US 3918839 A
A wind turbine rotatable about a shaft may include a drive rotor with one or more elongated blades each having a central outwardly curved portion of airfoil shape which produces rotary motion when the blade rotates in wind at a blade tip velocity to wind velocity ratio greater than about three or four, additional wind rotor means disposed at both ends of the curved portions of the elongated blade for rotatably accelerating the drive rotor to the desired velocity ratio, and means coupled to said rotors for utilizing the rotation thereof.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Blackwell et al.
1 Nov. 11, 1975 WIND TURBINE  Inventors: Bennie F. Blackwell; Louis V. Feltz;
Randall C. Maydew, all of Albuquerque, N. Mex.
 Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC.
 Filed: Sept. 20, 1974  Appl. No.: 508,016
 US. Cl. 416/175; 416/197; 416/203;
 Int. Cl. F031) 3/02  Field of Search 416/132. 175, 197, 203, 416/227, 240,110,1l1,1l9
 References Cited UNlTED STATES PATENTS 1.100.332 6/1914 Smith 416/175 1.697.574 l/1929 Savonius 416/110 1,744,924 l/l930 Sargent 416/175 1.835.018 12/1931 Darrieus 415/212 2.020.900 11/1935 Methvin 416/227 FORElGN PATENTS OR APPLICATIONS 961.999 5/1950 France 416/197 1.021.619 2/1953 France 416/197 39.680 3/1957 Poland 416/175 Primary Exunliner-Everette A. Powell, Jr. Attorney, Agent. or FirmDean E. Carlson; Dudley W. King; Richard E. Constant 57 ABSTRACT A wind turbine rotatable about a shaft may include a drive rotor with one or more elongated blades each having a central outwardly curved portion of airfoil shape which produces rotary motion when the blade rotates in wind at a blade tip velocity to wind velocity ratio greater than about three or four, additional wind rotor means disposed at both ends of the curved portions of the elongated blade for rotatably accelerating the drive rotor to the desired velocity ratio, and means coupled to said rotors for utilizing the rotation thereof.
16 Claims, 14 Drawing Figures UTILIZATION MEANS US. Patent Nov. 11, 1975 Shset 1 of 3 US. Patent Nov. 11, 1975 Sheet 3 of3 3,918,839
"F: 5 62c I m A e 56 FIG. IOb
WIND TURBINE BACKGROUND OF INVENTION Wind was one of the first natural energy sources to be harnessed by man with the use of various windmill driven apparatus. The use of windmills, however, declined drastically after the development of the steam engine, internal combustion engine, and other fossil fueled energy conversion machines. Recently, with the increasing cost of fossil and other presently widely used energy sources, interest is again being directed to the use of wind as a competitive source of energy.
For example, it has been estimated that greater than about kilowatt hours of electricity could be produced from practical wind power sites in the Unites States alone, the energy available being proportioned to the air density and wind speed, the latter affecting energy by the third power. Since the amount of energy available in wind may be significant when compared to the energy needs of the world, such wind driven power sources may become of increasing importance, especially if the location at which the energy is required is remote or where alternate energy sources require high cost fuel to produce power.
Various wind driven machines or turbines have been proposed or utilized, such as the well known horizontal axis windmills. These windmills have used various designs and arrangements of rotors which 'have achieved rotor tip velocity to wind velocity ratios of as great as 6 to l. However, because of the inherent limitations of such horizontal axis windmills which require the rotor to be aligned in a particular direction with respect to the wind direction (which of course is not constant) these windmills often include complex windmill rotation drive mechanisms to maintain the proper windmill rotor attitude or direction with respect to the wind direction. These mechanisms, besides being complex, generally must be attached to the windmill adjacent to the axis of the rotor and are thus supported well above ground level, at least as high as the radius of the rotor. This also adds to the complexity, cost and weight of the supporting towers and other previously referred to mechanisms of the entire windmill system.
Vertical axis wind turbines have been proposed and tested to overcome some of these shortcomings. Most vertical axis wind turbines, however, have very low rotor tip velocity to wind velocity ratios and are thus very inefficient or require an additional power source to accelerate the rotor to a velocity at which the rotor can produce positive torque. In addition, some prior vertical axis wind turbines have utilized rather complex and expensive rotor blade designs or have been of relatively low strength for practical applications. Even though vertical axis wind turbines are often capable of operating from a wind coming from any direction and with power generating equipment and tower structure which may be of relatively simple construction; vertical axis wind turbines have not been developed or widely used.
