Abstract:
Aspects of the disclosure are directed to a nacelle of an aircraft, comprising a surface that is profiled such that during cruise flight operation lines of constant static pressure of a boundary layer around the nacelle in a given region are substantially contained within a plane that is normal to an engine axis.

Description:
BACKGROUND 
       [0001]    A nacelle is the housing for a turbofan gas turbine engine used to power, for example, a commercial airliner. The nacelle and the engine together are referred to as the propulsion system or power plant of the aircraft. The nacelle forms the external aerodynamic surfaces of the propulsion system, and also helps form the duct for the bypass air from the fan, as well as encloses all the components and auxiliary devices surrounding and attached to the engine. The nacelle may also optionally provide a reverse thrust means to generate reverse thrust to slow the aircraft, for example during landing.  FIG. 1  illustrates a typical gas turbine engine inside a nacelle  22 , which are together attached via a pylon  21  to the underside of an aircraft wing  20 . The nacelle  22  includes a forward section  23  and an aft section  12 . The forward section  23  may be formed from an inlet and a fan cowl, which could be combined together. The aft section  12  may include a thrust reverser. 
         [0002]    In operation, an ambient air flow  56  on the exterior of the nacelle  22  generates drag. The drag force increases fuel consumption. Accordingly, it is desirable to minimize the drag. 
         [0003]    If the flow around the nacelle  22  is laminar the drag force will be reduced compared to a turbulent flow. Aircraft and nacelle designers have utilized nacelle external aerodynamic shapes that maintain a natural laminar flow over a portion of the first section  23  of a nacelle. By carefully selecting the aerodynamic profile (which is done through analysis including computational fluid dynamics, and through experimentation), natural laminar flow can be achieved beginning at the inlet lip and extending back as much as 10-30 inches. The portion where it is desired to achieve laminar flow much be clean of debris (e.g., bugs) and very aerodynamically smooth (e.g., fastener heads must be very flush and parallel with the surrounding surface). Steps and gaps can cause the laminar flow to trip and transition to turbulent flow. Many other active or hybrid natural/active techniques have been proposed for achieving and maintaining laminar flow on an aircraft nacelle and other aircraft surfaces. One such technique is through boundary layer ingestion or suction where the boundary layer next to the aircraft surface is pulled through small holes in the surface to remove the low energy boundary layer and regenerate it or maintain it at a minimum energy level. Active or hybrid laminar flow techniques may achieve and maintain laminar flow more consistently than natural means alone, and may be able to extend further aft the region on first section  23  of nacelle  22  which has laminar flow. 
         [0004]    While boundary layer suction or ingestion has been proposed for use on an aircraft surface such as an aircraft nacelle for drag reduction, no successful commercial systems have flown yet. There is a need for solutions to the many practical problems that remain to be solved. 
       BRIEF SUMMARY 
       [0005]    The following presents a simplified summary in order to provide a basic understanding of some aspects of the disclosure. The summary is not an extensive overview of the disclosure. It is neither intended to identify key or critical elements of the disclosure nor to delineate the scope of the disclosure. The following summary merely presents some concepts of the disclosure in a simplified form as a prelude to the description below. 
         [0006]    Aspects of the disclosure are directed to a nacelle of an aircraft, comprising a surface that is profiled such that during cruise flight operation lines of constant static pressure of a boundary layer around the nacelle in a given region are substantially contained within a plane that is normal to an engine axis. In some embodiments, a rib associated with one of the lines of constant static pressure is located at a substantially fixed location with respect to a reference direction associated with the nacelle. In some embodiments, the reference direction is an axial reference direction that is aligned with the engine axis. In some embodiments, the rib is oriented with a variation in an axial reference, where the axial reference direction is aligned with the engine axis. In some embodiments, the nacelle further comprises a first plenum, and a second plenum, where the first and second plenums are separated from one another by one of the lines of constant static pressure. In some embodiments, the first plenum is configured to traverse a substantial portion of a circumference of the nacelle. In some embodiments, the nacelle further comprises a zone formed in a skin of the nacelle, where the zone comprises perforations that are configured to enable a suction of air. In some embodiments, the perforations are configured to draw air from an air flow that is external to the nacelle into the nacelle. In some embodiments, a location of the zone on the nacelle corresponds to a location of a first plenum on the nacelle. 
         [0007]    Aspects of the disclosure are directed to a nacelle of an aircraft, comprising a plurality of plenums that are divided by axial and circumferential ribs, where the circumferential ribs are not straight. In some embodiments, at least one of the circumferential ribs does not lie in a plane that is normal to an engine axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present disclosure is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements. 
