GB2471466A

GB2471466A - Fan casing for a turbofan gas turbine engine

Info

Publication number: GB2471466A
Application number: GB0911253A
Authority: GB
Inventors: Clare Louise Pool; Julian Mark Reed
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-06-30
Filing date: 2009-06-30
Publication date: 2011-01-05
Also published as: GB0911253D0

Abstract

A fan casing is provided for a turbofan gas turbine engine. The casing has an annular shell 1 which, in use, extends circumferentially around the fan blades of the engine. The shell is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event. The casing further has a leading edge reinforcing rib 6 and a blade root 9 reinforcing rib each extending radially from and circumscribing the annular shell. The axial position of the leading edge reinforcing rib is between o first plane i perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running, and a second plane ii perpendicular to the casing axis and spaced axially forwards of the first plane by a distance which is half the chord length at the tips of the fan blades. The axial position of the blade root reinforcing rib is rearwards of the most rearwards predicted point of impact on the casing of the fan blade root during a blade-off event.

Description

FAN CASING FOR A TURBOFAN GAS TURBINE ENGINE

The present invention relates to a fan casing for a turbofan gas turbine engine.

Turbofan gas turbine engines for powering aircraft conventionally comprise a core engine which drives a fan.

The fan comprises a number of radially extending fan blades mounted on a fan rotor which is enclosed by a generally cylindrical, or frustoconical, fan casing. The fan blades are driven at high rotational speeds about an axis of rotation to provide the first stage of compression to the working medium gas (i.e. air in this region of the engine), and propulsive thrust.

The fan has an annular, radially outer casing, a primary function of which is to provide the flow path outer boundary for the ar passing through the fan.

The certifying regulations for turbofan engines require that if part, or all, of a fan blade becomes detached, the escape of the detached blade or blade fragments is prevented such that further damage to the aircraft or surrounding objects does not occur. If such a failure were to occur, the blade may be hurled outwardly with considerable energy and at high speed.

An important task of the fan casing is, therefore, to contain a detached blade or blade fragments. However, because of the size of the fan blades and the speeds at which they may be released in the event of failure, this is a non-trivial problem. One known approach to the design of the fan casing is for it to comprise a solid annular shell formed of a strong metal such as Armco or titanium. The shell is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event. The shell may be further strengthened by ribs in regions of predicted blade failure, as described for example in EP A 0965731, the ribs extending radially outwardly from the annulus to circumscribe its outer periphery. Another known approach to the design of the fan casing is for it to comprise an aluminium, typically an isogrid, shell around which is provided a fibrous wrapping formed of e.g. aramid fibres (such as Kevlar) . In the event of a blade-off event, the blade or blade fragments penetrate the aluminium shell, but are contained by the wrapping.

Other typical tasks of the fan casing are to: * Accommodate acoustic liner panels, a fan track liner and ice impact panels; * Support the intake attached to a front flange of the casing; * Support accessories, raft brackets, electrical harnesses, ground support equipment etc.; * Act as a firewall.

The fan casing can account for up to about 12% of the engine weight. Further, when engine bypass ratios are increased, fan blade radii are also increased and so larger containment structures are required.

Thus, it is desirable to provide improvements to fan casings which reduce the weight of the casing while not reducing its capabilities.

In a first aspect, the present invention provides a fan casing for a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and a leading edge reinforcing rib and a blade root reinforcing rib each extending radially from and circumscribing the annular shell; wherein: the axial position of the leading edge reinforcing rib is between a first plane perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running, and a second plane perpendicular to the casing axis and spaced axially forwards of the first plane by a distance which is half the chord length at the tips of the fan blades, and the axial position of the blade root reinforcing rib is rearwards of the most rearwards predicted point of impact on the casing of the fan blade root during a blade-off event.

The "chord length at the tip of a fan blade" is the distance between the radially outer point of the leading edge of the fan blade and the radially outer point of the trailing edge of the fan blade.

As the skilled person would recognise, the ability of the shell to contain the blade or blade fragments during a blade-off event, and the most rearwards predicted point of impact on the casing of the fan blade root event can be determined through computer simulations. Indeed, engine certification requires the performance of such simulations, as well as the performance actual blade-off tests.

Preferably, the axial position of the blade root reinforcing rib is from 200 to 400 mm axially rearwards of the most rearwards predicted point of impact.

The fan casing may further have a mid-chord reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the mid-chord reinforcing rib being between the first plane, and a third plane perpendicular to the casing axis and containing the radially outer mid-chord points of the fan blades when the engine is running.

The fan casing may further have a trailing edge reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the trailing edge reinforcing rib being between the third plane, and a fourth plane perpendicular to the casing axis and containing the radially outer points of the trailing edges of the fan blades when the engine is running.

