Propulsion engine for an aircraft

A propulsion system for an aircraft is provided having a propulsion engine configured to be mounted to the aircraft. The propulsion engine includes an electric machine defining an electric machine tip speed during operation. The propulsion system additionally includes a fan rotatable about a central axis of the electric propulsion engine with the electric machine. The fan defines a fan pressure ratio, RFP, and includes a plurality of fan blades, each fan blade defining a fan blade tip speed. The electric propulsion engine defines a ratio of the fan blade tip speed to electric machine tip speed that is within twenty percent of the equation, 1.68×RFP−0.518, such that the propulsion engine may operate at a desired efficiency.

FIELD OF THE INVENTION

The present subject matter relates generally to an aircraft propulsion system including an electric propulsion engine.

BACKGROUND OF THE INVENTION

A conventional commercial aircraft generally includes a fuselage, a pair of wings, and a propulsion system that provides thrust. The propulsion system typically includes at least two aircraft engines, such as turbofan jet engines. Each turbofan jet engine is mounted to a respective one of the wings of the aircraft, such as in a suspended position beneath the wing, separated from the wing and fuselage. Such a configuration allows for the turbofan jet engines to interact with separate, freestream airflows that are not impacted by the wings and/or fuselage. This configuration can reduce an amount of turbulence within the air entering an inlet of each respective turbofan jet engine, which has a positive effect on a net propulsive thrust of the aircraft.

However, a drag on the aircraft including the turbofan jet engines, also has an effect on the net propulsive thrust of the aircraft. A total amount of drag on the aircraft, including skin friction, form, and induced drag, is generally proportional to a difference between a freestream velocity of air approaching the aircraft and an average velocity of a wake downstream from the aircraft that is produced due to the drag on the aircraft.

Systems have been proposed to counter the effects of drag and/or to improve an efficiency of the turbofan jet engines. For example, certain propulsion systems incorporate boundary layer ingestion systems to route a portion of relatively slow moving air forming a boundary layer across, e.g., the fuselage and/or the wings, into the turbofan jet engines upstream from a fan section of the turbofan jet engines. Although this configuration can reduce drag by reenergizing the boundary layer airflow downstream from the aircraft, the relatively slow moving flow of air from the boundary layer entering the turbofan jet engine generally has a nonuniform or distorted velocity profile. As a result, such turbofan jet engines can experience an efficiency loss minimizing or negating any benefits of reduced drag on the aircraft.

Accordingly, a propulsion system including one or more components for reducing an amount of drag on the aircraft would be useful. More particularly, a propulsion system including an efficient propulsion engine for reducing an amount of drag on the aircraft without causing any substantial decreases in an efficiency of the aircraft engines would be especially beneficial.

BRIEF DESCRIPTION OF THE INVENTION

In one exemplary embodiment of the present disclosure, a propulsion system is provided for an aircraft having a fuselage and an aft end. The propulsion system includes a propulsion engine configured to be mounted to the aircraft. The propulsion engine defines a central axis and includes an electric machine defining an electric machine tip speed during operation of the propulsion engine. The propulsion engine additionally includes a fan rotatable about the central axis of the propulsion engine by the electric machine. The fan defines a fan pressure ratio (RFP) and includes a plurality of fan blades. Each fan blade defines a fan blade tip speed during operation of the propulsion engine, the propulsion engine defining a ratio of fan blade tip speed to electric machine tip speed. During operation of the propulsion engine, the ratio of fan blade tip speed to electric machine tip speed is within twenty percent of the following equation: 1.68×RFP−0.518.

In an exemplary aspect of the present disclosure, a method for operating a propulsion system for an aircraft is provided. The propulsion system includes a propulsion engine including an electric machine and a fan. The electric machine defines an electric machine tip speed and the fan defines a fan tip speed. The method includes operating the fan of the propulsion engine to define a fan pressure ratio (RFP) greater than one and less than about three. A ratio of fan blade tip speed to electric machine tip speed is within twenty percent of the equation 1.68×RFP−0.518.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “forward” and “aft” refer to the relative positions of a component based on an actual or anticipated direction of travel. For example, “forward” may refer to a front of an aircraft based on an anticipated direction of travel of the aircraft, and “aft” may refer to a back of the aircraft based on an anticipated direction of travel of the aircraft. 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.

