Abstract:
A gas generator for a reverse core engine propulsion system has a variable cycle intake for the gas generator, which variable cycle intake includes a duct system. The duct system is configured for being selectively disposed in a first position and a second position, wherein free stream air is fed to the gas generator when in the first position, and fan stream air is fed to the gas generator when in the second position.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of provisional application Ser. No. 61/781,778, filed Mar. 14, 2013. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The Government of the United States of America may have rights in the present invention as a result of NASA Cooperative Agreement Contract No. NNX11AB35A and Sub-Contract No. MIT/PW Subaward No. 5710002937 awarded by NASA. 
    
    
     BACKGROUND 
     The present disclosure is directed to a variable cycle intake for a propulsion system having a reverse core engine, which variable cycle intake has a first position for supplying free stream air to an inlet of the engine and a second position for supplying fan stream air to the inlet of the engine. 
     Typical multi-spool turbofan engines include a nested core, in which a high pressure, or core, spool is nested inside a low pressure spool. Such a nested core engine includes, in axial sequence, a low pressure compressor, a high pressure compressor, a combustor section, a high pressure turbine, and a low pressure turbine. The high pressure compressor is connected to the high pressure turbine with a high pressure shaft that extends through the combustor section. The low pressure compressor is connected to the low pressure turbine with a low pressure shaft that extends through the high pressure shaft. Increases in efficiency of the turbofan allow for the core to be reduced in size, such as by having a smaller diameter. The low pressure shaft, however, cannot be reduced in diameter because the rotational speeds of the low pressure spool are limited by critical speed. The shaft critical speed is proportional to the shaft diameter and inversely proportional to the shaft length. Thus, decreasing the shaft diameter with reduced core sizes is not possible without reducing the shaft length if the same critical speed is desired. Thus, reductions in the core size yields compromises in the high pressure spool to accommodate low pressure spool shaft diameters. For example, the size and weight of high pressure spool rotor disk need to be increased to accommodate openings for larger low pressure shaft sizes. As such, there is a need for improving engine architectures to allow for, among other things, decreased core sizes resulting from more efficient turbofan engines. 
     There has been proposed a gas turbine engine comprising a fan drive gear system, a low spool connected to the fan drive gear system, and a high spool disposed aft of the low spool. The low spool comprises a rearward-flow low pressure compressor disposed aft of the fan drive gear systems, and a forward flow low pressure turbine disposed aft of the low pressure compressor. The high spool comprises a forward flow high pressure turbine disposed aft of the low pressure turbine, a combustor disposed of aft of the high pressure turbine, and a forward-flow high pressure compressor disposed aft of the combustor. 
     One issue faced by designers of these new engine architectures is incorporation of the new engine architecture into an aircraft. 
     SUMMARY 
     In accordance with the present disclosure, there is provided a gas generator for a reverse core propulsion system, which broadly comprises a variable cycle intake for the gas generator, said variable cycle intake comprising a duct system which is configured for being selectively disposed in a first position and a second position, wherein free stream air is fed to the gas generator when in the first position and fan stream air is fed to the gas generator when in a second position. 
     In another and alternative embodiment, the duct system includes a free stream air inlet, a duct extending from the free stream air inlet, a slidable duct, a curved duct segment, and an outlet duct section. 
     In another and alternative embodiment, the slidable duct moves between a first position where the slidable duct communicates with the curved duct segment and a second position where the slidable duct is out of communication with the curved duct segment. 
     In another and alternative embodiment, the slidable duct surrounds a portion of the duct extending from the free stream air inlet. 
     In another and alternative embodiment, the curved duct segment surrounds a portion of the outlet duct section. 
     In another and alternative embodiment, the outlet duct section supplies one of free stream air and fan stream air to the gas generator. 
     In another and alternative embodiment, the outlet duct section is connected to an inlet of the gas generator. 
     In another and alternative embodiment, the gas generator further comprises a fan stream air inlet duct. 
     In another and alternative embodiment, the curved duct segment is moved from a free air stream position in contact with the slidable duct and out of contact with the fan stream air inlet duct to a fan air stream position in contact with the fan stream air inlet duct and out of contact with the slidable duct. 
     In another and alternative embodiment, the gas generator further comprises an actuator to move the curved duct segment from the free air stream position to the fan stream air position and from the fan stream air position to the free air stream position. 
     In another and alternative embodiment, the actuator has a first arm connected to a first surface of the curved duct segment and a second arm connected to a second surface of the curved duct segment. 
     In another and alternative embodiment, the gas generator further comprises a first link connected to the first surface of the curved duct segment and to a first surface of the slidable duct and a second link connected to the second surface of the curved duct segment and to a second surface of the slidable duct to move the slidable duct as the curved duct segment moves. 
     In another and alternative embodiment, the gas generator further comprises a particle separator connected to the free stream air inlet. 
     In another and alternative embodiment, the gas generator further comprises a cover plate for covering the free stream air inlet when the variable cycle intake is in the second position. 