           [0009]      FIG. 1  illustrates components and devices associated with an aircraft nacelle in accordance with the prior art. 
           [0010]      FIG. 2A  illustrates a side perspective view of a nacelle including a reference line denoting a maximum diameter associated with the nacelle. 
           [0011]      FIG. 2B  illustrates plenums formed by ribs in the nacelle of  FIG. 2A  for providing suction for boundary layer ingestion through small holes formed in the nacelle skin. 
           [0012]      FIG. 3A  illustrates a side perspective view of a nacelle including a reference line denoting lines of constant static pressure for the boundary layer around the nacelle. 
           [0013]      FIG. 3B  illustrates plenums formed by ribs in the nacelle of  FIG. 3A . 
           [0014]      FIG. 4A  illustrates a side perspective view of a nacelle including reference lines denoting lines of constant static pressure for the boundary layer around the nacelle. 
           [0015]      FIG. 4B  illustrates plenums formed by ribs in the nacelle of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    It is noted that various connections are set forth between elements in the following description and in the drawings (the contents of which are included in this disclosure by way of reference). It is noted that these connections are general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. A coupling between two or more entities may refer to a direct connection or an indirect connection. An indirect connection may incorporate one or more intervening entities. 
         [0017]    In accordance with various aspects of the disclosure, apparatuses, systems and methods are described for reducing/minimizing drag associated with a nacelle of the aircraft. For example, aspects of the disclosure may reduce/minimize drag attributable to a flow on the exterior of the nacelle. Aspects of the disclosure may be implemented as part of active laminar flow control (ALFC) system. 
         [0018]    Referring now to  FIG. 2A , a portion of a nacelle  222  is shown. Superimposed in  FIG. 2A  are axial, radial, and circumferential reference directions. Any point or location within or on the nacelle  222  may be specified by a value for each of these reference directions. 
         [0019]    Superimposed on the nacelle  222  is a reference line D max . D max  denotes the maximum diameter of the nacelle  222  (as measured from the center of the nacelle  222  along the axial direction) at every location along the nacelle  222  around the circumference of the nacelle  222 . As illustrated, the line D max  is often not fixed at a given axial location, i.e. it is not contained in the same plane normal to the axis of the engine. Instead, the line D max  appears “wavy” with substantial/appreciable variation in an axial direction. The position of D max  is likewise influenced by the many factors that shape the external aerodynamic surface of the nacelle, which include space claims for components housed within the nacelle (for instance, on some aircraft a gearbox mounted on the fan case causes a very noticeable bulge in the shape of the front section of the nacelle), interactions with the wing aerodynamics, the requirements for the air flow into the engine and fan, etc. This illustration of D max  helps visualize the relatively varying external aerodynamic shape of the nacelle. In some cases it can be difficult to achieve an external aerodynamic shape of the nacelle for natural laminar flow given all the constraints and trades. This illustration of D max  also helps to visualize the lines of constant static air pressure around the exterior of the nacelle. Similar to D max , the lines of constant static pressure vary substantially/appreciably in the axial direction, i.e. they are not contained in the same plane normal to the axis of the engine. 
         [0020]    Proposed boundary layer suction in active laminar flow systems would include small holes formed in the external surface of the nacelle where, according to analysis, it will be necessary to remove or ingest some of the boundary layer to prevent tripping to turbulent flow. A suction or pressure gradient/pressure differential is applied across these holes to pull boundary layer air through them and into the nacelle. The rate of flow through the holes, or rate of boundary layer ingestion, will be a factor of this pressure gradient, as well as the size and shape of the hole, among other factors. The rate of boundary layer ingestion is crucial in successfully actively maintaining the laminar flow. For manufacturing simplicity, the size and shape and spacing of the holes would ideally be the same or similar across the active laminar flow control surface. This means that in order to control the rate of ingestion, the principle variable to control would be the pressure differential across the holes. 
         [0021]    Maintaining the desired pressure gradient across each hole is complicated by the fact that, as previously discussed, the static pressure of the air in the boundary layer changes. The pressure gradient is the difference between the static pressure on the external of the nacelle surface immediately outside of the hole, and the lower or suction pressure inside. Because the external static pressure outside of the holes varies significantly, the internal lower or suction pressure must also vary in order to have a relatively consistent pressure gradient, or to achieve the pressure gradient needed for a given region. Thus, a single region of reduced air pressure or suction on the inside of the holes will not be sufficient, most likely varying regions of suction will be necessary. Cavities/plenums  224  formed under the nacelle external skin as shown in  FIG. 2B  may be used to achieve the needed regions of varying suction pressure. Cavities/plenums  224  may be defined in part by a first set of circumferential ribs  226  and a second set of axial ribs  228 . In combination with a bottom plenum surface and the external surface, the linear ribs  226  and  228  form cavities/plenums that will have individually tunable suction pressures to correspond with the external static pressure of the boundary layer on the other side of the external skin from the particular plenum. As long as the static pressure of the boundary layer on the outside of the plenum remains within a relatively narrow pressure band, the constant suction pressure on the inside will result in a relatively constant pressure gradient across the suction holes, and controlled boundary layer ingestion. While workable, this solution would require a large number of linear ribs  226  and  228  in some circumstances, which increases the weight and cost of the system. 
