WIND TURBINE BLADE HAVING A TENSILE-ONLY STIFFENER FOR PASSIVE CONTROL OF FLAP MOVEMENT

A wind turbine blade, including: an airfoil (50) having a pressure side (12), a suction side (14), and a trailing edge portion (20) deflectable from a trailing edge neutral position, and a tensile-only stiffener (52) secured to the trailing edge portion and having a tension center disposed toward the pressure side of and at a distance (54) from an elastic axis (30) of the airfoil when the airfoil is in an airfoil neutral position.

DETAILED DESCRIPTION OF THE INVENTION

The present inventor has devised a clever and unique arrangement for a trailing edge portion of a wind turbine blade airfoil that passively couples blade bending deformation resulting from wind force with trailing edge deflection to produce a desired aeroelastic effect. The arrangement allows for reduction of aerodynamic forces (reduction of lift) during positive (lee-ward, normal wind direction) load which reduces total airfoil flapwise deformation and fatigue loading. However, unlike the prior art, the arrangement does not also contribute to an increase in unwanted aerodynamic lift during negative (windward) load which tends to increase total airfoil flap-wise deformation and fatigue load amplitude. Specifically, the invention includes a stiffener disposed in the trailing edge portion that is effective in tension only. During positive load a tensile resistance of the tensile-only stiffener is coupled with (added to) an inherent tensile resistance of the trailing edge portion to change a curvature of the airfoil by deflecting the trailing edge portion toward the suction side with respect to the pressure side. This reduces airfoil chamber (i.e. flattens the curvature of the suction side) and that reduces an aerodynamic lift of the airfoil. The curvature of the airfoil is used herein to describe an overall shape of the pressure side and the suction side which can be seen in cross sections of the airfoil. Each cross section may have its own shape and each contributes to the curvature of the airfoil. The trailing edge portion may be a non-discrete end trailing end of the airfoil, or alternately it may be a discrete flap. During negative load the tensile-only stiffener offers no or negligible resistance to compression and hence does not contribute to the inherent compression resistance of the trailing edge portion. Thus, the stiffener has little or no effect on the trailing edge portion during negative load. This selective contribution of the tensile-only stiffener results in an airfoil that gains desirable aeroelastic characteristics without also gaining undesirable aeroelastic characteristics that have previously always accompanied the desirable aeroelastic characteristics.

FIG. 1shows a cross section of a prior art airfoil10having a pressure side12, a suction side14, a leading edge16, a trailing edge18, and a trailing edge portion20. Within a skin22of the airfoil10is a pressure side spar cap24, a suction side spar cap26, and a web28. This cross section is typical of many conventional airfoils in that there is a more convex curvature on the suction side14than on the pressure side12to generate lift through Bernoulli's principle. For a blade having such an airfoil10and spar caps24,26located near the maximum airfoil thickness, the trailing edge18is consequently usually located below (towards the pressure side from) a principle bending axis, also known as an elastic axis30. During positive airfoil flap-wise deformation32a first span-wise portion40of the airfoil10, located on a pressure side12of the elastic axis30, experiences a tensile load, and a second span-wise portion42of the airfoil10, located towards the suction side14from the elastic axis30, experiences a compressive load. The elastic axis remains neutral and serves as a transition between tensile load and the compressive load within the airfoil10. The dynamic is reversed during negative airfoil flap-wise deformation34, such that during negative airfoil flap-wise deformation34the first span-wise portion40experiences compressive load, while the second span-wise portion42experiences tensile load. The elastic axis30exists for each cross section. If each elastic axis lined up perfectly with adjacent elastic axes they may be considered to form an elastic plane (not shown). In the instance when the elastic axes do not line up perfectly they may be considered to form a neutral surface (similar to an elastic “ribbon” or “slice”) of the airfoil10. In case of non-linear elastic behavior of the main blade structure, the location of the neutral (bending-strain free) axis is not constant, but the same principles apply.

