Unducted propulsion systems having outlet guide vanes

Abstract

Unducted propulsion systems having outlet guide vanes are disclosed. An example unducted propulsion system includes a fan, and an outlet guide vane downstream of the fan, the outlet guide vane including a hub portion, a tip portion, and a peak positioned between the hub portion and the tip portion in a radial direction defined by the unducted propulsion system, the hub portion including a forward sweep, the tip portion including an aft sweep, wherein the outlet guide vane includes a first chord length in the tip portion, wherein the outlet guide vane includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

Description

UNDUCTED PROPULSION SYSTEMS HAVING OUTLET GUIDEVANESRELATED APPLICATION

[0001] This patent claims the benefit of U.S. Provisional Patent Application No. 63 / 673,554, which was filed on July 19, 2024. U.S. Provisional Patent Application No. 63 / 673,554 is hereby incorporated herein by reference in its entirety. Priority to U.S. Provisional Patent Application No. 63 / 673,554 is hereby claimed.FIELD OF THE DISCLOSURE

[0002] This disclosure relates generally to propulsion systems and, more particularly, to unducted propulsion systems having outlet guide vanes.BACKGROUND

[0003] Generally, a fan of an aircraft propulsion system produces thrust by accelerating air passing through the fan. Factors that are detrimental to efficiency of thrust production include losses in energy in air as it enters and passes through the fan, velocity contributions that do not contribute to thrust (such as swirl and vortices in the air leaving the fan), frictional drag forces on external surfaces of an aircraft propulsion system, and shockwave-related drag forces (e.g., wave drag) on external surfaces of the aircraft propulsion system. Thus, for an aircraft propulsion system, the goal is to generate a given amount of thrust without requiring excessive input power to the fan. As such, it is desirable to minimize inefficiency in the production of thrust.BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 is a cross-sectional view of a portion of an example unducted propulsion system including an outlet guide vane in accordance with teachings disclosed herein.

[0005] FIG. 2A is a perspective view of a portion of the unducted propulsion system including the example outlet guide vane of FIG. 1.

[0006] FIG. 2B is another perspective view of a portion of the unducted propulsion system including the example outlet guide vane of FIG. 1.

[0007] FIG. 3A is a perspective view of the outlet guide vane of FIGS. 1, 2A, and 2B.

[0008] FIG. 3B is a cross-sectional view of the outlet guide vane of FIGS. 1, 2A. 2B. and3A.

[0009] FIG. 4A is an example side view of the outlet guide vane of FIGS. 1, 2A-2B, and 3A-3B.

[0010] FIG. 4B is another example side view of the outlet guide vane of FIGS. 1, 2A-2B, and 3A-3B.

[0011] FIG. 5 is a plot representative of a relationship between a chord length of the outlet guide vane of FIGS. 1, 2A-2B, 3A-3B, and 4A-4B as a function of a distance from an axis of rotation 106 of the unducted propulsion system of FIG. 1 in a radial direction.

[0012] FIG. 6 is another plot representative of a relationship between a chord length of the outlet guide vane of FIGS. 1, 2A-2B, 3A-3B, and 4A-4B as a function of a span of the outlet guide vane in a radial direction defined by the unducted propulsion system of FIG. 1.

[0013] FIG. 7 is an isolated side view of the outlet guide vane of FIGS. 1, 2A-2B, 3A- 3B, and 4A-4B.

[0014] FIG. 8 is another isolated side view of the outlet guide vane of FIGS. 1, 2A-2B, 3A-3B, 4A-4B, and 7.

[0015] FIG. 9 is an isolated view of another example outlet guide vane having a wavy leading edge that can be implemented in the outlet guide vane of FIGS. 1, 2A-2B, 3A-3B, 4A- 4B, 7, and / or 8.

[0016] FIG. 10 is an isolated view of another example outlet guide vane having another wavy leading edge that can be implemented in the outlet guide vane of FIGS. 1, 2A-2B, 3A-3B, 4A-4B. 7, and / or 8.

[0017] FIG. 11 A is an example implementation of the outlet guide vane of FIGS. 1, 2A- 2B, 3A-3B, 4A-4B, 7, and / or 8 including a metal leading edge.

[0018] FIG. 1 IB is an example implementation of the outlet guide vane of FIGS. 1, 2A- 2B. 3A-3B, 4A-4B, 7, and / or 8 including a metal tip cap.

[0019] FIG. 11C is an example implementation of the outlet guide vane of FIGS. 1, 2A- 2B, 3A-3B, 4A-4B, 7, and / or 8 including the metal leading edge of FIG. 11A and the metal tip cap of FIG. 11B.

[0020] In general, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. The figures are not necessarily to scale.DETAILED DESCRIPTION

[0021] “Including” and “comprising” (and all forms and tenses thereof) are used herein to be open ended terms. Thus, whenever a claim employs any form of “include” or “comprise”(e.g., comprises, includes, comprising, including, having, etc.) as a preamble or within a claim recitation of any kind, it is to be understood that additional elements, terms, etc., may be present without falling outside the scope of the corresponding claim or recitation. As used herein, when the phrase “at least” is used as the transition term in, for example, a preamble of a claim, it is open-ended in the same manner as the term “comprising” and “including” are open ended. The term “and / or” when used, for example, in a form such as A. B, and / or C refers to any combination or subset of A, B, C such as ( 1) A alone, (2) B alone, (3) C alone, (4) A with B, (5) A with C, (6) B with C, or (7) A with B and with C. As used herein in the context of describing structures, components, items, objects and / or things, the phrase “at least one of A and B” is intended to refer to implementations including any of (1) at least one A, (2) at least one B, or (3) at least one A and at least one B. Similarly, as used herein in the context of describing structures, components, items, objects and / or things, the phrase “at least one of A or B” is intended to refer to implementations including any of (1) at least one A, (2) at least one B, or (3) at least one A and at least one B. As used herein in the context of describing the performance or execution of processes, instructions, actions, activities, etc., the phrase “at least one of A and B” is intended to refer to implementations including any of (1) at least one A, (2) at least one B, or (3) at least one A and at least one B. Similarly, as used herein in the context of describing the performance or execution of processes, instructions, actions, activities, etc., the phrase “at least one of A or B” is intended to refer to implementations including any of (1) at least one A, (2) at least one B, or (3) at least one A and at least one B.

[0022] As used herein, singular references (e g., “a”, “an”, “first”, “second”, etc.) do not exclude a plurality. The term “a” or “an” object, as used herein, refers to one or more of that object. The terms “a” (or “an”), “one or more”, and “at least one” are used interchangeably herein. Furthermore, although individually listed, a plurality of means, elements, or actions may be implemented by, e.g., the same entity or object. Additionally, although individual features may be included in different examples or claims, these may possibly be combined, and the inclusion in different examples or claims does not imply that a combination of features is not feasible and / or advantageous.

[0023] As used herein, unless otherwise stated, the term “above” describes the relationship of two parts relative to Earth. A first part is above a second part, if the second part has at least one part between Earth and the first part. Likewise, as used herein, a first part is “below” a second part when the first part is closer to the Earth than the second part. As noted above, a first part can be above or below a second part with one or more of: other partstherebetween, without other parts therebetween, with the first and second parts touching, or without the first and second parts being in direct contact with one another.

[0024] As used in this patent, stating that any part (e.g., a layer, film, area, region, or plate) is in any way on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, indicates that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween.

[0025] As used herein, connection references (e.g., attached, coupled, connected, and joined) may include intermediate members between the elements referenced by the connection reference and / or relative movement between those elements unless otherwise indicated. As such, connection references do not necessarily infer that two elements are directly connected and / or in fixed relation to each other. As used herein, stating that any part is in “contact” with another part is defined to mean that there is no intermediate part between the two parts.

[0026] Unless specifically stated otherwise, descriptors such as “first,” “second,” “third,” etc., are used herein without imputing or otherwise indicating any meaning of priority, physical order, arrangement in a list, and / or ordering in any way, but are merely used as labels and / or arbitrary names to distinguish elements for ease of understanding the disclosed examples. In some examples, the descriptor “first” may be used to refer to an element in the detailed description, while the same element may be referred to in a claim with a different descriptor such as “second” or “third.” In such instances, it should be understood that such descriptors are used merely for identifying those elements distinctly within the context of the discussion (e g., within a claim) in which the elements might, for example, otherwise share a same name.

[0027] The terms “upstream” and "downstream" refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows.

[0028] The terms “forward” and “aft” refer to relative positions within a turbine engine or vehicle, and refer to the normal operational attitude of the turbine engine or vehicle. For example, with regard to a turbine engine, forward refers to a position closer to an engine inlet and aft refers to a position closer to an engine nozzle or exhaust.

[0029] As used herein, a “radius” of a feature of an airfoil refers to a distance from an axial centerline (e.g., an axis of rotation) of an unducted propulsion system to the feature of the airfoil in a direction perpendicular to the axial centerline (i.e., a radial direction).

[0030] As used herein, a “tip” of an airfoil refers to an edge of the airfoil that defines a maximum radius of the airfoil.

