WO2014096839A1

WO2014096839A1 - An aerofoil structure with collapsible tip portion

Info

Publication number: WO2014096839A1
Application number: PCT/GB2013/053370
Authority: WO
Inventors: Nicholas Michael Merriman
Original assignee: Composite Technology And Applications Limited
Priority date: 2012-12-19
Filing date: 2013-12-19
Publication date: 2014-06-26
Also published as: GB201222974D0

Abstract

An aerofoil structure (26) for a gas turbine, the aerofoil structure comprising an aerofoil portion (38) and a tip portion (48), the tip portion being provided at a radially outermost end of the aerofoil portion, wherein the tip portion is configured to be provided adjacent to a casing structure (30), and wherein the tip portion is collapsible with respect to the aerofoil portion, the tip portion being configured such that the tip portion collapses due to an interaction with the casing structure occurring outside a normal mode of operation.

Description

AN AEROFOIL STRUCTURE WITH COLLAPSIBLE TIP PORTION

The present disclosure relates to an aerofoil structure, for example to a fan blade for a turbofan gas turbine engine and relates particularly, but not exclusively, to a blade arrangement having a modified tip.

Background Gas turbine blades are commonly provided with abradable or abrasive tips which, in combination with the surrounding casing which may be abrasive or abradable to match, cooperate to allow for the tip or the casing to be worn away in the event that the two come into contact with each other. Presently known arrangements provide perfectly acceptable solutions to normal contact conditions experienced during initial running-in of the engine, but excessive contact such as that experienced during a bird strike can result in a large amount of load being placed on the tip of the blade and as it distorts under load it can be difficult to accommodate the resulting contact purely through the removal of any abradable material. Consequently, it is possible that the blade itself may suffer mechanical distortion, which may adversely affect future performance or safety of the engine itself. Metal blades are more readily able to accommodate such distortion than a composite blade, e.g. made of carbon fibre, as the composite material, whilst being strong in normal use, is designed to be stiff in the radial direction and so tends to be insufficiently flexible to accommodate the degree and rate of deflection. The present disclosure therefore seeks to provide a novel blade, which at least reduces the above problem.

Statements of Invention According to the present invention there is provided an aerofoil structure for a gas turbine, the aerofoil structure comprising an aerofoil portion and a tip portion, the tip portion being provided at a radially outermost end of the aerofoil portion, wherein the tip portion is configured to be provided adjacent to a casing structure, and wherein the tip portion is collapsible with respect to the aerofoil portion. The tip portion may be configured such that the tip portion collapses due to an interaction with the casing structure occurring outside a normal mode of operation. The tip portion may be configured such that the tip portion may not collapse during the normal mode of operation. For example, during a normal mode of operation, the tip portion may withstand some rubbing with the casing structure without collapsing. For example, normal mode of operation events may include engine commissioning, running-in of the engine, take-off of an aircraft comprising a gas turbine engine, or manoeuvres of such an aircraft, and rubbing of the tip portion may be envisaged during such events. Such normal mode events may not result in repair of the gas turbine engine and/or aerofoil structure being required other than for routine maintenance,

Events outside a normal mode of operation may comprise a biade-off event, bird-strike or any other event not normally encountered. In other words, events outside a normal mode of operation may result in repair of the gas turbine engine and/or aerofoil structure being required.

The stress at which the tip portion collapses may be configured so as to avoid damage to the aerofoil portion and/or the casing structure. The tip portion may be collapsible in a substantially radial direction of the aerofoil structure. The tip portion may be collapsible in a substantially radial and circumferential direction of the aerofoil structure. The tip portion may be collapsible such that it is deformed in an inelastic manner.

The tip portion may be configured such that the tip portion may collapse in the event the aerofoil structure twists about a radial axis beyond a threshold value. The tip portion may be at least partially formed of an isotropic structure, e.g. an isotropic material. Additionally or alternatively, the tip portion may be at least partially formed of an anisotropic structure. The tip portion may have a first strength in a first direction L and a second strength in a second direction H. The strength in the first direction may be lower than the strength in the second direction,

In an installed configuration the aerofoil structure rotates in a direction R and wherein the first strength in the first direction L may be angled at an angle Θ relative to the direction of rotation R. Angle Θ may be perpendicular to the direction of rotation R. The collapsible tip portion may be configured such that the angle Θ varies along the chord of the tip portion. The second strength in the second direction H may be substantially parallel to the direction of rotation R. The tip portion may have a third strength in a third direction C. The strength in the third direction may be lower than the strength in the second direction H. The third strength in the third direction C may be substantially in a radial direction, e.g. towards a centreline of the gas turbine.