The present application is directed generally towards an electric propulsion engine having an electric motor and a fan configured for a desired efficiency and power density. The inventors of the present disclosure have discovered that coordinating a tip speed ratio (i.e., an electric machine tip speed relative to a fan blade tip speed) to a fan pressure ratio (RFP) may provide a desired increase in efficiency and power density across a range of fan sizes and electric motor sizes for the electric propulsion engine. Specifically, the inventors have discovered that a desired efficiency may be achieved by configuring the tip speed ratio of the electric propulsion engine within about twenty percent (20%) of the equation 1.68×RFP−0.518.

Referring now to the drawings, wherein identical numerals indicate the same elements throughout the figures,FIG. 1provides a top view of an exemplary aircraft10as may incorporate various embodiments of the present invention.FIG. 2provides a port side view of the aircraft10as illustrated inFIG. 1. As shown inFIGS. 1 and 2collectively, the aircraft10defines a longitudinal centerline14that extends therethrough, a vertical direction V, a lateral direction L, a forward end16, and an aft end18. Moreover, the aircraft10defines a mean line15extending between the forward end16and aft end18of the aircraft10. As used herein, the “mean line” refers to a midpoint line extending along a length of the aircraft10, not taking into account the appendages of the aircraft10(such as the wings20and stabilizers discussed below).

Moreover, the aircraft10includes a fuselage12, extending longitudinally from the forward end16of the aircraft10towards the aft end18of the aircraft10, and a pair of wings20. As used herein, the term “fuselage” generally includes all of the body of the aircraft10, such as an empennage of the aircraft10. The first of such wings20extends laterally outwardly with respect to the longitudinal centerline14from a port side22of the fuselage12and the second of such wings20extends laterally outwardly with respect to the longitudinal centerline14from a starboard side24of the fuselage12. Each of the wings20for the exemplary embodiment depicted includes one or more leading edge flaps26and one or more trailing edge flaps28. The aircraft10further includes a vertical stabilizer30having a rudder flap32for yaw control, and a pair of horizontal stabilizers34, each having an elevator flap36for pitch control. The fuselage12additionally includes an outer surface or skin38. It should be appreciated however, that in other exemplary embodiments of the present disclosure, the aircraft10may additionally or alternatively include any other suitable configuration of stabilizer that may or may not extend directly along the vertical direction V or horizontal/lateral direction L.

The exemplary aircraft10ofFIGS. 1 and 2includes a propulsion system100, herein referred to as “system100”. The exemplary system100includes one or more aircraft engines and one or more electric propulsion engines. For example, the embodiment depicted includes a plurality of aircraft engines, each configured to be mounted to the aircraft10, such as to one of the pair of wings20, and an electric propulsion engine. More specifically, for the embodiment depicted, the aircraft engines are configured as gas turbine engines, or rather as turbofan jet engines102,104attached to and suspended beneath the wings20in an under-wing configuration. Additionally, the electric propulsion engine is configured to be mounted at the aft end of the aircraft10, and hence the electric propulsion engine depicted may be referred to as an “aft engine.” Further, the electric propulsion engine depicted is configured to ingest and consume air forming a boundary layer over the fuselage12of the aircraft10. Accordingly, the exemplary aft engine depicted may be referred to as a boundary layer ingestion (BLI) fan106. The BLI fan106is mounted to the aircraft10at a location aft of the wings20and/or the jet engines102,104. Specifically, for the embodiment depicted, the BLI fan106is fixedly connected to the fuselage12at the aft end18, such that the BLI fan106is incorporated into or blended with a tail section at the aft end18, and such that the mean line15extends therethrough. It should be appreciated, however, that in other embodiments the electric propulsion engine may be configured in any other suitable manner, and may not necessarily be configured as an aft fan or as a BLI fan.