[0031] For a rotating propeller blade, a surface of the blade on an advancing side thereof, due to rotation, can be referred to as the pressure surface. A surface on the retreating side of the blade, due to rotation, can be referred to as a suction surface. The leading edge of a propeller blade is used herein to refer to a three-dimensional curve at which the suction surface and pressure surface meet on an upstream edge of the blade, based on the flight direction. A trailing edge refers to an intersection of the same suction surface and pressure surface on the downstream edge of the blade. The mean surface is used herein to refer to the imaginary surface connecting the leading edge to trailing edge, which lies between the pressure surface and suction surface. The leading and trailing edges for a stationary7vane are defined in the same way as for the rotating blade, with the vane suction and pressures sides being reversed from those of the blade.

[0032] As used herein, “approximately” and “about” modify their subjects / values to recognize the potential presence of variations that occur in real world applications. For example, “approximately” and “about” may modify dimensions that may not be exact due to manufacturing tolerances and / or other real world imperfections as will be understood by persons of ordinary skill in the art. For example, “approximately” and “about” may indicate such dimensions may be within a tolerance range of + / - 10% unless otherwise specified herein.

[0033] A turbofan engine operates on the principle that a central gas turbine core drives a bypass fan; the fan being located at a radial location between a fan duct and the engine core. An unducted propulsion system instead operates on the principle of having the bypass fan located outside of the engine nacelle. This permits the use of larger fan blades able to act upon a larger volume of air than for a turbofan engine, and thereby improves propulsive efficiency over conventional engine designs.

[0034] Unducted propulsion sy stems may take the form of a propeller system, as used on a wide range of aircraft (e.g., radio controlled model airplanes, drones, piston engine propeller aircraft, turboprop regional aircraft, and large turboprop military transports). Another type of unducted propulsion system, sometimes referred to as “open rotor”, includes two blade assemblies, one in a forward position and one in an aft position, in which at least one of them rotates about an axis to deliver power to the propulsive stream that generates thrust. As used herein, the term “propeller” may7refer to the single blade assembly of an unducted propulsion system or the forward blade assembly of an unducted propulsion system comprised of two blade assemblies. The term “fan” may refer to the either a propeller or both blade assemblies of an unducted propulsion sy stem.

[0035] An unducted propulsion system can enable high subsonic cruise flight speed. Cruise is a phase of the flight that occurs when the aircraft levels to a set altitude after a climb and before it begins to descend. Thus, as used herein, cruise represents a continuous, high speed, and stable condition of flight for which an aircraft is intended to operate. This description is to distinguish cruise from certain conditions that are abnormal or transient, such as dive, in which the aircraft can reach high flight speeds, but the aircraft is not intended to experience for a substantial portion of the mission from takeoff to landing.

[0036] An unducted propulsion system that enables highest subsonic cruise flight can have two blade assemblies positioned in aerodynamic relationship to one another. As used herein "‘aerodynamic relationship” means the blade assemblies are positioned such that one blade assembly is downstream of the other blade assembly so at least a portion of the air acted upon by the forward blade assembly is subsequently acted upon by the aft blade assembly. This allows the tangential velocity, also known as swirl, imparted to the air by the forward blade assembly to be counteracted, e.g., at least partially canceled, by the change in tangential velocity imparted by the aft blade assembly. At least one of the blade assemblies is a rotating assemblycarrying an array of airfoil blades that rotate about an axis of rotation and are located outside the engine nacelle. In some examples, the unducted propulsion system includes more than one rotating blade assembly (e.g., rotors, fans). The unducted propulsion system also includes an outlet guide vane assembly (e.g., a stator). In examples disclosed herein, an aftmost blade assembly is formed of outlet guide vanes. Without the aft blade assembly to cancel the swirl of the forward blade assembly, the high power per unit frontal or annular fan area for high speed flight w ould leave excessive swirl in the air that passes through the unducted propulsion system, resulting in poor efficiency in producing thrust. For this reason, single propeller propulsion systems, such as propellers on turboprop engines, typically do not power aircraft that exceed a cruise Mach number of 0.72.

[0037] Outlet guide vanes do not rotate about an axis and are placed aerodynamically downstream of the rotating blade assembly. The outlet guide vanes impart a change in tangential velocity that is opposite to the direction of tangential velocity imparted by the rotor, referred to as de-swirl. By de-swirling the air that it receives from the rotating blade assembly, the aft blade assembly reduces the magnitude of the tangential velocity of the air that passes through it, effectively converting the tangential momentum into axial momentum or thrust. The blades in a stator are often referred to as “vanes”. However, the general term “blade” and “blade assembly” are used herein to be used in either a rotating blade assembly or stationary blade assembly.

[0038] For a stationary blade assembly, the aircraft structure may be intermingled, integrated, or merged with the blade assembly. For example, the pylon used to mount an engine to an aircraft may occupy some of the same axial extent along the rotating blade assembly axis of rotation as at least some of the blades in the stationary blade assembly. Also, portions of the aircraft structure may be designed to serve the purpose of counter-swirl for a forward blade assembly or de-swirl for an aft blade assembly. Thus, aircraft structures may augment or even replace some blades in a stationary blade assembly.

[0039] Example unducted propulsion systems disclosed herein can be utilized in a subsonic aircraft having a cruise Mach number, Mo, of 0.7 or greater, for example 0.74 < Mo< 0.86. Referring now to the drawings, FIG. 1 is a partial cross-sectional view of an example unducted propulsion system 100. As is seen from FIG. 1, the unducted propulsion system 100 takes the form of an open rotor propulsion system and has a rotating element depicted as a propeller assembly which includes a fan 102 (e.g., an array or assembly of rotating fan blades, a propeller) affixed to a forward housing 104 (e g., a spinner) and configured to rotate around an axis of rotation 106 of the unducted propulsion system 100. The axis of rotation 106 is parallel to an axial direction A defined by the unducted propulsion system 100. The axis of rotation 106 is typically defined along a centerline of the unducted propulsion system 100. Further, the axis of rotation 106 is perpendicular to a radial direction R defined by the unducted propulsion system 100. The fan 102 includes outer radial edges, which are referred to as tips 108. A fan tip radius 110 is defined between the axis of rotation 106 and the tips 108 of the fan 102 in the radial direction R. The fan tip radius 110 is also referred to herein as RtiP,ProP. Specifically, the fan tip radius 110, and Rtip.prop. refers to a radius (e.g., a distance from the axis of rotation 106 in the radial direction R) of a leading edge of the tips 108 of the fan 102.

[0040] The unducted propulsion system 100 also includes an array (e.g., an assembly) of outlet guide vanes 112 (OGVs) (e.g., an array or assembly of non-rotating blades) positioned around the axis of rotation 106 downstream of the fan 102. The OGVs 112 may be arranged such that not all are equidistant from the fan 102. The OGVs are coupled to a stationary aft housing 114 (e.g., a nacelle) that is positioned around a flow path 116 defined by (e.g.. positioned around, surrounding) core engine components, w hich can include a compressor section, a combustor section, and a turbine section follow ed by an exhaust 118 (e.g., an outlet). Operations of the core turbomachinery help drive a rotation of the fan 102 and, thus, help produce thrust.

[0041] In the illustrated example of FIG. 1, an inlet 119 associated with the core turbomachinery is positioned betw een the fan 102 and the OGVs 112 in the axial direction A.This area is also referred to as the ‘‘splitter” as a first portion of the airflow produced by the fan 102 flows through the inlet 119 and a second portion of the airflow bypasses the inlet 119, flows over the unducted portion of the unducted propulsion system 100 and, thus, encounters the OGVs 112. The second portion of the airflow is herein referred to as the unducted fan stream or simply, fan stream.

[0042] The stationary’ aft housing 114 includes a nacelle bulge 120. As used herein, the ■‘nacelle bulge” is a portion of the nacelle downstream of the splitter (e.g., downstream of the inlet 119 for the core turbomachinery) at which the nacelle has a largest radius (e.g., a largest separation from the axis of rotation 106). When the unducted propulsion system 100 is operating (e.g.. producing thrust), air flowing adjacent the nacelle bulge 120 is energized by the fan 102, the radial distance R from the axis of rotation 106 is at a maximum radius, and an annular area of an airflow stream produced by' the fan 102 is the smallest relative to the other annular areas along the unducted propulsion system 100. As such, kinetic energy -per-area is highest proximate the nacelle bulge 120, which leads to increased noise and / or loads encountered in the area. Furthermore, the higher flow velocities result in increased skin friction drag on the nacelle bulge 120 and dow nstream portions of the stationary aft housing 114, which may result in reduced net thrust produced by the unducted propulsion system 100.

[0043] The OGVs 112 are shaped to minimize or otherwise reduce noise produced by the unducted propulsion system 100 and maximize or otherwise increase an efficiency of the unducted propulsion system 100. For example, the OGVs 112 can help the unducted propulsion system have a cruise Mach greater than 0.7. Rotation of the fan 102 causes an acceleration of the air stream that produces thrust. How ever, an increase in the acceleration of the airstream also tends to increase drag caused by the OGVs 112 and, thus, thrust losses within the array of OGVs 112. The OGVs 112 disclosed herein are configured to minimize or otherwise reduce drag at high Mach flight and maximize or otherwise increase thrust for a given power input.

