Patent Publication Number: US-11396822-B2

Title: Blade dovetail and retention apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent arises from U.S. Provisional Patent Application Ser. No. 63/070,259, which was filed on Aug. 25, 2020. U.S. Provisional Patent Application Ser. No. 63/070,259 is hereby incorporated herein by reference in its entirety. Priority to U.S. Provisional Patent Application Ser. No. 63/070,259 is hereby claimed. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to blade retention, and more particularly, to a retention apparatus for a blade dovetail. 
     BACKGROUND 
     In recent years, turbine engines have been increasingly utilized in a variety of applications and fields. Turbine engines are intricate machines with extensive availability, reliability, and serviceability requirements. Turbine engines include fan blades. The fan blades spin at high speed and subsequently compress the air flow. The high-pressure compressor then feeds the pressurized air flow to a combustion chamber to generate a high-temperature, high-pressure gas stream. In operation, various forces act on the blades and can cause the blades to become unseated and/or otherwise unsuitable for continued operation in the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates an example cross-section view of an example turbofan gas turbine engine. 
         FIG. 1B  illustrates an example cross-section view of an example open rotor engine. 
         FIG. 2  is a partial cross section of an example high-pressure compressor rotor depicting a compressor dovetail slot. 
         FIGS. 3A-4B  illustrate example forces and moments acting on a blade dovetail. 
         FIG. 5  illustrates an example in which a dovetail of a rotor or blade fits in a slot. 
         FIGS. 6A-6C  show views of an example self-adjusting, locking spacer apparatus. 
         FIG. 7  shows an example collect or holder for a blade dovetail. 
         FIG. 8  depicts an example engine cross-section in which the collect configuration of  FIG. 7  can be implemented. 
         FIG. 9  shows an example graph of amplification ratio versus frequency ratio experienced by an example blade dovetail. 
         FIGS. 10A-10C  illustrate differences in dovetail loading with respect to a baseline. 
         FIGS. 11A-11C  illustrate example blade dovetail axial and radial retention configurations. 
         FIGS. 12A-12B  illustrate an example parameterization with respect to ram loads. 
         FIGS. 13A-13C  illustrate an improved collect dovetail configuration in which a blade dovetail slot is provided in a collect and/or trunnion with combined axial retention and slot stiffeners on both sides of the slot. 
         FIGS. 14A-14D  show alternative example views of a collect and dovetail design, in which the dovetail surrounds and/or is otherwise attached to the collect, rather than being positioned in a slot. 
         FIGS. 15A-15B  show a conventional arrangement with a radial pitch axis around which a blade rotates that is oriented parallel or colinear to a radial center line of the engine. 
         FIGS. 16A-16B  show an example configuration in which the pitch axis is not parallel or colinear to the radial center line of the engine. 
         FIGS. 17-27  illustrate example configurations of a blade at an adjusted pitch axis. 
         FIG. 28  is a flow chart of a method to determine blade position to set a pitch axis relative to an engine radial axis. 
     
    
    
     The figures are not to scale. Instead, the thickness of regions may be enlarged in the drawings. 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. Connection references (e.g., attached, coupled, connected, and joined) are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated. As such, connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other. Stating that any part is in “contact” and/or “direct contact” with another part means that there is no intermediate part between the two parts. 
     Descriptors “first,” “second,” “third,” etc. are used herein when identifying multiple elements or components which may be referred to separately. Unless otherwise specified or understood based on their context of use, such descriptors are not intended to impute any meaning of priority, physical order or arrangement in a list, or ordering in time but are merely used as labels for referring to multiple elements or components separately 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 ease of referencing multiple elements or components. 
     DETAILED DESCRIPTION 
     Aircrafts include engines that act as a propulsion system to generate mechanical power and forces such as thrust. A gas turbine, also called a combustion turbine or a turbine engine, is a type of internal combustion engine that can be implemented in the propulsion system of an aircraft. For example, a gas turbine can be implemented in connection with a turbofan or a turbojet aircraft engine. Gas turbines also have significant applications in areas such as industrial power generation. 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific examples that may be practiced. These examples are described in sufficient detail to enable one skilled in the art to practice the subject matter, and it is to be understood that other examples may be utilized. The following detailed description is therefore, provided to describe example implementations and not to be taken limiting on the scope of the subject matter described in this disclosure. Certain features from different aspects of the following description may be combined to form yet new aspects of the subject matter discussed below. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “first,” “second,” and the like, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. As the terms “connected to,” “coupled to,” etc. are used herein, one object (e.g., a material, element, structure, member, etc.) can be connected to or coupled to another object regardless of whether the one object is directly connected or coupled to the other object or whether there are one or more intervening objects between the one object and the other object. 
     As used herein, the terms “system,” “unit,” “module,” “engine,” etc., may include a hardware and/or software system that operates to perform one or more functions. For example, a module, unit, or system may include a computer processor, controller, and/or other logic-based device that performs operations based on instructions stored on a tangible and non-transitory computer readable storage medium, such as a computer memory. Alternatively, a module, unit, engine, or system may include a hard-wired device that performs operations based on hard-wired logic of the device. Various modules, units, engines, and/or systems shown in the attached figures may represent the hardware that operates based on software or hardwired instructions, the software that directs hardware to perform the operations, or a combination thereof. 
     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. As used herein, “vertical” refers to the direction perpendicular to the ground. As used herein, “horizontal” refers to the direction parallel to the centerline of the gas turbine engine  100 . As used herein, “lateral” refers to the direction perpendicular to the axial and vertical directions (e.g., into and out of the plane of  FIG. 1 , etc.). 
     In some examples used herein, the term “substantially” is used to describe a relationship between two parts that is within three degrees of the stated relationship (e.g., a substantially colinear relationship is within three degrees of being linear, a substantially perpendicular relationship is within three degrees of being perpendicular, a substantially parallel relationship is within three degrees of being parallel, etc.). 
     As used herein, the terms “axial” and “longitudinal” both refer to a direction parallel to the centerline axis of an gas turbine (e.g., a turbofan, a core gas turbine engine, etc.), while “radial” refers to a direction perpendicular to the axial direction, and “tangential” or “circumferential” refers to a direction mutually perpendicular to the axial and radial directions. Accordingly, as used herein, “radially inward” refers to the radial direction from the outer circumference of the gas turbine towards the centerline axis of the gas turbine, and “radially outward” refers to the radial direction from the centerline axis of the gas turbine towards the outer circumference of gas turbine. As used herein, the terms “forward”, “fore”, and “front” refer to a location relatively upstream in an air flow passing through or around a component, and the terms “aft” and “rear” refer to a location relatively downstream in an air flow passing through or around a component. 
     The basic operation of a gas turbine implemented in connection with a turbofan engine of a propulsion system of an aircraft includes an intake of fresh atmospheric air flow through the front of the turbofan engine with a fan. In the operation of a turbofan engine, a first portion of the intake air bypasses a core gas turbine engine of the turbofan to produce thrust directly. A second portion of the intake air travels through a booster compressor (e.g., a first compressor) located between the fan and a high-pressure compressor (e.g., a second compressor) in the core gas turbine engine (e.g., the gas turbine). The booster compressor is used to raise or boost the pressure of the second portion of the intake air prior to the air flow entering the high-pressure compressor. The air flow can then travel through the high-pressure compressor that further pressurizes the air flow. The booster compressor and the high-pressure compressor each include a group of blades attached to a rotor and/or shaft. The blades spin at high speed relative to stationary vanes and each subsequently compresses the air flow. The high-pressure compressor then feeds the pressurized air flow to a combustion chamber (e.g., combustor). In some examples, the high-pressure compressor feeds the pressurized air flow at speeds of hundreds of miles per hour. In some instances, the combustion chamber includes one or more rings of fuel injectors that inject a steady stream of fuel into the combustion chamber, where the fuel mixes with the pressurized air flow. A secondary use of the compressors, particularly the high-pressure compressor, is to bleed air for use in other systems of the aircraft (e.g., cabin pressure, heating, and air conditioning, etc.) 
     In the combustion chamber of the core gas turbine engine, the fuel is ignited with an electric spark provided by an igniter, where the fuel in some examples burns at temperatures of more than 2000 degrees Fahrenheit. The resulting combustion produces a high-temperature, high-pressure gas stream (e.g., hot combustion gas) that passes through another group of blades called a turbine. The turbine can include a low-pressure turbine and a high-pressure turbine, for example. Each of the low-pressure turbine and the high-pressure turbine includes an intricate array of alternating rotating blades and stationary airfoil-section blades (e.g., vanes). The high-pressure turbine is located axially downstream from the combustor and axially upstream from the low-pressure turbine. As the hot combustion gas passes through the turbine, the hot combustion gas expands through the blades and/or vanes, causing the rotating blades couples to rotors of the high-pressure turbine and the low-pressure turbine to spin. 
