Blade Mounting

Abstract

The angular nature of corners 3 to roots 2 of a conventional blade 1 used in a gas turbine engine has resulted in relatively thick fan casings in order to contain any blade fragments from these corners 3 as a result of blade failure. By angling the end face 26 of the root 22 such that it is nearer to perpendicular to an axis of curvature 27 for a blade 21 the corners 23 are reduced in their angular nature. In such circumstances the potential for penetration of a fan casing through impingement with such corners 23 is reduced and the fan casing thickness can more confidently be thinned reducing overall weight. Furthermore by appropriate profiling of an aerofoil 24 adjacent to the front and trailing edges 20 of the aerofoil about the root 22 and face 26 further reductions in angular parts of the blade can be achieved.

Description

The present invention relates to blade mountings and blades utilised in gas turbine engines.

Operation of gas turbine engines is relatively well known and includes a number of aerofoil blades secured in mountings in different stages of the gas turbine engine. These blades are generally secured through root mountings which may take the form of dovetail root sections which enter a reciprocally shaped slot in order to secure the blade to a rotor disc. Normally the airfoils forming the blade are curved. In such circumstances front and rear edges of the blade root are cut to provide an orthogonal flat face for consistency with the rotor disc edge surfaces at the front and rear edges of the blade. Thus, these blade root edges include relatively sharp corners and angular parts.

It will be understood that blades within a gas turbine engine rotate at relatively high speeds. In such circumstances it is possible for these blades to fail and therefore sections of the blade to be projected with some force upon disintegration of the blade. Angular and pointed parts may exacerbate impact problems.

The acute corners if they impact against a casing when a fan blade fails can cause problems. The acute corners concentrate impact load from the relatively heavy root section of the blade upon disintegration. In order to prevent penetration though the engine casing it will generally be thicker in cross section to ensure that the blade will not puncture and pass through the casing. Clearly, additional thickness to a fan casing adds considerably to the necessary weight of the fan casing with detrimental effects upon engine operational efficiency. It is found that a 1 mm increase in thickness in a large fan casing can add approximately 16 kg to overall weight.

In accordance with a, first aspect of the invention, there is provided a blade for a gas turbine engine, the blade rotatable in use about an axis of rotation, the blade comprising a root for securing the blade, the root curved for alignment in use with a mounting slot, characterised in that an end of the root is substantially perpendicular to the axis of curvature of the root.

Alternatively, both ends of the root may be substantially perpendicular to the axis of curvature of the root.

According to a second aspect of the invention, there is provided a blade for a gas turbine engine, the blade rotatable in use about an axis of rotation, the blade comprising a root for securing the blade, the root curved for alignment in use with a mounting slot, characterised in that an end of the root makes an angle of substantially 25 degrees to the plane perpendicular to the axis of rotation.

Alternatively, both ends of the root may make an angle of substantially 25 degrees to the plane perpendicular to the axis of rotation.

In either the first or the second aspect of the invention, the blade may comprise an aerofoil which extends from the root and a portion of the aerofoil adjacent to the end of the root may be rounded so as to provide a smooth transition between the aerofoil and the end of the root.

At least one end of the root may be chamfered or truncated, so as to reduce the axial length of the root.

The blade may be part of a gas turbine engine.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:—

FIG. 1 is a schematic cross section of a prior blade;

FIG. 2 is a schematic cross section of a blade in accordance with the present invention;

FIG. 3 is a schematic end view of a prior blade;

FIG. 4 is a schematic end view of a blade in accordance with the present invention; and,

FIG. 5 is a schematic view of a blade mounting.

As indicated above, it is known to attach fan blades to a fan rotor disc using a curved dovetail root which is aligned axially. The fan blade root is generally in the form of as indicated a dovetail which locates in a corresponding reciprocal slot in the fan rotor disc. End surfaces of the root are formed so that they are parallel with the front and rear surfaces of the fan rotor disc and so orthogonal to the engine rotational axis. Such orthogonal presentation of the ends of the root sections for the blades creates sharp and angular portions which as indicated may concentrate imparted load upon impact with a fan casing should the blade become detached. Such potentially heavier impact forces require a thicker casing which in turn adds significantly to overall engine weight and therefore reduces efficiency.

FIG. 1 provides an illustration of a prior blade profile cross section. Thus, the blade 1 has a root 2 which incorporates angular corners 3 on the pressure side of an aerofoil 4. These acute corners as indicated previously will act through the potentially narrow impact zone of the acute point 5 of each corner 3 to impart relatively high impact loads. Such impact loads necessitate thicker and more robust casing profiles and therefore add to overall weight.

It would be appreciated that the corners 3 are created by desire to have end faces 6 of the root 2 which are orthogonal to the axis of rotation for an engine incorporating the blade 1 in a blade assembly. It will be understood that the root end surfaces 6 will generally be continuous and aligned with front and rear faces of a fan rotor disc as will be described later.

It is reducing the effects of these corners 3 in terms of their potential for detrimental impact and penetration of a fan casing which the present blade and blade mounting addresses.

FIG. 2 illustrates a blade mounting arrangement in accordance with the present invention. A blade 21 with a similar aerofoil 24 to that described with regard to FIG. 1 is provided. Thus, the aerofoil 24 extends from a root 22 which is utilised to secure the blade in a mounting arrangement comprising a number of aerofoil blades 24 secured to a fan rotor disc. As can be seen the root 22 now includes end surfaces 26 which are substantially perpendicular to an axis of curvature 27 for the blade 21 or at least turned towards that orientation. Thus, the ends 26 of the root 22 as indicated previously have a dovetail shaping and are relatively square ended so that the corners 23 of the roots 22 are less acute and of a smaller dimension. Thus, the impact forces should these corners 23 strike upon a fan casing as a result of failure of the blade 21 will be less severe. By such angling of the end faces 26 there is a reduction of the severity of the angle 23 in comparison with angle 3 in lever 1.

It will be appreciated that turning of the faces 26 between the orthogonal plane depicted in FIG. 1 toward the perpendicular orientation relative to the axis of curvature 27 in FIG. 2 will reduce the angular nature of the corner 23. In such circumstances the faces 26 may be presented at orientations other than perpendicular to the axis 27 in order to achieve a reduction in the angular nature of the corners 23 but it has been found that perpendicular orientation provides best results. In such circumstances typically, and this will depend upon the severity of the axis of curvature 27, the faces 26 will be at an angle 28 in the order of 25 degrees to the orthogonal plane, that is to say the plane 29 perpendicular to the axis of rotation for an engine incorporating the blades 21 and typically consistent with the front and rear surfaces of a mounting rotor disc in which the blades are secured through the roots 22.

Previously as shown in FIG. 1 the end face 6 of the root 2 will generally be in a plane 9 which extends orthogonally upwards from the end edges 10 of the blade 1. The end plane 29a in FIG. 2 is consistent with the plane 9 depicted in FIG. 1 that is to say orthogonal to the end edges 20 of the blade aerofoil 24. In such circumstances due to the angle of presentation of the end faces 26 the axial length of the blade 21 is increased. Thus, it may be necessary to increase the axial length of the fan rotor disc rim in order to accommodate the increased axial length of the blade 21 due to the presentation of the faces 26. Alternatively, a chamfer portion of the root 22 which extends beyond the plane 29a may be removed in order to truncate the root 22 in this section and so ensure that the blade 21 remains within the normal axial profile length or the slot of the rotor disc mounting but in such circumstances it will be appreciated that the length of engagement between the root 22 and the slot in the rotor disc may be reduced causing a reduction in mounting strength which may be unacceptable in a high speed rotating device.

FIGS. 3 and 4 respectively illustrate schematically end views of a prior blade (FIG. 3) and a present blade (FIG. 4). Thus, as can be seen in FIG. 3 a blade 31 again incorporates an aerofoil 34 extending from a root 32. As shall be described later the root 32 is of a dovetail nature and is slid into a reciprocally shaped slot in a rotor mounting disc in order to secure the blade 31 in use. In the prior blade based on FIG. 3 it will be noted that the root 32 at a front edge 35 is generally flat and as described above creates the angular corner 35 for the blade 31 which may cause disproportionate impact damage with a fan casing should the blade 31 fail and disintegrate. It is the severity of the angle at 35 which results in acute corners with sharp points (5 in FIG. 1) causing high impact loads.

In FIG. 4 a blade 41 in accordance with the present invention is illustrated with an end face 36 to the root 42. Thus, the blade 41 as previously incorporates an aerofoil blade 44 which develops from the root 42. As compared to the sharp edge 35 depicted in FIG. 3 it will be noted that there is profiling of the transition between the root 42 and in particular the face 46 to the adjacent portions 43 of the aerofoil 44. In these transition portions 43 there is less acute angling of the aerofoil 44 which in combination with the produced angular corners in the root 42 further limits the potential for sharp point impacts with a fan casing which may as indicated cause greater damage.

The actual profile cross section in the transition portions 43 may be chosen in accordance with operation requirements and it will be understood is dependant upon operational flow stressing on the aerofoil 44′ in use. It will be understood that the aerofoil 44 may be stressed such that the transitional profile in portions 43 should not reduce the overall operational efficiency or fatigue life of the blade 41 in use.

The present blade will generally reduce the potential for the blade root in particular to cause damage to incident portions of the fan casing within which the fan blade of the present mounting arrangement and a fan blade assembly is secured. It will be understood if there is greater control of potential impact damage it is possible to more confidently use thinner fan casing thicknesses giving a reduction in overall weight.

FIG. 5 provides a perspective view of a prior blade mounting 50. Thus, a blade 51 includes an aerofoil 54 and a root portion 52. The root 52 and therefore the blade 51 are secured in a fan rotor disc 60 through a slot 61. As can be seen the root 52 has a dovetail cross section and is slid along the length of the slot 51 during assembly such that an end face 56 is generally consistent with end surfaces 62 of the fan disc 60 about the slot 61. The root 52 is secured in the slot 51 through a slide end retainer assembly 63 to ensure appropriate presentation of the blade 51 in use. The fan rotor disc 61 incorporates annulus filler fixings for location of filler mountings between the blades 51 in use.

As can be seen the edges of the root 52 create angular corners (3 in FIG. 1) and it is the potential for these angular corners to impinge upon fan casings which is avoided by the present configuration for the root (22 in FIG. 2) and the transition to the blade overall.

The present blade and blade mounting arrangement achieves an overall reduction in weight by reducing the necessary thickness of fan casing to ensure that there is no penetration of that casing if the blade should fail. This advantage is achieved through altering the angle of the ends of the root portion along with additional profiling of the blade adjacent of these root ends. In short, by reducing the acuteness of the points the impact load area of any fragments from blade failure is broadened and therefore the impact force spread over a great area of the fan casing.

Alternations and modifications to the present blade and blade mounting will be appreciated by those skilled in the art and as indicated above in particular with regards to the actual variation in the root end face angle to achieve best effect in terms of maintaining fan disc axial width in the direction of critical length whilst reducing as indicated the angular nature of the corners for the root.

Claims

1. A blade for a gas turbine engine, the blade rotatable in use about an axis of rotation, the blade comprising a root for securing the blade, the root curved for alignment in use with a mounting slot, characterised in that an end of the root is substantially perpendicular to the axis of curvature of the root.

2. A blade as claimed in claim 1, in which both ends of the root are substantially perpendicular to the axis of curvature of the root.

3. A blade for a gas turbine engine, the blade rotatable in use about an axis of rotation, the blade comprising a root for securing the blade, the root curved for alignment in use with a mounting slot, characterised in that an end of the root makes an angle of substantially 25 degrees to the plane perpendicular to the axis of rotation.

4. A blade as claimed in claim 3, in which both ends of the root make an angle of substantially 25 degrees to the plane perpendicular to the axis of rotation.

5. A blade as claimed in claim 1, in which the blade comprises an aerofoil which extends from the root and a portion of the aerofoil adjacent to the end of the root is rounded so as to provide a smooth transition between the aerofoil and the end of the root.

6. A blade as claimed in claim 1, in which at least one end of the root is chamfered or truncated, so as to reduce the axial length of the root.

7. A gas turbine engine incorporating a blade as claimed in claim 1.

8. A gas turbine engine incorporating a blade as claimed in claim 2.

9. A gas turbine engine incorporating a blade as claimed in claim 3.

10. A gas turbine engine incorporating a blade as claimed in claim 4.

11. A gas turbine engine incorporating a blade as claimed in claim 5.

12. A gas turbine engine incorporating a blade as claimed in claim 6.