[0044] Expressing thrust non-dimensionally in a way that accounts for flight speed, ambient conditions, and fan annular area yields a thrust coefficient that can be calculated using Equation 1 below:

[0045] In Equation 1, Fnetis cruise unducted fan stream net thrust, p0is ambient air density7, Vois cruise flight velocity7, and Aanis fan stream tube cross-sectional area at the fan inlet. Fan annular area, Aan. is a cross-sectional area occupied by the fan 102 and can be computed based on the tip radius (e.g.. a maximum radius of the fan 102) of the forwardmost rotor blades (e.g., the fan tip radius 110) and the inner radius of the fan stream tube entering thefan (e.g., a radius at a leading edge of a root of the fan 102, a minimum radius of the fan). As used herein, "‘fan net thrust” refers to a total thrust produced by the unducted fan stream. The OGVs 112 can improve the efficiency of the unducted thrust producing system 100 having a cruise thrust coefficient greater than 0.06, which is higher than the cruise thrust coefficient associated with a propeller. The shape of the OGVs is discussed in greater detail below.

[0046] FIGS. 2A-2B are perspective views of a portion of the unducted propulsion system 100 of FIG. 1. Specifically, the views of FIGS. 2A-2B omit portions of the unducted propulsion system 100 forward and aft of the OGVs 112 (e.g., omits the fan 102 and the exhaust 118 of FIG. 1). As shown in FIGS. 2A-2B, a pylon 202 is coupled to the stationary aft housing 114. The pylon 202 can couple the unducted propulsion system 100 to an aircraft (e.g., a wing of an aircraft). In this example, the OGVs 112 are aligned with a forward portion 204 of the pylon 202 in the axial direction A defined by the unducted propulsion system 100. Although the illustrated example of FIGS. 2A-2B shows a certain quantity of the OGVs 112, it should be understood that the OGVs 112 can include a different quantity. Additionally, the OGVs 112 can be non-uniformly spaced apart and / or the OGV s 112 can have non-uniform radii (e.g., heights).

[0047] FIG. 3A illustrates an isolated perspective view of one of the OGVs 1 12 of the unducted propulsion system 100. The OGV 112 includes a leading edge 302, a trailing edge 304, a hub 306 (e.g., a root, an inner radial surface), and a tip 308 (e.g., an outer radial surface). The leading edge 302 is a portion of the OGV 112 that receives incoming airflow in typical operations performed by the unducted propulsion system 100 (e.g.. cruise). The trailing edge 304 is a portion of the OGV 112 from which the airflow departs in the typical operations. More particularly, for non-rotating blades, such as the OGV 112, a surface of the blade that produces a higher static pressure, typically due to having a concave shape, can be referred to as the pressure surface. A surface of the blade that produces a lower static pressure, typically due to having a convex shape, can be referred to as a suction surface. The leading edge of a blade (e.g., the leading edge 302) is used herein to refer to a three-dimensional curve at which the suction surface and pressure surface meet on an upstream edge of the blade, based on the flight direction. A trailing edge refers to an intersection of the same suction surface and pressure surface on the downstream edge of the blade.

[0048] FIG. 3B illustrates a cross-sectional view of the OGV 112 taken at a radius R1 (e.g., a distance from the axis of rotation 106 in the radial direction R) that is constant from the leading edge 302 to the trailing edge 304. Specifically, the cross-sectional view of FIG. 3B shows a chord length 310 of the OGV 112. The chord length 310 and a location of the chord length 310 along the height of the OGV 1 12 affect the aerodynamic performance and anefficiency of the OGV 112. Examples disclosed herein optimize the chord length 310 and a distribution of the chord length 310 to maximize or otherwise improve the aerodynamic performance and efficiency of the OGV 1 12 and, in turn, the unducted propulsion system 100.

[0049] As used herein, the “chord” or “chord length” of an airfoil, such as the OGV 112, is a straight line distance between a leading edge (e.g., the leading edge 302) and a trailing edge (e.g., the trailing edge 304) at a given radius R1 along a height of the airfoil (e.g., a line of constant radius between the hub 306 and the tip 308). FIG. 3B is representative of a crosssection of the OGV 112 taken along the radius R1. As such, the “chord” or “chord length” can also be referred to as a width of the airfoil at the radius R1. As used herein, because the OGV 112 does not have both a leading edge (e.g., the leading edge 302) and a trailing edge (e.g., the trailing edge 304) at some radii (e.g., at some distances from the axis of rotation 106) occupied by a portion of the OGV 112 (e g., at a minimum radius of the hub 306 and / or a maximum radius of the tip 308), the “chord” of the hub 306 and the tip 308 is discussed in the context of nearest radius at which both the leading edge 302 and the trailing edge 304 are positioned.

[0050] In some examples, a portion of the leading edge 302 is wavy’ (e.g., varies in position relative to an axial direction defined by the unducted propulsion system 100) to enhance incidence tolerance at off-design conditions. In some other examples, the entire leading edge may be wavy, as discussed further in connection with FIGS. 10-11. The amplitude and wavelength of these waves may be uniform over the span of the OGV 112 or may vary to provide optimal incidence tolerance. Incidence tolerance refers to a range across which an angle of incidence of an airfoil can vary during its operation without compromising aerodynamic performance of the airfoil. The angle of incidence is the difference betw een the incoming flow to the airfoil and the airfoil camber line (e.g., a difference in angle between airflow angle and airfoil meanline at the leading edge). In some examples, the leading edge may also be wavy in the circumferential direction.

[0051] A position along the leading edge 302 is defined by radial and axial locations along the radius of the OGV 112. If the OGV 112 has a variable pitch, the axial location at any radial location along the length of the OGV 112 will depend on the pitch setting. With respect to variable pitch OGV s, discussion of the shape of the OGV 112 herein pertains to an orientation of the OGV 112 during cruise operations. That is, the OGV 112 is at the pitch setting that minimizes the power input to the fan for the thrust produced by the unducted thrust producing system. How ever, it should be understood that the discussion of the shape of the OGV 112 may also pertain to other design points. For example, the shape of the OGV 112 can reduce noise encountered during other operations, such as takeoff.

[0052] FIG. 4A illustrates a side view of the OGV 112. The OGV 112 includes a peak 402, a valley 403, a hub portion 404, and a tip portion 406. The hub portion 404 extends from the hub 306 to a radial distance defined by the peak 402. The tip portion 406 extends from the radial distance defined by the peak 402 to the tip 308.

[0053] As used herein, the “peak” of an airfoil (e.g., the peak 402 of the OGV 112) refers to a location on the airfoil at which the leading edge of the airfoil is positioned furthest forward in the axial direction A where the airfoil has a forward sweep in a hub portion (e.g., the hub portion 404) and an aft sweep in a tip portion (e.g., the tip portion 406). Accordingly, a remainder of the leading edge 302 of the OGV 112 outside (e.g., radially inward and radially outward) of the peak 402 is positioned aft of the peak 402. That is, the peak 402 is positioned forward of the leading edge 302 in the hub portion 404 and the tip portion 406. As such, the peak 402 defines a forwardmost portion (e.g., a forwardmost point) of the OGV 112 along the axial direction. The section of the OGV 112 at which the leading edge of the airfoil is positioned furthest forward in the axial direction A where the airfoil has a forward sweep in a hub portion (e.g., the hub portion 404) and an aft sweep in a tip portion (e.g., the tip portion 406) can also be referred to as the “belly section.”

[0054] As used herein, the “forwardmost point” or “forwardmost portion” of an OGV (e.g., the OGV 112) refers to the forwardmost point of the leading edge of the OGV when the OGV is in a cruise orientation. In some examples, the OGV is actuated or pitched for certain conditions, such as takeoff, cruise, and other conditions, and the “forwardmost point” or “forwardmost portion” of the OGV may not be the forwardmost point of the leading edge of the OGV in other (e.g., non-cruise) conditions.

[0055] Returning to the illustrated example of FIG. 1, a peak 122 of the fan 102 is defined at a radius greater than or equal to a radius of the peak 402 of the OGV 112. Specifically, the fan 102 has a fan peak radius 124, and the OGV 112 has an OGV peak radius 126. The fan peak radius 124 is greater than or equal to the OGV peak radius 126. Further, the hub 306 of the OGV has an OGV hub radius 128.

