CA1042347A

CA1042347A - Wind turbine

Info

Publication number: CA1042347A
Application number: CA230,279A
Authority: CA
Inventors: Bennie F. Blackwell; Louis V. Feltz; Randall C. Maydew
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1974-09-20
Filing date: 1975-06-26
Publication date: 1978-11-14
Also published as: NL7508723A; FR2285527A1; IT1049691B; ES439834A1; DE2540757A1; US3918839A; JPS5166951A; FR2285527B3; AU8502675A; BE833581A; NO753023L; SE7509005L

Abstract

ABSTRACT OF DISCLOSURE
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 or curved blade 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 or curved blade of the elongated blade for rotatably accelerating the drive rotor to the desired velocity ratio, ant means coupled to said rotors for utilizing the rotation thereof.

Description

~04Z347 BACKGROU2~D OF INVENTlON
Wind was one of the fir~t natural energy sources to be harnessed by man with the use of variou~ windmlll driven apparatus. The use of wlndmllls, however, declined drastlcally after the development of the ~team englne, internal combustion engine, and other 08811 fueled energy conversion machine~. Recently, with the increaslng cost of fossll 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 1012 kilowatt hours of electricity could ::~
be produced from practlcal wind po~7er sites in the - - Unlted States alone, the energy available being proportional ~;~ to the air density and wind speed, the latter affecting ~ ., .
energy-by the third power. Since the amount of energy ~` available in wlnd m~y be slgnlficant when compared to the energy needs of the world, such wlnd driven power sources may become of lncr~sing importance, expeclally ~ ~ .
if the location at which the energy is required is remote or where alternate energy sources require hlgh cost fuel to produce power.
Various wind driven machines or turblnes have been proposed or utllized, such as the well known horlzontal - axia windmlll~. Th-ae wlndmlll~ hav~ w-d various design~ and arrangements of rotora whlch h ve achleved lQ4Z347 rotor tip velocity to w~nd velocity ratios of as 8reat as 6 to 1. However, becau~e of the inherent limitation~
of such horizontal axi~ windmill~ which require the rotor to be aligned in a particuiar direction with respect to the wlnd direction (which of course is not canstant) these windmills often include complex windmill rotation drive mechanisms to m~intain 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 ad~acent to the axis of the rotor and are thus supported well above ground level, at least as hi8h a~ the radiu~ of the rotor. This also adds to the complexity, cost and weight of the ~upporting towers and other previously referred to mechanisms of the entlre windmill system.
; ~
Vertic-l axis wind turbines have been proposed and ` ;tested to overcome some of these shortcomings. Most ,, .
vertical axls wind turbines, however, have very low roto~
tip velocity to wind velocity ratios and are thu~ very lnefficient or reqùire 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 dQsigns or have b-en of relatlvely low strength for practical applicatlon~. Even though vertlcal 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.

SUNNARY 0~ INVENTION

In view of the above, it i8 an ob~ect of this invention to provide a relatively simple and low cost wind turbine arrangement.
It is a further ob~ect 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 vestical axis wind turbine having a novel rotor blade configuration.
It is a further ob~ect of this invention to provide a high efficiency vertical axis wind turbine system.
This invention relates to a wind turbine having a drive rotor including an outwardly curved, airfoil shaped blade extending between end portions of a rotatable 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 continueo to contribute a driving force thereto at the higher opoeds.

1~4;~ 7 The invention comprises a wind turbine with rotatable shaft supporting a drive rotor that has an elongated blade with curved blade or central curved portion of airfoil shape transverse to its curvature, and means supporting the blade on the shaft with the airfoil shape directed along the path of movement of the blade for exerting significant driving force on the shaft when the curved blade portion attains a linear velocity to wind velocity ratio greater than about three, together with starter rotor means on the shaft having vanes out of registry with the curved portion or curved blade of the rotor for rotatably accelerating the shaft to the noted velocity ratio. There is coupled to the shaft some means for utilizing the shaft rotation.

