GB2083558A - Load Transfer Structure for Turbofan Aero Engines - Google Patents

Abstract

In a turbofan aeroengine, struts, such as outlet guide vanes 13' for the fan, extend across the fan duct 5 taking loads to and from the core of the turbofan. An annular member 63 is part of the core of the engine and is attached to the radially inner ends 71 of the struts, the function of the annular member 63 being to transfer loads between the core and the struts. Efficient transfer of large torsional loads as well as large tension and compression loads is enabled by making the annular member 63 channel-shaped in radial section, the open side being outermost, and fixing (preferably by automated electron or laser beam welding) the front and rear portions of the ends 71 to the inside surfaces of the front and rear walls 65, 67 respectively of annular member 63. <IMAGE>

Description

SPECIFICATION Load Transfer Structure for Turbofan Aero Engines The present invention relates to a structural assembly for a turbofan aero engine and to an annular component of that assembly the annular component being for transferring loads between the core of the turbofan and a plurality of struts comprising other components of that assembly and extending across the fan duct of the turbofan.

It is known to use an annular component for transferring loads between the core of a turbofan and struts in the form of fan outlet guide vanes which connect the core to surrounding engine structure, the annular component being part of the structure of the core. In the prior art, this annular component has a box-like cross-section, i.e. it is toroidal. To produce the prior structural assembly, the radially inner ends of the fan outlet guide vanes are butt-welded to stubs on the radially outer side of the toroid. This assembly is expensive and transfer loads mainly by compression and tension along the radial extent of the vanes.Emergency situations can arise where one or more of the fan blades become detached from their rotor, thus putting large unbalance loads on the core of the turbofan and producing large torsional loadings on the vanes as well as large compressive and tensile loadings. In order to increase the margin of safety it is therefore desirable to have a structure which can efficiently transfer large torsional loads as well as large compressive and tensile loads. Further, it is desirable to produce a structure with improved torsional characteristics which is also cheaper and quicker to manufacture than the prior construction.

According to the present invention, a structural assembly for a transfer of loads in a turbofan aero engine comprises a plurality of radially extending angularly spaced-apart struts which extend across the fan duct of the turbofan as parts of load paths for the support of the core of the turbofan within the fan duct, and an annular member attached to the radially inner ends of the struts as part of said core for transferring loads between the core of the turbofan and the struts, the annular member being channel-shaped in radial section with front, rear and radially inner walls, front and rear portions of the radially inner ends of the struts being fixed to the front and rear walls respectively of the annular member internally thereof.

Thus, each strut is fixed directly to front and rear sides of the channel sections, allowing efficient transference of loads by tension and compression along the strut and by torsional shear at its attachments to the channel.

Another advantage achieved by use of a channel section as opposed to a toroid is a reduction in the amount of material in the annular member and a reduction in the amount of time required for its manufacture. The invention also enables easier automation of the welding process, and a reduction in the amount of machining necessary in respect to the annular member once it has been manufactured, as discussed later in this specification.

The struts conveniently comprise outlet guide vanes for the fan, and preferably the struts are fixed to the annular member by weldments, welding being accomplished using laser beam or electron beam welding apparatus.

A specific embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which Figure 1 is a schematic part-sectional side elevation of a turbofan aero engine showing one likely location of a structural assembly according to the prior art and the invention; Figure 2 is an enlarged detailed view of the engine structure within the area 2 in Figure 1, showing a structural assembly according to the invention; Figure 3 is a view on arrow A in Figure 2; Figure 4 is a part of a view like Figure 2, showing a structural assembly according to the prior art; Figure 5 is a view on section B-B in Figure 2; and Figure 6 is a view on arrow C in Figure 5.

The drawings are not to scale.

Referring first to Figure 1, a turbofan aero engine 1 has a core engine 3, a fan duct 5 defined by fan duct casing and nacelle 7 surrounding the core 3, air intake 9, fan 11, a ring of fan outlet guide vanes 13, and propulsion nozzles 1 5 and 1 7 for fan air and core engine gases respectively.

