GB2630265A

GB2630265A - Electric propulsion systems

Info

Publication number: GB2630265A
Application number: GB2306927.1A
Authority: GB
Inventors: Monterzino Guido; Anmol Manohar Arava
Original assignee: Greenjets Ltd
Current assignee: Greenjets Ltd
Priority date: 2023-05-10
Filing date: 2023-05-10
Publication date: 2024-11-27
Also published as: WO2024231689A2; GB202306927D0; WO2024231689A3

Abstract

A propeller (fig.2,210) has multiple rotor blades 112 generating thrust when operating in a medium, the propeller has a thrust coefficient CT of between 0.15 to 0.5. The thrust coefficient performance space is defined as CT = T / (ρ A1 Umean2), where T is the generated fan thrust, ρ is the medium fluid density, A1 is the rotor blade actuator disc area, and Umean is the mean medium efflux velocity of the medium from an electric propulsion system 102. The rotor blades are preferably mounted within an annular shroud 114, and CT may be between 0.2 to 0.4. Also claimed is an electric propulsion system with a nacelle 104 housing a propeller with a number of rotor blades, at least one of a number of inlet guide vanes 140 and/or outlet guide vanes 142 which have a predetermined relationship with a number of rotor blades, such as a certain ratio of vanes to blades. The ratio of inlet vanes to blades may be 0 to 0.95, and the ratio of outlet vanes to blades may be 0.75 to 2, the number of rotor blades between 5 to 25.

Description

ELECTRIC PROPULSION SYSTEMS

BACKGROUND

[0001] There is an increasing drive to reduce carbon emissions to combat global warming. Part of that drive involves improving the efficiency of existing gas turbine engines and internal combustion engines of, for example, aircraft. Alternative propulsion technologies are also being investigated such as, for example, electric propulsion systems. Ground vehicles that use electric motors are well-established. However, aircraft that use electric motors to drive aerodynamic surfaces are less well established and face a number of technical challenges such as the relatively low energy density of electrical energy sources, lower power densities of batteries and inherent inefficiencies such as thermal management systems, power losses and other inefficiencies, as well as suboptimal operating conditions.

[0002] Furthermore, aircraft flying from major airports generate a significant amount of inconvenient engine noise during take-off. Furthermore, smaller aircraft such as, for example, drones, also have a significant adverse acoustic signature during operation.

[0003] It is well-established that a source of acoustic noise associated with propellers of aircraft is the open tips of the propeller blades. At moderate tip speeds, i.e., slightly below the onset of compressibility effects, both vortex noise and rotational noise due to thickness are lower than the rotational noise due to thrust and torque. Consequently, noise reduction for propellers, of both a theoretical and experimental nature, has concentrated on the effects of thrust and torque. In studies dealing with the reduction of overall propeller noise, however, vortex noise has been shown to be an important contributor and, in the case of electric flight, the level of thickness noise may exceed that of thrust and torque noise. Shrouded propellers have been designed that significantly reduce noise associated with propellers. However, propeller noise remains a significant problem both as a source of inefficiency and a source of non-compliance with noise emission regulations.

BRIEF INTRODUCTION OF THE DRAWINGS

[0004] Examples are described with reference to the accompanying drawings, in which: [0005] Figure 1 shows a sectional view of an electric propulsion system according to examples; [0006] Figure 2 illustrates a shrouded propeller for the electric propulsion system of figure 1 according to examples; [0007] Figure 3 depicts a graph showing the variation in thrust coefficient with advance ratio according to examples; [0008] Figure 4 shows a graph illustrating the variation of Total Pressure Rise Coefficient with Flow Coefficient according to examples; [0009] Figure 5 illustrates a graph showing the variation of Total Propeller Efficiency with Flow Coefficient according to examples, and [0010] Figure 6 depicts a graph illustrating the variation of Thrust with Flow Coefficient according to example.

