Electric ducted fan

An electric ducted fan for an aircraft is shown. A nacelle defines a duct that houses a propulsive fan having a fan diameter DF. An electric machine is configured to drive the fan, and has an electromagnetically active length LA and an electromagnetically active diameter DA defining an aspect ratio (LA/DA) of from 0.8 to 2. A ratio of the electromagnetically active diameter DA to the fan diameter DF (DA/DF) is from 0.45 to 0.60.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from British Application No. 1807769.3 filed on May 14, 2018 the entire contents of which are incorporated by reference.

BACKGROUND

Technical Field

This disclosure relates to configurations of electric ducted fans for aircraft.

Description of the Related Art

Ducted fan propulsors are used for a large proportion of aircraft due to their greater efficiency and reduced noise due to lower tip losses than open propellers. Turbofans utilise a gas turbine core to drive a fan, the fan being larger than the core to produce a bypass flow responsible for the majority of thrust.

Despite great advances in materials, compressor and turbine aerodynamics, and combustion efficiency, the gas turbine engines used in the cores of turbofans are still quite thermally inefficient. In particular, gas turbines are less efficient the smaller they are, which restricts possibilities in terms of the number of engines that may be installed on an airframe. Their fuel source is also not renewable. Further, the jet of high enthalpy exhaust from the core is responsible for a large amount of noise when it mixes with the exhausted bypass flow.

It is therefore desirable to utilise electric machines in place of gas turbine engines in ducted fan arrangements to alleviate some or all of the aforesaid issues.

SUMMARY

The present disclosure is directed towards electric ducted fans for aircraft.

One such electric ducted fan comprises a nacelle defining a duct, and a propulsive fan in the duct having a fan diameter DF. An electric machine is coupled to the fan. The electric machine has an electromagnetically active length LAand an electromagnetically active diameter DAdefining an aspect ratio (LA/DA) of from 0.8 to 2. A ratio of the electromagnetically active diameter DAto the fan diameter DF(DAto DF) is from 0.45 to 0.60.

Electric ducted fans constructed in such a manner have various advantages, such as reduced nacelle drag, a shorter pylon, and increased ground clearance.

DETAILED DESCRIPTION

An electric ducted fan propulsor is shown inFIG. 1.

The propulsor is shown generally at101, attached to a wing102of an aircraft (not shown) by a pylon103.

Being a ducted fan, the propulsor101comprises a nacelle104which defines a duct105having an inlet106and a nozzle107, and in which a propulsive fan108is located. In operation, the fan108raises the pressure of intake air, with swirl in the airflow being removed by outlet guide vanes109. The airflow is directed through the nozzle107to generate thrust. In the embodiment ofFIG. 1, the fan108is driven by an electric machine110.

In the present embodiment, the electric machine110is rated at a maximum continuous power of between 100 kilowatts and 100 megawatts. In a specific embodiment, the electric machine110is rated at a maximum continuous power of between 1 megawatt and 10 megawatts. In a more specific embodiment, the electric machine110is rated at 2 megawatts maximum continuous.

Thus, as illustrated inFIG. 2, the arrangement ofFIG. 1permits a combination of a fan having a high hub-tip ratio, and a high aspect ratio electric machine, in terms of its length-to-diameter.

The hub-tip ratio of the fan108in the present example is the ratio of the diameter DFof the leading edge of the fan blades201, to the diameter of the diameter DHof the hub202at the leading edge of the fan blades201, i.e. the diameter of the inner gas-washed surface of the fan108. It will be appreciated that the hub-tip ratio is a standard, well known property of a fan, compressor, or turbine stage.

The length of the electric machine as defined herein is the maximum length LAof the electromagnetically active components, whilst the diameter of the electric machine as defined herein is the maximum diameter DAof the electromagnetically active components. Examples of the definitions of length LAand diameter DAas applied to specific machine types will be described further with reference toFIG. 4.