Referring still to the embodiment ofFIGS. 1 and 2, in certain embodiments the propulsion system further includes one or more electric generators108operable with the jet engines102,104. For example, one or both of the jet engines102,104may be configured to provide mechanical power from a rotating shaft (such as an LP shaft or HP shaft) to the electric generators108. Although depicted schematically outside the respective jet engines102,104, in certain embodiments, the electric generators108may be positioned within a respective jet engine102,104. Additionally, the electric generators108may be configured to convert the mechanical power to electrical power. For the embodiment depicted, the propulsion system100includes an electric generator108for each jet engine102,104, and also includes a power conditioner109and an energy storage device110. The electric generators108may send electrical power to the power conditioner109, which may transform the electrical energy to a proper form and either store the energy in the energy storage device110or send the electrical energy to the BLI fan106. For the embodiment depicted, the electric generators108, power conditioner109, energy storage device110, and BLI fan106are all are connected to an electric communication bus111, such that the electric generator108may be in electrical communication with the BLI fan106and/or the energy storage device110, and such that the electric generator108may provide electrical power to one or both of the energy storage device110or the BLI fan106. Accordingly, in such an embodiment, the propulsion system100may be referred to as a gas-electric propulsion system.

It should be appreciated, however, that the aircraft10and propulsion system100depicted inFIGS. 1 and 2is provided by way of example only and that in other exemplary embodiments of the present disclosure, any other suitable aircraft10may be provided having a propulsion system100configured in any other suitable manner. For example, it should be appreciated that in various other embodiments, the BLI fan106may alternatively be positioned at any suitable location proximate the aft end18of the aircraft10. Further, in still other embodiments the electric propulsion engine may not be positioned at the aft end of the aircraft10, and thus may not be configured as an “aft engine.” For example, in other embodiments, the electric propulsion engine may be incorporated into the fuselage of the aircraft10, and thus configured as a “podded engine,” or pod-installation engine. Further, in still other embodiments, the electric propulsion engine may be incorporated into a wing of the aircraft10, and thus may be configured as a “blended wing engine.” Moreover, in other embodiments, the electric propulsion engine may not be a boundary layer ingestion fan, and instead may be mounted at any suitable location on the aircraft10as a freestream injection fan. Furthermore, in still other embodiments, the propulsion system100may not include, e.g., the power conditioner109and/or the energy storage device110, and instead the generator(s)108may be directly connected to the BLI fan106.

Referring now toFIG. 3, a schematic cross-sectional view of a propulsion engine in accordance with an exemplary embodiment of the present disclosure is provided. In certain exemplary embodiments, the propulsion engine may be configured a high-bypass turbofan jet engine200, herein referred to as “turbofan200.” Notably, in at least certain embodiments, the jet engines102,104may be also configured as high-bypass turbofan jet engines. In various embodiments, the turbofan200may be representative of jet engines102,104. Alternatively, however, in other embodiments, the turbofan200may be incorporated into any other suitable aircraft10or propulsion system100.

As shown inFIG. 3, the turbofan200defines an axial direction A1(extending parallel to a longitudinal centerline201provided for reference) and a radial direction R1. In general, the turbofan200includes a fan section202and a core turbine engine204disposed downstream from the fan section202.

The exemplary core turbine engine204depicted generally includes a substantially tubular outer casing206that defines an annular inlet208. The outer casing206encases, in serial flow relationship, a compressor section including a booster or low pressure (LP) compressor210and a high pressure (HP) compressor212; a combustion section214; a turbine section including a high pressure (HP) turbine216and a low pressure (LP) turbine218; and a jet exhaust nozzle section220. A high pressure (HP) shaft or spool222drivingly connects the HP turbine216to the HP compressor212. A low pressure (LP) shaft or spool224drivingly connects the LP turbine218to the LP compressor210.