It is known that under load conventional blades having airfoils10with the cross section similar to that shown inFIG. 1may react in a predictable manner during positive and negative loading. In particular, the trailing edge portion20of the conventional airfoil10may have an inherent stiffness response during tension and during compression. In the conventional airfoil10the trailing edge portion20is generally that portion toward the trailing edge18and there may be no distinct division between the trailing edge portion20and a remainder44of the airfoil10, which is also a leading portion of the airfoil10. The compressive stiffness response and the tensile stiffness response may or may not be the same as each other. In addition, the stiffness response of the trailing edge portion20may or may not be the same as a remainder44of the airfoil. During a positive airfoil flap-wise deformation32the trailing edge portion20will resist lengthening associated with the tensile loading. In this instance the trailing edge portion20wants to deflect toward the elastic axis30to accommodate. In conventional airfoils, this deflection is suppressed by the remainder of the airfoil. In airfoils with a flexible trailing edge or a hinged flap, this deflection would reduce a curvature of the suction side14and hence reduce aerodynamic lift and associated airfoil deformation. During negative loading34the trailing edge portion20would resist shortening associated with the compressive loading. In this instance the trailing edge portion20would again like to deflect toward the elastic axis30to accommodate, though the deflection is suppressed by the remainder of the airfoil. This would again decrease the curvature of the suction side14, but it would increase a curvature of the pressure side12, and this may reduce lift or create negative aerodynamic lift associated with the negative loading which is undesirable.

FIG. 2shows a cross section of an exemplary embodiment of an airfoil50as disclosed herein. In addition to that shown inFIG. 1there is a tensile-only stiffener52disposed in the trailing edge portion20. The tensile-only stiffener52and its associated tension center (an axis along which tensile forces are modeled) are disposed at a distance54from the elastic axis30when the airfoil10is in a neutral position66, which establishes a neutral position for the trailing edge18. The airfoil neutral position66is a reference position and may be a position that exists when there is no wind load, or alternately a position that exists when there exists a particular operating condition/wind load etc. It may be from this airfoil neutral position66that curvature changes are referenced. The tensile-only stiffener52may operate similar to a rope in that it offers significantly greater resistance to tension than to compression while being light weight and hence minimally contributing to the airfoil mass. An example includes unimpregnated aramid rovings contained in a PTFE conduit, a rubber impregnated aramid rope directly laminated or cast into the trailing edge, or braided rope which may be advantageous if non linear effects are desired. During tension a resistance to lengthening of the tensile-only stiffener52(stiffener effects) would be added to an inherent resistance of the trailing edge portion20. During compression the tensile-only stiffener52would simply buckle, or otherwise not contribute (or contribute negligibly) to an inherent resistance of the trailing edge portion20to shortening. The result is that a combined tensile stiffness response of the trailing edge portion20would be greater with the tensile-only stiffener52than without, while a combined compressive stiffness response of the trailing edge portion would be the same (or negligibly different) with or without the tensile-only stiffener52. While a rope or rope-like component may be used to envision how the tensile-only stiffener52contributes, any structure may be used so long as it results in the same effects described above.

Being so disposed, should the airfoil50undergo a positive airfoil flap-wise deformation32the first span-wise portion40would experience tensile loading and tend to elongate from a base to a tip of the airfoil50. Within the first span-wise portion40, lines that are parallel to the elastic axis30may indicate a constant amount of tensile strain and associated elongation. For example, constant strain line56is a line at the given distance54from the elastic axis30along which the tensile load is constant. Within the remainder44of the first span-wise portion40the line will remain approximately straight during tensile loading. However, due to the tensile stiffness in the trailing edge portion20due to the tensile-only stiffener52disposed therein, the trailing edge portion20will resist tensile expansion. To minimize the amount of elastic energy used to elongate the tensile-only stiffener52, the trailing edge portion20will move toward/deflect (advance toward) the elastic axis30relative to the remainder44of the first span-wise portion40along the constant strain line56. This movement is within the elastic range of the trailing edge portion20if not hinged, and hence the trailing edge portion will not be permanently deformed. Seen from another perspective, after experiencing a positive airfoil flap-wise deformation32, the trailing edge portion20would elongate less than the remainder44of the first span-wise portion40present along the constant strain line56. Since the airfoil50has a flexible or hinged connection between the trailing edge portion20and the remainder44of the airfoil50, the relatively shorter trailing edge portion20would seek the shortest path between its ends, or those points on the airfoil50where the tensile-only stiffener52is secured. Thus, while the remainder44of the first span-wise portion40along the constant strain line56would have a certain amount of arc for a given positive airfoil flap-wise deformation32, the arc of trailing edge portion20would be shorter. This can occur when the trailing edge portion20shifts toward the elastic axis30. This shifting will reduce a curvature of the suction side14and reduce positive lift. The reduction of positive lift will decrease airfoil deformation, and this will reduce the chances that the airfoil50will strike the support tower and will reduce blade fatigue loads.