DESCRIPTION OF DRAWINGS
~ he 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 ele~ents;
Fig. 2 ghows diagrammatically the preferred shape of the blades in the main drive rotor of the wind turbine;
Fig. 3 shows diagra~matically 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;

~04Z347 Fig. 6a and Fig. 6b illustrate by cros~ section variou~ shapes that the straight ~egment~ may have for the blades shown in Fig~. 1 and 2;
Fig. 7 illustrates diagrammatically in a top sectlon view the positions of the vanes of the starter rot~r .~
~; ueilized in the wind turbine as~embly of Fig. l;

Fig. 8 is a per~pective view of another starter rotor arrangoment which may be used with the turbine of Fig. l;

~i 10 Fig. 9 ~hows diagrammatically a modification to the ~ drlve rotor blade and blade shape;

-, Figs. lOa and lOb illustrate further modifications of the drlve rotor blade to effectively increase the , a~pect ratio of the rotor blade;

Fig. ll illustrates a modified version of the wind ?` ~ ~ ' turbine which utilizes a vertical ~tacking of drive `~ -rotors; and Fig. 12 is a simplified diagrammatic view of an ~1 . .
arrangement of the drive rotor blades in which the blade segments may be folded to reduce tho wind proflle of the turbine.

.
DETAILED DESCRIPTION
The wind turbine of thls invention lncludes a wind driven maln power or drlve rotor 10 and a pair of wind driven starter rotors 14 and 16 coupled to a rotatable shaft 12, a~ indicated in Fig. 1. It is preforred that the 104Z347wind turbine be supported in a vertical posltion as shown, 80 that any wind, regardless of direction, will always cause rotation o~ the wind turbine rotors without ad3ustment of the turbine axis. Each of the rotors 10, 14 and 16 are fixed to the shaft 12 so as to rotate together about fixed platform or tower 18 with the shaft 12 malntained in the desired vertical pos~tlon. Shaft 12 msy 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, depenting ,~ .
on the size of the wind turbine and the wind velocitie~
~in which it i8 to be operated. In addition, shaft 12, and consequently rotor~ 10, 14 and 16, may be coupled dlrectly or via an appropriate drive system, such aæ
- 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 any appropriate apparatus or mechanism which may convert the rotary motion of the wind turbine into electricity orsome other form of energy, for exsmple an alternator or generator, or which may provide some other operatlon or function, for example pumping of a fluid frcm a well or operating anoth-r appsratu~ or m-chanlsm.
The main drive or power rotor 10 may include one or more generally vertically disposed elongated bla~es, ~04Z;~47 such as the three blades 26a, 26b and 26c 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 po~itioned around ghaft 12 so as to balance each other or may be provided with appropriate counter-weights, or the like to provide this balance. Each blade, as indicated by blade 26a, may include a central outwardly curvet, 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, for convenience the portion 28 msy be referred to as the curved blade and the portions 30~ 32 as means for supporting the curved blade. 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 rot, 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 w ed to support the same.
It has beèn 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 veroical axis, the cable will assume the curvature indicated by the dotted , ~ .
line 34 shown in Fig. 2, referred to hereinafter as a troposkien shape, regardle-s of the angular velocity.

:;

