Patent Publication Number: US-9403599-B2

Title: Inlet section of an aircraft engine nacelle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. application No. 13/091,615, filed Apr. 21, 2011, which is a continuation of U.S. application No. 12/423,550, filed Apr. 14, 2009, now issued as U.S. Pat. No. 8,197,191 on Jun. 12, 2012, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to nacelles for aircraft engines, and more particularly relates to an improved nacelle for a turbofan engine having an inlet cowl that is designed to assist in achieving a stable fly-home configuration subsequent to a blade-out event. 
     2. Description of the Related Art 
     A nacelle for a turbofan engine must meet several basic design criteria. For example, the nacelle should direct air flow to the air intake of the engine while protecting the air flow from disturbances such as gusts, and the like. In addition, the exterior surface profile of the nacelle should minimize the aerodynamic drag caused by the engine and its related components. 
     As shown in  FIGS. 1A and 1B , a modern turbofan engine assembly  10  typically includes a nacelle  22  and a fan case  16 . The engine assembly, including the nacelle  22  and fan case  16 , can be suspended from an aircraft&#39;s wing by a pylon  12 . In  FIG. 1A , one side of the nacelle structure  22  is removed for ease of illustration. The fan case  16  surrounds the engine&#39;s fan  18 . The fan  18  includes a plurality of fan blades  19  attached to the engine&#39;s rotor. As shown in  FIG. 1A , a typical nacelle structure  22  includes a forward inlet portion  24  and an aft nacelle portion  25 . The inlet portion  24  is typically attached to a forward flange  14  on the fan case  16  by a plurality of circumferentially spaced fasteners, such as bolts or the like. As shown in  FIGS. 1A and 1B , the inlet portion  24  typically includes an outer barrel  32 , a rounded nose lip section  28 , an inner barrel  30 , and one or more spaced bulkheads  34 ,  36  disposed between the outer barrel  32  and the inner barrel portion  30 . The outer barrel portion  32  and nose lip portion  28  can be constructed of a thin metallic material, such as aluminum, for example, or can be constructed of composite materials. The inner barrel  30  typically is constructed of composite materials and includes acoustic treatment configured to attenuate at least some engine noise. Such an acoustically treated inner barrel  30  typically includes a honeycomb core  31  sandwiched between a perforated composite inner skin  33  and an imperforate composite outer skin  35 . The composite inner barrel  30  can be constructed in two or more circumferential segments joined together by fasteners, or can be an unsegmented, one-piece composite structure. Some advantages of a one-piece inner barrel  30  over a segmented inner barrel  30  include fewer parts and fasteners, a seamless aerodynamic inner surface, and lower manufacturing costs, for example. 
     A forward edge  39  of the outer barrel  32  can be connected to the nose lip portion  28  by a first plurality of circumferentially spaced fasteners  47 , such as rivets, or the like. Similarly, a forward edge of inner barrel  30  can be connected to the nose lip portion  28  by a second plurality of circumferentially spaced fasteners  37 , such as rivets, bolts, or the like. The fasteners  37 ,  47  secure the components of the inlet portion  24  together, and transmit loads between fastened components. In the embodiment shown in  FIG. 1B , a forward bulkhead  38  extends between the outer and inner walls of the nose lip  28 , and an intermediate bulkhead  34  and an aft bulkhead  36  connect portions of the outer barrel  32  and the inner barrel  30 . The bulkheads  34 ,  36  contribute to the rigidity and strength of the inlet portion  24 . In addition, the intermediate and aft bulkheads  34 ,  36  transmit loads between the inner barrel  30  and the outer barrel  32 . As shown in  FIG. 1B , an aft flange  41  on the inner barrel  30  can connect the inlet portion  24  to a forward flange  14  of a fan case  16 . Accordingly, the composite inner barrel  30  directly supports the outer barrel  32  and nose lip portion  28 . The weight of the inlet portion  24  and external loads borne by the inlet portion  24  are necessarily transferred to the fan case  16  through the inner barrel  30 . Therefore, the composite inner barrel  30  of a typical nacelle inlet  24  can substantially contribute to the overall rigidity, strength and stability of the inlet portion  24  of the nacelle  22 . 
