A lift generating system for an aerial vehicle

Abstract

A lift generating system 100 for an aerial vehicle (400, figure 13). The lift generating system comprises an airflow generator (120, figure 5), for example a ducted fan, for generating an airflow. The

Description

FIELD OF THE INVENTION The present disclosure relates to a lift generating system for an aerial vehicle. The present disclosure also relates to an aerial vehicle comprising the lift generating system. The present disclosure further relates to a kit of parts for assembling the lift generating system. BACKGROUND OF THE INVENTION The need for suitable vehicles to transport goods and people has grown in recent decades. Traditional transport methods, such as land vehicles or cargo ships, allow for the transportation of a large quantity of goods and / or people. However, these traditional methods can struggle to keep up with the demand for fast, high capacity transportation. Aircraft have been used for several decades to increase capacity for transportation of goods and people, and have shown significant improvements in the timeliness of transportation. However, the design of many aircraft can limit the locations that they are able to transport goods and / or people to and often necessitates the need for an additional last-mile delivery vehicle. Conventional fixed-wing aircraft use propulsion systems to accelerate the wing through the air to generate airflow over the wing, so that the wing can produce lift. This means that conventional fixed-wing aircraft typically require a runway having a length in excess of 100 or 1000 metres in order to achieve an air speed sufficient for flight. Similarly, conventional fixed-wing aircraft require runways in excess of 100 or 1000 metres in order to provide sufficient stopping distance during landing. On top of this, the take-off distance and the landing distance required fora conventional fixed-wing aircraft is not fixed and instead varies, sometimes significantly, based on factors such as payload, fuel on board, aircraft condition, meteorological conditions, and environmental conditions. The requirement for a runway having a suitable length can severely constrain the missions that can be operated by a conventional fixed-wing aircraft. For missions into remote or densely populated areas, it can make the use of conventional fixed-wing aircraft impossible. This issue can be difficult to overcome because building new runways, or adapting existing runways, can present significant financial, environmental and legal challenges. Aircraft capable of vertical take-off and landing (VTOL) have reduced, or in some cases eliminated, the need for conventional runways or landing strips. This has facilitated access to remote or hard-to-reach areas. VTOL technology has been particularly beneficial in unmanned aerial vehicles (UAVs), and VTOL UAVs are transforming the realm of unmanned aerial vehicles due to their unique capabilities. For example, their versatility and adaptability make them particularly valuable in commercial delivery applications, where navigating complex terrain and reaching customers in diverse environments is crucial. Typically, VTOL aircraft use either rotary wings or thrust-vectoring in order to provide VTOL capabilities. Examples of VTOL rotary-wing aircraft include: helicopters, autogyros, and gyrodynes. Rotary wing aircraft can be mechanically complex and the footprint of the rotors can become considerably large as the payload capacity or speed requirements of the aircraft increases. Additionally, rotary wing aircraft can also present a safety challenge when used in urban environments due to exposed high-velocity moving parts. Thrust vectoring for producing lift is typically found in military aircraft, rather than civilian aircraft. An example can be found in WO 2012 / 146931 A1 which describes a VTOL aircraft that generates lift through the Coanda effect and the direction of thrust downwards. However, the use of thrust-vectoring can limit take-off and landing to surfaces that can withstand exposure to the heat and high velocity thrust. Moreover, lift generation through thrust vectoring is not an efficient method for producing lift and, as the empty weight of the aircraft increases or the payload capacity of the aircraft increases, it becomes impractical to use thrust vectoring for VTOL due to how large and mechanically complex the thrust vectoring system would need to be. It is an objective of the present disclosure to provide a lift generating system for an aerial vehicle, in particular a VTOL aerial vehicle, which can efficiently produce lift whilst being relatively compact. It is also an objective of the present disclosure that the lift generating system may be less mechanically complex than the use of rotary wings or thrust-vectoring. It is further an objective of the present disclosure that the lift generating system may be suitable for use in urban environments. SUMMARY OF THE INVENTION According to the present disclosure, there is provided a lift generating system for an aerial vehicle. The lift generating system comprises an airflow generator for generating an airflow. The lift generating system also comprises an aerofoil for generating lift for the aerial vehicle. The lift generating system further comprises a duct defining a duct inlet, a duct outlet and an airflow path extending from the duct inlet to the duct outlet. The duct is arranged to direct the airflow from the airflow generator to the aerofoil. The duct may comprise an airflow management region. As used herein, the term “aerofoil” refers to a three-dimensional body that is shaped to generate more lift than drag. The aerofoil may be referred to as a wing. The lift equation defines that Lift = 0.5pClV2S\ where p is the density of the fluid, Cl is the lift coefficient of the lifting body, V is the relative velocity of the fluid and the lifting body, and S is the surface area of the lifting body. Therefore, the velocity term can be satisfied by the lifting body (such as an aerofoil) moving through the fluid (such as air), or by the fluid flowing over a static lifting body. In the present disclosure, the provision of an airflow generator allows the lift generating system to present the aerofoil with an airflow having sufficient velocity for the aerofoil to generate lift without the aerofoil needing to have any ground speed. Therefore, unlike conventional fixed-wing aircraft or rotary-wing aircraft, the aerofoil does not need to be moved through the air in order to generate lift because it is presented with a local airflow having sufficient velocity. Advantageously, this allows the lift generating system to achieve vertical take-off and landing. The provision of a duct means that the entire airflow generated by the airflow generator can be directed to the aerofoil and be used to generate lift, rather than a portion of the airflow being lost to the surroundings. Advantageously, this may increase the efficiency of the lift generating system. Moreover, the provision of the duct allows the duct to be designed to manipulate the airflow and enhance its properties before it reaches the aerofoil. For example, in the present disclosure, the duct may be configured such that the airflow reaching the aerofoil has one or more of the following properties: a substantially uniform velocity along the span of the aerofoil; negligible sideslip along the span of the aerofoil; minimal swirl along the span of the aerofoil; a minimal angle of attack variation along the span of the aerofoil; and being substantially laminar along the span of the aerofoil. Advantageously, by presenting the aerofoil with a uniform stream of airflow, the amount of lift produced by the aerofoil, and the efficiency of the lift generating system, may be significantly increased. According to the present disclosure, there is provided a lift generating system for an aerial vehicle. The lift generating system comprises an airflow generator for generating an airflow. The lift generating system also comprises an aerofoil for generating lift for the aerial vehicle. The lift generating system further comprises a duct defining a duct inlet, a duct outlet and an airflow path extending from the duct inlet to the duct outlet. The duct is arranged to direct the airflow from the airflow generator to the aerofoil. The duct comprises an airflow management region in which a thickness of the airflow path decreases in a downstream direction and a width of the airflow path increases in the downstream direction. At least a portion of the aerofoil is positioned within, or downstream of, the airflow management region. As used herein, the term “width”, when used with respect to the duct or component of the duct, refers to a dimension extending in a substantially spanwise direction of the aerofoil. So, a span of the aerofoil may extend in a substantially width direction of the duct or the airflow path. The width of the airflow path increasing in the downstream direction may ensure that the airflow is directed to a significant portion of the span of the aerofoil and it may also reduce the swirl caused by the airflow generator, thereby increasing the laminarity of the airflow and decreasing the angle of attack variation of the airflow. Advantageously, this may lead to increased lift production for the same airflow velocity reaching the aerofoil. The thickness of the airflow path decreasing in a downstream direction may limit or prevent deceleration of the airflow by allowing the duct to have an outlet to inlet ratio of 1 or less. It may also bring the airflow closer to the upper and lower surfaces of the aerofoil, which may increase efficiency of the lift generating system. The lift generating system may be for use in an aerial vehicle to enable vertical take-off and landing. In particular, the aerofoil may be the primary, or only, lift generating surface of the aerial vehicle. In other words, the lift generating system may be able to provide the entire lift required for vertical take-off and landing of the aerial vehicle. The aerofoil may be the primary, or only, lift generating surface during all phases of flight of the aerial vehicle. The duct may comprise a duct wall extending from the duct inlet to the duct outlet. The duct wall may circumscribe the airflow path, for example from the duct inlet to the duct outlet. The duct wall may define, or delimit, the airflow path. Therefore, the dimensions of the airflow path may correspond to the inner dimensions of the duct. The duct may be configured to present the airflow from the airflow generator to the aerofoil as a stream of air. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air in which a ratio between a thickness of the stream of air to a width of the stream of air is less than 1, preferably less than 0.5. For example, a cross-section of the airflow path at, or immediately upstream of, the aerofoil may have a thickness to width ratio of less than 1, preferably less than 0.5. Typically, the span of an aerofoil is much larger than a maximum thickness of the aerofoil. Therefore, presenting the airflow to the aerofoil as a stream of air having a similar width and thickness (such as having a square or circular cross-section) means that a significant volume of air may not interact with the aerofoil to generate lift, which is inefficient because energy has been expended in accelerating that airflow. Presenting the aerofoil with a stream of air having a thickness to width ratio of less than 1 means that the stream of air can be presented to at least a significant portion of the aerofoil whilst also being delivered close to one or both of the upper and lower surfaces of the aerofoil. Advantageously, this may lead to a lift generating system that is particularly efficient. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness greater than or equal to 150 percent of a maximum thickness of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness less than or equal to 300 percent of a maximum thickness of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness between 150 percent and 300 percent of a maximum thickness of the aerofoil. Advantageously, this may ensure the airflow is delivered close to one or both of the upper and lower surfaces of the aerofoil, thereby increasing efficiency. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness greater than or equal to 0.25 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness less than or equal to 0.5 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to the aerofoil as a stream of air having a thickness between 0.25 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil and 0.5 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. Advantageously, this may ensure the airflow is delivered close to one or both of the upper and lower surfaces of the aerofoil, thereby increasing efficiency. The duct may be arranged to direct the airflow from the airflow generator to both an upper surface of the aerofoil and a lower surface of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to at least 70 percent of a span of the aerofoil. The duct may be configured, such as dimensioned, to present the airflow to the entire span of the aerofoil. Advantageously, this may make use of the maximum area of the aerofoil to generate lift. The duct may comprise an upstream portion. The duct inlet may be located at an upstream end of the upstream portion. The duct may comprise a downstream portion. The duct outlet may be located at a downstream end of the downstream portion. The airflow management region may be at least a portion of the duct. Thus, may be referred to as an airflow management portion. The airflow management region may be positioned downstream of the upstream portion of the duct. The airflow management region may be positioned upstream of the downstream portion of the duct. The air flow management region may be positioned between the upstream portion and the downstream portion. Alternatively, the airflow management region may extend from the duct inlet to the duct outlet. Nevertheless, preferably, the duct outlet is located at, or in close proximity to, a downstream end of the airflow management region. Swirl intensity in the wake of the airflow generator, such as a fan or propeller, generally decreases in a downstream direction. Advantageously, by directing the airflow through the upstream portion of the duct before it enters the airflow management region, may allow for the swirl intensity to naturally reduce before entering into the airflow management region. The airflow management region may then further reduce the swirl intensity before the airflow is presented to the aerofoil. The duct may comprise a plurality of sub ducts arranged to form a continuous duct. For example, the upstream portion of the duct may be a first sub duct, and the airflow management region and the downstream portion of the duct may be a second sub duct. Alternatively, the duct may be a unitary duct, for example without any seam, join or connection between any sub ducts. The absence of any seam, join, or connection may mean that the inner surface of the duct is free from structures or imperfections that may introduce turbulence into the airflow. The upstream portion of the duct may have length greater than a thickness of the duct inlet. For example, a ratio between a thickness of the duct inlet and a length of the upstream portion of the duct may be greater than or equal to 1. A ratio between a thickness of the duct inlet and a length of the upstream portion of the duct may be greater than or equal to 2. A ratio between a thickness of the duct inlet and a length of the upstream portion of the duct may be less than or equal to 5. A ratio between a thickness of the duct inlet and a length of the upstream portion of the duct may be between 1 and 5, for example between 2 and 5. It has been found that swirl intensity may decrease to around 5% to 15% at a distance of 5 diameters downstream of the airflow generator. However, similar, or greater, reductions in swirl intensity may be achieved by providing the duct with an upstream portion having a length of at least 1 times the thickness of the duct inlet in combination with the airflow management region. Advantageously, an upstream portion having a length between 1 and 5 times the thickness of the duct inlet provides good natural swirl intensity reduction before the airflow reaches the airflow management region, without making the lift generating system excessively large which could result in cost and weight restrictions. The upstream portion of the duct may have a length greater than or equal to 200 millimetres. The upstream portion of the duct may have a length greater than or equal to 250 millimetres. The upstream portion of the duct may have a length less than or equal to 400 millimetres. The upstream portion of the duct may have a length less than or equal to 350 millimetres. The upstream portion of the duct may have a length between 200 millimetres and 400 millimetres. The upstream portion of the duct may have a length between 250 millimetres and 350 millimetres. The upstream portion of the duct may define a first portion of the airflow path. The first portion of the airflow path may have a cross-section that is constant in shape along the entire length of the upstream portion of the duct. The first portion of the airflow path may have constant cross-sectional dimensions along the entire length of the upstream portion. For example, both the width and thickness may be constant. That is, the first portion of the airflow path may not be convergent or divergent. Advantageously, this may allow the airflow to at least partially achieve uniformity and produce a boundary layer on the walls of the duct before entering into the airflow management region. This may assist in reducing the risk of flow separation during changes in dimensions of the duct in the airflow management region. However, in some embodiments, the upstream portion of the duct may be convergent. Therefore, the upstream portion of the duct may have a transverse cross-sectional area that decreases in a downstream direction. The first portion of the airflow path may have a cross-section that is circular or oval in shape. The first portion of the airflow path may have a thickness and a width that are both between 200 millimetres and 400 millimetres. Where the thickness and the width are the same, such as when the cross-section is circular in shape, the thickness or the width may be referred to as the diameter. The duct may have a length greater than or equal to 700 millimetres. The duct may have a length greater than or equal to 800 millimetres. The duct may have a length less than or equal to 1000 millimetres. The duct may have a length less than or equal to 900 millimetres. The duct may have a length between 700 millimetres and 1000 millimetres. A ratio between a length of the duct and a span of the aerofoil may be greater than or equal to 0.75. A ratio between a length of the duct and a span of the aerofoil may be greater than or equal to 0.85. Preferably, a ratio between a length of the duct and a span of the aerofoil may be greater than or equal to 1. Advantageously, it has been found that as the value of this ratio increases, the swirl intensity decreases. The thickness of the airflow path in the airflow management region may decrease in a downstream direction at the same time as the width of the airflow path in the airflow management region increases in the downstream direction. In other words, in the airflow management region, the thickness of the airflow path decreases and the width of the airflow path increases simultaneously. Thus, each subsequent cross-section of the airflow path in a downstream direction may have a thickness less than any previous cross-section of the airflow path in the airflow management region and a width greater than a width of any previous cross-section of the airflow path in the airflow management region. This may allow the airflow management region of the duct to be shorter than a duct which first decreases in thickness and then increases in width, or vice versa. Advantageously, a shorter duct may mean reduced energy losses of the airflow due to contact with the duct wall. It may also result in a lower weight and cost of the lift generating system. Furthermore, the decrease in the thickness of the airflow path may aid in preventing, or limiting, boundary layer separation as the width of the airflow path increases. A thickness of the airflow path at an upstream end of the airflow management region may be greater than a thickness of the airflow path at a downstream end of the airflow management region, and a width of the airflow path at the upstream end of the airflow management region may be less than a width of the airflow path at a downstream end of the airflow management region. Due to the thickness of the airflow path decreasing and a width of the airflow path increasing in the downstream direction, in the airflow management region, a cross-section of the