FIGS. 3-4schematically show the effect described above. InFIG. 3the airfoil50can be seen in the airfoil neutral position66and an angle a between the elastic axis and a tangent of the pressure side at the tensile-only stiffener52is zero. In this position the tensile-only stiffener52is disposed at the distance54from the elastic axis30associated with the airfoil neutral position66and the trailing edge neutral position. As shown inFIG. 4, during a positive airfoil flap-wise deformation32the airfoil deflects leeward and the trailing edge portion20deflects toward the suction side14. The angle a between the elastic axis and a tangent of the pressure side at the tensile-only stiffener52becomes greater than zero and a flattening of the curvature of the suction side14occurs. This, in turn, reduces aerodynamic lift during positive wind conditions that cause positive airfoil flap-wise deformation32and mitigates positive airfoil flap-wise deformation32. As can be seen inFIG. 5, during negative airfoil flap-wise deformation34the airfoil deflects windward but the trailing edge portion20does not deflect toward the suction side as inFIG. 4, and hence the curvature of the airfoil50is not altered. This is so because in this example the stiffener has not added compressive resistance to the trailing edge portion20. Hence, the airfoil50acts as it would were the stiffener not present. In other words, the stiffener does not contribute to the compressive stiffness of the trailing edge portion. Any spanwise compressive stiffness of the trailing edge portion20, exclusive of the tensile only stiffener52, will result in a corresponding force on the trailing edge portion20towards the elastic axis, as described above. Consequently, should this force be sufficient to result in unwanted deflection of the trailing edge portion20, such a compressive stiffness may be minimized, or the resulting force balanced. This may be achieved by a defined rotational stiffness of the hinge, or, as discussed later, by a preloading spring, or chordwise slots segmenting the trailing edge, or a membrane skin of the trailing edge portion20.

FIG. 6shows a cross section of the trailing edge portion20during the positive flap-wise deformation ofFIG. 4. The trailing edge portion20can be seen in the trailing edge neutral position (solid lines) prior to the positive airfoil flap-wise deformation32, and in a deflected position68(dashed lines) resulting from advancing toward the suction side14relative to the remainder44of the pressure side12. The trailing edge portion20may be a flexible trailing end that is not definitively demarked from the remainder44of the airfoil50, and a portion that simply flexes as described. Alternately, the trailing edge portion20may be a discrete trailing edge flap70having a pressure side72secured to the remainder44of the airfoil50via a hinge74. The hinge may be mechanical, or may simply be an area within the skin of the airfoil50configured to flex. For example, the hinge74may be a thinned laminate woven at ±45 degrees that forms the hinge74. The tensile-only stiffener52may be disposed at a distance L from the hinge74measured parallel to the elastic axis30.

If the trailing edge portion20cannot deflect relative to the remainder44of the airfoil50, the elongation of the tensile-only stiffener52in the trailing edge portion20is given by:

εZ=KX·B where KXis the curvature of the airfoil along the spanwise

direction, and B is the distance54of the tensile-only stiffener52from the elastic axis30in direction of the bending radius. If the trailing edge portion20has a hinge74, the equation may be expressed as:

εZ=KX* (B0−α*L) where B0describes the trailing edge neutral position with

respect to the elastic axis30, a is the flap deflection angle, and L is the distance of the tensile-only stiffener52from the hinge74measured parallel to the elastic axis30.

Under positive flap-wise airfoil deformation32, the trailing edge portion20will deflect toward the elastic axis30and the magnitude of the driving moment per spanwise (length of the flap can be calculated, for the example of an embedded stiffener with linear elastic response to tension, as:

MFlap=∂/∂ α(½*K*εZ2)=KX·K*(B0−α*L)*L, where K is the tensile stiffness of the tensile-only stiffener52.