         [0022]      FIGS. 3A-3B  illustrate a nacelle  322  in accordance with aspects of the disclosure. As shown in  FIG. 3A , the lines of constant static pressure in the boundary layer around the exterior of the nacelle are now generally straight and do not vary axially, i.e. they are generally contained in a plane normal to the engine axis. This straightening of the lines would be achieved by appropriately affecting the external aerodynamic profile of the nacelle, and would be accomplished through analysis such as computational fluid dynamics. Accordingly, and as shown in  FIG. 3B , the nacelle  322  may include or be associated with plenums  324  that are separated from one another basically only by linear, circumferential ribs  326  (where the ribs  326  may correspond to the ribs  226  of  FIG. 2B ). Each of the plenums  324  can be larger and extend over a greater surface area of the nacelle, and the weight and cost of the ribs to form the plenums, and the complexity of forming them (fewer fasteners, etc.), is greatly reduced. In the embodiment of the nacelle  322  shown in  FIG. 3B , the plenums  324  are tailored axially at station planes. 
         [0023]    The plenums  324  (which are shown as being separated from one another in  FIG. 3B  in essentially only one direction—illustratively, axially—by ribs  326 ) may be contrasted with the plenums  224  of  FIG. 2B  (which are shown as being separated from one another in two directions—axially and circumferentially—by the ribs  226  and  228 ). 
         [0024]    In some cases, sculpting the external shape of the nacelle to achieve generally straight lines of constant static pressure, as described above with respect to  FIGS. 3A and 3B , may not be feasible.  FIGS. 4A and 4B  illustrates a nacelle  422  where a similar advantage in the construction of the plenums is achieved, but in a slightly different manner. 
         [0025]    In  FIG. 4A , the nacelle  422  is shown with lines of constant static pressure of the boundary layer  426  superimposed. The constant pressure lines  426  are relatively straight, but remain somewhat wavy and are not in a single plane normal to the engine axis. 
         [0026]    The lines  426  may correspond to ribs, or serve to trace out the profile or shape of such ribs. Thus, ribs  426  used in connection with the nacelle  422  may be used to form hoop-shaped plenums such that each of the plenums traverses the entirety of the circumference of the nacelle  422 , similar to the plenums  324  of  FIG. 3B . However, unlike a given rib  326 , a rib  426  may be oriented with some variation in the axial direction (e.g., a rib  426  might not be located at a single/fixed axial position). 
         [0027]    Referring to  FIG. 4B , a (portion of a) nacelle  522  (where the nacelle  522  may correspond to one or more of the nacelles  222 ,  322 , or  422 ) is shown. The nacelle  522  may include one or more zones  524  formed in a skin of the nacelle  522 . The location of a zone  524  may correspond to a location of a plenum (e.g., the plenums described above). For example, a zone  524  may be located on the nacelle  522  at the same axial and/or circumferential positions as a corresponding plenum. As shown in  FIG. 4B , the nacelle  522  might not have any or very many axial ribs, but may include one or more circumferential ribs that follow closely the lines of constant static pressure  426  shown in  FIG. 4A . 
         [0028]    The zones  524  may include perforations/holes to enable a suction of air. For example, air associated with a flow that is external to the nacelle  522  (e.g., the flow  56  described above in connection with  FIGS. 1A-1B ) may be drawn into the nacelle  522  via the perforation/holes in the zones  524 , potentially as a result of a vacuum formed between the exterior and the interior of the nacelle  522 . The air that is drawn into the nacelle  522  may be discharged into a case compartment (not shown). 
         [0029]    Aspects of the disclosure have been described in terms of illustrative embodiments thereof. Numerous other embodiments, modifications, and variations within the scope and spirit of the appended claims will occur to persons of ordinary skill in the art from a review of this disclosure. For example, one of ordinary skill in the art will appreciate that the steps described in conjunction with the illustrative figures may be performed in other than the recited order, and that one or more steps illustrated may be optional in accordance with aspects of the disclosure. One or more features described in connection with a first embodiment may be combined with one or more features of one or more additional embodiments.