     The rotating blades of the high-pressure turbine and the low-pressure turbine serve at least two purposes. A first purpose of the rotating blades is to drive the fan, the high-pressure compressor, and/or the booster compressor to draw more pressured air into the combustion chamber. For example, in a dual-spool design of a turbofan, the low-pressure turbine (e.g., a first turbine) can be attached to and in force transmitting connection with the booster compressor (e.g., the first compressor) and fan via a first shaft, collectively a first spool of the gas turbine, such that the rotation of a rotor of the low-pressure turbine drives a rotor of the booster compressor and the fan. For example, a high-pressure turbine (e.g., a second turbine) can be attached to and in force transmitting connection with the high-pressure compressor (e.g., a second compressor) via a second shaft coaxial with the first shaft, collectively a second spool of the gas turbine, such that the rotation of a rotor of the high-pressure turbine drives a rotor of the high-pressure compressor. A second purpose of the rotating blades is to spin a generator operatively coupled to the turbine section to produce electricity. For example, the turbine can generate electricity to be used by an aircraft, a power station, etc. 
     It is generally an object of the design of aircraft engines such as turbofans to compress as much air as is feasible within the compressor of the a core gas turbine engine given the static, dynamic, centrifugal and/or thermal stress limitations and weight considerations of aspects of the core gas turbine engine and/or the turbofan engine. A metric defining the compressive action of a compressor is a compression ratio (e.g., pressure ratio) of a compressor. The compression ratio of a compressor of a turbofan engine is the ratio of pressure at an outlet of the compressor (e.g., the outlet of the high-pressure compressor at the combustion chamber of the gas turbine) to pressure at an inlet of a fan. A higher compression ratio increases a thermal efficiency of the turbine engine and decreases a specific fuel consumption of the turbine engine (e.g., a ratio of fuel used to thrust produced by the jet engine). Thus, an increase in the compression ratio of the compressor of a gas turbine can increase thrust produced by a jet engine, such as a turbofan, etc., and/or can increase fuel efficiency of the jet engine. In turn, it is an object of gas turbine design to minimize or otherwise reduce pressure losses through the compressors to maximize or otherwise improve the compression ratio. Though examples disclosed herein are discussed in connection with a turbofan jet engine, it is understood that examples disclosed herein can be implemented in connection with a turbojet jet engine, a turboprop jet engine, a combustion turbine for power production, or any other suitable application where it is desired to increase compression ratios across one or more compressors. 
     The example low-pressure compressor and high-pressure compressor of the turbine engine of the turbofan each include one or more stages. Each stage includes an annular array of compressor blades (e.g., first airfoils) mounted about a central rotor paired with an annular array of stationary compressor vanes (e.g., second airfoils) spaced apart from the rotor and fixed to a casing of the compressor. At an aft portion of a compressor stage, rotation of the rotor and accompanying blades provides an increase in velocity, temperature, and pressure of air flow. At a fore portion of the compressor stage, the air flow diffuses (e.g., loses velocity) across compressor vanes providing for an increase in pressure. The implementation of multiple stages across the low-pressure compressor and high-pressure compressor provides for the compression ratios to operate a jet engine such as a turbofan. 
     In the example of the high-pressure compressor and the low-pressure compressor, compressor blades (also referred to herein as blades and/or dovetail blades) are arrayed about a corresponding high-pressure compressor rotor and low-pressure compressor rotor, respectively. The high-pressure rotor and accompanying compressor blades (e.g., blades, dovetail blades, etc.) are typically fashioned from Titanium alloys (e.g., a Titanium-Aluminum alloy, a Titanium-Chromium alloy, etc.) and/or Steel alloys (e.g., a Steel-Chromium alloy), etc. For example, to increase ease of maintenance and assembly, replaceability of blades, and/or modularity of the high-pressure compressor, discrete compressor blades are mounted in series annularly about the high-pressure rotor to achieve a substantially uniform distribution annularly about the rotor. For this purpose, an example compressor blade implemented in accordance with the teachings of this disclosure includes an airfoil portion and a mounting portion. The airfoil portion of the compressor blade causes the velocity, pressure, and temperature increase to the air flow. The mounting portion of the compressor blade enables mounting of blade to the rotor. In some examples, the geometry of the airfoil portion and/or mounting portion can be different for the compressor blades of each stage of the high-pressure compressor and the same for the compressor blades within each stage of the high-pressure compressor. 
     In certain examples, the mounting portion of the example compressor blade includes a dovetail protrusion and a platform. In this example, the high-pressure compressor rotor is provided at each stage with a dovetail slot (e.g., also referred to herein as a slot) to receive the dovetail protrusions of a plurality of blades of the stage. For example, a compressor blade can be in a mounted state with a high-pressure rotor when the dovetail slot of the high-pressure compressor rotor receives the dovetail protrusion of the compressor blade. In this example, the dovetail protrusion of the blade defines a radially outer portion (e.g., a portion relatively radially outward when mounted) and a radially inner portion (e.g., a portion relatively radially inward when mounted). In this example, the radially outer portion is relatively less in axial length (e.g., when mounted, the length in the axial direction of the turbine engine and/or compressor) than the radially inner portion. The dovetail slot also includes a radially outer portion and a radially inner portion. For example, the radially outer portion can include a pair of annular flanges (e.g., a neck, a first neck, etc.) extending axially towards the center of the dovetail slot. The dimensions of the compressor blade and the dovetail slot are such that when the compressor blade is in a mounted state with the compressor blade, the annular flanges (e.g., a neck) of the dovetail slot interfere with the radially inner portion of the compressor blade, thereby retaining the compressor blade from radially outward movement. 
     Traditionally, a plurality of compressor blades of a stage are mounted annularly in a dovetail slot directly in series such that the platform of each blade interfaces with the platform of a first subsequent blade on a first circumferential side and interfaces with the platform of a second subsequent blade on a second circumferential side. 
     Though examples disclosed herein are discussed in connection with dovetail slots of a rotor of a high-pressure compressor of a core gas turbine engine of a turbofan engine, other examples can be implemented in accordance with the teachings of the present disclosure for a low-pressure compressor, an intermediate-pressure compressor, a sole compressor of a single spool gas turbine, a compressor with an alternative slot design, a compressor of a gas turbine for industrial power production, a turbine rotor and/or any other suitable application. 
     A challenge for an open rotor engine is to create a secure loading mechanism to retain blades in their slots. Examples disclosed and described herein provide various mechanisms (e.g., referred to as a blade retention apparatus, retention device, etc.) to keep a rotor blade in an engine slot. 
     Advantages to these configurations include keeping a blade loaded in a dovetail slot such as in instances of low rpm, where proper seating of the blade in the slot is reduced and an excitation force may be high such as in propeller or open-rotor applications. In propeller or open-rotor engine applications, for example, a high vibratory load is experienced during various phases of the flight due to asymmetric propeller loading (e.g., P-Factor or 1P loading). 1P loading, also referred to as +/−1P loading, refers to movement or force on a blade caused by a blade&#39;s excitation frequency relative to rotor revolution, which often occurs during takeoff rotation. 
     Further, existing turboprop or open rotor technology requires that when there is a failure of a blade, the single blade cannot be removed. Instead, a complex disassembly process must be completed to remove a single fan blade, which increases the time and work required to service the equipment. In contrast, certain examples enable blades to individually be retained and removed from a blade assembly for servicing, repair, replacement, etc. 
     Example retaining mechanisms can be applied to both closed and open rotor engine designs. For purposes of illustration only,  FIG. 1A  illustrates an example closed-rotor turbofan engine, and  FIG. 1B  illustrates an example open-rotor engine. 
       FIG. 1A  is a schematic partially cross-sectioned side view of an example turbofan gas turbine engine  10 . The engine  10  may particularly be configured as a gas turbine engine for an aircraft. Although further described herein as a turbofan engine, the engine  10  may define a turboshaft, turboprop, or turbojet gas turbine engine, including marine and industrial engines and auxiliary power units. As shown in  FIG. 1A , the engine  10  has a longitudinal or axial centerline axis  12  that extends therethrough for reference purposes. An axial direction A is extended co-directional to the axial centerline axis  12  for reference. The engine  10  further defines an upstream end  99  and a downstream end  98  for reference. In general, the engine  10  may include a fan assembly  14  and a core engine  16  disposed downstream from the fan assembly  14 . For reference, the engine  10  defines an axial direction A, a radial direction R, and a circumferential direction C. In general, the axial direction A extends parallel to the axial centerline  12 , the radial direction R extends outward from and inward to the axial centerline  12  in a direction orthogonal to the axial direction A, and the circumferential direction extends three hundred sixty degrees (360°) around the axial centerline  12 . 