The tip portion may include one or more internal cavities or cells. The internal cavities may have longitudinal axes L_x. The internal cavities may have a length and a breadth, in which the length may be greater than the breadth and the longitudinal axis may be defined in the lengthwise direction. The one or more internal cavities may have a cross-section formed of three or more sides.

The tip portion may comprise a plurality of tubes each having longitudinal axes L_x which extend substantially parallel to each other. The plurality of tubes may each have a vertical axis V_A and the vertical axis V_A may extend substantially perpendicular to the longitudinal axis L_x. The vertical axis V_A may extend in a radial direction of the aerofoil structure.

The plurality of tubes may have a substantially circular cross-section. The plurality of tubes may have a substantially oval cross-section. The plurality of tubes may comprise one or more tubes having a substantially circular cross-section and one or more tubes having a substantially oval cross-section. The plurality of tubes ma have a substantially rectangular or trapezoidal cross-section. The plurality of tubes may have a honeycomb cross section. The plurality of tubes may tessellate. The tubes may be joined to each other along their length.

The longitudinal axes L_x may be aligned at an angle Θ² to the first direction L. For example, the longitudinal axes L_x may be aligned substantially perpendicular to the first direction L. The longitudinal axes L_x may be substantially parallel to the second direction H.

The tip portion may comprise a plurality of planar wall sections. The planar wall sections may form the internal cavities and/or tubes. One or more of the plurality of planar wall sections may be aligned such as to extend substantially parallel with the second direction H. The planar wall sections may extend in a direction substantially parallel to the longitudinal axes L_x. Each planar wall section may comprise two or more connected portions. The connected portions may be angled with respect to one another at an angle Θ⁴ or Θ⁵. A line formed by the intersection of the connected portions may be substantially perpendicular to the radial direction. The line formed by the intersection of the connected portions may be substantially parallel to the longitudinal axes L_x and/or second direction H. The angle Θ⁴ or Θ⁵ may be selectable, e.g. to vary the strength of the internal cavities in a particular direction, such as the radial direction.

In an untwisted aerofoil structure condition, the longitudinal axes L_x may extend substantially in a direction of rotation R of the aerofoil structure. In a twisted aerofoil structure condition, the longitudinal axes L_x may extend at an angle ψ relative to the direction of rotation R of the aerofoil structure.

The tip portion may be hollow. The tip portion may include outer material portions having a first strength and one or more internal regions having a second strength. The second strength of the internal regions may be lower than the first strength of the outer material portions.

The aerofoil portion may have a composite structure. The aerofoil portion may have an end and the tip portion may be a separate item attached to the end of the aerofoil portion. The tip portion may be bonded to the end of the aerofoil portion. The tip portion may be replaceable with respect to the aerofoil portion.

The tip portion may comprise an abradable tip. Additionally or alternatively, the tip portion comprises an abrasive material.

In one arrangement said aerofoil structure may comprise a compressor blade, e.g. a compressor fan blade, of a gas turbine engine. In an alternative arrangement said aerofoil structure may comprise a turbine blade of a gas turbine engine. The present disclosure may also provide a gas turbine engine having an aerofoil structure as described above.

It will be appreciated that the above arrangement may allow for the tip or the aerofoil structure to collapse or crush under excessive tip loads which may be experienced during, for example, a bird strike, and that such collapsing may be in preference to the main body of the aerofoil structure experiencing load which might otherwise damage it. In essence, the present disclosure may limit the extent of any damage imparted as a result of excessive load on the aerofoil structure. In addition, a higher degree of strength in a second direction which may be, generally, in the same general direction as the direction of rotation R, may provide the aerofoil structure with sufficient strength to allow the tip to interact with an outer casing during lower load conditions during which it is able to abrade the tip or the casing without imparting any damaging or excessive load on the main body of the aerofoil structure itself.