airflow path at a downstream end of the air management region may have a width and a thickness in which the width of the cross-section is greater than the thickness of the cross-section. For example, a ratio between a thickness of the cross-section and a width of the cross-section may be less than 1, preferably less than 0.5. In the airflow management region, the rate of change of the thickness of the airflow path may vary in a downstream direction. In particular, in the airflow management region, the magnitude of a decrease in thickness per unit length of an upstream portion of the airflow path may be greater than the magnitude of a decrease in thickness per unit length of a downstream portion of the airflow path. In other words, the thickness of the airflow path decreases more quickly in the upstream portion of the airflow management region compared to the downstream portion of the airflow management region. Advantageously, this may ensure uniformity of the airflow before being presented to the aerofoil. In the airflow management region, the rate of change of the width may vary in a downstream direction. In particular, in the air flow management region, the magnitude of an increase in width per unit length of an upstream portion of the airflow path may be greater than the magnitude of an increase in width per unit length of a downstream portion of the airflow path. In other words, the width of the airflow path increases more quickly in the upstream portion of the airflow management region compared to the downstream portion of the airflow management region. Advantageously, this may ensure that the airflow is substantially uniform before being presented to the aerofoil. In the airflow management region, the width of the airflow path may increase more quickly than the thickness of the airflow path decreases. In particular, in the airflow management region, the magnitude of an increase in width per unit length of the airflow path may be greater than the magnitude of a decrease in thickness per unit length of the airflow path. ln the airflow management region, the airflow path may have a decreasing cross-sectional area in the downstream direction. A ratio between a cross-sectional area of the airflow path at a downstream end of the airflow management region and a cross-sectional area of the airflow path at an upstream end of the airflow management region may be less than or equal to 1. A ratio between a cross-sectional area of the airflow path at a downstream end of the airflow management region and a cross-sectional area of the airflow path at an upstream end of the airflow management region may be less than 0.9. A ratio between a cross-sectional area of the airflow path at a downstream end of the airflow management region and a cross-sectional area of the airflow path at an upstream end of the airflow management region may be less than 0.8. Advantageously, the airflow management region may therefore be convergent and accelerate the airflow. The airflow management region may have a length of greater than or equal to 300 millimetres. The airflow management region may have a length of greater than or equal to 350 millimetres. The airflow management region may have a length of greater than or equal to 400 millimetres. Advantageously, an airflow management region having a length of greater than or equal to 300 millimetres has been found to allow high velocity airflow at a downstream end of the airflow management region with low swirl intensity. The airflow management region may have a length of less than or equal to 500 millimetres. The airflow management region may have a length of less than or equal to 450 millimetres. The airflow management region may have a length of less than or equal to 400 millimetres. Advantageously, an airflow management region having a length of less than or equal to 500 millimetres may ensure that the lift generating system is compact. The airflow management region may have a length of between 300 millimetres and 500 millimetres. The airflow management region may have a length of between 350 millimetres and 450 millimetres. Advantageously, an airflow management region having a length of between 300 millimetres and 500 millimetres may provide a trade-off between high velocity airflow, low swirl intensity, and compactness. The downstream portion of the duct may extend from the downstream end of the airflow management region to the duct outlet. The downstream portion of the duct may have a constant thickness along its entire length. The downstream portion of the duct may have a constant width along its entire length. The downstream portion of the duct may have a length greater than or equal to 50 millimetres. The downstream portion of the duct may have a length greater than or equal to 75 millimetres. The downstream portion of the duct may have length less than or equal to 150 millimetres. The downstream portion of the duct may have length less than or equal to 125 millimetres. The downstream portion of the duct may have a length of between 50 millimetres and 150 millimetres. The downstream portion of the duct may have a length of between 75 millimetres and 150 millimetres. The duct may define a longitudinal axis extending through the geometric centre of, and perpendicular to the cross-section of, the duct inlet. The duct outlet may be offset from the longitudinal axis in the thickness direction. When viewed parallel to the longitudinal axis, at least 20 percent of the cross-sectional area of the duct outlet may overlap with the cross-sectional area of the duct inlet. When viewed parallel to the longitudinal axis, at least 30 percent of the cross-sectional area of the duct outlet may overlap with the cross-sectional area of the duct inlet. This may allow a portion of the airflow to be directed to the aerofoil with minimal change in direction. Advantageously, this may increase the efficiency of the lift generating system. The leading edge of the aerofoil may be positioned in proximity to the duct outlet. The span of the aerofoil may extend in a substantially width direction of the duct outlet. The maximum thickness of the aerofoil may extend in a substantially thickness direction of the duct outlet. The leading edge of the aerofoil may be positioned at, or immediately downstream of, the duct outlet. The leading edge of the aerofoil may be positioned at the duct outlet. That is, the position of the most upstream end of the aerofoil may coincide with the cross-section of the duct outlet. Advantageously, by positioning the leading edge at the duct outlet, the aerofoil does not interfere with the duct’s manipulation of the airflow and also the airflow may not interact with the free stream air before reaching the leading edge of the aerofoil. The leading edge of the aerofoil may be positioned greater than or equal to a quarter of a chord of the aerofoil from the duct outlet. The leading edge of the aerofoil may be positioned less than or equal to half of a chord of the aerofoil from the duct outlet. The leading edge of the aerofoil may be positioned between quarter of a chord of the aerofoil and half of a chord of the aerofoil from the duct outlet. This may be measured with respect to the maximum chord of the aerofoil. Thus, a quarter of the chord of the aerofoil may refer to a quarter of the maximum chord of the aerofoil, and half of a chord of the aerofoil may refer to half of the maximum chord of the aerofoil. Advantageously, the leading edge of the aerofoil being positioned greaterthan orequal to a quarter of a chord of the aerofoil from the duct outlet may cause flow separation from the aerofoil to occur at a higher angle of attack. Additionally, at these distances, the airflow may reach the aerofoil with optimum velocity. For example, when the leading edge of the aerofoil is too far within the duct, this can cause a blockage effect which may reduce the velocity of the airflow that reaches the aerofoil. As another example, when the leading edge of the aerofoil is too far downstream of the duct outlet, this may allow the airflow to expand and thereby reduce in velocity before reaching the aerofoil. The duct outlet may be shaped to conform to a spanwise shape of the leading edge. For example, if the aerofoil has an upwards ordownwards curvature, then the duct outlet may similarly have an upwards or downwards curvature. The duct outlet may have a width and a thickness in which the width of the duct outlet may be greaterthan the thickness of the duct outlet. For example, a ratio between a thickness of the duct outlet and a width of the duct outlet may be less than 1, preferably less than 0.5. The duct outlet may have a thickness greater than or equal to 50 percent of a maximum thickness of the aerofoil. The duct outlet may have a thickness greaterthan orequal to 150 percent of a maximum thickness of the aerofoil. The duct outlet may have thickness less than or equal to 300 percent of a maximum thickness of the aerofoil. The duct outlet may have a thickness less than or equal to 250 percent of a maximum thickness of the aerofoil. The duct outlet may have a thickness of between 50 percent and 300 percent of a maximum thickness of the aerofoil. The duct outlet may have a thickness of between 150 percent and 300 percent a maximum thickness of the aerofoil. The duct outlet may have thickness greater than or equal to 0.25 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. The duct outlet may have a thickness less than or equal to 0.5 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. The duct outlet may have a thickness between 0.25 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil and 0.5 multiplied by sin(8 degrees) multiplied by the maximum chord of the aerofoil. Advantageously, this may ensure the airflow is delivered close to one or both of the upper and lower surfaces of the aerofoil, thereby increasing efficiency. This may also ensure that the stream of air presented to the aerofoil is not too thick or too thin. The duct outlet may have a width greater than or equal to 70 percent of the span of the aerofoil. The duct outlet may have a width greater than or equal to 100 percent of the span of the aerofoil. The duct outlet may have a width less than or equal to 105 percent of the span of the aerofoil. The duct outlet may have a width less than or equal to 101 percent of span of the aerofoil. Preferably, the duct outlet may have a width of between 70 percent and 105 percent of the span of the aerofoil. More preferably, the duct outlet may have a width of between 