Also visible inFIG. 6is a preloading spring80disposed between a structural member82of the airfoil50as part of a delimiting stop84disposed on a suction side86of the trailing edge portion20. In this exemplary embodiment the trailing edge portion20is not secured to the suction side14of the remainder44of the second span-wise portion42. The preloading spring is one way the trailing edge portion20may be preloaded such that it stays in the trailing edge neutral position until a threshold amount of deflection force (i.e. a force urging the trailing edge portion20from the trailing edge neutral position) is experienced. In such a configuration the curvature of the airfoil50may change non linearly with the amount of airfoil flap-wise deformation. In an exemplary embodiment the tip may deflect as much as 10 meters from an ideal operating sweep before mitigation of the airfoil flap-wise deformation may be desired and an associated threshold deflection force exceeded. The preloading spring80may have a linear or non linear stiffness. Various other ways for applying such a preloading may be implemented as known to those in the art.FIG. 7shows an alternate exemplary embodiment of the trailing edge flap70where the suction side86of the trailing edge flap70is secured to the suction side of the remainder44of the second span-wise portion42via a deflection delimiter88which may be a sort of webbing and may also supply a preloading if desired.

As can be seen inFIGS. 8-9, which is a side view looking toward a trailing edge18of the airfoil50, positive flap-wise airfoil deformation32results in a certain curvature of the airfoil50.FIG. 8shows the airfoil50in a same condition as shown inFIG. 3, where the airfoil50is not experiencing positive flap-wise deformation32. Consequently, the airfoil50, and the trailing edge18are straight from the base60to the tip62. A dotted elastic edge line90represents an edge of the theoretical slice of the first span-wise portion40that includes all of the elastic axes30from the base60to the tip62. (The elastic axes30are running in and out of the sheet in this view.) Likewise, a constant strain edge line92represents an edge of a theoretical neutral surface (slice) of the first span-wise portion40that includes all of the neutral position constant strain lines56from the base60to the tip62. (The neutral position constant strain lines56are running in and out of the sheet in this view.) The distance94from the constant strain edge line92to the dotted elastic edge line90is also visible and in this exemplary embodiment may be equal to the given distance54from the elastic axis30from the base60to the tip62.

In this illustrative embodiment in the airfoil neutral position66the trailing edge18essentially aligns with the constant strain edge line92, and the dotted elastic edge line90, the constant strain edge line92, and the trailing edge18are all the same length. In contrast,FIG. 9shows the airfoil50in a same condition as the airfoil50inFIG. 4. InFIG. 9a length of the dotted elastic edge line90is the same as inFIG. 8(neutral strain means no elongation or compression). However, in order for the curvature ofFIG. 9to exist the first span-wise portion40must elongate under the tensile load. Consequently, a length of the first span-wise portion40must increase. The increase in length increases with increased distance from the dotted elastic edge line90. Thus, the increase in length for the constant strain edge line92, which is at the given distance54from the dotted elastic edge line90, is a quantifiable amount. Since the trailing edge18responds (deflects) with the trailing edge flap70, and since the trailing edge flap70resists elongation more than a remainder44of the airfoil50, the trailing edge flap70will shift toward the dotted elastic edge line90because the length of the dotted elastic edge line90has not increased (or decreased). The shorter trailing edge18can be envisioned as seeking a shorter distance between the base60and the tip62of the airfoil50because the trailing edge18has not elongated as much as the remainder44of the first span-wise portion40at the given distance54. This deflection reduces the curvature of the suction side which reduces aerodynamic lift and associated flap-wise deformation.

FIG. 10is a side view of the airfoil50showing the trailing edge flap70and the tensile-only stiffener52disposed therein. In this exemplary embodiment the trailing edge flap70does not span from the base60to the tip62as it may in another exemplary embodiment. Instead, the trailing edge flap70spans only a portion of the length from the base60to the tip62and hence changes the curvature of the airfoil50in this region only. In an exemplary embodiment the portion may include from 60% to 85% of the length because this region may experience a majority of the airfoil deformation. Consequently, placing the tensile-only stiffener52in this region may provide the greatest benefit.