Brief Description of the Drawings Exemplary embodiments of the present disclosure will be more fully described by way of example only with reference to the accompanying drawings in which:

Figure 1 , is a general cross-sectional view of a typical gas turbine engine; Figure 2, is a general side elevation of a compressor fan blade incorporating the present disclosure;

Figure 3, is a simplified view of a compressor fan blade illustrating the degree of twist often applied to them and further illustrating the tip region;

Figure 4, is a schematic front partial elevation of the gas turbine engine of figure 1 and identifies some of the directions and axes referred to in the application;

Figures 5 and 6 are schematic plan view representations of the compressor fan blades in normal and abnormal operation and illustrate the degree of deflection that may be experienced;

Figure 7 is a schematic side sectional view of the an aerofoil structure according to a first embodiment of the present disclosure;

Figure 8 is an exploded view of the tip portion of the blades shown in figures 2 and 3 and illustrates in hidden detail the region to which an anisotropic structure may be applied according to a second embodiment of the present disclosure; Figures 9 and 0 are schematic pian views of the compressor fan blade in normal and abnormal angular positions and illustrate the difference in alignment of the internal structure applied to the tip by the present disclosure; Figures 11 to 18 illustrate various forms of collapsible or crushable structure that may be applied to the tip of the blade of figures 2 and 3;

Figure 18b illustrates the structure of figure 18 in combination with features from figure 16 to form crushable wa!is; and

Figure 19 illustrates a still further angular arrangement of the anisotropic structure. Detailed Description Referring briefly to figures 1 to 4, a turbofan gas turbine engine 10, comprises in flow series an inlet 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and an exhaust 22. The fan section 14 comprises a fan rotor 24 carrying a plurality of circumferentially spaced radially outwardly extending fan blades 26. The fan blades 26 are arranged in a bypass duct 28 defined by a fan casing 30, which surrounds the fan rotor 24 and fan blades 26. The fan casing 30 is secured to a core engine casing 34 by a plurality of circumferentially spaced radially extending fan outlet guide vanes 32. The fan rotor 24 and fan blades 26 are arranged to be driven by a turbine (not shown) in the turbine section 20 via a shaft (not shown). The compressor section 16 comprises one or more compressors (not shown) arranged to be driven by one or more turbines (not shown) in the turbine section 20 via respective shafts (not shown). The engine includes a longitudinally extending centre line CL around which the blades rotate in a direction R.

An exemplary fan blade 26 to which the present disclosure can be applied is shown more clearly in figure 2. The fan blade 26 comprises a root portion 36 at a radially inner end and an aerofoil portion 38. The root portion 36 is arranged to locate in a slot 40 in the rim of the disc 42 of the fan rotor 24, and for example the root portion 36 may be dovetail shape, fir-tree shape, or other conventional shape, in cross-section and hence the corresponding slot 40 in the rim of the disc 42 of the fan rotor 24 is a similar shape. The aerofoil portion 38 has a leading edge 44, a trailing edge 46, a tip 48, and a chord 43 extending from the leading edge 44 to the tailing edge 46 at a radially outer end remote from the root portion 36 and the fan rotor 24. A concave pressure surface 50 extends from the leading edge 44 to the trailing edge 46 and a convex suction surface 52 also extends from the leading edge 44 to the trailing edge 46. Referring now more particularly to figures 5 and 6, which illustrate the compressor fan blades schematically and show the normal and abnormal modes of operation, it will be appreciated that incoming air is represented by arrow I and the direction of rotation by arrow R. In normal operation (figure 5) the incoming air is simply captured by blades 26 as they move perpendicular thereto and the pressure side 50 acts to compress the air and move it rearwardly in the general direction of arrow P. In normal use the blades 26 are not subjected to excessive loads and do not materially deflect. In the event that the blades are subjected to an adverse impact, such as may be experienced during a bird strike or blade off event, the blade may be caused to deflect or (un)twist into the abnormal position shown as 26b in figure 6. In the abnormal position of the blade the tip is twisted in the direction of arrow T such as to show an abnormal profile to the incoming air I and an abnormal profile to the casing 30 against which the tip of the blade operates. The change in profile will result in portions of the tip 48 coming into contact with the casing 30 or any abradable lining 54 provided on the inner surface 56 thereof. Whilst a certain degree of contact can normally be accommodated by abrading the abradable lining 54, excessive contact experienced during bird strikes, blade off events and the like may cause the tip of a normal blade or the casing itself to be damaged.