100 percent and 101 percent of the span of the aerofoil. The thickness of the duct inlet may be greater than the thickness of the duct outlet, and the width of the duct inlet may be less than the width of the duct outlet. The duct does not need to have overall convergence. For example, a ratio between a cross-sectional area of the duct inlet to a cross-sectional area of the duct outlet may be between 0.6 and 1.3. However, preferably, a ratio between a cross-sectional area of the duct inlet to a cross-sectional area of the duct outlet may be less than or equal to 1. The airflow management region may be immediately upstream of the duct outlet. The duct may be substantially empty downstream of the airflow generator. Advantageously, this may provide an unimpeded flow and avoid vortices being generated within the duct that could reduce the quality of the airflow delivered to the aerofoil. Alternatively, the duct may comprise one or more airflow guide vanes. The airflow generator may be positioned within the duct. For example, the airflow generator may be positioned within the upstream portion of the duct. Advantageously, this may mean that the lift generating system has a lower number of exposed moving parts that may present a safety hazard. This may make an aerial vehicle comprising the lift generating system particularly suitable for use in urban or densely populated environments compared to some other known VTOL aerial vehicles, such as helicopters. The airflow generator may not extend into the airflow management region. The airflow generator may not extend into the downstream portion of the duct. Advantageously, this may mean that the airflow generator does not interfere with the airflow management region’s manipulation of the airflow. The airflow generator may be configured to draw air into the duct through the duct inlet. The duct may be configured to direct the entire volume of air drawn into the duct to the aerofoil. Advantageously, this may increase the efficiency of the lift generating system. The airflow generator may comprise at least one fan. For example, the at least one fan may comprise a plurality of fans. Alternatively, the at least one fan may be a single fan. The, or each of the, at least one fan may be an axial fan. The, or each of the, at least one fan may have a diameter of between 90 millimetres and 250 millimetres. The, or each of the, at least one fan may comprise a plurality of fan blades. The tip clearance between the plurality of fan blades and the duct may be less than 10 millimetres. Advantageously, in embodiments comprising a plurality of fans, the plurality of fans may provide a degree of redundancy in case one or more of the fans should fail. The at least one fan may comprise contrarotating fans. In preferred embodiments, the tip clearance is between 0 percent and 30 percent of the diameter of the at least one fan. The duct inlet may have a diameter of between 100 percent and 130 percent of a diameter of the at least one fan. In embodiments in which the airflow generator comprises a single fan, the length of the duct may be defined by the following equation: cis — fd 1.5 * fd <dl <------J—— 2 * tan (35) where: fd is the fan diameter; dl is the length of the duct; and as is the aerofoil span. The airflow generator may comprise at least one motor for rotating the at least one fan. The, or each of the, at least one motor may be an electric motor. Advantageously, the use of an electric motor may result in the aerial vehicle being more environmentally-friendly during flight. This can be particularly beneficial in urban environments where exhausting combustion gases may present challenges with environmental regulations. Alternatively, the airflow generator may comprise at least one turbofan. The airflow generator may be configured to provide a mass flow rate of at least 3.5 kilograms per second. The airflow generator may be configured to provide a mass flow rate of at least 4.5 kilograms per second. The lift generating system may comprise control means for moving the airflow generator relative to the duct. For example, the control means may be configured to move the airflow generator upstream or downstream. Advantageously, this may allow the position of the airflow generator to be changed during different stages of flight in order to achieve an optimal velocity of airflow presented to the aerofoil. The lift generating system may comprise an air speed sensor arranged to determine a speed of the airflow presented to the aerofoil and a control unit connected to both the air speed sensor and the at least one motor. The control unit may be configured to control an RPM of the at least one fan to maintain the speed of the airflow presented to the aerofoil to be within a predetermined range. The predetermined range may be greater than or equal to 60 metres per second. The predetermined range may be greater than or equal to 80 metres per second. The predetermined range may be greater than or equal to 100 metres per second. The predetermined range may be greater than or equal to 120 metres per second. The predetermined range may be greater than or equal to 140 metres per second. The predetermined range may be greater than or equal to 180 metres per second. The predetermined range may be less than or equal to 200 metres per second The predetermined range may be less than or equal to 180 metres per second. The predetermined range may be less than or equal to 160 metres per second. The predetermined range may be less than or equal to 140 metres per second. The predetermined range may be less than or equal to 120 metres per second. The predetermined range may be between 60 metres per second and 200 metres per second. The predetermined range may be between 80 metres per second and 200 metres per second. The predetermined range may be between 100 metres per second and 200 metres per second. The predetermine range may be between 120 metres per second and 200 metres per second. The aerofoil may have a span of greater than or equal to 500 millimetres. The aerofoil may have a span of greater than or equal to 1000 millimetres. The aerofoil may have a span of greater than or equal to 2000 millimetres. The aerofoil may have a span of greater than or equal to 3000 millimetres. The aerofoil may have a span of less than or equal to 3000 millimetres. The aerofoil may have a span of less than or equal to 2000 millimetres. The aerofoil may have a span of less than or equal to 1000 millimetres. The aerofoil may have a span of between 500 millimetres and 3000 millimetres. The aerofoil may have a span of between 1000 millimetres and 3000 millimetres. The aerofoil may have a span of between 1000 millimetres and 2000 millimetres. The aerofoil may have a chord of greater than or equal to 100 millimetres. The aerofoil may have a chord of greater than or equal to 150 millimetres. The aerofoil may have a chord of less than or equal to 300 millimetres. The aerofoil may have a chord of less than or equal to 200 millimetres. The aerofoil may have a chord of between 100 millimetres and 200 millimetres. The aerofoil may have an aspect ratio of greater than or equal to 2.0. The aerofoil may have an aspect ratio of greater than or equal to 2.5. The aerofoil may have an aspect ratio of greater than or equal to 3. During operation of the lift generating system, the aerofoil may be moveable to increase or decrease an angle of attack of the aerofoil relative to the airflow presented to the aerofoil by the duct. For example, the lift generating system may comprise a motor or pneumatic assembly operable to actuate movement of the, or at least a portion of the, aerofoil. Advantageously, this may enable the lift generating system to not only control lift production through varying the speed of the airflow generator, but also by adjusting the angle of attack of the aerofoil. This may allow an aerial vehicle to react more quickly to changes in flight parameters. The aerofoil may comprise a stabiliser extending from a trailing edge of the aerofoil. A span of the aerofoil may comprise a first outboard section, a second outboard section, and a midsection positioned between the first outboard section and the second outboard section. Each of the first outboard section and the second outboard section may form an anhedral angle relative to the midsection. Alternatively, each of the first outboard section and the second outboard section may form a dihedral angle relative to the midsection. Alternatively, the aerofoil may have no anhedral or dihedral. During operation of the lift generating system, each of the first outboard section and the second outboard section may be movable independently of one another between a first position and a second position. The angle of attack relative to the airflow presented by the duct being different in the first position and the second position. Advantageously, this may enable to the aerofoil to control roll of the aerial vehicle. During operation of the lift generating system, each of the first outboard section and the second outboard section may be movable independently of one another between a first position having a first angle of attack relative to the airflow presented by the duct and a second position having a second angle of attack relative to the airflow presented by the duct. The second angle of attack may be different to the first angle of attack. The second angle of attack may be greater than the first angle of attack. According to the present disclosure, there is provided an aerial vehicle comprising the lift generating system as disclosed herein. In particular, the aerial vehicle may be capable of vertical take-off and landing. The aerofoil may be the primary lift generator of the aerial vehicle during all stages of flight. That is, the aerofoil may provide the aerial vehicle with the largest proportion of lift compared to any other lifting surfaces of the aerial vehicle. The airflow directed to the aerofoil may generate lift. However, a further effect may be the generation of thrust. The aerial vehicle may comprise one or more propulsion units for providing one or more of: yaw control of the aerial vehicle and counter-thrust to counter any thrust generated by airflow exhausted from the duct outlet. Preferably, the one or more propulsion units are positioned outside of the airflow exhausted from the duct outlet. That is, the one or more propulsion units are arranged so that that the airflow exhausted from the duct outlet does not interact with the one or more propulsion