FIG. 11shows a close up of the trailing edge flap70ofFIG. 10, including optional chordwise oriented gaps100disposed adjacent to trailing edge portion segments102(for example, sections of airfoil skin which are relatively structurally rigid compared to the gaps100). These gaps100may be left open or the segments102may be joined across the gaps100via flexible and compressible material, for example, textile-reinforced rubber, or through a tape fixed to the surface of one segment and sliding on the surface of the adjacent segment. This would provide a continuous aerodynamic skin surface while providing compressible gaps100in the trailing edge flap70oriented transverse to compressive loads felt during airfoil deformation. These gaps100reduce an extensional stiffness (rigidity) of the trailing edge flap70. During compression of the first span-wise portion40that is experienced during negative airfoil flap-wise deformation34the reduced compressive stiffness of the gaps100translates into a reduced compressive stiffness response of the trailing edge flap70, and hence a greater compressibility. The greater compressibility of the trailing edge flap70may help the trailing edge flap70compress as much as the remainder44of the first span-wise portion40during negative airfoil flap-wise deformation34, and this may help the airfoil50keep its aerodynamic shape (curvature) during the compression. This, in turn, further alleviates any negative lift associated with negative flap-wise deformation. This arrangement may readily be used together with the tensile-only stiffener52extending through each of the rigid segments to create an airfoil50having an increased tensional stiffness during positive airfoil flap-wise deformation32when compared to the airfoil without the stiffener, and a decreased compression stiffness when compared to the airfoil without the gaps100. This yields an optimal result of mitigation of both unwanted positive flap-wise deformation and unwanted negative flap-wise deformation.

FIGS. 11-14show various implementations of the tensile-only stiffener52in the trailing edge flap70. In FIG. lithe tensile-only stiffener52is secured to the blade at locations outside the trailing edge flap70and traverses the trailing edge flap70, forming a continuous component spanning the segments102and connected to one or more of the rigid segments102.FIG. 12shows an exemplary embodiment of a base end mounting arrangement110and a tip end mounting arrangement112for the stiffener. The base end mounting arrangement110may include a base spring arrangement114that may include one or more base springs116,118. Each base spring116,118may have a unique spring rate. For example, the base springs116,118may be configured to provide little resistance (spring stiffener effects) until a threshold amount of tensile elongation is experienced. At that point the springs may begin to increase resistance as desired to induce tension in the tensile-only stiffener52and the associated desired flap deflection. This may amount to “slack” in the stiffener arrangement. Likewise, the tip end mounting arrangement112may include a tip spring arrangement120that may include a tip spring122that may have a linear or a non linear spring rate. The base end mounting arrangement110and the tip end mounting arrangement112may be used together or individually. Delaying the onset of the stiffener effects in this manner may be implemented alone or together with the pre-loading of the trailing edge flap as shown inFIG. 6.

In an alternate exemplary embodiment shown inFIG. 13, the tensile-only stiffener52is shown having slack130. This allows the tensile-only stiffener52to lengthen to accommodate an amount of positive airfoil flap-wise deformation32, but after a threshold amount the tensile-only stiffener52will begin to function as detailed above.FIG. 14shows another alternate exemplary embodiment of a stiffener arrangement where the stiffener is secured to each rigid segment102via a respective segment spring140. Each segment spring may be tailored such that it has a spring rate desired for the respective rigid segment102to which it is secured. In this way an amount of tension in the tensile-only stiffener52may be transferred to each segment102in a manner most suited for the respective segment102to mitigate the amount of lengthening the trailing edge flap70undergoes at the respective segment102. For example, a segment spring140toward the base60may have a higher or lower spring rate than a different segment spring140toward the tip62. In this way a curvature of the airfoil50from the base60to the tip62can be tailored to respond differently to airfoil deformation132depending on its radial location from the base60.

In another exemplary embodiment (not shown), in contrast to delaying the onset of the stiffener effects, the blade may be pre-bent leewards to initiate the onset of the stiffener effects so that stiffener effects are felt at normal operating conditions. Conversely, windward prebend is used in many upwind turbines to increase tower distance, and doing so here will result in an effect similar to providing slack as described above. Various other early-onset configurations are envisioned, as well as configurations where effects may be front-loaded (occur most at lighter loading), middle loaded, end loaded, or any combination thereof.

In light of the foregoing it can be seen that the inventor has developed a new and unique way to reduce flap-wise deformation and fatigue loading on a wind turbine blade airfoil via a tensile-only stiffener that reduces aerodynamic lift during positive loading, but contributes negligible or no effect during negative loading. The tensile-only stiffener uses materials and practices known to those in the art and hence is easy to implement and economically feasible. Consequently, the disclosure represents an improvement in the art.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein, so long as the stiffener is configured to add to a bending stiffness of the trailing edge portion during positive flap-wise deformation and not to add to the bending stiffness of the trailing edge portion during negative flap-wise deformation. While it has been disclosed as a tensile-only stiffener herein, it is appreciated that various other structures and materials may be used to effect the same results. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.