     The bulkheads  34 ,  36 ,  38  shown in  FIG. 1B  typically are constructed of a thin metallic material such as aluminum, for example. The bulkheads  34 ,  36 ,  38  can be welded to the metallic outer barrel  32  and metallic nose lip portion  28 , or can be connected to the outer barrel  32  and/or nose lip portion  28  by mechanical fasteners, such as rivets, or the like. The aft bulkhead  36  and intermediate bulkhead  34  can be fastened to the composite inner barrel  30  by mechanical fasteners such as rivets, bolts, or the like. As shown in  FIG. 1B , one or more circumferentially extending reinforcement ribs  21  can be welded or otherwise attached along the inner surface of the outer barrel  32  to stiffen the thin metal skin and maintain an aerodynamic shape. 
     As discussed below, a typical nacelle structure like that shown in  FIGS. 1A and 1B  and described above can be improved. U.S. Federal Aviation Administration (FAA) regulations set forth numerous design objectives for aircraft. For example, the structural integrity of an aircraft engine nacelle should be sufficient to permit an associated aircraft to be safely flown and landed following a blade-out event. More specifically, a nacelle  22  should maintain a stable and aerodynamic configuration that will not impede the fly-home capability of an aircraft following a blade-out event. As is known in the art, a “blade out event” arises when a blade is accidentally released from a turbine&#39;s rotor, such as when a first-stage fan blade  19  is accidentally released from a rotor of a high-bypass turbofan engine  10 . When suddenly released during flight, a fan blade  19  can impact a surrounding fan case  16  with substantial force, and resulting loads on the fan case  16  can be transferred to surrounding structures, such as to the inlet portion  24  of a surrounding nacelle  22 . These loads can cause substantial damage to the nacelle inlet  24 , including damage to an adjoined inner barrel  30 . In addition or alternatively, a released fan blade  19  can directly impact a portion of an adjacent inner barrel  30 , thereby causing direct damage to the inner barrel  30 . Because the inner barrel  30  directly supports the inlet portion  24  on the fan case  16 , including the outer barrel  32  and nose lip portion  28 , damage to the inner barrel  30  can compromise the structural integrity and stability of the nacelle inlet  24 , and may negatively affect the fly-home capability of an aircraft. 
     A blade-out event also causes the rotational balance of an engine&#39;s fan  18  to be lost. After a damaged engine  10  is shut down following a blade-out event, airflow impinging on the unbalanced fan  18  can cause the fan  18  to rapidly spin or “windmill.” Such wind-milling of an unbalanced fan  18  can exert substantial vibrational loads on the engine  10  and fan case  16 , and at least some of these loads can be transmitted to an attached inlet portion  24  and inner barrel  30  of the nacelle  22 . In addition, following a blade-out event, aerodynamic forces and a suction created by a windmilling fan  18  can exert substantial loads on a damaged inlet portion  24  of the nacelle  22 . Such loads can cause substantial deformation of a damaged inlet portion  24  and can result in unwanted aerodynamic drag. 
     Such loads also can cause cracks or breaks in a damaged composite inner barrel  30  to propagate, further compromising the structural integrity and stability of a damaged inlet portion  24  of a nacelle  22 . Without crack-stopping longitudinal joints or reinforced flanges between adjoined circumferential segments of an inner barrel  30 , such crack propagation can be more severe in a one-piece inner barrel than in a segmented inner barrel. 
     As discussed above, the inner barrel  30  of a typical nacelle inlet  24  substantially contributes to the overall strength and rigidity of nacelle inlet&#39;s structure. Accordingly, when the inner barrel  30  of an inlet portion  24  of a nacelle is substantially damaged subsequent to a blade-out event, the structural integrity and rigidity of a nacelle&#39;s inlet portion  24  may not be sufficient to adequately withstand such suction and/or aerodynamic loads, or to maintain a stable and aerodynamic configuration of the nacelle inlet  24  that is sufficient to support the fly-home capability of an aircraft. 
     Accordingly, there is a need for a nacelle structure for a turbofan aircraft engine that is capable of maintaining a substantially stable and aerodynamic configuration subsequent to a blade-out event, and which thereby supports an aircraft&#39;s fly home capability following such an incident. In particular, there is a need for a nacelle inlet structure for a high-bypass turbofan aircraft engine that maintains its structural integrity and a stable aerodynamic configuration even though its composite inner barrel has been substantially damaged due to a blade-out event. Preferably such an improved nacelle inlet will include a minimal number of components in order to minimize weight and minimize manufacturing costs. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention includes a nacelle inlet for an aircraft engine of a type having an engine fan case with a forward flange. The nacelle inlet can include an acoustic inner barrel having a forward edge and an aft edge, and an outer shell including a nose lip portion having a trailing inner edge and an outer barrel portion having an aft portion. The nacelle inlet can further include an aft attachment flange configured to attach the inlet to the forward flange of the engine fan case, and an aft bulkhead having an aft end and connecting the outer barrel portion of the outer shell to the aft attachment flange. The forward edge of the acoustic inner barrel can be connected to the trailing inner edge of the nose lip portion, and the aft portion of the inner barrel can be attached to the aft attachment flange. 
