GB2471465A

GB2471465A - Fan casing for a turbofan gas turbine engine

Info

Publication number: GB2471465A
Application number: GB0911252A
Authority: GB
Inventors: Clare Louise Pool
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: GB0911252D0

Abstract

A fan casing is provided for a turbofan gas turbine engine. The casing has an annular shell 1. The casing further has a leading edge reinforcing rib 6 extending radially from the annular shell. The axial position of the leading edge reinforcing rib is between a first plane i perpendicular to the casing axis and 60 mm forward of the leading edges of the fan blades L, and a second plane ii perpendicular to the casing axis and 20 mm forward of the leading edges of the fan blades. The casing further has a fan track liner 11 mounted to the radially inward side of the annular shell. The liner provides a shroud for the tips of the blades. In the event of a blade-off event, the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade from penetrating the full thickness of the liner. The annular shell is sufficiently strong to contain those parts of the blade which do penetrate the liner.

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.

A further task of the casing is to provide adequate strength against ice impact events. Ice forms on and sheds from the fan blades regularly in use. Thus, typically, ice impact panels are mounted to the casing downstream of the blades.

Other typical tasks of the fan casing are to: * Accommodate acoustic liner panels and a fan track liner; * 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 about 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, a leading edge reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the leading edge reinforcing rib being between a first plane perpendicular to the casing axis and 60 mm forward of 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 20 mm forward of the radially outer points of the leading edges of the fan blades when the engine is running, and a fan track liner mounted to the radially inward side of the annular shell and extending circumferentially around the fan blades, the liner providing a shroud for the tips of the blades; wherein, in the event of a blade-off event, the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the trailing edge from penetrating the full thickness of the liner, the annular shell being sufficiently strong to contain those parts of the blade which do penetrate the liner.

The fan casing is particularly useful for use in combination with fan blades, such as swept fan blades, where ice impact areas are relatively far forward. In such cases ice can impact on the fan track liner. To provide protection against these impacts the fan track liner can be strengthened, but such strengthening effects the performance of the casing in its important task of containing a released blade. For example, a strengthened fan track liner can change the position at which a released blade strikes the annular shell.

In the fan casing of the present invention, the fan track liner is significantly strengthened relative to conventional liners, such that the liner is not only able to resist ice impacts, but actually absorbs a significant portion of the impact energy of a released blade. This multiple role for the liner is facilitated by the tendency towards lighter blades.

Thus in the fan casing of this aspect, the strengthening of the fan track liner is combined with careful positioning for the leading edge reinforcing rib.

In particular, the absorption of significant amounts of impact energy by the liner effects the position at which the released blade reaches the shell.

The fan casing may further have a root reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the root reinforcing rib being rearwards of the most rearwards predicted point of impact on the casing of the fan blade root during a blade-off event.

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 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 a third plane perpendicular to the casing axis and containing points on the radially outer chords of the fan blades which points are one third of the radially outer chordal distances from the leading edges when the engine is running, and a fourth plane perpendicular to the casing axis and containing points on the radially outer chords of the fan blades which points are two thirds of the radially outer chordal distances from the leading edges when the engine is running.

The "radially outer chord" of a blade is the straight line connecting the radially outer point of the leading edge of the blade and the radially outer point of the trailing edge of the blade. The "radially outer chordal distance" is then the length of that line.

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 a fifth plane perpendicular to the casing axis and 60 mm forward of the radially outer points of the trailing edges of the fan blades when the engine is running, and a sixth plane perpendicular to the casing axis and 20 mm forward of the radially outer points of the trailing edges of the fan blades when the engine is running.

Again, the mid-chord and/or the trailing edge reinforcing ribs are positioned forward of conventional positions for such ribs because the penetration of the liner by the released blade is delayed by the relative strength of the liner, whereby the released blade is typically further forward in the casing by the time it reaches the shell.

The fan casing may have no other reinforcing ribs extending radially from and circumscribing the annular shell.

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 fan casing may further have a containment hook which extends radially inwardly from the annular shell and circumscribes its inner periphery, the containment hook being axially positioned forward of the first plane.

Preferably, the minimum thickness of the shell between the containment hook and the leading edge reinforcing rib is at least 1.5 times greater than the minimum thickness of the shell between the leading edge reinforcing rib and a plane (i.e. the third plane) perpendicular to the casing axis and containing radially outer points on the chords of the fan blades which points are one third of the chordal distances from the leading edges when the engine is running. Such a thickness ratio can help to optimise the weight and strength of the casing.

