Wind turbine blade comprising a root end structure with a pultruded element having a transition portion

Abstract

This invention relates to a root end structure, a wind turbine blade comprising such a root end structure and a method of manufacturing such a wind turbine blade. The root end structure comprises a plurality of fastening members distributed along a root end of a blade part, wherein a first plurality of pultruded elements are arranged in between the fastening members and a second pultruded element is further arranged at the blade joint ends adjacent to an outermost fastening member. Each first pultruded element has opposite facing second sides each facing a first side of an adjacent fastening member. The second pultruded element has one second side facing the outermost fastening member and another second side facing the blade joint interface. The second pultruded element comprises a transition portion forming a smooth transition for the inner layers extending further along the mould edge surface.

Claims

1. A root end structure (19) of a wind turbine blade (5), the root end structure (19) extending from a blade root (7) into a root end portion (14) of a blade shell (11) formed by at least one blade part having an inner surface (21), an outer surface (22) and at least one blade joint edge, the root end structure (19) comprising: a plurality of fastening members (23) distributed along the root end (7) of said at least one blade part in a circumference direction, wherein each of the fastening members (23) has a first inner surface (39), a first outer surface (40) and opposite facing first side surfaces (41a, 41b) each extending in a longitudinal direction, wherein a second pultruded element (28′) is arranged between an outermost fastening member (23a) and a blade joint edge (20a, 20b) of the at least one blade part, wherein the second pultruded element (28′) has a second inner surface (34), a second outer surface (35) and opposite facing second side surfaces (33a, 33b) further extending in the longitudinal direction, wherein the root end structure (19) further comprises a number of outer layers (44) of a fibre material extending along the outer surface (22) and a number of inner layers (29) of a fibre material extending along the inner surface (21), and wherein the second pultruded element (28′) comprises a transition portion (31, 31′) extending from one second side surface (33b) towards the other one of the second side surfaces (33a), and wherein a local thickness of said transition portion (31, 31′) tapers towards the blade joint edge (20a, 20b) and/or said one second side surface (33a) in the circumference direction.

2. The root end structure according to claim 1, characterised in that said transition portion (31, 31′) forms a transitional contact surface (32) contacting the inner layers (29), wherein said transitional contact surface (32) forms at least a part of the second inner surface (34).

3. The root end structure according to claim 2, characterised in that said transitional contact surface (32) further forms at least a part of said one second side surface (33b).

4. The root end structure according to claim 1, characterised in that the second pultruded element (28″) is terminated at an inner point (37) in a thickness direction, wherein the outermost fastening member (23a) extends beyond the inner point (37) of the second pultruded element (28″) in the thickness direction.

5. The root end structure according to claim 1, characterised in that the transition portion (31) has a planar surface profile arranged in an angle relative to the second outer surface (35) of the second pultruded element (28″).

6. The root end structure according to claim 1, characterised in that the transition portion (31) has a curved surface profile.

7. The root end structure according to claim 6, wherein the curved surface profile comprises a circular or elliptical arc profile having a predetermined radius.

8. The root end structure according to claim 1, wherein the local thickness of said transition portion (31, 31′) tapers from the other one of the second side surfaces (33) towards the blade joint edge (20a, 20b) and/or from the one of the second side surfaces (33b) towards the other one of the second side surface (33a) in the circumference direction.

9. The root end structure according to claim 1, characterised in that the transition portion (31) extends along partly or fully along a length of the second pultruded element (28′).

10. The root end structure according to claim 1, characterised in that the second pultruded element (28′″) comprises a first sub-piece (28a) and at least one second sub-piece (28b) arranged relative to the first sub-piece (28a), wherein said first sub-piece (28a) and said at least one second sub-piece (28b) extend in the longitudinal direction.

11. The root end structure according to claim 1, further comprising a first pultruded element (27) arranged between at least one pair of fastening members (23), wherein the first pultruded element (27) has a second inner surface, a second outer surface and opposite facing second side surfaces further extending in the longitudinal direction.

