Unmanned aerial vehicle

Abstract

An unmanned aerial vehicle 500, operable in a first mode in which the vehicle is hovering, the vehicle has a body 502 with a leading edge 504 and a trailing edge 506 defining an angle of attack 110. T

Description

Technical Field The present disclosure relates to an unmanned aerial vehicle; in particular the disclosure relates to an unmanned aerial vehicle operable in a first mode in which the vehicle is hovering. Background Drone inspection is a practise used in the energy industry to assess and maintain remote installations, such as offshore wind turbines at long distance. It involves the deployment of unmanned aerial vehicles (UAVs) equipped with specialized sensors, cameras, and data collection tools. The primary purpose of this technology is to evaluate the structural integrity, condition, and performance of remote installations, such as offshore wind turbines while enhancing operational efficiency and safety. The inspection process includes pre-flight planning, drone launch and flight, data collection, real-time analysis, post-flight data analysis, and report generation. The benefits of wind turbine drone inspection are extensive, including improved safety by reducing the need for human climbers, cost-effectiveness through lower operational expenses, enhanced efficiency due to faster inspections, early issue detection, and better maintenance planning based on comprehensive data. This technology finds applications in routine inspections, pre- and postconstruction surveys, damage assessment, performance monitoring, and environmental impact assessments. Operators must comply with local regulations and obtain necessary permits to ensure safety and adherence to airspace restrictions. However, existing systems have limited flight endurance and weather resistance, limiting their applications It is an object of the disclosure to address one or more of the above limitations. Summary According to a first aspect of the disclosure, there is provided an unmanned aerial vehicle operable in a first mode in which the vehicle is hovering, the vehicle comprising: a body having a leading edge and a trailing edge defining an angle of attack; a propulsion mechanism coupled to the body and configured to provide thrust substantially parallel to a chord line of the body; one or more tiltrotors coupled to the body; wherein the body is configured to provide a first lift and the said one or more tiltrotors are configured to provide a second lift; wherein a total lift of the vehicle is defined by the sum of the first lift and the second lift; and a controller configured to control at least one of the tiltrotors and the propulsion mechanism to adjust the angle of attack; and wherein in the first mode the controller is configured to control the one or more tiltrotors to provide and maintain the second lift substantially perpendicular to a freestream in front of the body. For instance the body may be a wing body having a wing profile. Optionally, the controller is configured to adjust the angle of attack based on a detected air speed. Optionally, when the air speed increases the first lift increases for a defined angle of attack; the controller being configured to reduce the angle of attack when the first lift reaches a first lift threshold value to maintain the first lift at a substantially constant value. Optionally, the first lift threshold value is a pre-determined value, or wherein the controller is configured to calculate the first lift threshold value. Optionally, wherein the first lift threshold value is a percentage of the total lift. Optionally, wherein the controller is configured to increase the angle of attack to increase the first lift, up to a maximum angle of attack beyond which the vehicle would stall. Optionally, wherein each tiltrotor comprises a rotor coupled to a tilt mechanism configured to tilt an axis of rotation of the rotor. Optionally, comprising a sensor module configured to sense one or more physical properties and provide a sensor data signal to the controller. For instance the physical properties may include properties of the environment such as speed and direction of the freestream; location properties such as GPS data; and structural properties of the body such as acceleration of the body, strain or deformation of the body, surface pressure on the body, etc... Optionally, the sensor module comprises a global-positioning system configured to provide a geo-spatial coordinate of the unmanned aerial vehicle. Optionally, the sensor module comprises a strain gauge configured to detect a strain on the body. Optionally, the sensor module comprises at least one of: a gyroscope; an accelerometer; a compass configured to detect a cardinal direction; a barometer; an airspeed sensor; an airflow sensor configured to detect an air mass flowrate; a wind vane sensor configured to detect the orientation of the freestream; and a stall detector configured to detect an angle of attack stall point. Optionally, the controller is configured to adjust the angle of attack based on the sensor data signal. Optionally, the controller is configured to implement a predictive algorithm, the predictive algorithm being configured to receive the sensor data signal and to predict a future position of the unmanned aerial vehicle. Optionally, the predictive algorithm comprises a machine learning model. Optionally, the controller is configured to combine the predicted position with a geo-spatial coordinate position to provide a hybrid position. Optionally, wherein the controller is configured to compare the hybrid position with a target position to obtain a difference value; and the controller