     The core engine  16  may generally include a substantially tubular outer casing  18  that defines an annular inlet  20 . The outer casing  18  encases or at least partially forms, in serial flow relationship, a compressor section having a booster or low pressure (LP) compressor  22 , a high pressure (HP) compressor  24 , a heat addition system  26 , an expansion section or turbine section including a high pressure (HP) turbine  28 , a low pressure (LP) turbine  30  and a jet exhaust nozzle section  32 . A high pressure (HP) rotor shaft  34  drivingly connects the HP turbine  28  to the HP compressor  24 . A low pressure (LP) rotor shaft  36  drivingly connects the LP turbine  30  to the LP compressor  22 . The LP rotor shaft  36  may also be connected to a fan shaft  38  of the fan assembly  14 . In certain examples, as shown in  FIG. 1A , the LP rotor shaft  36  may be connected to the fan shaft  38  via a reduction gear  40  such as in an indirect-drive or geared-drive configuration. 
     As shown in  FIG. 1A , the fan assembly  14  includes a plurality of fan blades  42  that are coupled to and that extend radially outwardly from the fan shaft  38 . An annular fan casing or nacelle  44  circumferentially may surround the fan assembly  14  and/or at least a portion of the core engine  16 . It should be appreciated by those of ordinary skill in the art that the nacelle  44  may be configured to be supported relative to the core engine  16  by a plurality of circumferentially-spaced outlet guide vanes or struts  46 . Moreover, at least a portion of the nacelle  44  may extend over an outer portion of the core engine  16  so as to define a fan flow passage  48  therebetween. However, it should be appreciated that various configurations of the engine  10  may omit the nacelle  44 , or omit the nacelle  44  from extending around the fan blades  42 , such as to provide an open rotor or propfan configuration of the engine  10  depicted in  FIG. 1B . 
     It should be appreciated that combinations of the shafts  34 ,  36 , the compressors  22 ,  24 , and the turbines  28 ,  30  define a rotor assembly  90  of the engine  10 . For example, the HP shaft  34 , HP compressor  24 , and HP turbine  28  may define a high speed or HP rotor assembly of the engine  10 . Similarly, combinations of the LP shaft  36 , LP compressor  22 , and LP turbine  30  may define a low speed or LP rotor assembly of the engine  10 . Various examples of the engine  10  may further include the fan shaft  38  and fan blades  42  as the LP rotor assembly. In certain examples, the engine  10  may further define a fan rotor assembly at least partially mechanically de-coupled from the LP spool via the fan shaft  38  and the reduction gear  40 . Still further examples may further define one or more intermediate rotor assemblies defined by an intermediate pressure compressor, an intermediate pressure shaft, and an intermediate pressure turbine disposed between the LP rotor assembly and the HP rotor assembly (relative to serial aerodynamic flow arrangement). 
     During operation of the engine  10 , a flow of air, shown schematically by arrows  74 , enters an inlet  76  of the engine  10  defined by the fan case or nacelle  44 . A portion of air, shown schematically by arrow  80 , enters the core engine  16  through a core inlet  20  defined at least partially via the outer casing  18 . The flow of air is provided in serial flow through the compressors, the heat addition system, and the expansion section via a core flowpath  70 . The flow of air  80  is increasingly compressed as it flows across successive stages of the compressors  22 ,  24 , such as shown schematically by arrows  82 . The compressed air  82  enters the heat addition system  26  and mixes with a liquid and/or gaseous fuel and is ignited to produce combustion gases  86 . It should be appreciated that the heat addition system  26  may form any appropriate system for generating combustion gases, including, but not limited to, deflagrative or detonative combustion systems, or combinations thereof. The heat addition system  26  may include annular, can, can-annular, trapped vortex, involute or scroll, rich burn, lean burn, rotating detonation, or pulse detonation configurations, or combinations thereof. 
     The combustion gases  86  release energy to drive rotation of the HP rotor assembly and the LP rotor assembly before exhausting from the jet exhaust nozzle section  32 . The release of energy from the combustion gases  86  further drives rotation of the fan assembly  14 , including the fan blades  42 . A portion of the air  74  bypasses the core engine  16  and flows across the fan flow passage  48 , such as shown schematically by arrows  78 . 
     It should be appreciated that  FIG. 1A  depicts and describes a two-stream engine having the fan flow passage  48  and the core flowpath  70 . The example depicted in  FIG. 1A  has a nacelle  44  surrounding the fan blades  42 , such as to provide noise attenuation, blade-out protection, and other benefits known for nacelles, and which may be referred to herein as a “ducted fan,” or the entire engine  10  may be referred to as a “ducted engine.” 
       FIG. 1B  provides a schematic cross-sectional view of an example open-rotor turbine engine according to one example of the present disclosure. Particularly,  FIG. 1B  provides an aviation three-stream turbofan engine herein referred to as “three-stream engine  100 ”. The three-stream engine  100  of  FIG. 1B  can be mounted to an aerial vehicle, such as a fixed-wing aircraft, and can produce thrust for propulsion of the aerial vehicle. The architecture of the three-stream engine  100  provides three distinct streams of thrust-producing airflow during operation. Unlike the engine  10  shown in  FIG. 1A , the three-stream engine  100  includes a fan that is not ducted by a nacelle or cowl, such that it may be referred to herein as an “unducted fan,” or the entire engine  100  may be referred to as an “unducted engine.” 
     For reference, the three-stream engine  100  defines an axial direction A, a radial direction R, and a circumferential direction C. Moreover, the three-stream engine  100  defines an axial centerline or longitudinal axis  112  that extends along the axial direction A. In general, the axial direction A extends parallel to the longitudinal axis  112 , the radial direction R extends outward from and inward to the longitudinal axis  112  in a direction orthogonal to the axial direction A, and the circumferential direction extends three hundred sixty degrees (360°) around the longitudinal axis  112 . The three-stream engine  100  extends between a forward end  114  and an aft end  116 , e.g., along the axial direction A. 
     The three-stream engine  100  includes a core engine  120  and a fan section  150  positioned upstream thereof. Generally, the core engine  120  includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. Particularly, as shown in  FIG. 1B , the core engine  120  includes a core cowl  122  that defines an annular core inlet  124 . The core cowl  122  further encloses a low pressure system and a high pressure system. In certain examples, the core cowl  122  may enclose and support a booster or low pressure (“LP”) compressor  126  for pressurizing the air that enters the core engine  120  through core inlet  124 . A high pressure (“HP”), multi-stage, axial-flow compressor  128  receives pressurized air from the LP compressor  126  and further increases the pressure of the air. The pressurized air stream flows downstream to a combustor  130  where fuel is injected into the pressurized air stream and ignited to raise the temperature and energy level of the pressurized air. It will be appreciated that as used herein, the terms “high/low speed” and “high/low pressure” are used with respect to the high pressure/high speed system and low pressure/low speed system interchangeably. Further, it will be appreciated that the terms “high” and “low” are used in this same context to distinguish the two systems, and are not meant to imply any absolute speed and/or pressure values. 
     The high energy combustion products flow from the combustor  130  downstream to a high pressure turbine  132 . The high pressure turbine  132  drives the high pressure compressor  128  through a high pressure shaft  136 . In this regard, the high pressure turbine  132  is drivingly coupled with the high pressure compressor  128 . The high energy combustion products then flow to a low pressure turbine  134 . The low pressure turbine  134  drives the low pressure compressor  126  and components of the fan section  150  through a low pressure shaft  138 . In this regard, the low pressure turbine  134  is drivingly coupled with the low pressure compressor  126  and components of the fan section  150 . The LP shaft  138  is coaxial with the HP shaft  136  in this example. After driving each of the turbines  132 ,  134 , the combustion products exit the core engine  120  through a core exhaust nozzle  140  to produce propulsive thrust. Accordingly, the core engine  120  defines a core flowpath or core duct  142  that extends between the core inlet  124  and the core exhaust nozzle  140 . The core duct  142  is an annular duct positioned generally inward of the core cowl  122  along the radial direction R. 
     The fan section  150  includes a fan  152 , which is the primary fan in this example. For the depicted example of  FIG. 1B , the fan  152  is an open rotor or unducted fan. However, in other examples, the fan  152  may be ducted, e.g., by a fan casing or nacelle circumferentially surrounding the fan  152 . As depicted, the fan  152  includes an array of fan blades  154  (only one shown in  FIG. 1B ). The fan blades  154  are rotatable, e.g., about the longitudinal axis  112 . As noted above, the fan  152  is drivingly coupled with the low pressure turbine  134  via the LP shaft  138 . The fan  152  can be directly coupled with the LP shaft  138 , e.g., in a direct-drive configuration. Optionally, as shown in  FIG. 1B , the fan  152  can be coupled with the LP shaft  138  via a speed reduction gearbox  155 , e.g., in an indirect-drive or geared-drive configuration. 