SUMMARY OF THE INVENTION In view of the above, it is an object of this invention to provide a relatively simple and low cost wind turbine arrangement. e
It is a further object of this invention to provide a vertical axis wind turbine which is self-starting and which is capable of providing a relatively high blade tip velocity to wind velocity ratio.
It is a still further object of this invention to provide a vertical axis wind turbine having a novel rotor blade configuration.
7 It is a further object of this invention to provide a high efficiency vertical axis wind turbine system.
Various other objects and advantages will appear from the following description of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims. It will be understood that various changes in the details, materials and arrangements of the parts, which are herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principles and scope of this invention.
This invention relates to a wind turbine having a driverotor including an outwardly curved, airfoil shaped blade extending between end portions of amtatable shaft; and additional rotor means disposed at both end portions of the shaft and out of registry with torque producing portions of the drive rotor to bring the entire rotor assembly up to a speed at which the drive rotor may maintain a rotary driving force to the shaft and which thereafter continues to contribute a driving force thereto at the higher speeds.
DESCRIPTION or DRAWINGS The invention is illustrated in the accompanying drawing wherein;
FIG. 1 is a somewhat simplified perspective view of the wind turbine assembly of this invention showing the relative positions of the rotor elements;
' FIG. 2 shows diagrammatically the preferred shape of the blades in the main drive rotor of the wind turbine;
FIG. 3 shows diagrammatically a comparison of the blade shape of this invention with other possible blade curvatures;
FIG. 4 is a cross-sectional view of the airfoil portion of the blade shown in FIG. 2;
FIG. 5 is a graph of efficiency versus velocity ratios for the respective rotor portions of the present wind turbine; I I
FIG. 6a and FIG. 6b illustrate by cross section various shapes that the straight segments may have 'for the blades shown in FIGS. 1 and 2;
FIG. 7 illustrates diagrammatically in a top section view the positions of the vanes of the starter rotor utilized in the wind turbine assembly of FIG. 1; 7
FIG. 8 is a perspective view of another starter rotor arrangement which may be used with the turbine of FIG. .1;
FIG. 9 shows diagrammatically a modification driverotor blade and blade shape;
FIGS. 10a and 10b illustrate further modifications of the drive rotor blade to effectively increase the aspect ratio of the rotor blade;
FIG. 11 illustrates a mofified version of the wind turto the him which utilizes a vertical stacking of drive rotors;
' and DETAILED DESCRIPTION The wind turbine of this invention includes a wind driven main power or drive rotor and a pair of wind driven starter rotors 14 and 16 coupled to a rotatable shaft 12, as indicated in FIG. 1. It is preferred that the wind tubine be supported in a vertical position as shown, so that any wind, regardless of direction, will always cause rotation of the wind turbine rotors without adjustment of the turbine axis. Each of the rotors l0, l4 and 16 are fixed to the shaft 12 so as to rotate together about fixed platform or tower 18 with the shaft 12 maintained in the desired vertical position. Shaft 12 may be rotatably mounted on platform 18 by appropriate rotary bearings, and the like and may be stabilized by appropriate guys or other supports 19 from upper portions of the shaft, if such is desirable, depending on the size of the wind turbine and the wind velocities in which it is to be operated. In addition, shaft 12, and consequently rotors 10, 14 and 16, may be coupled directly or via an appropriate drive system, such as represented by gears 20 and 22, to a suitable utilization means 24 which may convert or otherwise utilize the energy produced by the rotation of shaft 12. Utilization means 24 may be an appropriate apparatus or mechanism which may convert the rotary motion of the wind turbine into electricity or some other form of energy, for example an alternator or generator, or which may provide some other operation or function, for example pumping of a fluid from a well or operating another apparatus or mechanism.