DETAILED DESCRIPTION

[0011] Referring to figure 1 there is shown a view 100 of an electric propulsion system 102. The electric propulsion system can be for an aircraft. The electric propulsion system 102 comprises a nacelle 104. The nacelle 104 comprises an aerodynamic annular housing. The annular housing defines an internal annular volume 106. The annular housing also defines an inner duct indicated generally as 108. The inner duct is external to the housing and forms an airflow path through which air passes. The electric propulsion system 102 comprises at least one propeller 110. The propeller has a plurality of aerodynamic surfaces or blades 112 for generating thrust. The blades 112 are mounted to an annular propeller shroud 114 to form a shrouded propeller. The shrouded propeller 114 is mounted on a central hub 116 so that the propeller comprising the blades 112 can rotate about a central shaft 118. Each of the blades 112 is mounted to the propeller shroud 114 via a respective fixture 122.

[0012] Using a propeller shroud can reduce noise generation that is otherwise associated with open tip blades. The noise reduction can be realised, at least in part, using sound absorbing materials to form the nacelle housing. Further noise reductions can be realised by profiling a profiled trailing edge geometry to the nacelle 104. The profiled trailing edge bears features such as, for example, a Helmholtz Resonator muffler 123, that reduce aero-electric noise associated with operating the propulsion system 102. The Helmholtz Resonator muffler 123 is arranged to supress large amplitude and mid to low frequency pressure fluctuations such as, for example, mechanical vibratory noise of aerodynamic bodies immersed in a propulsive medium and buffeting noise arising at an engine intake.

[0013] The electric propulsion system 102 comprises a powertrain having at least one electric motor to drive the propeller 110 via the propeller shroud 114. Other than the electric motor, the powertrain is, at least partly, housed within the inner annular volume 106. Housed within the inner annular volume 106 is at least one electrical power source to power the at least one electric motor. In the example illustrated, a single electric motor 124 is provided. A electric motor 124 comprises a set of stator coils (not shown) that are arranged to cooperate with a set of rotor magnets (not shown). The propeller shroud is driven by the electric motor 124. The nacelle 104, as well as producing thrust, acts as structural housing for the powertrain. The nacelle 104 also functions as a safety shroud that protects the vehicle against blade failure.

[0014] The internal volume 106 of the nacelle 104 houses part of the powertrain for driving the propeller 110. Examples can be realised in which the powertrain comprises at least one or more than one of: at least one electric motor, at least one electrical power source to power the at least one electric motor, an engine control unit, at least one inverter, a power distribution system, a thermal management system, a fuel cell, taken jointly and severally in any and all permutations. While the electric motor 124 is housed in the central hub 116, other parts of the powertrain are housed within the interior annual volume 106 defined by the nacelle 104.

[0015] In the example depicted, the powertrain comprises at least one electrical power source 136. The at least one electrical power source 136 can comprise an energy storage system. The energy storage system can comprise at least one battery. The at least one electrical power source 136 is housed within the internal volume 106 of the nacelle 104. The at least one electrical power source 136 is arranged to supply power to one or more inverters 137 for driving the electric motor 124. Alternatively, the one or more inverters 137 can be housed within the interior volume 106 of the nacelle 104 with the drive signals being coupled to the electric motor 124 via cabling.

[0016] The nacelle 104 comprises a further internal volume 138. The further internal volume 138 is arranged to accommodate system, a fire protection system, a de-icing system, taken jointly and severally in any and all permutations.

[0017] The central hub 116 is coupled to the nacelle 104 via a number of load transfer members. Examples can be realised in which the number of load transfer members comprises a set of fore, or inlet, guide vanes 140. Examples can be realised additionally, or alternatively, in which the load transfer members comprise a set of aft, or outlet, guide vanes 142. The load transfer members are arranged to transfer thrust generated by the blades 112 to the rest of the electric propulsion system 102 and, ultimately, to a vehicle bearing such a propulsion system. The exit guide vanes can be used as control surfaces to enable the thrust to be vectored, which can be used to manoeuvre an associated vehicle such as an aircraft.

