Abstract:
A dual boundary layer engine inlet for a turbofan propulsion engine of an aircraft having a first air inlet positioned generally within the boundary layer flowing around the exterior surface of the aircraft. A first passageway fluidly interconnects the first air inlet and the turbofan propulsion engine to provide air from the boundary layer to the bypass to reduce aerodynamic drag. A second air inlet is positioned generally outside of the boundary layer. This second passageway fluidly interconnecting the second air inlet and the turbofan propulsion engine to provide air outside of the boundary layer to the core and compressor of the turbofan engine to maintain engine efficiency.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an engine inlet system for a turbofan propulsion engine and, more particularly, to an engine inlet system that is capable of separately diverting boundary layer air and free-stream air into a turbofan propulsion engine 
     BACKGROUND OF THE INVENTION 
     In conventional aircraft design, the wings of the aircraft provide aerodynamic lift and further support the weight of the fuselage. Engines are then coupled to the wings and/or the fuselage to provide thrust for propelling the aircraft. 
     However, recently there have been significant developments into the design of “blended wing-body” aircraft. In a blended wing-body aircraft, the fuselage and wings are joined to form a smooth curve along the exterior of the aircraft with no discrete interface between the fuselage and the wing. In order to maintain the aerodynamic efficiency and lift characteristics of a blended wing-body aircraft, it has been determined that an aft-mounted engine configuration provides the least disturbance of airflow over the wing-body surface, thereby maintaining the aerodynamic efficiencies and advantages of the blended wing-body design. 
     Aerodynamic lift is the result of the movement of fluid (e.g. air) over the surface of the wing. According to the laws of fluid dynamics, such fluid movement produces a boundary layer between a region of low static pressure and a region of high static pressure. According to current wing design technology, it is preferable to keep this boundary layer attached along a wing surface in order to delay or totally prevent flow separation. Such delay or prevention of the flow separation improves the aerodynamic characteristics of the wing surface, thereby providing a wing that produces less drag relative to a wing having a separated flow field. 
     During flight, the boundary layer air that typically forms along the wing surfaces and fuselage is of low velocity and low static pressure. Because low energy air causes poor engine performance, some aircraft have employed some type of boundary layer diverter system to prevent the boundary layer air from entering the engine inlet. 
     Present boundary layer diverters require various subsystems or add on baffles to make them work properly. Such subsystems and/or baffles may increase the weight, the cost of production, mechanical complexity, and the cost of maintenance of the aircraft. Also, the engines would be mounted higher up, causing nose-down moments and increased wetted area. 
     On the other hand, in the case of a blended wing-body aircraft, when the engines are mounted generally flush with a trailing edge of the effective wing, the mixture of boundary layer air and free stream air causes distortion in a combined inlet. That is, simply aft mounting engines to a blended wing-body aircraft may produce poor aerodynamic efficiency of the effective wing surface and may cause poor engine efficiency due to the intake of low energy boundary layer air. 
     Accordingly, there exists a need in the relevant art to provide an engine inlet system for a turbofan propulsion engine that is capable of separately diverting boundary layer air and free-stream air to a turbojet propulsion engine. Furthermore, there exists a need in the relevant art to provide an engine inlet system that is capable of maximizing the aerodynamic efficiency of the wing surface and, simultaneously, maximizing the engine efficiency of the jet propulsion engine. Still further, there exists a need in the relevant art to provide an engine inlet system that overcomes the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     A dual boundary layer engine inlet for a turbofan propulsion engine of an aircraft having an advantageous construction is provided. The engine inlet includes a first air inlet positioned generally within the boundary layer flowing around the exterior surface of the aircraft. A first passageway fluidly interconnects the first air inlet and the jet propulsion engine to provide air from the boundary layer to the bypass to reduce aerodynamic drag. A second air inlet is positioned generally outside of the boundary layer. This second passageway fluidly interconnecting the second air inlet and the turbofan propulsion engine to provide free-stream air outside of the boundary layer to the core and compressor of the turbofan engine to maintain engine efficiency. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a perspective view illustrating a blended wing-body aircraft employing a dual boundary layer engine inlet system according to the principles of the present invention; 
     FIG. 2 is an enlarged side view, with portions in cross-section, illustrating the dual boundary layer engine inlet system; and 
     FIG. 3 is an enlarged perspective view of the inlets of the dual boundary layer engine inlet system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For example, the dual boundary layer engine inlet system of the present invention may find utility in a variety of different aircraft applications, such as subsonic aircraft, supersonic aircraft, and conventional fuselage-wing aircraft. The following disclosure simply relates to the preferred embodiment as illustrated in the drawings, however, such description should not be interpreted as a limitation of the scope of the present application. 
     Referring to FIG. 1, a blended wing-body aircraft  10  is illustrated having a fuselage  12  and a pair of wings  14 . Blended wing-body aircraft  10  of the preferred embodiment is characterized by the smooth shallow curve formed by the exterior structural panels between fuselage  12  and wings  14 . Unlike conventional aircraft designs, blended wing-body aircraft  10  has no discrete interface between fuselage  12  and wings  14 . The exterior skin of fuselage  12  and wings  14  join together to form a blended region  16 . Fuselage  12 , wings  14 , and blended region  16  cooperate to define a substantially uninterrupted wing member capable of providing aerodynamic lift to blended wing-body aircraft  10  according to known aerodynamic principles. 