units. The one or more propulsion units may be attached or mounted to the duct or the aerofoil. The one or more propulsion units may comprise a ducted fan or ducted propeller. The aerial vehicle may comprise one or more payload compartments. Additionally, or alternatively, the aerial vehicle may comprise one or more attachment points for releasably attaching a payload to the aerial vehicle. According to the present disclosure, there is provided a kit of parts for assembling the lift generating system as disclosed herein. The kit of parts comprising the airflow generator and the duct. The kit of parts may comprise the aerofoil. The invention is defined in the claims. However, below there is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein. Example 1. A lift generating system for an aerial vehicle, comprising: an airflow generator for generating an airflow; an aerofoil for generating lift for the aerial vehicle; and a duct defining a duct inlet, a duct outlet and an airflow path extending from the duct inlet to the duct outlet, the duct being arranged to direct the airflow from the airflow generator to the aerofoil; wherein the duct comprises an airflow management region in which a thickness of the airflow path decreases in a downstream direction and a width of the airflow path increases in the downstream direction; and wherein at least a portion of the aerofoil is positioned within, or downstream of, the airflow management region. Example 2. A lift generating system according to Example 1, wherein the duct is configured to present the airflow to the aerofoil as a stream of air in which a ratio between a thickness of the stream of air to a width of the stream of air is less than 1, preferably less than 0.5. Example 3. A lift generating system according to Example 1 or 2, wherein the duct is configured to present the airflow to the aerofoil as a stream of air having a thickness less than or equal to 300 percent of a maximum thickness of the aerofoil. Example 4. A lift generating system according to any preceding Example, wherein the duct is arranged to direct the airflow from the airflow generator to both an upper surface of the aerofoil and a lower surface of the aerofoil. Example 5. A lift generating system according to any preceding Example, wherein the duct is configured to present the airflow to at least 70 percent of a span of the aerofoil. Example 6. A lift generating system according to any preceding Example, wherein the duct is configured to present the airflow to the entire span of the aerofoil. Example 7. A lift generating system according to any preceding Example, wherein the airflow management region is immediately upstream of the duct outlet. Example 8. A lift generating system according any preceding Example, wherein a leading edge of the aerofoil is positioned at, or immediately downstream of, the duct outlet. Example 9. A lift generating system according to any preceding Example, wherein the duct outlet has a width and a thickness, the width of the duct outlet being greater than the thickness of the duct outlet. Example 10. A lift generating system according to Example 9, wherein the thickness of the duct outlet is between 50 percent and 300 percent of a maximum thickness of the aerofoil. Example 11. A lift generating system according to Example 9 or 10, wherein the duct outlet has a width greater than or equal to 100 percent of the span of the aerofoil. Example 12. A lift generating system according to any preceding Example, wherein a span of the aerofoil extends in a substantially width direction of the duct outlet. Example 13. A lift generating system according to any preceding Example, wherein the airflow management region comprises one or more airflow guide vanes. Example 14. A lift generating system according to any preceding Example, wherein a crosssection of the airflow path at a downstream end of the air management region has a width and a thickness, the width of the cross-section being greater than the thickness of the cross-section. Example 15. A lift generating system according to any preceding Example, wherein, in the airflow management region, the thickness of the airflow path decreases and the width of the airflow path increases simultaneously. Example 16. A lift generating system according to any preceding Example, wherein, in the air flow management region, the magnitude of a decrease in thickness per unit length of an upstream portion of the airflow path is greater than the magnitude of a decrease in thickness per unit length of a downstream portion of the airflow path. Example 17. A lift generating system according to any preceding Example, wherein, in the air flow management region, the magnitude of an increase in width per unit length of an upstream portion of the airflow path is greater than the magnitude of an increase in width per unit length of a downstream portion of the airflow path. Example 18. A lift generating system according to any preceding Example, wherein, in the airflow management region, the magnitude of an increase in width per unit length of the airflow path is greater than the magnitude of a decrease in thickness per unit length of the airflow path. Example 19. A lift generating system according to any preceding Example, wherein, in the airflow management region, the airflow path has a decreasing cross-sectional area in the downstream direction. Example 20. A lift generating system according to any preceding Example, wherein the airflow generator is positioned within the duct and is configured to draw air into the duct through the duct inlet. Example 21. A lift generating system according to Example 20, wherein the duct is configured to direct the entire volume of air drawn into the duct to the aerofoil. Example 22. A lift generating system according to Example 20 or 21, wherein the duct comprises an upstream portion positioned upstream of the airflow management region, wherein the airflow generator is positioned within the upstream portion of the duct. Example 23. A lift generating system according to Example 22, wherein the airflow path within the upstream portion of the duct has a constant cross-sectional area. Example 24. A lift generating system according to Example 22 or 23, wherein the airflow generator does not extend into the airflow management region of the duct. Example 25. A lift generating system according to any preceding Example, wherein the airflow generator comprises a fan and a motor for rotating the fan. Example 26. A lift generating system according to Example 25, wherein the fan has a diameter of between 90 millimetres and 250 millimetres. Example 27. A lift generating system according to Example 25 or 26, wherein the motor is an electric motor. Example 28. A lift generating system according to any one of Examples 25 to 27 comprising an air speed sensor arranged to determine a speed of the airflow presented to the aerofoil and a control unit connected to both the air speed sensor and the motor; wherein the control unit is configured to control an RPM of the fan to maintain the speed of the airflow presented to the aerofoil to be within a predetermined range. Example 29. A lift generating system according to Example 28, wherein the predetermined range is 60 metres per second to 200 metres per second. Example 30. A lift generating system according to any preceding Example, wherein the aerofoil has a span of between 500 millimetres and 3000 millimetres. Example 31. A lift generating system according to any preceding Example, wherein the aerofoil has a chord of between 100 millimetres and 200 millimetres. Example 32. A lift generating system according to any preceding Example, wherein the aerofoil has an aspect ratio greater than or equal to 2.0. Example 33. A lift generating system according to any preceding Example, wherein, during operation of the lift generating system, the aerofoil is moveable to increase or decrease an angle of attack of the aerofoil relative to the airflow presented to the aerofoil by the duct. Example 34. A lift generating system according to any preceding Example, wherein the aerofoil comprises a stabiliser extending from a trailing edge of the aerofoil. Example 35. A lift generating system according to any preceding Example, wherein a span of the aerofoil comprises a first outboard section, a second outboard section, and a midsection positioned between the first outboard section and the second outboard section. Example 36. A lift generating system according to Example 35, wherein each of the first outboard section and the second outboard section form an anhedral angle relative to the midsection. Example 37. A lift generating system according to Example 35, wherein each of the first outboard section and the second outboard section form a dihedral angle relative to the midsection. Example 38. A lift generating system according to anyone of Examples 35 to 37, wherein, during operation of the lift generating system, each of the first outboard section and the second outboard section are movable independently of one another between a first position having a first angle of attack relative to the airflow presented by the duct and a second position having a second angle of attack relative to the airflow presented by the duct, wherein the second angle of attack is different to the first angle of attack. Example 39. A lift generating system according to any preceding Example, wherein a span of the aerofoil extends in a substantially width direction of the airflow path. Example 40. An aerial vehicle comprising the lift generating system according to any preceding Example. Example 41. An aerial vehicle according to Example 40, wherein the aerofoil is the primary lift generator of the aerial vehicle during all stages of flight. Example 42. An aerial vehicle to according to Example 40 or 41 comprising one or more propulsion units for providing one or more of: yaw control of the aerial vehicle and counter-thrust to counter any thrust generated by airflow exhausted from the duct outlet. Example 43. An aerial vehicle according to Example 42, wherein the one or more propulsion units are positioned outside of the airflow exhausted from the duct outlet. Example 44. An aerial vehicle according to Example 42 or 43 wherein the one or more propulsion units are attached or mounted to the duct or the aerofoil. Example 45. An aerial vehicle according to any one of Examples 42 to 44, wherein the one or more propulsion units are duct fans or propellers. Example 46. A kit of parts for assembling the lift generating