator 
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0082]    The present invention uses two or more relatively stiff vanes fixedly attached to a rotating hub. The pitch of the vanes is varied cyclically in order to produce a desired sound. The pitch variation needed is actually created by twisting the vane. Once the twisting force is relaxed, the natural stiffness of the vane and aerodynamic force tends to return it to its neutral position (possibly assisted by other restoring forces). By creating the vane with a stiff structure, high frequency pitch variations are possible. 
         [0083]      FIG. 4  shows one example of a vane  34  suitable for use in the present invention. Hub interface  52  is configured to mount to a rotating hub—in this case using a pair of bolts passed through mounting holes  54 . Root  46  extends outward from the hub interface and blends into middle region  70 . Tip  44  lies at the vane&#39;s distal extreme. Leading edge  48  lies in the direction of rotation while trailing edge  50  lies in the opposite direction. 
         [0084]    The shape of vane  34  in this example is similar to that used for an aircraft propeller. The reader will note that the angle of attack decreases as one proceeds from root  46  out toward tip  44  (two representative cross sections are shown as dashed lines). This variation in the angle of attack compensates for the fact that the tip travels further per revolution of the vane than the root.  FIG. 4  actually shows the “relaxed” or “neutral” position for this particular vane, which is defined as the state of the vane when no external twisting forces are applied. The reader will note that the “neutral” position in this embodiment still retains a non-zero angle of attack. This need not always be the case. However, for the example shown, the “neutral” position generates thrust as the vane is rotated. 
         [0085]    A torsional actuator is added to the vane to drive the desired pitch variations.  FIGS. 5-8  illustrate an exemplary embodiment for such a torsional actuator.  FIG. 5  presents an exploded perspective view showing the components employed. Torsion rod  56  is a long metal rod with a bent portion lying at is distal end. Sleeve  58  slips over most of the length of torsion rod  56 . Hub bearing  60  slips over the proximal end of the rod. Rod receiver  64  is designed to receiver and lock to she proximal portion of torsion rod  56 . Rotation is prevented between the rod receiver and the torsion rod. In this example, pitch arm  62  is formed integrally with rod receiver  64 . Once pitch arm  62  is attached, it may he used to apply torque to torsion rod  56 . 
         [0086]      FIG. 6  shows the torsional actuator in an assembled state. The portion of torsion rod  56  lying within sleeve  58  (the middle portion) and the portion lying within hub bearing  60  (the proximal portion are free to rotate with respect to both the sleeve and the hub bearing. Thus, the sleeve and hub bearing can remain fixed while rotating pitch arm  62  and torsion rod  56 . Rotating the torsion rod tends to translate the position of rod tip  66  (through an arc). 
         [0087]      FIG. 7  shows a detailed view of the distal end of the torsion rod, including rod Up  66 . Most of the torsion rod is cylindrical so that it can freely rotate within the sleeve and bearing. However, the cylindrical shape is preferably flattened into an oval for the portion of the rod extending out the distal end of sleeve  58 . 
         [0088]      FIG. 8  shows the torsional actuator in the position it occupies within vane  34 . Vane  34  may be made of a wide variety of materials assembled in many different ways. As an example, it may be created as a fiber-reinforced core matrix with a stiff exterior layer. The exterior layer may be made of woven carbon fiber. The torsional actuator is preferably laid into the composite structure as the composite structure is created. Sleeve  58  and hub bearing  60  may be bonded to the composite core material. Rod tip  66  preferably is bonded to the core material (and possibly part of the outer layer of material if it extends outward far enough). 
         [0089]    Pitch arm  62  lies outside of vane  34  at the vane&#39;s proximal end. It is preferable to secure the vane&#39;s hub interface rigidly to the hub so that the hub interface itself does not twist significantly. Those skilled in the art will realize that a torque applied to pitch arm  62  will be transmitted via torsion rod  56  to the bent portion of the torsion rod lying distal to the distal end of sleeve  38 . Sleeve  58  and hub bearing  60  will not rotate. However, rod tip  66  (and the bent portion of the rod in its vicinity) will rotate as pitch arm  62  is rotated. This rotation will twist a portion of vane  34 . Thus, in this case, pitch arm  62  provides a torque input interface—meaning that it provides a mechanism for an external force to apply a torque to the torsional actuator. As will be explained, many different types of torque input interlace can be provided. 
         [0090]      FIG. 9  shows vane  34  with the torsional actuator installed. Applying a torque to the pitch arm as indicated by the arrow increases the pitch of the vane in variable pitch region  68 . Some twist will be experienced for most of the vane&#39;s length. However, the vane is made thicker and stiffer between middle region  70  and root  46 . Thus, that portion of the vane will not tend to twist as much. Additional twist will of course not be added beyond the distal extreme of the torsion rod. As a result, most of the variable twist occurs within variable pitch region  68 . 