[0018] The aft portion of the nacelle 104 comprises at least one heat exchanger. In the example shown the aft portion of the nacelle 104 comprises sets of heat exchangers. A first set of heat exchangers 144 is disposed on an inward facing surface of the nacelle housing. The first set of heat exchangers 144 is arranged to couple heat from the powertrain to the air contained within the inner duct 104. Coupling heat from the powertrain to the air within the inner duct 108 causes that air to expand and, therefore, to contribute to generating thrust. The aft portion of the nacelle comprises a further set of heat exchangers 146. The further set of heat exchangers 146 is arranged to couple heat from the powertrain and vent it to atmosphere in order to manage the thermal conditions within the powertrain.

[0019] Any and all examples can be realised in which the heat exchangers 144 have surface features that supress noise associated any boundary layer of the downstream flow within the inner duct. The surface features can comprise undulations on the inwardly directed surfaces of the heater exchangers 114. Examples can be realised in which the undulations present a ribbed texture surface or airflow interface. The undulations can comprise microscale structures, such as, for example, dentils, arranged to create longitudinal grooves. The size and pitch of these grooves is a function of an expected local velocity and density of the medium flowing through the nacelle. Any or each of the examples described herein can be realised comprising such microstructures in the range of 100 to 400 pm in depth and 30 to 40 pm in width. Examples presenting such a textured or profiled surface realise boundary layer noise attenuation as well as local skin friction reduction. Examples can be realised in which such attenuation follows without incurring a specific heat exchange penalty.

[0020] A thermal management system 148 is provided to manage the thermal conditions associated with the powertrain. A common thermal management system is used to manage the thermal conditions associated with each element, or at least with selectable elements, of the powertrain. The thermal management system 148 uses liquid coolant, such as, for example, a dielectric fluid to transfer heat away from any such elements of the powertrain and to direct that heat to at least one, or both, of the first 144 and second 146 sets of heat exchangers. Having a common thermal management system 148 supports collecting or harvesting heat from one or more than one component of the electric propulsion system, in particular, the powertrain such as, for example, one or more than one of: any motor or motors, any battery or batteries, any power electronics such as any inverter or inverters taken jointly and severally in any and all permutations, which saves weight and complexity.

[0021] The thermal management system 148 can also direct heat generated by the powertrain to a set of heaters, or to at least one set of heaters. For example, the thermal management system 148 can direct heat generated by the powertrain to de-icing heaters 150 associated with the leading edge of a fore section of the nacelle 104. The de-icing heaters 150 are arranged to deice the nacelle. Examples can also be realised in which the thermal management system 148 can couple heat to at least one, or both, of the load transfer members 140 and/or 142, or the blades 112. The thermal management system 148 can couple heat from the powertrain to heaters or heat exchangers 152 and/or 154 in the inlet 140 and exit 142 guide vanes. Furthermore, the thermal management system 148 can couple heat from the powertrain to the central hub 116 via a respective heater or respective heat exchanger 158.

[0022] An effective way of realising lightweight motors for a given power is to increase the revolutions per minute and to reduce the torque since electric motors are sized according to torque. To introduce electric redundancy into the electric propulsion system 102, examples can be realised in the electric motor 124 is capable of spinning at a high RPM, that is, that are capable of spinning above a threshold level RPM. Examples can be realised in which the threshold level RPM would be a speed above 2,000 RPM and below 60,000 RPM. Examples can be realised in which the motors are arranged to generate propeller speeds of between 2,000 and 3,000 RPM. It can be appreciated that electric motor 124 is used to drive the central shaft 118 carrying the propeller shroud 114. Aerodynamic efficiency is realised by ensuring that the single propeller 110 operates at RPMs of above 2000.

