Abstract:
A method and apparatus is provided, including an actuator system that may be connected to a wing frame for controlling an active element. The actuator system may include sliding elements movable along an axis parallel to the span-wise axis of the wing. The sliding elements may be connected to fixed elements and a crank element, the crank element generally comprising a beam element and a cross-axis flexure pivot element. The beam element may be offset from the pivot element so that the crank element is rotatable about the pivot element with a negative stiffness under an external force that tends to pull the sliding elements away from the fixed elements.

Description:
TECHNICAL FIELD 
     This disclosure relates in general to the field of heavier-than-air aircraft, and more particularly to a method and apparatus for actively manipulating aerodynamic surfaces. 
     DESCRIPTION OF THE PRIOR ART 
     Emerging and future generations of rotary-wing and tilt-rotor aircraft have active elements on the blade or wing, such as trailing edge flaps and leading edge droops, which can provide a number of enhancements over passive designs. For example, active elements can be used for vibration reduction, noise reduction, and performance improvements. Actuator systems are needed to operate active elements, but actuator systems also add weight and complexity to the aircraft. Accordingly, the design of powerful, light-weight actuator systems presents significant challenges to engineers and manufacturers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features believed characteristic and novel of a method and apparatus (collectively, a system) for active manipulation of aerodynamic surfaces are set forth in the appended claims. However, the system, as well as a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an example embodiment of a helicopter according to the present specification; 
         FIG. 2  is a partial top view of an example embodiment of a helicopter having an active blade element and actuator system according to the present specification; 
         FIG. 3  is a simple top-view schematic of an example embodiment of an actuator system according to the present specification having a span-wise orientation and a parallel configuration of linear actuators in a rotor blade; 
         FIG. 4  is a simple side-view schematic of an example embodiment of an actuator system according to the present specification having a span-wise orientation and a parallel configuration of linear actuators in a rotor blade; 
         FIG. 5  is a cut-away view of an example embodiment of a linear motor actuator according to the present specification; 
         FIG. 6  is a simple top-view schematic of another example embodiment of an actuator system according to the present specification having a span-wise orientation and a serial configuration of linear actuators in a rotor blade; 
         FIG. 7  is a simple schematic of an example embodiment of an actuator system having a cross-axis flexure pivot element and parallel actuators, according to the present specification; 
         FIG. 8  is a simple schematic of an example embodiment of an actuator system having a cross-axis flexure pivot element and serial actuators, according to the present specification; 
         FIG. 9  is a perspective view of an example embodiment of an assembled actuator system having a cross-axis flexure pivot element and parallel actuators, according to the present specification; 
         FIG. 10  is a perspective view of an example embodiment of an assembled actuator system having a cross-axis flexure pivot element and serial actuators, according to the present specification; 
         FIG. 11  is a top view of an example embodiment of an assembled actuator system having a cross-axis flexure pivot element and parallel actuators, according to the present specification; 
         FIG. 12  is an exploded top view of an example embodiment of an assembled actuator system having a cross-axis flexure pivot element and parallel actuators, according to the present specification; and 
         FIG. 13  is an exploded bottom view of the example embodiment in  FIG. 12 . 
     
    
    
     While the system and apparatus for active manipulation of aerodynamic forces is susceptible to various modifications and alternative forms, the novel features thereof are shown and described below through specific example embodiments. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the system or apparatus to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the novel system are described below. In the interest of clarity, not all features of such embodiments may be described. It should be appreciated that in the development of any such system, numerous implementation-specific decisions must be made to achieve specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it should be appreciated that such decisions might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components as the system is depicted in the attached drawings. However, as should be recognized by those skilled in the art, the elements, members, components, etc. described herein may be positioned in any desired orientation. Thus, the use of terms such as “above,” “below,” “upper,” “lower,” or other like terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components, respectively, as the example embodiments described herein may be oriented in any desired direction. 