The fan duct 5 receives air from fan 1 which also supplies core engine 3 through core intake 19, part of the flow path through the core compressor 21 being indicated by dashed lines.

The fan 11 is driven from a turbine (not shown) in the core 3.

The core 3 of the turbofan 1 is supported within fan duct 5 by means of the fan outlet guide vanes 13, which act as struts, and other means (not shown) well known to those skilled in the art.

Vanes 13 are thus included in load paths taking tensile, compressive and torsional loads to and from core 3. These loads may be categorised as suspension loads the normal loads originating in the core 3, and emergency out-of-balance loads resulting from loss of one or more of the fan blades 11, e.g. as a result of impact with a large bird.

Normal loads originating in core 3 are gas thrust forces, "g" forces and torsional forces due, for example, to changes in rotor speeds and gyroscopic effects. Emergency out-of-balance loads would exert large tensile, compressive and torsional loads on the vanes 1 3.

As shown in the prior art drawing (Figure 2), the vanes 1 3 are connected to the core engine 3 through an annular member which in the prior art takes the form of a toroid 23. As seen in the figure the radial cross-section of the toroid 23 is an irregular quadrilateral or "box" with front side 25, rear side 27, radially outer side 29 and radially inner side 31. Toroid 23 is directly or indirectly connected to various portions of the core 3, loading from the core being brought to the toroid mainly through boss ring 33 and a frusto-conical extension 35, though flanges 37 and 39 also connect the toroid to the outer skins 41, 41' of the core 3, which forms the inner boundary of the fan duct 5.

If the loading experienced by the toroid 23 through boss 33 is considered it will be seen that the boss 33 takes loading from the radially outer wall 43 of the compressor passage, and hence also from the stator blades 45, 47 etc., which are held therein at their radially outer ends. Through the stator blades 45, boss 33 also takes loading from radially inner parts of the core engine which in turn transmit loads from the fan 11 and the compressor blades 53, 55 etc. Frusto-conical extension 35 transmits loads from the rear of the compressor.

Fan outlet guide vanes 13 are welded along weld line 59 to the radially outer side 29 of toroid 23 through aerofoil stub portions 61, which are unitary with radially outer side 29 and have been machined from it. This construction can be more clearly seen in Figure 3. Vanes 13 are welded to stub portions 61 rather than directly to the outer face of side 29 in order to avoid introducing stress concentrations into the highly stressed transition area between the toroid 23 and the vane 13.

In the invention, toroid 23 of Figure 2 is replaced in Figure 4 by annular channel 63 comprising a front side 65, a rear side 67 and a radially inner side 69. The channel 63 takes the same loads from the same structure as shown in Figure 2, but is cheaper to make than the toroid 23 with its aerofoil stubs 61, uses less material, and can be manufactured from a smaller number of parts. In Figure 4, adjacent details of the core 3 have been omitted for convenience of illustration, but as before, flanges 37' and 39', which are similar to flanges 37 and 39 in Figure 3, support core outer skins 41, 41', and boss 33' with frusto-conical extension 35' support portions of the core 3 in the same way as boss 33 and extension 35 in Figure 3.However, transference of the loads between core 3 and vanes 13' is accomplished differently according to the invention in that vanes 13' are provided with radially inner thickened "root" portions 71 which fit inside the channel 63, the front and rear ends of these root portions 71 being welded along weldment lines 73 and 75 to lands 77 and 79 on the front and rear walls 65 and 67 respectively of the channel (see also Figures 5 and 6). This construction gives the advantages mentioned before in efficient transfer of torsional loads as well as compressive and tensile loads from the core 3 to vanes 13', the vanes 13' being fixed directly to the stiffest most important members of the load transfer structure.

Note that the channel configuration of the annular load transfer member allows the thickened root portions 71 to be given considerable radial extent, enabling efficient transfer of torsional loads from the vanes to the channel via lands 77, 79 which are of the same radial extent as the root portions. The shear stresses in weldments 73, 75 due to torsional loads are thus lower than the corresponding stresses in the corresponding welds 59 of Figure 2.