12. A wind turbine blade, the wind turbine blade (5) extending from a blade root (7) to a tip end (8) in a longitudinal direction and further from a leading edge (9) to a trailing edge (10) in a chordwise direction, the wind turbine blade (5) comprising a blade shell (11) formed by at least two blade parts each having an inner surface (21) and an outer surface (22), wherein said at least two blade parts are connected along at least one blade joint interface in the longitudinal direction, each blade joint interface is defined by a first joint edge (20a) of one blade part and a second joint edge (20b) of another blade part, characterised in that said at least two blade parts comprises a root end structure (19) configured according to claim 1.

13. A method of manufacturing a wind turbine blade, comprising the steps of: laying up a number of outer layers (44) of a fibre material along a blade mould surface (26); providing a plurality of fastening members (23) configured to mount the wind turbine blade (5) to a rotor hub interface or a pitch bearing unit; positioning said fastening members (23) relative to said outer layers (44) at a root end (7) of said wind turbine blade (5); positioning a second pultruded element (28′) between an outermost fastening member (23a) and a blade joint edge (20a, 20b) of the at least one blade part; further laying up a number of inner layers (29) of a fibre material along said second pultruded element (28′) and said fastening members (23) to form a root end structure (19); enclosing at least the root end structure (19) via a vacuum bag material; introducing resin into said fibre material; and curing said resin to form a cured blade part, wherein the inner layers (29) extends over a transitional contact surface (32) of the second pultruded element (28′) and further along a mould edge surface (25) during the lay-up, wherein the second pultruded element (28′) has a second inner surface (34), a second outer surface (35) and opposite facing second side surfaces (33a, 33b) further extending in the longitudinal direction, and wherein the second pultruded element (28′) comprises a transition portion (31, 31′) extending from one second side surface (33b) towards the other one of the second side surfaces (33a), and wherein a local thickness of said transition portion (31, 31′) tapers towards the blade joint edge (20a, 20b) and/or said one second side surface (33a) in the circumference direction.

14. The method according to claim 13, characterised in that the inner layers (29) further extends over a portion of the outermost fastening member (23a).

15. The method according to claim 13, characterised in that excess fibre material of at least the inner layers (29) are trimmed off in a post lay-up step or in a post-moulding step.

16. The method according to claim 13, further comprising the step of positioning a first pultruded element (27) between at least one pair of fastening members (23), and wherein the step of further laying up the number of inner layers (29) of the fibre material along said second pultruded element (28′) further comprises laying up the number of inner layers (29) of the fibre material along said first pultruded element (27) and said fastening members (23) to form the root end structure (19).

17. A root end structure (19) of a wind turbine blade (5), the root end structure (19) extending from a blade root (7) into a root end portion (14) of a blade shell (11) formed by at least one blade part having an inner surface (21), an outer surface (22) and at least one blade joint edge, the root end structure (19) comprising: a plurality of fastening members (23) distributed along the root end (7) of said at least one blade part in a circumference direction, wherein each of the fastening members (23) has a first inner surface (39), a first outer surface (40) and opposite facing first side surfaces (41a, 41b) each extending in a longitudinal direction, wherein a second pultruded element (28′) is arranged between an outermost fastening member (23a) and a blade joint edge (20a, 20b) of the at least one blade part, wherein the second pultruded element (28′) has a second inner surface (34), a second outer surface (35) and opposite facing second side surfaces (33a, 33b) further extending in the longitudinal direction, wherein the root end structure (19) further comprises a number of outer layers (44) of a fibre material extending along the outer surface (22) and a number of inner layers (29) of a fibre material extending along the inner surface (21), wherein the second pultruded element (28′) comprises a transition portion (31, 31′) extending from one second side surface (33b) towards the other one of the second side surfaces (33a), and wherein a local thickness of said transition portion (31, 31′) tapers towards the blade joint edge (20a, 20b) and/or said one second side surface (33a), and wherein the second pultruded element (28′) is terminated at an inner point (37) in a thickness direction, wherein the outermost fastening member (23a) extends beyond the inner point (37) of the second pultruded element (28′) in the thickness direction.