being configured to generate one or more control signals to reduce the difference value. Optionally, the controller is configured to reduce the difference value of the unmanned aerial vehicle thereby maintaining the unmanned aerial vehicle in a stable hovering mode. Optionally, the control signals are configured to control at least one of the tiltrotors, the propulsion mechanism and one or more body control surfaces. Optionally, the controller is configured to orient the leading edge of the body to face the freestream. For instance, this may include controlling the yaw of the unmanned aerial vehicle. Optionally, the body comprises one or more control surfaces wherein the control surfaces comprise at least one of an aileron, a rudder; and an elevator. Optionally, the unmanned aerial vehicle comprises one or more arm members coupled to the body, each arm member being adapted receive at least one tilt mechanism. Optionally, the unmanned aerial vehicle comprises four tiltrotors. Optionally, the four tiltrotors are provided in a quadcopter configuration. Optionally, the unmanned aerial vehicle comprises a first arm member and a second arm member coupled to the body; wherein the first arm member is adapted to receive a first tiltrotor distal to a second tiltrotor, so that the first tiltrotor is located in front of the body leading edge and the second tiltrotor is located behind the body trailing edge; wherein the second arm member is adapted to receive a third tiltrotor distal to a fourth tiltrotor, so that the third tiltrotor is located in front of the body leading edge and the fourth tiltrotor is located behind the body trailing edge. Optionally, the unmanned aerial vehicle comprises a freestream detector configured to detect an orientation of the freestream in front of the body Optionally, the propulsion mechanism comprises a propeller. Optionally, the propeller is forwardly mounted to the leading edge of the body or wherein the propeller is rearwardly mounted to the trailing edge of the body. Optionally, the unmanned aerial vehicle comprises a battery arranged to provide energy for the one or more tiltrotors and the propulsion mechanism. Optionally, wherein the battery comprises a lithium-ion battery or a fuel cell or a gas turbine generator. According to a second aspect of the disclosure, there is provided a method of controlling an unmanned aerial vehicle in a first mode in which the vehicle is hovering, wherein the vehicle comprises a body having a leading edge and a trailing edge defining an angle of attack; a propulsion mechanism coupled to the body and configured to provide thrust substantially parallel to a chord line of the body; one or more tiltrotors coupled to the body; wherein the body is configured to provide a first lift and the said one or more tiltrotors are configured to provide a second lift; wherein a total lift of the vehicle is defined by the sum of the first lift and the second lift; the method comprising controlling at least one of the tiltrotors and the propulsion mechanism to adjust the angle of attack; and controlling the one or more tiltrotors to provide and maintain the second lift substantially perpendicular to the freestream in front of the body. Optionally, the method comprises detecting an orientation of a freestream in front of the body. Description of the drawings The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure 1 is a side view of a conventional wing for an aircraft or UAV; figure 2 is a perspective view of the wing of figure 1; figure 3(a) is a side view of a conventional UAV operating in a first mode; figure 3(b) is another side view of the UAV of figure 3(a) in the first mode; figure 3(c) is a side view of the UAV of figure 3(a) operating in a second mode; figure 4(a) is a side view of another conventional UAV in operation in an erratic airflow at a first point in time; figure 4(b) is a side view of the UAV of figure 4(a) in operation in an erratic airflow at a second point in time; figure 4(c) is a side view of the UAV in operation in an erratic airflow at a third point in time; figure 5(a) is a side view of a UAV according to the disclosure operating in fixed-wing mode; figure 5(b) is a side view of the UAV of figure 5(a) operating in a hover mode; figure 6(a) is a perspective view of a tiltrotor for use in the UAV of figure 5, in a non-tilting configuration; figure 6(b) is a perspective view of the tiltrotor of figure 6(a) in a tilting configuration ; figure 7 is a perspective view of another UAV according to the disclosure ; figure 8 is a flow chart of a control process for the UAV of figure 7 figure 9 is another flow chart of a control process for the UAV of figure 7. figure 10A is plot of the lift as a function of the freestream velocity for the UAV of figure 5; figure 10B is plot of the angle of attack as a function of the freestream velocity for the UAV of figure 5. Description It will be appreciated that references to aerodynamic forces refer to the four forces of flight: lift, weight, thrust and drag. These forces control the motion of a body in flight. For example, if the lift force exceeds the weight force, the body in flight will move up. If the weight force exceeds the lift, a body will move down. If lift and weight are equal, the body will hover. Similarly, if thrust exceeds the drag the body will move forward and if drag exceeds the thrust, the body will move backwards. If thrust and drag are equal, the body will neither move forward nor backward. Figure 1 is a side view of a known aerodynamic body 100 for an aircraft or UAV, which may be referred to as a wing, having a leading edge 102 and a trailing edge 104. The leading edge 