     Moreover, the fan blades  154  can be arranged in equal spacing around the longitudinal axis  112 . Each blade  154  has a root and a tip and a span defined therebetween. Each blade  154  defines a central blade axis  156 . For this example, each blade  154  of the fan  152  is rotatable about its respective central blade axes  156 , e.g., in unison with one another. One or more actuators  158  can be controlled to pitch the blades  154  about their respective central blade axes  156 . However, in other examples, each blade  154  may be fixed or unable to be pitched about its central blade axis  156 . 
     The fan section  150  further includes a fan guide vane array  160  that includes fan guide vanes  162  (only one shown in  FIG. 1B ) disposed around the longitudinal axis  112 . For this example, the fan guide vanes  162  are not rotatable about the longitudinal axis  112 . Each fan guide vane  162  has a root and a tip and a span defined therebetween. The fan guide vanes  162  may be unshrouded as shown in  FIG. 1B  or may be shrouded, e.g., by an annular shroud spaced outward from the tips of the fan guide vanes  162  along the radial direction R. Each fan guide vane  162  defines a central blade axis  164 . For this example, each fan guide vane  162  of the fan guide vane array  160  is rotatable about its respective central blade axes  164 , e.g., in unison with one another. One or more actuators  166  can be controlled to pitch the fan guide vane  162  about their respective central blade axes  164 . However, in other examples, each fan guide vane  162  may be fixed or unable to be pitched about its central blade axis  164 . The fan guide vanes  162  are mounted to a fan cowl  170 . 
     As shown in  FIG. 1B , in addition to the fan  152 , which is unducted, a ducted fan  184  is included aft of the fan  152 , such that the three-stream engine  100  includes both a ducted and an unducted fan that both serve to generate thrust through the movement of air without passage through core engine  120 . The ducted fan  184  is shown at about the same axial location as the fan guide vane  162 , and radially inward of the fan guide vane  162 . Alternatively, the ducted fan  184  may be between the fan guide vane  162  and core duct  142 , or be farther forward of the fan guide vane  162 . The ducted fan  184  may be driven by the low pressure turbine  134  (e.g., coupled to the LP shaft  138 ), or by any other suitable source of rotation, and may serve as the first stage of booster or may be operated separately. 
     The fan cowl  170  annularly encases at least a portion of the core cowl  122  and is generally positioned outward of the core cowl  122  along the radial direction R. Particularly, a downstream section of the fan cowl  170  extends over a forward portion of the core cowl  122  to define a fan flowpath or fan duct  172 . Incoming air may enter through the fan duct  172  through a fan duct inlet  176  and may exit through a fan exhaust nozzle  178  to produce propulsive thrust. The fan duct  172  is an annular duct positioned generally outward of the core duct  142  along the radial direction R. The stationary struts  174  may each be aerodynamically contoured to direct air flowing thereby. Other struts in addition to the stationary struts  174  may be used to connect and support the fan cowl  170  and/or core cowl  122 . In many examples, the fan duct  172  and the core duct  122  may at least partially co-extend (generally axially) on opposite sides (e.g., opposite radial sides) of the core cowl  122 . For example, the fan duct  172  and the core duct  122  may each extend directly from the leading edge  144  of the core cowl  122  and may partially co-extend generally axially on opposite radial sides of the core cowl. 
     The three-stream engine  100  also defines or includes an inlet duct  180 . The inlet duct  180  extends between an engine inlet  182  and the core inlet  124 /fan duct inlet  176 . The engine inlet  182  is defined generally at the forward end of the fan cowl  170  and is positioned between the fan  152  and the array of fan guide vanes  160  along the axial direction A. The inlet duct  180  is an annular duct that is positioned inward of the fan cowl  170  along the radial direction R. Air flowing downstream along the inlet duct  180  is split, not necessarily evenly, into the core duct  142  and the fan duct  172  by a splitter or leading edge  144  of the core cowl  122 . The inlet duct  180  is wider than the core duct  142  along the radial direction R. The inlet duct  180  is also wider than the fan duct  172  along the radial direction R. 
     Example Dovetail and Slot Configurations 
       FIG. 2  is a partial cross section of an example high-pressure compressor rotor depicting a compressor dovetail slot. In the example of  FIG. 2 , one or more compressor blades  202  are mounted on the high-pressure compressor rotor  200 . In the view of  FIG. 2 , the fore direction is to the left, and the aft direction is to the right. An example dovetail slot  204  includes a lower cavity  206 . A dovetail protrusion  208  fits in the lower cavity  206  of the example dovetail slot  204 . 
     Certain examples provide improved dovetail seating in a slot. Certain examples provide dovetail seating adding a radial load to the dovetail of the blade. In certain examples, a radial load is added to the dovetail in combination with an axial retention feature. The radial loading keeps the dovetail seated during large applied moments at low revolutions per minute (RPM). In certain examples, in an open rotor configuration, a high radial load is provided to prevent unseating of the blade and its dovetail from its slot. 
     Current configurations, such as beam or clank springs can provide some radial load under windmill conditions but cannot provide sufficient radial load to reliably maintain the blade in the slot. Current configurations are not locking or self-adjusting. In contrast, certain examples provide sufficient radial load to maintain the dovetail of the blade in the slot. Certain examples are locking and self-adjusting. 
     In certain examples, an open rotor blade may be subjected to a variety of conditions such as low tip speed, high propeller alternating loads, reverse thrust, etc., which can cause the dovetail to unseat from the slot or socket and result in rapid wear of the dovetail and blade. Certain examples provide a mechanism to increase the seating force on the dovetail to prevent movement under normal vibratory and/or reverse thrust load conditions. 
     Certain examples provide a spacer that fits underneath the blade and has a tapered slot with a wedge inside. Movement of the wedge changes the effective thickness of the spacer. A pre-loaded spring pulls on the wedge expanding the spacer so exerting force on the blade root. Manufacturing variations are automatically compensated for by the nature of the wedge/spring arrangement. Changes in part dimensions during service due to wear or creep are similarly accommodated. 
     Certain examples provide a radial clamp, which is locking such that it does not compress under reaction load. The radial clamp is also self-adjusting to compensate for component wear, compliance, creep, temperature, manufacturing variance, etc., while operating in a small envelope. Certain examples provide expanding wedges. 
     As such, certain examples provide a preload mechanism to prevent unseating and/or wear in blade dovetails. Alternatively or additionally, certain examples adjust a pitch axis to reduce or minimize a range of moments. 
       FIG. 3A  illustrates an example dovetail/slot attachment  300  with rigid materials and a radial load. As shown in the example  300 , a contact force distribution  310  across pressure faces of the dovetail  320  is approximately uniform. As shown in the example of  FIG. 3B , addition of a moment  330  to a shank  340  of the blade changes the contact force distribution  315  to react the moment  330 . 
     As shown in the example of  FIG. 4A , if the moment  330  becomes large enough, the contact force  317  becomes zero along the edges of the pressure-faces, which lose or loosen contact and start to open up. If this is the result of a vibratory load (1P), it may lead to movement and rapid wear of the blade. As shown in the example of  FIG. 4B , by adding to the radial load  410 , the moment carrying capability  420  before loss of edge contact can be increased, at the expense of average and peak contact pressure. 
     Propeller blades have a big range of moments that have to be reacted. A dovetail attachment can be provided with a radial force  410  to keep the dovetail in the slot and prevent or reduce rocking due to the moment  330 , which in turn reduces wear and associated failure and maintenance, for example. 
       FIG. 5  illustrates an example in which a dovetail  510  of a rotor or blade  520  fits in a slot, also referred to as a trunnion  530 . A space  540  between the bottom of the dovetail  510  and the cavity of the trunnion  530  can contribute to motion of the dovetail  510 . To compensate for this motion (e.g., due to pressure, vibration, etc.), an insert, such as a lock, etc., can be added. 
     A locking spacer, for example, can fill the space  540  and apply a preload to the dovetail  510  in the slot/socket/trunnion  530 . The locking spacer can provide a radial clamping force in a small envelope to compensate for component wear, compliance, creep in the trunnion  530 , etc. The locking spacer can be insensitive to temperature, lightweight, and enable dis-assembly without lock up or complication to remove a blade  520  from its trunnion  530 , for example. 