18. A wind turbine blade, the wind turbine blade (5) extending from a blade root (7) to a tip end (8) in a longitudinal direction and further from a leading edge (9) to a trailing edge (10) in a chordwise direction, the wind turbine blade (5) comprising a blade shell (11) formed by at least two blade parts each having an inner surface (21) and an outer surface (22), wherein said at least two blade parts are connected along at least one blade joint interface in the longitudinal direction, each blade joint interface is defined by a first joint edge (20a) of one blade part and a second joint edge (20b) of another blade part, wherein said at least two blade parts comprises a root end structure (19) configured according to claim 16.

19. A root end structure (19) of a wind turbine blade (5), the root end structure (19) extending from a blade root (7) into a root end portion (14) of a blade shell (11) formed by at least one blade part having an inner surface (21), an outer surface (22) and at least one blade joint edge, the root end structure (19) comprising: a plurality of fastening members (23) distributed along the root end (7) of said at least one blade part in a circumference direction, wherein each of the fastening members (23) has a first inner surface (39), a first outer surface (40) and opposite facing first side surfaces (41a, 41b) each extending in a longitudinal direction, wherein a second pultruded element (28′) is arranged between an outermost fastening member (23a) and a blade joint edge (20a, 20b) of the at least one blade part, wherein the second pultruded element (28′) has a second inner surface (34), a second outer surface (35) and opposite facing second side surfaces (33a, 33b) further extending in the longitudinal direction, wherein the root end structure (19) further comprises a number of outer layers (44) of a fibre material extending along the outer surface (22) and a number of inner layers (29) of a fibre material extending along the inner surface (21), wherein the second pultruded element (28′) comprises a transition portion (31, 31′) extending from one second side surface (33b) towards the other one of the second side surfaces (33a), and wherein a local thickness of said transition portion (31, 31′) tapers towards the blade joint edge (20a, 20b) and/or said one second side surface (33a), and wherein the transition portion (31) has a planar surface profile arranged in an angle relative to the second outer surface (35) of the second pultruded element (28′).

20. A wind turbine blade, the wind turbine blade (5) extending from a blade root (7) to a tip end (8) in a longitudinal direction and further from a leading edge (9) to a trailing edge (10) in a chordwise direction, the wind turbine blade (5) comprising a blade shell (11) formed by at least two blade parts each having an inner surface (21) and an outer surface (22), wherein said at least two blade parts are connected along at least one blade joint interface in the longitudinal direction, each blade joint interface is defined by a first joint edge (20a) of one blade part and a second joint edge (20b) of another blade part, wherein said at least two blade parts comprises a root end structure (19) configured according to claim 19.

Description

DESCRIPTION OF DRAWINGS

(1) The invention is explained in detail below with reference to embodiments shown in the drawings, in which

(2) FIG. 1 shows a wind turbine,

(3) FIG. 2 shows an exemplary embodiment of the wind turbine blade,

(4) FIG. 3 shows an exemplary embodiment of a root end structure,

(5) FIG. 4 shows a cross-sectional view of a first embodiment of the root end structure placed in a blade mould,

(6) FIG. 5 shows a cross-sectional view of a second embodiment of the root end structure placed in the blade mould,

(7) FIG. 6 shows a third embodiment of the second pultruded element,

(8) FIGS. 7a-c show three views of a fourth embodiment of the second pultruded element,

(9) FIG. 8 shows a fifth embodiment of the second pultruded element,

(10) FIGS. 9a-b show two views of a sixth embodiment of the second pultruded element,

(11) FIG. 10 shows a side view of the fastening member,

(12) FIG. 11 shows a side view of a first embodiment of the first pultruded element,

(13) FIG. 12 shows a side view of a second embodiment of the first pultruded element,

(14) FIGS. 13a-c show three views of the root end structure comprising a third embodiment of the first pultruded element,

(15) FIG. 14 shows the root end structure comprising a fourth embodiment of the first pultruded element,

(16) FIGS. 15a-b show two views of the root end structure comprising a fifth embodiment of the first pultruded element,

(17) FIG. 16 shows a sixth embodiment of the first pultruded element,

(18) FIGS. 17a-b show two views of the root end structure comprising a seventh embodiment of the first pultruded element,

(19) FIG. 18 shows a longitudinal view of the root end structure with a misalignment between the outer layers and the first pultruded element,

(20) FIG. 19 shows a longitudinal view of the root end structure with the first pultruded element conforming to the shape of the outer layers, and

(21) FIG. 20 shows a transverse view of the root end structure with the first pultruded element arranged between a pair of adjacent fastening members.