102 and trailing edge 104 have a vector plane known as the chord line 106, which is the shortest distance between the leading edge 102 and the trailing edge 104. A freestream 108 is the air upstream of an aerodynamic body 100, that is airstream before the aerodynamic body 100 has a chance to deflect, slow down or compress the air. The freestream 108 and the aerodynamic body chord line 106 form an angle of attack 110 for the body (or wing) 100 for the aircraft, or UAV. Updrafts, downdrafts and turbulence may cause the freestream direction to change. Figure 2 is a perspective view of the wing 100 of figure 1 , displaying an upper airflow 200 and lower airflow 202 and the resulting lifting force 204. The freestream 108 separates into the upper airflow 200 passing over the body 100 and the lower airflow 202 passing under the wing. The upper airflow 200 has a faster airflow velocity and lower pressure than lower airflow 202, thereby generating an upward lifting force 204. Figures 3(a-c) illustrate an Unmanned Aerial Vehicles (UAV) 300 Vertical Take Off and Landing (VTOL) drone, capable of operating in a fixed wing mode or a hover mode. Figure 3(a) shows the UAV 300 operating in a fixed wing mode and traveling against an oncoming freestream 108. The Vertical-Take-Off-And-Landing (VTOL) architecture of the UAV 300 for traveling into a freestream 108 is an aerodynamic body 304 providing for fixed-wing lift and one or more fixed vertical rotors 306 for rotary-wing lift. The UAV 300 also has a forward propulsion mechanism 308. In a fixed-wing flight mode, the UAV 300 flies in a similar fashion as a fixed-wing aircraft into an oncoming freestream 108. Figure 3 (b) shows the UAV 300 operating in a fixed wing mode and displaying the trailing drag force 310 from the aerodynamic body. Figure 3 (c) shows the UAV 300 operating in a hover mode where the angle of attack is lowered relative to the freestream 108. In hover mode the UAV 300 remains stationary relative to the ground. In hover mode the aerodynamic body 304 lowers the angle of attack of the body 304 into the oncoming freestream 108 resulting in a reduction in the air velocity passing over the aerodynamic body 304, and reducing the lift 312. When the UAV 300 lowers the angle of attack it in turn lowers the rotor's 306 angle of attack simultaneously. The rotors 306 must therefore provide additional thrust 314 to compensate for the reduction in lift from the body 304. Figures 4 (a-c) illustrate another known UAV 400 , capable of operating in a fixed wing mode or a hover mode. The UAV utilises a control scheme that responds to an erratic airflow 402. Figure 4 (a) shows the UAV 400 having an aerodynamic body 304 providing for fixed-wing lift and one or more fixed vertical rotors 306 for rotary-wing lift. The UAV 400 has a forward propulsion mechanism 308. The UAV 400 is exposed to an erratic airstream 402, such as a gust of wind in which the airstream velocity and the angle of incidence to the UAV 400 may fluctuate rapidly. Figure 4 (b) shows the UAV 400 having sensors 404 which sense a property that varies in response to the erratic airstream 402. For example, the sensors 404 may be wind vane sensors that sense the direction of the erratic airstream 402. Figure 4 (c) shows the UAV 400 having oriented its position 406 by adjusting the rotors 306 based on sensing a property of the airstream 402. For example, the rotors 306 output may be adjusted based upon the sensors 404 detecting an adjustment in the direction of the erratic airstream 402. Figures 5 (a-b) illustrate an exemplary embodiment of a UAV according to the disclosure. The UAV 500 also referred to as Vertical Take Off and Landing (VTOL) drone, is operable in a first mode referred to as hovering mode (figure 5b) in which the vehicle is hovering, and a second mode also referred to as fixed wing mode (figure 5a) in which the vehicle is travelling. The UAV 500 has a body 502, which may be referred to as a wing, having a leading edge 504 and a trailing edge 506 defining an angle of attack similar to that previously described for the wing 100 in figure 1. In this example the UAV 500 is provided with two pairs of tiltrotors, a first pair provided at one end of the body / wing 502; and a second pair provided the other end of the body / wing 502 in a quadcopter configuration. The quadcopter configuration has 4 tiltrotors. In this example the UAV 500 has a freestream detector 524 configured to detect an orientation of a freestream in front of the body 502. For instance the freestream detector may be provided by a vane sensor. For instance the vane sensor may be mounted in the freestream, forward of the body. Typically this could be at the front of the nose or leading edge of wings, and may protrude outwards from the body. In this example the freestream detector 524 is provided on one of the front legs of the drone but could be provided at a different location on the drone. A wind direction sensor may be provided to sense a direction of the wind in the horizontal plane (i.e. North East). This allows the UAV to keep pointing into the wind. In some alternative embodiments the tilt of the tiltrotors may be adjusted based on orientation data of the drone. For instance orientation data may be obtained from a gyroscope. The side views 5(a) and 5(b) only show the first pair made of a front tiltrotor 510a provided at the front of the body 502, and a rear tiltrotor 510b provided at the rear of the body 502. Similarly the second pair (not shown) includes a front tiltrotor 510’a provided at the front of the body 502, and a rear tiltrotor 510’b provided at the rear of the body 502. Each tiltrotor 510 can orient its position