The fan casing may have no other reinforcing ribs extending radially from and circumscribing the annular shell. Thus the casing may have (i) only the leading edge and root ribs, (ii) only the leading edge, root and mid-chord ribs, or (iii) only the leading edge, root, mid-chord and trailing edge ribs.

The possible arrangements of ribs according to this aspect of the invention allow a relatively lightweight casing to be produced, but one which still meets all performance requirements. The casing may be particularly beneficial to use, for example, in an engine with relatively long fan blades, the leading edge and blade root reinforcing ribs strengthening and stiffening the shell against the impacts of those parts of the blade which generally carry the most weight. Other flanges and attachment features may contribute to the reinforcement of the casing, but generally to a lesser extent.

In a second aspect, the present invention provides a fan casing for a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and a leading edge reinforcing rib and a mid-chord reinforcing rib each extending radially from and circumscribing the annular shell; wherein: the casing has no other reinforcing ribs extending radially from and circumscribing the annular shell, the axial position of the leading edge reinforcing rib is between a first plane perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running, and a second plane perpendicular to the casing axis and spaced axially forwards of the first plane by a distance which is half the chord length at the tips of the fan blades, and the axial position of the mid-chord reinforcing rib is between the first plane, and a third plane perpendicular to the casing axis and containing the radially outer mid-chord points of the fan blades when the engine is running.

The casing of this aspect can be beneficial to use, for example, when the shell and the leading edge and mid-chord ribs provide sufficient strength and rigidity to withstand the blade root impact without the benefit of a specific blade root reinforcing rib.

In a third aspect, the present invention provides a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and only one reinforcing rib extending radially from and circumscribing the annular shell; wherein the axial position of the reinforcing rib is at the point on the casing which would experience the greatest deflection during a blade off event were the rib not present.

In some circumstances a single rib can sufficiently strengthen and rigidify the shell such that further ribs are not required and weight savings achieved.

The following optional features pertain to each of the above aspects of the invention.

The fan casing typically further has a containment hook which extends radially inwardly from the annular shell and circumscribes its inner periphery, the containment hook being axially positioned forward of a plane perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running (i.e. the first plane of the first and second aspects) . The fan casing typically further has a front flange extending from the shell for attaching the engine intake to the casing. The fan casing typically further has a rear flange extending from the shell for attaching the casing to the engine, e.g. via a rear engine casing.

Preferably, the or each rib extends radially outwardly from the annular shell and circumscribes its outer periphery. This prevents the ribs from interfering with e.g. the fan track liner, ice impact panels and acoustic liner panels mounted on the inside of the casing. However, in some circumstances a radially inwardly extending rib may be adopted, particularly when there is limited space to the outside of the casing e.g. due to the proximity of externals or cowl doors.

Preferably, the annular shell is metallic. For example, the shell may be formed of titanium-based alloy.

Typically, the annular shell is solid. However, in some circumstances, the shell may have internal cavities, e.g. it may be formed as a foamed metal sandwich.

The or each rib may have a C-shaped cross-section, as this generally provides the best solution for strength, ease and cost of manufacture, and weight.

The annular shell may have frustoconical section which extends between at least a plane (i.e. the first plane of the first and second aspects) perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running and a plane perpendicular to the casing axis and containing the radially outer points of the trailing edges of the fan blades when the engine is running (i.e. the fourth plane of the first aspect), the section converging from the first plane to the fourth plane.

Further aspects of the invention provide (i) a combination of the fan casing of any one of the previous claims and the fan blades around which the fan casing circumferentially extends, and (ii) a turbofan gas turbine engine having the fan casing of any one of the previous aspects.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic meridional cross-section through a fan casing of a turbofan gas turbine engine according to an embodiment of the present invention; and Figures 2(a) to (j) show respective cross-sectional shapes of various possible reinforcing ribs.

Figure 1 shows a schematic meridional cross-section through a fan casing of a turbofan gas turbine engine according to an embodiment of the present invention. The fan casing has an annular shell 1 which extends circumferentially around the fan blades of the engine. The cross-section of the shell on only one side of the casing axis is shown in Figure 1, the opposite cross-section on the other side of the axis being a substantially identical mirror image.

In operation, fan blades tend to "unwind". The positions swept by the radially outer points of the leading edges, trailing edges and mid-chord positions of the fan blades when the engine is running are indicated by respective arrows L, T and M. Dashed lines indicate the positions of four planes which are perpendicular to the casing axis. First plane (i) passes through point L. Second plane (ii) is spaced axially forward of the first plane by a distance which is half the chord length at the tips of the fan blades. Third plane (iii) passes through point M. Fourth plane (iv) passes through point T. The shell 1 has front 2 and rear 3 flanges. The front flange attaches to the engine intake (not shown) by a series of bolts. The front flange is scalloped and circumferentially non-continuous to reduce weight and ensure that only local casing deformations affect the bolts. The rear flange attaches the casing to the rear engine casing (not shown) by a further series of bolts.