system according to any one of Examples 1 to 39, the kit of parts comprising the airflow generator and the duct. BRIEF DESCRIPTION OF THE DRAWINGS Examples will now be further described with reference to the figures in which: Figure 1 shows a schematic perspective view of a lift generating system according to a first embodiment of the present disclosure; Figure 2 shows a schematic side view of the lift generating system of Figure 1; Figure 3 shows a schematic plan view of the lift generating system of Figure 1; Figure 4 shows a schematic rear end view of the lift generating system of Figure 1; Figure 5 shows a schematic front end view of the lift generating system of Figure 1; Figure 6 shows a plan view of a computational fluid dynamics analysis of the lift generating system of Figure 1; Figure 7 shows a schematic perspective view of a lift generating system according to a second embodiment of the present disclosure; Figure 8 shows a schematic side view of the lift generating system of Figure 7; Figure 9 shows a schematic plan view of the lift generating system of Figure 7; Figure 10 shows a schematic front end view of the lift generating system of Figure 7; Figure 11 shows a plan viewof a computational fluid dynamics analysis of the lift generating system of Figure 7; Figure 12 shows a schematic plan viewof an aerial vehicle according to a third embodiment of the present disclosure; Figure 13 shows a schematic plan view of an aerial vehicle according to a fourth embodiment of the present disclosure; and Figure 14 shows a schematic front view of the aerial vehicle of Figure 13. DETAILED DESCRIPTION Figures 1 to 5 show different views of a lift generating system 100 according to a first embodiment of the present disclosure. Figure 6 shows a plan view of a computational fluid dynamics analysis of the lift generating system 100. The lift generating system 100 comprises an airflow generator 120 for generating an airflow and an aerofoil 140 for generating lift from the generated airflow. The lift generating system 100 further comprises a duct 160 defining a duct inlet 162, a duct outlet 164 and an airflow path extending from the duct inlet 162 to the duct outlet 164. The duct 160 comprises a duct wall 165 extending from the duct inlet 162 to the duct outlet 164. The duct wall 165 circumscribes and delimits the airflow path along the entire length of the duct 160, so the dimensions of the airflow path correspond to the inner dimensions of the duct 160. The duct 160 has three sections: an upstream portion 166, a downstream portion 168, and an airflow management region or portion 167 positioned between the upstream portion 166 and the downstream portion 168. The upstream portion 166 has a length of about 302 millimetres, the airflow management region 167 has a length of about 350 millimetres and the downstream portion 168 has a length of about 100 millimetres. Therefore, the duct 160 is relatively compact and has an overall length of about 752 millimetres. The airflow generator 120 is positioned within the upstream portion 166 of the duct 160 and is proximal to the duct inlet 162. The airflow generator 120 comprises a ducted fan and an electric motor to power the fan. Therefore, as the fan rotates, it generates an airflow within the duct 160 by drawing air into the duct 160 via the duct inlet 162. The duct 160 then directs the airflow from the airflow generator 120 to the aerofoil 140 via the upstream portion 166, the airflow management region or portion 167 and the downstream potion 168. In this embodiment, the airflow generator 120 has the properties set out in Table 1 below. Properties Value Ducting inside diameter 195 mm Fan swept area 215 cm2 Total weight (including motor) 3400 g Static thrust range 215 N to 250 N Exhaust speed 84 m / s to 98 m / s RPM range 12,000 rpm to 14,000 rpm Electrical input power 9.8 kWto 15.6 kW Battery 12-14S 20000 mAh LiPo Table 1: Airflow generator properties A suitable airflow generator having these properties is the DS-215-DIA HST®, which at the date of filing, is manufactured and sold by Schubeler GmbH, Germany. However, the skilled person will appreciate that the lift generating system 100 is not limited to having an airflow generator 120 with the above properties. The airflow within the duct 160 is constrained by the duct wall 165 and forms a stream of air that passes through the duct 160 before exiting the duct 160 at the duct outlet 164, where it is then delivered to the aerofoil 140. However, the airflow downstream of the airflow generator 120 has a high swirl intensity which can decrease the amount of lift that the aerofoil 140 can generate. Additionally, the dimensions of the duct 160 at, or towards, the duct inlet 162 are generally dictated by the dimensions of airflow generator 120. However, delivering the airflow to the aerofoil 140 as a stream of air having the dimensions of the duct 160 at, or towards, the duct inlet 162 would reduce the efficiency of the lift generating system 100 as the aerofoil 140 is generally much thinner and much wider than these dimensions. Therefore, the air flow management region 167 of the duct 160 is designed to spread out the stream of air along the span of the aerofoil 140 and decrease the thickness of the stream of air. The air flow management region 167 is also shaped to manipulate the airflow to have a substantially uniform velocity, negligible sideslip, minimal swirl, a minimal angle of attack variation, and to be substantially laminar. As seen in at least Figures 2 and 3, in the airflow management region 167, the dimensions of the duct 160 (and therefore the airflow path) markedly change. In particular, in the airflow management region 167, the thickness of the airflow path decreases in the downstream direction at the same time as the width of the airflow path increases in the downstream direction. Thus, each subsequent transverse cross-section of the airflow path in a downstream direction has a thickness less than any previous transverse cross-section of the airflow path in the airflow management region 167 and also has a width greater than a width of any previous transverse cross-section of the airflow path in the airflow management region 167. After passing through the airflow management region 167, the airflow passes through the downstream portion 168 of the duct 160 which has a constant width and thickness. The airflow then exits the duct 160 via the duct outlet 164 and is delivered to the aerofoil 140. As seen in Figure 4, the duct outlet 164 has a transverse cross-sectional shape generally corresponding to the transverse cross-sectional shape of aerofoil 140, and a width and thickness similar to the width and thickness of the aerofoil 140. In more detail, the duct outlet 164 has a width of about 652 millimetres and a thickness of about 40 millimetres. Due to the airflow management region 167, the airflow delivered to the aerofoil 140 is generally uniform. For example, it can be seen from the air flow streamlines 170 in Figure 6 that the spanwise components of the air flow at the duct outlet 164 are negligible. The vertical components of the air flow at the duct outlet 164 are also negligible. The aerofoil 140 has an Eppler420 shape having a thickness of about 8 percent, a span of about 652 millimetres, a chord length of about 187 millimetres and an aspect ratio of about 3.49. The aerofoil 140 is coupled to the duct 160 via a support frame (not shown) and is positioned downstream of the duct outlet 164. In more detail, the leading edge 141 of the aerofoil 140 is positioned about 30 millimetres downstream of the duct outlet 164. As seen in Figure 4, the aerofoil 140 is vertically aligned with the duct outlet 164, although in some embodiments the aerofoil 140 may be moveable in a vertical direction during operation of the lift generating system 100. Also as seen in Figure 4, the span of the aerofoil 140 extends in a width direction of the duct outlet 164 and the thickness of the aerofoil 140 extends in a thickness direction of the duct outlet 164. The aerofoil 140 comprises a horizontal stabiliser 148 extending from the trailing edge of the aerofoil 140. The aerofoil 140 is also divided into three main sections along its span: a first outboard section 142, a second outboard section 144, and a midsection 146 positioned between the first outboard section 142 and the second outboard section 146. The first outboard section 142 and the second outboard section 144 are each coupled to the midsection 146 of the aerofoil 140 at an anhedral angle. Therefore, each of the first outboard section 142 and the second outboard section 144 are directed downward and, in this embodiment, each form an angle of about 45 degrees with respect to the midsection 146 of the aerofoil 140. The first outboard section 142 and the second outboard section 144 are not fixed relative to the midsection 146 of the aerofoil 140 but are, instead, each coupled to the midsection 146 of the aerofoil 140 with a movable coupling operated by an actuator housed within the aerofoil 140. This means that each of the first outboard section 142 and the second outboard section 146 are movable independently of one another and the midsection 146 to increase or decrease their angle of attack with respect to airflow exiting the duct outlet 164. In this embodiment, the first outboard section 142 and the second outboard section 146 pivot about coupling points positioned towards the leading edge 141 of the aerofoil 140. This allows the first outboard section 142 and the second outboard section 144 to provide roll control. Additionally, the entire aerofoil 140 is also movable by an actuator housed within the aerofoil 140 to increase or decrease the angle of attack of the aerofoil 140 with respect to the airflow exiting the duct outlet 164. Other components of the lift generating system 100 are also housed within the aerofoil 140. For example, electronic components, such a flight control computer, and batteries for powering the airflow generator 120 and the aerofoil 140 actuators are also housed within the aerofoil 140. Velocity [m / s] Alpha [deg] Lift Force [N] 60 0 155 8 286 80 0 275 8 509 90 0 348 8 644 100 0 430 8 796 110 0 520 8 963 120 0 619 8 1146 Table 2: Lift force produced by the aerofoil 140 Table 2 above shows the lift force produced by the aerofoil 140 during operation of the lift generating system 100. The first column shows the velocity, in metres per second, of the airflow delivered to the aerofoil 140. The second column shows the angle of attack, in