to provide directional lift. The UAV 500 has a propulsion mechanism 508 that is coupled to the body 502 and provides thrust substantially parallel to a chord line of the body 502. In this example the propulsion mechanism 508 is a forward propeller. The UAV 500 has a controller 512 arranged to control the flight of the UAV 500. The controller is configured to control at least one of the tiltrotors 510 and the propulsion mechanism 508 to adjust the angle of attack in first mode. For instance the angle of attack may be adjusted when an air speed measurement is equal or above a threshold value. This may be achieved by altering any characteristic of the tiltrotor 510 or propulsion mechanism 508, such as thrust or orientation. Different types of airspeed sensors may be provided on the UAV. These can be pitot tube sensors or air mass flow rate sensors, all of which are based on measuring pressure differences to calculate airspeed. Figure 5(a) shows the UAV 500 in a fixed wing mode. In this mode the tiltrotors 510 may be activated or deactivated. When activated the tiltrotors 510 provide a vertical lift 518 substantially perpendicular to the freestream 501. Figure 5 (b) shows the UAV 500 operating in the hover mode. To switch from the fixed wing mode as shown in figure 5(a) to the hover mode of figure 5(b) the controller 512 adjusts the angle of attack relative to the freestream 501. The orientation of the tiltrotors is changed so that the tiltrotor vertical lift 518 remains substantially perpendicular to the freestream 501. The forward propulsion mechanism 508 provides a thrust 514 as required by the mode. For example, when in hover mode the propulsion mechanism 508 provides a thrust 522 that opposes, and can be the equal to, a force of the drag 514 thereby maintaining the UAV in a stationary hovering position. The controller 512 controls the tiltrotors 510 and / or the propulsion mechanism 508 to adjust the angle of attack. In the first mode the controller controls the tiltrotors 510 to provide and maintain the lift 518 provided by the tiltrotors substantially perpendicular to the freestream 501 in front of the body 502. The freestream 501 separates into an upper airflow passing over the body 502 and a lower airflow passing under the body 502. The upper airflow has a faster airflow velocity and lower pressure than lower airflow thereby generating an upward lifting force. The additional lift force means that the tiltrotors 510 do not have to produce as much lift 518 to stay in the air. In alternative embodiments, to further improve the energy efficiency at low speeds, high lift devices like vortex generators, turbulators, flaps and slats can be used. In use, the UAV 500 may enter a Hover mode that is configured to enhance flight endurance where the wind passes over the wings to create lift and thus reduce power consumption from the one or more tiltrotors 510. The drone faces the freestream 501 in hover mode 1 as seen in figure 5b, this allows the freestream (wind) to pass over the body 512 and produce a wing lift 520. By producing extra lift from the body, the rotors subsequently do not need to provide as much lift 518 and therefore use less power, hence improving efficiency. In order to counteract the drag from the freestream (wind), the propulsion system provides a force parallel to the body chord line. The controller 512 rotates the tilt rotors 510, so that the blades of the tiltrotors 510 are parallel to the freestream 501 whilst also rotating the angle of attack of the body to improve the wing lift 520 of the body. The controller 512 increases the angle of attack of the body and also changes the tilt angle to keep it parallel to the freestream 501. The tilt angle is decided by the controller depending on airspeed thresholds as explained below. The UAV 500 has a first threshold which splits the hover mode into low lift regime, and a high lift regime. For both high lift regimes and low lift regimes the angle of attack will always be lower than the stall angle and will be adjusted during flight automatically by the controller 512. The amount of lift created is a key property in splitting these flight regimes. The lift L is a product of the velocity (V) of the air passing over the body, the shape and angle of attack (CL) and area (A) of the body 502 and the density of the air P (rho) , and defined as : pV2 L = Cl A L = Produced Lift CL = Experimentally determined coefficients P = Density of air. V = Velocity of air A = Body / Wing Surface Area. Low lift Regime (below a first threshold). Low lift occurs at low airspeeds ranging from a 0 mph airspeed (where no lift is produced by the wing), to a first threshold. The first threshold is defined as the point in which lift produced is over a set percentage of the weight of the drone (e.g a 1 kg drone at a 60% set point, once the wings are producing 60% lift and over (+0.6kg) the threshold one is reached). The first threshold may also be defined as a point where the UAV 500 becomes unstable, such as when the body produces a large ratio of the total lift and tiltrotors are no longer providing significant portion of the lift, thereby losing a measure of control over the flight of the UAV 500. To maximise the lift in a low lift flight regime (to minimise power consumption from the one or more tiltrotors) the angle of attack 110 of the body 502 is increased as high as possible without inducing stall. Stall occurs when angle of attack 110 is increased beyond the stall angle in which airflow separation occurs and causes reduced lift to be produced and increases drag. Stall occurs at higher angles of attack, typically in the 15- 20-degree region. The stall angle