         [0091]      FIG. 10  shows a section through the vane assembly where the bend in the torsion rod lies. The rod is embedded within the core material of the vane and is thereby mechanically locked to the vane, in this region (Note how the flattened portion of rod tip  66  allows it to fit more easily within the vane&#39;s trailing edge). 
         [0092]      FIG. 11  shows a section through the same assembly in the region of sleeve  58 . In this region sleeve  58  is likely bonded to the core material of the vane but the torsion rod—being shielded by the sleeve—is not. Thus, the rotation of the torsion rod in this region will not impart any twisting force to the vane. The reader will thereby understand that he position and length of sleeve  58  helps to determine where the twisting force imparted by the torsion rod will actually be applied to the vane. 
         [0093]      FIG. 12  shows a transverse section through the vane in the region of the hub interface (see the section “call out” in  FIG. 9 ). Those skilled in the art will realize that the actuation of pitch arm  62  can produce significant reaction forces where torsion rod  56  enters the vane. In order to resist wear in this vicinity, a durable hub bearing  60  is preferably used. Sleeve  58  will experience significantly lower reaction forces. A polymer such as extruded NYLON may be used for this component. 
         [0094]    Having now described some of the component in significant detail, the reader may wish to known how an assembly comprising the present invention can be created.  FIG. 13  shows an exemplary assembly. Two or more vanes are preferably included in order to create a balanced rotating mass. In the embodiment shown, three vanes  34  are used. 
         [0095]    Drive shaft  74  provides rotational power to hub assembly  72 . Both the drive shaft and hub assembly rotate about a central axis of rotation centered on the drive shaft. Each of the three vanes  34  is attached to hub assembly  72 . The attachment can be made using many different devices but in the example shown several bolts  78  are passed through center disk  76 , through the bolt holes in the vanes themselves, and into threaded holes in the hub assembly. 
         [0096]      FIG. 14  shows a detailed plan view of the area of the hub with the three vanes  34  in place and ready to be attached to the hub  72 . The hub assembly in this example includes the mechanism for applying force to the torsional actuator in each vane (in order to selectively vary the pitch of each vane). The exemplary mechanism is somewhat simplistic and is only intended to represent one example of the type of actuating mechanisms that are possible. 
         [0097]    A pitch arm  62  from each of the varies lies within the hub. Each pitch arm  62  includes an actuator pin  80  which allows torque to be easily applied to the pitch arm by the actuating mechanism. Mounting holes  54  in each of the vanes align with threaded receivers in the hub assembly itself Center bore  82  passes into the hub assembly. 
         [0098]      FIG. 15  shows a perspective view of the same general area. Actuator shaft  86  slides into center bore  82  in the hub assembly (The center bore is shown in  FIG. 14 ). Actuator plate  84  is attached to the end of actuator shaft  86 . Actuator shaft  86  is provided with a key protrusion that slides in a keyway provided in center bore  82  (see  FIG. 14 ). The interface of the key protrusion and the keyway prevents the rotation of the actuator shaft and the attached actuator plate. 
         [0099]    Actuator plate  84  is configured to bear against the three actuator pins  80  when actuator plate  84  is urged downward (in the orientation shown in the view). The actuator plate is intended to contact the actuator pins, but not the pitch arms themselves. This objective explains why the rotation of the actuator plate needs to be limited. If the actuator plate is allowed to rotate an interference would likely result. 
         [0100]    The configuration shown in  FIG. 15  represents the position of the actuator plate before assembly is complete. Once the assembly is completed, actuator plate  84  preferably rests directly on the three actuator pins  80 —thereby eliminating “backlash” in the system.  FIG. 16  shows the final stage of the assembly. Center disk  76  is placed over the top of the three vanes and bolts  78  are passed through center disk  76 , through the bolt holes in the vanes themselves, and into the hub. The reader will thereby appreciate that the portion of each vane actually attached to the hub is attached in a rigid fashion. Though the present invention is actually intended to twist the vanes in order to provide the desired pitch variation, the portion of each vane proximate the hub will not experience much twist. 
         [0101]      FIGS. 17 and 18  illustrate the operation of the exemplary pitch-actuating mechanism.  FIG. 17  is a section view taken through the center of actuator shaft  86  (The plane of the section view is called out in  FIG. 15 ).  FIG. 17  represents the “neutral” position in which no (or very little) torque is placed on the torsional actuator in each vane (it is optional to maintain a low pre-load torque in some instances). Actuator plate  84  rests against actuator pin  80  on the pitch arm  62  shown. 
         [0102]    Actuator plate  84  is—as explained previously—attached to the outer end of actuator shaft  86 . The inner end of actuator shaft  86  forms part of a linear actuator. Magnet  86  is attached to the inner end of the actuator shaft. Electromagnetic actuator  90  is attached to the hub and stays in place. 