[0056] A relationship of the OGV hub radius 128 at the leading edge 302 normalized by the fan tip radius 110 provides a normalized OGV hub radius (NOGVHR) that can be calculated using Equation 2 below:OGV hub radius 128Equation 2: NOGVHR fan tip radius 110

[0057] A relationship of the OGV peak radius 126 normalized by the fan tip radius 110 provides a normalized OGV peak radius (NOGVPR) that can be calculated using Equation 3 below:OGV peak radius 126Equation 3: NOGVPR fan tip radius 110

[0058] In some examples, the NOGVPR herein is less than or equal to 0.4. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR (e.g., 110% of the NOGVHR). In some examples, the NOGVPR is less than or equal to 0.4. In some such examples, the NOGVPR is greater than or equal to 1.15*NOGVHR (e.g., 115% of the NOGVHR). In some examples, the NOGVPR is less than or equal to 0.4. In some such examples, the NOGVPR is greater than or equal to 1.2*NOGVHR (e.g., 120% of the NOGVHR). In some examples, the NOGVPR is less than or equal to 0.5. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.5. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR In some examples, the NOGVPR is less than or equal to 0.5. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.55. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.55. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.55. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.6. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.6. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.6. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.65. In some such examples, the NOGVPR is greater than 1 . 1 *NOGVHR. In some examples, the NOGVPR is less than or equal to 0.65 and greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.65. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.68. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.68. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.68. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.72. In some such examples. the NOGVPR is greater than 1.1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.72. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.72. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.75. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.75. Insome such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.75. In some such examples, the NOGVPR is greater than 1.2*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.8. In some such examples, the NOGVPR is greater than 1. 1*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.8. In some such examples, the NOGVPR is greater than 1. 15*NOGVHR. In some examples, the NOGVPR is less than or equal to 0.8. In some such examples, the NOGVPR is greater than 1.2*NOGVHR.

[0059] Similarly, a normalized fan peak radius (NFPR) can be calculated using Equation4 below: fan peak radius 124Equation 4: NFPR fan tip radius 110

[0060] In examples disclosed herein, the NOGVPR is less than or equal to the NFPR. In some aspects, the NOGVPR is less than the NFPR. Specifically, reducing open fan noise includes maximizing fan sweep in the outer radial portions of the fan 102, where noise radiates most efficiently. Accordingly, the fan peak radius 124 is selected such as to maximize or otherwise increase sweep in the acoustically sensitive portion of the fan 102 to minimize the noise radiated by the fan loading. Configuring the NOGVPR to be less than the NFPR maximizes or otherwise increases the OGV sweep radially outward of the peak 402, where the fan wake interaction with the OGV 112 radiates most efficiently to the surroundings, which reduces the fan-wake OGV interaction noise and, in combination with the choice of NFPR, reduces the total noise produced by the unducted propulsion system 100.

[0061] Returning to the illustrated example of FIGS. 4A-4B, the valley 403 defines a forwardmost portion of the trailing edge 304. More generally, as used herein, the “valley” of an airfoil (e.g., the valley 403 of the OGV 112) refers to a location on a trailing edge of the airfoil at which the trailing edge of the airfoil is positioned furthest forward in the axial direction A where the airfoil has a forward sweep in a hub portion (e.g., the hub portion 404) and an aft sweep in a tip portion (e.g., the tip portion 406). Thus, a remainder of the trailing edge 304 of the OGV 112 outside (e.g., radially inward and radially outward) of the valley 403 is positioned aft of the vallev 403.

[0062] In some examples, the peak 402 and the valley 403 are positioned at different radii. For example, a radius of the valley 403 can be greater than a radius of the peak 402, as shown in FIGS. 4A-4B.

[0063] In some examples, the OGV 112 is formed of metal. In some examples, the leading edge 302 of the OGV 112 is formed of metal. In some examples, a trailing portion of the OGV 1 12 (e.g., a portion of the OGV 112 aft of the metal at the leading edge 302, a portionof the OGV 112 extending from the trailing edge 304 to the metal of the leading edge 302 in the axial direction A) is formed of a composite material. In some examples, the composite material includes a braided fabric and / or braided fiber with a composite skin surrounding the braided fabric and / or fiber. In some examples, the composite material includes a polymer matrix composite (PMC), a ceramic matrix composite (CMC), metal matrix composite (MMC), carbon fiber, polymeric resin, thermoplastic, bismaleimide (BMI), polyimide materials, epoxy resin, glass fiber, and / or silicon matrix materials. In some examples, the composite materials includes metallic and non-metallic composites. For example, the composite material can include a unidirectional tape material and an epoxy resin matrix. In some examples, the composite material includes composite materials of the non-metallic type made of a material containing a fiber such as a carbonaceous, silica, metal, metal oxide, or ceramic fiber embedded in a resin material such as Epoxy, PMR15, BMI, PEED, etc. In some examples, the composite material includes fibers unidirectionally aligned into a tape that is impregnated with a resin, formed into a part shape, and cured via an autoclaving process or press molding to form a light-weight, stiff, relatively homogeneous article having laminates within. In some examples, the tip 308 includes a metal tip cap.

[0064] The hub portion 404 includes a forward sweep 408. As used herein, “forward sweep” refers to a tilt or angular orientation of a portion of the OGV 112 that positions the leading edge 302 further forward as a separation from the axis of rotation 106 increases. That is, the leading edge 302 extends further forward in the hub portion 404 as the separation of the hub portion 404 from the axis of rotation 106 increases in the radial direction R. In other words, an axial position of the leading edge 302 moves further forward in the hub portion 404 as separation from the axis of rotation 106 in the radial direction R increases.

[0065] The tip portion 406 includes an aft sweep 410. As used herein, “aft sweep” refers to a tilt or angular orientation of a portion of the OGV 1 12 that positions the leading edge 302 further aft as a separation from the axis of rotation 106 increases. As such, the leading edge 302 extends further aft in the tip portion 406 as the separation of the tip portion 406 from the axis of rotation 106 increases in the radial direction R. That is, an axial position of the leading edge 302 moves further aft in the tip portion 406 as separation from the axis of rotation 106 in the radial direction R increases.

[0066] The aft sweep of the tip portion 406 improves an efficiency and aerodynamics of the unducted propulsion system 100 and reduces noise produced by the unducted propulsion system 100. The forward sweep of the hub portion 404 limits an aft extension of the tip portion 406. In some examples, the forward sweep of the hub portion 404 enables the tip 308 to bepositioned forward of the nacelle bulge 120 (FIG. 1), which would otherwise reduce an efficiency of the unducted propulsion system 100. In some examples, the forward sweep of the hub portion 404 reduces an area of the tip portion 406 that aligns with the nacelle bulge 120 to improve an efficiency of the unducted propulsion system 100.

[0067] Additionally, as shown in the illustrated example of FIGS. 4A-4B, the chord length 310 of the OGV 112 varies with a radial distance from the axis of rotation 106. In the illustrated example of FIG. 4A, a maximum chord length 412 of the OGV 112 is defined in a portion of the hub portion 404. Specifically, the hub portion 404 has an increased chord length to provide the OGV 112 with support against higher aerodynamic loads encountered by the hub portion 404. In the illustrated example of FIG. 4B, the peak 402 has a same radius as the maximum chord length 412 of the OGV 112. The tip portion 406 encounters lower aerodynamic loads and includes a reduced chord length to limit skin friction losses encountered by the OGV 112 during cruise operations. Aerodynamic loads encountered by the tip 308 of the OGV 112 in the unducted propulsion system 100 at cruise conditions are lower than OGVs in ducted engines, which enables the tip portion 406 to have the reduced chord length without negatively affecting the structural integrity of the OGV 112. In this example, the tip 308 includes a minimum chord length 414 of the OGV 112.

[0068] FIG. 5 is a plot 500 representative of a relationship between the chord length 310 of the OGV 112 as a function of a distance from the axis of rotation 106 in the radial direction R. Specifically, ay-axis 502 of the plot 500 is representative of a ratio of (i) a distance between the axis of rotation 106 and the particular radial location of the OGV 112 at which the chord length is observed to (ii) the fan tip radius 110 between the axis of rotation 106 and the tip 108 of the fan 102 in the radial direction R (e.g., Rtip,ProP). That is, the y-axis 502 represents the ratio of (i) the radius along the OGV 112 to (ii) the radius of the tip 108 of the fan 102. As such, the y-axis can be represented by Equation 5 below:

[0069] In Equation 5, R is the radius along the OGV 112, and Rtlp, prop is the radius of the tip 108 of the fan 102 (e.g., the fan tip radius 110). In some examples, the OGV 112 and the fan 102 have different heights. As such, the ratio of (i) the distance of the tip 308 of the OGV 112 from the axis of rotation 106 to (ii) the fan tip radius 110 can be in a range that is greater than 0.75 and less than 1.1. Reference herein to the distance and / or the radius of the tip 308 of the OGV 112 from the axis of rotation 106 refers to a distance between the leading edge 302 at the tip 308 and the axis of rotation 106. In the illustrated example of FIG. 5, the x-axis 504 is representative of a chord variation ratio of (i) a chord length at the particular radial location ofthe OGV 112 at which the chord length is observed (e.g., at a particular distance from the axis of rotation 106 in the radial direction R) to (ii) a maximum chord length of the OGV 112. That is, the x-axis 504 represents the ratio of the chord along the OGV 112 to the maximum chord of the OGV 112. As such, the x-axis can be represented by Equation 6 below:Equation 6: X = - . chord.max

[0070] In Equation 6, chord is the chord length of the OGV 112 along the radial distance across which the OGV 112 extends, and chord, max is the maximum chord length of the OGV 112. As shown in the illustrated example of FIG. 5, a chord variation ratio of the minimum chord length of the OGV 112 (e.g., at the tip 308) to a maximum chord length of the OGV 112 (e.g., in the hub portion 404) is less than or equal to 0.25. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV is less than or equal to 0.3. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV is less than or equal to 0.4. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV is less than or equal to 0.5. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV is less than or equal to 0.6. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV is less than or equal to 0.7. In some examples, the chord variation ratio of the minimum chord length to the maximum chord length of the OGV 112 is less than or equal to 0.8. The low chord variation ratios of the minimum chord length of the OGV 1 12 to the maximum chord length of the OGV 1 12 enables the OGV 112 to have structural support for high loads encountered in the hub portion 404 while also maximizing an aerodynamic performance of the tip portion 406 and, thus, an efficiency of the unducted propulsion system through a reduction in skin friction losses encountered by the OGV 112 in the tip portion 406.