Typically, the rear flange is circumferentially continuous, although it may have weight reduction features.

Front acoustic panels 4 are attached to the inner surface at front of the shell 1.

A containment hook 5 abuts the rearward edges of the front acoustic panels 4. The hook extends radially inwardly from the shell 1 and circumscribes its outer periphery. During a blade-off event, the blade tends to carry forward, as well as outward, in the casing. This forward movement is arrested by the hook, which traps the tip of the blade. The released blade then rotates about the trapped tip, with the root of the blade impacting the casing at a rearward position in the casing. The position of the most rearwards predicted point of impact on the casing of the fan blade root is indicated by the arrow I. Reinforcing ribs 6, 7, 8, 9 (described in more detail below) extend radially outwardly from the shell and circumscribe its outer periphery. The ribs strengthen and stiffen the shell in the event of blade-off events Rearward from the hook 5, the shell 1 has a largely frustoconical section 10 which extends to beyond position T and converges in the axial direction from the hook to position T. This section carries a fan track liner 11 which has a radially inner abradable honeycomb layer filled with epoxy resin and a radially outer polypropylene or aluminium alloy unfilled honeycomb layer. The fan track liner is formed from a circumferential arrangement of individually replaceable panels.

Rearward of the fan track liner 11, the honeycomb structure supports ice impact panels 13 which absorb the impact of ice shed from the fan blades.

Rearward of the ice impact panels 13, the shell diverges in a further frustoconical section 14. Rear acoustic panels 15 are positioned at the inner surface of the shell 1 in this section. Also an accessory flange 16 for fixing accessories and other externals to the casing extends radially outwardly from the shell in this section.

Typically, the flange is circumferentially non-continuous, although a continuous flange may be adopted at a reduced cost of manufacture but increased weight.

Reinforcing rib 6 is positioned between the first plane (i) and second plane (ii) . With the tendency of a released blade to carry forward, this rib strengthens and stiffens the shell at or close to the point of impact of the leading edge of the blade. The relative density of a blade varies along its chord, with a denser, stronger part of the blade typically being at the leading edge (where foreign object impact forces are generally higher) . Thus the casing distortions produced by the impact of the blade leading edge can be severe and reinforcing rib 6 typically plays an important role in containing the released blade and reducing the distortions experienced by the front flange 2.

Reinforcing rib 9 is positioned 200-400 mm rearwards of point I, i.e. behind the most rearwards predicted point of impact of the fan blade root during a blade-off event.

The blade root is a relatively massive part of the blade, and the larger fan blades found in high bypass engines have correspondingly more massive roots. Thus, particularly in such engines, the impact of the root on the casing can produce large distortions. Reinforcing rib 9 can assist in containing the root of the released blade and can reduce the distortions experienced by the rear flange 3.

Reinforcing rib 7 is positioned between the first plane (i) and third plane (iii), and reinforcing rib 8 is positioned between the third plane (iii) and fourth plane (iv) . These ribs help to further strengthen the shell 1 against the impact of a released blade and to reduce distortions in the casing.

The arrangement of ribs according to this embodiment of the invention allows a relatively lightweight casing to be produced. However, the shell 1 can still be strong enough to contain the blade or blade fragments in the event of a blade-off event. The careful positioning of ribs 6, 7, 8, 9, while locally strengthening the shell, also reduces the magnitude of shell distortions, particularly at important locations such as the front 2 and rear 3 flanges.

Indeed, a variant embodiment of the casing has only reinforcing rib 6 and reinforcing rib 9, as these ribs are particularly effective at reducing the magnitude of shell distortions at the front 2 and rear 3 flanges, and are located at or near to impact positions of relatively massive portions of the detached blade. Another variant embodiment of the casing includes reinforcing rib 7 along with reinforcing rib 6 and reinforcing rib 9, but no other ribs (i.e. does not include reinforcing rib 8) . When additional strengthening and stiffening is required relative to the two-rib variant, it is sometimes possible for rib 7 alone to provide the extra reinforcement.

Another embodiment of the casing has all the features of the embodiment shown in Figure 1, except for reinforcing rib 8 and reinforcing rib 9, i.e. the shell 1 has only reinforcing rib 6 and reinforcing rib 7. Such an embodiment, can be deployed when, for example, the further frustoconical section 14 of the shell is sufficiently strong and stiff to withstand the blade root impact without the benefit of rearward reinforcing rib 9.