could be calculated beforehand or detected by stall detectors during flight or through the controller measuring a drop in lift and increase in drag. If stall or near stall is detected the angle of attack is reduced to prevent stall. Maintaining the body lift 520 below a certain percentage of the drone weight will allow for the some of the lift to always be produced by the one or more tiltrotors 510, hence providing better hovering control. High Lift Regime (above a first threshold). High lift flight regime occurs at higher airspeed above the first threshold (as defined previously). In high lift flight regimes, the drone will attempt to maintain the lift first threshold (maintaining lift at a certain percentage x% of the drone’s weight). Air velocity influences lift 520 produced by the body, therefore as the air velocity increases, lift 520 increases above the first threshold. In order to reduce lift, the angle of attack 110 is reduced, which maintains the lift produced by body 520 in a target ratio with the lift 518 produced by the one or more tiltrotors 510. The preferred angle of attack 110 may be predefined from airspeed calculations or calculated during flight by measuring the lifting force and adjusting the angle of attack 110 to ensure constant lift. Figures 6(a-b) show an exemplary embodiment of a tiltrotor 510 attached to an arm 608 in fixed wing mode (6(a)) and hovering mode (6b), respectively. The arm 608 extends along a longitudinal axis 609. The tiltrotor 510 has rotor 600 driven by a propulsion 602. The rotor 600 has an axis of rotation 601. The motor 602 is coupled to a tilt mechanism 604. The tilt mechanism 604 is movable to change the orientation of the axis of rotation 601 of the rotor blade 600 with respect to the longitudinal axis 609. As explained above the tilt angle may be selected to provide directional lift substantially perpendicular to a freestream in front of the body 502. The tilt mechanism 604 can maintain the rotor blades 600 parallel to the freestream whilst increasing the angle of attack for the rest of the body 502. In one example the tilt mechanism 604 provides a pivot joint that pivots the motor 602, thereby orienting the rotor blades 600. The arm 608 is coupled to the body 502 at the proximal end (not shown here) and the tilt mechanism 604 is mounted at the arm distal end 606. Figure 6(b) shows the tiltrotor 510 in hovering mode. The tilt mechanism 604 tilts the axis of rotation 601 with respect to the longitudinal axis 609 to maintain the rotor blades in a desired orientation. Figure 7 is an example implementation of a UAV 700 according to the disclosure. The UAV 700 of this embodiment has all the features previously discussed with respect to the UAV 500 of figure 5. The UAV 700 has several flight control surfaces, including aileron 702 and 702’, two flaps 704 and 704', two elevators 706 and 706' and a rudder 708. The ailerons 702 and 702' control the roll movement about the longitudinal or roll axis 701 of the UAV 700. The flaps 704 may be made of a hinged panel or panels mounted on the trailing edge of the wing body 502. When extended, they increase the camber and, in most cases, the chord and surface area of the wing resulting in an increase of both lift and drag and a reduction of the stall speed. The elevators 706 are used to control the UAV 700 pitch about the pitch axis 703 and thereby adjust the angle of attack of the body 502. The rudder 708 is mounted on the trailing edge of a vertical stabilizer and is used to control the yaw about the yaw axis of the aircraft. The UAV 700 is arranged in a quadcopter configuration with 2 forward arms 608 mounting tiltrotors 510a and 510’a, and 2 rearward arms 608 mounting tiltrotors 510b and 510’b. The quadcopter configuration is where 4 tiltrotors are arranged such that each tiltrotor rotor 510a, 510b, 510’a, 510'b produces a torque about its centre of rotation. In one illustrative example the quadcopter configuration has two tiltrotors 510’a, 510b spinning clockwise (CW) and two tiltrotors 510’b, 510a spinning counterclockwise (CCW). Flight control is provided by independently varying the speed and pitch of each tiltrotor 510a, 510b, 510'a, 510’b, thereby adjusting the lift and torque of each tiltrotor. The UAV 700 has a controller (not shown here) for controlling the fight path of the UAV 700. For example; some controller movement combinations are listed below: Move Right • Power up left-side tiltrotors 510a and 510b • Power down right-side tiltrotors 510’a and 510’b • Position left aileron down 702 • Position right aileron up 702' This will roll the UAV 700 clockwise, allowing for right-horizontal motion supplied by the body 502 and the four tiltrotors 510 Move Forward • Power up the forward propulsion motor 508 Move Up • Increase the pitch of the four tilt rotors 510 relative to the body (which has the effect of pitching the body up). • Increase power to the one or more tiltrotors 510 • Position the elevators 706 and 706' up This will pitch the wind and body of the UAV upwards, allowing for upwards motion supplied by the body 502 and the four tilt rotors 510. Yaw Clockwise • Power up the tiltrotors 510’b and 510a. • Power down the tiltrotors 510'a and 510b. • Position the Rudder 708 to the right • Lower the pitch of the left tiltrotors 510a, 510b downwards relative to the body • Raise the pitch of the right tiltrotor 510’a and 510’b upwards relative to the body. In an alternative example, the quadcopter configuration has two tiltrotors rotation reversed, 510’a, 510b spinning CCW and two tiltrotors 510’b, 510a spinning CW. A non-limiting example of the UAV 700 in use is described in which the UAV 700 is flown