LIST OF REFERENCES

(22) 1, Wind turbine 2. Wind turbine tower 3. Nacelle 4. Hub 5. Wind turbine blades 6. Pitch bearing 7. Blade root 8. Tip end 9. Leading edge 10. Trailing edge 11. Blade shell 12. Pressure side 13. Suction side 14. Blade root portion 15. Aerodynamic blade portion 16. Transition portion 17. Length of wind turbine blade 18. Chord length of wind turbine blade 19. Root end structure 20a. First blade joint edge 20b. Second blade joint edge 21. Inner surface 22. Outer surface 23. Fastening members, bushings 23a, Outermost fastening member, bushing 24. Blade mould 25. Blade mould surface 26. Mould edge surface 27. First pultruded element, retaining member 27a-b. First and second portions 28. Second pultruded element, retaining member 28a-c. First, second and third sub-pieces 29. Inner layers 30. Wrinkles 31. Transition portion 32. Transitional contact surface 33a-b. Local second sides 34. Local inner side 35. Local outer side 36. Recess 37. Inner point 38. Second side surface 39. Local inner side 40. Local outer side 41a-b. Local first sides 42. Outer layers 43. Gap 44. First side surface 45. Contact area 46. First sub-piece 47. Second sub-piece 48. Wrinkles 49. Spacer elements

(23) The listed reference numbers are shown in abovementioned drawings where no all reference numbers are shown on the same figure for illustrative purposes. The same part or position seen in the drawings will be numbered with the same reference number in different figures.

DETAILED DESCRIPTION OF THE DRAWINGS

(24) FIG. 1 shows a modern wind turbine 1 comprising a wind turbine tower 2, a nacelle 3 arranged on top of the wind turbine tower 2, and a rotor defining a rotor plane. The nacelle 3 is connected to the wind turbine tower 2, e.g. via a yaw bearing unit. The rotor comprises a hub 4 and a number of wind turbine blades 5. Here three wind turbine blades are shown, but the number of blades may be greater or smaller. The hub 4 is connected to a drive train located in the wind turbine 1 via a rotation shaft.

(25) The hub 4 comprises a mounting interface for each wind turbine blade 5. A pitch bearing unit 6 is optionally connected to this mounting interface and further to a blade root of the wind turbine blade 5.

(26) FIG. 2 shows a schematic view of the wind turbine blade 5 which extends in a longitudinal direction from a blade root 7 to a tip end 8. The wind turbine blade 5 further extends in a chordwise direction from a leading edge 9 to a trailing edge 10. The wind turbine blade comprises a blade shell 11 having two opposite facing side surfaces defining a pressure side 12 and a suction side 13 respectively. The blade shell 11 further defines a root portion 14, an aerodynamic portion 15, and a transition portion 16 between the root portion 14 and the aerodynamic portion 15.

(27) The root portion 14 has a substantially circular or elliptical cross-section (indicated by dashed lines). The root portion 14 together with a load carrying structure (not shown) are configured to add structural strength to the wind turbine blade 5 and transfer the dynamic loads to the hub 4. The load carrying structure extends between the pressure side 12 and the suction side 13 and further in the longitudinal direction.

(28) The aerodynamic blade portion 15 has an aerodynamically shaped cross-section (indicated by dashed lines) designed to generate lift. The cross-sectional profile of the blade shell 11 gradually transforms from the circular or elliptical profile into the aerodynamic profile in the transition portion 16.

(29) The wind turbine blade 5 has a longitudinal length 17 of at least 35 metres, preferably at least 50 metres. The wind turbine blade 5 further has a chord length 18 as function of the length 17, wherein the maximum chord length is found between the aerodynamic portion 15 and the transition portion 16. The wind turbine blade 5 further has a blade thickness as function of the chord length 18, wherein the blade thickness is measured between the pressure side 12 and the suction side 13.

(30) FIG. 3 shows an exemplary embodiment of a root end structure 19 formed by two blade parts where one blade part comprises two first blade joint edges 20a and the other blade part comprises two second blade joint edges 20b. The blade joint edges 20a, 20b extend in the longitudinal direction and, when joined together, the first and second blade joint edges 20a, 20b form a leading edge joint interface and further a trailing edge interface.

