The present invention relates to propulsion systems for aircraft and in particular to an electric propulsion system (EPS) for aircraft, and to aircraft using such a system.

There is at present increasing interest in providing aircraft with electrical propulsion systems. Many flying vehicles already use electric power; in particular, it has found application in multirotor vehicles such as quadcopters, unmanned aerial vehicles (UAVs), and drones, amongst others. There is also a demand for electrical propulsion systems for larger aircraft, with the aim of reducing environmental harm arising from emissions from fossil-fuelled aircraft

A feature that is common to existing designs of EPS is that a thrust fan (such as a propeller or a helicopter rotor) is directly connected to an output shaft of an electric motor. That means that the fan rotates at the same speed as internal rotating components of the motor. There are many reasons why this arrangement is an attractive one: it uses a minimal number of components (so minimising weight), it is easy to manufacture, it is reliable, and the design of such systems is well-understood. On the face of it, therefore, a direct drive (without gearbox) EPS provides a good solution for the problem of providing electric propulsion for an aircraft

The present applicants have realised that the use of a direct drive EPS may not be optimal in some cases. The optimal speed of rotation of a fan is typically low in comparison with the optimal speed of an electric motor. For example, a fan may optimally operate at around 1 000 rpm, while an electric motor may operate at speeds of up to 30 000 rpm. Operating a motor at such speeds can result in a significant saving in size and weight, when compared with a motor operating at round 1 000 rpm.

From a first aspect, this invention provides an electric propulsion system for an aircraft comprising: a. an electric motor assembly that includes an electric motor being a prime mover for the system, the electric motor assembly having a rotary output; b. a gearbox assembly that includes a gear train having an input connected for rotation with the output of the electric motor assembly and a rotary output that can be connected, for use, to cause a rotordynamic thrust system to rotate; wherein c. the gear train is a reduction gear train that causes the system output to rotate at a speed less than that of the electric motor.

The motor in a system embodying the invention operates at a higher speed than would be possible in a direct drive system, which can allow the use of a motor that is smaller, lighter and/or is more power dense than the direct drive equivalent

The gear train typically has a reduction ratio in the range 3.5:1 to 216:1 and more typically in the range 12;1 to 36:1. As an example, this allows a rotordynamic thrust system connected to the EPS output to be driven between 500 and 2,000 rpm while the motor operates in the range 10 000 to 40 000 rpm.

The gear train typically includes a planetary gearset Most typically, it includes two or more planetary gearsets arranged in series to provide a sequential ratio reduction. This has the advantage of providing a compact and efficient reduction gear train. In such embodiments, the input of the gearbox assembly may be coaxial with the output of the gearbox assembly.

In an alternative configuration, the gear train may alternatively or additionally include a cylindrical gearset The gear train is, typically, contained within a gearbox housing, formed most usually of metal. The gearbox housing may contain a suitable liquid, such most usually a gear oil, to lubricate and cool the gear train. The gearbox assembly most advantageously includes a cooling arrangement that serves to remove heat from within the housing. The cooling arrangement may include a heat exchanger arrangement, such as external fins formed on the gearbox housing. The cooling arrangement may further include components that interact with the heat exchanger to promote removal of heat from the gearbox housing, those components including, for example, an air duct that surrounds at least part of the heat exchanger arrangement and a fan that drives air through the air duct. In a preferred embodiment the airflow is provided from a mixed flow or centrifugal fan mounted on the motor. The fan may also be an axial-flow fan that is mounted on the output of the gearbox assembly. In addition, the gearbox assembly may include a pump that circulates gear oil within the gearbox to move the gear oil between the gear train and the heat exchanger.

The electric motor assembly typically includes drive circuitry that is operative to receive electric power from an electrical system of an aircraft and to supply a drive current to the electric motor. The drive circuitry preferably includes silicon carbide MOSFET (or other suitably low loss devices e.g. Gallium Nitride) switches to deliver power to motor phase windings.