     Referring to the appended drawings,  FIG. 1  is a perspective view of an example embodiment of a helicopter  10  according to the present specification. In general, helicopter  10  has a fuselage  12  and a main rotor assembly  14 , which includes main rotor blades  16   a - c  and a main rotor shaft  18 . Helicopter  10  may also include a tail rotor assembly  20 , which generally includes tail rotor blades  22  and a tail rotor shaft  24 . Main rotor blades  16   a - c  may rotate about a longitudinal axis  26  of main rotor shaft  18 . Tail rotor blades may rotate about a longitudinal axis  28  of tail rotor shaft  24 . Also illustrated in  FIG. 1  are flaps  32   a - b  and actuator systems  36   a - b  on main rotor blades  16   a - b , respectively. Not visible in  FIG. 1  are flap  32   c  and actuator system  36   c  on main rotor blade  16   c.    
       FIG. 2  is a partial top view of helicopter  10 , including main rotor blade  16   a , connected to a hub  30  on main rotor shaft  18 . In the example embodiment of helicopter  10 , main rotor blade  16   a  may include additional active elements that may be used to manipulate aerodynamic surfaces, such as flap  32   a . Flap  32   a  in the example embodiment of helicopter  10  is placed outboard along the trailing edge  34   a , but may be placed in other positions according to particular design criteria. And while flap  32   a  is illustrated and described herein as a distinct component of main rotor blade  16   a , it may also be any movable or flexible portion of main rotor blade  16   a . An example embodiment of actuator system  36   a  is also depicted in the cut-away section  FIG. 2 , generally oriented parallel to a span-wise axis  17   a  of main rotor blade  16   a . During operation, main rotor blade  16   a  may rotate about hub  30 , while actuator system manipulates flap  32   a . The rotation causes a number of reactive forces, including lift and centrifugal forces (CF). 
       FIG. 3  is a simple top-view schematic of actuator system  36   a  in main rotor blade  16   a . Actuator system  36   a  may include linear actuators  38   a - b . Each linear actuator  38   a - b  typically includes a fixed or stationary element, such as stators  40   a - b , and a moving or sliding element, such as sliders  42   a - b . Stators  40   a - b  in the example embodiment are rigidly connected to the frame of main rotor blade  16   a , and they may be identical elements or may have distinct properties for certain applications. Likewise, sliders  42   a - b  may be identical or have distinct properties for certain applications. Linear actuators  38   a - b  each has an elongated shape with a lengthwise axis  39   a - b  that is generally oriented parallel with span-wise axis  17   a  of main rotor blade  16   a . In the example embodiment of  FIG. 3 , linear actuators  38   a - b  are also generally oriented parallel to each other along the span of main rotor blade  16   a . Such a span-wise orientation is generally preferable to other orientations as it generally provides larger space in the blade for larger, more powerful motors with longer strokes, and better mass placement. 
     In actuator system  36   a , a crank  44  is connected to sliders  42   a - b . Crank  44  includes a beam element  46 , a pivot element  48 , and an arm element  50 . Examples of pivot element  48  include a conventional bearing with rolling elements, an elastomeric element, a sleeve bushing, or a structural flexure. Pivot element  48  may be positioned coincident with beam element  46 , or may be offset a distance L relative to beam element  46 , as shown in  FIG. 3 . By adjusting distance L, the large centrifugal force acting on sliders  42   a - b  may be used advantageously to create a negative stiffness spring effect, wherein the negative spring constant, k, is proportional to the centrifugal force CF, distance L, and angular displacement θ (−k=CF*L*sin(θ)/θ). The negative spring effect may counteract aerodynamic forces and reduce actuator power requirements, thereby also potentially reducing the mass of actuator system  36   a . Arm element  50  may be rigidly attached to beam element  46 , or beam element  46  and arm element may  50  be fabricated as a single element. 
       FIG. 4  is a simple side-view schematic of actuator system  36   a . Stators  40   a - b  are preferably placed within the frame of main rotor blade  16   a  in parallel. Connecting rod  52  connects actuator system  36   a  to flap  32   a  through crank  44  (see  FIG. 3 ) and sliders  42   a - b  (see  FIG. 3 ). Flap  32   a  may rotate about an axis  33  in response to force from connecting rod  52 . Alternate positions of flap  32   a  as it rotates about axis  33  are illustrated in phantom as flaps  32   a - 1  and  32   a - 2 . 
