Abstract:
A propulsor apparatus for a mobile platform, for example an aircraft is provided. The propulsor apparatus includes an aerodynamically shaped propeller duct that houses at least a portion of a turboprop engine, and fully houses a propeller driven by the engine. Inside the propeller duct is a circumferential ring that closely surrounds the propeller such that only a small clearance is provided between the outermost tips of the propeller blades and an inner surface of the circumferential ring. The circumferential ring includes sound deadening material that attenuates noise generated by the tip vortices created at the outermost tips of the propeller blades. The propeller duct and circumferential ring are supported by a plurality of structural rods that couple to structure of the mobile platform, for example to the spars within a wing of an aircraft. The apparatus and method significantly reduces the noise associated with a turboprop engine.

Description:
FIELD 
       [0001]    The present disclosure relates generally to engines used with various forms of aircraft. More particularly, the present disclosure relates to an engine for an aircraft, where the engine has a ducted propeller. 
       BACKGROUND 
       [0002]    Over the years, performance demands on commercial transport aircraft has increased in the area of noise, fuel economy and reduced weight. Various technological advances continue to improve aircraft performance in each of the above mentioned areas. For example, continuous improvement in engines leads to better fuel economy. Even more, shifting from propeller powered systems to turbofans has led to quieter aircraft, while using more composite material in primary structures has yielded weight savings. 
         [0003]    Today, however, there is increasing interest in going back to the use of turboprop engines from turbofan engines, especially for commercial transport aircraft applications. While significant strides have been made over the years to improve the fuel economy of turbofan engines, such engines still are not as fuel efficient as turboprop engines. On the other hand, while turboprop engines enjoy a fuel efficiency advantage over turbofan engines, turboprop engines generate higher noise levels during operation. Present day turboprop engines will not likely be able to meet the increasingly stringent noise level regulations that are expected to be enacted at airports around the world over the next several years. Thus, a challenge exists in providing a turboprop engine that generates less noise than present day turboprop engines. 
         [0004]    The increased noise associated with turboprop engines is generally due to the propeller blade tips and the vortices associated with the blade tips. This increased noise impacts the communities surrounding an airport, as well as the ambient environment within the cabin of a commercial passenger transport aircraft. The increased noise is most noticeable during takeoff, when power is highest and altitude is lowest. When power input to the propeller of a turboprop engine is large, the thrust produced by the propeller is large, and the noise thus generated is commensurately greater. 
         [0005]    Another drawback with turboprop engines is the speed disadvantage the turboprop engines suffer when compared to turbofan engines. In some instances, depending on the route being flown, this may not be a concern. For example, on shorter flight routes, the desired number of missions being flown per day may still be within the performance capabilities of an aircraft employing turboprop engines. However, on longer flight routes, the increased length of time needed for a given flight would likely be viewed negatively by paying passengers, if the aircraft was a commercial passenger transport aircraft. 
         [0006]    What would be highly desirable is a new propulsive apparatus for powering transport aircraft that combines the benefits of better fuel economy of turboprop engines, with the reduced noise of a turbofan engine. 
       SUMMARY 
       [0007]    In accordance with the present disclosure, a propulsor for a mobile platform is disclosed. In one implementation, the mobile platform is an aircraft, and the propulsor forms a turboprop engine having a propeller and associated duct structure that surrounds the propeller. In one aspect, a structure for securing the duct to a wing of an aircraft includes first laterally spaced mounts on the duct secured to a first spar of the wing, and at least one second mount secured to a second wing spar by means of a diagonal brace. 
         [0008]    A principal advantage of the described embodiments is reducing noise from the propeller of the turboprop engine. Another advantage is that the duct that shrouds the propeller can be independently secured to the wing. Since the engine nacelle and the duct are independently secured to the aircraft wing, this allows all loads experienced by the duct to be transmitted directly to the wing spars. Because the engine nacelle does not support the duct, a lighter, slightly less robust engine nacelle may be used, which enables a weight savings to be achieved. Additionally, another advantage is that the duct helps to protect the fuselage of the aircraft against damage, if a propeller should fail. 
         [0009]    Still another advantage is conveyed by using the duct to shroud the propeller. As the forward speed of a propeller-driven aircraft increases, rotational speed of the propeller also must increase. The propeller tip speed becomes a key factor in this progression when that speed nears Mach 1, the speed of sound. For the propeller tips operating in open air, large efficiency losses are encountered in this speed range, effectively limiting the maximum economical cruising speed of the aircraft. The presence of the duct around the propeller can be used to expand the incoming airflow, reducing its velocity as the propeller is encountered. This allows higher cruise speeds to be achieved without encountering the Mach problems described above. 