The electric motor assembly further includes a cooling arrangement that is operative to maintain components of the electric motor assembly within an operating temperature range. The cooling arrangement may include a heat exchanger that operates to remove heat from the drive circuitry, for example, including a heatsink that is in contact with an active component of the drive circuitry. The cooling arrangement may further include components that interact with a heat exchanger to promote removal of heat from the heat exchanger, those components including, for example, an air duct that surrounds at least part of the heat exchanger and a fan that drives air through the air duct In a preferred embodiment, the fan is a mixed flow fan that is carried on the input of the electric motor assembly.

The motor typically has 20 poles or fewer. Most typically, it may have 8, 6, 4 or 2 poles. In some embodiments of the invention, each of the gearbox assembly, converter assembly and the electric motor assembly each include individual cooling fans. Alternatively, a single fan may be provided that provides cooling air to both the gearbox assembly, converter assembly and the electric motor assembly; in such examples, the air ducts of the gearbox assembly and the electric motor assembly are typically interconnected such that air can flow from one into the other.

The electric motor assembly may include more than one electric motor, each having a rotary output. Typically, the output of each electric motor is connected to a respective input of the gearbox assembly. Alternatively, the output of each electric motor may be connected to a common input of the gearbox assembly.

From a second aspect, this invention provides an aircraft propulsion system that incorporates an electric propulsion system embodying the first aspect of the invention and a thrustgenerator connected to the output of the gearbox assembly.

The rotordynamic thrust system may be one of: a ducted fan, a propeller or a rotor.

From a third aspect, this invention provides an aircraft that includes one or more propulsion systems each embodying the second aspect of the invention.

An aircraft embodying this aspect of the invention may typically be a rotary-wing aircraft in which the thrust generator driven by at least one propulsion system may be a rotor. Such an aircraft may have one, two, three, four or more rotors.

Alternatively, an aircraft embodying this aspect of the invention may be a fixed-wing aircraft, in which at least one rotordynamic thrust system may be a propeller.

Typical embodiments of this aspect of the invention are drones (unmanned aerial vehicles (UAVs)), which are capable of flight without a human pilot on board. Further embodiments may be capable of carrying one or more person and may optionally be piloted. Embodiments may be capable of carrying a payload such as telemetric or monitoring equipment and/or an object to be delivered to and left at a specific location. An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which:

Figures 1 and 2 are external views of an electric propulsion system for an aircraft;

Figures 3 and 4 are cross-sectional views of the embodiment of Figures 1 and 2; Figure 5 is an end view of a gearbox assembly of the embodiment of Figures 1 and 2;

Figure 5 is a sectional view of a gearbox assembly of the embodiment of Figures 1 and 2;

Figure 6 is an end view of a motor assembly of the embodiment of Figures 1 and 2;

Figure 7 shows the power electronic portion of the converter integrated with the motor assembly of Figure 6; Figure 8 shows a gearbox assembly of the embodiment of Figures 1 and 2;

Figure 9 shows the gearbox assembly of Figure 6 with an outer casing removed;

Figure 10 shows an embodiment of the invention in which cooling of the motor assembly and converter assembly takes place in parallel, and subsequently in series with the gearbox.

Figure 11 is a cross-sectional view of the embodiment of Figure 10; Figure 12 shows an embodiment of the invention in which cooling of the motor assembly and gearbox assembly takes place in series;

Figure 13 is a cross-sectional view of the embodiment of Figure 12; and

Figure 14 is a sectional view of a second embodiment of the invention. With reference to the figures, an electric propulsion system for an aircraft comprises a motor assembly 10 and a gearbox assembly 12. The motor assembly 10 and a gearbox assembly 12 are arranged along a common machine axis A.

The motor assembly 10 includes an electric motor. In this embodiment, the electric motor is a permanent magnet machine with a rotor 22 surrounded by stator windings 24. The rotor 22 is carried on a motor shaft 30. The rotor 22 is fixed for rotation with the motor shaft 30, which is carried on rolling element bearings 32, 34 such that it is free to rotate about the machine axis A. The motor is contained within a tubular housing 36, the housing having an air inlet opening 38 at one end centred on the machine axis A.

In this embodiment, the motor is intended to operate at speeds of up to approximately 35,000 rpm and has 4 to 8 poles. It is an optimised low-loss machine, with as high pole count as possible, high slot count, high strength magnets and low loss laminations.