Benefits are obtained by specifying the following parameters for the propulsor101:

(i) the aspect ratio of electric machine (110LA/DA, i.e. the value of LAdivided by DA) being from 0.8 to 2;

(ii) the ratio of the diameters of the electromagnetically active components in the electric machine and the fan (DA/DF, i.e. the value of DAdivided by DF) being from 0.45 to 0.60.

In particular, the inventor has discovered that this combination of values advantageously enables the propulsor101to have a smaller diameter nozzle107for the same overall fan pressure ratio, which allows the pylon103to be made shorter. This results in, for a particular fan diameter DF, greater ground clearance, or a greater fan diameter DFfor a particular ground clearance. Further, the reduction in pylon length and height results in a reduced moment on the wing102, and a reduction in weight.

In a specific embodiment, the ratio LA/DAis from 1.1 to 1.7. The inventor has discovered that this allows a narrowed nozzle, along with a higher speed machine. In another specific embodiment, LA/DAis from 1.3 to 1.5. In a more specific embodiment, LA/DAis 1.4. In an additional or an alternative embodiment, the ratio DA/DFis from 0.50 to 0.55. In a more specific embodiment, the ratio DA/DFmay be 0.525. The hub-tip ratio of the fan108, i.e. the value of DT/DH, may be from 0.24 to 0.32. In a specific embodiment, the hub-tip ratio may be from 0.26 to 0.3. In a more specific embodiment, the hub-tip ratio may be 0.28.

In an embodiment, the fan108has a tip pressure ratio (i.e. the ratio of the stagnation pressure immediately upstream of the tip of a fan blade201, and immediately downstream of the fan blade201) of from 1.3 to 1.7 at an altitude of 35,000 feet above sea level and a temperature of minus 54 degrees Celsius (i.e. ISA+0 standard conditions), and a true airspeed of Mach 0.85, i.e. during cruise conditions. In a specific embodiment, the tip pressure ratio may be from 1.4 to 1.6 in the aforesaid conditions. In a more specific embodiment, the tip pressure ratio may be 1.5 in the aforesaid conditions.

An example of a configuration of the electric machine101is shown in cross-section through its central axis A-A inFIG. 3. This particular configuration is a radial flux electric machine301, which comprises a stator302which surrounds a rotor303. The stator302comprises a lamination stack of the known type. In the present example, the electric machine is a permanent-magnet machine and thus the rotor303comprises permanent magnets, which interact with the magnetic field generated by windings in the stator302to generate torque. Alternative machine types such as induction machines may also be employed. As a radial flux machine, end windings304emerge axially from the lamination stack of the stator302.

As described previously, the electric machine301may be described as having an electromagnetically active length LAand an electromagnetically active diameter DA. As used herein, “electromagnetically active” refers to the region responsible for generating torque upon the rotor. Thus in the present example, the length LAis the length of the lamination stack of the stator302, and does not include the end windings304as they do not generate an appreciable torque upon the rotor303. Similarly, the diameter DAis the diameter of the lamination stack of the stator302.

Put another way, the “electromagnetically active” parts of the electric machine301may be characterised as those components which form part of the torque-generating magnetic circuit in the machine.

Those skilled in the art will appreciate how this definition may read across to other machine types such electromagnet-based synchronous motors, and axial flux machines, with the definitions of aspect ratio as set out herein also applying to these types of machine.

Another example configuration of the electric machine101is shown in cross-section through its central axis A-A inFIG. 4. This particular configuration is a radial flux electric machine401, which comprises a rotor402which surrounds a stator403. As with electric machine301, the electromagnetically active length LAof this machine is the length of the lamination stack of the stator403, not including the end windings404. The electromagnetically active diameter DAis, however, in this embodiment, the diameter of the rotor402. Again, machine401is a permanent-magnet machine, and thus the diameter DAis bounded by the greatest radial extent of the magnets therein. Should the machine instead be an induction machine, the diameter DAwould be defined by the greatest radial extent of the rotor iron.

More generally, as described with reference toFIG. 3, it will be appreciated the “electromagnetically active” parts of the electric machine401may be characterised as those components which form part of the torque-generating magnetic circuit in the machine.

Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein.