GEARBOX BREATHER OUTLET

Abstract: A breather outlet for the gearbox of an aircraft mounted gas turbine engine comprises an aerodynamic mast (15) which extends from the engine nacelle (10). The mast contains a breather duct (17) which interconnects the interior of the gearbox with an exhaust outlet (18). The exhaust outlet (18) is generally rearwardly facing and is spaced apart from the engine nacelle (10) so as to avoid oil staining of the nacelle (10).

Description: This invention relates to a gearbox breather outlet and in particular to a gearbox breather outlet suitable for an aircraft mounted gas turbine engine enclosed within a nacelle. Aircraft mounted gas turbine engines are usually provided with a gearbox which is driven by the engine and provides drive for certain engine accessories. Such gearboxes are oil lubricated and are provided with so-called "breather" outlets to provide communication between the gearbox interior and the exterior of the engine. This is to ensure that the operation of the gearbox does not result in an air pressure build-up within the gearbox casing. Inevitably operation of the gearbox results in severe agitation of the oil within the gearbox to the extent that an oil mist is usually formed. This oil mist can escape through the gearbox breather outlet and so it is common to provide a centrifuge device in the breather to separate out the oil mist before it is ejected from the engine. Unfortunately such structures are not completely effective in providing the capture of all of the oil mist so that some oil is inevitably lost through the breather outlet. The magnitude of the oil loss under these circumstances is not great and does not normally present any problems in the effective operation of the gearbox. However the oil which is ejected from the breather outlet, which is typically fitted flush with the engine nacelle, tends to cause dark coloured stains along the engine nacelle. The problem is particularly acute in the case of nacelles which are light coloured. Such stains are seen as being highly undesirable since they are unsightly and are very difficult and time consuming to remove by normal cleaning methods. Consideration has been given to providing a breather outlet which is in the form of a mast extending normal to the engine nacelle, the oil/air mixture from the gearbox being exhausted from the end of the mast remote from nacelle in a direction generally normal to the airflow over the nacelle. It has been found with such masts, however, that the air turbulence which they create tends to result in at least some of the exhausted oil to flow into contact with the nacelle, thereby causing staining. A further undesirable feature of such masts is that they cause a certain degree of drag which provides a penalty in terms of overall engine performance. It is an object of the present invention to provide a gas turbine engine breather outlet which substantially avoids oil staining of the engine nacelle in the region of the outlet. Accordingly the present invention comprises a mast which is located on the exterior of the nacelle and generally aerodynamically shaped and contains a breather duct having an exhaust outlet and operationally interconnected with the interior of the gearbox, said exhaust outlet being generally rearwardly facing with respect to the operational flow of air over said mast and additionally being positioned so as to be spaced apart from the engine nacelle to minimise interaction between any efflux from said exhaust outlet and said nacelle. The minimising of interaction between any such efflux from the exhaust outlet and the engine nacelle ensures that any oil contained in the efflux causes minimal staining of the nacelle. The invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a general view of an aircraft mounted gas turbine engine nacelle incorporating a breather outlet in accordance with the present invention. Figure 2 is a close-up view of the breather outlet mounted on the nacelle shown in Figure 1, shown in ghost form to reveal its internal configuration. Figure 3 is a sectioned plan view of a part of the breather outlet shown in Figure 2. With reference to Figure 1, a gas turbine engine nacelle 10 containing a propulsive gas turbine engine is suspended from an aircraft (not shown) by means of a mounting pylon 11. The gas turbine engine within the nacelle 10 is provided with a gearbox having a breather, the outlet of that breather being indicated at 12. The breather outlet can be seen more clearly in Figure 2. Referring to Figure 2, the breather outlet 12 extends through an apertured seal plate 13 provided on the surface of the nacelle 10. That portion of the breather outlet 12 within the nacelle includes a transition piece connector 14 which facilitates an interconnection between the breather outlet 12 and the interior of the previously mentioned gearbox. That portion of the breather outlet 12 which lies outside of the nacelle 10 comprises an aerodynamic mast 15. The mast 15 contains a duct 17 which interconnects the gearbox interior with an exhaust outlet 18. The exhaust outlet 18 is so positioned in the mast 15 that it is generally rearwardly facing with respect to the operational air flow over the nacelle. Moreover the exhaust outlet 18 is so positioned that it is spaced apart by approximately 12 cms from the surface of the nacelle 10. The breather duct 17, which is shown in greater clarity in Figure 3, is so arranged that its cross-sectional configuration changes from a slot shape as it exits from the nacelle 10 to a circular cross-sectional shape at its exhaust outlet 18. It can also be seen from Figure 3 that the plane of the exhaust outlet 18 is canted so as to be directly slightly away from the nacelle 10. This is to decrease still further the possibility of oil coming into contact with the nacelle 10. The mast 15 has a leading edge 19, with respect to the operational flow of air thereover, which is curved in a convex manner. Specifically the leading edge 19 extends from the most upstream part 20 of the mast 15 to the exhaust outlet 18. The mast trailing edge 21 on the other hand is straight and extends from the most downstream part 22 of the mast 15 which intersects with nacelle 10, to the exhaust outlet 18. These configurations of the leading and trailing edges 19 and 21 enhance the aerodynamic performance of the breather outlet 12 thereby decreasing its drag. During operation of the gas turbine engine mounted within the nacelle 10, oil is expelled in small quantities from the engine gearbox into the breather outlet duct 17. That oil and the air in which it is suspended flows through the duct 17 to be finally exhausted in a generally downstream direction through the exhaust outlet 18. By exhausting the oil in generally downstream direction, in this manner the oil is caused to follow a path which ensures that at least the majority of it does not come into contact with the nacelle 10 to an extent likely to lead to staining of the nacelle 10. A further benefit arising from exhausting the oil/air mixture in this way is that it provides a limited degree of thrust which goes at least some way towards off-setting the aerodynamic drag resulting from the presence of the mast 15. Additional benefits which accrue from breather outlets 12 in accordance with the present invention are that they are not prone to blockage through icing and also that they do not result in the generation of back pressure within the breather duct 17. Such a back pressure could result in an undesirable effect on internal air blown seals located around bearing chambers within the engine.

