as shown in the drawings , the present invention is embodied in a vertical windmill 10 that includes an omni - directional diffuser 12 defined by a plurality of vertical stators 14 . the stators are equally spaced circumferentially about a vertical central axis 18 at a uniform radial distance from the axis . a central shaft 20 is rotatably mounted in a gearbox 22 and bearings 24 along the central axis of the windmill . radial struts 28 attach a airfoils 30 to the central shaft inward of the omnidirectional diffuser at a fixed radial distance from the central shaft . a support frame 32 that includes upper and lower support bars 34 supports the central shaft and the stators . the cross - section of each airfoil 30 is designed to provide aerodynamic lift . the airfoils are preferably equally spaced circumferentially about the central shaft 20 . as is evident from comparing fig1 and 2 , the windmill 10 of the present invention may contain one more levels of stators 14 and airfoils . according to one aspect of the present invention , the stators 14 may rotate from radial alignment to variable alignment , either to increase efficiency or to withstand high winds , as depicted by the dashed lines in fig3 . although several mechanisms could be used , the mechanism presently contemplated for turning the stators is best depicted in fig4 and 6 . each stator is attached to steel tubes 38 and column fairing bearings 40 allow them to rotate . each stator is mounted so the centroid 44 of the stator is offset from the steel tube centerline . wind resistance will act against the stators 14 at or near the centroid 44 , causing the stator to rotate about the steel tube 38 in the direction of the wind . a weight 48 is attached to the stator at the point offset from the steel tube by a cable 50 passing through pulleys 52 . tension in the cable , caused by the weight , restrains rotation of the stator at wind velocities a predetermined magnitude and direction . when the vertical plane containing the cordwise dimension of the stator 14 is coincident with the vertical plane containing the weight 48 , the stator is the null or operational position , i . e . aligned radially to the central axis 18 of the windmill 10 . when the stator is perturbed by the wind to rotate ( as shown in fig6 ) in either direction , a return moment is formed when the cable 50 is forced to turn 90 ° around pulley 52 in the horizontal plane measured from the vertical plane defined by the weight vector and the windmill radial dimension . at small angular perturbations of the stator from this plane , the tension in the cable acts perpendicular to the stator cordwise dimension . in very high winds , the stator seeks the angular attitude of least air resistance . the stator will rotate so that its cordline aligns with the wind vector , raising the weight . the weight 48 attached to the cable 50 can move from its static position very near ground level ( as shown in fig5 ), to a height limited by the windmill structure , providing enough cable length to allow the stator 14 one revolution . if further rotation is required , the cable is released by disconnecting the mechanical connection 54 between the cable and the stator , allowing the stator to freely rotate . when wind velocity reduces , the stators gradually return to their radially oriented position for normal and efficient windmill operation . thus , the vertical windmill presents a reduced cross - section to high winds and the structural support necessary for the windmill is significantly reduced , which can provide as much as a 50 % reduction in the amount of steel necessary to support to the windmill . as shown in fig7 each of the airfoils 30 has a thickness t and a cord length c . according to a preferred embodiment of the invention , the thickness ratio t / c is 0 . 18 or greater to provide self - starting and sustained operation for the windmill . although the prior art teaches that 0 . 15 is frequently too large a ratio , it has been experimentally determined that a thickness 0 . 18 or greater is preferable in the vertical windmill environment described to provide self - starting in low wind , and sustained operation in higher winds . according to another aspect of the present invention , each of the airfoils 30 may turn about a vertical elongated airfoil shaft 58 to reduce the lift the airfoils and slow rotation of the windmill 10 . as depicted in fig8 - 12 , a cam - cable braking mechanism 60 is one method for reducing or eliminating the lift on the airfoils and thereby slowing the rotation of the central shaft 20 of the windmill 10 . each airfoil is attached to the end of top and bottom radial struts 28 mounted to the central shaft . the airfoils are mounted in such a way that they are free to rotate about the airfoil shaft , which passes through the blade end of the airfoil , perpendicular and intersecting to the airfoil mean cord line at some distance between the leading and trailing edge of the airfoil ( see fig8 ). the airfoil shafts 58 are mounted in bearings located on the ends of each top and bottom strut 28 . a rail 62 ( shown in fig9 ) is attached to the bottom of the airfoil 30 , i . e . that end closest to the ground . the rail extends from near the airfoil shaft 58 aft to the trailing edge coincident with the airfoil mean cord line . a rail bearing 64 fits