     In another embodiment, the invention includes a nacelle for an aircraft engine of a type having a fan case. The nacelle can include an inlet portion having an acoustic inner barrel, an outer shell including a nose lip portion and an outer barrel portion, and a mounting means for mounting the inlet portion to the fan case. The mounting means can provide a load path from an aft portion of the outer shell to the fan case through the mounting means such that no substantial portion of the load path passes through the acoustic inner barrel. 
     In a further embodiment, an aircraft engine nacelle can include an inlet portion having an outer shell with a nose lip portion and an outer barrel portion. The inlet portion can further include an inner barrel, an aft bulkhead, and an aft mounting flange. The outer barrel can be connected to the aft bulkhead by a first connection, the aft bulk head can be connected to the aft mounting flange by a second connection, and the inner barrel can be connected to the mounting flange by a third connection. The second connection can be separate from the third connection. 
     These and other aspects of the invention will be understood from a reading of the following detailed description together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a turbofan aircraft engine having a nacelle with a typical prior art inlet portion. 
         FIG. 1B  is a cross sectional view of the prior art nacelle inlet portion shown in  FIG. 1A . 
         FIG. 2  is a cross-sectional view of one embodiment of a nacelle inlet portion according to the invention. 
         FIG. 3  is a cross-sectional view of another embodiment of a nacelle inlet portion according to the invention. 
         FIG. 4  is a cross-sectional view of an additional embodiment of a nacelle inlet portion according to the invention. 
         FIG. 5A  is a cross sectional view of a form tool for producing an outer shell of a nacelle inlet. 
         FIG. 5B  is a cross-sectional view of a portion of the tool shown in  FIG. 5A . 
         FIG. 5C  is a cross sectional view of another form tool for producing an outer shell of a nacelle inlet. 
         FIG. 6  is a longitudinal cross section of a segmented inner barrel. 
         FIG. 7  is a perspective view of a segmented inner barrel. 
         FIG. 8  is a longitudinal cross-sectional view of a longitudinal joint between segments of an inner barrel. 
         FIG. 9  is a perspective view of another embodiment of a segmented inner barrel. 
         FIG. 10  is a longitudinal cross-sectional view of another longitudinal joint between segments of an inner barrel. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A cross section of one embodiment of a nacelle inlet portion  200  according to the invention is shown in  FIG. 2 . In this embodiment, the inlet portion  200  includes an outer shell  204  having a rounded nose lip portion  223 , an outer barrel portion  225 , and an aft bulkhead portion  227 . In  FIG. 2 , the rounded nose lip portion  223  extends from an inner trailing edge  221  at point “A” to a point “B,” the outer barrel portion  225  extends between point “B” and point “C,” and the aft bulkhead portion  227  extends between point “C” and a trailing edge  229  at point “D.” In the embodiment shown in  FIG. 2 , the outer shell  204  can be constructed of a continuous piece of material that extends between points “A” and “D.” The outer shell  204  preferably has a contoured shape that facilitates laminar airflow and minimizes aerodynamic drag. The outer shell  204  can be constructed in a single piece that circumferentially extends a full 360 degrees around the nacelle inlet portion  200 , or the outer shell  204  can be constructed in two or more circumferential segments joined together along longitudinal joints (not shown in the drawings). In one embodiment, the outer shell  204  is formed from a graphite composite. Such a graphite composite may be formed by layering sheets of a resin impregnated graphite fabric on a contoured forming tool, and then bagging and-curing the layered fabric and tool in an autoclave in a manner known in the art. Alternatively, the outer shell  204  may be formed from sheet metal, such as aluminum, such as by stretch forming, spin forming, or the like. 
