Abstract:
An aircraft auxiliary power unit assembly includes an aircraft skin that provides a cavity. It is secured to a structure in an assembled condition and it provides an opening. An auxiliary power unit is arranged within the cavity and is secured to the structure. The aircraft skin substantially covers the auxiliary power unit in the assembled condition. An inlet duct that is removable is secured within the opening and is selectively connected to the auxiliary power unit between the installed and service positions. The installed and service positions are with the aircraft skin in the assembled condition.

Description:
[0001]    This disclosure is a continuation application of U.S. application Ser. No. 13/173447 filed on  30  Jun. 2011. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to an aircraft auxiliary power unit assembly, which includes an inlet duct arranged at an aircraft skin. The disclosure also relates to a method of servicing the auxiliary power unit while keeping the aircraft skin in place. 
         [0003]    Auxiliary power units (APU) are commonly used in aircraft to provide emergency power for various aircraft systems. Typically, the APU is mounted to structural members within a tail cone of the aircraft. Aircraft skin is secured to the structural members, such as the airframe, to enclose the APU. To service components of the APU, such as line replaceable units (LRUs), portions of the skin must be removed to provide sufficient access to the LRUs. 
         [0004]    An inlet duct is typically integral with a housing of the APU. The inlet duct extends from the APU and seals against an interior surface of the aircraft skin within the cavity. The inlet duct cannot be removed until the skin is removed from the airframe. 
       SUMMARY 
       [0005]    In one exemplary embodiment, an aircraft auxiliary power unit assembly includes an aircraft skin that provides a cavity. It is secured to a structure in an assembled condition and it provides an opening. An auxiliary power unit is arranged within the cavity and is secured to the structure. The aircraft skin substantially covers the auxiliary power unit in the assembled condition. An inlet duct that is removable is secured within the opening and is selectively connected to the auxiliary power unit between the installed and service positions. The installed and service positions are with the aircraft skin in the assembled condition. 
         [0006]    In a further embodiment of the above, the aircraft skin provides a tail cone. An outer surface is provided on an exterior side opposite the cavity. The aircraft skin includes a skin flange that defines the opening. The inlet duct has a duct flange secured to the skin flange in the installed position with fastening elements. 
         [0007]    In a further embodiment any of the above, the inlet duct includes a passage and a screen arranged over the passage. The outer surface provides a contour. The screen is generally arranged along the contour and affixed within the opening in the installed position. 
         [0008]    In a further embodiment any of the above, the auxiliary power unit includes line replaceable units including at least one of a fuel control unit, a speed sensor, and an igniter. The line replaceable units are removable through the opening with the inlet duct removed to the service position. 
         [0009]    In another exemplary embodiment, an inlet duct for an auxiliary power unit includes a duct which defines a passage. First and second flanges are arranged on the duct at opposite ends of the passage. The first flange includes apertures that are configured to receive fastening elements. A seal is secured to the second flange. Silencing elements are arranged within the passage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0011]      FIG. 1  is a schematic view of an APU arranged within a tail cone. 
           [0012]      FIG. 2  is a partial cross-sectional view of the APU and an inlet duct. 
           [0013]      FIG. 3  is a perspective view of the inlet duct. 
           [0014]      FIG. 4  is a one perspective view of the APU with the inlet duct removed to service the APU. 
           [0015]      FIG. 5  is another perspective view of the APU with the inlet duct removed to service the APU. 
           [0016]      FIG. 6A  is a cross-sectional view of a first arrangement of the inlet duct secured to an aircraft skin. 
           [0017]      FIG. 6B  is a cross-sectional view of a second arrangement of the inlet duct secured to the aircraft skin. 
           [0018]      FIG. 6C  is a cross-sectional view of a third arrangement of the inlet duct secured to the aircraft skin. 
           [0019]      FIG. 6D  is a cross-sectional view of a fourth arrangement of the inlet duct secured to the aircraft skin. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Referring to  FIG. 1 , an auxiliary power unit (APU)  10  is shown arranged in a cavity  11  of a tail cone  12 . The APU  10  is mounted to a structural member  13 . The APU  10  includes a gearbox  14  through which a generator  16  is mechanically driven in response to rotation of a shaft-mounted compressor and turbine (not shown). Air from the exterior of the tail cone  12  is supplied through an inlet duct  18  to the compressor. The compressed air, once expanded across the turbine, is expelled through an exhaust duct  20 . 
         [0021]    An aircraft skin  22  is secured to the structural member  13  in an assembled condition in which the aircraft is prepared for flight. The skin  22  provides an outer surface  23  on an exterior side of the skin  22  opposite the cavity  11 . The outer surface  23  provides an aerodynamic outer mold line or contour of the tail cone  12 . The inlet duct  18  is secured to the skin  22 . 