(31) The blade shell 11 of the root end structure 19 forms an inner surface 21 and an outer surface 22. A plurality of fastening members 23 are distributed along the root end 7 in a first circumference direction. A plurality of retaining members (shown in FIGS. 4 and 5) are arranged relative to each of the fastening members 23, wherein the fastening members 23 and the retaining members are sandwiched between a number of inner layers (shown in FIGS. 18-19) and a number of outer layers (shown in FIGS. 18-19).

(32) FIG. 4 shows a cross-sectional view of the root end structure 19 placed in a blade mould 24. The blade mould 24 has a blade mould surface 25 having a predetermined surface profile and a mould edge surface 26.

(33) The outer layers (shown in FIGS. 18-19) extend along the blade mould surface 26 and further along the mould edge surface 25 during the lay-up, but are removed in FIGS. 4 and 5 for illustrative purposes. The outer layers form an outer skin of the blade shell 11 defining the outer surface 22.

(34) The fastening members 23 are here formed as bushings each arranged between a pair of adjacent retaining members. The retaining members are here formed as pultruded elements. A first pultruded element 27 is arranged between a pair of adjacent bushings 23 while an outermost bushing 23a is arranged between a first pultruded element 27 and a second pultruded element 28.

(35) The first pultruded element 27 may be formed as a single continuous element, or by a number of sub-pieces. Here, two symmetrical sub-pieces are shown which are arranged back-to-back, as indicated by the dashed line. The first pultruded element 27 has a butterfly shaped cross-sectional profile in the width direction.

(36) The second pultruded element 28 has a semi-butterfly shaped cross-sectional profile in the width direction. The second pultruded element 28 is arranged at the first and/or second blade joint edge 20a, 20b, FIG. 4 shows a prior art embodiment of the second pultruded element 28 where a sharp transition is formed between the second side surface (see FIG. 6) and the second inner surface (see FIG. 6). Wrinkles 30 are thus formed in the inner layers 29 at the blade joint edge 20a, 20b due to this sharp transition.

(37) During lay-up, the inner layers 29 extend along the local inner sides (shown in FIGS. 6 and 16) of the first pultruded elements 27, over the second pultruded element 28 and further along the mould edge surface 25. Excess material of the inner 29 and outer layers are trimmed off after curing. The inner layers 29 form an inner skin of the blade shell 11 defining the inner surface 21.

(38) FIG. 5 shows a cross-sectional view of the root end structure 19 still placed in the blade mould 24, wherein the root end structure 19 comprises a first embodiment of the second pultruded element 28′.

(39) Here, the second pultruded element 28′ comprises a transition portion 31 forming a transitional contact surface 32 for contacting the inner layers 29. The local thickness of this transition portion 31 tapers towards the second side surface (see FIG. 6) facing the blade joint edge 20a, 20b. The local width of the transition portion 31 further tapers towards the second inner side (shown in FIG. 6). The transitional contact surface 32 forms a smooth transition of the inner layers 29, thereby reducing the risk of wrinkles 30.

(40) FIG. 6 shows a second embodiment of the second pultruded element 28′, wherein the transition portion 31′ extend along the entire width of the second pultruded element 28′. The local thickness tapers from one second side 33a to an opposite second side 33b.

(41) Here, the transition portion 31′ extends along the entire local inner side 34 of the second pultruded element 28′ and partly along the local second side 33b. The transition port 31′ may extend fully along both the local inner side 34 and the local second side 33b. The transition port 31′ may also extend partly along both the local inner side 34 and the local second side 33b, as indicated in FIG. 5.

(42) The second pultruded element 28′ further has a local outer side 35 facing the outer layers.

(43) A recess 36 is formed in the local second side 33a of the second pultruded element 28 and in both local second sides 33a, 33b of the first pultruded element 27. The recesses 36 are configured to partly receive the outermost bushing 23a, as indicated in FIG. 4. The recess 36 may extend partly along the local second side 33a, as indicated in FIG. 6, or alternatively along the entire local second side 33a.