     As shown in  FIG. 2 , the nacelle inlet portion  200  can include an acoustically treated composite inner barrel  202 . The inner barrel  202  can be either a one-piece  360  degree structure, or can include a plurality of joined circumferential segments. The inner barrel  202  can include a honeycomb core  210  sandwiched between a perforated composite inner skin  212  and an imperforate composite outer skin  214 . The manufacture of such an acoustically treated composite inner barrel  202  is known to those skilled in the art. As shown in  FIG. 2 , the inner barrel  202  can include a forward flange  216  for connecting the inner barrel  202  to the inner trailing edge  221  of the nose lip portion  223  of the outer shell  204 . The forward flange  216  of the inner barrel  202  can be attached to the inner trailing edge  221  of the outer shell  204  by a plurality of circumferentially spaced fasteners  230 , such as rivets or the like. As shown in  FIG. 2 , the forward flange  216  of the inner barrel  202  and the trailing edge  221  of the nose lip portion  223  of the outer shell  204  can be configured to provide a substantially continuous aerodynamic surface along their juncture at point “A.” 
     One or more circumferential stiffeners  285  can be attached along the inner surface of the outer barrel portion  225  in order to stiffen the outer barrel portion  225  and maintain its aerodynamic shape. The circumferential stiffeners  285  can be attached to the outer barrel portion  225  with adhesive and/or fasteners, such as rivets, for example. Alternatively, when the outer shell  204  is metal, the stiffeners  285  can be attached by welding. As shown in  FIG. 2 , the nacelle inlet  200  can include a forward bulkhead  260  that extends between the inner trailing edge  221  of the nose lip portion  223  at point “A” and the transition between the nose lip portion  223  and the outer barrel portion  225  at point “B.” The forward bulkhead  260  can be formed of aluminum, titanium or another suitable material. The forward bulkhead  260  combines with nose lip portion  223  to form a D-channel  270 . Heated air can be forced through the D-channel  270  in a known manner to prevent or eliminate ice formations on the outer surface of the nose lip portion  223 . 
     As shown in  FIG. 2 , the aft bulkhead portion  227  of the outer shell  204  can extend radially inwardly and rearwardly from an aft edge (point “C”) of the outer barrel portion  225 , and can include a rearwardly extending trailing edge  229 . The trailing edge  229  of the aft bulkhead portion  227  can be configured for connection to an aft portion  218  of the inner barrel  202  and for connection to a mounting flange  250 . In the embodiment shown, the trailing edge  229  of the aft bulkhead portion  227  can have a substantially cylindrical shape. The mounting flange  250  can be configured for fastening the nacelle inlet  200  to a forward flange of a fan case (not shown), and can include a circle of bolt holes corresponding to a matching circle of bolt holes in the fan case (not shown in  FIG. 2 ). In the embodiment shown in  FIG. 2 , the mounting flange  250 , the trailing edge  229  of the aft bulkhead portion  227  and the aft portion  218  of the outer skin  214  of the inner barrel  202  overlap or underlap each other, and are secured together by a plurality of circumferentially spaced fasteners  252 , such as rivets, bolts, or the like. As also shown in  FIG. 2 , the mounting flange  250  can also be directly fastened to the aft bulkhead portion  227  by another plurality of circumferentially spaced fasteners  254 , such as rivets, bolts, or the like. 
     Unlike prior nacelle inlet designs, the mounting flange  250  can be directly connected to the aft bulkhead portion  227  of the outer shell  204  rather than to only the inner barrel  202 , and thus, the outer shell  204  is directly supported by an associated fan case when the mounting flange  250  is bolted to the fan case. Accordingly, a direct load path is provided from the outer shell  204  to a supporting fan case whereby loads on the outer shell  204  can be transmitted to the fan case without having to pass through the inner barrel  202 . This direct load path ensures that the structural integrity and stability of the nacelle inlet  200  can be maintained even though the structural integrity of the inner barrel  202  may be compromised as the result of a blade-out event. In other words, by providing a direct connection between the outer shell  204  and a supporting fan case that is independent from a connection between the inner barrel  202  and the fan case, the strength, rigidity and stability of the nacelle inlet  200  does not substantially depend upon the structural integrity of the inner barrel  202 . Accordingly, the nacelle inlet  200  can support the fly-home capability of an aircraft despite substantial damage to its inner barrel  202  due to a blade-out incident. In addition, by providing a direct load path from the outer shell  204  to a supporting fan case, loads on the nacelle inlet  200  can be transmitted to the fan case without substantial stress on the inner barrel  202 , thereby reducing the likelihood that damage to the inner barrel  202  will propagate. 
     Furthermore, in contrast to a prior art nacelle inlet  24  like that shown in  FIG. 1B  and discussed above that employs rivets  47  to secure its nose lip  28  to its outer barrel  32 , the one-piece outer shell  204  of the nacelle inlet  200  can be devoid of such fasteners. This elimination of fasteners along the outer surfaces of the nose lip  223  and outer barrel  225  enhances laminar air flow over the outer shell  204  and reduces aerodynamic drag on the nacelle inlet  200 . 
     A cross section of a second embodiment of a nacelle inlet  300  according to the invention is shown in  FIG. 3 . In this embodiment, the nacelle inlet  300  includes an acoustically treated inner barrel  302 , an outer shell  304 , and an aft bulkhead  390 . The outer shell  304  can include a nose lip portion  323  and an outer barrel portion  325 . The nose lip portion  323  can include a trailing edge  306  that connects to a forward flange  316  on the inner barrel  302 . The aft bulkhead  390  connects an aft portion of the outer shell  304  to a mounting flange  350 . In one embodiment, the connection between the aft bulkhead  390  and the mounting flange  350  includes a plurality of circumferentially spaced rivets  397 , or the like. As shown in  FIG. 3 , the aft bulkhead  390  can be connected to the aft edge of the outer barrel portion  325  of the outer shell  304  by a connecting ring  380  which can have a substantially T-shaped cross section. The aft bulkhead  390  and the outer barrel portion  325  can be connected to the connecting ring  380  by pluralities of circumferentially spaced fasteners, such as rivets, bolts or the like. 
     The mounting flange  350  can be secured to an aft portion  318  of the inner barrel  302  using rivets  399 , bolts, or other fastening means. The acoustically treated inner barrel  302  can be substantially like the composite inner barrel  202  described above. As shown in  FIG. 3 , at least a portion  320  of the acoustically treated inner barrel  302  can extend aft of the mounting flange  350  in order to provide acoustic treatment to a forward portion of a mating fan case (not shown in  FIG. 3 ). 
     The outer shell  304  can be constructed of a composite material using known methods, or can be spin formed or stretch formed from sheet metal, such as aluminum. The outer shell  304  can be constructed in one piece, or can be formed in two or more circumferential segments. As shown in  FIG. 3 , the aft bulkhead  390  can be canted or inclined in a forward and outward direction such that the aft bulkhead  390  has a generally frusto-conical shape. A frusto-conically shaped aft bulkhead  390  has greater inherent stiffness than the substantially planar aft bulkhead  36  shown in  FIG. 1B , for example, due to its three dimensional shape and curvature. Due to such greater rigidity, the frusto-conically shaped aft bulkhead  390  is less likely to flex under applied loads than the more flexible planar bulkhead  36 , and is better able to react and directly transfer loads between the mounting flange  350  and the outer barrel portion  325 . In addition, the forward inclination of the aft bulkhead  390  permits the bulkhead  390  to at least partially transmit loads between the mounting flange  350  and the outer barrel portion  325  in tension or compression, rather than primarily in shear and bending. As a consequence, following structural damage to the inner barrel  302  due to a fan blade-out event, undamaged portions of the structure can transmit loads and retain sufficient structural stability to maintain a satisfactory inlet configuration and minimize the propagation of damage, thereby permitting an aircraft to safely fly home. 
     A cross section of a third embodiment of a nacelle inlet  400  according to the invention is shown in  FIG. 4 . The nacelle inlet  400  can include an outer shell  404  and an aft bulkhead  490  that are substantially similar to the outer shell  304  and aft bulkhead  390  described above. In this embodiment, however, the mounting flange  491  is integrally formed as part of the composite outer skin  710  of the inner barrel  700 . The aft bulkhead  490  can be connected to the integral mounting flange  491  by a bracket  492  and a plurality of fasteners  497 , such as rivets, bolts, or the like. As shown in  FIG. 4 , a forward edge of the inner barrel  700  can be joined to an interior trailing edge  421  of the outer shell  404  by a substantially Z-shaped bracket  427  and pluralities of fasteners  402 , such as rivets or the like. In the embodiment shown in  FIG. 4 , the inner barrel  700  can be constructed in a plurality of segments joined together along longitudinally extending flanges  750 , for example. Alternatively, the inner barrel  700  can be constructed as a one-piece, non-segmented barrel. 
       FIGS. 5A and 5B  show tooling which can be used to form a composite outer shell  204  having an integral aft bulkhead portion  227  like that described above and shown in  FIG. 2 .  FIG. 5A  shows a cross sectional view of a 360° lay-up tool  900 . The tool  900  can include a forward tool portion  904 , an aft bulkhead tool portion  920 , and a break-away ring tool portion  940 . The forward tool portion  904  can include a front flange  906 , a rounded nose lip portion  910 , an outer barrel portion  912 , and an aft attach flange  914 . A front flange  906  can be located at a free forward end  905  of the forward tool portion  904 . The nose lip portion  910  can forwardly extend (in the direction “F”) from the front attach flange  906  to a forwardmost point  907  of the forward tool portion  904 , and then rearwardly extend to the outer barrel portion  912 . The outer barrel portion  912  can extend rearwardly from the nose lip portion  910  to an aft end  911  of the forward tool portion  904 . The aft flange  914  outwardly extends from the outer barrel portion  912 . Thus, the forward tool&#39;s nose lip portion  910  and outer barrel portion  912  are shaped similarly to the nose lip portion  223  and outer barrel portion  225 , respectively, of the outer barrel  204  described above and shown in  FIG. 2 . 
     As shown in  FIG. 5A , the forward tool portion  904  can be formed from a plurality of circumferential segments  903   a,    903   b  which can be detachably secured together. In this embodiment, there are three  120  degree circumferential segments (only two segments  903   a,    903   b  are shown), though more or fewer segments can be used. Thus, the front flange  906  and the aft flange  914  can be formed from a plurality of forward end flange segments  913  and aft flange segments  917 , respectively. First and second axially extending flanges  919 ,  921  can extend between the forward end flange segment  913  and the aft flange segment  917 , and can be provided at opposite side edges of each tool portion segment  903   a,    903   b.  The first axially extending flange  919  on one tool portion segment  903   a  can be detachably secured to the second axially extending attach flange  921  of an adjacent tool segment  903   b  to form axially extending flange joints  999 . Adjacent first and second axially extending flanges  919 ,  921  can be secured together with bolts or other suitable fasteners. 
     The aft bulkhead tool portion  920  shown in  FIG. 5A  includes a front attach flange  922 , a canted portion  924  extending aft of the front attach flange  922 , and an aft end flange  926  extending from the canted portion  924 . The forward tool&#39;s aft attach flange  914  can be detachably secured to the aft bulkhead tool&#39;s front attach flange  922  via bolts or other fasteners, for example. The canted portion  924  of the aft bulkhead tool portion  920  is shaped similarly to the aft bulkhead portion  227  of an outer shell  204  (see  FIG. 2 ), and can be used to form the outer shell&#39;s aft bulkhead portion  227 . Although shown as a single, unitary piece, the aft bulkhead tool portion  920  can be formed from a plurality of circumferential segments. 
     As also shown in  FIGS. 5A and 5B , the tool assembly  900  can further include a break-away tool portion or ring  940  for forming an inner trailing edge  221  of an outer shell  204  like that&#39;shown in  FIG. 2 . The break-away tool portion  940  can be a one-piece ring (not shown in  FIG. 5A ) if a sufficient negative draft angle or set-back feature is provided in the front attachment leg inner trailing edge  221  of the laid-up outer shell  204  to allow removal of the of the laid-up outer shell  204  from the front tool portion  904 . Otherwise, the break-away tool portion  940  can be a segmented ring like that shown in  FIG. 5A  and can include separable ring segments  931  to allow the segments  931  to be individually removed prior to removing a laid-up outer shell  204  (not shown) from the forward tool portion  904 . The break-away tool portion  940  can be detachably secured to the front attach flange  906  using fasteners (not shown). As shown in  FIG. 5B , the break-away tool portion  940  can be radially offset from the front attach flange  906  for forming an inner trailing edge  221 . 