offshore to inspect a damaged wind turbine in relatively windy conditions. In this example the wind turbine is located at a distance of 20 km. Launch - The UAV 700 takes off vertically using the four tiltrotors 510, which may occur using a known hover mode. The tiltrotors 510 are spun using electric motors 602 which may be powered by lithium-ion batteries. The tiltrotors can be used to take the aircraft up in hover mode to 30 meters above the ground before transitioning to fixed-wing mode. During the transition to fixed wing mode, the forward propulsion 508 requires a time interval to bring the forward propulsion mechanism 508 up to the required speed. In fixed-wing mode the drone may cruise at 20 meters per second at a much lower power draw. This allows the UAV to travel a 20km distance without reducing much battery power. Inspection - Once the UAV reaches the wind farm, it will reduce it’s fixed-wing speed and enter the hover mode of the present disclosure, producing more lift form the body thereby reducing power draw from the battery and sustaining flight endurance. Without any wind the UAV will require more power to stay hovering. The drone will then fly up and down the sides of the wind turbine taking high quality images for the inspection. If there is no wind, this will be done purely in hover mode with only the tiltrotors running. Windy conditions - If the conditions become windier it will be beneficial for the UAV 700 to increase the angle of attack of the body 502 while maintaining the tiltrotors 510 perpendicular to the incoming freestream, thereby generating additional lift from the body 502. The UAV can then continue to inspect the wind turbine with extra control manoeuvrability. However, the yaw axis 701 will be kept facing into the wind so that lift can be produced via the wing body 502. The UAV 700 will be controlled to face the wind and the forward propulsion mechanism 508 will provide thrust to counteract the wind force. To enhance this effect the body’s 502 pitch is raised depending on the wind conditions. The high lift devices (flaps 704 and ailerons 702) are deployed to increase the lift provided at low airspeeds. The body 502 pitch is determined using the wind speed and wind direction: at low wind speeds the maximum angle of attack (before stall) will be used to provide as much lift as possible. At higher airspeeds, the angle of attack can be reduced so that the UAV can still be controlled by the tiltrotors 510. Figure 8 is a flow chart of a control process 800 for the UAV 700 in which a controller 512 is arranged to operate a predictive control system. To improve the lift produced by the body 502, the UAV first sense the aerodynamic forces on the body 502 and combine this with structural forces. It will be appreciated that references to structural forces refers to forces acting within the UAV 700, for example a strain deformation on the body 502. The UAV 700 has a sensor module (not shown) that includes a variety of sensors. These sensors may include: gyroscopes, accelerometers, GPS, compass, barometers, airspeed sensors, airflow sensors, wind vane sensors, stall detectors, pressure sensors and strain gauge, current sensors, among others. The controller receives data from the sensor module and controls and orient the UAV 700 based on the received data to maximise power efficiency while maintaining the UAV stable. The sensor data is used to predict the best way to move the UAV to avoid instability, rather than react after the UAV has become unstable. The control process 800 illustrates the operation of an algorithmic predictive control system. The control process 800 shows the steps applied to the control system to produce predictive control. The predictive control system may be an implementation of a machine learning model, a neural network, bayes classifiers or a physics based model such as a predictive simulation or analytical analysis of the aerodynamic forces. When the predictive control system implementation is a Machine learning model it may be trained using prior flight data. When the predictive control system implementation is a physics based model, data may be derived from physics models which may be constructed from the knowledge of aerodynamic equations etc. At step 810 the algorithmic predictive control system (which may have already been trained on previous flight data) receives aerodynamic sensor data 804, and use these data to predict a future position of the UAV 700. At step 812, the Predicted Position is then combined with the Actual Position (positional sensor data 802 measured by the GPS sensor) to give a hybrid position. For example, the machine learning model may output a position which is 50% predicted and 50% actual, this would make the UAV 'think' that is 50% closer to the Predicted Position than it is. At step 814 the hybrid position is compared to a desired or target position 808. At step 816 the control system determines an output action based on the comparison. The output action may include an orientation of the tiltrotors 510. In an alternative embodiment, the control process 800 describing a prediction of a position is applied a position velocity controller, or the acceleration and attitude controller or an angular controller, and the angular rate controller to produce a combined predicted property as input to the controller. An example of the control process 800 is now provided for gusty conditions where the wind speed or direction is changing often, the predictive control process 800 will reduce UAV sway and vibration. The machine learning model may be trained on the flight data stored from previous flights operating the predictive control