To this end, the present disclosure relates to an aerofoil structure, e.g. blade 26, comprising an aerofoil portion 38 and a tip portion 48, wherein the tip portion is collapsible with respect to the aerofoil portion. The tip portion is configured such that the tip portion collapses due to an interaction with the casing 30 occurring outside a normal mode of operation, e.g. during bird strikes, blade off events and the like. Figure 7 shows an aerofoil structure according to a first embodiment of the present disclosure. The tip portion 48 comprises an isotropic structure 49 which is made from a material with isotropic properties. The isotropic structure 49 is collapsible with significantly less stiffness and strength than the aerofoil portion 38, for example a carbon composite blade. The isotropic structure 49 may comprise for example a metal foam material. Alternatively, the isotropic structure may comprise a low density sintered metal (such as titanium or nickel), a closed or open cell foam (again such as titanium or nickel) with or without cells of varying size, plastic materials or any other isotropic material. The isotropic material may be chosen to be sufficiently stiff so that it can operate as a cutting edge and enable the tip to cut into the fan casing liner during normal engine operation.

In a second embodiment of the present disclosure, the aforementioned problem may also be addressed by providing the tip 48 in the form of an anisotropic structure provided within at least a region of the tip (shown at 58 in figure 8). The anisotropic structure may have a first lower strength in a first direction L and a second higher strength in at least a second direction H.

In the particular arrangement shown in figures 9 and 10, the blade according to the second embodiment has a direction of rotation R and the first direction L is selected to be angled at an angle Θ relative thereto. This may be perpendicular to the direction of rotation or may be otherwise, but it is generally selected to provide a first, lower, degree of strength L in a direction angled to the normal direction of rotation R of the blade 26. The anisotropic structure is also arranged to provide a second, higher, strength in a second direction H which, as shown in figure 9, may be generally close to or parallel to the normal direction of rotation R. It will be appreciated that the second direction H may be other than parallel to the direction of rotation R but that it is generally selected such as to ensure the tip has a greater degree of strength in a direction generally parallel to direction R than it would have when the tip is distorted from its normal profile. Figure 10 illustrates the blade 28 in an abnormal tip position and where the angular relationship between the direction of rotation R and directions L and H have been altered accordingly, in this arrangement the anisotropic structure is now presenting a side profile to the direction of rotation R such that the low strength direction L is now at an angle Θ³ to direction R and the tip of the blade is now less strong and less able to resist deformation or collapse if it comes into contact with the casing 30 or the abradabie lining 56 provided thereon. The longitudinal axes L_x extend at an angle ψ relative to the direction of rotation R of the aerofoil structure when the blade is in this abnormal position. It will be appreciated that by presenting the anisotropic structure such that the lower strength direction thereof L is exposed to any tip to casing contact when in the abnormal position that the tip is configured such that it will prefer to crush or collapse rather than deform or dig into the casing or abradabie lining and that, consequently, the blade and casing will survive an abnormal loading and distortion by deformation or partial collapse of the tip rather than the blade experiencing catastrophic loading and suffering the consequences thereof. Whilst there exists a number of suitable anisotropic structures that may be employed in the present disclosure it has been found that the following examples are very effective and each is described herein by way of example and with reference to figures 11 to 18.

Figure 1 1 illustrates a first arrangement in which the anisotropic structure is formed of a plurality of cavities, which may for example be formed by tubular members 60, which may or may not be connected or linked to each other. Each tubular member 60 includes a longitudinal axis L_x and is preferably of a length sufficient to impart the desired degree of strength in the high strength direction H. It will be appreciated that the hollow nature of the tubular structure will provide a natural high strength along the tube that would resist tip bending during normal operation but the tube has significantly less strength in a direction perpendicular to axis L_x. if the tubular members 60 are aligned within region 58 of the tip 48 such that axis L_x extends in a direction substantially parallel to the direction of rotation R (as shown in figure 9) then this will provide the tip with the maximum strength possible for normal modes of operation of the blade 26. Twisting of the tip will expose the sides of the tubes to any forces exerted thereon and as the structure is less strong in this direction the tubes will preferably collapse rather than resist the experienced load. This collapse will allow the tip to collapse in preference to the excessive load being transmitted to the blade and / or casing. Compression of the tip may be accommodated by the collapse of the tubes 60 in the direction of arrow C which represents a radial direction of the blade as shown in figures 1 and 2. It will be appreciated that the tubes 60 have very little strength in this direction and would collapse relatively readily and that this would allow for the tip of the blade 26 to collapse under impact with the casing rather than be damaged or damage the casing or liner associated therewith. Figure 12 illustrates an alternative tubular arrangement the same as figure 11 with the exception of the tubes being oval in cross-section and having a vertical axis V_A extending substantially perpendicular to axis L_x and radially relative to the centre line CL of the engine itself. Such an arrangement has even less strength from the side than the arrangement of figure 1 but greater strength in the high strength direction H and may be used to good effect when these enhanced properties are required. Further, it will be appreciated that this arrangement will have a lower strength in the direction of arrow C and, thus, may be the preferred arrangement if casing damage is to be limited without overly distorting the blade tip shape.