[0023] As indicated above the interior volume 106 of the nacelle 104 can comprise a powertrain having a fuel cell, that is, can comprise a fuel cell powertrain. Several advantages follow from including a fuel cell powertrain in the interior of the nacelle 104. For example, electrical cabling within an aircraft needed by the propulsion system can be reduced thereby leaving space for hydrogen storage and distribution systems. Furthermore, plug and play propulsion, that is, electricity generation to thrust generation in a propulsor, supports quick replacement of the powertrain that, in turn, can reduce aircraft downtime for operators.

[0024] It can be appreciated that the load transfer members 140 and 142 are coupled to respective structures 156 and 158 for transferring load from the rotor propeller 114 to the inlet guide vanes 140 and exit guide vanes 142. The respective structures can comprise a central hub or boss 156, 158.

[0025] It will be appreciated that the motor, inverter and battery share at least one of: power management, thermal management, structural housing, and protection equipment, taken jointly and severally in any and all permutations, thereby leading to weight and efficiency gains. . Furthermore, as the motors are direct drive, the rotational speeds are relatively low. Normally low rotational speeds require a high torque and heavy electric motor because, in traditional systems, the motor has to have a large radius.

[0026] The electric propulsion system 102 is arranged so that the propeller 110 has a thrust coefficient, CT, of 0.15 CT 0.5, optionally, 0.2 Cr 0.4 where the thrust coefficient performance space is defined by: CT [0027] wherein [0028] T is the thrust generated by the propulsor, [0029] p is the fluid density of the target medium, [0030] Al is the actuator disc area formed by the plurality of rotor blades, and [0031] Umean is is the mean medium efflux velocity of the medium from the electric propulsion system 102.

[0032] Furthermore, the thrust coefficient, CT, is associated with an advance ratio, J, of the propeller 110 given by: vf of [0033] = -

T

Umean nD' [0034] wherein [0035] Vf is the freestream fluid velocity of the medium, [0036] n is the rotation speed of the propeller in revolutions per second, and [0037] D is the rotational diameter of the blades of the propeller.

[0038] It will be appreciated that the above parameters can be varied to accommodate thrust coefficients in the above ranges according to performance requirements. As non-limiting examples, the above parameters can take values within one or more than one of the following ranges taken jointly and severally in any and all permutations: [0039] 5 T 2000, [0040] 1.3kg/m3 p 0.3 kg/m3, depending on altitude performance, [0041] Tr. < d", where rm2 in is the minimum rotor radius and rtii." is the maximum rotor radius, which will be linked to diameter, D, below, [0042] Umean will be linked to Vf and will have a range that is dependent on Vf + 5, where 8 is sufficient to realise a target action-reaction effect, [0043] 8 ms-1 < Vf < 150 ms-1, [0044] 350 rpm *S n < 90,000 rpm, and [0045] 30 mm < D < 4000 mm.

[0046] However, there will be cases where the performance of any given parameter is outside of the above example ranges. In such cases, appropriate adjustments to the other parameters might be needed to achieve a target thrust coefficient within the above ranges, or within some other range.

[0047] Preferred examples of the electric propulsion system 102 have a designed flow coefficient, cp, of 0.75, and still work efficiently in a range of 0.6 < cp < 0.9 However, examples can be realised in which the designed flow coefficient is in the range of 0.45 < cp < 1.3. . [0048] Figure 2 shows a schematic view 200 of shrouded propeller 202 according to some examples for the above electric propulsion system 102. Therefore, the propeller 202 is an example of the above-described propeller 110, which can be driven from the hub (hub-drive) or optionally from the rim (rim-drive). The propeller 202 comprises a circumferential shroud 204. The circumferential shroud 204 comprises a radially inwardly directed surface 206. The radially inwardly directed surface 206 hosts a set of propeller blades 208. The set of propeller blades comprises a plurality of blades 210; only one of which has been labelled for clarity. In the example depicted, the set of propeller blades comprises nine propeller blades 210. Each propeller blade can optionally be removable and/or replaceable independently. However, examples can be realised in which the set of propeller blades comprises 5 to 25 blades.