     Further in accordance with the present disclosure, there is provided an aircraft which broadly comprises a fuselage having a tail section; a pair of gas generators located in the tail section; each of the gas generators having a variable cycle intake for supplying one of free stream air and fan stream air to a respective one of the gas generators; and variable cycle intake comprising a duct system which feeds free stream air to the respective one of the gas generators when in a first position and which feeds fan stream air to the respective one of the gas generators when in a second position. 
     In another and alternative embodiment, the duct system includes a free stream air inlet, a duct extending from the free stream air inlet, a slidable duct, a curved duct segment, and an outlet duct section. 
     In another and alternative embodiment, the slidable duct moves between a first position where the slidable duct communicates with the curved duct segment and a second position where the slidable duct is out of communication with the curved duct segment. 
     In another and alternative embodiment, the slidable duct surrounds a portion of the duct extending from the free stream air inlet. 
     In another and alternative embodiment, the curved duct segment surrounds a portion of the outlet duct section. 
     In another and alternative embodiment, the outlet duct section supplies one of free stream air and fan stream air to the respective one of the gas generators. 
     In another and alternative embodiment, each of the gas generators comprises a reverse core engine and the outlet duct section is connected to an inlet of the respective one of the gas generator. 
     In another and alternative embodiment, the duct system further comprises a fan stream air inlet duct. 
     In another and alternative embodiment, the curved duct segment is moved from a free air stream position in contact with the slidable duct and out of contact with the fan stream air inlet duct to a fan air stream position in contact with the fan stream air inlet duct and out of contact with the slidable duct. 
     In another and alternative embodiment, the duct system further comprises an actuator to move the curved duct segment from the free air stream position to the fan stream air position and from the fan stream air position to the free air stream position. 
     In another and alternative embodiment, the actuator has a first arm connected to a first surface of the curved duct segment and a second arm connected to a second surface of the curved duct segment. 
     In another and alternative embodiment, the duct system further comprises a first link connected to the first surface of the curved duct segment and to a first surface of the slidable duct and a second link connected to the second surface of the curved duct segment and to a second surface of the slidable duct to move the slidable duct as the curved duct segment moves. 
     In another and alternative embodiment, the duct system further comprises a particle separator connected to the free stream air inlet. 
     In another and alternative embodiment, the duct system further comprises a cover plate for covering the free stream air inlet when the variable cycle intake is in the second position. 
     In another and alternative embodiment, the duct system is at least partially embedded within an aerodynamic fairing. 
     In another and alternative embodiment, the aircraft further comprises a pair of free turbines and a pair of fans fan driven by said free turbines, wherein said gas generators provide air for driving said pair of free turbines. 
     Other details of the variable cycle intake for reverse core engines are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic representation of an aircraft having a propulsion system with two gas generators in the form of reverse core engines; 
         FIG. 1B  illustrates a portion of the tail section of the aircraft of  FIG. 1A ; 
         FIG. 2  is a sectional view of the propulsion system for propelling the aircraft of  FIG. 1 ; 
         FIG. 3  is a sectional view of a fairing having the variable cycle intake embedded therein; 
         FIG. 4  is a schematic representation of the variable cycle intake in a first position where free stream air is supplied to a gas generator; 
         FIG. 5  is a schematic representation of the variable cycle intake of  FIG. 4  in a second position where fan stream air is supplied to the gas generator; 
         FIGS. 6A-6C  are schematic representation of the variable cycle intake as it moves from the first position to the second position; 
         FIG. 7  is a rear view of the propulsion system showing the fairing blended into a bi fi wall; 
         FIG. 8  illustrates the variable cycle intake in the first position; 
         FIG. 9  illustrates the variable cycle intake in the second position; 
         FIG. 10  illustrates the flow through the variable cycle intake when in the first position; 
         FIG. 11  illustrates the flow through the variable cycle intake when in the second position; and 
         FIG. 12  illustrates a cover which can be slid over an air inlet of the variable cycle intake when not in use. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  illustrate an aircraft  10  having a fuselage  12 , wings  14 , and a tail  15  having vertical tail surfaces  16  and a tail wing  18  mounted to the tail surfaces  16 . A propulsion system having a pair of propulsors  20 , which are gas turbine engines, is mounted to the fuselage  12  at the base of the tail  15 . The inlet  44  to each of the propulsors  20  includes a channel  46  in the fuselage  12  for delivering atmospheric air to the propulsors  20 . An aerodynamic fairing  22  may extend from each side of the fuselage  12  adjacent the tail  15 . 
     Referring now to  FIG. 2 , each of the propulsors  20  may comprise a propulsor section  23  which has a free turbine  52 , a fan  48  having a plurality of fan blades  49  which is driven by the free turbine  52 , and a plurality of fan exit guide vanes  47 . The free turbine  52  and the fan  48  rotate about a central axis  24 . Each of the propulsors  20  further has a gas generator  26  which has a longitudinal axis or central axis  28  which is at an angle to the fan central axis  24 . 