     Example Self-Adjusting, Locking Spacer 
       FIGS. 6A-6C  show views of an example self-adjusting, locking spacer apparatus  600 . The example spacer apparatus  600  can be inserted into the space  540  and locked to reduce or inhibit movement of the dovetail  510  in the trunnion  530  (e.g., serving as a preload or retaining device helping to retain the dovetail  510  in the trunnion/slot/socket  530 ). The example spacer apparatus  600  includes a spacer  610 , a wedge  620 , and a spring  630 . The spring  630 , which can be set at assembly, maintenance, etc., pulls the wedge  620  through the spacer  610  to provide a pre-load to keep the dovetail  510  of the blade  520  in the slot/trunnion  530  to help ensure compliance and reduce wear. The material, tension, bend, etc., of the spacer  610 , wedge  620 , and/or spring  630  can vary based on a size of the gap or space  540 , the dovetail  510 , the blade  520 , and/or the trunnion  530 , for example. The wedge  620  fits inside the spacer  610 , and, when the spring  630  tightens, the spring  630  pulls the wedge  620  to expand the spacer  610  (e.g., in the space or gap  540 ). The spring  630  creates a radial force (e.g., a radial preload force) on the bottom of the blade  520  and can accommodate changes in the space  540  by greater or lesser pulling of the wedge  620  with the spring  630 . The wedge  620  generates an axial load on the blade  520  as well, so that the combination of radial and axial forces from the spring  630  and the wedge  620  act to keep the blade  520  in position. As force increases, the wedge  620  will not slide back due to strength of friction exerted on the wedge  620  with respect to the spacer  610  (e.g., making the spacer  610  self-locking). If reaction increases, the wedge  620  does not compress. The combination of spacer  610 , the wedge  620 , and the spring  630  can accommodate changes in distance due to wear, heat, etc. Choice of spacer  610 , wedge  620 , and spring  630  determine a range of the locking spacer apparatus  600 , for example. 
     In certain examples, the spacer  610 , wedge  620 , and/or spring  630  are metallic. However, depending on the space  540  and total force involved, one or more of the spacer  610 , the wedge  620 , and/or the spring  630  can be non-metallic. For example, the spacer  610  and/or the wedge  620  can be formed of titanium, aluminum, a composite, etc. The spring  630  can be formed of a laminated composite, for example. Coating(s) can be applied to surface(s) to control wear and friction (e.g., e.g., Teflon®-impregnated Nomex® or glass cloth, molybdenum paint, no coating (e.g., bare), etc. In certain examples, the spacer  610  is formed in a curve (e.g., could also be an S-curve). The spacer  610  can be formed as a single piece that is bent, etc., and/or as two parts joined together with a hinge and/or other connection. The spacer  610 , the wedge  620 , the spring  630 , etc., can be machine, injection molded, additively manufactured, etc. 
     As such, rather than a flexible beam spring, the locking spacer assembly  600  provides a locking, self-adjusting mechanism to apply a radial load to the dovetail  510  in the trunnion  530 . The example apparatus  600  forms an expanding wedge that increases a seating force on the dovetail  510  to help keep the dovetail  510  in the trunnion or slot  530  under vibratory and/or reverse thrust load conditions, for example. The spacer wedge assembly  600  fits underneath the blade dovetail  510  and has a tapered slot  610  with the wedge  620  inside. Movement of the wedge  620  changes the effective thickness of the spacer  610 . A pre-loaded spring  630  pulls on the wedge  620  to expand the spacer  610  and exert force on the blade root  510 . Manufacturing variations are automatically compensated for by the wedge  620 /spring  630  arrangement. Changes in part dimension during service due to wear or creep are similarly accommodated by the wedge  620 /spring  630  combination with the spacer  610 , for example. The added axial force, alone or in conjunction with a radial force, depending on the configuration, overcome an unseating moment to hold the blade dovetail  510  in position. 
     As such, certain examples provide a retaining system for a blade of an engine. The example retaining system includes means, such as the spacer  610 , for spacing to be positioned in a socket with an end of a blade. The example retaining system includes means, such as the wedge  620 , for wedging positioned inside the means for spacing. The example retaining system includes means, such as the spring  630 , for tightening to pull the means for wedging to expand the means for spacing. 
     Other examples can be implemented as alternatives and/or in addition to the locking spacer apparatus  600  described above. For example, a collect or holder can be used with a trunnion, socket, or other slot to secure the dovetail of a blade. 
     Example Radial and/or Axial Preloading 
       FIG. 7  shows an example collect or holder  700  for a blade dovetail. Rather than an integral or circular collect, certain examples provide a slotted, counterweighted collect  700  to secure the dovetail  510  of the blade  520 . Using the example counterweighted collect  700 , the dovetail slides into a fixture  710  and the blade slips into a slot  720  forming the collect  700 . The counterweighted collect  700  provides a radial force to keep the dovetail  510  seated during large applied moments at low RPM, for example. 
       FIG. 8  depicts an example engine cross-section  800  in which the collect  700  configuration can be implemented. The example cross-section  800  shows an example blade  805  and its associated radial load  810 . The blade radial load  810  can be reacted using a variety of radial and/or axial preload devices as described herein. 
       FIG. 9  shows an example graph of amplification ratio versus frequency ratio experienced by an example blade dovetail when exposed to no damping, some damping critical damping, etc. (as indicated by variable A large frequency/amplification margin (1F/1R) can be accommodated according to certain examples disclosed herein to handle increased blade loading, particularly in an open rotor configuration such as shown in the example of  FIG. 8 , while preventing the dovetail from being unseated. For example, an open rotor P factor is caused by an angle of attack (AoA) of the blade during rotation. Increased blade loading (1/F) can result, which has a tendency to unseat a traditional dovetail. As such, there is an incentive to maintain a large 1F/1R margin. The P factor and associated blade loading and unseating can be countered by a preload mechanism such as described herein. 
       FIGS. 10A-10C  illustrate differences in dovetail loading with respect to a baseline, plus or minus P loading. For example,  FIG. 10A  illustrates a load or force at takeoff or baseline.  FIG. 10B  shows an example takeoff force plus an asymmetric blade effect P (e.g., takeoff+1P).  FIG. 10C  shows an example takeoff force minus asymmetric blade effect P (e.g., takeoff−1P). 
     Example Collet-Based Configurations 
     In certain examples, both radial and axial preloads can be applied to a blade dovetail using a ram.  FIGS. 11A-11C  illustrate example blade dovetail axial and radial retention configurations that can be used alternatively and/or additionally to the configurations of  FIGS. 6A-6C , etc. By using the configurations in combination, for example, both radial and axial forces can be applied to retain the blade in its trunnion or slot. 
     The example of  FIG. 11A  depicts a double wedge configuration with a mechanical screw to secure the dovetail of the blade  1110  with respect to a collect  1120 . In the example of  FIG. 11A , turning a screw  1130  pulls in a wedge or ram  1140  to separate an upper taper  1  and a lower taper  2 . The screw  1130  can be turned and set at assembly, upon maintenance, etc. Pulling the wedge/ram  1140  separates the tapers  1  and  2 , seats the blade  1110  in the collect  1120 , and engages axial tabs  1150 ,  1155  to secure the blade  1110 . 
     In the example of  FIG. 11B , a screw or plunger  1130  is movable from the bottom of the collect  1120  to push upward on the ram  1140 . Pressure from the ram  1140  created by the screw/plunger  1130  helps to secure the blade  1110 . As such, rather than pulling the ram  1140  as in the example of  FIG. 11A , the configuration of  FIG. 11B  positions the screw  1130  underneath the ram  1140  and utilizes movement of the screw to push upward on the ram  1140  to apply pressure to blade  1110  to keep the blade  1110  seated with respect to the collect  1120 . 
     In the example of  FIG. 11C , a spring  1160  is provided instead of the tapers of  FIG. 11A . The ram  1140  is pushed in to push out the spring  1160  and seat the blade  1110  with respect to the collect  1120 . The spring  1160  allows some movement while providing at least an axial force with the ram  1140  to keep the blade  1110  seated with respect to the collect  1120 . 
     As such, the examples of  FIGS. 11A-11C  generate axial and/or radial force greater than an unseating moment (e.g., greater than an unseating moment by at least 1.5, etc.) to hold the blade  1110  in place with respect to the collect  1120 . The example designs accommodate low RPM blade seating with vibrational pressure loading (e.g., between 1.5-4× aero load, etc.), while being serviceable for on-wing blade removal. Axial retention is incorporated to aid in collect sizing, for example. The example configurations can be applied to open rotor blades/propellers, short nacelles (e.g., with significant loading &gt;1.2× aero load, etc.), dovetail interfaces, etc., with combined axial and radial retention, for example. 
       FIGS. 12A-12B  illustrate an example parameterization with respect to ram loads (e.g., the ram  1140  of the examples of  FIGS. 11A-11C ). For example, airfoil and aero loads want to unseat an airfoil from its slot. Certain examples provide a ram force to keep the airfoil seated at all times. As shown in the example of  FIGS. 12A-12B , a minimum ram force can be determined to provide a workable spring-dovetail-collect configuration. For example, a conventional spring puts a few thousand pounds of force on a blade to keep the blade seated during windmilling of the rotor on the tarmac (e.g., when wind is blowing and moving the blades). When the blade is spun up with power, the weight of the blade seats the blade in the slot, and the associated conventional spring has no effect on the blade. To remedy the deficiencies of a conventional spring, certain examples provide an improved spring that is applying force to the blade throughout windmilling, engaged spinning, and other exposure to force. This can be parameterized as illustrated in the example of  FIGS. 12A-12B . 
     For example, as shown in  FIG. 12A , an aero load is placed on an airfoil, which is affixed to an attachment. The aero load is applied at a radial center of pressure (CP) for the airfoil or blade. A moment (M) generated at the point of attachment of the airfoil can be calculated as:
 
 M =Aero load×CP  (Eq. 1).
 
An unseating moment (Mp) for the airfoil can be calculated as:
 
Mp=&gt;1.5 Aero load×CP  (Eq. 2).
 
     As shown in the example of  FIG. 12B , a ram force can react the moment (M) affecting the dovetail of the blade at the attachment point. The ram force relates to a force (F) being applied to the blade dovetail at a distance (d) with an angle alpha. For example, the force (F) relates to the ram force and the unseating moment (Mp) as follows:
 
 F =Ram Force/2×cos(alpha)  (Eq. 3); and
 
 F×d=Mp   (Eq. 4).
 
A desired ram force can then also be related to the unseating moment (Mp):
 
Ram Force/2×cos(alpha)× d=Mp   (Eq. 5); and
 
Ram Force=2[Mp/(2 cos(alpha)× d )]  (Eq. 6).
 
As such, a ram force can be calculated to react an unseating moment for a blade dovetail and used to apply a ram and/or other preload to the dovetail, as described herein.
 
     Returning to the example counterweight collect  700  of  FIG. 7 , certain examples provide a dovetail interface to a collect design. That is, alternatively or in addition to providing a preload force to a blade dovetail in a slot/socket/trunnion, a collect can be employed to retain the dovetail in the slot/socket/trunnion. Certain examples apply this configuration to an open rotor turbofan engine in which a pitch-controlled blade requires that blade attachment be maintained. Using a dovetail interface, the modified collect provides axial retention in both forward and aft directions and adds stiffness to collect posts. Since the collect  700  is circular, stiffness in the posts counters less stiffness at the dovetail slot breakout ends compared to at mid-span of the collect  700 . Certain examples enable use of a dovetail slot in a collect design. A dovetail slot allows for flight-line removal of blades and also allows the blade to be much thinner at the root, which limits the blade hub flow blockage, for example. Additionally, certain examples with dovetail use enable a pitch change configuration, allowing a thinner blade shank near the hub, adjustment of the blade, etc. 
     The initial collect  700  and dovetail configuration of  FIG. 7  can be modified for better retention of the dovetail in the slot.  FIGS. 13A-13C  illustrate an improved collect dovetail configuration  1300  in which a blade dovetail slot is provided in a collect and/or trunnion with combined axial retention and slot stiffeners (also referred to herein as retainers or retention straps) on both sides of the slot. The example of  FIGS. 13A-13C  applies to a variable pitch collect with a dovetail slot, for example, and/or other collect configuration on a rotatable hub (e.g., an open rotor engine hub, etc.) and provides axial retention for a blade in the slot. Post stiffening can be provided via a bolted connection. Certain examples incorporate optional machined ramped interfaces for additional stiffening and/or optional axial bolting to retain collect interfaces. 
     For example, a round collect can be machined to flatten forward and aft faces and form pockets with ramps to hold one or more stiffening straps in position and react moments to keep the blade in place. In certain examples, the collect can be formed of titanium or other similar material, with components formed of the same material or at least material matching a coefficient of thermal expansion (CTE). Other parts of the trunnion or socket could be formed of titanium as well, rather than steel, because the improved collect reinforces and provides support, rendering the heavier metal in the trunnion unnecessary. As such, the improved configuration not only provides increased support for the blade but also reduces weight of the materials. 
     More specifically,  FIG. 13A  illustrates an example perspective view of a collect/trunnion base  1300  for a blade dovetail. The collect/trunnion socket  1300  includes a slot  1310  into which the dovetail of a blade is positioned. Retainers or stiffeners  1320 ,  1325  are provided at the ends of the slot  1310 . The retainers  1320 ,  1325  combine axial retention and slot stiffening to help keep the blade seated in the slot  1310 . 
       FIG. 13B  shows a top view of the arrangement  1300  of  FIG. 13A . As shown in the example of  FIG. 13B , the blade slot  1310  in the collect/trunnion  1300  is bounded by the axial retainer  1320 . As shown in the example top view of  FIG. 13B , ramps  1330 ,  1335  can (optionally) be machined into the retainer  1320  for additional stiffening support. Ramps  1330 ,  1335  can be aligned with the dovetail as it fits in the slot  1310 , for example. A pocket  1340 ,  1345  can be machined in the collect/trunnion  1300  and the retainer  1320  on each end of the retainer  1320 . A bolt  1350 ,  1355  is (optionally) positioned through the respective end of the retainer  1320  through the pocket  1340 ,  1345  and into the collect/trunnion  1300  to secure the retainer  1300  with respect to the collect/trunnion  1300  and secure the dovetail in the slot  1310 . While the retainer  1320  is shown as an example, the retainer  1325  (not shown in this view) can be similarly configured at the other end of the slot  1310  of the collect/trunnion  1300 . 
       FIG. 13C  shows an axial view of the arrangement  1300  of  FIGS. 13A-13B . As shown in the example of  FIG. 13C , pockets  1340 ,  1345  are machined into the collect/trunnion fixture  1300  to accommodate the retainers  1320 ,  1325  and machined ramps  1330 ,  1335  can be provided for additional stiffening. 
     As such, the example of  FIGS. 13A-13C  provides axial retention for a blade in the slot and provides post stiffening via a bolted connection. The example of  FIGS. 13A-13C  can incorporate machined ramp interfaces for additional stiffening. The example of  FIGS. 13A-13C  can incorporate axial bolting to retain collect interfaces. The example of  FIGS. 13A-13C  can apply to a variety of collect/trunnion configurations including a variable pitch collect with a dovetail slot, for example. 
     Certain examples provide a blade retention apparatus including a receiving means, such as the collect/trunnion  1300 , including a slot to receive an end of a blade, and a retention means, such as the retainers  1320 ,  1325 , taken alone or in conjunction with the ramps  1330 ,  1335  and/or the pockets  1340 ,  1345 , positioned at a first end and a second end of the receiving means to provide axial retention for the blade in the slot. 
       FIGS. 14A-14D  show alternative example views of a collect and dovetail design, in which the dovetail surrounds and/or is otherwise attached to the collect, rather than being positioned in a slot. The example of  FIG. 14A  shows an alternative collect  1400  design including a blade dovetail  1405  positioned around the collect  1400  and in the slot  1410  of the collect  1400 . In the example of  FIG. 14A , a radial force  1415  is added, impacting the blade dovetail  1405  to keep the dovetail  1405  seated in the collect  1400 . As shown in the example of  FIG. 14B , a bolt  1420  can connect or affix the blade and/or its dovetail  1405  to the collect  1400  in single shear. Placement of the bolt  1420  reduces a likelihood that the dovetail  1405  can be forced apart or pried open. However, the bolt  1420  can be removed to service or replace the blade and/or the collect  1400  (e.g., while on-wing). 
       FIG. 14C  illustrates an example configuration in which the dovetail is positioned in a clamped, split arrangement with respect to the collect  1400 . The dovetail has a primary portion  1430  which interacts with the collect  1400  and a split portion  1435  that works with the primary portion  1430  to engage and clamp the collect  1400 . The bolt  1420  connects the primary portion  1430  and the split portion  1435  around the collect  1400 . As shown in the example of  FIG. 14D , a variation of the clamped split dovetail can include a shim or taper  1440  engaging with a clamping dovetail  1450  that is bolted to the shimmed/tapered collect  1440  with the bolt  1420 . The example configuration of  FIG. 14D  provides a double shear connection with the bolt  1420 , for example. 
     Thus, using one or more of the example configurations of  FIGS. 14A-14C , dovetail faces can be angled (e.g., 60-90 degrees), parallel, etc., and positioned with respect to a collect to react vibratory forces, for example. One or more bolts can be used to attach the dovetail to the collect and carry friction and shear (single or double shear). If a blade is to be repaired or replaced, the dovetail can be unbolted from the collect to remove the blade, for example. 