     According to one embodiment, a composite lay-up can be made within the tool  900  to make a continuous composite outer shell  204  like that shown  FIG. 2 , for example. Alternatively, circumferential shell segments can be laid-up individually on separate tool portions and/or tool segments and assembled to form a complete outer shell  204 . According to another embodiment, outer shell segments can be partially laid-up on separate tool portions and/or tool segments, and the tool portions/tool segments can then be assembled together and lap-seams applied to bond the outer shell segments together to form a complete outer shell  204 . Once an outer shell  204  has cured in the tool assembly  900 , the forward tool  904 , the aft bulkhead tool  920 , the break-away ring tool  940 , and other tool segments such as the forward tool segments  905  and the break-away ring tool segments  931 , can be separated to facilitate removal of a cured outer shell  204  from the tools  904 ,  920 ,  940 . One of ordinary skill in the art would understand how to manufacture an outer shell in the disclosed lay-up tool  900 . 
     In some embodiments, the trailing inside portion of a nose lip portion of an outer shell can be extended farther aftward than the nose lip portions  223 ,  323 ,  423  shown in  FIGS. 2-4 . In such a case, a lay-up tool  1000  like that shown in  FIG. 5C  can be used. The tool  1000  can be substantially similar to the tool  900  shown in  FIG. 5A , and like components shown in  FIGS. 5A and 5C  have like reference numerals. Unlike the tool  900  described above, however, the tool  1000  does not include an aft bulkhead tool  920  or a break-away tool portion  940 . As shown in  FIG. 5C , the tool  100  can include a nose lip extension tool portion  960  for forming an extended nose lip portion. The nose lip extension tool portion  960  can include a front attach flange  962 , an axially extending portion  964  extending rearwardly from the front attach flange  962 , and an aft end flange  966  extending transversely from the axially extending portion  964 . The front attach flange  906  of the forward tool  904  can be detachably secured to the front attach flange  962  of the nose lip extension tool portion  960  using fasteners  968 , for example. The nose lip extension tool portion  960  can be substituted for the break-away tool portion  940  in previously described tool  900  for forming an outer shell having both an aft bulkhead portion and an extended nose lip portion. The lay-up process for forming a composite outer shell using the lay-up tool  1000  can be substantially similar to that discussed above with respect to tool  900  shown in  FIGS. 5A and 5B . 
       FIG. 6  shows a cross-sectional view of one embodiment a circumferentially segmented acoustic inner barrel  700  of a type that can be employed in conjunction with a nacelle inlet  400  like that described above and shown in  FIG. 4 , for example. As shown in  FIG. 6 , the circumferentially segmented acoustic inner barrel  700  can include three circumferential segments  704 ,  706 ,  708 . Alternatively, a circumferentially segmented acoustic inner barrel can include a different number of circumferential segments, such as two, four, or more. Each circumferential end of a circumferential segment  704 ,  706 ,  708  is provided with a pair of radially outwardly projecting flanges  750 ,  751  that extend in a longitudinal direction substantially entirely along the length of the acoustic inner barrel  700 . Adjacent circumferential segments mate along a longitudinal splice  780  where their radially outwardly projecting flanges  750 ,  751  oppose one another. 
     As shown in  FIG. 7 , each circumferential segment of the acoustic inner barrel  700  can include an upstanding attachment flange  790  at its aft end. The upstanding attachment flange  790  can be formed from a plurality of circumferential upstanding attachment flange segments  791 ,  793 ,  795 , and can be machined with one or more circumferentially spaced apart bolt holes for attachment to a forward flange of an engine fan case (not shown). Circumferentially facing flanges  751 ,  753  of adjacent circumferential segments can extend substantially along the length of the acoustic inner barrel  700 , and can be secured to a mating flange by a plurality of spaced fasteners  792 . 
       FIG. 8  shows a detailed view of a cross-section of one embodiment of a longitudinal splice joint  780  which can be used to join the longitudinal edges of two adjacent inner barrel segments, such as segments  704  and  706 , for example. In one embodiment, each circumferential segment  704 ,  706  includes an inner skin  707 , an acoustic core  709 , and an outer skin  710 . As shown in  FIG. 8 , the outer skin  710  can include a thickened portion  712  in the region where the two circumferential segments  704 ,  706  meet at the splice  780 . As shown in  FIG. 8 , facing inner skin portions  714 ,  715  of adjacent segments  704 ,  706  can be wrapped around the edges of their core sections  709  such that they extend into the splice joint  780 . Additional plies of a prepeg fabric can be provided on the outer shell  710  at the circumferential edges of each segment  704 ,  706  to form the thickened regions  712  and facing flanges  750 ,  751 . The segments  704 ,  706  can then be cured in an autoclave using known methods. When assembled to form a complete inner barrel  700 , the effective gap  730  between the opposed inner skin portions  714 ,  715  (i.e., the acoustical discontinuity) can have an acoustically negligible width, designated P 1 . In one embodiment, the gap width P 1  is less than or equal to approximately 0.2 inches, or about 5 mm. Providing such a minimal acoustic splice effectively creates the acoustic benefits of a continuous 360° one-piece acoustic inner barrel, while retaining the damage propagation stop features of a circumferentially segmented inner barrel. Alternatively, the inner barrel  700  can be assembled by bonding an inner skin  707 , cellular core  709  and outer skin  710  to form an integral unit having no longitudinal joints. 