[0071] To facilitate certain advantages, such as noise reduction, one or more of the OGVs 112 can be clipped such that the distance of the tip 308 from the axis of rotation 106 is reduced. For a given OGV 112 example indicated by a curve in FIG. 5, clipping reduces the ratio indicated on the y-axis 502, thus increasing the ratio indicated on the x-axis 504. Thus, any of the curves for the example OGVs represented in the plot of FIG. 5 can be clipped (e.g., cut off or stopped at 0.7 on the y-axis, cut off or stopped at 0.8 on the y-axis, cut off or stopped at 0.9 on the y-axis) to represent reductions to the radius of tip 308. Accordingly, a ratio indicated on the x-axis 504 at 0.7, 0.8, and / or 0.9 on the y-axis can correspond to the ratio at the rip 308 of the OGV 112.

[0072] The plot of FIG. 5 shows the taper in chord from the max chord location (e.g., where the chord variation ratio indicated on the x-axis 504 equals 1.0) towards the tip 308. Another ratio of (i) the change in quantity represented on x-axis 504 to (ii) the change in the parameter represented on the y-axis 502, where the changes are taken from the maximum chord location to the tip 308, is a negative number. The magnitude of the negative number indicates an amount the chord decreases from the maximum chord location to the tip 308. As such, the ratio of the change in the x-axis 504 to the change in the y-axis 502 is less than -0.5. The ratio of the change in the x-axis 504 to the change in the y-axis 502 is preferably less than -1.0. The ratio of the change in the x-axis 504 to the change in the y-axis 502 is even more preferably less than -1.5.

[0073] Additionally, as shown in FIG. 5, the OGV 112 includes a ratio of (i) an absolute value of a change in a percentage of the chord length 310 from the maximum chord length to a chord length in the tip portion (e.g., the chord length 310 at the tip 308) to (ii) a radius difference between the maximum chord length and the third chord length normalized by a radius of the tip 108 of the fan 102 that is greater than 0.5. In some examples, the OGV 112 includes a ratio of (i) an absolute value of a change in a percentage of the chord length 310 from the maximum chord length to a chord length in the tip portion (e.g., the chord length 310 at the tip 308) to (ii) a radius difference between the maximum chord length and the third chord length normalized by a radius of the tip 108 of the fan 102 that is greater than 1.0. In some examples, the OGV 112 includes a ratio of (i) an absolute value of a change in a percentage of the chord length 310 from the maximum chord length to a chord length in the tip portion (e.g., the chord length 310 at the tip 308) to (ii) a radius difference between the maximum chord length and the third chord length normalized by a radius of the tip 108 of the fan 102 that is greater than 1.5.

[0074] FIG. 6 is another plot 600 representative of a relationship between the chord length 310 of the OGV 112 as a function of a span of the OGV 112 in the radial direction R. Specifically, some OGVs 112 can be clipped to reduce a height of the OGV 112 in the radial direction R (e.g., a separation of the tip 308 from the axis of rotation 106). The plot 600 provides the relationship between the chord length 310 of the OGV 112 as a function of a span of the OGV 112 in the radial direction R for a baseline OGV 602 (e.g., a nominal OGV). a first clipped OGV 604, and a second clipped OGV 606. Specifically, a tip (e.g., the tip 308) of the first clipped OGV 604 is positioned closer to the axis of rotation 106 than a rip (e.g., the tip 308) of the baseline OGV 602 in the radial direction R, and a tip of the second clipped OGV 606 is positioned closer to the axis of rotation 106 than the tip of the first clipped OGV 604 in theradial direction R. In some examples, the unducted propulsion system 100 (FIG. 1) includes the baseline OGV 602. the first clipped OGV 604, and / or the second clipped OGV 606.

[0075] In the illustrated example of FIG. 6, ay-axis 608 of the plot 600 is representative of a span of the OGV s 602, 604, 606. Along the y-axis 608, 0.0 is a hub (e.g., a root) at which the OGV 602, 604, 606 couples to the nacelle 114, and 1.0 is a tip (e.g., the tip 308) of the OGV 602, 604. 606. Accordingly, the y-axis 608 (Y2) can be represented by Equation 7 below:Equat .i.on - 7:vr2= - radius radius, max .

[0076] In Equation 7, the radius is the radius across which the OGV 1 12 extends (e.g., the distance across which the OGV 112 extends between the axis of rotation 106 and the tip 308 of the OGV 112), and the radius, max is the radius of the tip 308 of the OGV 112 (e.g., the distance between the axis of rotation 106 and the tip 308). The x-axis 610 of the plot 600 is representative of a normalized chord length of the OGVs 602, 604, 606. The normalized chord length is representative of a ratio between a chord length of the OGV 602, 604, 606 at the particular span location along the y-axis 608 relative to a minimum chord length of the OGV 602, 604. 606. Accordingly, the x-axis (X2) 610 can be represented by Equation 8 below:Equation 8: X2= -c,lord- chord, minimum

[0077] In Equation 8, chord is the chord length of the OGV 112 along the radial distance across which the OGV 1 12 extends, and chord, minimum is the minimum chord length of the OGV 112 (e.g., at the tip 308). In the illustrated example of FIG. 6, the maximum chord length for the OGVs 602, 604, 606 occurs at approximately 0.2 of the span. The baseline OGV 602 has a maximum chord length greater than 4 times the minimum chord length at the tip (e.g., the tip 308). The first clipped OGV 604 has a maximum chord length greater than 2 times the minimum chord length at the tip (e.g., the tip 308). The second clipped OGV 606 has a maximum chord length approximately 1.5 times the minimum chord length at the tip (e.g., the tip 308). The difference between the maximum chord length to minimum chord length ratios of the OGVs 602, 604, 606 is a result of the tips of the OGVs 602, 604, 606 having a greater chord length when clipped.

[0078] FIG. 7 is an isolated view of the OGV 112. The illustrated example of FIG. 7 will be utilized for discussion of a leading edge aft sweep 700 of the tip portion 406 of the OGV 112. In the illustrated example of FIG. 7, the leading edge aft sweep 700 along the leading edge 302 of the tip portion 406 is characterized by a ratio of (i) an axial separation 702 between the leading edge 302 of the tip portion 406 and the peak 402 in the axial direction A to (ii) a radial separation 704 from the peak 402 to the tip 308. Accordingly, the leading edge aft sweep 700 from the peak 402 to the tip 308 can be represented by a ratio of (i) a first distance from the peak402 to the leading edge 302 at the tip 308 in the axial direction A to (ii) a second distance from the peak 402 to the tip 308 in the radial direction R. Thus, the leading edge aft sweep 700 can be represented by a leading edge sweep characteristic ratio (LESCR) defined by Equation 9 below: axial separation 702Equation 9. LESCR radial separation 704'

[0079] As shown, the leading edge aft sweep 700 results in an axial separation between the peak 402 and the leading edge 302 of the tip portion 406 that increases with increased separation from the axis of rotation 106 in the radial direction R. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0. 1. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0.2. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0.3. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0.5. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0.7. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 0.9. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 1. 1. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 to be greater than or equal to 1.4. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 (e.g., at the tip 308) to be greater than or equal to 1 .7. In some examples, the leading edge aft sweep 700 causes the ratio in a portion of the tip portion 406 (e.g., at the tip 308) to be greater than or equal to 1.8.

[0080] The leading edge aft sweep 700 can also be quantified in degrees (°). For example, the leading edge aft sweep 700 (e.g., an average angle of the leading edge aft sweep 700) is equivalent to or between 10° and 60°. The leading edge aft sweep 700 in degrees can be determined by taking the arctangent of the LESCR and converting to degrees. Additionally or alternatively, the leading edge aft sweep 700 in degrees can be determined by measuring an angle between a straight line from the peak 402 to the leading edge 302 at the tip 308 and the radial direction R (FIG. 1) (e.g., the radial separation 704 of FIG. 7). Table 1 below shows how the degree quantification corresponds with the LESCR.Table 1

[0081] The example ratio values provided above generally correspond to an unducted propulsion system (e.g., the unducted propulsion system 100) that cruises at approximately Mach 0.8. The ratios can increase with an increase in the cruise Mach and / or an increase in the thrust coefficient. Additionally, the ratios can increase with an increase in the thrust coefficient.