     The illustrated gas generator  26  is a reverse core engine which includes a compressor section  50  having one or more stages such as a low pressure compressor and a high pressure compressor, a combustion section  51  having one or more combustors, and a turbine section  53  having one or more stages such as a low pressure turbine and a high pressure turbine. The low pressure compressor in the gas generator  26  is driven by a low pressure turbine via a low pressure spool and a high pressure compressor in the gas generator  26  is driven by a high pressure turbine via a high pressure spool. The gas generator  26  delivers combusted fluid to the free turbine  52 , for driving same, via a plenum  55  connected to the outlet of the gas generator  26 . The free turbine  52  drives the fan  48 . 
     Referring now to  FIG. 3 , there is shown a variable cycle air intake  60  which is at least partially embedded within the aerodynamic fairing  22 . As can be seen from  FIG. 3 , the aerodynamic fairing has a leading edge  62 , a trailing edge  64 , an upper aerodynamic surface  66 , and a lower aerodynamic surface  68 . 
     Referring now to  FIGS. 4 and 5 , the variable cycle intake  60  has a duct system which includes a free stream air inlet  70 , a duct  72  extending from the air inlet  70 , a slidable duct section  74  which surrounds a portion of the duct  72  and which moves relative to the duct  72 , a curved duct segment  76 , and an outlet duct section  78  which connects to an inlet of a low pressure compressor section of the gas generator  26 . 
     The curved duct segment  76  overlaps and surrounds a portion of the outlet duct section  78 . The curved duct segment  76  is movable relative to the outlet duct section  78  between a first position (see  FIG. 4 ) and a second position (see  FIG. 5 ). In the first position, the curved duct segment  76  is in communication with the slidable duct section  74 . In the second position (see  FIG. 5 ), the curved duct segment  76  is in communication with a fan stream air inlet duct  80 . 
     As can be seen from  FIGS. 4 and 5 , the curved duct segment  76  is rotated about an axis  82  by a U-shaped actuator  84 . As shown in  FIGS. 6A-6C , the U-shaped actuator  84  has a first arm  86  connected to a first surface  88  of the curved duct segment  76  and a second arm  90  connected to a second surface  92  of the curved duct segment  76 . The actuator  84  may be rotated about the axis  82  by a motor (not shown) or any other suitable power source. 
     An upper link  94  is connected at a first end  96  to the first surface  88  of the curved duct segment  76 . At a second end  98 , the upper link  94  is connected to a first surface  95  of the slidable duct section  74 . As shown in  FIG. 5 , a lower link  102  is connected to at a first end to the second surface  92  of the curved duct segment  76 . At a second end, the lower link  102  is connected to a second surface  108  of the slidable duct section  74 . 
     Referring now to  FIGS. 6A-6C , as the actuator  84  rotates about the axis  82  towards the air inlet  70 , the rotation of the actuator causes the slidable duct section  74  to move from a first free stream air position to a second fan stream position. In the first free stream position the slidable duct section  74  is in contact with the curved duct segment  76 . In the second fan stream position, the duct  74  is out of contact with the curved duct segment  76 . 
     When moving from the first position to the second position, the slidable duct section  74  moves relative to the duct  72  by siding in a direction toward the air inlet  70  and assume the position shown in  FIG. 5  and  FIG. 6C . As shown in  FIGS. 6B and 6C , movement of the slidable duct section  74  creates a gap  110  which allows the curved duct segment  76  to rotate and come into fluid communication with the fan stream inlet duct  80 . When the curved duct segment  76  is in the position shown in  FIG. 6C , fan stream air is supplied to the inlet of the gas generator  26 . 
     When the actuator  84  rotates about the axis  82  away from the air inlet  70 , the rotation of the actuator causes the curved duct segment  76  to rotate into the position shown in  FIG. 4  and causes the slidable duct section  74  to slide over the duct  72  and into the position shown in  FIG. 4  where the slidable duct section  74  is in communication with the curved duct segment  76  and the curved duct segment is out of contact with the fan stream inlet duct  80 . In this position, free stream air is provided to the inlet of the gas generator  26 . 
     The variable cycle intake  60  may include a particle separator  112  (see  FIG. 3 ) which separates solid particles from the free air stream. The particle separator  112  may be provided with a first, upstream outlet that communicates with an internal channel  114  and a second downstream outlet  116  in the external lower aerodynamic surface  68 . Particles within the free air stream tend not to follow the curvature of the intake  30  and continue on straight into the particle separator  112 . 
     As shown in  FIG. 7 , the aerodynamic fairing  22  may be blended into a bi-fi wall  118  surrounding the core  120  of the gas generator  26 . 
       FIGS. 8 and 10  illustrate the variable cycle intake  60  in a first position where free air stream may be provided to a low pressure compressor section of the gas generator  26 . 
       FIGS. 9 and 11  illustrate the variable cycle intake  60  in a second position where fan air stream may be provided to the low pressure compressor section of the gas generator  26 . 
     As shown in  FIG. 12 , a cover plate  122  may be provided within the fairing  22  to cover the free stream air inlet  70  when the variable cycle intake  60  is in the fan stream air position. An actuator (not shown) may be provided to slide the cover plate  122  over the air inlet  70 . 
     The primary benefit of the variable cycle intake  60  is the dual cycle capability that it provides. 
     There has been provided in accordance with the present disclosure a variable cycle intake for a reverse core engine. While the variable cycle intake has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.