       FIG. 9  shows a perspective view of another embodiment of a segmented inner barrel  700 ′. The inner barrel  700 ′ includes a plurality of circumferential segments  704 ′,  706 ′,  708 ′ and is similar to the inner barrel  700  described above except the inner barrel  700 ′ includes a reduced diameter aft end portion  794  formed from the aft ends  791 ,  793 ,  799  of the circumferential segments  704 ′,  706 ′,  708 ′. The aft end portion  794  receives a 360-degree engine attach ring  796  which helps to secure the aft ends  791 ,  793 ,  799  together. The engine attach ring  796  can be received over and secured to the reduced diameter portion  794  of the inner barrel  700 ′ using known techniques, such as by adhesives or mechanical fasteners, for example. 
     As described above with reference to  FIG. 4 , a nacelle inlet  400  can include an outer skin  404  and an acoustic inner barrel  700  constructed from a plurality of circumferential segments  704 ,  706 ,  708 . Alternatively, a nacelle inlet can include an outer skin  404  and an acoustic inner barrel  700 ′ constructed from a plurality of circumferential segments  704 ′,  706 ′,  708 ′. In the case of a fan blade-out event, only one of the plurality of circumferential segments  704 ,  706 ,  708  or  704 ′,  706 ′,  708 ′ is likely to be damaged initially, and this damage generally will not propagate to an adjacent circumferential segment due to the longitudinal discontinuities between segments. Since undamaged circumferential segments are affixed to the remaining undamaged structure, they can help augment the stability and fly-home capability of the nacelle inlet  400 . 
       FIG. 10  shows a cross-section of an alternative construction for a segmented inner barrel  800  having an acoustically negligible gap P 2  between two adjacent segments  804 ,  805 . Each circumferential segment  804 ,  805  can include a perforated inner skin  806 , an acoustic core  808 , and an imperforate outer skin  810 . As shown in the figures, the outer skin  810  can include a thickened region  812  where the two circumferential segments  804 ,  805  meet. The facing inner skin portions  814 ,  815  can be wrapped radially outward as shown. An acoustically negligible gap  830  of width length P 2  can exist between the inner skin portions  814 ,  815 . In one embodiment, P 2  is less than or equal to 0.2 inches, or about 5 mm. 
     In the embodiment shown in  FIG. 10 , the acoustic inner barrel  800  includes reinforcement members  850  in the form of splice plates  852  secured to the outer shell  810  by fastening members  880 . As seen in  FIG. 10 , the reinforcement member  850  is attached to the barrel  800  such that an inner surface  854  of the splice plate  852  is joined to the outer surface of the outer shell  810  of the inner barrel  800 . The fastening members  880  extend transversely through the splice plate  852 , through the outer shell  810  of the inner barrel  800  and at least partially into the cellular core  808 . The splice plate  852  can be secured to the outer shell  810  by blind fasteners installed from the back side and penetrating the thickened portion  812  of the outer shell  810 . The fastening members  880  occupy only a small amount of the cross-sectional area of the cellular core  810  and do not extend to or block perforations in the inner skin  806  of the inner barrel  800 . Thus, such a spliced connection will not detrimentally affect the acoustic performance of the barrel  800 . 
     The circumferentially segmented acoustic inner barrel  800  can be used in a manner similar to the circumferentially segmented acoustic inner barrel  700 ,  700 ′ described above. The circumferentially segmented acoustic inner barrel  800  can help provide a nacelle inlet  400  with fly-home capability in addition to that provided by other improvements to the nacelle inlet  400 . 
     While the present invention has been described herein above in connection with a plurality of aspects and embodiments, it is understood that these aspects and embodiments were presented by way of example with no intention of limiting the invention. Accordingly, the present invention should not be limited to any specific embodiment or aspect, but rather construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.