GAS TURBINE ENGINE NACELLE

Abstract: A nacelle structure (14) for a gas turbine engine (4) ex­ tends upstream from the engine air inlet (10) and includes an external surface (18) and an internal surface (16). The external surface (18) is concentric about a linear centerline (22) which is disposed parallel to the free airflow stream (6) encountered by the nacelle (14) in flight. The internal surface (16) is con­ centric about a curved centerline (30), the curved centerline (30) being parallel to the free airflow stream (6) at the nacelle inlet (12) and parallel to the engine centerline (26) at the en­ gine air inlet (10) for smoothly direction the internal airflow stream (32) into the engine air inlet (10).

Description: The present invention relates to a nacelle for a gas turbine engine, and more particularly, to a nacelle for a gas turbine engine mounted on an aircraft. Gas turbine engines used to propel today's subsonic commercial aircraft are typically surrounded by a nacelle structure having a drooped or angled inlet opening aligned relative to the direction of the surrounding airflow. This is particularly true for those engines mounted under the wings of an aircraft, wherein the influence of the surrounding aircraft structure, airfoil angle of attack, and other aerodynamic factors result in a nacelle inlet droop angle of approximately 3-5 degrees with respect to the engine centerline. Prior art nacelle structures include a double wall intake portion extending upstream of the gas turbine engine air inlet and having internal and external surfaces defined by the end points of radii drawn perpendicular to a linear centerline parallel to the encountered airflow. The nacelle opening divides the free airstream into an internal portion which is directed into the gas turbine engine air inlet and an external portion which flows around the nacelle structure. The external surface configuration is defined so as to reduce the occurrence of shock waves or separation, thereby avoiding the creation of undesirable drag forces. The interior surface, also symmetrical about the linear drooped centerline, forms a duct for directing internal flow in a direction generally parallel to the drooped nacelle centerline, but abruptly turning at the engine inlet to the direction of the engine centerline. Experience has shown that this turn introduces a circumferential static pressure gradient in the engine inlet region which decreases engine efficiency as well as increases generated noise. US-A-4,220,171 which discloses a nacelle structure according to the precharacterizing portion of claim 1 recognized this deficiency in the prior art nacelle structure and discloses the improvement wherein the internal and external nacelle surfaces are defined by the end points of radii drawn perpendicular to a curved centerline. The curved centerline is parallel to the free airstream flow at the nacelle inlet opening and parallel to the engine centerline at the engine air inlet opening. The structure of US-A-4,220,171 thus smoothly turns the internal airflow prior to entering the engine inlet, increasing engine efficiency by reducing internal static pressure loss and noise generation. The curved external nacelle surface of US-A-4,220,171, however, has been found not to produce the same benefit. By forming the exterior surface about a curved centerline, the nacelle of US-A-4,220,171 increases the local air velocity along the upper forward portion of the nacelle, increasing the likelihood of forming local shock waves and/or boundary layer separation. As noted above, such phenomena increase the undesirable surface drag on the nacelle structure resulting in higher aircraft operating costs. Thus, the object of the invention is to provide a nacelle structure having reduced external aerodynamic drag while simultaneously avoiding the creation of internal static pressure losses. According to the present invention, to achieve this, there is provided a nacelle structure for a gas turbine engine substantially disposed in a unidirectional flow of air, the engine having a centerline inclined with respect to the direction of the flow of air, said nacelle structure comprising an annular inlet duct extending upstream of the gas turbine engine, the duct defining a nacelle inlet opening perpendicular to the airflow, and further including an internal surface for receiving air flowing through the nacelle inlet opening and conducting the received air into the gas turbine engine air inlet, the internal surface further being defined by the end points of radii drawn perpendicular to a curved internal surface centerline, the curved centerline being off-set from the engine centerline adjacent the nacelle inlet opening and being colinear with the engine centerline at the engine inlet, and an external surface for directing air flowing around the nacelle structure, the external surface further being defined by the end points of radii drawn perpendicular to an external surface centerline being off-set from the engine centerline at the nacelle inlet opening and converging to the engine centerline from the nacelle inlet opening toward the engine inlet, characterized in that the external surface centerline is a straight line disposed at an angle with respect to the engine centerline. The nacelle structure is internally configured to efficiently conduct a flow of air into a gas turbine engine mounted on an aircraft and is externally configured to reduce external aerodynamic drag and/or shock wave formation. By providing separately developed internal and external aerodynamic surfaces for interacting with the respective internal and external airflow, the nacelle structure according to the present invention achieves efficient engine operation without the creation of external airflow shock waves. Each surface is thus shaped to perform its particular task without compromise, resulting in an overall fuel savings due to increased engine efficiency over prior art canted nacelle structures. Preferably, the external surface centerline is tangential to the internal surface curved centerline adjacent the nacelle inlet opening. The nacelle structure will now be described in greater detail with reference to the accompanying drawings, wherein: Fig. 1 is a side view of an aircraft with a gas turbine engine mounted under the wing. Fig. 2 is a cross-sectional view of the nacelle inlet structure. Referring to Fig. 1, an aircraft 2 is shown with a gas turbine engine 4 mounted under the wing. The general effect of the wing and engine pylon 5 aerodynamics on the free external airflow is indicated by an arrow 6. For such configurations, the free airflow stream is deflected upward in a region just upstream of the engine and nacelle structure to follow an upward slant, blending with the wing airfoil aerodynamics. The gas turbine engine 4 is mounted such that the nacelle inlet extends into that portion of the airflow that has been deflected upwards. Fig. 2 shows a cross section of the forward portion of a nacelle. The nacelle extends upstream of the gas turbine inlet 10 having a nacelle inlet opening 12 which is oriented perpendicularly with respect to the free airstream flow 6. The nacelle inlet structure 14 includes an internal surface 16 and an external surface 18. The external surface 18 is defined by the loci of the end points of a plurality of radii 20 drawn perpendicular to the external surface centerline 22. As can be seen in Fig. 2, the external surface centerline 22 is parallel with the flow direction of the free external airflow 6 and forms an angle 24 with respect to the engine centerline 26. This angle 24, also termed the droop angle, has been exaggerated for clarity, typically being on the order of 3 to 5 degrees. The internal surface 16 of the nacelle structure 14 is defined by the loci of the end points of a plurality of radii 28 drawn perpendicular to a curved centerline 30. The curved centerline 30 is colinear wich the external surface centerline 22 at the forward end of the nacelle structure 14 adjacent the nacelle inlet 12, and is colinear with the engine centerline 26 adjacent the engine air inlet 10 at the downstream end of the nacelle structure 14. During operation of an aircraft 2 equipped with a nacelle structure 14, free-flowing external air 6 is separated between an internal flow 32 and an external flow 34. The internal flow 32 enters the nacelle inlet 12 and is smoothly turned by the internal surface 16, generally following the curved internal surface centerline 30. The result is that the internal airflow 32 is flowing substantially parallel to the engine centerline 26 by the time it enters the gas turbine air inlet 10. The external portion 34 of the divided airflow does not experience this gradual turning, instead continuing to flow substantially linearly about the nacelle external surface 18 as defined by the linear external surface centerline 22. By maintaining the generally linear flow of the external air 34, the nacelle 14 reduces the likelihood of the formation of shock waves at the external surface 18 of the nacelle. Such shock waves result in an increased aerodynamic drag and an overall performance penality for the operating aircraft. Similarly, by providing a gradually turning intake duct, the internal surface 16 aligns and directs the internal airflow 32 with the gas turbine engine centerline 26, reducing losses which occurred in prior art linear duct structures having an abrupt redirection of the internal airflow at the engine inlet plane. The overall result is to improve overall performance of the engine and airframe combination without incurring the external aerodynamic drag losses associated with nacelle structures having exterior surfaces developed from curved centerlines as disclosed in the prior art.