[0082] FIG. 8 is another isolated view of the OGV 112. The illustrated example of FIG. 8 will be utilized for discussion of a trailing edge aft sweep 800 of the trailing edge 304 betw een the valley 403 and the tip 308. In the illustrated example of FIG. 8, the trailing edge aft sw eep 800 along the trailing edge 304 is characterized by a ratio of (i) an axial separation 802 between the valley 403 and the trailing edge 304 at the tip 308 in the axial direction A to (ii) a radial separation 804 between the valley 403 and the trailing edge 304 at the tip 308 in the radial direction R. Accordingly, the trailing edge aft sweep 800 from the valley 403 to the tip 308 can be represented by a ratio of (i) a first distance (e.g., a third distance) from the valley 403 to the trailing edge 304 at the tip 308 in the axial direction A to (ii) a second distance (e.g., a fourth distance) from the valley 403 to the tip 308 in the radial direction R. Thus, the trailing edge aft sweep 800 can be represented by a trailing edge sweep characteristic ratio (TESCR) defined byEquation 10 below : axial separation 802Equation 10. TESCR = radial separation 804’The TESCR is greater than or equal to 0. 1 and less than or equal to 1.8. Additionally, the TESCR is less than or equal to the LESCR.

[0083] FIG. 9 illustrates another example OGV 900 including a first example wavy leading edge 902. The wary leading edge 902 enhances incidence tolerance at off-design conditions (e.g., non-cruise). In the illustrated example of FIG. 9, the wavy leading edge 902 includes convex portions 904 contiguous with the hub 306 and the tip 308.

[0084] FIG. 10 illustrates another example OGV 1000 including a second example wavy leading edge 1002. The wavy leading edge 1002 enhances incidence tolerance at off-design conditions (e g., takeoff). In the illustrated example of FIG. 10, the wavy leading edge 1002 includes concave portions 1004 contiguous with the hub 306 and the tip 308. In some examples, the OGV 1000 includes one of the convex portions 904 contiguous with the hub 306 instead ofone of the concave portions 1004. In some examples, the OGV 1000 includes one of the convex portions 904 contiguous with the tip 308 instead of one of the concave portions 1004.

[0085] The amplitude and wavelength of waves in the first wavy leading edge 902 of FIG. 9 and the second wavy leading edge 1002 of FIG. 10 may be uniform over the span of the OGV 112 or may vary to provide the optimal incidence tolerance. As mentioned above, the optimal incidence tolerance refers to a range across which an angle of incidence of an airfoil can vary during its operation without compromising aerodynamic performance of the airfoil. In some examples, the first wavy leading edge 902 of FIG. 9 and / or the second wavy leading edge 1002 of FIG. 10 may also be wavy in the circumferential direction. The example OGV 900 of FIG. 9 is preferable to the example OGV 1000 of FIG. 10 to enable the OGV to have a leading edge forward sweep contiguous with the hub 306 and a leading edge aft sweep contiguous with the tip 308.

[0086] While provided with a different reference number as a result of a difference between shapes of the leading edge 302 of the OGV 112 of FIGS. 1-4 and 7-8, the first wavy- leading edge 902 of the OGV 900 of FIG. 9. and the second wavy leading edge 1002 of the OGV 1000 of FIG. 10, it should be understood that the first wavy leading edge 902 or the second wavy leading edge 1002 can be incorporated in the OGV 112 (i.e., in place of a non- wavy shape of the leading edge 302) and the unducted propulsion system 100 of FIG. 1. Moreover, the OGV 112 can maintain other features discussed herein in connection with FIGS. 1-8 above and FIGS. 1 1 A-C below with incorporation of the first wavy leading edge 902 or the second wavy leading edge 1002.

[0087] FIG. 11 A illustrates an example implementation of the OGV 112 including a metal leading portion 1102 (e.g.. a metal leading edge) and a composite trailing portion 1104 (e.g., a composite trailing edge). The metal leading portion 1102 of the OGV 112 is formed of a metal. The composite trailing portion 1104 of the OGV 1 12 is formed of a composite material. The metal leading portion 1102 extends from the leading edge 302 towards the trailing edge 304. Thus, the metal leading portion 1102 defines a leading portion of the OGV 112 (e.g., a leading portion of the chord length 310 (FIG. 3)), and the composite trailing portion 1104 defines a trailing portion of the OGV 112 (e.g., a trailing portion of the chord length 310) distinct from and contiguous with the metal leading portion. Thus, the metal occupies a portion of a length of the OGV 112 in the axial direction A from the leading edge 302. The metal leading portion 1102 provides the leading edge 302 with erosion protection while reducing a weight of the OGV 112 relative to the OGV 1 12 being formed of metal in the composite trailing portion 1 104 in addition to the metal leading portion 1 102.

[0088] FIG. 1 IB illustrates another example implementation of the OGV 112 including a metal tip cap 1106 that occupies a portion of a height of the OGV 112 in the radial direction R from the tip 308 (e.g., the outer radial surface of the OGV 112). That is, the metal tip portion extends from the tip 308 towards the hub 306. In FIG. 1 IB, the metal tip cap 1106 includes a metal, and a portion of the OGV 112 outside of the metal tip cap 1106 includes anon-metal (e.g., a composite). The metal tip cap 1106 provides the tip 308 with increased protection against collision (e.g., bird strike). Further, as the metal tip cap 1 106 is smaller than the metal leading portion 1102 (FIG. 11A), the example implementation of the OGV 112 in FIG. 1 IB has a reduced weight relative to the example implementation of the OGV 112 in FIG. 11 A.

[0089] FIG. 11C illustrates another example implementation of the OGV 112 including both the metal leading portion 1102 and the metal tip cap 1106. As such, the example implementation of the OGV 112 of FIG. 11C provides increased protection against debris impact and increased erosion protection at the leading edge 302. However, the example implementation of the OGV 112 of FIG. 11 C has an increased weight relative to the example implementations of the OGV 112 of FIGS. HA and 11B.

[0090] From the foregoing, it will be appreciated that example unducted propulsion systems and associated outlet guide vanes have been disclosed that improve an efficiency and aerodynamics of an engine. Further, the example unducted propulsion systems and associated outlet guide vanes disclosed herein can reduce noise produced by unducted propulsion systems.

[0091] Example unducted propulsion systems having outlet guide vanes are disclosed herein. Further examples and combinations thereof include the following clauses:

[0092] An unducted propulsion system comprising a fan, and an outlet guide vane downstream of the fan. the outlet guide vane including a hub portion, a tip portion, and a belly section between the hub portion and the tip portion in a radial direction defined by the unducted propulsion system, the hub portion including a forward sweep, the tip portion including an aft sweep, wherein the belly section includes a forwardmost point of the outlet guide vane, wherein at least a portion of the tip portion includes a first chord length, wherein the outlet guide vane includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

[0093] The unducted propulsion system of any preceding clause, wherein the second chord length is a maximum chord length of the outlet guide vane, and wherein the hub portion includes the maximum chord length.

[0094] The unducted propulsion system of any preceding clause, wherein the second chord length is a maximum chord length of the outlet guide vane, and wherein the hub portionincludes the maximum chord length or the peak is defined at a same radius as the maximum chord length.

[0095] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the tip is positioned a first distance from an axis of rotation of the fan in a radial direction defined by the unducted propulsion system, wherein the maximum chord length is positioned a second distance from the axis of rotation of the fan in the radial direction, and wherein the second distance is approximately half of the first distance.

[0096] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the tip is positioned at a first distance from an axis of rotation of the fan in a radial direction defined by the unducted propulsion system, wherein the first chord length is positioned at a second distance from the axis of rotation of the fan in the radial direction, and wherein the second distance is greater than or equal to 75% of the first distance.

[0097] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.7.

[0098] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.6.

[0099] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.5.

[0100] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.4.

[0101] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.3.

[0102] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.25.

[0103] The unducted propulsion system of any preceding clause, wherein the ratio is a first ratio, wherein a second ratio of a maximum chord length of the outlet guide vane to a minimum chord length of the outlet guide vane is at least 1.5.

[0104] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 2.0.

[0105] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 2.5.

[0106] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 3.0.

[0107] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 3.5.

[0108] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 4.0.

[0109] The unducted propulsion system of any preceding clause, wherein the second ratio is at least 4.4.

[0110] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a leading edge and a trailing portion, wherein the leading edge is formed of a metal, wherein the trailing portion is formed of a composite material.[OHl] The unducted propulsion system of any preceding clause, wherein an outer radial surface of the tip portion of the outlet guide vane includes a metal tip cap.

[0112] The unducted propulsion system of any preceding clause, wherein an outer radial edge of the tip portion defines a tip of the outlet guide vane, wherein the ratio is a chord variation ratio, wherein a sweep from the belly section to the tip is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the forwardmost point to a leading edge of the tip in an axial direction defined by the unducted propulsion system to (ii) a second distance from the belly portion to the tip in the radial direction, wherein the leading edge sweep characteristic ratio is greater than or equal to 0. 1 and less than or equal to 1.8.

