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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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1. A wind turbine comprising:a rotor with a plurality of blades which rotate about a substantially horizontal axis wherein said rotor has a diameter defined by the area swept by said blades; a mechanism for varying the diameter of said rotor; and a controller for controlling the mechanism that varie

1. A wind turbine comprising:a rotor with a plurality of blades which rotate about a substantially horizontal axis wherein said rotor has a diameter defined by the area swept by said blades; a mechanism for varying the diameter of said rotor; and a controller for controlling the mechanism that varies the diameter of said rotor, wherein said controller monitors the power output of said wind turbine and wherein said controller decreases the diameter of said rotor when the power output exceeds a first predetermined level and increases the diameter of said rotor when the power output is less than a second predetermined level, so as to avoid undue exercising of the mechanism for varying in variable wind conditions. 2. A wind turbine blade comprising:an inboard end that is proximate an axis of rotation; an outboard end that is distant from said axis of rotation; a structure extending between said inboard end and said outboard end wherein at least a portion of said structure has an airfoil shaped cross-section that provides aerodynamic lift; and a mechanism for varying the distance between said inboard end and said outboard end, wherein said blade defines a longitudinal axis between said inboard end and said outboard end and further comprising a fixed section proximate said inboard end, a moveable section proximate said outboard end, and a mechanism for moving said moveable section relative to said fixed section along a path that is substantially parallel to said longitudinal axis, wherein at least a portion of both said fixed section and said moveable section has an airfoil shaped cross-section, and wherein said moveable section is sized and shaped to nest inside of said fixed section wherein said mechanism for moving said moveable section relative to said fixed section comprises a threaded rod, the threaded rod being adapted to rotate to cause movement of the moveable section of said wind turbine blade; further wherein said mechanism for varying the distance between said inboard end and said outboard end comprises a slider track and a slider, the slider being guided within the slider track dining movement of the moveable section. 3. The wind turbine blade of claim 2, wherein each slider track and slider is one of a pair of slider tracks and sliders, the sliders being guided within the slider tracks to restrain the moveable section from moving in any direction other than said path substantially parallel to said longitudinal axis.4. The wind turbine blade of claim 2, wherein said mechanism for moving said moveable section relative to said fixed section further comprises an actuator adapted to rotate said threaded rod.5. The wind turbine blade of claim 2, wherein the slider track and the slider are in only sliding contact with each other.6. A method of controlling a wind turbine comprising the steps of:providing a rotor with a plurality of blades and a diameter defined by the area swept by said blades; providing a mechanism for adjusting the diameter of said rotor; measuring the power generated by said wind turbine; and adjusting the diameter of said rotor during operation of said wind turbine based on said power measurement such that turbulence and gusts do not cause undue exercising of the mechanism for adjusting the diameter of said rotor. 7. The method of claim 6 wherein the step of adjusting the diameter of said rotor comprises following a control strategy that includes three operating regions wherein the first region is defined by power generated by said wind turbine being below a first predetermined level and the diameter of said rotor being at a maximum, the second region is defined by power generated by said wind turbine being between said first predetermined level and a second predetermined level and the diameter of said rotor varying as a function of said power measurement, and the third region is defined by power generated by said wind turbine being above said second predetermined level and the diameter of said rotor being at a minimum.8. A wind turbine blade comprising:an inboard end that is proximate an axis of rotation; an outboard end that is distant from said axis of rotation; a structure extending between said inboard end and said outboard end wherein at least a portion of said structure has an airfoil shaped cross-section that provides aerodynamic lift; and a mechanism for varying the distance between said inboard end and said outboard end, wherein said blade defines a longitudinal axis between said inboard end and said outboard end and further comprising a fixed section proximate said inboard end, a moveable section proximate said outboard end, and a mechanism for moving said moveable section relative to said fixed section along a path that is substantially parallel to said longitudinal axis, wherein at least a portion of both said fixed section and said moveable section has an airfoil shaped cross-section, and wherein said moveable section is sized and shaped to nest inside of said fixed section; wherein said mechanism for varying the distance between said inboard end and said outboard end comprises a cable supported on the fixed section of the wind turbine blade by a pulley and controlled by a winch, further wherein the mechanism for varying the distance is able to maintain the moveable section in an extended position relative to the fixed section when the wind turbine blade is not rotating about said axis of rotation. 9. The wind turbine blade of claim 8, wherein the mechanism for varying the distance is adapted to move the moveable section in one direction when the winch advances the cable and in an opposite direction when the winch retracts the cable.10. The wind turbine blade of claim 8, wherein the cable is double-wound onto the winch such that one end of the cable is pulled in by the winch while another end of the cable is being let out by the winch.11. The wind turbine blade of claim 8, further comprising a retention block attached to the moveable section and to the cable such that the moveable section is moved longitudinally when the winch operates.