When the cable assumes this shape and i8 rotated about the vertlcal axis, the stresses produced in the cable sre essentlally tenslle stresses. It has further been found that for purposos of this lnvention, the tropo~kien ; shape can be approximated by a clrcular src 34a, at the outermost portion of the troposkien shape, and a pair of stralght segments 34b and 34c coupled between the ends of the circular arc 34a and the rotation axis. With this approxlmation, the cable is still sub~ected to essentially 10~ tensile stresses with only negligible bending ~tresses.
TbiJ approximatlon i8 utilized as the desired shape for the power rotor 10 blades illustrated in Fig. 1.
., ~
~ Flg. 3 illustrates the diffèrences between a :
; ; tropo~kien-shaped curve 34 and that of a circular arc 36 and a catenary-shaped curve 38~ The catenary-shaped k.
curve 38 -pproximates the shape assumed by a perfectly flexible cable of uniform den8ity and cross section hanging freely from two fi~ed points. A rotated blade having elth-r of-the shape~ 36 or 38 will produce greater -~ ~ 20bending stresses than the shape 34 or its approximation.
. ~
described above, the troposkien-shaped curve 34 -- minimiz the bending stresses produced in the vertical .
blade when sub~ected to rotary motion, while the approxima-tion of a tropo~kien ahape, as illu~trated by the circular arc ~-gment 34a nd th- tra$8ht ~egment sectlons 34b and 34c in Flg. 2 and the correspondlng curved portion 28 and straight segments 30 and 32 of the blade 26a in _ g _ ; i04Z347 - Fig 1, provide minimized bendlng stresses while insuring a blsde configuratlon which may be manufactured with a relatively simpl- shape at relatively low cost The blade configuration shown may be selected to provide a close; approximQtion of the troposkien shape to minimize ~ bending`~tresses by minimizing the maximum separation .. . ~. . .
~` di-t nce'beeween curve 34 and the pproximatlon segments : 34a,'34b and 34C? or by otherwlse ad3usting thë approximation hap~ In additlon, since th~ rotors 14 and 16 are ~ located ln po-ition where they may normally interfere with an sir sèream or wind directed against the blades of rotor I0 t the upper nd lo~er tr_ities th-reof the trsight segm~nts 30 and 32 of the rotor lO'blades '' ' c~n be"i'or èd a8 structur 1 members with little or no rod~nu ic lift or touque producing effects Further, Inc- the torque or rotary force produced by the blades of rotor 10 increà~-~ a~ the blade distance from the rot tion a is -ncr a-es, the U8- of tho curved portion 28 as th- principal or only drive section m~kes more eff tiv- u~e of wind nergy as other portions of the blade~ o ely the ~traight s-gmcnts, inherently produce .
- lower torque levels fro~ equal wind energy ~- Th- curved portion 28 of th- blad-s 26a, 26b and ~:
'~ 26c r provid-d with an irfoil ~hap or cros~ 8 ction ~' tranfv-r~- to th- blad- curv tur and facing tha dlr ction of rot-tion of rotor 10 ~o ~ to provide li~t , .

104Z;~47 force when the rotor 10 turns in a wind. A typical cross section is shown in Fig. 4 which i8 selected to provide an optimum Lift-to-Drag ratio, thus increasing power producing performance.
Because of the nature of rotor 10 and the circular movemont of the blades, each blade curved airfoil section 28 will experience both positive and negative angles of attack during a revolution 60 that there i~ no apparent atvantage in ~sing a nonsymmetrical airfoil. In addition, the lift for airfoil~ increases with increasing angle of attàck up to the point where the flow separates from the airfoil, which condition may cau~e a stall and is gen rally to be avoided, the maximum lift being higher for increa~ing ~ spect 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 ab~olute wind ~peed or velocity but rather the absolute wind velocity minus vectorially the absolute blade velocity. Also, in a rotating airfoil, the angle of attack i~ the angle between the relative wind speed (that is the apparont 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 re~pect to the turbine. For a given blade position, the angle of attack decrea~es with increasing blade velocity to wind velocity ratio. Therofore, for a sufficiently high ratio, the airfoil may never stall during a revolution while at low ratios the airfoil may be stalled ovcr an àppreciable portion of the blade revolution. At high ratios, the angle of attack decrease~ 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 detenmined by analytical studies and wind-tunnel testing. It has been found that the most efficient velocity ratios for ; ~ - the rotor 10 of this invention to produce maximum power ~ is from about 5 to 7, typically with a msximum at about 6.
"`'`J` A symmetrical airfoil shape which has a large lift-- to-drag ratio msy be the NACA 0012 airfoil (National Advisory Committee for Aeronautics). Such an airfoil or similar irfoil 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 80 as to act as the supporting : ~ :
element for the curved portion 28 and as the strength member to withstand the tensile force~ produced in the blade from tho rotatlon of rotor 10. The rigld foam core 44 may be formed from lightweight polyurethane or the like foam bodies, as described below. Suitsble ~, --fa~teners or attachments, such as hinges or pin~ (not shown), may be affixed to the ends of element 42 at thi~
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 conflguration and suitably adhered to the stress element 42, such as by forming the core 44 by machining ~r the like two separate ri8id foam body balves from sultable foam blades into the de~ired complementary shapes or -~ 10 ~ections 44a and 44b and then attaching the sections on either side of the curved stress element 42. The outer surf~ce of the core 44 may then be appropriately coated, ' :
~ ~ such as with a fiberglass resin skln 46 in either m~t, : ;
cloth or sprayed form, to provide a s~ooth and erosion resistant surface around the core 44 which will protect ~; ~ the same from impacts by ob~ects carried by the wind nd from rain, hail, or the like. The skin 46 may be ~ .
smoothed and polished and further coated to minimize fricion and other aerodynamic losses and to provide :
~ 20 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 undsr ; maxlmum otress condltions and are attached ln an approprlate manner to the fasteners connected to curved portion 28.