       FIG. 5  is a cut-away view of an example embodiment of a linear actuator  60 . In this embodiment, linear actuator  60  is an electromagnetic linear motor having a fixed element, stator  62 , having electric coils, and an elongated, high-power permanent magnetic slider  64 . The slider  64  moves and converts electrical power to useful work. The motion, position, and retention of slider  64  are controlled with electromagnetic force generated with the electric coils of stator  62 . Such an actuator may provide benefits in certain applications where high bandwidth and large stroke with a small footprint are desirable. For example, an electromagnetic motor such as linear actuator  60  may be advantageous in a helicopter rotor blade where vibrations and noise are counteracted with relatively small flap deflections at high frequency, but performance is enhanced with larger deflections at a lower frequency. 
     During rotation of main rotor blade  16   a , the centrifugal forces are carried across beam element  46  and reacted by pivot  48 , effectively canceling the tendency of sliders  42   a - b  to sling outward because of the centrifugal forces. Crank  44  is similar to a common bell crank, and as it rotates it converts the span-wise motion of sliders  42   a - b  into chord-wise motion that may be used to manipulate an active element, such as flap  32   a , which is connected to arm element  50  through a connecting rod  52  or similar linkage. 
     In operation, sliders  42   a - b  are actuated such that each reciprocates generally parallel to axis  17   a  and slider  42   a  moves opposite to slider  42   b . Thus, as slider  42   a  moves in the outboard direction of main rotor blade  16   a , slider  42   b  moves inboard. And as slider  42   a  moves outboard and slider  42   b  moves inboard, crank  44  rotates about pivot element  48 , causing arm element  50  to advance toward trailing edge  34   a  of main rotor blade  16   a . The movement of arm element  50  toward trailing edge  34   a  in turn causes connecting rod  52  to act on flap  32   a , which may rotate about axis  33  to position  32   a - 1 . 
     Conversely, as slider  42   a  moves inboard and slider  42   b  moves outboard, crank  44  rotates in the opposite direction about pivot element  48 , causing arm element  50  to retreat from trailing edge  34   a . The movement of arm element  50  away from trailing edge  34   b  in turn causes connecting rod  52  to act on flap  32   a , which may rotate about axis  33  to another position, such as  32   a - 2 . 
       FIG. 6  is a simple top-view schematic of another example embodiment of an actuator system  70  in a main rotor blade  72  according to the present specification. Actuator system  70  may include linear actuators  74   a - b . Each linear actuator  74   a - b  typically includes a fixed or stationary element, such as stators  76   a - b , and a moving element or sliding element, such as sliders  78   a - b . Stators  76   a - b  in the example embodiment are rigidly connected to the frame of main rotor blade  72 , and they may be identical elements or may have distinct properties for certain applications. Likewise, sliders  78   a - b  may be identical or have distinct properties for certain applications. Linear actuators  74   a - b  each has an elongated shape with a lengthwise axis  75   a - b  that is generally oriented parallel with span-wise axis  73  of main rotor blade  72 . In contrast to linear actuators  38   a - b  in  FIG. 3 , linear actuators  74   a - b  are generally oriented in series along the span of main rotor blade  72 . 
     In actuator system  70 , a crank  80  is connected to sliders  78   a - b . Crank  80  includes a beam element  82 , a pivot element  84 , and an arm element  86 . Extension elements  79   a - b  may be used to connect sliders  78   a - b  to beam element  82 . Examples of pivot element  84  include a conventional bearing with rolling elements, an elastomeric element, a sleeve bushing, or a structural flexure. Pivot element  84  may be positioned coincident with beam element  82 , or may be positioned a distance L relative to beam element  82 , as shown in  FIG. 6 . By adjusting distance L, the large centrifugal force acting on sliders  78   a - b  may be used advantageously to create a negative stiffness spring effect, wherein the negative spring constant, k, is proportional to the centrifugal force CF, distance L, and angular displacement θ (−k=CF*L*sin(θ)/θ). The negative spring effect may counteract aerodynamic forces and reduce actuator power requirements, thereby also potentially reducing the mass of actuator system  70 . Arm element  86  may be rigidly attached to beam element  82 , or beam element  82  and arm element  86  may be fabricated as a single element. 
     During rotation of main rotor blade  72 , the centrifugal forces are carried across beam element  82  and reacted by pivot element  84 , effectively canceling the tendency of sliders  78   a - b  to sling outward because of the centrifugal forces. Crank  80  is similar to a common bell crank, and as it rotates it converts the span-wise motion of sliders  78   a - b  into chord-wise motion that may be used to manipulate an active element, such as flap  88 , which is connected to arm element  86  through a connecting rod  90  or similar linkage. 