[0049] The blades 210 of the set of propeller blades 208 are coupled to a central hub 212. The central hub 212 can be mounted to a central shaft (not shown) and can optionally comprise a set of bearings 214 for rotating the optionally rim-driven shrouded propeller 202 about an axis. [0050] The rim-driven shrouded propeller 202 has a radially outwardly directed surface 216. The radially outwardly directed surface 216 can bear a set of drive elements. The set of drive elements comprises at least one set of a plurality of circumferentially disposed drive elements. The at least one set of a plurality of circumferentially disposed drive elements can comprise a plurality of sets of circumferentially disposed drive elements. In the specific example depicted in figure 2, the rim-driven shrouded propeller 202 comprises two sets of circumferentially disposed drive elements 218 and 220. The drive elements can comprise at least permanent magnets. The permanent magnets are responsive to a time-varying magnetic field to rotate the rim-driven shrouded propeller 202 about a respective axis of rotation 222.

[0051] Referring to figure 3, there is shown a view 300 of a graph 302 showing the variation in thrust coefficient 304 with advance ratio 306 according to examples. The thrust coefficient 304 and the advance ratio 306 have been defined above. The graph 302 comprises several sets of performance data 308 to 316.

[0052] A first set 308 of performance data is associated with a flight speed of the electric propulsion system 102 of zero m/s. This demonstrates a high thrust coefficient under static airflow or flight speed conditions, that is, static thrust data. It should be noted that the first set of performance data is experimental data, whereas the second 310 to fifth 316 sets of performance data are theoretic performance data derived from simulations.

[0053] A second set 310 of performance data is associated with a flight speed of the electric propulsion system 102 of 7.14 m/s. It can be appreciated that the thrust coefficient varies from about 0.4 to -0.1.

[0054] A third set 312 of performance data is associated with a flight speed of the electric propulsion system 102 of 14.08 m/s. It can be appreciated that the thrust coefficient varies from about 0.3 to -0.7.

[0055] A fourth set 314 of performance data is associated with a flight speed of the electric propulsion system 102 of 21.60 m/s. It can be appreciated that the thrust coefficient varies from about 0.25 to -1.65.

[0056] A fifth set 316 of performance data is indicated on the graph 302 using the dashed box 316. It can be appreciated that there is a negligible advance ratio 306 but a corresponding thrust coefficient of between 0.3 and 0.4, which corresponds to the performance data for a static electric propulsion system, that is, a flight speed of zero m/s.

[0057] The performance of an electric propulsion system such as, for example, the above-described electric propulsion system 102, varies with a number of parameters of the electric propulsion system 102. A performance measure of interest comprises propeller efficiency. Examples can be realised in which a predetermined propeller efficiency is realised at a predetermined flow coefficient, p. Examples can be realised in which the predetermined flow coefficient, p, takes values 0.45 e co < 1.3. Examples can be realised in which the predetermined flow coefficient, cp, takes values 0.6 < p < 0.9. Preferred examples can be realised in which the predetermined flow coefficient, co, takes values co = 0.75.

[0058] Values or ranges of flow coefficients, such as those given above, provide significant flexibility in the design for target performance metrics such as, for example, acoustic performance.

Accordingly, the propeller 110 of the electric propulsion system 102 comprise an rotor blade count of between 5 and 25 blades inclusive. Preferred examples of the electric propulsion system 102 have a blade counts that will depend on performance such that preferred numbers of rotor blades will vary with application. Accordingly, examples can be realised in which 9 or 17 rotors can be used. However, it will be recognised that there might be applications or performance requirements that use a number or rotor blades that are outside of the range of 5 to 25 blades 112.

[0059] Performance metrics can be influenced by at least one, or both, of: the number of inlet guide vanes (IGVs) and outlet guide vanes (OGVs) such as the above described IGVs such as the above-described set of IGVs 140 and the set of OGVs 142. Examples can be realised in which the electric propulsion system comprises at least one, or both, of: a set of IGVs comprising a predetermined number of IGVs and a set of OGVs comprising a predetermined number of OGVs. Accordingly, examples can be realised in which the set of IGVs 140 comprises a predetermined number of IGVs. Examples can be realised in which the predetermined number of IGVs is in the range of 3 to 14 inclusive. Similarly, examples can be realised in which the set of OGVs 142 comprises a predetermined number of OGVs. Examples can be realised in which the predetermined number of OGVs is in the range of 10 to 50 inclusive.

[0060] Examples can be realised in which there is a predetermined relationship between at least one, or both, of: (a) the predetermined number of IGVs and the rotor blade count, and (b) the predetermined number of OGVs and the rotor blade count. Accordingly, examples can be realised in which there is a predetermined relationship between the predetermined number of IGVs and the rotor blade count. Examples can be realised in which there is a predetermined relationship between the predetermined number of OGVs and the rotor blade count. Examples can be realised in which there is a predetermined relationship between (a) the predetermined number of IGVs and the rotor blade count and (b) the predetermined number of OGVs and the rotor blade count.

[0061] Preferred examples can be realised in which the predetermined relationship between the predetermined number of IGVs and the rotor blade count is a predetermined ratio (IGV:Rotor blade count ratio). Examples can be realised in which that ratio is 1:2 such that the predetermined number of IGVs is half the rotor blade count. Although a predetermined ratio of 1:2 has been given, other ratios can be realised for the IGV:Rotor blade count ratio such as, for example, 0 to 0.95.

[0062] Alternatively, or additionally, preferred examples can be realised in which the predetermined relationship between the predetermined number of OGVs and the rotor blade count is a predetermined ratio (OGV:Rotor blade count ratio). Examples can be realised in which that ratio is 2:1 such that the predetermined number of OGVs is twice the rotor blade count.

Although a predetermined ratio of 2:1 has been given, other ratios can be realised for the OGV:Rotor blade count ratio such as, for example, 0.75 to 3.

[0063] Still further preferred examples can be realised in which: [0064] -the predetermined relationship between the predetermined number of IGVs and the rotor blade count (IGV:Rotor blade count ratio) is a predetermined ratio such as given above, and [0065] -the predetermined relationship between the predetermined number of OGVs and the rotor blade count (OGV: Rotor blade count ratio) is a predetermined ratio such as given above. [0066] Referring to figure 4, there is shown a view 400 of a graph 402 illustrating the variation of Total Pressure Rise Coefficient 404 with Flow Coefficient 406 according to examples. The Total Pressure Rise Coefficient (iyo) is defined as the difference in circumferentially averaged total pressure at rotor exit to that at the rotor inlet normalized with the dynamic pressure based on rotor tip speed. The graph 402 comprises three curves 408 to 412. Each curve 408 to 412 corresponds to a respective engine 414 to 418. The graphs relate to a specific configuration of IGV:Rotor blade count:OGV, which is 0:9:10, that is, zero IGVs, 9 rotor blades and 10 OGVs. In all instances the diameter of the rotor blades were 161.6 mm.

[0067] A first performance curve 408 corresponds to a first example 414 of electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total Pressure Rise Coefficient varies from a high of about 0.45 at a corresponding Flow Coefficient of 0.4 to a Total Pressure Rise Coefficient of about 0.19 at a corresponding Flow Coefficient of about 0.75.

[0068] A second performance curve 410 corresponds to a second example 416 of an electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total Pressure Rise Coefficient varies from a high of about 0.65 at a corresponding Flow Coefficient of 0.63 to a Total Pressure Rise Coefficient of about 0.35 at a corresponding Flow Coefficient of about 0.93.