The main drive or power rotor 10 may include one or more generally vertically disposed elongated blades, such as the three blades 26a, 26b and 260 shown, which are fastened or coupled to shaft 12 at their extremities through an appropriate collar or other support. The blade or blades may be positioned around shaft 12 so as to balance each other or may be provided with appropriate counterweights, or the like to provide this balance. Each blade, as indicated by blade 26a, may include a central outwardly curved, arcuate portion 28 connected through straight segment 30 to an upper portion of shaft 12 and through another straight segment 32 to a lower portion of shaft 12. More or less blades than the three shown may be utilized in rotor 10 but with some lowering of efficiency and/or inordinate increase in cost, the efficiency of drive rotor 10 being a function of the ratio of blade area to the blade swept area. The shaft 12 may be a single solid or hollow rod, concentric rods rotatable with respect to each other, or a lattice or truss-like structure, depending on its proposed strength and size and the apparatus used to support the same.
It has been found that if a perfectly flexible cable of uniform density and cross section is attached at its ends to two points on a vertical axis and is then spun at a constant angular velocity about the vertical axis, the cable will assume the curvature indicated by the dotted line 34 shown in FIG., 2 referred to hereinafter as a troposkien shape, regardless of the angular velocity. When the cable assumes this shape and is rotated about the vertical axis, the stresses produced in the cable are essentially tensile stresses. It has further been found that for purposes of this invention, the troposkien shape can be approximated by a circular arc 34a, at the outermost portion of the troposkien shape, and a pair of straight segments 34b and 340 coupled between the ends of the circular are 340 and the rotation axis. With this approximation, the cable is still subjected to essentially tensile stresses with only negligible bending stresses. This approximation is utilized as the desired shape for the power rotor 10 blades illustrated in FIG. 1.
FIG. 3 illustrates the differences between a troposkien-shaped curve 34 and that of a circular are 36 and a catenary-shaped curve 38. The catenary-shaped curve 38 approximates the shape assumed by a perfectly flexible cable of uniform density and cross section hanging freely from two fixed points. A rotated blade having either of the shapes 36 or 38 will produce greater bend ing stresses than the shape 34 or its approximation. As described above, the troposkien-shaped curve 34 minimizes the bending stresses produced in the vertical blade when subjected to rotary motion, while the approximation of a troposkien shape, as illustrated by the circular arc segment 34a and the straight segment sections 34b and 340 in FIG. 2 and the corresponding curved portion 28 and straight segments 30 and 32 of the blade 26a in FIG. 1, provide minimized bending stresses while insuring a blade configuration which may be manufactured with a relatively simple shape at relatively low cost. The blade configuration shown may be selected to provide a close approximation of the troposkien shape to minimize bending stresses by minimizing the maximum separation distance between curve 34 and the approximation segments 34a, 34b and 340, or by otherwise adjusting the approximation shape. In addition, since the rotors l4 and 16 are located in a position where they may normally interfere with an air stream or wind directed against the blades of rotor 10 at the upper and lower extremities thereof, the straight segments 30 and 32 of the rotor 10 blades can be formed as structural members with little or no aerodynamic lift or torque producing effects. Further, since the torque or rotary force produced by the blades of rotor 10 increases as the blade distance from the rotation axis increases, the use of the curved portion 28 as the principal or only drive section makes more effective use of wind energy as other portions of the blade, namely the straight segments, inherently produce lower torque levels from equal wind energy.
The curved portion 28 of the blades 26a, 26b and 26c are provided with an airfoil shape or cross section transverse to the blade curvature and facing the direction of rotation of rotor 10 so as to provide a lift force when the rotor 10 turns in a wind. A typical cross section is shown in FIG. 4 which is selected to provide an optimum Lift-to-Drag ratio, thus increasing power producing performance.
Because of the nature of rotor 10 and the circular movement of the blades, each blade curved airfoil section 28 will experience both positive and negative angles of attack during a revolution so that there is no apparent advantage in using a nonsymmetrieal airfoil. In addition, the lift for airfoils increases with increasing angle of attack up to the point where the flow separates from the airfoil, which condition may cause a stall and is generally to be avoided, the maximum lift being higher for increasing aspect ratios (the ratio of airfoil length to airfoil chord length). However, with the rotor 10, the wind felt on curved portion 28 is not simply the absolute wind speed or velocity but rather the absolute wind velocity minus vectorially the absolute blade velocity. Also, in a rotating airfoil, the angle of attack is the angle between the relative wind speed (that is the apparent wind direction) and the chord line of the airfoil blade, the angle of attack being dependent on wind velocity, rotational blade velocity and the blade position with respect to the turbine. For a given blade position, the angle of attack decreases with increasing blade velocity to wind velocity ratio. Therefore, for a sufficiently high ratio, the airfoil may never stall during a revolution while at low ratios the airfoil may be stalled over an appreciable portion of the blade revolution. At high ratios, the angle of attack decreases consequently decreasing the chord-wise component of lift. There is thus maximum rotor efficiency at some tip velocity (linear velocity of blade or vane at its maximum diameter) to wind velocity ratio as indicated by curve 40 in FIG. 5, as determined by analytical studies and wind-tunnel testing. It has been found that the most efficient velocity ratios for the rotor of this invention to produce maximum power is from about 5 to 7, typically with a maximum at about 6.