~4Z347 For example, the straight segments may be fonmed with an airfoil shape to aid in providing a drive force or to minimize drag resi6tance to rotor 10 by bending a sheet into an airfo~l shape and welding ~he trailing edge~
of the sheet, a~ shown by the straight segment cross section 50a in Fig. 6a. Howevsr, since the str~ight segments may contribute very little drive force due to thelr position wlth respect to the rotors 14 and 16 and with respect to shaft 12, economy may dictate the use of .
a s~mple circular hollow or solid rod or other sh~pe as indicated by the cross section 50b in Fig. 6b. The straight segments are generally made of rigid msterials 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 turblne. In these arrangoments, 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 - mu~t be driven to a blade tip speed to wind velocity - ratlo of about 3 before the rotor 10 blados bogin to e~ort or provld- a slgniflcant driving forco sufflcient to offset drag, inertia, and othor 108808 and to accelorate ~04Z347 the turb~ne to peak operating lèvels. In order to achleve this velocity, starter rotor~ 14 and 16 are appr~prlately supported at upper and lower portions of rotor 10 coupled to tha common shaft 12 and out of registry with curved portions 28 of the drive rotor 10. A particularly effective starter rotor ie illustrated in Fig. 7 in which a pair of arcuate or semicircular ~haped 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-~hape fashion. With the vanes 80 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 g2 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 e~hibits an efficiency to rotational velocity ratio charscteristic as indicated by the curve 62 in Fig. 5 -~ showing that the peak performsnce of the rotor shown in `f~ Fig. 7 occurs at a ratio of approximately one. The ratio of the diameter of rotor 10 to rotors 14 and 16 should , . . .
-~ thus be sized to be from about 5 to 6 to 1, 80 that both the starting and drlv~ rotors ar~ op-rating at th-ir peak perfor~ance at about th- same rotatlonal velocities.

,- , 1~4Z347 It has also been found that the starter rotors 14 and 16 may be provided with a height which i8 approximately the same as their diameter to minimize blocking of the most effective portion, that ~8 the curved portlon 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 vaneæ 52 and 54 of the starter ... .
rotors may be made in the fonm shown or with variable thlckness in an airfoil shape to provide increased efficiency. For purpose of economy, and since the additional aerodynamic perfonmance may not be significantly greater to warrant the additional fabrication cost~, the vanes 52 and 54 are preferably fonmed from sheet metal with the~vane chamber or hollow portion forming a se~ment of an arc having constant radius. The vanes of the upper starter rotor 14 should be positioned, as shown in Flg. 2, 80 as to be out of phase with the vanes of the lower startor rotor 16, that is, interdigitated or perpendicular one with re~pect to the other, 80 that ., .
-~ 20 the wind turbine is self-starting from wind coming from - any direction and 80 as to ~mooth out the starting torque produced by the starting rotors. Other types of starter ; rotors, such as certain drag-type rotors msy be utilizod but with lower over all effici-nci-s and drive power, such as the type ~hown in Fig. 8 utilizing thre8 bucket8 ;~ 62a, 62b and 62c appropriately connected to shaft 12.