(44) FIGS. 7a-c show three views of a third embodiment of the second pultruded element 28″, wherein the profile of the second pultruded element 28′, 28″ is terminated at the local inner side 34 or at an inner point 37 forming an inner edge.

(45) In FIG. 7a, the transitional contact surface 32′ has a planar surface profile which extends perpendicularly from the local outer side 35. The transitional contact surface 32′ intersects a second side surface 38 of the local second side 33a, thereby forming a reduced recess 36′ for receiving the outermost bushing 23a. The recess 36 may have a semi-circular shaped profile while the recess 36′ may have circular segment shaped profile. The inner layers 29 may thus contact a portion (dashed line) of the outer surface of the outermost bushing 23a, as indicated in FIG. 7a.

(46) In FIG. 7b, the transitional contact surface 32″ has a curved profile extending from the local outer side 35 to the inner point 37. In FIG. 7c, the transitional contact surface 32′ has a planar surface profile which extends in an inclined angle relative to the local outer side 35.

(47) FIG. 8 shows a fourth embodiment of the second pultruded element 28′″ comprises a number of sub-pieces which together form a substantial semi-butterfly shaped cross-sectional profile in the width direction. A first sub-piece 28a extends in the thickness direction and a set of second sub-pieces 28b projects from a side surface of the first sub-piece 28a. Here, a second sub-piece 28b is arranged at both the local inner and outer sides 34, 35.

(48) The individual sub-pieces 28a, 28b together form the recess 36 for partly receiving the outermost bushing 23a.

(49) FIGS. 9a-b show two views of a fifth embodiment of the second pultruded element 28″″, wherein the transition portion 31″ is integrally formed by one of the sub-pieces 28a-c.

(50) In FIG. 9a, the first sub-piece 28a′ has a substantial rectangular profile in the width direction, wherein the transition portion 31″ is facing the local inner side 34. The second sub-pieces 28b have a substantial triangular profile in the width direction.

(51) In FIG. 9b, the first sub-piece 28a″ extends in the width direction and has a substantial parallelogram shaped profile. Here, the transition portion 31″ form one end of the first sub-piece 28a″. The second sub-pieces 28b′ extends in the thickness direction and has a substantial rectangular profile in the width direction. Further, a third sub-piece 28c projects from a side surface of the second sub-piece 28b and has a substantial triangular profile in the width direction.

(52) FIG. 10 shows a side view of the fastening member 23, 23a having a predetermined length measured between a local root end 7′ and an opposite end. Here, the fastening member 23, 23a has a uniform cross-sectional profile in the length direction. However, the cross-sectional profile may instead vary or taper along the length. As indicated in FIGS. 4-5, the fastening member 23, 23a has a circular cross-sectional profile. However, the fastening member 23, 23a may another suitable cross-sectional profile, such as an elliptical or polygonal profile. The fastening member 23, 23a thus has a uniform or variable outer diameter or thickness along its length.

(53) The fastening member 23, 23a has a local inner side 39, a local outer side 40 and two opposite facing local first sides, as indicated in FIG. 13b-c. Here, only one local first side 41a is show.

(54) The exterior surface of the fastening member 23, 23a is optionally wrapped in a fibre material, as indicated with dashed lines, wherein the wrapped fibre material extends along at least a part of the length of the fastening member 23, 23a.

(55) FIGS. 11-12 show a side view of a first and a second embodiment of the first pultruded element 27 comprising a first portion 27a and a second portion 27b. The first portion 27a extends from a local root end 7″ towards an opposite end while the second portion 27b extends from the first portion 27a to said opposite end.

(56) The first portion 27a has a uniform thickness along its local length, as indicated in FIGS. 11-12. The local length of the first portion 27a corresponds substantially to the length of the fastening member 23, 23a, as indicated in FIGS. 18-19.

(57) The second portion 27b has a tapered profile extending beyond the fastening member 23, 23a where the local thickness tapers from a maximum thickness to a minimum thickness. As indicated in FIG. 11, the first and second portions 27a, 27b may form a continuous local outer side 35 where the second portion 27b tapers from the inner side 34 to the local outer side 35. As indicated in FIG. 11, the second portion 27b may form inclined local inner and outer sides 34, 35 where the second portion 27b tapers towards a local central line.