         [0010]    Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating various embodiments of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a front view of an aircraft power plant installation apparatus in accordance with an embodiment of the present disclosure, as well as showing the apparatus being supported from a wing of an aircraft; 
           [0013]      FIG. 2  is a side view of the aircraft power plant installation apparatus of  FIG. 1  in accordance with directional arrow  2  in  FIG. 1 ; 
           [0014]      FIG. 3  is a cross-sectional side view of the aircraft power plant installation apparatus of  FIG. 1  in accordance with section line  3 - 3  in  FIG. 1 ; 
           [0015]      FIG. 3A  is an enlarged side view of the aircraft power plant installation apparatus in accordance with region  3 A in  FIG. 3 ; 
           [0016]      FIG. 4  is an enlarged front view of the aircraft power plant installation apparatus of  FIG. 1 ; 
           [0017]      FIG. 5  is a top view in accordance with directional arrow  5  in  FIG. 1  illustrating a plurality of structural tie rods of the aircraft power plant installation apparatus of  FIG. 1 ; 
           [0018]      FIG. 5A  is an enlarged side view of an exemplary structural tab or mount of a structural ring that is coupled to an aircraft wing via a structural rod in accordance with region  5 A in  FIG. 3 ; 
           [0019]      FIG. 5B  is an enlarged side view of an exemplary structural tab of the structural ring that is coupled directly to the aircraft wing in accordance with region  5 B in  FIG. 3 ; and 
           [0020]      FIG. 6  is a cross-sectional side view of another aspect of the aircraft installation apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following description of various embodiments is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. 
         [0022]    In  FIGS. 1 and 2 , there is shown an aircraft  10  having a fuselage  12 , two conventional wings  14  (only one of which is shown), and two wing mounted power plant installations or propulsor apparatuses  16  (again only one of which is shown). 
         [0023]    As shown in  FIGS. 3 and 5 , each wing  14  has a main front spar  18  that extends spanwise along the wing  14  and a structural rib  20  that extends chordwise along the wing  14 . It is to be understood that the wing  14  includes a plurality of such ribs  20  spaced at suitable locations along the span of the wing  14 . The front portion of the rib  20  is conventionally joined to the front spar  18 . A leading edge panel  21  is conventionally affixed to the front spar  18  and is shaped to form the leading edge section of a conventional airfoil  22 . Likewise, a rear portion of the rib  20  is conventionally joined to a rear spar  24 . A rear edge panel  26  is conventionally affixed to the rear spar  24  and is shaped to form the rear edge section of the airfoil  22 . The wing  14  has an upper airfoil surface skin  22   a  and a lower airfoil surface skin  22   b  that are affixed in a conventional manner to the front and rear spars  18 ,  24  and to the ribs  20 . 
         [0024]    Generally, as illustrated in  FIG. 3 , each propulsor apparatus  16  includes a typical power plant or turboprop engine  33 , an engine nacelle  34 , a propeller  36 , and a propeller duct  38 . The turboprop engine  33  has a longitudinal centerline axis and is generally coaxially disposed within the nacelle  34  and supported from its wing  14  by a conventional engine mounting device  40 , such as a typical cowl. Although the aircraft  10  comprises a two-propulsor configuration, a multiple-propulsor configuration could be incorporated on the aircraft  10  as well. For example, if a four-propulsor configuration (not shown) is used, then each nacelle  34  may include two engines with their respective drive shafts connected in parallel in the drive train. The engine  33  and the engine nacelle  34  is typically mounted below the wing  14 . However, referring to  FIG. 4 , the engine  33  and the engine nacelle  34  may be also mounted above the wing  14  as shown in phantom at  35 . 
         [0025]    Referring further to  FIGS. 3 and 4 , the propeller  36  is mounted on a drive shaft  32   a  that extends from the engine  33  in a conventional manner. The propeller  36  may comprise at least one single-rotating or counter-rotating propeller  36  having a desired diameter and a plurality of blades  42 . The blades  42  have a common, predetermined length. For illustrative purposes only, a 6-blade propeller  36  is shown in  FIGS. 1 and 4 . 
         [0026]    Again referring to  FIGS. 3 and 5 , the propeller duct  38  includes a housing  39  having an internally mounted structural ring  46  that shrouds the propeller  36 . The propeller duct  38  reduces noise derived from propeller tips  36   a  and the related vortices existing at the propeller tips  36   a . The propeller duct  38  is an independent structure from the engine nacelle  34  and has an aerodynamic surface. Additionally, the propeller duct  38  includes a plurality of diagonal rods  50  to couple the propeller duct  38  to the aircraft  10 . The propeller duct  38  may be made from aluminum or any other suitably strong material typically used in airframe construction. 