A symmetical airfoil shape which has a large lift-todrag ratio may be the NACA 0012 airfoil (National Advisory Committee for Aeronautics). Such an airfoil or similar airfoil may be formed, as shown in FIG. 4, with a high-strength backbone or tensile stress element 42 surrounded by a rigid foam core 44. The stress element 42 may be a steel, aluminum or fiber composite leaf or strap which is roll or otherwise formed in the desired arcuate curvature shown in FIG. 2 by curve 34a so as to act as the supporting element for the curved portion 28 and as the strength member to withstand the tensile forces produced in the blade from the rotation of rotor 10. The rigid foam core 44 may be formed from light-weight polyurethane or the like foam bodies, as described below. Suitable fasteners or attachments, such as hinges or pins (not shown), may be affixed to the ends of element 42 at this time for convenience in connecting the curved portion 28 of the blade to the straight segments 30 and 32. The rigid core 44 may be shaped in the desired airfoil configuration and suitably adhered to the stress element 42, such as by forming the core 44 by machining or the like two separate rigid foam body halves from suitable foam blades into the desired complementary shapes or sections 44a and 44b and then attaching the sections on either side of the curved stress element 42. The outer surface of the core 44 may then be appropriately coated, such as with a fiberglass resin skin 46 in either mat, cloth or sprayed form, to provide a smooth and erosion resistant surface around the core 44 which will protect the same from impacts by objects carried by the wind and from rain, hail, or the like. The skin 46 may be smoothed and polished and further coated to minimize friction and other aerodynamic losses and to provide the desired final shaping and balancing of the airfoil.
The straight segments 30 and 32 of the blades 26a, 26b and 26c may be formed of any convenient shape which provides minimal wind resistance and which has sufficient tensile strength to support the curved portion 28 under maximum stress conditions and are attached in appropriate manner to the fasteners connected to curved portion 28. For example, the straight segments may be formed with an airfoil shape to aid in providing a drive force or to minimize drag resistance to rotor 10 by bending a sheet into an airfoil shape and welding the trailing edges of the sheet, as shown by the straight segment cross section 50a in FIG. 6a. However, since the straight segments may contribute very little drive force due to their position with respect to the rotors l4 and 16 and with respect to shaft 12, economy may dictate the use ofa simple circular hollow or solid rod or other shape as indicated by the cross section 50b in FIG. 6b. The straight segments are generally made of rigid materials to support the blades when the turbine is at rest and may include suitable supports (not shown) from shaft 12 to aid in this support. There may also be applications where it would be desirable to form the segments 30 and 32 out of a flexible material, such as a steel cable, which would assume the troposkien shape upon rotation of the turbine. In these arrangements, some other support of the airfoil portions may have to be provided, as needed, when the turbine is at rest.