In the fabrication of structures such as the present one it is advantageous to automate the welding process if possible using electron beam or laser beam welding for reasons of productivity and consistent weld quality. Unfortunately, the necessary requirement in the prior art of Figures 2 and 3, that vanes 13 should be welded to stub portions 61 having the same aerofoil shape as the vanes, rendered the process difficult to automate due to the curved surface of the aerofoil, the varying thickness of metal to be welded across the breadth of the aerofoil, the problem of other vanes getting in the way of the welding beam, and the necessity of welding bot sides of the aerofoil to give a constant weld width and depth.

Consequently, it was better to weld by hand using the argon arc welding process. However, the construction shown in Figures 4 to 6 allows automatic welding using electron- or laserbeams, the welding beam paths WB being shown in Figures 4 and 5. It will be seen that automatic welding is facilitated because: the welds 73, 75 are all straight lines; the metal to be welded is of constant thickness; the vanes 13' do not get in the way of the welding beams WB; and the welds can be made from one side only of the root portions 71, since the welding beam WB can be set to penetrate the entire thickness of the constant thickness root portions 71 as it traverses over angie 0 during each weld pass (see Figure 5, the limits of traverse being shown by dashed lines), thus giving welds of constant width and depth.The components of the assembly can of course be held in contact with each other in a suitable jig during the welding process and the jig can be made rotatable so as to bring a fresh joint under the welding beam after each weld is completed. Preferably, both ends of the root portions 71 are welded to their respective lands 77 and 79 simultaneously using dual welding beams.

Finally, as will be clear from an examination of Figures 4 to 6, it is necessary to fill in the channel 63 by some means in order to allow a smooth flow of air over the top of the channel in continuity with the flow of air over skins 41, 41'.

This is easily accomplished by casting or injecting a foamed plastic (e.g. polyurethane) filler material into the channel 63 to produce the smooth flow surface indicated by dashed line 81 in Figure 4 so that the root portions 71 are embedded therein.

Alternatively, fibre-reinforced plastic sheet components with suitable flanges to engage with the internal wall surfaces of the channel can be used to produce the smooth flow surface 81 across the channel, these being held in place by fasteners or a suitable adhesive.

Claims (7)

Claims

1. A structural assembly for transfer of loads in a turbofan aero engine, comprising a plurality of radially extending angularly spaced-apart struts, which extend across the fan duct of the turbofan as parts of load path for the support of the core of the turbofan within the fan duct, and an annular member attached to the radially inner ends of the struts as part of said core for transferring loads between said core and the struts, the annular member being channel-shaped in radial section with front rear and radially inner walls, front and rear portions of the radially inner ends of the struts being fixed to the front and rear walls respectively of the annular member internally thereof.

2. A structural assembly according to claim 1 in which the struts comprise outlet guide vanes for the fan.

3. A structural assembly according to claim 1 or claim 2 in which the struts are fixed to the annular member by weldments.

4. A structural assembly for a turbofan aero engine substantially as described in this specification with reference to and as illustrated in Figures 4 to 6 of the accompanying drawings.

5. An annular member for transferring loads between the core of a turbofan aero engine and a plurality of radially extending angularly spaced apart struts extending across the fan duct of the turbofan as parts of load paths for the support of the core of the turbofan within the fan duct, said annular member being channel-shaped in radial section with front, rear and radially inner walls and said annular member being adapted to receive the radially inner ends of the struts for fixing of front and rear portions of said radially inner ends to said front and rear walls respectively of said annular member internally thereof.

6. An annular member according to claim 5 having lands on the internal faces of the front and rear walls, the lands being adapted for joining to the front and rear portions of the radially inner ends of the struts.

7. An annular member for transferring loads between the core of a turbofan aero engine and a plurality of struts extending across the fan duct of the aero engine, substantially as described in this specification with reference to and as illustrated in Figures 4 to 6 of the accompanying drawings.