         [0027]    Performance requirements for weight and structure regarding most aircraft dictate that the propeller duct  38  is lightweight yet structurally strong. The propeller duct  38  must be capable of withstanding severe inertial loads, especially transverse loads that are exerted on the propeller duct  38  during normal flight of the aircraft  10 . The inertial loads can be especially severe in turbulent flight conditions, and upon landing of the aircraft  10 . 
         [0028]    Referring further to  FIGS. 3 ,  3 A,  5 A and  5 , the housing  39  includes a forward portion  39   a  and an aft portion  39   b , and an interior and an exterior surface extending longitudinally over and beneath the engine nacelle  34  and a portion of the wing  14 . As shown in  FIG. 3A , the propeller duct  38  also includes an internal surface geometry  43  configured to increase cruise speeds by expanding incoming airflow and reducing airflow velocity. Again referring to  FIGS. 3 and 5 , the housing  39  also includes a forward air inlet opening  39   c , adjacent to which is mounted the propeller  36  as described above. An aft end  39   d  of the housing  39  includes a trailing edge  39   e  that extends spanwise almost a full length of the housing  39  and aerodynamically merges to skin  22  of the wing  14 . 
         [0029]    With specific reference to  FIGS. 3 and 3A , the housing  39  is mounted on the structural ring  46  via any conventional mounting means, such as by bolting, riveting, or bonding. The structural ring  46  encircles the propeller  36  and maintains the concentricity of the housing  39  relative to the propeller  36 . The structural ring  46  also maintains a desired clearance  41  from the propeller tips  36   a , as shown in  FIG. 3A . The clearance  41  is configured to ensure optimal operating conditions, and typically is about 1% of a diameter of the propeller  36 . 
         [0030]    Now referring to  FIGS. 3 and 4 , the structural ring  46  includes a length to sufficiently shroud the propeller  36  fore-to-aft, and more preferably slightly longer than the fore-to-aft length of the propeller  36 , as represented by arrow  47 . In addition, the structural ring  46  is sufficiently rigid to maintain performance requirements as stated above. The structural ring  46  may comprise ultra-stiff composites, for example carbon or boron fibers. Positioned at spaced apart points along a circumference of the structural ring  46  are a plurality of attachment points or laterally spaced structural tabs  48 . Each structural tab  48  includes a thickness equal to, or about equal to, a thickness of the structural ring  46 . Additionally, each structural tab  48  is preferably molded or formed as part of the structural ring  46  to provide sufficient strength and rigidity to maintain connection of the structural ring  46  to the wing  14  under the above-mentioned loads. The structural tabs  48  will be further discussed later. 
         [0031]    With further reference to  FIG. 3A , the structural ring  46  also comprises an inner layer of a sound absorbing, acoustic lining  45 . For example, the acoustic lining  45  may be disposed around the inner peripheral of the ring  46  and may be a conventional type of single or double layered acoustic lining  45  used in turbofan jet engines, or other type of acoustic lining  45 . More specifically, the acoustic lining  45  surrounds outermost tips  36   a  of the propeller  36 . The acoustic lining  45  includes a layer in the form of a perforated sheet of aluminum. 
         [0032]    The acoustic lining  45  absorbs noise generated within the interior of the propeller duct  38 . As will be appreciated, the propeller  36  includes the blades  42 , as shown in  FIG. 3 , that rotate about the central axis of the engine  33  during operation of the engine  33 , and that produce sound waves (i.e. noise) from the propeller tips  36   a . Rotation of the blades  42  serves to draw airflow into the engine  33 . Sound waves generated by airflow of the engine  33  and by the propeller tips  36   a  are absorbed by the lining  45 . 
         [0033]    Referring to  FIGS. 3-5B , the diagonal rods  50  are configured to provide a means of securing the propeller duct  38  to the wing  14 . The diagonal rods  50  form a plurality of structural tie rods that couple the propeller duct  38  to the wing  14 . Each structural tie rod  50  attaches to one of the plurality of structural tabs  48  of the structural ring  46  via a coupling device (not shown) and secures the propeller duct  38  to the wing  14 . For example, the coupling device may comprise a standard lug mount. Referring to  FIGS. 2 and 4 , using a first pair of lug mounts (not shown), first and second structural tie rods  50   a  link first and second structural tabs  48   a  of the structural ring  46  to the rear spar  24  atop of the wing  14 . Additionally, third and fourth structural tabs  48   b  link the propeller duct  38  to the front spar  18  of the wing  14  via a second pair of lug mounts (not shown). Using a third pair of lug mounts (not shown), third and fourth structural tie rods  50   b  couple fifth and sixth structural tabs  48   c  to the rear spar  24  beneath the wing  14 . Each structural tie rod  50  transmits propeller duct loads arising from internal and external drag, and inertial force conditions, to the wing  14 . Since the propeller duct  38  is independently coupled to the wing  14 , the structural tie rods  50  allow a direct transmission for shorter load paths to the wing  14 . 