within this rail having a vertical axis of rotation such that it can freely rotate as a wheel along the rail . the rail bearing is mounted on a short shaft 68 that is attached to a flat cam plate 70 , which lies in a horizontal plane and is mounted on the strut 28 so that it can be rotated ( as shown in fig1 ). the rotational axis 72 of the cam plate 70 is parallel to the axis of the airfoil 30 , but offset from it ( as shown in fig9 , and 11 ) in a direction toward the central shaft 20 and behind ( down wind of ) the strut 28 . rotation of the cam plate moves the rail bearing 64 aft of the airfoil shaft 58 ( as shown in fig1 ) toward the airfoil trailing edge , causing the airfoil trailing edge to rotate toward the strut and central shaft 20 . in this manner , the airfoil is rotated or pitched to an angle so as to cause air flow over the airfoil to separate and the airfoil to stall , transforming the airfoil from a positive torque producing element to a negligible or negative torque producing element . the cam plate 70 is rotated by a first cable 74 ( shown in fig9 ) attached to a central disc 78 that is mounted around the central shaft 20 . the first cable is attached in a direction aft or down wind of the cam axis 72 to cause the cam to rotate away from the airfoil shaft 28 . rotation of the central disc 78 relative to the central shaft 20 as the blade pitches causes tension in the first cable that rotates the cam plate . the cam plate 70 is held in the null operational condition by a spring 80 , attached to a second cable 84 ( shown in fig9 ), which acts against the cam plate . when tension in the first cable is small , the spring action of the second cable returns the cam plate to its stop position . the central disc 78 is held in the null operational condition by retaining springs 86 . when the central disc is acted on by an external force , the spring tension is overcome and the central disc rotates . as shown in fig8 the retaining springs are attached at one end the central disc and at the other end to the struts 28 . two central disc spring retention stops 88 for the springs to act against in the null operational condition are also shown in fig8 . the cam plate 70 in its stop position is oriented so that a line ( as shown in fig9 ) intersecting both the rail bearing 64 centerline and the cam axis 72 is parallel to the radial centerline of the strut 28 . moments acting on the airfoil 30 cannot act to rotate the cam plate in this configuration . the airfoil is held fast and not subject to cyclic aerodynamic pitching moments . a cam - cable braking mechanism 60 connects each of the airfoils 30 to the central disc 78 . consequently , rotation of the central disc relative to the central shaft 20 causes all the airfoils to rotate at equal angles relative to the struts . because each of the airfoils has an associated cam - cable braking mechanism as described , the rotational speed of the windmill can be controlled with only a slight exertion of pressure by a disc brake 90 ( see fig1 ) against the central disc , thereby improving the longevity of the disc brake and greatly increasing the safety of the windmill . as depicted in fig1 and 14 , another aspect of the present invention is a stop / release mechanism for the disc brake 90 . specifically , a right angle bracket 94 is pivotally mounted to a sleeve 98 around the central shaft 20 . a cable 100 is attached between the right angle of the bracket and a spring 102 . the spring is attached to a plate 104 that is mounted on the central shaft above the sleeve . as shown in fig1 , in normal operation , centrifugal force pivots the right angle bracket 94 from the central shaft 20 and extends the spring 102 . a gate shaft 108 mounted by two pillow blocks 110 and 112 to the bottom of the right angle bracket is held away from a first stop 114 on the central disc 78 in the approximate angle theta indicated in fig1 . when the brake shoe 118 is applied against the central disc 78 , the central disc rotates relative to the central shaft 20 through the angle theta so that the gate shaft pushes against the first stop . this causes the central disc to rotate at the same speed as the central shaft , and results in the relative position of the gate shaft , first stop and central disc shown in fig1 . the slowing of the central disc , as shown in fig1 , actuates the cam - cable braking mechanism 60 to rotate the airfoils 30 and cause the airfoils to lose lift . as depicted in fig9 and 10 , the rail bearing 64 on the cam plate 70 is used to position the airfoil 30 with respect to the strut 28 and thus the windmill 10 . to stop the rotation of the windmill , the disc brake 90 is applied sufficiently to slow the rotation of the central shaft 20 . at a certain rotational speed , the centrifugal force acting against the right angle bracket 94 no longer overcomes the force exerted on the bracket by the spring 102 . the right angle bracket thus pivots toward the central shaft to the position shown in fig1 . pivoting of the right angle bracket lifts the gate shaft 108 so that the gate shaft passes over the first stop 114 . the central disc 78 rotates through an angle phi to the relative position shown in fig1 . this additional rotation causes the first cables 74 to pull the cam plates 70 completely off the rails 62 , as shown in fig1 , so that the