     Blended wing-body aircraft  10  further includes a plurality of turbofan propulsion engines  18 . As illustrated in the figures, the presently preferred embodiment includes three turbofan propulsion engines  18  generally mounted to an aft region  20  of blended wing-body aircraft  10 . It should be appreciated, however, that the principles of the present invention may be employed in aircraft having any number of engines. 
     As best seen in FIG. 2, turbofan propulsion engine  18  served by the presently preferred embodiment is a turbofan-type jet engine. For instance, an “aft fan” arrangement was featured on the GENERAL ELECTRIC CF 700-1. Specifically, turbofan propulsion engine  18  includes an aft-mounted bypass fan section  22  and a turbine section  24 . Turbine section  24  is disposed concentrically within bypass fan section  22 . Turbine section  24  generally includes a compressor casing  28  and an exhaust nozzle  30 . A turbine rotor  31  is operably mounted within compressor casing  28  and is mechanically linked to a compressor  32 . Compressor  32  is disposed within a compressor casing  28 . Finally, a rear cone  34  is mounted within exhaust nozzle  30  so as to provide proper thrust flow from turbofan propulsion engine  18 . 
     Bypass fan section  22 .includes a plurality of fan blades  21  in a fan casing  38  so as to provide “cold” flow thrust from outlet  40  of bypass fan section  22 . 
     In operation, feed air is supplied to turbofan propulsion engine  18  via a dual boundary layer engine inlet system  42 . Engine inlet system  42  includes a compressor air inlet duct  44  and a bypass air duct  46 . 
     Bypass air duct  46  includes an inlet end  48  and an outlet end  50 . In the presently preferred embodiment of FIGS. 1 and 3, inlet end  48  of bypass air duct  46  is generally rectangular in shape such that it is positioned and substantially follows the curvature of an upper surface  52  of blended wing-body aircraft  10 . It should be understood that upper surface  52  of blended wing-body aircraft and, consequently, inlet end  48  of bypass air duct  46  may include any inlet end profile that is conducive to the curvature shape of the aircraft or other aerodynamic requirements. Outlet end  50  of bypass air duct  46  is generally circular in cross-section so as to provide a proper fit with an inlet end  54  of bypass fan section  22  of turbofan propulsion engine  18 . Therefore, bypass air duct  46  includes a generally complex three-dimensional transition from the generally rectangular inlet end  48  to the generally circular outlet end  50 . 
     Compressor.air inlet duct  44  of engine inlet system  42  is generally S-shaped having an inlet end  56  and an outlet end  58 . Inlet end  56  of compressor air inlet duct  44  is generally semi-circular in shape (FIG. 3) and is positioned on top of or in a “piggy-back” position relative to bypass air duct  46 . That is, a generally flat surface  60  of inlet end  56  of compressor air inlet duct  44  is positioned upon a corresponding top surface  62  of bypass air duct  46 . Outlet end  58  of air inlet duct  44  is generally circular in shape and of sufficient size so as to be coupled to an inlet end  64  of compressor casing  28 . A grid  61  serves as a trap for moisture and foreign objects, before the boundary layer air enters the compressor air inlet duct. 
     According to the principles of the present invention, air inlet duct  44  is positioned within a more high energy free-stream air. Accordingly, during flight, boundary layer air, generally indicated at  66  (FIG.  2 ), flows over upper surface  52  of blended wing-body aircraft  10 . Inlet end  48  of bypass air duct  46  is generally disposed within this boundary layer air  66  so as to provide fluid communication of boundary layer air  66  to bypass fan section  22  of turbofan propulsion engine  18 . 
     An advantage of this arrangement is that the operation of bypass fan  21  in bypass fan section  22  produces a reduced pressure at inlet end  54  of bypass fan section  22 . This reduced pressure condition further exists within bypass air duct  46  and serves to scavenge the flow of boundary layer air  66  over upper surface  52  of blended wing-body aircraft  10 . That is, the reduced pressure condition within bypass air duct  46  helps to enhance or promote the flow of boundary layer air  66  over a larger longitudinal portion of upper surface  52  relative to aircraft of conventional design not utilizing this reduced pressure condition. 
     In order.to supply higher energy free-stream air to turbine section  24  of turbofan propulsion engine  18 , inlet end  56  of turbine air duct  44  is positioned substantially above boundary layer air  66  (FIG. 2) and, thus, is open to free-stream air, generally indicated at  68 . Such free-stream air  68  is supplied to inlet end  56  of compressor inlet  58 . As is well known in the art, free-stream air serves to improve the engine efficiency of known jet propulsion engines. 
     As should be appreciated from the foregoing discussion, the dual boundary layer engine inlet system according to the principles of the present invention provides a number of aerodynamic and commercial advantages. For instance, the dual boundary layer engine inlet system of the present invention provides a method of supplying high energy free-stream air to the engine&#39;s compressor inlet while, simultaneously, supplying boundary layer air to a bypass fan inlet. The bypass fan produces reduced pressure that scavenges and promotes the attached relationship of the boundary layer air to the aircraft lift surfaces. Furthermore, the dual boundary layer engine inlet system of the present invention enables the aft mounting of the turbofan propulsion engines so as to facilitate simple and convenient repair and/or maintenance in a commercial environment. Simple and convenient repair and maintenance of the jet engines is a prerequisite to commercial viability within the passenger and military transport arenas. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.