For the embodiment depicted, the fan section202includes a variable pitch fan226having a plurality of fan blades228coupled to a disk230in a spaced apart manner. As depicted, the fan blades228extend outwardly from disk230generally along the radial direction R1. Each fan blade228is rotatable relative to the disk230about a pitch axis P by virtue of the fan blades228being operatively coupled to a suitable actuation member232configured to collectively vary the pitch of the fan blades228in unison. The fan blades228, disk230, and actuation member232are together rotatable about the longitudinal axis12by LP shaft224across a power gear box234. The power gear box234includes a plurality of gears for stepping down the rotational speed of the LP shaft224to a more efficient rotational fan speed. Additionally, it will be appreciated that the fan226generally defines a fan pressure ratio (i.e., a fan pressure ratio rating for the fan, such as a maximum fan pressure ratio of the fan as may be achieved through an entire flight envelope), and that the plurality of fan blades228each define a fan blade tip speed at an outer tip along the radial direction R1.

Referring still to the exemplary embodiment ofFIG. 3, the disk230is covered by rotatable front hub236aerodynamically contoured to promote an airflow through the plurality of fan blades228. Additionally, the exemplary fan section202includes an annular fan casing or outer nacelle238that circumferentially surrounds the fan226and/or at least a portion of the core turbine engine204. It should be appreciated that the nacelle238may be configured to be supported relative to the core turbine engine204by a plurality of circumferentially-spaced outlet guide vanes240. Moreover, a downstream section242of the nacelle238may extend over an outer portion of the core turbine engine204so as to define a bypass airflow passage244therebetween.

Additionally, the exemplary turbofan200depicted includes an electric machine246rotatable with the fan226. Specifically, for the embodiment depicted, the electric machine246is configured as an electric generator co-axially mounted to and rotatable by the LP shaft224(the LP shaft224also rotating the fan226through, for the embodiment depicted, the power gearbox234). The electric machine246includes a rotor248and a stator250. As will be appreciated, the rotor248may be attached to the LP shaft224and the stator250may remain static within the core turbine engine204. During operation, the electric machine may define an electric machine tip speed (i.e., a linear speed of the rotor248at a radially outer edge of the rotor). Notably, when the turbofan engine200is integrated into the propulsion system100described above with reference toFIGS. 1 and 2, the electric generators108may be configured in substantially the same manner as the electric machine246ofFIG. 3.

Further, in still other embodiments, the turbofan engine200may be designed such that a ratio of the fan blade tip speed to electric machine tip speed may have a specific relationship to the fan pressure ratio. Notably, the relationship of these factors described below in the context of the electric propulsion device300, may also apply to the relationship of these factors described with respect to the turbofan engine200(i.e., the relationship of the ratio of the fan blade tip speed to electric machine tip speed of the turbofan engine200to the fan pressure ratio of the turbofan engine200).

It should be also appreciated, however, that the exemplary turbofan engine200depicted inFIG. 3is by way of example only, and that in other exemplary embodiments, the turbofan engine200may have any other suitable configuration. Further, it should be appreciated, that in other exemplary embodiments, the jet engines102,104may instead be configured as any other suitable aeronautical engine, such as a turboprop engine, turbojet engine, internal combustion engine, etc.

Referring now toFIG. 4, a schematic, cross-sectional side view of an electric propulsion engine in accordance with various embodiments of the present disclosure is provided. The electric propulsion engine depicted is mounted to an aircraft10at an aft end18of the aircraft10and is configured to ingest a boundary layer air. Accordingly, for the embodiment depicted, the electric propulsion engine is configured as a boundary layer ingestion (BLI), aft fan (referred to hereinafter as “BLI fan300”). The BLI fan300may be configured in substantially the same manner as the BLI fan106described above with reference toFIGS. 1 and 2and the aircraft10may be configured in substantially the same manner as the exemplary aircraft10described above with reference toFIGS. 1 and 2.