The respect~ve rotors 10, ~4 and 16 connected to the common shaft 12 may be rotated in a wlnd to a velocity ; of from 3 to 4 times that of the wind by the proper proportioning of the size and radiu~ of the starter rotors and the power rotor, as described above. The stsrter 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 0 and limitations of the turbine. The starter rotor may continue to produce driving power even at the operating .
velocity of the power rotor witbout 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 ~- utillzation 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 10 may be modified by positioning appropriate mas~
~- or weight members at the ~unctlon between the straight segments and the curv-d portion of the blade, such as shown by weight mo~bers 64 nd 66 in Flg. 9. These masses will tend to straighten out and change the arc of the curved portion of the blades of the previous troposkien descrlption into a new arc ~hape or curved portion 2&
whlch increases the swept area of the rotor 10 blado~.
In other words, the airfoil portion of the blades are more vertical and thus provide a greater average rad~u~
from the rotor ~haft to the drive portion of the power blade and a greater area of blade ~weep. Sin~e the blade curved portion is still in the form of an arc, the stres-ses within the curved portion will still be tensile but m~y require a higher strength ~oint or iunction between the curve portion 28a and the straight segments of ehe blade.
The blades of rotor 10 may be further modified by installing tip plates of larger dimension than the blade C~88 section at the 3unction be~ween the curved po~tion ., ~ .
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 ~ide.
The tip plates may be installed perpendicular to the blade a~ sho~n in Fig. lOa by tip 68a or perpendicular to th- vertlcsl axls or shaft 12 of the turbine as indlcated by tip 68b in Fig. lOb. In the latter configur-1~4Z347ation, the tip plate 68b wo;uld minlmize interference with the alr flow over the blade itself and would not have to rotate against the air stream at the rotstional velocity of rotor 10.
Since the fabrication cost of a wind turbine of the type described above may increase ~ubstantlally 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 `-10: bove the other on a common shaft 72, as indicated in Fig. 11 by turbines 70a and 70b. Because of this increase in wlnd velocity with height, it may also be desirable that the upper wind turbine 70b be provided ; ;
~ with a diameter greater than lower t~rbines to provide .; :
more efficient ut~lization of the wind energy. The turbines 70a and 70b (and additional stacked turbines) and their common sbaft 72 may be appropriately supported i ~
at the ground and with suitable guy and collar arrange-ments 74a and 74b at intermediate and upper positions of - 20 ~ the turbines. The turbines can thus be positioned 80 `~ - as to occupy a limited area of ground without any wind -1 intererence between turbines. It will be understood -`:that these turbines may be provided with one or more similar starter rotors as doscribed above.
In ord-r to prot-ct tho wind turbines of thi~
invention from excessive winds, tho turbinos may be providod with demount~ble or foldable ~unction~ or fasteners at the connection between the curved portion~ and strsight segments of the blades and between the blades and shaft 12 ~o that the blades may be folded or collap~ed to a much smaller dlameter which will have significantly lower wind reslstance and which may be suitably c~vered, 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 segment~ 30' and 30" and curved portions 28' and 28" and between the ,; .
lower straight ~egments 32' and 32" and the vertical shaft 12', and if the lbwer 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 ~haft while the upper straight segment and curved portion are pivoted against the shaft and :
appropriately fastened or strapped thereto. A~ can be ~` seen, the wind profile of the turbine is thus drastically - reduced.
~, .

':,' -

Claims

What is claimed is:

1 A wind turbine comprising a rotatable shaft; a drive rotor having a curved blade of airfoil shape transverse to said curvature and means for supporting each end of said curved blade in spaced relationship on said shaft at a radial distance from 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 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 blade 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 of similar configuration.

3. The turbine of claim 2 wherein said curved blades are arcuately shaped approximating a portion of a troposkein shape.

4. The turbine of claim 1 wherein said supporting means includes substantially straight segments connecting and supporting between them said curved blade.

5. The turbine of claim 4 including means for separating the ends of said curved blade from said segments.

6. The turbine of claim 4 including tip plate spoilers at each end of said curved 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 blade.

9. The turbine of claim 4 wherein said curved blade includes a high strength, elongated strap disposed at the center thereof, 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 5 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 blade and a second self-starting rotor disposed below said curved blade; each of said self-starting rotors including a plurality of hollow-shaped vanes facing in opposite 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 1 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 succeeding drive rotor having a diameter greater than the next adjacent lower rotor.