[0069] A third performance curve 412 corresponds to a third example 418 of an electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total Pressure Rise Coefficient varies from a high of about 0.78 at a corresponding Flow Coefficient of 0.85 to a Total Pressure Rise Coefficient of about 0.525 at a corresponding Flow Coefficient of about 1.25.

[0070] Referring to figure 5, there is shown a view 500 a graph 502 showing the variation of Total Propeller Efficiency 504 with Flow Coefficient 506 according to examples.

[0071] A first performance curve 508 corresponds to a first example 514 of electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total propeller Efficiency varies from over a range of about 0.7 at a corresponding Flow Coefficient of 0.4 to a Total propeller Efficiency of about 0.65 at a corresponding Flow Coefficient of about 0.75.

It can be seen that operating the propeller over a flow coefficient range of about 0.5 to 0.68 would give a corresponding propeller efficiency of over 72%.

[0072] A second performance curve 510 corresponds to a second example 516 of an electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total propeller Efficiency varies over a range of about 0.755 at a corresponding Flow Coefficient of 0.63 to a Total propeller Efficiency of about 0.35 at a corresponding Flow Coefficient of about 0.93. It can be seen that operating the propeller over a flow coefficient range of about 0.66 to 0.825 would give a corresponding propeller efficiency of 80%+.

[0073] A third performance curve 512 corresponds to a third example 518 of an electric propulsion system 102 comprising 9 blades each of diameter 161.3 mm. It can be seen that the Total propeller Efficiency varies over a range of about 0.67 at a corresponding Flow Coefficient of 0.85 to a Total propeller Efficiency of about 0.7 at a corresponding Flow Coefficient of about 1.25. It can be seen that operating the propeller over a flow coefficient range of about 0.9 to 1.5 would give a corresponding propeller efficiency of 71%+.

[0074] Figure 6 depicts a view 600 of a graph 602 illustrating the variation of Thrust 604 with Flow Coefficient 606 according to examples as shown by the Thrust-Flow Coefficient curve 608. [0075] It can be appreciated that the thrust 604 generated by the electric propulsion system rapidly increases with flow coefficient 606 initially from a value of about 28N at a flow coefficient of 0.28 to 40 N at a flow coefficient of 0.48, followed by a slowly declining variation of thrust with flow coefficient from the 40 N at a flow coefficient of 0.48 to 25 N at a flow coefficient of 1.08, that is, in turn, followed by a rapid decline in thrust with flow coefficient from 25 N at a flow coefficient of 1.08 to almost 0 N at a flow coefficient of above 1.4.

[0076] The graph 602 shows a maximum operating point 610 at flow coefficient of 0.48 at which the electric propulsion system 102 generates about 40 N. [0077] The graph 602 also shows an operating point 612 at flow coefficient of 0.6 at which the electric propulsion system 102 generates about 38.3 N. [0078] A further operating point 614 is shown that corresponds to the beginning of the rapid decline from a thrust of 25 N at a flow coefficient of about 1.08.

[0079] It can be appreciated that operating with a flow coefficient of 0.45 < q < 1.3 provides thrust of between about 15 N and 40 N, optionally, operating with a flow coefficient of 0.6 < co 0.9, provides thrust of between about 38.3 N and about 30.8 N, operating with a preferred (I) = 0.75. provides thrust of 31.7 N. [0080] Although the propellers described herein are rim-driven shrouded propellers, examples are not limited thereto. Examples can be realised in which the propellers are hub-driven propellers. Furthermore, examples can be realised in which the propellers are open-tip propellers as opposed to being shrouded propellers. 11.