As shown inFIG. 4, the BLI fan300defines an axial direction A2extending along a longitudinal centerline axis302(or center axis) that extends therethrough for reference, as well as a radial direction R2and a circumferential direction C2(a direction extending about the axial direction A2, not shown). Additionally, the aircraft10defines a mean line15extending therethrough.

In general, the BLI fan300includes a fan304rotatable about the centerline axis302and a structural support system308. The structural support system308is configured for mounting the BLI fan300to the aircraft10, and for the embodiment depicted generally includes an inner frame support310, a plurality of forward support members312, an outer nacelle314, a plurality of aft support members316, and a tail cone318. As is depicted, the inner frame support310is attached to a bulkhead320of the fuselage12. The plurality of forward support members312are attached to the inner frame support310and extend outward generally along the radial direction R2to the nacelle314. The nacelle314defines an airflow passage322with an inner casing324of the BLI fan300, and at least partially surrounds the fan304. Further, for the embodiment depicted, the nacelle314extends substantially three hundred and sixty degrees (360°) around the mean line15of the aircraft10. The plurality of aft support members316also extend generally along the radial direction R2from, and structurally connect, the nacelle314to the tail cone318.

In certain embodiments, the forward support members312and the aft support members316may each be generally spaced along the circumferential direction C2of the BLI fan300. Additionally, in certain embodiments the forward support members312may be generally configured as inlet guide vanes and the aft support members316may generally be configured as outlet guide vanes. If configured in such a manner, the forward and aft support members312,316may direct and/or condition an airflow through the airflow passage322of the BLI fan300. Notably, one or both of the forward support members312or aft support members316may additionally be configured as variable guide vanes. For example, the support member may include a flap (not shown) positioned at an aft end of the support member for directing a flow of air across the support member.

It should be appreciated, however, that in other exemplary embodiments, the structural support system308may instead include any other suitable configuration and, e.g., may not include each of the components depicted and described above. Alternatively, the structural support system308may include any other suitable components not depicted or described above.

The BLI fan300additionally defines a nozzle326between the nacelle314and the tail cone318. The nozzle326may be configured to generate an amount of thrust from the air flowing therethrough, and the tail cone318may be shaped to minimize an amount of drag on the BLI fan300. However, in other embodiments, the tail cone318may have any other shape and may, e.g., end forward of an aft end of the nacelle314such that the tail cone318is enclosed by the nacelle314at an aft end. Additionally, in other embodiments, the BLI fan300may not be configured to generate any measurable amount of thrust, and instead may be configured to ingest air from a boundary layer of air of the fuselage12of the aircraft10and add energy/speed up such air to reduce an overall drag on the aircraft10(and thus increase a net thrust of the aircraft10).

Referring still toFIG. 4, the fan304includes a plurality of fan blades328and a fan shaft330. The plurality of fan blades328are attached to the fan shaft330and spaced generally along the circumferential direction C2of the BLI fan300. As depicted, the plurality fan blades328are, for the embodiment depicted, at least partially enclosed by the nacelle314.

The fan304of the BLI fan300defines a fan pressure ratio (RFP). As used herein, the term “fan pressure ratio” refers to a ratio of a discharge pressure of a fan to an inlet pressure of the fan during operation of the fan. Accordingly, for the exemplary BLI fan300depicted inFIG. 4, the fan pressure ratio, RFP, refers to a ratio of a pressure downstream of the fan304to a pressure upstream of the fan304during operation of the BLI fan300. An amount of thrust generated by the BLI fan300may be directly related to the fan pressure ratio, RFP, of the fan304. Notably, as used herein, the term “fan pressure ratio” generally refers to a fan pressure ratio rating for the fan, such as a maximum fan pressure ratio of the fan as may be achieved through an entire flight envelope.

In certain exemplary embodiments, the plurality of fan blades328may be attached in a fixed manner to the fan shaft330, or alternatively, the plurality of fan blades328may be rotatably attached to the fan shaft330. For example, the plurality of fan blades328may be attached to the fan shaft330such that a pitch of each of the plurality of fan blades328may be changed, e.g., in unison, by a pitch change mechanism (not shown). Changing the pitch of the plurality of fan blades328may increase an efficiency of the BLI fan300and/or may allow the BLI fan300to achieve a desired thrust profile. With such an exemplary embodiment, the BLI fan300may be referred to as a variable pitch BLI fan.

Additionally, each of the plurality of fan blades328define a tip332at an outer end of the respective fan blade328along the radial direction R2. During operation of the BLI fan300, each fan blade328may accordingly define a fan blade tip speed SFB. As will be appreciated, the fan blade tip speed SFBmay generally be determined by multiplying a rotational speed of the fan blades328and fan shaft330by a radius334of the tip332of the respective fan blade328along the radial direction R2, relative to the centerline axis302. Further, the fan blade tip speed SFBmay, in certain embodiments, be corrected to a standard day condition, such that fan blade tip speed SFMreferred to herein may be equal to: SFB,ACT×√{square root over (TAMB÷518.67°Ra)}, where SFB,ACTequals the actual fan blade tip speed and TAMBequals an ambient temperature in Rankine.

Moreover, for the embodiment depicted, the fan304is rotatable about the centerline axis302of the BLI fan300by an electric machine. More particularly, for the embodiment depicted, the electric machine is an electric motor336and the BLI fan300additionally includes a power gearbox338mechanically coupled to the electric motor336. Additionally, the fan304is mechanically coupled to the power gearbox338. For example, for the embodiment depicted, the fan shaft330extends to and is coupled to the power gearbox338, and a driveshaft340of the electric motor336extends to and is also coupled to the power gearbox338. Accordingly, for the embodiment depicted, the fan304is rotatable about the central axis302of the BLI fan300by the electric motor336through the power gearbox338.

The power gearbox338may include any type of gearing system for altering a rotational speed between the driveshaft340and the fan shaft330. For example, the power gearbox338may be configured as a star gear train, a planetary gear train, or any other suitable gear train configuration. Additionally, the power gearbox338may define a gear ratio, which as used herein, refers to a ratio of a rotational speed of the driveshaft340to a rotational speed of the fan shaft330. In certain exemplary embodiments, the gear ratio of the power gearbox338may be greater than about 1:1 and less than about 1:5. For example, in certain embodiments, the gear ratio of the power gearbox338may be between about 1:1.5 and about 1:3.5, such as between about 1:1.2 and about 1:2.75. It should be appreciated, that as used herein, terms of approximation, such as “about” or “approximately,” refer to being within a 10% margin of error.

Referring still to the exemplary embodiment ofFIG. 4, the electric motor336is located at least partially within the fuselage12of the aircraft10. More specifically, the fan304is positioned forward of the power gearbox338along the central axis302of the BLI fan300, and the electric motor336is positioned forward of the fan304along the central axis302of the BLI fan300. Conventional design teaches that the power gearbox338should be positioned at a forward location, as the power gearbox may require lubrication oil to operate, and a forward position may provide convenience with regard to supplying lubrication oil and scavenging lubrication oil. However, the inventors of the present disclosure have discovered that positioning the power gearbox338proximate an aft end of BLI fan300may allow for the power gearbox338to be relatively accessible for, e.g., repair of the power gearbox338. For example, in certain embodiments, the tail cone318of the BLI fan300may be removable (or may include one or more of movable sections), such that the power gearbox338is exposed for repair.

Further, in certain exemplary embodiments, the BLI fan300may be configured with a gas-electric propulsion system, such as the gas-electric propulsion system100described above with reference toFIGS. 1 and 2. In such an embodiment, the electric motor336may receive power from one or both of an energy storage device or an electric generator—such as the energy storage device110or electric generator108ofFIGS. 1 and 2.

Furthermore, referring now also toFIG. 5, providing a schematic, cross-sectional view of the electric motor336, the electric motor336is generally configured as an inrunner electric motor336. More particularly, the exemplary electric motor336depicted includes a stator342, a rotor344positioned within the stator342, and an outer casing346enclosing the stator342and rotor344. However, in other embodiments, the electric motor336may instead be configured as an outerunner (or out-running) electric motor, such that a position of the stator342and rotor344are reversed and the rotor344is instead location radially outside of the stator342.

As will be appreciated, the stator342may include one or more electromagnetic coils (not shown), and the rotor344may include a plurality of segmented permanent magnets (not shown) having alternating magnetic poles. The rotor344and stator342together define an air gap348therebetween along the radial direction R2. The rotor344is mounted to an axle or output shaft, which for the embodiment depicted is configured as, or coupled to, the driveshaft340. The electric motor336additionally includes a plurality of bearings350within the outer casing346facilitating rotation of the rotor344. It should be appreciated, however, that in other exemplary embodiments, the electric motor336may instead have any other suitable configuration. For example, in other exemplary embodiments, the electric motor336may be positioned at any other suitable location within, e.g., the fuselage12of the aircraft10or the BLI fan300.

The exemplary electric motor336additionally defines an electric machine tip speed SEMduring operation of the BLI fan300and electric motor336. The electric machine tip speed SEMrefers to a speed of the rotor344at an interface of the rotor344with the air gap348(e.g., a surface speed or linear speed of the rotor344). Accordingly, as the electric motor336depicted is configured as an inrunner electric motor, the electric machine tip speed SEMrefers to a speed of an outer edge352of the rotor344. As will be appreciated, such an electric machine tip speed SEMmay be calculated by multiplying a rotational speed of the rotor344of the electric motor336by a radius354of the outer edge352of the rotor344along the radial direction R2relative to the axial centerline302.

Notably, the exemplary BLI fan300depicted defines a ratio of the fan blade tip speed SFBto electric machine tip speed SEM(SFB:SEM, “tip speed ratio, RTS”). The tip speed ratio, RTS, defines a relationship to the fan pressure ratio, RFP, which the inventors of the present disclosure have determined may have an effect on an efficiency of the fan304and BLI fan300. Specifically, the inventors of the present disclosure have found that in order to increase an efficiency of the BLI fan300, it may be beneficial to design the BLI fan300to have a tip speed ratio, RTS, that accounts for a desired fan pressure ratio, RFP. For example, during operation of the BLI fan300, the inventors have found that a desired efficiency may be accomplished by configuring the fan304to have a tip speed ratio, RTS, within about twenty percent (20%) of the following equation: 1.68×RFP−0.518. More particularly, in certain embodiments, a desired efficiency may be accomplished by configuring the fan304to have a tip speed ratio, RTS, within about ten percent (10%), or further, within about five percent (5%) of the following equation: 1.68×RFP−0.518.

In certain exemplary embodiments, the above relationship between fan pressure ratio, RFP, and tip speed ratio, RTS, may provide for a fan304having a desired efficiency when the fan pressure ratio, RFP, is greater than one (1) and less than about three (3). For example, the above relationship between fan pressure ratio, RFP, and tip speed ratio, RTS, may provide for a fan304having a desired efficiency when the fan pressure ratio, RFP, is greater than one (1) and less than about 2.75, less than about 2.5, or less than about two (2).

Additionally, an electric machine configured according to the exemplary electric machine, or rather electric motor336, described above may operate during standard day, maximum speed conditions with an electric machine tip speed SEMbetween about 350 feet per second (ft/s) and about 750 ft/s. More particularly, an electric machine in accordance with one or more exemplary embodiments of the present disclosure may operate during standard day, maximum speed conditions with an electric machine tip speed SEMbetween about 450 feet per second (ft/s) and about 700 ft/s, such as between about 550 ft/s and about 650 ft/s.

Accordingly, in a non-limiting, example embodiment of the present disclosure, the fan304of the BLI fan300may define a fan pressure RFPof about 1.25. Using the equation RTS=1.68×RFP−0.518, the tip speed ratio RTSis approximately 1.58. The electric motor336, power gearbox338, and fan304may then be sized such that the effective tip speed ratio RTSis approximately 1.58. An electric machine tip speed SEMof an electric machine in accordance with the present disclosure (e.g., electric motor336) may be approximately 600 ft/s. Accordingly, with this exemplary embodiment, the fan blade tip speed SFBmay be approximately 948 ft/s.

As an illustrative aspect of the present disclosure, a method for operating a propulsion system for an aircraft is provided. The propulsion system may be the exemplary propulsion system100described above with reference toFIGS. 1 through 5. Accordingly, the exemplary propulsion system may include an electric propulsion engine, such as the exemplary BLI fan300, including an electric motor and a fan. The electric motor may define an electric machine tip speed and the fan may define a fan tip speed. The method includes operating the fan of the electric propulsion engine to define a fan pressure ratio greater than one (1) and less than about three (3). A ratio of the fan blade tip speed to the electric machine tip speed is within 20% of the equation: 1.68×RFP−0.518.

An electric propulsion engine in accordance with one or more embodiments of the present disclosure may provide for an electric propulsion engine having a desired efficiency and power density (e.g., an amount of thrust per unit volume of air flowing through the electric propulsion machine). Specifically, the inventors of the present disclosure have found that an electric propulsion engine defining a tip speed ratio having a relationship to a fan pressure ratio of a fan of the electric propulsion engine in accordance with one or more the embodiments described above may result in an electric propulsion engine having an increased efficiency and power density across various sizes of fans and electric motors. Such a configuration may be accomplished by sizing the electric motor, fan blades, and if necessary, inclusion of a power gearbox having a specified gear ratio, based on an anticipated fan pressure ratio during operation of the engine.

Referring now toFIG. 6, a BLI fan300in accordance with another exemplary embodiment of the present disclosure is depicted. The exemplary BLI fan300depicted inFIG. 6may be configured in substantially the same manner as exemplary BLI fan300depicted inFIG. 4and described above. Accordingly, the same or similar numbering may refer to the same or similar part.

As depicted, the exemplary BLI fan300generally includes a fan304and a structural support system308. As with the embodiment described above, the structural support system308includes a plurality of struts310, a plurality of forward support members312, an outer nacelle314, a plurality of aft support members316, and a tail cone318. Additionally, the fan304generally includes a plurality of fan blades328at least partially enclosed within the outer nacelle314and a fan shaft330. For the embodiment depicted, the exemplary fan304is rotatable about a central axis302of the BLI fan300by an electric motor336, or more particularly, the fan304is rotatable about the central axis302of the BLI fan300by the electric motor336through a power gearbox338. The electric motor336is mechanically coupled to a driveshaft340, which extends to an attaches to the power gearbox338. The power gearbox338is, in turn, attached to the fan shaft330for rotating the fan shaft330.

However, for the embodiment depicted inFIG. 6, a relative positioning of the fan304, the power gearbox338, and the electric motor336has been altered. More specifically, for the embodiment depicted inFIG. 6, the power gearbox338is positioned forward of the fan304, and forward of the electric motor336along the central axis302of the BLI fan300. Further, the fan304is, for the embodiment depicted, positioned forward of the electric motor336along the central axis302of the BLI fan300. However, in other embodiments, the fan304may overlap with the electric motor336along the central axis302, or may even be positioned aft of the electric motor336along the central axis302. Regardless, such a configuration may allow for greater ease of access to the electric motor336during, e.g., maintenance or repair of the electric motor336. For example, in certain exemplary embodiments, the tail cone318, or a part thereof, may be removable, such that the electric motor336may be relatively easily accessible during maintenance or repair operations.