A cooling fan 40 is carried on and fixed for rotation with the motor shaft 30 such that it is located adjacent to the air inlet opening 38. The cooling fan 40 is disposed such that, during operation of the motor, it drives air into the tubular housing 36 through the air inlet opening 38. The cooling fan 40 rotates at the same speed as the motor so must be capable of operating efficiently at speeds ofup to 25 000 rpm. To provide a compact and lightweight cooling fan 40, the fan is single-stage high flow rate, high pressure drop, mixed-flow fan. (Multi stage axial or centrifugal fans may be a less-efficient alternative.)

A power controller frame, shown in Figure 6, is carried on the tubular housing 36. The power controller frame is formed from thermally-conductive metal and includes a heat sink region 44 that has flat outer mounting surfaces 46 (three, in this example) from which fins 48 project radially inwardly. The mounting surfaces 46 are disposed circumferentially around and tangentially to an outer surface of the tubular housing 36. The power controller frame also includes securing bands 47 that surround the tubular housing 36 to secure the power controller frame in place.

The mounting surfaces 46 each carry a respective silicon carbide power MOSFET switches 50.

The MOSFET switches 50 are in close contact with the mounting surfaces 46 such that heat generated by the MOSFET switches 50 is conducted through the power controller frame to the fins 48 from which it is dissipated into air flowing over the fins 48. Spacer studs 52 extend from the periphery of each mounting surface 46, the spacer studs of each mounting surface supporting a respective switching PCB 54 radially outwardly of each MOSFET switch 50. Each switching PCB 54 contains the switching circuitry necessary to control the adjacent MOSFET switch 50. Also carried on the power controller frame is a busbar 56 of laminated copper conductors that delivers power to the MOSFET switches 50, which operate to supply power to phase windings of the electric motor under control of the circuitry on the switching PCBs 54. Also carried on the power controller frame is a control PCB 57 that controls operation of the switching circuitry on the switching PCBs 54.

The gearbox assembly 12 has a gearbox 14 which has an input shaft 58 that is connected to the motor shaft 30. Within the gearbox assembly 12, there are two planetary gearsets: an input gearset 60 and an output gearset 62, each having an annulus, a planet carrier that carries several planet gears and a sun wheel. The input gearset 60 and the output gearset 62 are connected in series, each providing a reduction ratio between its input and its output. The annulus 70 of the input gearset 60 is driven by the input shaft 58. An intermediate shaft 64 is driven by the planet carrier 72 of the input gearset 60 and drives the sun wheel 74 of the output gearset 62. The output gearset 62 drives an output shaft 66 of the gearbox assembly 12 which is connected to the planet carrier 76 of the output gearset The input shaft 58, intermediate shaft 64 and the output shaft 66 are all carried on rolling element bearings 78 within the gearbox.

Components of the gearbox 14 are contained within a structural metal case 80 that is bolted to the stator of the motor to form a single integrated unit. A cylindrical outer wall of the case 80 is formed with radial fins 82, formed of aluminium in this example. A plurality of mounting lugs 84 extend from the case 80. The case 80 is contained within an outer casing. The outer casing has a cylindrical section 90 that closely surrounds the fins 82 of the case 80. A section of the outer casing of reduced diameter 92 surrounds the part of the output shaft 66 that projects from the case 80 and ends in an open-air outlet in those embodiments in which the gearbox assembly is cooled in series with the motor and converter unit. (In embodiments in which the gearbox assembly has its own fan, this is an air inlet 94 for the gearbox assembly. Within the section of reduced diameter 92 adjacent to the air inlet 94 a fan 96 is carried on the output shaft 66. The cylindrical part 90 of the outer casing is open at its end opposite to that of the air outlet such that rotation of the fan causes air to be drawn into it, over the fins 82, and then exiting the casing through the air outlet 94.

The gearbox case 80 contains gear oil that serves to lubricate and cool the gearsets 60, 62 and other internal components of the gearbox. The gear oil transfers heat from the components of the gearbox to the fins 82 from which heat is removed by the airflow generated by the fan 96.