Claims: 1. A nacelle structure for a gas turbine engine (4) substantially disposed in a unidirectional flow of air, the engine (4) having a centerline (26) inclined with respect to the direction of the flow of air, said nacelle structure comprising: an annular inlet duct extending upstream of the gas turbine engine (4), the duct defining a nacelle inlet opening (12) perpendicular to the airflow, and further including an internal surface (16) for receiving air flowing through the nacelle inlet opening (12) and conducting the received air into the gas turbine engine air inlet (10), the internal surface (16) further being defined by the end points of radii (28) drawn perpendicular to a curved internal surface centerline (30), the curved centerline (30) being off-set from the engine centerline (26) adjacent the nacelle inlet opening (12) and being colinear with the engine centerline (26) at the engine inlet (10), and an external surface (18) for directing air flowing around the nacelle structure (14), the external surface (18) further being defined by the end points of radii (20) drawn perpendicular to an external surface centerline (22) being off-set from the engine centerline (26) at the nacelle inlet opening (12) and converging to the engine centerline (26) from the nacelle inlet opening (12) toward the engine inlet (10), characterized in that the external surface centerline (22) is a straight line disposed at an angle with respect to the engine centerline (26). 2. A nacelle structure according to claim 1, characterized in that the external surface centerline (22) is tangential to the internal surface curved centerline (30) adjacent the nacelle inlet opening (12).

Inventors: WYNOSKY, THOMAS ALBERT
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1987
Main classification: B64D33
IPC codes: F02C7, B64D33