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A wind turbine blade made of a fixed blade section with an integral mounting flange for attachment to a wind turbine hub. A moveable blade section is attached to the fixed blade section and is free to move in a longitudinal direction relative to the fixed blade section. A positioning device controll

A wind turbine blade made of a fixed blade section with an integral mounting flange for attachment to a wind turbine hub. A moveable blade section is attached to the fixed blade section and is free to move in a longitudinal direction relative to the fixed blade section. A positioning device controllably positions the moveable blade section to vary the overall length of the blade. This allows the wind turbine's rotor diameter to be adjusted. The rotor diameter can be increased in order to provide high power output in low wind conditions and it can be decreased in order to minimize loads in high wind conditions.

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We claim: 1. A wind power generation system comprising: a turbine including a rotor having a plurality of blades which rotate about a substantially horizontal axis, wherein each of the blades has a separately variable length; a mechanism configured to separately vary the length of each of the blade

We claim: 1. A wind power generation system comprising: a turbine including a rotor having a plurality of blades which rotate about a substantially horizontal axis, wherein each of the blades has a separately variable length; a mechanism configured to separately vary the length of each of the blades; a controller configured to: control the mechanism that varies the length of each of the blades, and selectively alter the length of any number of the plurality of blades; and a sensor configured to measure inequalities in the balance of the rotor, and wherein in response to a signal from the sensor, the controller is further configured to separately alter the length of any number of the plurality of blades to adjust inequalities in the balance of the rotor. 2. The wind power generation system of claim 1, wherein the sensor is configured to measure at least one of mass balance and aerodynamic balance of the rotor. 3. The wind power generation system of claim 1, wherein in response to the output of the sensor, the controller is configured continuously adjust inequalities in the balance of the rotor. 4. The wind power generation system of claim 1, further comprising a sensor configured to measure power output of the turbine, and wherein in response to a signal from the sensor, the controller is configured to alter the length of all of the plurality of blades to maintain the power output of the turbine above a predetermined power level. 5. The wind power generation system of claim 4, wherein the predetermined power level is about 80% of rated power of the turbine. 6. A wind power generation system comprising: a turbine including a rotor having a plurality of blades which rotate about a substantially horizontal axis, wherein each of the blades has a separately variable length; a mechanism configured to separately vary the length of each of the blades; and a controller configured to control the mechanism that varies the length of each of the blades, wherein the controller is further configured to differently alter the length of at least one of the plurality of blades by altering the length of one of the plurality of blades by one distance, and altering the length of another of the plurality of blades by another distance. 7. The wind power generation system of claim 6, further comprising a sensor configured to measure power output of the turbine, and wherein in response to a signal from the sensor, the controller is configured to alter the length of all of the plurality of blades to maintain the power output of the turbine above a predetermined power level. 8. A method of controlling a wind turbine comprising: providing a rotor having a plurality of blades which rotate about a substantially horizontal axis, wherein each of the blades has a variable length; providing a mechanism for separately and independently varying the length of each of the blades; measuring inequalities in the balance of the rotor; and selectively altering the length of any number of the plurality of blades including differently altering the length of at least one of the plurality of blades in response to measured inequalities in the balance of the rotor. 9. The method of claim 8, wherein measuring inequalities in the balance of the rotor comprises measuring at least one of mass balance and aerodynamic balance of the rotor. 10. The method of claim 8, wherein differently altering the length of at least one of the plurality of blades comprises varying the length of one of the plurality of blades by one distance, and varying the length of an other of the plurality of blades by an other distance. 11. The method of claim 8 further comprising: measuring power output of the wind turbine; in response to the measured power output, altering the length of all of the plurality of blades to maintain the power output of the turbine above a predetermined power level. 12. The method of claim 11, wherein the predetermined power level is about 80% of rated power of the turbine. 13. A machine for producing power comprising: a stationary wind turbine; a rotor attached to said wind turbine, wherein said rotor has a plurality of blades which rotate about a substantially horizontal axis of rotation, wherein said rotor has a diameter defined by the area swept by said blades, and wherein a portion of each of said blades has an airfoil-shaped cross section; a mechanism for varying the diameter of said rotor, wherein said mechanism changes a length of the airfoil-shaped portion of each of said blades as the diameter of said rotor is changed, and wherein said mechanism for varying the diameter of said rotor is able to maintain the diameter of said rotor in an extended position when said rotor is not rotating about said substantially horizontal axis; a controller configured to selectively change the length of the airfoil-shaped portion of any number of said plurality of blades; and a sensor configured to measure inequalities in the balance of said rotor, and wherein in response to a signal from said sensor, the controller is further configured to differently change the length of the airfoil-shaped portion of at least one of said blades. 14. The machine of claim 13, wherein the controller is further configured to: control the mechanism that varies the diameter of said rotor, responsive to a power output of the turbine, decreases the length of the airfoil-shaped portion when the power output is greater than a first predetermined level, and responsive to the power output of the turbine, increases the length of the airfoil-shaped portion when the power output is less than a second predetermined level. 15. The machine of claim 13, wherein each of said blades comprises: a fixed inboard section proximate to said axis of rotation; and a movable outboard section distant from said axis of rotation; wherein said mechanism for varying the diameter of said rotor moves the movable section relative to the fixed section, and wherein said mechanism comprises a winch mounted on said fixed section and a cable that is controlled by said winch that is fastened to said moveable section, wherein said moveable section is moved relative to said fixed section when said winch is actuated. 16. The machine of claim 13, wherein each of said blades comprises: a fixed inboard section proximate to said axis of rotation; and a movable outboard section distant from said axis of rotation; wherein said mechanism for varying the diameter of said rotor moves the movable section relative to the fixed section, and wherein said mechanism comprises a threaded rod mounted on said fixed section for rotation about an axis that is substantially parallel to a longitudinal axis of said blade and a nut in driving contact with said threaded rod, said nut being fastened to said moveable section wherein said moveable section is moved relative to said fixed section when said threaded rod is rotated. 17. A machine for producing power comprising: a stationary wind turbine; a rotor attached to said wind turbine, wherein said rotor has a plurality of blades which rotate about a substantially horizontal axis of rotation, wherein a portion of a length of each of said blades has an airfoil-shaped cross section; a mechanism for extending and retracting the length of each blade, said mechanism comprising a separate mechanism on each said blade; and a controller configured to control the extension and retraction of each said blade, wherein said controller is adapted to cause said blades to be extended and retracted simultaneously by the same amount during normal operation of said machine, wherein said mechanism for extending and retracting the length of each blade is capable of extending and retracting each said blade independently, and wherein said controller is capable of extending and retracting each blade independently to balance said rotor. 18. The machine of claim 17 wherein said mechanism for extending and retracting the length of each blade changes the length of the airfoil-shaped portion of each of said blades as each said blade is extended and retracted. 19. The machine of claim 17 further comprising a sensor for measuring a power output of the turbine and a controller that extends and retracts said blades in response to an output of said sensor. 20. The machine of claim 17 wherein said mechanism for extending and retracting said blades is capable of maintaining said blades in an extended position when said rotor is not rotating about said substantially horizontal axis.