(58) The second pultruded element 28 has a similar configuration as the first pultruded element 27 shown in FIGS. 11-12.

(59) FIGS. 13a-c show three views of the root end structure 19′ comprising a third embodiment of the first pultruded element 27′. FIGS. 13b-c show a simplified view of one first pultruded element 27′ where the local first sides 41a, 41b of a pair of adjacent bushings 23 extend into the local second sides 33a, 33b of the first pultruded bushing 27′.

(60) The first pultruded element 27′ has a thickness greater than the outer diameter of the fastening member 23. The inner layers 29 extend along the local inner sides 34 and the outer layers 42 extend along the local outer sides 35 of the first pultruded elements 27′.

(61) The local second sides 33a, 33b of a pair of adjacent first pultruded elements 27′ may contact each other, as indicated in FIG. 13a, or be spaced apart, as indicated in FIG. 14.

(62) A number of gaps 43 are formed between a first side surface 44 of the bushing 23 and the second side surface 38 of the first pultruded element 27′. Here, a first gap 43′ and a second gap 43′ are formed on opposite sides of the bushing 23. The first and second gaps 43′ extend in the longitudinal direction and further along a second circumference direction defined by the second side surface 38.

(63) Here, the second side surface 38 has an elliptical arc profile while first side surface 44 has a circular profile. The second side surface 38 and the adjacent local side surfaces may form a sharp transition, as indicated in FIG. 13b, or a smooth transition, as indicated in FIG. 13c.

(64) The elliptical arc profile of the second side surface 38 forms part of an inscribed profile having a predetermined height, h.sub.2, and width, w.sub.2, as illustrated in FIG. 13b. Further, the circular profile of the first side surface 44 forms part of a circumscribed profile having a predetermined height, h.sub.1, and width, w.sub.1, as also illustrated in FIG. 13b.

(65) Here, the first pultruded element 27′ contacts the adjacent bushings 23 at a contact area formed on the second side surface 38, as indicated in FIGS. 13b-c. The first and second gaps 43′ has a radial distance that varies along the second side surface 38, as indicated in FIGS. 13a-c. This allows for an adaptive positioning of the first pultruded element 27′ in the longitudinal direction.

(66) FIG. 14 shows the root end structure 19″ comprising a fourth embodiment of the first pultruded element 27″ where the first pultruded element 27″ has a local thickness smaller than the outer diameter of the bushings 23. The inner and outer layers 29, 42 are here contacting both the local inner sides 35 of the first pultruded elements and the local inner sides 43 of the bushings 23.

(67) FIGS. 15a-b show two views of the root end structure 19′″ comprising a fifth embodiment of the first pultruded element 27′″. FIG. 15b shows a simplified view of one first pultruded element 27′″ where a pair of adjacent bushings 23 extends partly in the recesses 36 formed in the first pultruded bushing 27′″.

(68) Here, a central gap 43″ is formed between the first and second side surfaces 38, 44 wherein the radial distance varies along the first circumference direction. A first contact area 45′ and a second contact area 45″ are further formed between the first and second side surfaces 38, 44 where the bushings 23 are contacting the pultruded element 27′″ at these first and second contact areas 45″. This allows for an adaptive positioning of the first pultruded element 27′″ in the width direction.

(69) Here, the second side surface 38 has an alternative elliptical arc profile and the first side surface 44 has a circular profile.

(70) FIGS. 16a-b show a sixth embodiment of the first pultruded element 27″″ where the second side surface of the recesses 36 comprises a planar surface portion 38′ arranged between two curved surface portions 38″. The curved surface portions 38″ may be shaped as a circular arc segments, as indicated in FIG. 16a. The inner arc segment has a first radius, r.sub.1, and outer arc segment has a second radius, r.sub.2. The first and radiuses r.sub.1, r.sub.2 have the same or different values.

(71) The curved surface portions 38″ may also be shaped as elliptical or super-elliptical arc segments, as indicated in FIG. 16b. The two elliptical arc segments have the same or different major and minor radiuses.

(72) The planar surface portion 38′ functions as contact areas for contacting the bushings 23. Unlike the embodiments of FIGS. 15 and 17, the gaps 43 can be formed while maintaining a minimum width between the recesses 36 in the width direction, as indicated by dashed lines in FIG. 16a.

