Abstract:
An exhaust system for an aircraft has a primary exhaust duct for communicating exhaust gas from an engine exhaust exit and is configured for movement with the engine. A secondary exhaust duct is in fluid communication with the primary exhaust duct and is movably mounted to the airframe. The secondary duct has a portion selectively rotatable relative to the remainder of the secondary duct for directing the exhaust gas vector. The system has means for maintaining a generally consistent relative alignment between the primary duct and the secondary duct.

Description:
TECHNICAL FIELD 
       [0001]    The technical field is engine exhaust systems for aircraft. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    In conventional aircraft exhaust systems, an exhaust ejector has a primary exhaust gas duct attached to an engine flange for receiving exhaust gas from the engine and passing exhaust gas through the primary exhaust gas duct. The conventional exhaust ejector also has a secondary engine exhaust gas duct attached to the airframe and initially concentric with the primary exhaust duct. However, shifting, vibrating, or other relative movement of the engine with respect to the airframe often results in the primary exhaust gas duct becoming non-concentric with the secondary exhaust gas duct. 
         [0003]    For example,  FIG. 1  shows a prior-art engine exhaust system comprising a primary exhaust duct  13 , a secondary exhaust duct  15 . Primary exhaust duct  13  is attached directly to engine  17  and moves with engine  17 , whereas secondary exhaust duct  15  is attached to airframe  19  and remains in a generally fixed position relative to airframe  19 . When engine  17  moves relative to airframe  19 , primary duct  13  and secondary duct  15  may become non-coaxial and non-concentric.  FIG. 1  illustrates this, as axis  21  of primary duct  13  is not coaxial with axis  23  of secondary duct  15 . 
         [0004]    When primary duct  13  is not concentric with secondary duct  15 , the exhaust gas flow in secondary duct  15  may be directionally biased, resulting in poor ejector performance. The misalignment can cause several undesirable conditions, including turbulent exhaust gas flow within secondary duct  15  and/or direct impinging of portions of the flow of hot exhaust gas  25  on inner surface  27  of secondary duct  15 . Both of these conditions can result in overheating of portions of secondary duct  15 . In addition, less than optimal exhaust gas ejection may include higher engine compartment temperatures, higher exhaust gas temperatures, and these effects may negatively impact other components of the aircraft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a cross-sectional side view of a prior-art engine exhaust system. 
           [0006]      FIG. 2  is a partially cross-sectioned side view of the preferred embodiment of an engine exhaust system according to the present invention. 
           [0007]      FIG. 3  is a cross-sectional view (taken at cutting plane A-A of  FIG. 2 ) of the engine exhaust system of  FIG. 2 . 
           [0008]      FIG. 4  is a schematic end view of a rotatable nozzle of the engine exhaust system of  FIG. 2  shown in a first position and in phantom in a second position. 
           [0009]      FIG. 5  is a schematic detail view of a joint of the engine exhaust system of  FIG. 2 . 
           [0010]      FIG. 5  is a simplified end view of an actuator assembly of the engine exhaust system of  FIG. 2 . 
           [0011]      FIG. 6  is a simplified schematic view of a limit switch system of the engine exhaust system of  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    To resolve the issue of aircraft exhaust ducts undesirably becoming non-concentric due to relative movement between the engine and the airframe of an aircraft and the issue of hot aircraft exhaust flow unduly impacting aircraft components, an exhaust system provides (1) a means for linking the two ducts together so that even with engine movement relative to the airframe, the two ducts remain concentric, and (2) a means for rotating an exhaust duct/nozzle to direct exhaust flow in an optimal direction. Therefore, if engine movement occurs for any reason, the airframe mounted secondary duct is pushed or pulled into consistent alignment with the primary duct for maintaining maximum ejector performance while also providing for selective control of the exhaust gas vector. Rotation of the gas vector allows for redirecting of hot exhaust gas from impinging on composite parts (such as rotor blades) or other heat-sensitive components during near-idle conditions when the aircraft id on the ground. In addition, the exhaust vector may be redirected while the aircraft is on the ground or in flight to alter the infra-red (heat) signature of the aircraft for avoiding heat-seeking devices, such as missiles. 