Claims

CLAIMS1. A propeller bearing a plurality of rotor blades to generate thrust when operable within a target medium; optionally, the plurality of rotor blades being mounted within a propeller shroud, wherein a. the propeller has a thrust coefficient, CT, of 0.15 CT 0.5, optionally, 0.2 CT 0.4 where the thrust coefficient performance space is defined by: pAiumeam.L CT -T.ii. wherein 1. T is the thrust generated by the fan, 2. p is the fluid density of the target medium, 3. is the area of the actuator disc formed by the plurality of rotor blades, and 4. Um"" is the mean medium efflux velocity of the medium from the electric propulsion system 102.
2. The propeller of claim 1, in which the thrust coefficient, CT, is associated with an advance ratio, J, of the propeller given by: J _ Vf = Vf mean flD' b. wherein i. 17f is the freestream fluid velocity of the medium, i. n is the rotation speed of the propeller in revolutions per second, and D is the rotational diameter of the rotor blades of the propeller.
3. The propeller of any preceding claim, comprising a respective flow coefficient, yo.
4. The electric propulsion system of claim 3, in which the respective flow coefficient, (p, takes values in the range: 0.45 < cp < 1.3, optionally, 0.6 < cp < 0.9, preferably, cp = 0.75.
5. The propeller of any preceding claim, in which the plurality of rotor blades comprises a predetermined number of rotor blade of between 5 and 25 blades inclusive.
6. An electric propulsion system; the system comprising a. a nacelle comprising an aerodynamic annular housing defining an internal annular volume and defining an inner duct, b. at least one propeller comprising a propeller as claimed in any preceding claim, and c. a powertrain comprising at least one, or both, of: an electric motor to drive the propeller and at least one electrical power source to power the electric motor. a.
IGV-Rotor-OGV Count and Ratios 7. An electric propulsion system comprising a nacelle; the nacelle housing: a. a propeller having a predetermined number of rotor blades, b. at least one, or both, of: i. a set of a predetermined number of inlet guide vanes and ii. a set of a predetermined number of outlet guide vanes; c. wherein at least one, or both, of: i. the predetermined number of inlet guide vanes has a predetermined relationship with the predetermined number of rotor blades, and ii. the predetermined number of outlet guide vanes has a predetermined relationship with the predetermined number of rotor blades.
8. The electric propulsion system of claim 7, in which predetermined relationship between the predetermined number of inlet guide vanes and the predetermined number of rotor blades is a predetermined IGV: Blade count ratio.
9. The electric propulsion system of claim 8, in which the predetermined IGV: Blade count ratio takes a value in the range of 0 to 0.95 and is preferably 0.5, that is, 1:2.
10. The electric propulsion system of any of claims 7 to 9, in which predetermined relationship between the predetermined number of outlet guide vanes and the predetermined number of rotor blades is a predetermined OGV: Blade count ratio.
11. The electric propulsion system of claim 10, in which the predetermined OGV:Blade count ratio takes a value in the range of 0.75 to 2 and is preferably 2, that is, 2:1.
12. The electric propulsion system of any of claims 7 to 11, in which the predetermined number of rotor blades is between 5 and 25.
13. The electric propulsion system of any of claims 7 to 12, having a thrust coefficient, CT, of 0.15 < CT 0.5, optionally, 0.2 e CT e 0.4, where the thrust coefficient performance space is defined by: CT = T, wherein pAiUmea,, a. T is the thrust generated by the propeller, b. p is the fluid density of the target medium, c. Al is the area of the actuator disc formed by the plurality of rotor blades, and d. U,"a" is the mean medium efflux velocity of the medium from the electric propulsion system 102.
14. The electric propulsion system of any of claims 7 to 14, in which the thrust coefficient, CT, is associated with an advance ratio, J, of the propeller by: J = - wherein Untecm 70 ' a. Vf is the freestream fluid velocity of the medium, b. n is the rotation speed of the propeller in revolutions per second, and c. D is the rotational diameter of the rotor blades of the plurality of rotor blades.
15. The electric propulsion system of any of claims 7 to 14, comprising a flow coefficient, cp, of 0.45 < W < 1.3, optionally, 0.6 < W < 0.9, preferably, yo = 0.75.