Combinations of the above arrangements of figures 1 and 12 are also possible. For example, one section of the blade tip may be provided with the arrangement of figure 11 whilst another section may be provided with the arrangement of figure 12. Still further combinations are shown in figures 13 to 15 which, for convenience, show the tubes in cross-section only. In the arrangement of figure 13, intermediate and oval shaped spacer tubes separate the circular upper and lower tubes whilst figure 14 shows the opposite arrangement. Figure 15 shows a combination of shaped tubes may be used and further illustrates both circular and oval tubes may be used in combination with different sizes of tube. Oval and circular tubes may be used on top of each other as well as side by side. Such arrangements would allow for the local modification of the strength of the tip and may be desirable when complex tip twisting and deformation must be accommodated. (NB, figure 14 makes reference to the relative strengths of the structure in different directions and illustrate the differences by way of arrows in which the following abbreviations apply: S= stiff, L_s= less stiff.)

The tubes of the above arrangements may be joined to each other along their lengths. Joining can be by bonding or welding or any other such suitable joining technique.

Still further arrangements are shown in figures 16 and 17, both of which illustrate planar wail sections 70 extending in a plane P and being arranged such as to be aligned in a manner such that the plane P extends in a direction substantially parallel to the second, high strength, direction H. In this way, the planar sections act in the same manner as the tubes of figures 1 1 and 12 in as much as the structure has greater strength within the plane P than they have in a direction perpendicular or angled relative thereto, it is this property that is used to endow the structure with the high strength required in direction H and the lower strength in direction L. To further enhance the strength in direction H and weakness in direction L, the wail sections may be formed as two connected portions 72, 74 angled at an angle Θ⁴ or Θ⁵ as shown in figures 16 and 17 respectively. The greater the angle the greater the strength in direction H. It will also be appreciated that these structures will have relatively little strength in the direction of arrow C which, as discussed above, represents a direction substantially radial to the centre line CL of the engine. It will be appreciated that the smaller the angle Θ⁴ or Θ⁵ the less the strength will be in the direction of arrow C too. These can be arranged such that the strength in the direction opposite to C can be high in order to resist radial growth and creep, and apparently lower in direction of C due to lower stiffness in direction L. Figure 18 illustrates a still further arrangement in which a generally rectangular structure 80 having a length L_E, a width W and a longitudinal axis L_x is used to replace the individual arrangements of figures 11 to 17 and wherein the longitudinal axis L_x is aligned in the manner of figures 1 and 12. In effect, one or more of the structures 80 will have the same strength arrangements as discussed above and, when suitably aligned as shown they will provide the necessary high strength in the direction H and low strength in the direction L. Aligning the structures 80 accordingly will provide the desired strengths in the normal and abnormal conditions of the blade 26. As will be realised, other shapes are possible with three or more sides in cross-section, and these may be offset to give a bias direction for the collapse or crush. On a square or rectangular section, for instance, this bias can be imparted by making one of the sides shorter, by skewing the angles to for rhombic or parallelogram sections. Figure 18b illustrates one such combination, where the features of Figure 16 have been combined with Figure 18, (NB, figures 16 to 18 make reference to the relative strengths of the structure in different directions and illustrate the differences by way of arrows in which the following abbreviations apply:

weak.)

In addition to the above, it is noted that cavities or cells of varying sizes may also be used. This may provide anisotropic properties in an additional direction. For example, if smaller cells (i.e. the least stiff) are used at the very tip of the tip structure, then the collapse of the structure will emanate at the very tip thereby ensuring that the tip structure progressively collapses from the very tip, and discourages leaning of the tip structure. It will be appreciated that the above structures could comprise hollow structures in which the interior thereof 82 is either empty (including full or partial vacuum) or filled with a less strong material, for instance this material could be an open cell carbon foam, or Aerogel. For example, in applications where heating (e.g. due to the tip cutting into the casing liner) is significant, the cells may be filled with materials of high heat capacity and/or low thermal conductivity, e.g. Aerogel glass. Cooling of the tip 48 may be provided by collecting air at the leading edge 44 and/or aerofoil pressure surface 50, and guiding the air through the tip structure using channels formed with the cell structure before exhausting on the aerofoil suction surface. Alternatively or additionally, enhanced tip sealing may be provided by collecting high pressure air (for example from the stagnation point on the leading edge 44) and re-directed using channels within the tip structure to form an air curtain at the tip.