[0113] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a tip, wherein the ratio is a chord variation ratio, wherein a sweep from the peak to the tip is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the peak to a leading edge of the tip in an axial direction defined by the unducted propulsion sy stem to (ii) a second distance from the peak to the tip in the radial direction, wherein the leading edge sweep characteristic ratio is greater than or equal to 0. 1 and less than or equal to 1.8.

[0114] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.2.

[0115] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.3.

[0116] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.4.

[0117] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.5.

[0118] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.6.

[0119] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.7.

[0120] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.8.

[0121] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.9.

[0122] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.0.

[0123] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.1.

[0124] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.2.

[0125] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.3.

[0126] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.4.

[0127] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.5.

[0128] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.6.

[0129] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.7.

[0130] The unducted propulsion system of any preceding clause, wherein the outlet guide vane is a first outlet guide vane of an outlet guide vane assembly, wherein the ratio is a first chord variation ratio, and wherein the outlet guide vane assembly includes a second outlet guide vane downstream of the fan.

[0131] The unducted propulsion system of any preceding clause, wherein the second outlet guide vane includes a third chord length and a fourth chord length, wherein the third chord length is at a tip of the second outlet guide vane, and wherein a second chord variation ratio of the third chord length to the fourth chord length is approximately equivalent to the first chord variation ratio.

[0132] The unducted propulsion system of any preceding clause, wherein the fourth chord length is a maximum chord length of the second outlet guide vane.

[0133] The unducted propulsion system of any preceding clause, wherein the second outlet guide vane includes a third chord length and a fourth chord length, wherein the third chord length is at a tip of the second outlet guide vane, and wherein a second chord variation ratio of the third chord length to the fourth chord length is different than the first chord variation ratio.

[0134] The unducted propulsion system of any preceding clause, wherein the fourth chord length is a maximum chord length of the second outlet guide vane.

[0135] The unducted propulsion system of any preceding clause, wherein the outlet guide vane is a first outlet guide vane that has a first radial height, further including a second outlet guide vane that has a second radial height, wherein the second radial height is different than the first radial height.

[0136] The unducted propulsion system of any preceding clause, wherein the unducted propulsion sy stem provides a cruise flight Mach greater than 0.7.

[0137] The unducted propulsion system of any preceding clause, wherein the unducted propulsion system provides a cruise flight Mach greater than 0.75.

[0138] The unducted propulsion system of any preceding clause, wherein the unducted propulsion sy stem provides a cruise flight Mach greater than 0.78.

[0139] The unducted propulsion system of any preceding clause, wherein the unducted propulsion system provides a thrust coefficient greater than 0.06.

[0140] The unducted propulsion system of any preceding clause, wherein a forwardmost portion of the outlet guide vane is at a radius normalized by a tip radius of the fan that is less than or equal to 0.5.

[0141] The unducted propulsion system of any preceding clause, wherein a forwardmost portion of the outlet guide vane is at a radius normalized by a tip radius of the fan is less than or equal to 0.55.

[0142] The unducted propulsion system of any preceding clause, wherein a forwardmost portion of the outlet guide vane is at a radius normalized by a tip radius of the fan is less than or equal to 0.6.

[0143] The unducted propulsion system of any preceding clause, wherein the ratio is a chord variation ratio, wherein the outlet guide vane includes a second ratio of (i) an absolute value of a change in a percentage of the chord length from a maximum chord length to a third chord length in the tip portion to (ii) a radius difference between the maximum chord length and the third chord length normalized by a radius of atip of the fan that is greater than 0.5.

[0144] The unducted propulsion system of any preceding clause, wherein the ratio is a chord variation ratio, wherein the outlet guide vane includes a second ratio of (i) an absolute value of a change in a percentage of the chord length from a maximum chord length to a third chord length in the tip portion to (ii) a radius difference between the maximum chord length and the third chord length normalized by a distance betw een a tip of the fan that is greater than 1.0.

[0145] The unducted propulsion system of any preceding clause, wherein the ratio is a chord variation ratio, wherein the outlet guide vane includes a second ratio of (i) an absolute value of a change in a percentage of the chord length from a maximum chord length to a third chord length in the tip portion to (ii) a radius difference between the maximum chord length and the third chord length normalized by a distance between a tip of the fan that is greater than 1.5.

[0146] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the belly section is positioned a second distance from the axis of rotation, and wherein the second distance is in a range of 50% to 60% of the first distance.

[0147] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the dp of the fan is positioned a first distance from an axis of rotation of the fan, wherein the belly section is positioned a second distance from the axis of rotation, and wherein the second distance is in a range of 50% to 65% of the first distance.

[0148] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the belly section is positioned a second distance from the axis of rotation, and wherein the second distance is in a range of 50% to 70% of the first distance.

[0149] The unducted propulsion system of any preceding clause, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the second chord length is positioned a second distance from the axis of rotation of the fan, and wherein the second distance is at least 70% of the first distance.

[0150] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.6, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the second chord length is positioned a second distance from the axis of rotation of the fan, and wherein the second distance is at least 80% of the first distance.

[0151] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.4, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the second chord length is positioned asecond distance from the axis of rotation of the fan, and wherein the second distance is at least 90% of the first distance.

[0152] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.3, wherein the fan includes a tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the second chord length is positioned a second distance from the axis of rotation of the fan, and wherein the second distance is at least 90% of the first distance.

[0153] An outlet guide vane for an unducted propulsion system, the outlet guide vane comprising a leading edge, a trailing edge, a hub to couple to a nacelle of the unducted propulsion system, and a tip including a first chord length from the leading edge to the trailing edge, wherein a portion of the outlet guide vane between the hub and the tip in a radial direction defined by the unducted propulsion system includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

[0154] The outlet guide vane of any preceding clause, further including a belly between the hub and the tip, wherein a leading edge of the outlet guide vane at the belly defines a forwardmost portion of the outlet guide vane.

[0155] The outlet guide vane of any preceding clause, wherein the outlet guide vane is actuated to a first position for a first flight operation, and wherein the outlet guide vane is actuated to a second position for a second flight operation, wherein the second position is different than the first position, and wherein the second flight operation is different than the first flight operation.

[0156] An unducted propulsion system comprising a fan, and an outlet guide vane downstream of the fan. the outlet guide vane including a hub portion, a tip portion, and a peak positioned between the hub portion and the tip portion in a radial direction defined by the unducted propulsion system, the hub portion including a forward sweep, the tip portion including an aft sweep, wherein the outlet guide vane includes a first chord length in the tip portion, wherein the outlet guide vane includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

[0157] The unducted propulsion system of any preceding clause, wherein the second chord length is a maximum chord length of the outlet guide vane, and wherein the hub portion includes the maximum chord length.

[0158] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.6.

[0159] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.4.

[0160] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.25.

[0161] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a leading edge and a trailing portion, wherein the leading edge is formed of a metal, wherein the trailing portion is formed of a composite material, and wherein an outer radial surface of the tip portion of the outlet guide vane includes a metal tip cap.

[0162] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a tip, wherein the ratio is a chord variation ratio, wherein a sweep from the peak to the tip is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the peak to a leading edge of the tip in an axial direction defined by the unducted propulsion system to (ii) a second distance from the peak to the tip in the radial direction, wherein the leading edge sweep characteristic ratio is greater than or equal to 0. 1 and less than or equal to 1.8.

[0163] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.5.

[0164] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.9.

[0165] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.4.

[0166] The unducted propulsion system of any preceding clause, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.7.

[0167] An unducted propulsion system comprising a fan. and an outlet guide vane downstream of the fan, the outlet guide vane including a leading edge, a trailing edge, a hub to couple to a nacelle of the unducted propulsion system, and a tip including a first chord length from the leading edge to the trailing edge, wherein a portion of the outlet guide vane between the hub and the tip in a radial direction defined by the unducted propulsion system includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

[0168] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip. wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan. wherein the outlet guide vane includes a peakpositioned a second distance from the axis of rotation, and wherein the second distance is equivalent to or between 50% and 60% of the first distance.

[0169] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein the second distance is less than or equal to 70% of the first distance.

[0170] The unducted propulsion system of any preceding clause, w herein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.8.

[0171] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.75.

[0172] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.72.

[0173] The unducted propulsion system of any preceding clause, wherein the tip is a first tip. wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.68.

[0174] The unducted propulsion system of any preceding clause, wherein the tip is a first tip. w herein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and w herein a ratio of the second distance to the first distance is less than or equal to 0.65.

[0175] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, w herein the fan includes a second tip, wherein the second tip of the fan is positioned a firstdistance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.6.

[0176] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.55.

[0177] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.5.

[0178] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation, and wherein a ratio of the second distance to the first distance is less than or equal to 0.4.

[0179] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a first peak, wherein the first peak is positioned at a first radius, wherein the fan includes a second peak positioned at a second radius during cruise operations, wherein the second radius is greater than the first radius.