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, only one reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the reinforcing rib being at the point on the casing which would experience the greatest deflection during a blade off event were the rib not present, and a fan track liner mounted to the radially inward side of the annular shell and extending circumferentially around the fan blades, the liner providing a shroud for the tips of the blades; wherein, in the event of a blade-off event, the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the trailing edge from penetrating the full thickness of the liner, the annular shell being sufficiently strong to contain those parts of the blade which do penetrate the liner.

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

Typically, the fan casing may further have 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.

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

Preferably, the fan track liner is sufficiently strong to prevent at least the trailing edge of the blade and adjacent parts of the blade up to one half of a chordal distance from the trailing edge from penetrating the full thickness of the liner.

Preferably, the fan track liner is sufficiently weak to allow at least the leading edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the leading edge to penetrate the full thickness of the liner. This can help the liner to provide better resistance to foreign object damage, hail and ice, and ice shed from blades and structures.

The fan track liner may have a radially outer honeycomb layer, a radially inner honeycomb layer, and a septum layer separating the two honeycomb layers.

Preferably, the cells of the radially inner honeycomb layer are filled with epoxy resin. Preferably, the cells of the radially outer honeycomb layer (which can be a metallic, e.g. aluminium alloy, or a polymeric, e.g. polypropylene, honeycomb) are unfilled. The septum layer, which can help to spread the loads between the honeycomb layers, may be formed of glass fibre reinforced plastic. The liner is typically abradable by the tips of the fan blades. The liner is typically formed from a plurality of separately replaceable, circumferentially arranged liner panels.

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 the fan track liner, and with ice impact panels and acoustic liner panels which may be 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 perpendicular to the casing axis and containing the radially outer points of the leading edges and a plane perpendicular to the casing axis and containing the radially outer points of the trailing edges of the fan blades, the section converging from the leading edge plane to the trailing edge 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 Figure 2 shows a schematic diagram of the tip of a fan blade of the engine of Figure 1 as viewed along a radial direction of the engine.

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 and trailing edges positions of the fan blades when the engine is running are indicated by respective arrows L and T. Dashed lines indicate the positions of six planes which are perpendicular to the casing axis. First plane (i) is 60 mm forward of position L. Second plane (ii) is mm forward of position L. Third plane (iii) contains points on the radially outer chords of the fan blades which points are one third of the radially outer chordal distances C from the leading edges when the engine is running. Fourth plane (iv) contains points on the radially outer chords of the fan blades which points are two thirds of the radially outer chordal distances C from the leading edges when the engine is running. Fifth plane (v) is 60 mm forward of position T. Sixth plane (vi) is 20 mm forward of position T. For clarity, Figure 2 shows a schematic diagram of the tip of a blade as viewed along a radial direction of the engine, the positions of the six planes being indicated, and the casing axis and the plane of Figure 1 being denoted by the horizontal dashed line A-A.

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 I and converges in the axial direction from the hook to position I. 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. A septum layer formed of glass fibre reinforced plastic spreads loads between the two honeycomb layers. The fan track liner is formed from a circumferential arrangement of individually replaceable panels.

A first function of the fan track liner 11 is to reduce leakage of air over the tips of the blades by accommodating rotational variation in the fan blades. A second function of the liner is to absorb the impacts of ice shed from the fan blades. Particularly with swept fan blades, such impacts can occur forward of the trailing edges of the blades. A third function of the liner (discussed in more detail below) is to absorb a significant portion of the energy of a released fan blade in the event of a fan blade-off event.

Rearward of the fan track liner panels 11, are mounted ice impact panels 13 which absorb the impacts of ice shed from the fan blades when the impacts occur rearward of the trailing edges of the 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.

As indicated above, the fan track liner 11 plays a role in containing the blade in the event of a blade-off event. Specifically, the liner is designed to prevent at least the most rearward third of the span of the aerofoil portion of the released blade from penetrating the thickness of the liner. This blade containment capability is facilitated by a tendency to use lighter fan blades.

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 point of impact is affected by the impact absorbing capability of the liner 11, and the positioning of the rib takes that into account.

The relative density of the aerofoil portion of a blade can vary 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. Because of its higher density, the leading edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the trailing edge may fully penetrate the thickness of the fan track liner 11. However, these parts of the blade will then be contained by the shell 1.

