Abstract:
A nacelle inlet lip has a simplified geometry. The nacelle inlet lip has a constant cross-section all along a circumference thereof and is wholly concentric relative to the engine centerline, thereby allowing for the use of simple and economical manufacturing processes.

Description:
TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to the inlet lip of a turbofan engine nacelle. 
     BACKGROUND OF THE ART 
     Gas turbine engine nacelles typically have a bottom eccentric hollow space or loft which bulges radially outwardly with respect to the engine centerline in order to accommodate engine accessories. To conform to the eccentric geometry of the nacelle, the inlet lip, which is mounted to the forward end of the nacelle, is formed as an asymmetric part having a constantly changing curvature along the circumference thereof. This results in expensive and complex manufacturing processes. 
     Accordingly, there is a need to simplify the nacelle inlet lip geometry. 
     SUMMARY 
     In one aspect, there is provided a nacelle inlet for a turbofan engine having an engine centerline, comprising an annular inlet lip concentric relative to the engine centerline, the annular inlet lip having a constant cross-section all along a circumference thereof, the annular inlet lip having an external surface and an internal surface joined by a rounded leading edge, the external surface being spaced radially outwardly from the internal surface relative to the engine centerline, the external surface being axially shorter than the internal surface, and a forwardly canted bulkhead extending between said external surface and said internal surface. 
     In a second aspect, there is provided a nacelle surrounding a turbofan engine having an engine centerline, the nacelle comprising an inlet duct section having a nacelle external surface and a nacelle internal surface defining an annular cavity therebetween, the nacelle external surface having a bottom eccentricity relative to the engine centerline to provide for the formation of a radially outwardly bulging loft in a bottom portion of the nacelle for accommodating engine accessories; and an inlet lip projecting forwardly from said inlet duct section, said inlet lip being concentric relative to the engine centerline and having a constant cross-section along all a circumference thereof, the inlet lip having ah inlet lip external surface and an inlet lip internal surface joined by a rounded edge forming a leading edge of the nacelle, the inlet lip internal and external surfaces respectively blending with said nacelle internal and external surfaces. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a partially sectioned side elevation schematic of an aircraft engine mounted within a nacelle having a symmetrical inlet lip wholly concentrically disposed about the engine centerline; 
         FIG. 2  is an enlarged cross-sectional isometric view of the inlet lip mounted to an inlet duct section of the nacelle; and 
         FIG. 3  is a cross-sectional view of the nacelle inlet lip. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a nacelle  10  of an aircraft power plant  14  is fixed to a mounting structure  12  of an aircraft (hot shown). The power plant  14  is described herein as a gas turbine engine, and more particularly as a turbofan, however the exemplary nacelle inlet lip that will be described hereinafter can be used with other suitable aircraft power plant. The turbo fan engine  14 , as illustrated in  FIG. 1 , has an upstream fan  16  that provides, initial compression of the engine inlet airflow which is subsequently split into the outer annular bypass airflow passage  18  and the inner annular engine core airflow passage  20 . Generally, inlet guide varies  24  are disposed at least within the engine core airflow passage  20 , upstream of the next compressor stage  22 . 
     The nacelle  10  is generally tubular and has an external surface  31  and an internal surface  33  radially spaced apart to define a hollow cavity  29  therebetween. The external and internal surfaces can be made of sheet metal. As can be appreciated from  FIG. 2 , the hollow cavity  29  has a substantially greater cross-section at the lower portion of the nacelle  10 . This provides an eccentric lower nacelle loft  34  designed to accommodate engine accessories, such as pumps, gear and lubricating systems (not shown). This extra internal volume is obtained by causing the nacelle external surface  31  to bulge radially outwardly with respect to the centerline axis of the engine  14  in the bottom region of the nacelle  10 . The external tubular surface  31  of the nacelle  10  is thus eccentric relative to the engine centerline (i.e. it has a variable curvature along its circumference). In contrast, the circumferential internal surface  33  of the nacelle  10  is concentric relative to the engine centerline and defines the air inlet flow passage to the engine at the upstream end thereof, and defines the annular bypass airflow passage  18  further downstream. 