degrees, of the aerofoil 140 relative to the duct outlet 162. The third column shows the lift force, in Newtons, produced by the aerofoil 140. As seen from Table 2, despite its compact size, the lift generating system 100 can produce considerable amounts of lift. Moreover, even higher lift forces have been achieved at higher airflow velocities and with alternative embodiments. For example, the lift force produced by the aerofoil in one alternative embodiment, in which the aerofoil has a span of 2540 millimetres and a chord of 1000 millimetres, is set out in Table 3 below. Velocity [m / s] Alpha [deg] Lift Force [N] 60 0 2365 8 4374 80 0 4204 8 7775 90 0 5320 8 9841 100 0 6568 8 12149 110 0 7948 8 14700 120 0 9458 8 17495 Table 3: Lift force produced by an alternative embodiment Figures 7 to 10 show different views of a lift generating system 200 according to a second embodiment of the present disclosure. Figure 11 shows a plan view of computational fluid dynamics analysis of the lift generating system 200. As with the lift generating system 100 according to the first embodiment, the lift generating system 200 according to the second embodiment comprises an airflow generator 220 for generating an airflow and an aerofoil 240 for generating lift from the airflow. The lift generating system 200 further comprises a duct 260 defining a duct inlet 262, a duct outlet 264 and an airflow path extending from the duct inlet 262 to the duct outlet 264. As with the first embodiment of the lift generating system 100, the duct 260 has an upstream portion 266, a downstream portion 268, and an airflow management region or portion 267 positioned between the upstream portion 266 and the downstream portion 268. The upstream portion 266 has a length of about 300 millimetres, the airflow management region 267 has a length of about 350 millimetres and the downstream portion 268 has a length of about 100 millimetres. The airflow generator 220 is positioned within the upstream portion 266 of the duct 260 and is proximal to the duct inlet 262. The airflow generator 220 comprises a first fan, a first electric motorto power the first fan, a second fan, and a second electric motor to power the second fan. Each fan has a radius of about 105 millimetres. Therefore, the duct inlet 262 has a thickness of about 210 millimetres and a width of about 420 millimetres. The fan blade tip clearance between each of the first fan and the second fan is minimal. Similarly, the fan blade tip clearance between the duct 260 and each of the first fan and the second fan is minimal. The airflow from the airflow generator 220 passes through the airflow management region 267 of the duct 260. As seen in at least Figures 8 and 9, in the airflow management region 267, the dimensions of the duct 260 (and therefore the airflow path) markedly change. In particular, in the airflow management region 267, the thickness of the airflow path decreases in the downstream direction at the same time as the width of the airflow path increases in the downstream direction. Thus, each subsequent transverse cross-section of the airflow path in a downstream direction has a thickness less than any previous transverse cross-section of the airflow path in the airflow management region 267 and a width greater than a width of any previous transverse cross-section of the airflow path in the airflow management region 267. After passing through the airflow management region 267, the airflow passes through the downstream portion 268 of the duct 260 which has a constant width and thickness, where it exits the duct 260 via the duct outlet 264 and is delivered to the aerofoil 240. As seen in Figure 7, the duct outlet 264 has a transverse cross-sectional shape generally corresponding to the transverse cross-sectional shape of aerofoil 240 and a width and thickness similar to the width and thickness of the aerofoil 240. In more detail, the duct outlet 264 has a width of about 739 millimetres and a thickness of about 40 millimetres. Due to the airflow management region 267, the airflow delivered to the aerofoil 240 is generally uniform. For example, it can be seen from the air flow streamlines 270 in Figure 11 that the spanwise components of the air flow at the duct outlet 264 are negligible. The vertical components of the air flow at the duct outlet 264 are also negligible. The aerofoil 240 is coupled to the duct 260 via a support frame (not shown) and is positioned downstream of the duct outlet 264. In more detail, the leading edge of the aerofoil 240 is positioned about 30 millimetres downstream of the duct outlet 264. As seen in Figure 8, the aerofoil 240 is vertically aligned with the duct outlet 264 but, as with the first embodiment, the aerofoil 240 may be moveable in a vertical direction during different stages of flight. Moreover, the entire aerofoil 240 is movable by an actuator housed within the aerofoil 240 to increase or decrease the angle of attack of the aerofoil 240 with respect to the airflow exiting the duct outlet 264 The aerofoil 240 is generally planar in a spanwise direction and does not have angled outboard sections like the aerofoil 140 in the first embodiment. The aerofoil 240 has an Eppler420 shape having a thickness of about 8 percent, a span of about 739 millimetres, a chord length of about 187 millimetres and an aspect ratio of about 3.95. A horizontal stabiliser 248 extends from the trailing edge of the aerofoil 240. Figure 12 shows a schematic plan view of an aerial vehicle 300 according to a third embodiment of the present disclosure. The aerial vehicle 300 comprises the lift generating system 200 of Figures 7 to 10. The aerial vehicle 300 further comprises counter-thrusters 370 positioned at the end of booms 372 extending from each side of the stabiliser 248. The counter-thrusters 370 are electrically powered ducted fans. The counter-thrusters 370 are arranged so that their thrust-line is substantially parallel to that of the forward thrust produced by air exiting the duct 260 at the duct outlet 264. This allows the counter-thrusters 370 to oppose forward thrust produced by air exiting the duct 260. Therefore, the aerial vehicle 300 can be moved forward, moved rearward or be held stationary by controlling the difference in thrust produced by the air exiting the duct outlet 264 and thrust produced by the counter-thrusters 370. The counter-thrusters 370 are positioned towards the rear of the aerial vehicle 300, which helps to move the centre of gravity of the aerial vehicle 370 rearwards and at least partially counteracts the forward movement of the centre of gravity due to the mass of the airflow generator 220 and the duct 260. The centre of gravity of the aerial vehicle 300 can be moved further rearward by having mass, such as batteries or ballast, inside the aerofoil 240 rearward of the centre of pressure of the aerofoil 240. Laterally offsetting the counter-thrusters 370 from the longitudinal axis of the aerial vehicle 300 ensures that air exiting the duct 260 does not impinge on their operation. Additionally, this allows the counter-thrusters 370 to be individually operated to control yaw of the aerial vehicle 300. The aerial vehicle 300 can perform VTOL, and climbs and descents, by controlling the velocity of the airflow over the aerofoil 240 using the airflow generator 220 whilst maintaining a groundspeed of zero with the counter-thrusters 370. The aerial vehicle 300 can also perform forward or rearward climbs and descents whilst remaining substantially level, even if the angle of attack of the aerofoil 240 is increased or decreased with respect to the air exiting the duct outlet 264. That is, the aerial vehicle 300 can achieve forward or rearward climbs and descents with a pitch angle of zero. Banked turns can be achieved by manipulating the ailerons 241,243. However, yawed turns are preferable at zero or slow ground speeds and are achieved with asymmetric thrust from the counter-thrusters 370. The aerial vehicle 300 also comprises an attachment point (not shown) on a lower surface of the duct 260 for attaching a payload. The payload being underslung lowers the centre of gravity and adds stability. The aerial vehicle 300 is remote controlled although it could alternatively be autonomous. Figure 13 shows a schematic plan view of an aerial vehicle 400 according to a fourth embodiment of the present disclosure and Figure 14 shows a schematic front view of the aerial vehicle 400. The aerial vehicle 400 comprises a plurality of lift generating systems 200 attached to a payload compartment 480 via mounting arms 490. In more detail, the mounting arms 490 couple to a lower surface of the ducts 260 via a releasable coupling. As with the third embodiment, the aerial vehicle 400 comprises counter-thrusters 470 which extend from the payload compartment 480 via booms 472. The counter-thrusters 470 are positioned so as not to interfere with the airflow flowing into the duct inlets of the lift generating systems and are also positioned to avoid significant pitch changes of the aerial vehicle 400 when they are operated. The lift generating systems 200 operate together to achieve flight. For example, the aerial vehicle 400 will start to lift off as power is increased to all lift generating systems 200. The aerial vehicle 400 can: control pitch by adjusting the power equally to the front two or rear two lift generating systems 200; control roll by adjusting the power equally to the port two or starboard two lift generating systems 200; and control yaw by adjusting power to the counter-thrusters 470. Further controllability is achieved through control of each individual lift generating system 200 to manipulate the direction and magnitude of their respective lift vector. As an example, by tilting some or all of the lift generating systems 200 so that their lift vector is directed at least partially sideways, the aerial vehicle 400 can move sideways. For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. In this context, therefore, a number A is understood as A ± 10% of A. Within this context, a number A may be considered to include numerical values that are within general standard error for the measurement of the property that the number A modifies. The number A, in some instances as used in the appended claims, may deviate by the percentages enumerated above provided that the amount by which A deviates does not materially affect the basic and novel characteristic(s) of the claimed invention. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. 16 02 26