As illustrated by the curve 40 in FIG. 5, rotor 10 must be driven to a blade tip speed to wind velocity ratio of about 3 before the rotor 10 blades begin to exert or provide a significant driving force sufficient to offset drag, inertia, and other losses and to accelerate the turbine to peak operating levels. In order to achieve this velocity, starter rotors l4 and 16 are appropriately supported at upper and lower portions of rotor 10 coupled to the common-shaft l2 and out of registry with curved portions 28 of the drive rotor 10. A particularly effective starter rotor is illustrated in FIG. 7 in which a pair of arcuate or simicircular shaped rectangular vanes 52 and 54 are supported on shaft 12 with hollowed portions facing in opposite directions with a portion of each vane overlapping the shaft 12 and the other vane in a generally S-shape fashion. With the vanes so positioned, wind directed against the hollow portion or chamber on the inside of one of the vanes, such as the portion 56 of vane 52, will apply a driving force against vane 52 in the direction of the arrow 58 and will be directed through the channel 60 between vane 52 and shaft 12 against the hollow portion of vane 54, again producing a driving force in the direction of arrow 58. Such a rotor exhibits an efficiency to rotational velocity ratio characteristic as indicated by the curve 62 in FIG. 5 showing that the peak performance of the rotor shown in FIG. 7 occurs at a ratio of approximately one. The ratio of the diameter of rotor 10 to rotors l4 and 16 should thus be sized to be from about 5 to 6 to 1, so that both the starting and drive rotors are operating at their peak performance at about the same rotational velocities. It has also been found that the starter rotors l4 and 16 may be provided with a height which is approximately the same as their diameter to minimize blocking of the most effective portion, that is the curved portion 28 of rotor 10 as indicated in FIG. 1, or they may extend from said curved portion 28 to beyond the ends of the drive rotor 10 blades. The vanes 52 and 54 of the starter rotors may be made in the form shown or with varible thickness in an airfoil shape to provide increased efficiency. For purpose of economy, and since the additional aerodynamic performance may not be significantly greater to warrant the additional fabrication costs, the vanes 52 and 54 are preferably formed from sheet metal with the vane chamber or hollow portion forming a segment of an are having constant radius. The vanes of the upper starter rotor 14 should be positioned, as shown in FIG. 2, so as to be out of phase with the vanesof the lower starter rotor 16, that is, interdigitated or perpendicular one with respect to the other, so that the wind turbine is self-starting from wind coming from any direction and so as to smooth out the starting torque produced by the starting rotors. Other types of starter rotors. such as certain drag-type rotors may be utilized but with lower over all efficiencies and drive power. such as the type shown in FIG. 8 utilizing three buckets 62a, 62b and 620 appropriately connected to shaft 12.
The respective rotors l0, l4 and 16 connected to the common shaft 12 may be rotated in a wind to a velocity of from 3 to 4 times that of the wind by the proper proportioning of the size and radius of the starter rotors and the power rotor, as described above. The starter rotors will self-start without any external application of power (other than wind) and will automatically regulate the correct airfoil starting velocity as a function of any wind velocity within the operating range and limitations of the turbine. The starter rotor may continue to produce driving power even at the operating velocity of the power rotor without degrading the latter operation. With the blade design described above, the forces produced in the blade are substantially tensile in nature and readily absorbed by the system. The utilization means 24 may then be operated to provide whatever power, energy or operation desired from the rotation of the wind turbine in a highly efficient, simple and low cost system.
If it is desired to provide increased driving torque but with somewhat higher tensile stresses, the blades of rotor may be modified by positioning appropriate mass or weight members at the junction between the straight segments and the curved portion of the blade, such as shown by weight members 64 and 66 in FIG. 9. These masses will tend to straighten out and change the arc of the curved portion of the blades of the previous troposkien description into a new are shape or curved portion 28a which increases the swept area of the rotor 10 blades. In other words, the airfoil portion of the blades are more vertical and thus provide a greater average radius from the rotor shaft to the drive portion of the power blade and a greater area of blade sweep. Since the blade curved portion is still in the form of an ace, the stresses within the curved portion will still be tensile but may require a higher strength joint or junction between the curve portion 28a and the straight segments of the blade.
The blades of rotor 10 may be further modified by installing tip plates of larger dimension than the blade cross section at the junction between the curved portion 28 and the straight segments 30 and 32 of the power blades of rotor 10. These tip plates are most effective when the angles of attack are high to increase the effective aspect ratio (ratio of blade length to blade chord length) of the blade airfoil by preventing the higher pressure air inside the airfoil from spilling around the end of the airfoil into the low pressure side. The tip plates may be installed perpendicular to the blade as shown in FIG. 10a by tip 68a or perpendicular to the vertical axis or shaft 12 of the turbine as indicated by tip 68b in FIG. 10b. In the latter configuration, the tip plate 68b would minimize interference with the air flow over the blade itself and would not have to rotate against the air stream at the rotational velocity or rotor 10.