     In operation, sliders  78   a - b  are actuated such that each reciprocates generally parallel to axis  73  and slider  78   a  moves opposite to slider  78   b . Thus, as slider  78   a  moves in the outboard direction of main rotor blade  72 , slider  78   b  moves inboard. And as slider  78   a  moves outboard and slider  78   b  moves inboard, crank  80  rotates about pivot element  84 , causing arm element  86  to advance toward trailing edge  92  of main rotor blade  72 . The movement of arm element  86  toward trailing edge  92  in turn causes connecting rod  90  to act on flap  88 , which may rotate about axis  89 . 
     Conversely, as slider  78   a  moves inboard and slider  78   b  moves outboard, crank  80  rotates in the opposite direction about pivot element  84 , causing arm element  86  to retreat from trailing edge  92 . The movement of arm element  86  away from trailing edge  92  in turn causes connecting rod  90  to act on flap  88 , which may rotate about axis  89 . 
       FIG. 7  is a simple schematic of an example embodiment of an actuator system  100  having a cross-axis flexure pivot element, which may be deployed in a main rotor blade  101  or other wing structure having an active element  103 . A cross-axis flexure pivot element may simulate a pinned joint while providing certain potential advantages over alternative elements, such as reducing weight and moving parts. A cross-axis flexure pivot element may also be advantageous where high frequency motion and low hysteresis is needed. Actuator system  100  may include moving or sliding elements, such as sliding elements  102   a - b , which may be driven by input forces F 1  and F 2 , respectively. Sliding elements  102   a - b  may be identical or have distinct properties for certain applications. Sliding elements  102   a - b  each has an elongated shape with a lengthwise axis  105   a - b  that may be oriented parallel with a span-wise axis  107  of main rotor blade  101 . In the example embodiment of  FIG. 7 , sliding elements  102   a - b  are also generally oriented parallel to each other along the span of main rotor blade  101 . 
     In actuator system  100 , a crank  104  may be connected to sliding elements  102   a - b . Crank  104  includes a beam element  106 , a cross-axis flexure pivot element  108 , and arm elements  110   a - b . Cross-axis flexure pivot element  108  may include flexure straps  112   a - b , which may be made of fiberglass or other suitable flexure material. Each flexure strap  112   a - b  can be fastened on one end to crank  104 , and on the other end to the frame of main rotor blade  101  or other fixture that may be rigidly attached to the frame. The flexure straps  112   a - b  intersect at a pivot point  113 . Arm elements  110   a - b  may each be fastened to flexure straps  114   a - b , respectively, which may in turn be fastened to sleeve elements  116   a - b . The use of flexure straps  114   a - b  for connecting crank  104  to sleeve elements  116   a - b  allows transverse displacement between crank  104  and sliding elements  102   a - b . Each sleeve element  116   a - b  may be fastened to a sliding element  102   a - b , respectively. The length of flexure straps  114   a - b  may be adjusted to control the offset d 3  between pivot point  113  and the points of attachment  115   a - b  with arm elements  110   a - b . The offset d 3  may be used advantageously to create a negative stiffness spring effect as discussed above. The negative spring effect may counteract aerodynamic forces and reduce actuator power requirements, thereby also potentially reducing the mass of actuator system  100 . Arm elements  110   a - b  may be rigidly attached to beam element  106 , or beam element  106  and arm elements  110   a - b  may be fabricated as a single element. A link element  118  may be fastened on one end to crank  104  and on the other to active element  103 . 
     In operation, sliding elements  102   a - b  may be actuated such that each reciprocates generally parallel to a span-wise axis of a wing structure and sliding element  102   a  moves opposite to sliding element  102   b . Thus, as sliding element  102   a  moves in the outboard direction of a main rotor blade, sliding element  102   b  moves inboard. And as sliding element  102   a  moves outboard and sliding element  102   b  moves inboard, crank  104  may rotate about pivot point  113 , causing beam element  106  to translate in a first direction (e.g., away from a trailing edge). The movement of beam element  106  in turn may cause link element  118  to act on active element  103 , which may rotate about an axis  109 . 