process 800. If a gust of wind were to intersect the UAV 700 head on it would be picked up by the sensor modules; aerodynamic sensors (airspeed sensors, airflow sensors, pressure sensors] as well as the structural sensors in the wing (strain gauges, accelerometers, position sensors). The predictive control process 800 will then use these data to predict where the UAV is expected to be. This takes a short time for instance about 0.2 second. In an example case, the predictive system expects the aircraft to be pushed backwards by 1 meter. This value is then combined with the actual position of the UAV to give a hybrid position change of 0.8 meters backwards. When this is compared with the desired position, the UAV 'thinks' it has already been pushed back and will start powering up the forward propulsion mechanism 508 to counteract this. This will result in the controller 512 cancelling out the effects of the gust before the aircraft has moved out of position. This can all act in conjunction with fixed wing mode or hover mode described above. Figure 9 is a flow chart of another control process for the UAV 700. The process 900 is similar to the control process 800 but in this example, additional sensor data 902 is fed through the Machine Learning Model at step 904 to produce a Predicted Position (PP) atthe same time as the sensors 802, 804, 806 output the Actual Position (AP). At step 906 these two positions AP and PP are combined using the equation X = A*AP+ B*PP in which the coefficients A and B are defined such that A+B = 1. An RC transmitter 907 may be used to provide a desired position signal for the position control 908. At step 908 the position control compares the output X obtained at step 906 with the desired position. At step 910 a velocity control receives the output of the position control 908. In turn the velocity controller determines what speed the drone must travel at in order to get to the new position. At step 912 the Acceleration and yaw controller determines what acceleration the drone must provide to get to this new speed. At step 914 the Angle controller determines what angle the drone must rotate to in order to produce this new acceleration. At step 916 the angular rate controller determines what angular acceleration is required to get to this new angle. At step 918 the mixer translates the angular acceleration into motor rotation speeds. For example, the mixer may be a microprocessor that is arranged to implement a portion of software that serves the function of a hardware mixing: inputting desired angular acceleration from the angular rate controller 916 to provide output tiltrotor and control surface signals. An example of this working would be if the UAV 700 gets hit by an up draft of air while trying to hover in the same position. The actual position is measured at 5m in altitude, but after the gust hits, the predictive position is estimated at 6m (e.g this is where the drone is expected to be after a short time period). These two positions could be combined (50:50) to give a new position of 5.5m. Since the drone is trying to stay stationary, the desired position must also be 5m. Therefore, the UAV will 'think' that it is too high and so it will reduce its altitude just as the up draft takes effect, thereby providing a way to cancel out the effects of the gust without having to wait for the UAV 700 to move out of position. Figure 10A is plot of the lift as a function of the freestream velocity for the UAV of the present disclosure discussed in Figure 5(b). Figure 10B is plot of the angle of attack as a function of the freestream velocity for the UAV of the present disclosure discussed in Figure 5(b). When the air speed (freestream velocity ) increases the first lift, that is the lift provided by the body or wing increases for a defined angle of attack (see Figure 10A). When the first lift reaches a first lift threshold value 1002, the controller reduces the angle of attack to maintain the first lift at a substantially constant value. In figure 10B the angle of attack reduces from a maximum angle of attack above which the drone would stall. The first lift threshold value 1002 may be a pre-determined value stored in the controller. Alternatively, the controller may be the configured to calculate the first lift threshold value dynamically. For instance the first lift threshold value 1002 may be set to a predefined percentage of the total lift, for instance 60% of the total lift. The UAV 500, 700 described above is kept facing towards the freestream. The controller adjusts the angle of attack 110 relative to the freestream and adjusts the tiltrotor angle to remain substantially perpendicular to the freestream which maximises the lift produced by the body (first lift) up to the lift threshold 1002. By maximizing the lift produced by the body (first lift), the tiltrotors 510 do not need to provide as much lift (second lift) and therefore use less power, hence improving the drone efficiency. To counteract the drag from the freestream the propulsion system provides a force parallel to the body chord line. It will be appreciated the UAV described with reference to figures 5 to 10 could include several additional sensors depending on the application. For instance, the UAV may be provided with high-resolution cameras, LiDAR 24 sensors, ultrasonic sensors, Inertial measurement unit (IMU), and communication systems. A skilled person will therefore appreciate that variations of the disclosed 5 arrangements are possible without departing from the disclosure. Accordingly, the above description of the specific embodiments is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described. 10

