BACKGROUND TO THE INVENTION

Technical Field

The present invention relates to the field of electric and electric- hybrid aviation and in particular to the design of electric ducted fans.

1. Description of the Prior Art

Historically electric ducted fans (EDFs) started to appear with the invention of brushless DC motors; nowadays they are commonplace in model aviation. In addition larger ducted fans are now starting to be used in commercial aviation such as in Airbus's E-FAN project.

Most electric ducted fans currently on the market today are built for linear flight. There are few fans optimized for vertical take-off and landing flight (VTOL).

If one defines the effective power loading of an EDF as (the thrust produced - weight of fan including motor and control electronics)/electrical power consumed, then virtually all EDFs today are characterized by values less than 1.5g/W.

This is to be contrasted with free propeller propulsion units employed in multi-copters where typical effective power loadings are in the region 5-10g/W.

For VTOL flight, the flight duration of a wingless drone powered by a plurality of EDF units without payload is directly proportional to the average effective power loading of the EDFs employed given simple assumptions concerning the drone structure weight. Wingless drones based on EDFs rather than free propellers are often to be preferred for a variety of reasons including their smaller size and their much-improved safety afforded by the absence of free blades.

However current EDFs all have too low an effective power loading to support useful flight times. In addition the best current EDFs known (with the highest values of effective power loading) are either too large or suffer from thermal overheating. They also lack real-time engine health monitoring, have no deicing facilities and are critically sensitive to insect ingress and rain.

Most of these criticisms may also be applied to EDFs designed for linear flight. Specifically, many such commercially available fans suffer from overheating problems due to the necessity of keeping to a small hub size in which a high power, low weight motor is installed. Often such overheating problems limit the possible flight time dramatically. To compound this problem very few commercial fans offer real-time motor temperature telemetry, something which might be regarded as almost essential in the presence of overheating. In addition sensitivity to water and insect ingress is a major issue for the lighter weight ducted fans where heat dissipation is often by direct air cooling of the motor coils.

The present invention therefore describes an improved type of EDF which can be designed to have either a large, medium or small effective power loading, is intrinsically thermally cool, is compact, has inbuilt comprehensive remote engine health monitoring, intrinsically provides deicing and is also rain and insect resistant.

2. BRIEF SUMMARY OF THE INVENTION

The present invention describes an electric ducted fan (EDF) comprising a rotor and a stator within a duct for the production of thrust. The central part of the motor comprises: a centrifugal fan mounted directly in front of the rotor for recirculation of hot post-rotor air, a cylindrical rotor hub, a cylindrical stator hub and a tapering compressive tail cone.

The rotor is driven by a single brushless electric motor. The base of this motor is mounted within the cylindrical stator hub in the centre of the EDF.

Stator blades radiate from the stator hub out to the duct wall thereby securing the stator hub to the duct. The front end of the motor is mounted into the cylindrical rotor hub such that the rotor blades broadly straddle the two bearings of the motor.

Air enters the fan and is compressed and accelerated by the rotor. The induced swirl is greatly diminished by the action of the stator blades. Thrust is generated by the duct, the rotor and the stator.

A small proportion of the accelerated air is redirected into the base of the motor via a compressive tail cone and is pumped back through the centre of the motor (flowing directly over the motor coils) by the centrifugal fan mounted in front of the main rotor; this fan acts to depressurise the air above the motor coils and under the front cap so dragging back air towards the main inlet of the duct where it is expelled into the main flow.

In this design the motor is very effectively cooled by the recirculated air and hot air is also injected into the EDF inlet, heating the blades and preventing icing.

The electronic speed controller (ESC) required to run the motor is mounted within the tail-cone of the EDF. Here too are mounted an infrared sensor to measure the rear axle temperature of the motor, a microcontroller with Wi-Fi (e.g. ESP-32), a voltage regulator for said micro-controller, an accelerometer, a voltage and current sensor and an integrated RPM sensor. This electronics package is cooled by the air recirculated by the compressive tail-cone.

Electrical connection is through two power wires and two signal wires, all of which are routed through a stator blade and are accessible from within a water resistant connector system mounted into the duct which also incorporates mechanical attachment points permitting an external structure to securely hold the EDF.

Insect, particle and rain ingestion is not possible through the front of the EDF as high velocity air is expelled through the centrifugal fan thereby insuring that insects, water and other debris do not enter and fowl the motor.

The rotor hub/stator hub junction is usually engineered in the present invention to be angled in a reverse direction to the main flow and the diagonal gap between the two hubs is kept to an absolute minimum. The compressive tail cone then insures that the pressure difference across the stator hub - rotor hub interface is either small or in such a direction that air is expelled from this interface into the main flow. As such ingress of rain, particles or insects cannot occur at the stator hub-rotor hub interface. A person skilled in the art will understand that there are many similar methods of insuring absence of particulate and water ingress at this junction including for example arranging for a larger rotor hub to overlap a smaller stator hub...