     Conversely, as sliding element  102   a  moves inboard and sliding element  102   b  moves outboard, crank  104  may rotate in the opposite direction about pivot point  113 , causing beam element  106  to translate in a second direction (e.g., toward a trailing edge). The movement of beam element  106  in turn may cause link element  118  to act on active element  103 , which may rotate about axis  109 . 
       FIG. 8  is a simple schematic of an example embodiment of an actuator system  200  having a cross-axis flexure pivot element, which may be deployed in a main rotor blade  201  or other wing structure having an active element  203 . Actuator system  200  may include moving or sliding elements, such as sliding elements  202   a - b , which may be driven by input forces F 1  and F 2 , respectively. Sliding elements  202   a - b  may be identical or have distinct properties for certain applications. Sliding elements  202   a - b  each has an elongated shape with a lengthwise axis  205   a - b  that may be oriented parallel with a span-wise axis  207  of main rotor blade  201 . In the example embodiment of  FIG. 8 , sliding elements  202   a - b  are also generally oriented in series with each other along the span of main rotor blade  201 . 
     In actuator system  200 , a crank  204  may be connected to sliding elements  202   a - b . Crank  204  includes a beam element  206 , a cross-axis flexure pivot element  208 , and arm elements  210   a - b . Cross-axis flexure pivot element  208  may include flexure straps  212   a - b , which may be made of fiberglass or other suitable flexure material. Each flexure strap  212   a - b  may be fastened on one end to crank  204 , and on the other end to the frame of main rotor blade  201  or other fixture that may be rigidly attached to the frame. The flexure straps  212   a - b  intersect at a pivot point  213 . Arm elements  210   a - b  may be each fastened to flexure straps  214   a - b , respectively, which may be in turn fastened to sleeve elements  216   a - b . The use of flexure straps  214   a - b  for connecting crank  204  to sleeve elements  216   a - b  allows transverse displacement between crank  204  and sliding elements  202   a - b . Each sleeve element  216   a - b  may be fastened to a sliding element  202   a - b , respectively. The length of flexure straps  214   a - b  may be adjusted to control the offset d 3  between pivot point  213  and the points of attachment  215   a - b  with arm elements  210   a - b . The offset d 3  may be used advantageously to create a negative stiffness spring effect as discussed above. The negative spring effect may counteract aerodynamic forces and reduce actuator power requirements, thereby also potentially reducing the mass of actuator system  200 . Arm elements  210   a - b  may be rigidly attached to beam element  206 , or beam element  206  and arm elements  210   a - b  may be fabricated as a single element. A link element  218  may be fastened on one end to crank  204  and on the other to active element  203 . 
     In operation, sliding elements  202   a - b  may be actuated such that each reciprocates generally parallel to a span-wise axis of a wing structure and sliding element  202   a  moves opposite to sliding element  202   b . Thus, as sliding element  202   a  moves in the outboard direction of a main rotor blade, sliding element  202   b  moves inboard. And as sliding element  202   a  moves outboard and sliding element  202   b  moves inboard, crank  204  may rotate about pivot point  213 , causing beam element  206  to translate in a first direction (e.g., away from a trailing edge). The movement of beam element  206  in turn may cause link element  218  to act on active element  203 , which may rotate about an axis  209 . 
     Conversely, as sliding element  202   a  moves inboard and sliding element  202   b  moves outboard, crank  204  rotates in the opposite direction about pivot point  213 , causing beam element  206  to translate in a second direction (e.g., toward a trailing edge). The movement of beam element  206  in turn causes link element  218  to act on active element  203 , which may rotate about axis  209 . 
       FIG. 9  is a perspective view of an example embodiment of an assembled actuator system  300  according to the present specification. Actuator system  300  is representative of a system having a parallel configuration of linear actuators, similar to the system illustrated in  FIG. 3  or  FIG. 7 . As shown, actuator system  300  includes a crank  304 , a base  306 , and a pivot element  308 . Pivot element  308  generally includes flexure straps  312   a - b , which may be fastened or clamped on one end to crank  304  with fasteners  305   a - d  and on the other to base  306 . Base  306  may be fastened to a wing frame (not shown) with bolts  307   a - b . Each sleeve element  316   a - b  may be fastened near one end to a sliding element (not visible) and near the other end to another flexure strap (not visible). Similar to flexure straps  114   a - b  in  FIG. 7 , these flexure straps may be fastened on the other end to crank  304 . Sleeve element  316   a  may further include a hole  317  to permit passage of a link element  318  from crank  304  to an active element (not shown). 