Claims

1. An unmanned aerial vehicle operable in a first mode in which the vehicle is hovering, the vehicle comprising:a body having a leading edge and a trailing edge defining an angle of attack;a propulsion mechanism coupled to the body and configured to provide thrust substantially parallel to a chord line of the body;one or more tiltrotors coupled to the body;wherein the body is configured to provide a first lift and the said one or more tiltrotors are configured to provide a second lift; wherein a total lift of the vehicle is defined by the sum of the first lift and the second lift; anda controller configured to control at least one of the tiltrotors and the propulsion mechanism to adjust the angle of attack; and wherein in the first mode the controller is configured to control the one or more tiltrotors to provide and maintain the second lift substantially perpendicular to a freestream in front of the body.

2. The unmanned aerial vehicle of claim 1, wherein the controller is configured to adjust the angle of attack based on a detected air speed.

3. The unmanned aerial vehicle of claim 2, wherein when the air speed increases the first lift increases for a defined angle of attack; the controller being configured to reduce the angle of attack when the first lift reaches a first lift threshold value to maintain the first lift at a substantially constant value.

4. The unmanned aerial vehicle of claim 3, wherein the first lift threshold value is a pre-determined value, or wherein the controller is configured to calculate the first lift threshold value.

5. The unmanned aerial vehicle of any of the preceding claims, wherein the controller is configured to increase the angle of attack to increase the first lift up to a maximum angle of attack beyond which the vehicle would stall.

6. The unmanned aerial vehicle of any preceding claims, wherein each tiltrotor comprises a rotor coupled to a tilt mechanism configured to tilt an axis of rotation of the rotor.

7. The unmanned aerial vehicle of any preceding claim, comprising a sensor module configured to sense one or more physical properties and provide a sensor data signal to the controller.

8. The unmanned aerial vehicle of claim 7, wherein the sensor module comprises a global-positioning system configured to provide a geo-spatial coordinate of the unmanned aerial vehicle.

9. The unmanned aerial vehicle of claim 7 or 8, wherein the sensor module comprises a strain gauge configured to detect a strain on the body.

10. The unmanned aerial vehicle of any of the claims 7 to 9, wherein the sensor module comprises at least one ofa gyroscope;an accelerometer;a compass configured to detect a cardinal direction;a barometer;an airspeed sensor;an airflow sensor configured to detect an air mass flowrate;a wind vane sensor configured to detect the orientation of the freestream; anda stall detector configured to detect an angle of attack stall point.

11. The unmanned aerial vehicle of any of the claims 7 to 10, wherein the controller is configured to adjust the angle of attack based on the sensor data signal.

12. The unmanned aerial vehicle of claim 11, wherein the controller is configured to implement a predictive algorithm, the predictive algorithm being configured to receive the sensor data signal and to predict a future position of the unmanned aerial vehicle.

13. The unmanned aerial vehicle of claim 12, wherein the controller is configured to combine the predicted position with a geo-spatial coordinate position to provide a hybrid position.

14. The unmanned aerial vehicle of claim 13, wherein the controller is configured to compare the hybrid position with a target position to obtain a difference value; and the controller being configured to generate one or more control signals to reduce the difference value.

15. The unmanned aerial vehicle of claim 14, wherein the control signals are configured to control at least one of the tiltrotors, the propulsion mechanism and one or more body control surfaces.

16. The unmanned aerial vehicle of any preceding claim, wherein the controller is configured to orient the leading edge of the body to face the freestream.

17. The unmanned aerial vehicle of any preceding claim, wherein the body comprises one or more control surfaces wherein the control surfaces comprise at least one of an aileron, a rudder; and an elevator.

18. The unmanned aerial vehicle of any of the claims 6 to 17, comprising one or more arm members coupled to the body, each arm member being adapted receive at least one tilt mechanism.

19. The unmanned aerial vehicle as claimed in any of the preceding claims comprising four tiltrotors.

20. The unmanned aerial vehicle of claim 19, wherein the four tiltrotors are provided in a quadcopter configuration.

21. The unmanned aerial vehicle of any of the claims 18 to 20, comprising a first arm member and a second arm member coupled to the body;wherein the first arm member is adapted to receive a first tiltrotor distal to a second tiltrotor, so that the first tiltrotor is located in front of the body leading edge and the second tiltrotor is located behind the body trailing edge;wherein the second arm member is adapted to receive a third tiltrotor distal to a fourth tiltrotor, so that the third tiltrotor is located in front of the body leading edge and the fourth tiltrotor is located behind the body trailing edge.

22. The unmanned aerial vehicle of any preceding claim, comprising a freestream detector configured to detect an orientation of the freestream in front of the body.

23. The unmanned aerial vehicle of any preceding claims, wherein the propulsion mechanism comprises a propeller.

24. The unmanned aerial vehicle of any preceding claim comprising a battery arranged to provide energy for the one or more tiltrotors and the propulsion mechanism.

25. A method of controlling an unmanned aerial vehicle in a first mode in which the vehicle is hovering, wherein the vehicle comprises a body having a leading edge and a trailing edge defining an angle of attack; a propulsion mechanism coupled to the body and configured to provide thrust substantially parallel to a chord line of the body; one or more tiltrotors coupled to the body; wherein the body is configured to provide a first lift and the said one or more tiltrotors are configured to provide a second lift; wherein a total lift of the vehicle is defined by the sum of the first lift and the second lift; the method comprisingcontrolling at least one of the tiltrotors and the propulsion mechanism to adjust the angle of attack; andcontrolling the one or more tiltrotors to provide and maintain the second lift substantially perpendicular to the freestream in front of the body.

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