It will also be appreciated that the tip 48 may also be provided with an abrasive coating 90, or an abradabie coating 92. As for the isotropic material, the anisotropic structure may be provided with a cutting edge to cut into the fan casing liner during normal engine operation. The anisotropic structure may be sufficiently stiff to withstand such an interaction. It will also be appreciated that the longitudinal axis L_x of the above arrangements may be presented such that, in a normal mode of the blade 26, they are not parallel to the direction of rotation R, but angled at a first angle ψ_Ρ. The first angle ψ_Ρ may be less than the angle ψ at which the longitudinal axis L_x extends relative to direction R when the blade is in the abnormal position. Whilst such an arrangement may not provide maximum strength in the direction of rotation R in the normal mode, it may nonetheless still provide a greater degree of strength in the normal mode than it will in the abnormal mode.

Still further, with reference to Figure 19, it will be appreciated that one may vary the angular position of axis L_x depending on the position of the element along the chord 43 relating thereto on the tip 48. Such an arrangement would allow the tip to be modified such as to provide the tip with different degrees of strength in different directions at different points on the tip itself. This may be of advantage if the tip of the blade distorts unevenly when subjected to an adverse loading. Such an arrangement is shown diagrammaticaily in figure 19 in which the different angular relationships are shown at !_xi and L_X2 respectively.

The blade 26 as described above may be a compressor blade, such as a fan blade 26, or may, in certain circumstances, be a turbine blade (not shown). The blade 26 may be part of an engine 10 as discussed with reference to figure 1 above and the present disclosure extends to such an engine with such a blade 26. The blade 26 as described above may also be applied to ducted fans, e.g., future aircraft engine architectures, hovercraft propelling fans, ducted helicopter rear rotors, air-conditioning fans, wind tunnel propulsors, marine propulsors and marine power generators or any other ducted fan,

It will be appreciated that individual items described above may be used on their own or in combination with other items shown in the drawings or described in the description and that items mentioned in the same sentence as each other or the same drawing as each other need not be used in combination with each other. In addition the expression "means" may be replaced by actuator or system or device as may be desirable. In addition, any reference to "comprising" or "consisting" is not intended to be limiting any way whatsoever and the reader should interpret the description and claims accordingly. Index of Reference Numerals

S stiff

Ls less stiff

S_E equal stiffness

CL centre line

R direction of rotation

L first lower strength in a first direction

H second higher strength in at least a second direction

Θ angle between R and L

Θ³ angle between R and L during abnormal operation

L_x longitudinal axis

C direction of collapse

V_A vertical axis

P plane

e⁴ angle between connected portions 72, 74

Θ⁵ angle between connected portions 72, 74

I_E length

W width

Ψ angle between L_x and R 10 a turbofan gas turbine engine

12 inlet

14 fan section

16 compressor section

18 combustion section

20 turbine section

22 exhaust

24 fan rotor

26 fan blade

26b fan blade abnormal position

28 bypass duct

30 fan casing

32 fan outlet guide vanes

34 core engine casing

36 root portion

38 aerofoil portion

40 slot

42 disc

43 chord

44 leading edge

46 trailing edge

48 tip / tip portion

49 isotropic structure

50 concave pressure surface

52 convex suction surface

54 abradable lining

56 inner surface

58 anisotropic structure region of the tip 60 tubular members

70 planar wall sections

72 connected portion

74 connected portion

80 rectangular structure

82 interior

90 abrasive coating

92 abradable coating

Claims

1. An aerofoil structure for a gas turbine, the aerofoil structure comprising an aerofoil portion and a tip portion, the tip portion being provided at a radially outermost end of the aerofoil portion, wherein the tip portion is configured to be provided adjacent to a casing structure, and wherein the tip portion is collapsible with respect to the aerofoil portion, the tip portion being configured such that the tip portion collapses due to an interaction with the casing structure occurring outside a normal mode of operation.

2. An aerofoil structure as claimed in claim 1 , wherein the tip portion is configured such that the tip portion does not collapse during the normal mode of operation.