In contrast, the more rearward parts of the span of the aerofoil portion are typically hollow and therefore less dense. The fan track liner 11 is designed to capture the trailing edge of the released blade and adjacent parts of the blade up to one third, and preferably up to one half, of a chordal distance from the trailing edge. The captured blade parts, typically being hollow and thin, generally crumple on impact with the liner.

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 third plane (iii) and fourth plane (iv), and reinforcing rib 8 is positioned between the fifth plane (v) and sixth plane (vi) . These ribs help to further strengthen the shell 1 against the impact of a released blade and to reduce distortions in the casing. The positions of the ribs again take account of the impact absorbing capability of the liner 11.

The arrangement of ribs according to this embodiment of the invention allows a relatively lightweight casing to be produced. Further, by devolving significant impact absorbing functionality to the liner 11, parts of the shell 1 can be made thinner and lighter. However, the shell 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 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.

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, a leading edge reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the leading edge reinforcing rib being between a first plane perpendicular to the casing axis and 60 mm forward of 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 20 mm forward of the radially outer points of the leading edges of the fan blades when the engine is running, and a fan track liner mounted to the radially inward side of the annular shell and extending circumferentially around the fan blades, the liner providing a shroud for the tips of the blades; wherein, in the event of a blade-off event, the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the trailing edge from penetrating the full thickness of the liner, the annular shell being sufficiently strong to contain those parts of the blade which do penetrate the liner.
2. A fan casing according to claim 1 further having a root reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the root reinforcing rib being rearwards of the most rearwards predicted point of impact on the casing of the fan blade root during a blade-off event.
3. A fan casing according to claim 2, wherein the axial position of the root reinforcing rib is from 200 to 400 mm axially rearwards of the most rearwards predicted point of impact.
4. A fan casing according to any one of the previous claims 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 a third plane perpendicular to the casing axis and containing points on the radially outer chords of the fan blades which points are one third of the radially outer chordal distances from the leading edges when the engine is running, and a fourth plane perpendicular to the casing axis and containing points on the radially outer chords of the fan blades which points are two thirds of the radially outer chordal distances from the leading edges when the engine is running.
5. 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 a fifth plane perpendicular to the casing axis and 60 mm forward of the radially outer points of the trailing edges of the fan blades when the engine is running, and a sixth plane perpendicular to the casing axis and 20 mm forward of the radially outer points of the trailing edges of the fan blades when the engine is running.
6. A fan casing according to any one of the previous claims further having a containment hook which extends radially inwardly from the annular shell and circumscribes its inner periphery, the containment hook being axially positioned forward of the first plane, wherein the minimum thickness of the shell between the containment hook and the leading edge reinforcing rib is at least 1.5 times greater than the minimum thickness of the shell between the leading edge reinforcing rib and the third plane perpendicular to the casing axis and containing radially outer points on the chords of the fan blades which points are one third of the chordal distances from the leading edges 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, only one reinforcing rib extending radially from and circumscribing the annular shell, the axial position of the reinforcing rib being at the point on the casing which would experience the greatest deflection during a blade off event were the rib not present, and a fan track liner mounted to the radially inward side of the annular shell and extending circumferentially around the fan blades, the liner providing an shroud for the tips of the blades; wherein, in the event of a blade-off event, the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the trailing edge from penetrating the full thickness of the liner, the annular shell being sufficiently strong to contain those parts of the blade which do penetrate the liner.
8. A fan casing according to any one of the previous claims, wherein the fan track liner is sufficiently strong to prevent at least the trailing edge of the released blade and adjacent parts of the blade up to one half of a chordal distance from the trailing edge from penetrating the full thickness of the liner.
9. A fan casing according to any one of the previous claims, wherein the fan track liner is sufficiently weak to allow at least leading edge of the released blade and adjacent parts of the blade up to one third of a chordal distance from the leading edge to penetrate the full thickness of the liner.
10. A fan casing according to any one of the previous claims, wherein the fan track liner has a radially outer honeycomb layer, a radially inner honeycomb layer, and a septum layer separating the two honeycomb layers.
11. 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.
12. A fan casing according to any one of the previous claims, wherein the annular shell is metallic.
13. A fan casing according to any one of the previous claims, wherein the or each rib has a C-shaped cross-section.
14. A combination of the fan casing of any one of the previous claims and the fan blades around which the fan casing circumferentially extends.
15. A turbofan gas turbine engine having the fan casing of any one of the previous claims.
16. 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.