airfoils 30 react freely to the wind and lose all lift . continued pressure on the disc brake will then stop the rotation of the windmill . as the rotation of the central shaft 20 slows , the central disc 78 rotates until a second stop 120 mounted on the central disc contacts an angle bracket 122 that is mounted to the bottom of the sleeve 98 . thereafter , the central disc rotates at the same speed as the central shaft . as a result , the first and second cables 74 and 84 are not damaged by continuing relative angular rotation between the central disc and the central shaft . as depicted in fig8 the retaining springs 86 are attached to the central disc 78 to counteract the effect of the disc brake 90 on the central disc . in other words , the retaining springs 86 exert a force that tends to pull the central disc to the normal operating position . if the disc brake 90 is actuated ( causing the central disc to slow rotation relative to the central shaft 20 and the first cables 74 to turn the airfoils 30 ) and then released , the retaining springs pull the central disc back into the normal operational position . this results in the cam plates 70 rotating the airfoils from the position shown in fig1 having reduced lift or no lift to the position shown in fig9 having maximum lift . thus , the retaining springs countermand the effect of the disc brake and hold the airfoils in the proper position for normal operation that is shown in fig9 . as is diagrammatically shown in fig1 , the brake shoe 118 and disc brake 90 are connected by a series of joints 130 to a lever arm 132 having a weighted block 134 on its outward end , away from the central shaft 20 . the block is connected by a line 138 to a winch 140 . by activating the winch to lower the block , the brake shoe 118 acts against the bottom of the central disc 78 and the rotational speed of the rotor formed by the central shaft , the struts 28 , and the airfoils 30 is slowed as described above with reference to fig8 - 17 . another aspect of the present invention is a high speed fail - safe centrifugally actuated braking mechanism designed to protect the windmill 10 from the damage that may be caused by extremely high winds . according to this aspect of the invention , a weighted lever 144 is pivotally mounted to one of the struts 28 or to the central shaft 20 , and a spring 148 connects an inner portion of the strut to the lever . as the airfoils 30 rotate , centrifugal force gradually pivots the end of the weighted lever 144 that is connected to the spring 148 through an angle alpha toward the circumference of the windmill . a target 150 is mounted on the end of a piece of angle iron 152 . a notch 154 in the angle iron holds a stop 158 that is fixed to a bar 160 . in normal operation , the stop fixed in the notch counteracts the biasing of a compression spring 162 that pushes against a fixed housing 164 and pulls the bar and a connecting tether 168 . the end of the tether attaches to a winch release 170 on the winch 140 . if a critically high rotational speed is reached , the lever 144 rotates through the angle alpha to strike the target 150 , the angle iron 152 pivots about a rod 172 and releases the stop 158 . the compression spring 162 pulls the bar 160 and the tether 168 , releasing the winch release 170 . as a result , the winch 140 immediately lowers the weighted block 134 , causing the brake shoe 118 to push against the central disc 78 and completely stop the windmill 10 , as described above . thus , the centrifugally activated brake mechanism provides a safety brake to stop dangerous high speed rotation of the windmill in extremely high winds . the central disc 78 is rotated relative to the central shaft 20 through the action of external forces acting against the retaining springs 86 . the external forces are either caused by the brake shoe 118 pressing against the bottom surface of the central disc , or a centrifugal arm 174 activated by centrifugal force to move against the retaining springs 86 and against the central disc , causing the central disc to rotate . as shown in fig1 , the centrifugal arm 174 is held in a fixed position against a stop 176 , until rotor speed increases to a predetermined value , by a spring 178 that is attached to one of the struts 28 . centrifugal acceleration pivots the centrifugal arm 174 about a hinge point 182 . once the centrifugal arm commences pivoting , the moment arm between the center of gravity of the centrifugal arm and the hinge point increases and the moment arm between the spring line of tension and the hinge line decreases , resulting in a rapidly increasing net moment forcing the center gravity of the arm in a direction away from the central disc 78 , as shown in fig2 . a cable 184 is attached to the centrifugal arm 174 and the central disc 78 . the cable is slack when the centrifugal arm is in the unactivated position and becomes tense when the centrifugal arm can produce a high actuation torque and consequent high tension in the cable . the cable rotates the central disc relative to the central shaft 20 , which in turn activates the camcable braking mechanism 60 to rotate the airfoils 30 and slow rotor speed . rotor speed is thus governed to remain within a structurally safe range . speed governing occurs if the brake system fails , or in the case where the central shaft rotates at variable speed under loads , as is the case with most applications except with line excited ac generators . according to another aspect of the present invention , the slow , high torque motion of the central shaft 20 is transformed to a high speed , low torque motion by a shaft - mounted gear transmission 188 and two v - belts 190 and 192 that form a transmission link with the transmission sheave 194 ( shown in fig2 and 23 ). generator shaft speeds are many times higher than the central shaft speed of the transmission sheave 194 . the v - belts provide a transmission link that reduces shock loads on the transmission . the pitch diameter ratios of the v - belt transmission link can be easily changed to allow for a range of rotor speeds given a constant speed induction generator . the transmission 188 and v - belts 190 and 192 are adapted to provide low belt tension at low or no torque and high belt tension at high torque . the transmission is mounted on the central shaft 20 and would rotate freely if it were not restrained by an external frame 196 through transmission frames 198 and 199 , bolts 200 and 201 and retention spring 202 . the transmission is held against low torque stops 204 and 206 , as shown in fig2 , by spring retention and adjusted to maintain a predetermined tension in the v - belts 190 and 192 such that under high shock generator starting torques , the v - belts can slip and provide a shock buffer between the transmission and a main generator 208 . under high torque input , the transmission pivots the transmission frames 198 and 199 and bolts 200 and 201 away from the low torque stops to contact high torque stops 210 and 212 shown in fig2 , adjusting v - belt tension to allow the belt 190 to slip if a predetermined torque is exceeded . belt slippage causes the rotation of the central shaft 20 to speed up , which activates the cam - cable braking mechanism 60 and slows the rotation of the central shaft 20 , thereby stopping the windmill 10 in high winds . an induction motor / generator 218 is mounted on the opposite side of the transmission 188 from the main generator 208 , as shown in fig2 and 23 . the small v - belt 192 , which has a very high pitch diameter ratio , links the motor / generator to the transmission so the rotor speed is about one - fourth of its operational speed when the motor / generator is functioning and the main generator is turned off . the motor / generator is turned on , unless the main generator has been turned on . consequently , the motor / generator operates when wind speeds are below the main generator threshold . when the motor / generator 218 is turned on and driving the sheave 194 and thus the central shaft 20 ( which occurs at wind speeds below 5 m . p . h . ), tension in the v - belt 192 between the motor / generator and the transmission is highest . the transmission rests against the low torque stops 204 and 206 , under the influence of the retention spring 202 . when wind speed rises , the motor / generator becomes a generator and torque flowing through the transmission causes the transmission to push against the retention spring 202 . when this torque reaches a predetermined value , the retention spring force is overcome and the transmission pivots the bolts 200 and 201 away from the low torque stops 204 and 206 and toward the high torque stops 210 and 212 , loosening the v - belt 192 until , at some point , that belt begins to slip , causing the sheave 194 to accelerate . the sheave 194 gradually accelerates to the main generator 208 speed . at that time , an electronic tachometer switch 222 turns the main generator 208 on and concurrently turns the motor / generator 218 off . the main generator operates until the rotor torque goes to zero , or is negative . the electronic tachometer switch then shuts the main generator off and the motor / generator on . the motor / generator decelerates the central shaft 20 to the motor / generator speed , which is about one - fourth of the main generator speed . this motor / generator arrangement simplifies the electric controls on the windmill 10 , allows for energy production in low winds , prevents stalls , improves cold climate operation because the motor / generator 218 keeps the bearings 24 and gearbox 22 moving , and results in greater net energy output . the motor / generator also provides good public relations , because viewers generally do not understand that if the wind is too low , the central shaft 20 cannot turn , and so the motor / generator drives the airfoils 30 for demonstrative purposes . furthermore , because of the &# 34 ; soft &# 34 ; coupling between the main generator and the gearbox , gearbox life is increased significantly and high wind braking may be made smoother . from the foregoing , it will be appreciated that the present windmill includes numerous safety and structural features . the windmill meets construction codes with significantly less structural strength , utilizes a drop in aerodynamic lift to slow or stop rotation of the airfoils , and includes a safety centrifugal braking mechanism and a motor / generator to drive the windmill and produce power in low winds . while several particular forms of the invention have been illustrated and described , various modifications can be made without departing from the spirit and scope of the invention .