3. An aerofoil structure as claimed in claim 1 or 2, wherein the tip portion is configured such that the tip portion collapses in the event the aerofoil structure twists about a radial axis beyond a threshold value.

4. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion is at least partially formed of an isotropic structure.

5. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion is at least partially formed of an anisotropic structure.

6. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion has a first strength in a first direction L and a second strength in a second direction H; the strength in the first direction being lower than the strength in the second direction.

7. An aerofoil structure as claimed in claim 8, wherein in an installed configuration the aerofoil structure rotates in a direction R and wherein the first strength in the first direction L is angled at an angle Θ relative to the direction of rotation R.

8. An aerofoil structure as claimed in claim 7, wherein the collapsible tip portion is configured such that the angle Θ varies along the chord of the tip portion.

9. An aerofoil structure as claimed in any of claims 6 to 8, wherein in an installed configuration the aerofoil structure rotates in a direction R and wherein the second strength in the second direction H is substantially parallel to the direction of rotation R.

10. An aerofoil structure as claimed in any of claims 6 to 9, wherein the tip portion has a third strength in a third direction C; the strength in the third direction being lower than the strength in the second direction H.

11. An aerofoil structure as claimed in claim 10, wherein the third strength in the third direction C is substantially in a radial direction.

12. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion includes one or more internal cavities.

13. An aerofoil structure as claimed in claim 12, wherein the internal cavities have longitudinal axes L_x.

14. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion comprises a plurality of tubes each having longitudinal axes L_x which extend substantially parallel to each other.

15. An aerofoil structure as claimed in claim 4, wherein the plurality of tubes have a substantially circular cross-section.

16. An aerofoil structure as claimed in claim 14, wherein the plurality of tubes have a substantially oval cross-section.

17. An aerofoil structure as claimed in claim 14, wherein the plurality of tubes comprises one or more tubes having a substantially circular cross-section and one or more tubes having a substantially oval cross-section.

18. An aerofoil structure as claimed in claim 14, wherein the plurality of tubes have a substantially rectangular cross-section.

19. An aerofoil structure as claimed in any of claims 14 to 18, wherein the tubes are joined to each other along their length.

20. An aerofoil structure as claimed in claim 13, wherein the internal cavities are formed by planar wall sections extending in a direction substantially parallel to the longitudinal axes L_x.

21. An aerofoil structure as claimed in claim 20, wherein each planar wall section comprises two or more connected portions angled with respect to one another at an angle Θ⁴ or Θ⁵.

22. An aerofoil structure as claimed in claim 21 , wherein the angle Θ⁴ or Θ⁵ is selectable to vary the strength of the internal cavities in a particular direction.

23. An aerofoil structure as claimed in any of claims 13 to 22, when dependent on claim 6, wherein the longitudinal axes L_x are aligned at an angle Θ² to the first direction L.

24. An aerofoil structure as claimed in any of claims 13 to 23, wherein, in an untwisted aerofoil structure condition, the longitudinal axes L_x extend substantially in a direction of rotation R of the aerofoil structure and, in a twisted aerofoil structure condition, the longitudinal axes L_x extend at an angle ψ relative to the direction of rotation R of the aerofoil structure.

25. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion includes outer material portions having a first strength and one or more internal regions having a second strength, the second strength of the internal regions being lower than the first strength of the outer material portions.

26. An aerofoil structure as claimed in any of the preceding claims, wherein the aerofoil portion has a composite structure.

27. An aerofoil structure as claimed in any of the preceding claims, wherein the aerofoil portion has an end and wherein the tip portion is a separate item attached to the end of the aerofoil portion.

28. An aerofoil structure as claimed in claim 27, wherein the tip portion is bonded to the end of the aerofoil portion.

29. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion is replaceable with respect to the aerofoil portion.

30. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion comprises an abradable tip.

31. An aerofoil structure as claimed in any of the preceding claims, wherein the tip portion comprises an abrasive material,

32. A compressor fan blade of a gas turbine engine comprising the aerofoil structure as claimed in any of claims 1 to 31.

33. A turbine blade of a gas turbine engine comprising the aerofoil structure as claimed in any of claims 1 to 31.

34. A gas turbine engine comprising an aerofoil structure as claimed in any of claims 1 to 3 .

35. An aerofoil structure, compressor fan blade, turbine blade or gas turbine, substantially as described herein, with reference to and as shown in the accompanying drawings.