         [0103]    When the electromagnetic actuator is activated, actuator shaft  86  is pulled into the hub (downward in the orientation of the view).  FIG. 18  shows this position. Actuator shaft  86  moves in the direction indicated by the arrow. Actuator plate  84  bears against actuator pin  80  and pulls it downward as shown. This motion rotates pitch arm  62  and thereby applies torque to the torsional actuator in the vane. The result is a change in the pitch of the vane. 
         [0104]    Although only one pitch arm is visible in the section view of  FIG. 18 , the motion of actuator plate  84  simultaneously moves all three pitch arms in this embodiment. Thus, the pitch of all three vanes is varied simultaneously. 
         [0105]    Electromagnetic actuator  90  should not be viewed as an “on/off” device. Rather, it is preferably a device that is able to smoothly provide any desired amount of linear force within a defined range in either direction. For example, if the electrical power signal fed info electromagnetic actuator  90  is a 200 Hz sinusoidal signal the actuator will move the actuator shaft sinusoidally tracking the phase and amplitude of the signal. 
         [0106]    Electromagnetic actuator  90  may be capable of producing linear force in both directions and also the restoring force. On the other hand, the stiffness of the vanes will tend to rapidly return the assembly to the “neutral” position ( FIG. 17 ) once the force is removed. Thus, the actuator may be configured to apply force in only one direction and allow the stiffness of the vanes to act as a restoration force for aerodynamic stability. Even if force is applied in both directions, the vane stiffness is helpful in impedance matching. 
         [0107]    Returning now to  FIG. 8 , the reader will recall that each vane includes a torsional actuator embedded within a naturally stiff structure. The resonant frequency of the vane itself is preferably fairly high. Exemplary blades using carbon fiber may have a torsional resonant frequency above 2,000 Hz. The inherent stiffness of the structure means two things. First, frequencies below the resonant frequency may be applied to the vanes by the twisting mechanism without, tear of exciting a resonant frequency or blade bending or flap mode that possibly creates aerodynamic flutter. Thus, the vanes may be excited by input frequencies over a very wide range (lower than 20 Hz and up to the vicinity of 2,000 Hz for a structure having resonance above 2,000 Hz). Second, the vanes are able to respond to the relatively high-frequency input signal phase and amplitude without substantial distortion or reductions in the amplitude of the output. Third, the vanes become easy to twist near the torsional natural frequency which reduces the effects of inertia and increases the high frequency output. 
         [0108]    This latter phenomenon represents one of the significant features of the present invention. The proposed structure: 
         [0109]    (1) Adjusts pitch on the faster traveling part of the vane, thereby imparting a more forceful pressure variation; 
         [0110]    (2) Avoids having to change the pitch of the entire vane, thereby avoiding significant polar moment of inertia delays; and 
         [0111]    (3) Uses the structural stiffness of the vane itself as a restoring force to improve high frequency output. 
         [0112]    The linear actuating mechanism shown in  FIGS. 17 and 18  is rather simplistic. Additional features could be substituted or added, including: 
         [0113]    (1) Using hydraulic power to drive the linear actuator rather than electromagnetic power; 
         [0114]    (2) Locating the actuator pins  80  in a slot in actuator plate  84  so that the linear actuator could drive the actuator pins in both directions; 
         [0115]    (3) Including a roller bearing on each actuator pin to minimize friction; 
         [0116]    (4) Using a purely mechanical device for driving the linear actuator, such as a moving cam; and 
         [0117]    (5) Locating the actuating mechanism outside the hub, such as out near the vane tips. 
         [0118]    As explained previously, the neutral, position of the vanes need not be a zero-thrust state. The embodiments depicted all produce some thrust in the neutral position (though this need not always be the case). Thus, it is possible to use the inventive rotary transducer as both a mass-moving device and a sound producing device. Depending upon the desired output, one may even configure the input signal that produces the vane twisting to reduce the amount of sound produced by the rotating assembly. 
         [0119]    Although an example provided has used only three vanes, the reader should bear in mind that the invention may be implemented using four, five, or even more vanes.  FIG. 19  shows an embodiment using many more vanes. The functional, operation of the embodiment of  FIG. 19  is the same. The vanes still include a torsional actuator. However, many more vanes are involved. Additional variations on the invention are possible, including: 
         [0120]    (1) Only varying the pitch on some of the vanes in a rotating assembly; 
         [0121]    (2) Containing the rotating vanes within a duct to minimize tip losses or account for other phenomena; 
         [0122]    (3) Placing the pitch varying mechanism near the outer perimeter of the vanes rather than the hub; and 
         [0123]    (4) Using more direct actuation methods—such as a magnet embedded in each vane responding to an electromagnetic force. 
         [0124]    The preceding descriptions contain significant detail regarding the novel aspects of the present, invention. They should not be construed, however, as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.