     An annular inlet lip  28  is mounted to the most upstream end of the nacelle inlet duct section shown in  FIG. 2 . The inlet lip  28  defines the engine air inlet and is coaxial with the engine centerline. As can be appreciated from  FIGS. 2 and 3 , the inlet lip  28  has a substantially C-shaped or rearwardly open cross-section defined by an aerodynamic external surface  38 , an aerodynamic internal surface  40  spaced radially inwardly from the external surface  38 , and a rounded leading edge  36  integrally interconnecting the external and internal surfaces  38 ,  40 . The inlet lip external surface  38  provides a smooth forward extension of the nacelle external surface  31 . Likewise, the inlet lip internal surface  40  provides a smooth forward extension of the nacelle internal surface  33  for channeling incoming air to the engine  14 . 
     While the nacelle external and internal surfaces  31  and  40  define an eccentric bottom portion (i.e. the outwardly bulging loft  34 ), the inlet lip  28  is wholly concentric with respect to the engine centerline (the nacelle inlet lip  28  has a constant cross-section all along its circumference). Blending of the inlet lip external surface  38  with the nacelle external surface  31 , which respectively have a constant and a variable curvature in a circumferential direction, is rendered possible in part by the short axial profile of the inlet lip external surface  38 . As can be appreciated from  FIGS. 2 and 3 , the inlet lip external surface  38  is substantially axially shorter than the internal lip surface  40 . The inlet lip shortened external surface  38  is interrupted in a curved portion having a tangent angled to the centerline of the engine, whereas the inlet lip internal surface  40  ends in a substantially axially straight portion nearly parallel to the engine centerline. The inlet lip external surface  38  ends short after the rounded edge  36  at a forward location where the inlet lip starts to describe a curve from radial to axial. In this manner, the length of the external surface  38  of the lip may be shortened by approximately 70% of the axial length of the lip internal surface  40 . 
     By so forwardly shifting the interface or junction between the inlet lip external surface  38  and the nacelle external surface  31  closer to the rounded leading edge  36  that is in a curved lip region which still has a substantial radial component as opposed to only or close to only an axial component, it is possible to slightly change the curvature of the nacelle in the axial direction all along the circumference thereof so that the top and bottom parts of the nacelle external surface  31  be substantially tangent to the inlet lip external surface  38 . In this way the eccentric nacelle lower loft  34  can reasonably blend with the concentric inlet lip  28 . 
     The rear end portion of the inlet lip external and internal surfaces  38  and  40  is connected to the nacelle external and internal surfaces  31  and  33  by means of a forwardly canted annular bulkhead  42 . The bulkhead  42  closes the open-rear end of the inlet lip  28  so as to define therewith an anti-icing cavity  44  adapted to receive an anti-icing system (not shown). The bulkhead  42  can be provided at opposed ends thereof with flanges  42   a  and  42   b  to facilitate the attachment of the inlet lip  28  to the nacelle external and internal surfaces  31  and  33 . The radially outer flange  42   a  extends forwardly to interconnect the shortened inlet lip external surface  38  to the nacelle external surface  31 . The radially inner flange  42   b  extends rearwardly to interconnect the inlet lip internal surface  40  to the nacelle internal surface  33 . Like the inlet lip  28 , the bulkhead  42  is wholly concentric with respect to the engine centerline. The inlet lip  28  and the bulkhead  42  can thus be economically formed by standard spinning techniques, and where required, polishing for the inlet lip  28  is also simplified. Indeed, symmetrical rotation allows for ease of manufacture. Accordingly, sheet material such as high temperature aluminium or stainless steel can be use to spin form a wholly concentric one-piece inlet lip. 
     The forwardly canted bulkhead  42  also provides improved birdstrike survivability. The short inlet lip outer surface design together with the forwardly canted bulkhead  42  contributes to reduce the volume to be heated for the purposes of de-icing the inlet lip  28 . The volume of the anti-icing cavity  44  is indeed small as compared to the volume of a conventional inlet lip anti-icing cavity. The forward trimming of the inlet lip outer surface also result in weight savings and reduced polishing area. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.