In this example, the input gearset 60 has a ratio of 4.8:1 and the output gearset 63 has a ratio of 4.4:1, giving a total reduction ratio of 21:1.

In an alternative configuration, air may also be fed from the cooling fan 40 of the motor assembly 10 and power electronics, to provide airflow over the fins 82 to cool the gearbox assembly 12 either to suppiementor to replace the airflow caused by the fan 96. This air flow may require additional cooling following exit from the power electronics by using an intercooler. In these embodiments, the air flow from the single fan 40 may cool the motor assembly 10 and the gearbox assembly 12 in parallel or in series.

Figures 10 and 11 show an arrangement in which cooling of the motor assembly 10 and the gearbox assembly 12 occur in series. Air from the fan 40 is divided into inner and outer coaxial flow paths. The outer path, shown in black arrows in Figure 11, passes through the heat sink 44 that is disposed to cool drive circuitry within the motor assembly, and is then directed to the gearbox. The inner path, shown in white arrows in Figure 11, passes through the motor assembly 10, combines with air from the electronics heat sink and into the case 80 of the gearbox assembly 12. Within the case 80, the air passes over cooling elements (a heat sink with fins 82, in this embodiment) before being exhausted to atmosphere.

Figures 12 and 13 show an arrangement in which cooling of the motor assembly 10 and the gearbox assembly 12 occur in series. The flow of air from the cooling fan 40 follows the arrows in Figure 13. From the fan, the air passes through the motor 10, and then passes back through the heat sink 44 surrounding the motor. Air leaving the heat sink 44 is received in a collecting chamber 90 and is carried from the receiving chamber 130 through a duct 132. The duct 132 conveys the air to an annular receiver 134 that surrounds an end portion of the heat sink 82 of the gearbox assembly 12. From the receiver 134, the air passes through the heat sink 82 and from there is exhausted to atmosphere.

In a further alternative arrangement shown in Figure 14, the gearbox is predominantly liquid cooled. In this embodiment, the gearbox assembly 112 incorporates a mechanical oil pump 115 driven by the output shaft 166. The pump 115 circulates oil from the gearbox to a radiator 117. In this embodiment, the radiator 117 is positioned around the perimeter of the gearbox case 180 and force air cooled with a mechanical fan 196 on the output shaft 166 of the gearbox or in series with the motor and converter.

Alternatively or additionally, a radiator 117 positioned around the perimeter of the gearbox case 180 may be cooled by the propeller or rotor attached to the output shaft: typically, rotor downwash in VTOL applications or forward flight air flow in fixed wing applications.

A radiator may alternatively be positioned behind an air intake scoop and be cooled by airflow generated by an electric fan. In such an arrangement, the fan may be required to operate when there is no significant forward flight movement to generate a flow of air, such as when hovering or on ground.

The pump 115 may also circulate oil through pipes 145 to a heat sink 146 associated with the motor assembly 110 to supplement or to replace air cooling of the motor.

An electrical oil pump may alternatively or additionally be provided. This allows coolant flow to be maintained when the motor is not running.

The motor used in embodiments of the invention are typically high-pole machines (high for the rotational speed, e.g., 6 to 8 poles) which allows low motor mass and results in a high fundamental frequency. The high fundamental frequency is mitigated by a high switching frequency. The high switching frequency is achieved by use of SiC transistors and high-speed software in the control and drive circuitry. SiC transistors also permit a high switching frequency and allow low losses. Either or both cooling fans 40, 96 may be implemented as a high-pressure-drop mixed-flow mechanical fan, which allows combined cooling of the entire motor assembly 10. The high pressure drop allows the mass of cooled heat sinks to be minimised. Effective cooling maximises reliability of mechanical and electrical systems. Embodiments may have many alternative configurations of motor (pole count, operating speed, etc.) and gearboxes (single stage, multistage, epicyclic, spur, etc.,) amongst many other options.

Scalability may allow embodiments to be provided that develop between 6 kW and 500 kW. At some point supplementary liquid cooling may be required, however; its requirement will be minimised by using this architecture. In general, smaller machines may have higher operating speeds and larger machines lower speeds, within the range of the invention.