     Example Changes in Pitch Axis 
     Blades are positioned in their sockets or slots at a certain angle or pitch. Pitch refers to an angle of the blade in air, for example. Pitch corresponds to a blade&#39;s angle of incidence, which affects the blade&#39;s angle of attack when in motion. In many configurations, an axis by which the pitch is measured (a radial pitch axis) aligns with a radial center of the engine (e.g., referred to as a radial pitch axis). However, in certain examples, instead of or in addition to locking/securing the blade in the collect, a change in pitch axis can also be used to react moments on the blade in the slot. A non-radial pitch axis can be used to mitigate moment reaction, for example. Certain examples provide a pitch change device for an aircraft engine, such as an open-rotor propeller driven engine or other bladed engine. 
       FIG. 15A  shows a conventional arrangement with a radial pitch axis  1500  around which the blade  1510  rotates that is oriented parallel or colinear to a radial center line of the engine  1520 . As shown in  FIG. 15B , the blade  1510  in motion retains a pitch axis  1500  consistent with the radial center line of the engine. 
     However,  FIG. 16A  illustrates an example configuration in which the pitch axis  1600  has a forward tilt and/or a tangential lean such that the pitch axis  1600  is not parallel or colinear to the radial center line of the engine  1620 . As shown in the example of  FIG. 16B , adding a tilt and/or lean (e.g., a tangential lean, etc.) to the pitch axis  1600  changes the inertial component of loading at off-design conditions due to pitch change. Such configuration can be used to reduce peak values of a moment reacted at the dovetail or root attachment and associated bearing(s), making values smaller and lighter, for example. 
     In certain examples, the pitch axis  1600  is tilted fore or aft, and/or leaned into or away from a direction of rotation. In response, the inertial component of a moment reaction at off-design conditions can be changed, allowing a reduction of the peak load (e.g., a 30% reduction for a lean/tilt combination of 5.6/1.6 degrees, etc.). A change in pitch axis angle  1600  by moving the blade  1600  relative to a line of action of force generates an artificial inertial reaction at a root or base of the blade  1610 , and that reaction can be used to offset some of the load affecting the blade  1610 , for example. 
     In certain examples, a hub provides a load-bearing attachment point for a plurality of blades. The hub rotates with a plurality of blades attached in an open- or closed-rotor engine design, for example. The hub can include one or more bearings, separately connected to or integrated with one or more trunnions (e.g., trunnion bearings if integrated). Each trunnion is associated with a blade and carries the radial load of the blade in connection with the hub. In certain examples, the hub can be machined such that the axis of the bearings is tilted or leaned relative to the engine center line to cause a change in the pitch axis. Rotation of the blades about an inclined axis reduce the peak bending moment and associated load on the blade. 
     For example, reverse thrust results in a greatly increased range of moment reaction involved at a blade attachment point. These moments are reacted by both the connection between the airfoil (e.g., blade) and the trunnion (e.g., a dovetail, etc.) and by the bearings between the trunnion and the hub. The size and weight of the dovetail attachment and the bearing arrangement are proportional to the maximum load, which also determines a radius ratio of the fan, which is a key performance metric. Certain examples reduce size and weight of the blade attachment and allow for improved stability and performance. 
       FIGS. 17-27  illustrate example configurations of a blade  1710  at an adjusted pitch angle with respect to a trunnion  1720  and its hub with bearing(s)  1730 . As shown in the examples of  FIGS. 17-27 , a position at which the blade  1710  is mounted to hub  1730  of the trunnion  1720  sets a pitch angle of the blade  1710  and affects its load (e.g., radial load, etc.). The examples of  FIGS. 17-27  illustrate that a variety of placements of the blade  1710  with respect to the trunnion  1720  are envisioned to create an adjusted pitch axis that aligns or diverges from a radial center line of the engine to create a desired moment reaction. By tilting, leaning, and/or otherwise shifting the blade  1710  with respect to the hub  1730  and/or the trunnion  1720 , an artificial inertial reaction can be generated at the root of the blade  1710  to offset radial load. 
     For example,  FIG. 17  positions a first end of the blade  1710  near the connection to the trunnion  1720 . In the example of  FIG. 18 , the blade  1710  is rotated with respect to the trunnion  1720 . In  FIG. 19 , the first end of the blade  1710  is removably affixed to the trunnion  1720 .  FIGS. 20-27  provide additional examples of tiling, rotating, shifting, and/or otherwise moving the blade  1710  in the trunnion  1720  to generate a pitch axis that is offset from a radial axis of the hub  1730  and associated engine. 
     In certain examples, positioning of the blade  1710  with respect to the trunnion  1720  and/or the hub  1730  can be determined through finite element (FE) modeling to simulate and resolve reactions based on point of attachment between the blade  1710  and the trunnion  1720 . The point of attachment and/or the associated pitch axis can be modified to determine a blade  1710  orientation and attachment point to the trunnion  1720  that reduces or minimizes reaction on the blade  1710 , for example. 
     As such, certain examples provide a rotor apparatus including movement means, such as a hub, to facilitate movement of blades in an engine, the engine having a radial center line, and connection means, such as a trunnion, to accommodate a first blade, at least one of the connection means or the first blade positioned with at least one of a tilt or a lean with respect to the radial center line to form a pitch axis offset from the radial center line such that the pitch axis is not parallel to or co-linear with the radial center line, the movement means to rotate the first blade about the pitch axis. In certain examples, retention means includes at least one of a ram, a spring, a wedge, taper(s), screw(s), bolt(s), etc. 
       FIG. 28  is a flow chart of a method  2800  to determine blade position to set a pitch axis relative to an engine radial axis. In certain examples, at block  2802 , a finite element (FE) model of the blade  1710  is created, and a dovetail attachment of the blade  1710  is connected to the trunnion  1720  and/or other attachment mechanism at a chosen location (e.g., centered about a pitch axis). At block  2804 , the blade is rotated about the chosen location (e.g., pitch axis) to represent different operating conditions, and, at block  2806 , corresponding pressure and temperature loads can be applied. At block  2808 , the model can be solved and reactions resolved at the point of attachment, for example. Maximum and minimum values of critical reactions can be determined. At block  2810 , a location of the dovetail and/or pitch axis can be modified and the analyses repeated to find a location with minimum absolute value of critical reaction (e.g., balance dovetail). 
     In certain examples, at block  2812 , a Design of Experiments (DoE) study of design space is created in which the pitch axis is leaned and/or tilted over a narrow range (+/−10 deg). At block  2814 , results can be tabulated, and a statistical analysis/data processing tool is used to create meta-models (e.g., surrogate models) predicting output based on lean/tilt. At block  2816 , a weighted output parameter can be created by combining individual outputs using a relative importance function. At block  2818 , an optimization tool can be used to run the meta-models and determine the combination of lean/tilt resulting in the minimum weighted output parameter. 
     In certain examples, at block  2820 , results are weighted by combining two results to get one final result (e.g., one result is more valuable than the other so weight it, etc.). At block  2822 , inputs can be examined to give the most attractive combined result, and various inputs can be weighted to determine what is most important/what provides the best outcome. For example, pitch/lean can be varied by 10% to evaluate an impact on force, moment, etc. 
     As described herein, a variety of devices, positioning, and associated techniques can be applied to maintain or retain a blade in a collect, trunnion, slot, and/or other socket. Certain examples provide a variety of blade retention apparatus that can be used separately or in combination to reduce a likelihood of movement of a dovetail in a socket (e.g., trunnion, collect, slot, etc.). Certain examples provide a locking spacer or insert. Certain examples provide a ram, wedge, spring, retention strip, and/or other device to apply radial and/or axial preload to the dovetail or base of a blade/airfoil. Certain examples adjust the pitch axis. Certain examples both adjust the pitch axis and provide a spacer to secure the blade in its slot. 
     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, and (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, and (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, and (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 and/or steps, 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, and (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 and/or steps, 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, and (3) at least one A and at least one B. 
     As used herein, singular references (e.g., “a”, “an”, “first”, “second”, etc.) do not exclude a plurality. The term “a” or “an” entity, as used herein, refers to one or more of that entity. The terms “a” (or “an”), “one or more”, and “at least one” can be used interchangeably herein. 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. 
     From the foregoing, it will be appreciated that example methods, apparatus and articles of manufacture have been disclosed that improve design and configuration of a blade in a slot. Certain examples improve positioning and maintenance of positioning of the blade in the slot when subjected to force. 
     Although certain example methods, apparatus and articles of manufacture have been disclosed herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims of this patent. 