Ingress of particles, rain and insects can however occur at the compression inlet(s) of the compressive tail cone. Here however one or more small outlets towards the rear of the cone (less preferably a small single aperture at the extreme end) can be used to act to create a partial exit flow of the heavier entrained particles such as water droplets and insects; as all such heavy entrained matter has considerable inertia in the main flow direction and cannot simply reverse direction and flow back towards the motor. Various combinations of vents and mesh filters are then used to evacuate any rain entering the tail cone and to either evacuate or trap insects and particles ingested; in the latter case these can then be removed at a later stage by cleaning and changing of said mesh filters.

In summary, the design philosophy of the current invention is to actively create a reverse axial airflow in the core of the EDF. This flow can then be used to effectively and very efficiently cool a brushless DC electric motor by direct flow of cool air over the motor coils. The motor heat is then also used usefully as a de-icing system rather than being radiated away as is usually the case with prior-art EDF designs. In addition the reverse flow makes it much easier to prevent particulate and water ingress into the motor and indeed allows for the only such place of ingress to effectively be at the rear of the EDF downstream of the electronics package and in a location where inertial and physical filtering can easily deal with such ingress in a safe way.

The majority of the components used to produce the current invention lend themselves extremely well to production by FDM/FFF printing using preferably polycarbonate. The exception is the rotor blades, which are best produced using injection moulding as glass-reinforced plastic or by using carbon fibre fabrication techniques. The duct is particularly important, as it must be strong and yet very light. FDM and other additive manufacturing techniques are of great use here.

3.0 BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l Electric Ducted fan described in the first embodiment of the invention. The fan is composed of a duct (1) with inlet lip (6); a rotor hub (2) in which are mounted seven rotor blades (e.g. 4); a stator hub (3) attached to the duct by 5 stator blades (e.g. 5); a compressive cone(7); a centrifugal fan with cap (10) comprising 18 impeller blades (e.g.

11); two electromechanical ports (12) incorporating M4 hex attachment points (13) an RC connector plug (15) and an XJ-60 high current socket (14); a T-Motor BDLC motor MN4014-9 kv 400 (not shown - see Fig.2) mounted to the interior of both rotor and stator hubs; an electronic speed controller T-motor HV-60 (ESC - not shown) to drive the brushless DC motor; and an electronics diagnostics package (not shown) to measure and transmit by Wi-Fi the motor temperature, motor RPM, motor voltage, motor current and fan vibration characterisation.

Fig.2. Electric Ducted fan described in the first embodiment of the invention showing main air flow and reverse cooling flow. Air enters at the front of the duct and is accelerated by the rotor, stator and duct, thereby creating thrust, to exit at the rear (e.g. streamlines 21,22). A small proportion of the main accelerated flow enters the compressive tail-cone at inlet(s) 25/26, thereby pressurizing the interior of the cone and stator hub (27) and in particular the region immediately below the motor (28). Low pressure is simultaneously arranged at the top of the motor (28) via action of the centrifugal fan (Figl./lO with Blades Figl./ll). In this way a pressure difference is sustained across the motor driving flow from the base of the motor to the top where it is expelled by the centrifugal fan (e.g. streamlines 29,30), thereby cooling said motor.

Fig.3. Electric Ducted fan described in the first embodiment of the invention showing detail of the blade design used in the centrifugal fan. 18 blades (e.g. 32) form a centrifugal fan expelling air sideways. Air from the main flow is compressed and enters the compressive cone 35 creating a reverse axial flow through the stator 34 and Rotor/Motor assembly 31, before being expelled aby the centrifugal fan blades 32 into the primary airstream.

4.0 DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENT

Figs 1, 2 and 3 illustrate the preferred embodiment of the invention. The EDF described here has been designed for VTOL applications as is characterised by a value of effective power loading which exceeds 3g/W. Someone skilled in the art will however understand that the EDF could be designed instead to be more suitable for linear flight by designing the blades and shroud to give a lower effective power loading and more dynamic thrust. All other design elements would remain then the same. Most of the major mechanical components of the EDF were 3D printed using Ultimaker 2+ machines and a Raise3D Pro2 machine. All 3D printed parts were made from PolyPC polycarbonate material which confers high mechanical strength and good temperature characteristics. Semi-automated abrasive post processing was used to improve surface quality to that required for aeronautical applications. Rotor and Stator Blades were designed using analytic Euler Theory using a Vortex free swirl distribution and cascade formulae after which optimisation was applied from CFD analysis. The initial design point was for 700W shaft power/ 9000 RPM / 35N Thrust at STP with a hub diameter of 60mm and an inner shroud diameter of 180mm. The outer shroud diameter was 220mm. The 7 rotor blades were injection moulded using Polycarbonate incorporating 20% glass fibre and were post-processed to an SPI A3 finish. The 5 stator blades were 3D printed in PC. One of the stator blades was slightly over sized to accommodate passage of the electrical wires.