(73) FIGS. 17a-b show three views of the root end structure 19″″ comprising a seventh embodiment of the first pultruded element 27′″″ where a continuous gap 43′″ is formed between the first and second side surfaces 38, 44.

(74) The gap 43′″ has a uniform radial distance along the second side surface 38, as indicated in FIG. 17b. The second side surface 38 may have a circular arc profile with an equal height, h.sub.w, and width, w.sub.w, and the first side surface 44 may further have a circular profile with an equal height, h.sub.b, and width, w.sub.b, as indicated in FIG. 17b. The first and second side surfaces 38, 44 may also have an elliptical arc profile, as indicated in FIG. 17c. The first and second side surfaces 38, 44 have a common centre point, but different radiuses. This increases the flexibility of the adaptive positioning of the first pultruded element 27′″″.

(75) Here, the bushings 23 are not in a firm and close contact with the first pultruded element 27′″″ as the first pultruded element 27′″″ is able to move to relative to the bushings in both the thickness direction and in the width direction.

(76) FIGS. 18a-b shows a further alternative embodiment of the first pultruded element 27. Here, the first pultruded element 27 is formed by a number of sub-pieces are arranged relative to each other.

(77) A first sub-piece 46 having a rectangular cross-sectional profile in the width direction is arranged between a pair of adjacent bushings 23. A first set of second sub-pieces 47 is arranged at the local inner side 34 and a second set of second sub-pieces 47 is arranged at the local outer side 35. The individual second sub-pieces 47 of each set are positioned on opposite facing sides of the first sub-piece 46.

(78) In conventional root end design, as indicated in FIG. 18a, all the first and second sub-pieces 46, 47 are contacting the bushing 23 to prevent any relative movements. The first and second sub-pieces 46, 47 together form an inscribed circular profile having equal height and width.

(79) In the present invention, as indicated in FIG. 18b, only the first sub-piece 46 is contacting the bushing 23 and the second sub-pieces 47′ are spaced apart from the bushing 23 to form the gaps 43. The first and second sub-pieces 46, 47′ together form an inscribed substantial elliptical profile having a height, h″, that is greater than its width, w″. Here, the bushing 23 forms a circumscribed circular profile having a height, h′, and a width, w′, of equal values. This also allows for an adaptive positioning of the first pultruded elements 27 in the longitudinal direction,

(80) FIG. 19 shows a longitudinal view of the root end structure 19 with a misalignment between the outer layers 42 and the first pultruded element 27 in the longitudinal direction. This misalignment results in wrinkles 48 forming in the transition area between the inner and outer layers 29, 42.

(81) This misalignment may occur when evacuating the root end structure 19 during the vacuum assisted resin infusion process.

(82) FIG. 20 shows a longitudinal view of the root end structure 19 with the first pultruded element 27 conforming to the shape of the outer layers 42. This is achieved by providing one or more gaps 43 between the first and second side surfaces 38, 44. The gaps 43 in turn enable the first pultruded element 27 to move relative to the bushings 23 (indicated by arrow) during the vacuum assisted resin infusion. Thereby, allowing the first pultruded elements 27 to passively adapt its longitudinal position relative to the outer layers 42 during the vacuum assisted resin infusion.

(83) FIG. 21 shows a transverse view of the root end structure 19 with the first pultruded element 27 arranged between a pair of adjacent bushings 23.

(84) Here, the first pultruded element 27 is prevented from moving within the width plane (indicated by arrow) relative to the bushings 23 and spacer elements 49 while being able to move relative to the bushings 23 in the thickness plane, as indicated in FIG. 20.

(85) The spacer elements 49 are positioned relative to the bushings 23 and extend further in the longitudinal direction. The spacer element 49 has a length substantially corresponding to the local length of the second portion 27b of the first pultruded element 27. The spacer element 49 has a tapered profile in the longitudinal direction corresponding to the tapered profile of the second portion 27b. The abovementioned gaps 43, optionally, extend along the length of the bushings 23 and further along at least a part of the length of the spacer element 49.

(86) The abovementioned embodiments may be combined in any combinations without deviating from the present invention.