         [0034]    As the structural ring  46  is directly coupled to the wing  14  via the third and fourth structural tabs  48   b , a portion of the wing  14  is notched out as far back as the front spar  18  as shown in  FIG. 3 . Additionally, the housing  39  may comprise other attachment points (not shown) that are added between the front and rear spars  18 ,  24 . The other attachment points may aid the propeller duct  38  in even better absorbing the primary load and/or any secondary loads. 
         [0035]    Another significant benefit that can be realized by incorporating the duct  38  is forming the internal geometry of the duct  38  to expand the incoming airflow into the engine  33  during flight, much as like what is presently done with turbofan engines. This is important because with turboprop engines, as the forward speed of the aircraft increases, the rotational speed of the propeller  36  must also increase. The speed of the propeller tip  36   a  becomes a key factor in this progression when the speed of the propeller tips  36   a  nears Mach 1 (i.e., the speed of sound). For propeller tips  36   a  operating in open air (i.e., without being surrounded by any form of shroud or ducting), large efficiency losses are encountered at this speed range. The efficiency losses effectively limit the maximum economical cruising speed of the aircraft. 
         [0036]    In practice, today&#39;s turboprop aircraft cruise at speeds in the range of about 0.4 Mach-0.6 Mach. Turbofans can economically cruise at up to 0.8 Mach or even greater. The same type of internal duct geometry as used with present day turbofans could easily be used with the duct  38  to expand incoming airflow, and thus reduce the velocity of the incoming airflow as it encounters the propeller  36 . This will allow higher cruise speeds than that obtained by conventional turboprops to be achieved without encountering the above-described Mach problems. While the aircraft cruise speeds attainable with a ducted propeller may not as great as with a conventional turbofan engine, cruise speeds would be higher than what could be achieved with a conventional un-shrouded turboprop engine. 
         [0037]    Referring to  FIGS. 1 ,  3  and  4 , another aspect of the present disclosure includes the propeller duct  38  having a conventional inlet guide vane  78 . The inlet guide vane  78  guides the incoming airflow before it strikes the propeller  36 . The inlet guide vane  78  may include a plurality of radial struts  79  from a diameter within the inlet guide vane  78  to a point along a circumference of the inlet guide vane  78 . As shown in  FIGS. 1 ,  3  and  4 , the inlet guide vane  78  is conventionally mounted ahead of the propeller  36  and within the propeller duct  38  and received by the bearing that supports the propeller  36 . Additionally, the inlet guide vane  78  is conventionally mounted to the forward air inlet opening  39   c  of the propeller duct  38 . Dimensions of the inlet guide vane  78  will vary from one application to another depending on the physical dimensions of the flow intake and flow passages as well as fluid flow conditions throughout exterior fluid flow and inlet passages. 
         [0038]    Referring to  FIG. 6 , still another aspect of the present disclosure includes the propeller duct  38  further comprising a conventional stator  80 . The stator  80  may alternatively be used instead of the inlet guide vane  78  (see  FIG. 5 ). Additionally, the stator  80  is conventionally mounted aft of the propeller  36  and received by the drive shaft  32   a  that supports the propeller, and before the engine nacelle  34  with all remaining parts being the same. The stator  80  directs airflow from the propeller  36 . The stator  80  ensures efficiency by eliminating any rotation of airflow. 
         [0039]    The apparatus and method of the present disclosure provides a means of reducing much of the noise related to the vortices around each propeller tip  36   a , and any other noise from the propeller  36 , by surrounding the propeller  36  with the aerodynamic duct  38  that is coupled independently to the wing  14  of the aircraft  10 , rather than the engine nacelle  34 . In addition to noise attenuation provided by the sound-absorbing lining  45 , the contour of the duct  38  further aids in reducing noise. In particular, the duct  38  restricts the formation of vortices at the propeller tips  36   a  and physically shields this noise and any other noise from leaving the duct  38 . A further advantage is that any first or secondary loads imposed on the propeller duct  38  are transmitted directly to the wing  14  instead of the engine nacelle  34 , which allows the engine nacelle  34  to be made less robust and with a reduced weight. 
         [0040]    While various embodiments have been described, those skilled in the art will recognize modifications or variations that might be made without departing from the inventive concept. The examples illustrate the apparatus and method and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.