Claims: A breather outlet (12) suitable for use with the gearbox of an aircraft mounted gas turbine engine enclosed within a nacelle (10), comprising a mast (15) containing a breather duct (17) said mast (15) being adapted to be located on the exterior of said nacelle (10), said breather duct (17) being operationally interconnected with the interior of said gearbox characterised in that said mast (15) is generally aerodynamically shaped, the exhaust outlet (18) of said breater duct (17) being generally rearwardly facing with respect to the operational flow of air over said mast (15), and additionally positioned so as to be spaced apart from said nacelle (10) to minimise interaction between any efflux from said exhaust outlet (18) and said nacelle (10). A breather outlet as claimed in claim 1 characterised in that the leading edge (19) of said mast (15), with respect to the operational flow of air over said nacelle (10), is curved in a convex manner and extends from the most upstream part (20) of said mast (10), with respect to said operational flow of air, to said exhaust outlet (18). A breather outlet as claimed in claim 2 characterised in that the trailing edge (21) of said mast (15), with respect to the operational flow of air over said nacelle (10), is straight and extends from the most downstream part (22) of said mast (15), with respect to said operational flow of air, which is operationally adjacent said nacelle (10), to said exhaust outlet (18). A breather outlet as claimed in any one preceding claim characterised in that the plane of said exhaust outlet (18) is canted so as to be directed slightly away from said nacelle (10) when said mast (15) is mounted on said nacelle (10). A breather outlet as claimed in any one preceding claim characterised in that said exhaust outlet (18) of said breather duct (17) is of circular cross-sectional shape. A gas turbine engine characterised in that said engine is provided with a breather outlet (12) as claimed in any one preceding claim.

Inventors: HUTTON, PETER ELSTOB
Applicants: ROLLS-ROYCE PLC
Application year: 1990
Main classification: B64D33
IPC codes: F02C7, F01M13, B64D33