     Further aspects are provided by the subject matter of the following clauses: 
     Example 1 provides an apparatus including a socket to receive an end of a blade; and a retaining device to interact with the socket and the blade for retention of the end of the blade in the socket. The retaining device includes: a spacer to be positioned in the socket with the end of the blade; a wedge positioned inside the spacer; and a spring to tighten to pull the wedge to expand the spacer. 
     Example 2 is the apparatus of any preceding clause, wherein the socket includes a trunnion. 
     Example 3 is the apparatus of any preceding clause, wherein the socket is positioned on a hub of an open-rotor engine. 
     Example 4 is the apparatus of any preceding clause, wherein the spring is tightened to lock the wedge with respect to the spacer in the socket. 
     Example 5 is the apparatus of any preceding clause, wherein the spacer, the wedge, and the spring are metallic. 
     Example 6 is the apparatus of any preceding clause, wherein at least one of the spacer, the wedge, or the spring has a coating. 
     Example 7 is the apparatus of any preceding clause, wherein the spacer is formed in a curve. 
     Example 8 is the apparatus of any preceding clause, wherein the spacer is formed as a single bent piece. 
     Example 9 is the apparatus of any preceding clause, wherein the spacer is formed from a plurality of parts joined together. 
     Example 10 is the apparatus of any preceding clause, further including a ram positioned with respect to the end of the blade in the socket. 
     Example 11 is a retaining apparatus including: a spacer to be positioned in a socket with an end of a blade; a wedge positioned inside the spacer; and a spring to tighten to pull the wedge to expand the spacer. 
     Example 12 is the apparatus of any preceding clause, wherein the socket includes a trunnion. 
     Example 13 is the apparatus of any preceding clause, wherein the socket is positioned on a hub of an open-rotor engine. 
     Example 14 is the apparatus of any preceding clause, wherein the spring is tightened to lock the wedge with respect to the spacer in the socket. 
     Example 15 is the apparatus of any preceding clause, wherein at least one of the spacer, the wedge, or the spring has a coating. 
     Example 16 is the apparatus of any preceding clause, wherein the spacer is formed in a curve. 
     Example 17 is the apparatus of any preceding clause, wherein the spacer is formed as a single bent piece. 
     Example 18 is the apparatus of any preceding clause, wherein the spacer is formed from a plurality of parts joined together. 
     Example 19 is the apparatus of any preceding clause, further including a ram positioned with respect to the end of the blade in the socket. 
     Example 20 is a retaining system for a blade of an engine. The example retaining system includes: means for spacing to be positioned in a socket with an end of a blade; means for wedging positioned inside the means for spacing; and means for tightening to pull the means for wedging to expand the means for spacing. 
     Example 21 is a blade apparatus including a collect including a slot to receive an end of a blade; and a plurality of retainers including a first retainer positioned on a first end of the collect and a second retainer positioned on a second end of the collect to provide axial retention for the blade in the slot. 
     Example 22 is the apparatus of any preceding clause, wherein the plurality of retainers includes a plurality of straps. 
     Example 23 is the apparatus of any preceding clause, wherein the collect includes pockets to receive the retainers. 
     Example 24 is the apparatus of any preceding clause, wherein the collect includes ramps to retain and support the retainers. 
     Example 25 is the apparatus of any preceding clause, wherein the ramps align with the end of the blade that is positioned in the slot. 
     Example 26 is the apparatus of any preceding clause, wherein the collect is round with the first end and the second end flattened. 
     Example 27 is the apparatus of any preceding clause, wherein the plurality of retainers is secured to the collect using bolts. 
     Example 28 is the apparatus of any preceding clause, wherein at least one bolt extends into the blade. 
     Example 29 is the apparatus of any preceding clause, wherein the at least one bolt is in at least one of single shear or double shear. 
     Example 30 is the apparatus of any preceding clause, further including a ram positioned with respect to the end of the blade in the socket. 
     Example 31 is an open rotor engine apparatus including: a rotatable hub; a collect positioned on the hub, the collect including a slot to receive an end of a blade; and a plurality of retainers including a first retainer positioned on a first end of the collect and a second retainer positioned on a second end of the collect to provide axial retention for the blade in the slot. 
     Example 32 is the apparatus of any preceding clause, wherein the plurality of retainers includes a plurality of metal straps. 
     Example 33 is the apparatus of any preceding clause, wherein the collect includes pockets to receive the retainers. 
     Example 34 is the apparatus of any preceding clause, wherein the collect includes ramps to retain and support the retainers. 
     Example 35 is the apparatus of any preceding clause, wherein the collect is round with the first end and the second end flattened. 
     Example 36 is the apparatus of any preceding clause, wherein the plurality of retainers is secured to the collect using bolts. 
     Example 37 is the apparatus of any preceding clause, wherein at least one bolt extends into the blade. 
     Example 38 is the apparatus of any preceding clause, wherein the at least one bolt is in at least one of single shear or double shear. 
     Example 39 is the apparatus of any preceding clause, further including a ram positioned with respect to the end of the blade in the socket. 
     Example 40 is a blade retention apparatus including: a receiving means including a slot to receive an end of a blade; and a retention means positioned at a first end and a second end of the receiving means to provide axial retention for the blade in the slot. 
     Example 41 is a blade apparatus including: a hub to facilitate movement of blades in an engine, the engine having a radial center line; and a trunnion connected to the hub, the trunnion including a slot to accommodate a first blade, at least one of the trunnion or the first blade positioned with at least one of a tilt or a lean with respect to the radial center line to form a pitch axis offset from the radial center line such that the pitch axis is not parallel to or co-linear with the radial center line, the hub to rotate the first blade about the pitch axis. 
     Example 42 is the apparatus of any preceding clause, further including at least one bearing, the at least one bearing mounted to the hub and at least one of connected to or integrated with the trunnion. 
     Example 43 is the apparatus of any preceding clause, wherein the tilt includes a forward tilt. 
     Example 44 is the apparatus of any preceding clause, wherein the lean includes a tangential lean. 
     Example 45 is the apparatus of any preceding clause, wherein the tangential lean is into or away from a direction of rotation. 
     Example 46 is the apparatus of any preceding clause, wherein a position of the first blade is modeled to simulate and resolve reactions based on a point of attachment between the first blade and the trunnion. 
     Example 47 is the apparatus of any preceding clause, wherein at least one of the point of attachment or an orientation of the first blade are determined based on the simulated reactions on the first blade. 
     Example 48 is the apparatus of any preceding clause, further including a ram positioned with respect to the first blade in the trunnion. 
     Example 49 is the apparatus of any preceding clause, further including at least one of a spring or a wedge positioned with respect to the first blade in the trunnion. 
     Example 50 is a rotor apparatus including: movement means to facilitate movement of blades in an engine, the engine having a radial center line; and connection means to accommodate a first blade, at least one of the connection means or the first blade positioned with at least one of a tilt or a lean with respect to the radial center line to form a pitch axis offset from the radial center line such that the pitch axis is not parallel to or co-linear with the radial center line, the movement means to rotate the first blade about the pitch axis. 
     Example 51 is the apparatus of any preceding clause, wherein the tilt includes a forward tilt. 
     Example 52 is the apparatus of any preceding clause, wherein the lean includes a tangential lean. 
     Example 53 is the apparatus of any preceding clause, further including a retention means to retain the blade in the connection means. 
     Example 54 is a computer-implemented method to determine blade position to set a pitch axis relative to a radial engine axis. The example method includes: adjusting a position of a model of a blade at a location with respect to a trunnion attached to a hub of an engine; applying loads to the model; solving the model to resolve reactions caused by the loads; predicting one or more outputs based on the model solution; determining at least one of a lean or a tilt of the blade based on the one or more outputs; generating a blade position to form a pitch axis offset from a radial center line of the engine such that the pitch axis is not parallel to or co-linear with the radial center line, the blade to rotate about the pitch axis. 
     Example 55 is the method of any preceding clause, wherein the at least one output includes a plurality of outputs, and further including combining the outputs to create a weighted output parameter. 
     Example 56 is the method of any preceding clause, further including evaluating an outcome of the blade position. 
     Example 57 is the method of any preceding clause, wherein the model is a finite element model and wherein the position of the blade is modeled to simulate and resolve reactions based on a point of attachment between the blade and the trunnion. 
     Example 58 is the method of any preceding clause, wherein at least one of the point of attachment or an orientation of the blade are determined based on the reactions simulated and resolved on the blade. 
     Example 59 is the method of any preceding clause, wherein the location is a first location, and further including modifying the location to a second location and repeating the method. 
     Example 60 is the method of any preceding clause, further including creating a design space to at least one of lean or tilt the pitch axis within a range.