A brushless DC outrunner T-motor 4014400kv was used as the motor and was operated at 8S/33V at 900W average power. Maximum instantaneous power was 1300W. The control electronics consisted of a T-motor HV-60 ESC which was mounted behind the motor within the stator. Also mounted in the stator compartment were an ESC32 microcontroller and WIFI chip, a custom 5V switched mode regulator, an MPU6000 accelerometer, a custom current sensor, an HW86060041 RPM sensor, a custom voltage sensor and an optical GY906 temperature sensor which was secured to the rear of the stator hub and which pointed directly at the motor axle. The ESP32 was programmed to broadcast UDP packets every 1 second containing information about the motor temperature, voltage, current, RPM and vibration to a Raspberry Pi microcomputer which was part of an inhouse test bench for EDFs. The test bench also measured Thrust. As a result, graphs of temperature, RPM, Power, Vibration and thrust could be plotted versus time over extended periods. The total weight of the EDF including motor, ESC, all "SMART" electronics and connectors was measured at 85Og. The maximum thrust measured at 1290W was 40.5N.

Testing of the EDF was performed initially outside under a variety of weather conditions, including occasionally heavy rain and wind over a continuous period of 1 month. The operation of the EDF was controlled automatically by a computer to regular 15-minute periods of simulated VTOL flight data (average power 900W with high frequency excursions up to 1300W recorded from a real drone flight), followed by 5 minutes of inactivity after which another period of 15 mins ensued. This regime was continued Monday-Friday 9.00am to 5pm for 1 month over the summer period. The ambient temperature during the test period ranged from 15C to 38C. At the end of each day, insects were removed from the blades and shroud and the EDF cleaned with water.

During the test period the motor temperature remained very stable at approximately 28C above ambient temperature. No increase in vibration or evolution of Thrust/Power, RPM/Power or Motor Temperature was observed within measurement errors except as transients caused by large insect ingress, strong wind gusts and heavy rain. No damage from the sometimes heavy insect ingestion was observed.

An alternative set of indoor tests were employed to characterise the motor temperature behaviour with power and time and to compare these with tests in which the centrifugal cap fan and the compressive cone were replaced by a normal cone with an open exit aperture end and with a normal open hub. The control open cone/open hub is the solution most often used in the prior art.

The results (at ambient temperature 17C) showed that at only 500W average power the control test showed a time-stabilised motor temperature of greater than lOOC. In contrast the EDF, when fitted with the centrifugal cap fan and compressive tail cone described in this invention showed a time-stabilised motor temperature of only 60C at a continuous electrical motor power of 1200W. At 500W, the stabilised motor temperature was measured as 35C.

A final series of experiments were undertaken to see if it were possible to modify the motor temperature so as to produce a higher or lower temperature exit air stream from the centrifugal fan. The rationale behind these experiments was that at very cold ambient temperatures and in bad icing conditions, a warmer rather than cooler centrifugally ejected airstream would be preferable for de icing. It was found that indeed by making the inlet aperture of the compressive cone smaller, the ejected air temperature could be easily controlled. Less usefully, by changing the number of centrifugal blades, a higher or lower ejected air temperature could be arranged.

The results of these tests show clearly that the concept of reverse- flow axial cooling of a rotor-stator EDF represents an enormous improvement over the prior-art. Light weight BLDC motors, where all electrical coils are exposed to the airflow may be used effectively in this concept. Such motors may be run at significantly higher powers than would otherwise be possible due to the effective cooling proved by reverse flow axial cooling. In addition, the hot air produced by the motor is now used beneficially to prevent icing and insect/rain ingress. This solves very large problems with the prior art where blades are intrinsically cool and susceptible to icing and where normal front-to-back air-cooling of the motor leads to insect and water ingress to the motor with often catastrophic effect.

Very little thrust reduction for a given power is observed with the present design and the inclusion of a simple servo system to control the entrance aperture of the compressive cone provides an obvious way to accurately control motor temperature and icing.

The addition in the present design of the "SMART" electronics package (total extra weight 22g) allows engine performance to be monitored and controlled in real-time. There are many scenarios where timely intervention will save an EDF and even the entire aircraft. By monitoring RPM as a function of power for example one can understand if bearings are starting to wear or if blades have become chipped. By measuring vibration one can also understand whether icing is accumulating or whether a blade has been lost. And motor temperature is of course the most important sensor. A rising and uncontrolled motor temperature can lead to the complete destruction of the EDF in seconds, with blade parts causing additional loss of adjacent EDF units. By measuring in real time the diagnostic data provided by the SMART electronics of all the EDF units on an aircraft, the flight control or engine management computer can make quick and timely decisions which fundamentally improve safety.

The present embodiment has targeted an EDF which has been designed for VTOL use. As such this EDF produces a higher "Static" thrust at low efflux speed rather than a more dynamic thrust characterised by a faster efflux air speed. However exactly the same methodology can be applied to even highly dynamic fans. Here the thermal power which must be dissipated is higher - but then the air-velocity and pressure increase produced by the rotor is also greater in this case. As such the concept of reverse-flow axial cooling is likely to find application is virtually all forms of EDF.