Yet another embodiment of the casing has all the features of the embodiment shown in Figure 1, except the shell 1 has only one reinforcing rib. The axial position of the reinforcing rib is at the point on the casing which is predicted to experience the greatest deflection during a blade off event were the rib not present. Having only one rib, such an embodiment has the potential for significant weight savings.

In Figure 1 reinforcing rib 6 and reinforcing rib 9 are shown with rectangular-shaped cross-sections, while reinforcing rib 7 and reinforcing rib 8 have C-shaped cross-sections. In general, C-shaped cross-sections are preferred for all the ribs. However, other possible cross- sections for the ribs are L-shaped, I-shaped, half T-shaped, Y-shaped, half Y-shaped, triangular-shaped, rectangular-shaped etc. Further they may be solid or hollow. They may also follow a circular course around the shell 1 (i.e. parallel to the containment hook 5) or may follow a wavy course. Although the ribs circumscribe the shell, and are thus circumferentially continuous, they may have occasional cut-outs or holes which locally reduce their cross-sectional areas in order to reduce weight.

Figures 2(a) to (j) show respective cross-sectional shapes of various possible reinforcing ribs. Figures 2(b) and (d) show respectively a I-shape, and a half I-shape, both of which are used in conventional casings. Figures 2(a) and (h) are variants on a Y-shape. Figure (f) is a half Y-shape. Figures 2(c), (e), (g), (i) and (j) are variants on the preferred C-shape.

The ribs can be machined from a larger piece of material integral with the shell 1, or can be subsequently added to the shell by a process such as welding or material deposition. Indeed, a rib can be partly machined and partly formed by subsequent addition.

In general, the rib cross-sections avoid sharp corners, particularly where the rib joins to the shell, as these can produce high stresses which may lead to premature rib failure.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure.

Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

CLAIMS1. A fan casing for a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and a leading edge reinforcing rib and a blade root reinforcing rib each extending radially from and circumscribing the annular shell; wherein: the axial position of the leading edge reinforcing rib is between a first plane perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine s running, and a second plane perpendicular to the casing axis and spaced axially forwards of the first plane by a distance which is half the chord length at the tips of the fan blades, and the axial position of the blade root reinforcing rib is rearwards of the most rearwards predicted point of impact on the casing of the fan blade root during a blade-off event.
2. A fan casing according to claim 1, wherein the axial position of the blade root reinforcing rib is from 200 to 400 mm axially rearwards of the most rearwards predicted point of impact.
3. A fan casing according to claim 1 or 2 further having a mid-chord reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the mid-chord reinforcing rib being between the first plane, and a third plane perpendicular to the casing axis and containing the radially outer mid-chord points of the fan blades when the engine is running.
4. A fan casing according to any one of the previous claims further having a trailing edge reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the trailing edge reinforcing rib being between the third plane, and a fourth plane perpendicular to the casing axis and containing the radially outer points of the trailing edges of the fan blades when the engine is running.
5. A fan casing according to any one of the previous claims having no other reinforcing ribs extending radially from and circumscribing the annular shell.
6. A fan casing for a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and a leading edge reinforcing rib and a mid-chord reinforcing rib each extending radially from and circumscribing the annular shell; wherein: the casing has no other reinforcing ribs extending radially from and circumscribing the annular shell, the axial position of the leading edge reinforcing rib is between a first plane perpendicular to the casing axis and containing the radially outer points of the leading edges of the fan blades when the engine is running, and a second plane perpendicular to the casing axis and spaced axially forwards of the first plane by a distance which is half the chord length at the tips of the fan blades, and the axial position of the mid-chord reinforcing rib is between the first plane, and a third plane perpendicular to the casing axis and containing the radially outer mid-chord points of the fan blades when the engine is running.
7. A fan casing for a turbofan gas turbine engine, the casing having: an annular shell which, in use, extends circumferentially around the fan blades of the engine and which is sufficiently strong to contain the blade or blade fragments in the event of a blade-off event, and only one reinforcing rib extending radially from and circumscribing the annular shell; wherein the axial position of the reinforcing rib is at the point on the casing which would experience the greatest deflection during a blade off event were the rib not present.
8. A fan casing according to any one of the previous claims, wherein the or each rib extends radially outwardly from the annular shell and circumscribes its outer periphery.
9. A fan casing according to any one of the previous claims, wherein the annular shell is metallic.
10. A fan casing according to any one of the previous claims, wherein the or each rib has a C-shaped cross-section.
11. A combination of the fan casing of any one of the previous claims and the fan blades around which the fan casing circumferentially extends.
12. A turbofan gas turbine engine having the fan casing of any one of the previous claims.
13. A fan casing for a turbofan gas turbine engine as any one herein described with reference to and/or as shown in the accompanying drawings.