ACCESSORY GEARBOX FOR A GAS TURBINE ENGINE

Abstract

A gas turbine engine arrangement includes an accessory gearbox which is mounted so as to be aligned in an axial direction along the engine. The accessory gearbox may be recessed at least partly into a casing of the engine.

Claims

1. A gas turbine engine arrangement, the engine comprising an accessory gearbox, the arrangement characterised in that the accessory gearbox is aligned in an axial direction along the engine.

2. The arrangement of claim 1, in which the accessory gearbox is mounted on a casing of the engine, and in which the accessory gearbox is recessed at least partly into the casing.

3. The arrangement of claim 2, in which the casing comprises a recess to accommodate the accessory gearbox.

4. The arrangement of claim 3, in which the recess causes a reduction in the bypass duct cross-sectional area in the region of the recess.

5. The arrangement of claim 4, in which the inner wall of the bypass duct comprises a corresponding recess to mitigate the reduction in cross-sectional area.

6. The arrangement of claim 1, in which the axial alignment of the accessory gearbox permits a reduction in the outer casing line of the nacelle and therefore a reduction in the overall length of the nacelle.

Description

[0017] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0018] FIG. 1 is a sectional side view of a gas turbine engine, as already described;

[0019] FIG. 2(a) is a schematic sectional side view of a gas turbine engine, showing the nacelle cross-section with a conventional accessory gearbox; and

[0020] FIG. 2(b) is a schematic sectional side view of a gas turbine engine, showing the nacelle cross-section with an axially-aligned accessory gearbox.

[0021] FIG. 2(a) shows a gas turbine engine 110 comprising a nacelle air intake 112, a fan casing 113, an IMC 114, and a core engine 180 mounted within a bypass casing 182. (For other engine architectures, an ICC may take the place of the IMC 114.) The core engine 180 and the bypass casing 182 together define an annular bypass duct 121. As in the engine shown in FIG. 1, air entering the nacelle air intake 112 is accelerated by a fan (not shown in FIG. 2(a)) to produce two air flows: a first air flow into the core engine 180 and a second air flow which passes through the bypass duct 121 to provide propulsive thrust. The core engine air flow is exhausted through a nozzle 118; the bypass air flow is exhausted through a bypass nozzle 119 defined by a nacelle 120.

[0022] Mounted to the IMC 114 is an accessory gearbox 140, on which are mounted a number of accessories 190 (only two shown). The accessories 190 may be of any known type (for example starter, hydraulic pump, heat exchanger, breather, oil pump, drains tank, hydro-mechanical unit (HMU), fuel pump, alternator).

[0023] The nacelle outer contour (i.e. the profile of the outer skin of the nacelle 120 around and along the whole engine) is constrained by the need to maintain a minimum clearance 196 between the dressed engine and the nacelle outer skin. Because the accessory gearbox 140 protrudes from the lower part of the IMC and because there is no corresponding protrusion from the upper part of the IMC, the lower part of the nacelle extends further away 198 from the bypass duct casing 182 than does the upper part 199, so that the outer contour of the nacelle 120 is non-circular. The nacelle outer contour along the lower part of the engine therefore constrains the loft lines and thereby the overall length 160 of the nacelle, which is greater than if it were constrained by the nacelle outer contour along the upper part of the engine.

[0024] FIG. 2(b) shows a gas turbine engine 210. Like the engine of FIG. 2(a), the gas turbine engine 210 comprises a nacelle air intake 112, a fan casing 113, an IMC 114, and a core engine 180 mounted within a bypass casing 282. The core engine 180 and the bypass casing 282 together define an annular bypass duct 121. As in the engine shown in FIG. 1, air entering the nacelle intake 112 is accelerated by a fan to produce two air flows: a first air flow into the core engine 180 and a second air flow which passes through the bypass duct 121 to provide propulsive thrust. The core engine air flow is exhausted through a nozzle 118; the bypass air flow is exhausted through a bypass nozzle 219 defined by a nacelle 220.

[0025] Mounted to the bypass casing 282 is an axially-aligned accessory gearbox 240, on which are mounted a number of accessories 290 (only three shown). The accessories 290 may be of any known type, as described above.

[0026] In the arrangement of FIG. 2(b), the bypass casing 282 comprises a recess configured to accommodate the accessory gearbox 240, so that the accessory gearbox 240 is partially inset into the bypass casing 282 and protrudes less far in a radial direction than if the recess were not present.

[0027] The radially inward contouring of the bypass casing 282 to accommodate the accessory gearbox 240 results in a corresponding contouring on the inner wall of the bypass casing 282, which locally reduces the cross-sectional area of the bypass duct 121. This inevitably increases the bypass losses, but because the bypass flow velocity is lower than the flow velocity outside the nacelle, a geometrically identical blockage will cause less loss in the bypass and hence result in an overall reduced loss (at engine/power plant level).

[0028] The increased bypass losses can be mitigated by a corresponding radially inward contouring of the bypass duct 121 inner wall (formed by, for example, the fairings of the core engine 180 or inner fixed structure (not shown in FIG. 2(b)) so that there is no or less local reduction in the bypass duct 121 cross-sectional area. The benefit of the axially mounted accessory gearbox can thus be maximised.

[0029] As in the engine shown in FIG. 2(a), it is necessary to maintain a minimum clearance 296 between the dressed engine and the nacelle outer skin. The axially-extending accessory gearbox 240 protrudes less far from the bypass casing 282 than does the conventional accessory gearbox 140 in the arrangement of FIG. 2(a). Therefore, the lower part of the nacelle 220 extends less far 298 from the bypass casing 282 than in the arrangement of FIG. 3(a). The upper dimension 299 is the same as the corresponding dimension 199 in FIG. 3(a), so the outer casing line of the nacelle 220 is smaller than the outer casing line of the nacelle 120.

[0030] Because the outer casing line of the nacelle 220 is smaller than the outer casing line of the nacelle 120, the overall length 260 of the nacelle 220 can be made shorter than the overall length 160 of the nacelle 120 without introducing aerodynamically unacceptable curvatures. The reduced length and reduced outer casing line of the nacelle 220 deliver a significant reduction in weight compared with the conventional nacelle 120, and the reduced outer casing line also delivers an aerodynamic benefit.

[0031] In an alternative arrangement, the bypass casing 282 may comprise no recess, so that the accessory gearbox 240 is simply mounted on the bypass casing 282. This will cause the accessory gearbox 240 to protrude further in a radial direction than in the arrangement of FIG. 2(b), and therefore the outer casing line will be larger than for the arrangement of FIG. 2(b).

[0032] It will be understood that the invention is not limited to the embodiments described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.