Since the fabrication cost of a wind turbine of the type described above may increase substantially as the size of the wind turbine is increased and since wind velocities often increase with distance above ground level, it may be desirable to stack wind turbines one above the other on a common shaft 72, as indicated in FIG. II by turbines a and 70h. Because of this increase in wind velocity with height, it may also be desirable that the upper wind turbine 70b be provided with a diameter greater than lower turbines to provide a more efficient utilization of the wind energy. The turbines 70a and 70b (and additional stacked turbines) and their common shaft 72 may be appropriately supported at the ground and with suitable guy and collar arrangements 74a and 74b at intermediate and upper positions of the turbines. The turbines can thus be positioned so as to occupy a limited area of ground without any wind interference between turbines. It will be un derstood that these turbines may be provided with one or more similar starter rotors as described above.
In order to protect the wind turbines of this invention from excessive winds, the turbines may be provided with demountable or foldable junctions or fasteners at the connection between the curved portions and straight segments of the blades and between the blades and shaft 12 so that the blades may be folded or collapsed to a much smaller diameter which will have significantly lower wind resistance and which may be suitably covered, if desired. For example, if the blades of rotor 10 are provided, as shown in FIG. 12 with a hinge-like connector between each of the upper straight blade segments 30' and 30" and curved portions 28' and 28" and between the lower straight segments 32 and 32" and the vertical shaft 12, the lower straight segments are demountable from the curved portion, the lower straight segments may be detached from the curved portion of the blade and folded against the shaft while the upper straight segment and curved portion are pivoted against the shaft and appropriately fastened or strapped thereto. As can be seen, the wind profile of the turbine is thus drastically reduced.
What is claimed is:
l. A wind turbine comprising a rotatable shaft; a drive rotor having an elongated blade with a central curved portion of airfoil shape transverse to said curvature, and means for supporting said blade on said shaft with said airfoil shape directed along the path of movement of said blade for exerting significant driving force on said shaft when said curved blade portion attains a linear velocity to wind velocity ratio greater than about three; starter rotor means disposed on said shaft having vanes out of registry with the curved portion of said drive rotor for rotatably accelerating said shaft to said velocity ratio; and means coupled to said shaft for utilizing the rotation of said shaft.
2. The turbine of claim 1 wherein said drive rotor includes a plurality of said blades, each having a central outwardly curved portion of airfoil shape.
3. The turbine of claim 2 wherein said outwardly curved portions of said blades are arcuately shaped approximating a portion of a troposkien shape.
4. The turbine of claim 1 wherein said blade supporting means includes substantially straight blade segments connecting and supporting between them said curved portion of airfoil shape.
5. The turbine of claim 4 including means for separating the ends of said curved portion of said blade from said blade segments.
6. The turbine of claim 4 including tip plate spoilers at each end of said curved portion of said blade.
7. The turbine of claim 6 wherein said tip plate spoilers are positioned generally perpendicular to said shaft.
8. The turbine of claim 4 including weight members positioned at each end of said curved portion of said blade.
9. The turbine of claim 4 wherein said curved portion of said blade includes a high strength. elongated strap disposed at the center of said curved portion, a foam core disposed about said strap in said airfoil shape, and an outer substantially impervious skin thereover.
10. The turbine of claim 9 wherein said strap is bent into an arcuate shape, said foam comprises inner and outer airfoil shape segments adhered to each other and to said strap on both sides of said strap, and the outer surface of said foam segments is coated with said impervious skin.
11. The turbine of claim 1 wherein said velocity ratio is from about to 7.
12. The turbine of claim 1 wherein said shaft is generally vertical and said starter rotor means include a first self-starting rotor disposed above said curved portion and a second self-starting rotor disposed below said curved portion; each of said self-starting rotors including a plurality of hollow-shaped vanes facing in oppositc directions with respect to each other, and means for supporting said vanes on said shaft partially overlapping each other in a generally S-shaped fashion for directing wind caught by the hollow portion of one vane into the hollow portion of at least another vane in each rotor.
13. The turbine of claim 12 wherein the outer radii of said self-starting rotors are less than the outer radius of said drive rotor.
14. The turbine of claim 13 wherein the ratio of said radii is from about 5 to 6 to l and aid in rotation of said turbine at and above said velocity ratio.
15. The turbine of claim 12 wherein said vanes of said self-starting rotors are interdigitated with respect to each other.
16. The turbine of claim 1 including a plurality of said drive rotors supported one above the other on said shaft, each succeding drive rotor having a diameter greater than the next adjacent lower rotor.