         [0013]    Referring now to  FIGS. 2 and 3 , an embodiment of an engine exhaust system  101  is illustrated. Exhaust system  101  comprises a tubular primary exhaust duct  103 , a tubular secondary exhaust duct  105 , and a slip joint  107  for allowing relative axial movement between ducts  103 ,  105 . Primary exhaust duct  103  and secondary exhaust duct  105  are also held in alignment by a drag link  109 . Secondary exhaust duct  105  is connected to an airframe  111  and is supported by vertical struts  113  and a lateral strut  115 , which are preferably struts connected at each end with uni-ball connectors. Vertical struts  113  carry vertical loads, and lateral strut  115  carries side loads. Preferably, the strut attachments are located on or very close to the center of gravity to avoid any undesired moments. 
         [0014]    Primary exhaust duct  103  is attached directly to engine  117  for allowing exhaust gas to flow from engine  117  through primary exhaust duct  103  and into secondary duct  105 . A forward end of secondary exhaust duct  105  is slipped concentrically into and sealably joined to an engine flange  119  through the use of o-ring type seal  121  in slip joint  107 , and use of o-ring seal  121  allows for thermal expansion of ducts  103 ,  105 . O-ring seal  107  is preferably a high-temperature o-ring type seal. Slip joint  107  also supports duct  105  in both vertical and horizontal directions, but not in an axial direction. Axial control of duct  105  is accomplished with drag link  109 , which connects flange  119  to secondary duct  105 . The mounts of drag link  109  are preferably uni-ball connectors, and this configuration allows for relative axial movement between engine  117  and secondary duct  105 , but this does not allow for vertical or lateral movement. The single degree of freedom associated with drag link  109  allows engine movement to push or pull secondary exhaust duct  105  consistently with primary duct  103  and keeps the system in the desired alignment. 
         [0015]    Secondary exhaust duct  105  has a curved portion  123  for altering the direction of exhaust flow from its original flow path along central axis  124  of a fixed portion  125  of duct  105  to a direction off the central axis of fixed portion  125 . In the nominal orientation, cured portion directs flow in the direction shown relative to fixed portion  125 . However, curved portion  123  is rotatable generally about axis  124  through a range of motion of about 90 degrees to either side (or to the extent of the range of motion available in the particular application).  FIG. 4  shows an end view of secondary duct  105  with curved portion  123  in the nominal position (solid lines) and rotated to one side (broken lines). To enable rotation while retaining the function of the means for keeping ducts  103  and  105  concentric, a bearing band  127  is disposed between curved portion  123  and struts  113 ,  115 . 
         [0016]    A joint  129 , which is shown in  FIG. 5 , joins curved portion  123  and fixed portion  125  together. Joint  129  is configured to retain curved portion  123  axially by a retainer  131  while allowing curved portion  123  to rotate relative to fixed portion  125  along central axis  124 . Also, a self-lubricated bearing coating  133 , preferably Rexton, is disposed at the interface of curved portion  123  and fixed portion  125 . 
         [0017]      FIG. 6  is an end view of an actuator system  135 , comprising an electric motor type actuator  137  connected to curved portion  123  through the use of wire rope  139  and chain  141 . Actuator  137  is selectively controlled to rotate curved portion  123  into a position yielding optimal exhaust gas flow direction, thereby controlling or eliminating aircraft component overheating or altering the IR signature of the aircraft. Actuator  137  may be controlled manually, but is preferably controlled using a micro-processor-based flight control computer. 
         [0018]      FIG. 7  shows an adjustable stop  143  operably associated with wire rope/cable  139  and/or chain  141  for interacting with limit switch system  145 . Of course, any other means for limiting, controlling, or causing rotation of curved portion may be used in alternative embodiments of the present invention. 
         [0019]    The exhaust system provides for several advantages, including: (1) the ability to maintain primary and secondary exhaust ducts in a desired orientation; (2) selectively control the exhaust gas vector; (3) low weight; (4) increased reliability and durability; and (5) easy installation. 
         [0020]    This description includes reference to illustrative embodiments, but it is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments, will be apparent to persons skilled in the art upon reference to the description.