[0180] The unducted propulsion system of any preceding clause, wherein the tip is a first tip. wherein the fan includes a second tip, wherein the tip of the fan is positioned at a first radius, wherein the outlet guide vane includes a peak positioned at a second radius, and wherein the second radius is in a range of 50% to 65% of the first radius.

[0181] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the tip of the fan is positioned at a first radius, wherein the outlet guide vane includes a peak positioned at a second radius, and wherein the second radius is in a range of 50% to 70% of the first radius.

[0182] The unducted propulsion system of any preceding clause, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned at a first radius, wherein the second chord length is positioned at a second radius, and wherein the second radius is at least 70% of the first radius.

[0183] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.6, wherein the tip is a first tip. wherein the fan includes a second tip, wherein the second tip is positioned at a first radius, wherein the second chord length is positioned at a second radius, and wherein the second radius is at least 80% of the first radius.

[0184] The unducted propulsion system of any preceding clause, wherein the ratio is less than or equal to 0.3, wherein the tip is a first tip. wherein the fan includes a second tip, wherein the second tip is positioned at a first radius, wherein the second chord length is positioned at a second radius, and wherein the second radius is at least 90% of the first radius.

[0185] The unducted propulsion system of any preceding clause, wherein the second distance is greater than or equal to 40% of the first distance.

[0186] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a first peak, wherein the first peak is positioned at a first radius, wherein the fan includes a second peak positioned at a second radius, wherein the second radius is greater than the first radius.

[0187] An outlet guide vane for an unducted propulsion system, the outlet guide vane comprising a hub a tip, and a leading edge extending from the hub to the tip, wherein the leading edge includes a peak positioned between the hub and the tip in a radial direction defined by the unducted propulsion system, wherein the leading edge includes a leading edge aft sweep from the peak to the tip, wherein the leading edge aft sweep is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the peak to the leading edge at the tip in an axial direction defined by the unducted propulsion system to (ii) a second distance from the peak to the tip in the radial direction, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.4 and less than or equal to 1.8.

[0188] The outlet guide vane of any preceding clause, further including a trailing edge extending from the hub to the tip, wherein the trailing edge includes a valley between the hub and the tip in the radial direction, wherein the trailing edge includes a trailing edge aft sweep from the valley to the tip, wherein the trailing edge aft sweep is characterized by a trailing edge sweep characteristic ratio of (i) a third distance from the valley to the trailing edge at the tip in the axial direction to (ii) a fourth distance from the valley to the tip in the radial direction, wherein the trailing edge sweep characteristic ratio is greater than or equal to 0.2 and less than or equal to 1.8.

[0189] The outlet guide vane of any preceding clause, further including a trailing edge extending from the hub to the tip, wherein the trailing edge includes a valley between the hub and the tip in the radial direction, wherein the trailing edge includes a trailing edge aft sweepfrom the valley to the tip, wherein the trailing edge aft sweep is characterized by a trailing edge sweep characteristic ratio of (i) a third distance from the valley to the trailing edge at the tip in the axial direction to (ii) a fourth distance from the valley to the tip in the radial direction, wherein the trailing edge sweep characteristic ratio is greater than or equal to 0.3 and less than or equal to 1.8.

[0190] The unducted propulsion system of any preceding clause, wherein the outlet guide vane includes a wavy leading edge.

[0191] The unducted propulsion system of any preceding clause, wherein the wavy leading edge includes a convex portion contiguous with at least one of a hub or a tip of the outlet guide vane.

[0192] The unducted propulsion system of any preceding clause, wherein the wavy leading edge include a concave portion contiguous with at least one of a hub or a tip of the outlet guide vane.

[0193] The following claims are hereby incorporated into this Detailed Description by this reference. Although certain example systems, apparatus, articles of manufacture, and methods have been disclosed herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all systems, apparatus, articles of manufacture, and methods fairly falling within the scope of the claims of this patent.

Claims

What Is Claimed Is:

1. An unducted propulsion system, comprising: a fan; and an outlet guide vane downstream of the fan, the outlet guide vane including a hub portion, a tip portion, and a peak positioned between the hub portion and the tip portion in a radial direction defined by the unducted propulsion system, the hub portion including a forward sweep, the tip portion including an aft sweep, wherein the outlet guide vane includes a first chord length in the tip portion, wherein the outlet guide vane includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

2. The unducted propulsion system of claim 1, wherein the second chord length is a maximum chord length of the outlet guide vane, and wherein the hub portion includes the maximum chord length or the peak is defined at a same radius as the maximum chord length.

3. The unducted propulsion system of claim 1, wherein the ratio is less than or equal to 0.6.

4. The unducted propulsion system of claim 1, wherein the ratio is less than or equal to 0.4.

5. The unducted propulsion system of claim 1, wherein the ratio is less than or equal to0.25.

6. The unducted propulsion system of claim 1, wherein the outlet guide vane further includes a leading edge and a trailing portion, wherein the leading edge is formed of a metal, wherein the trailing portion is formed of a composite material, and wherein an outer radial surface of the tip portion of the outlet guide vane includes a metal tip cap.

7. The unducted propulsion system of claim 1, wherein the tip portion of the outlet guide vane further includes a tip, wherein the ratio is a chord variation ratio, wherein a sweep from the peak to the tip is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the peak to a leading edge of the tip in an axial direction defined by the unducted propulsion system to (ii) a second distance from the peak to the tip in the radial direction, wherein the leading edge sweep characteristic ratio is greater than or equal to 0. 1 and less than or equal to 1.8.

8. The unducted propulsion system of claim 7, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.5 and less than or equal to 1.8.

9. The unducted propulsion system of claim 7, wherein the leading edge sweep characteristic ratio is greater than or equal to 0.9 and less than or equal to 1.8.

10. The unducted propulsion system of claim 7, wherein the leading edge sweep characteristic ratio is greater than or equal to 1.4 and less than or equal to 1.8.1 1. An unducted propulsion system, comprising:a fan; and an outlet guide vane downstream of the fan, the outlet guide vane including: a leading edge; a trailing edge; a hub to couple to a nacelle of the unducted propulsion system; and a tip including a first chord length from the leading edge to the trailing edge, wherein a portion of the outlet guide vane between the hub and the tip in a radial direction defined by the unducted propulsion system includes a second chord length, and wherein a ratio of the first chord length to the second chord length is less than 0.8.

12. The unducted propulsion system of claim 11, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the tip of the fan is positioned a first distance from an axis of rotation of the fan, wherein the outlet guide vane includes a peak positioned a second distance from the axis of rotation of the fan, and wherein the second distance is less than or equal to 70% of the first distance.

13. The unducted propulsion system of claim 12, wherein the second distance is greater than or equal to 40% of the first distance, and wherein the second distance is less than or equal to 70% of the first distance.

14. The unducted propulsion system of claim 11, wherein the outlet guide vane includes a first peak, wherein the first peak is positioned at a first radius, wherein the fan includes a second peak positioned at a second radius, wherein the second radius is greater than the first radius.

15. The unducted propulsion system of claim 11, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the tip of the fan is positioned at a first radius, wherein the outlet guide vane includes a peak positioned at a second radius, and wherein the second radius is in a range of 50% to 70% of the first radius.

16. The unducted propulsion system of claim 11, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip of the fan is positioned at a first radius, w herein the second chord length is positioned at a second radius, and wherein the second radius is at least 70% of the first radius.

17. The unducted propulsion system of claim 11, wherein the ratio of the first chord length to the second chord length is less than or equal to 0.6, wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip is positioned at a first radius, wherein the second chord length is positioned at a second radius, and wherein the second radius is at least 80% of the first radius.

18. The unducted propulsion system of claim 11, wherein the ratio of the first chord length to the second chord length is less than or equal to 0.

3. wherein the tip is a first tip, wherein the fan includes a second tip, wherein the second tip is positioned at a first radius, wherein the second chord length is positioned at a second radius, and wherein the second radius is at least 90% of the first radius.

19. An outlet guide vane for an unducted propulsion system, the outlet guide vane comprising: a hub a tip; and a leading edge extending from the hub to the tip, wherein the leading edge includes a peak positioned between the hub and the tip in a radial direction defined by the unducted propulsion system, wherein the leading edge includes a leading edge aft sweep from the peak to the tip, wherein the leading edge aft sw eep is characterized by a leading edge sweep characteristic ratio of (i) a first distance from the peak to the leading edge at the tip in an axial direction defined by the unducted propulsion system to (ii) a second distance from the peak to the tip in the radial direction, wherein the leading edge sw eep characteristic ratio is greater than or equal to 0.4 and less than or equal to 1.8.

20. The outlet guide vane of claim 19, further including a trailing edge extending from the hub to the tip, wherein the trailing edge includes a valley between the hub and the tip in the radial direction, wherein the trailing edge includes a trailing edge aft sweep from the valley to the tip, wherein the trailing edge aft sweep is characterized by a trailing edge sweep characteristic ratio of (i) a third distance from the valley to the trailing edge at the tip in the axial direction to (ii) a fourth distance from the valley to the tip in the radial direction, wherein the trailing edge sweep characteristic ratio is greater than or equal to 0.2 and less than or equal to 1.8.

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