         [0022]    Referring to  FIG. 2 , the APU  10  includes an inlet housing  24  having a housing flange  26 . The inlet duct  18  defines a passage  38  provided by a neck  31  and includes first and second duct flanges  28 ,  30  arranged on opposite ends of the neck  31 . The first duct flange  28  is secured to a skin flange  62  (see  FIGS. 4 and 5 ) provided by the skin  22 . In the example, the skin flange  62  is recessed from the adjoining outer surface  23  ( FIG. 1 ). A seal  32  is secured to the second duct flange  30  with fastening elements  34 . In one example, the seal  32  is a bellow-type seal that seals against the housing flange  26 , enabling removal of the inlet duct  18  while the APU  10  remains secured to the structural member  13 . 
         [0023]    The inlet housing  18  is configured to reduce inlet noise during operation of the APU  10 . The body of the inlet duct  18  may be constructed from a fiberglass with a phenolic resin or carbon fiber composite. It should be understood that any suitable material may be used, for example, metallic and/or non-metallic materials. A first silencing element  36 , in the example, two silencing elements, is arranged within the passage  38  and extends between a wall  40  and opposite ends of the inlet duct  18 . The leading edge of the first silencing element  36  includes a wear resistant structure. The wall  40  includes a second silencing element  42  circumscribing at least a portion of the passage  38 . The wall  40  protrudes outward from the passage  38  to enable the second silencing element  42  to be arranged generally flush with the adjoining passage wall. In one example, the first and second silencing elements  36 ,  42  may include a sound-deadening structure constructed from an aluminum honeycomb to which a porous mesh is adhered with resin. 
         [0024]    Referring to  FIGS. 2 and 3 , a screen assembly  44  is arranged over the entry of the inlet duct  18 . The screen assembly  44  may be a separate structure from the inlet duct  18  (as shown in  FIGS. 2 and 3 ) or integrated therewith. In one example, the screen assembly  44  includes a perimeter element  43  surrounding and supporting a screen  45 . The screen assembly  44  generally lies within the contour provided by the outer surface  23  in an installed position, as illustrated in  FIG. 3 . 
         [0025]    Apertures  46  extend through the perimeter element  43  (if a separate screen assembly is used), the first duct flange  28  and the skin flange  62  (as shown in  FIG. 4 ). Fastening elements  66  (see  FIGS. 6A-6D ) secure the inlet duct  18  to the skin  22 . The fastening elements  66  may be a flat machine screw and nut, for example. The outermost structure (in the example, the perimeter element  43 ) includes a chamfer  48  circumscribing each aperture  46  to accommodate a countersunk head and provide a more aerodynamic surface. 
         [0026]    Referring to  FIGS. 4 and 5 , with the inlet duct  18  and screen assembly  44  removed from the skin  22  in a service position in which the inlet duct  18  is remote from the skin  22 , APU components, such as line replaceable units (LRUs), may be serviced through the opening  50 . Example LRUs include a fuel control unit  52 , a speed sensor  56  and an igniter  60 . The fuel control unit  52  may be replaced by loosening a clamp  54 . An oil cooler  58  also may be serviced and cleaned through the opening  50 . 
         [0027]    Referring to  FIG. 6A-6D , multiple configurations of securing the inlet duct  18  to the skin flange  62  of the skin  22  are shown. The arrangements are exemplary, and it should be understood that different arrangements or combinations of the illustrated arrangements may be used. The inlet duct/screen assembly is arranged generally flush with the outer surface  23 . 
         [0028]    In an example shown in  FIGS. 6A , the screen  145  is integrated with the inlet duct  18 . The first duct flange  128  is seated against the skin flange  62 . A retainer  64  is provided over the first duct flange  128  and the outer surface  23  and secured by fastening elements  66  to retain the inlet duct  118  in the installed position. 
         [0029]    Referring to  FIG. 6B , a reinforcing washer  68  is embedded into the first duct flange  228 , which prevents composite-constructed inlet ducts from becoming crushed or cracked during installation of the inlet duct. The inlet duct  218  is secured to the skin flange  62  by the fastening element  66 . 
         [0030]      FIG. 6C  illustrates a cross-sectional view of the inlet duct  18  and screen assembly  44 , shown in  FIGS. 2 and 3 , secured to the skin flange  62  with fastening element  66  arranged through the first duct flange  28 . 
         [0031]    Referring to  FIG. 6D , a gasket  70  is provided between the inlet duct  118  and the skin flange  62  and secured by fastening elements  66 . 
         [0032]    In operation, the method of servicing the APU  10  includes removing the inlet duct  18  (from the installed position) from the opening  50  in the skin  22  while the skin  22  remains secured to the structural member  13  in the assembled condition. The fastening elements  66  are removed, and the first duct flange  28  is unseated from the skin flange  62  that is provided around the opening  50 . If the screen assembly  44  is separate, it is removed from the opening  50 . The neck  31  is withdrawn from the cavity  11  through the opening  50  to the exterior side of the  22  (service position). The seal  32  is unseated from the housing flange  26 , in the example. 
         [0033]    The APU  10  is exposed with the inlet duct  18  removed, and a portion of the APU  10  is serviceable through the opening  50 . An auxiliary power unit component can be serviced with the APU  10  in place within the cavity  11 . For example, LRUs, such as the fuel control unit  52 , the speed sensor  56 , and the igniter  60 , can be replaced. 
         [0034]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.