       FIG. 10  is a perspective view of another example embodiment of an assembled actuator system  400  according to the present specification. Actuator system  400  is representative of a system having a serial configuration of linear actuators, similar to the system illustrated in  FIG. 6  or  FIG. 8 . As shown, actuator system  400  includes a crank  404 , a base  406 , and a pivot element  408 . Pivot element  408  generally includes flexure straps  412   a - b , which may be fastened or clamped on one end to crank  404  and on the other to base  406 . Each sleeve element  416   a - b  may be fastened near one end to a sliding element  402   a - b , respectively, and near the other end to another flexure strap  414   a - b , respectively. Similar to flexure straps  214   a - b  in  FIG. 8 , these flexure straps may be fastened on the other end to crank  404 . 
       FIG. 11  is a top view of an example embodiment of an actuator system  500 , according to the present specification. Actuator system  500  is representative of a system having a parallel configuration of linear actuators, similar to the system illustrated in  FIG. 9 . As shown, actuator system  500  includes a crank  504 , a base  506 , and a pivot element  508 . Crank  504  is configured with multiple attachment points  515   a - d , and may include a beam element  506  and arm elements  510   a - b . Pivot element  508  generally includes flexure straps  512   a - b , which may be fastened or clamped at attachment points  515   c - d  to crank  504  with fasteners  505   a - b  and at attachment points  515   e - f  to base  506  with fasteners  505   c - d . Flexure straps  512   a - b  intersect at a pivot point  513 , and the length of flexure straps  512   a - b  may be adjusted to control the offset between pivot point  513  and attachment points  515   e - f . The offset may be used advantageously to create a negative stiffness spring effect as discussed above. Base  506  may be fastened to a wing frame (not shown) with bolts, such as bolt  507 . Each sleeve element  516   a - b  is fastened near one end to a sliding element  502   a - b , respectively, and near the other end to flexure straps  514   a - b , respectively. Flexure straps  514   a - b  may also be fastened to crank  504  with fasteners  505   e - f  at attachment points  515   a - b , respectively. Shoe elements  520   a - d  may be used to control curvature and bending strain. Sleeve element  516   b  may further include a hole (not visible) to permit passage of a link element  518  from crank  504  to an active element (not shown). 
       FIG. 12  is an exploded top view of an actuator system  600 , according to the present specification. Actuator system  600  is also representative of a system having a parallel configuration of linear actuators. As shown, actuator system  600  includes linear actuators  602   a - b , a crank  604 , a base  606 , and a pivot element  608 . Pivot element  608  generally includes flexure straps  612   a - b , which may be fastened or clamped on one end to crank  604  with fasteners  605   a - b  and on the other to base  606  with fasteners  605   c - d . Base  606  may be fastened to a wing frame (not shown) by inserting bolts (not shown) through holes  607   a - b . Each sleeve element  616   a - b  may be fastened near one end to a sliding element  602   a - b , respectively, and near the other end to flexure straps  614   a - b , respectively. Flexure straps  614   a - b  may also be fastened to crank  604  with fasteners  605   e - f . Shoe elements  620   a - d  may be used to control curvature and bending strain. Sleeve element  616   b  may further include a hole (not visible) to permit passage of a link element  618  from crank  604  to an active element (not shown). 
       FIG. 13  is an exploded bottom view of actuator system  600 . 
     Alternatively or additionally, an actuator system may include hydraulic, piezoelectric, or electromechanical components. For example, a linear actuator may have a fixed element such as a hydraulic cylinder and a moving element such as a hydraulic ram. 
     The system and apparatus described herein provides significant advantages, including: (1) reducing or eliminating the adverse effects of centrifugal forces on linear actuators in a span-wise orientation; (2) more powerful motors; (3) longer stroke and greater bandwidth than other systems; and (4) improved mass distribution characteristics. 
     Certain example embodiments have been shown in the drawings and described above, but variations in these embodiments will be apparent to those skilled in the art. The principles disclosed herein are readily applicable to a variety of aircraft, including many types of rotary wing, tilt-rotor, and fixed wing aircraft, as well as a variety of other active wing elements, including leading edge droops. The preceding description is for illustration purposes only, and the claims below should not be construed as limited to the specific embodiments shown and described.