Claims

1. A lift generating system for an aerial vehicle, comprising:an airflow generator for generating an airflow;5 an aerofoil for generating lift for the aerial vehicle; anda duct defining a duct inlet, a duct outlet and an airflow path extending from the duct inlet to the duct outlet, the duct being arranged to direct the airflow from the airflow generator to the aerofoil;wherein the duct comprises an airflow management region in which a thickness of the10 airflow path decreases in a downstream direction and a width of the airflow path increases in the downstream direction; and wherein at least a portion of the aerofoil is positioned within, or downstream of, the airflow management region; andwherein the duct is arranged to direct the airflow from the airflow generator to both an upper surface of the aerofoil and a lower surface of the aerofoil.

152. A lift generating system according to claim 1, wherein the duct is configured to present the airflow to the aerofoil as a stream of air having a thickness less than or equal to 300 percent of a maximum thickness of the aerofoil.20 3. A lift generating system according to claim 1 or 2, wherein the duct is configured to presentthe airflow to the entire span of the aerofoil.

4. A lift generating system according any preceding claim, wherein a leading edge of the aerofoil is positioned at, or immediately downstream of, the duct outlet.

255. A lift generating system according to any one of claims 1 to 3, wherein a leading edge of the aerofoil is positioned between quarter of a chord of the aerofoil and half of a chord of the aerofoil from the duct outlet.30 6. A lift generating system according to any preceding claim, wherein the duct outlet has awidth and a thickness, the width of the duct outlet being greater than the thickness of the duct outlet.

7. A lift generating system according to claim 6, wherein the thickness of the duct outlet is35 between 50 percent and 300 percent of a maximum thickness of the aerofoil.

8. A lift generating system according to claim 6 or 7, wherein the duct outlet has a width between 100 percent and 101 percent of the span of the aerofoil.16 02 269. A lift generating system according to any preceding claim, wherein, in the air flow management region, the magnitude of a decrease in thickness per unit length of an upstream portion of the airflow path is greater than the magnitude of a decrease in thickness per unit length 5 of a downstream portion of the airflow path.

10. A lift generating system according to any preceding claim, wherein, in the air flow management region, the magnitude of an increase in width per unit length of an upstream portion of the airflow path is greater than the magnitude of an increase in width per unit length of a 10 downstream portion of the airflow path.

11. A lift generating system according to any preceding claim, wherein, in the airflow management region, the airflow path has a decreasing cross-sectional area in the downstream direction.1512. A lift generating system according to any preceding claim, wherein the airflow generator is positioned within the duct and is configured to draw air into the duct through the duct inlet.

13. A lift generating system according to claim 12, wherein the duct is configured to direct the 20 entire volume of air drawn into the duct to the aerofoil.

14. A lift generating system according to claim 12 or 13, wherein the duct comprises an upstream portion positioned upstream of the airflow management region, wherein the airflow generator is positioned within the upstream portion of the duct.2515. A lift generating system according to claim 14, wherein the airflow generator does not extend into the airflow management region of the duct.

16. A lift generating system according to any preceding claim, wherein the airflow generator 30 comprises a fan and a motor for rotating the fan.

17. A lift generating system according to claim 16, wherein the motor is an electric motor.

18. A lift generating system according to any preceding claim, wherein the aerofoil has an 35 aspect ratio greater than or equal to 2.0.16 02 2619. A lift generating system according to any preceding claim, wherein, during operation of the lift generating system, the aerofoil is moveable to increase or decrease an angle of attack of the aerofoil relative to the airflow presented to the aerofoil by the duct.5 20. A lift generating system according to any preceding claim, wherein a span of the aerofoilcomprises a first outboard section, a second outboard section, and a midsection positioned between the first outboard section and the second outboard section.

21. A lift generating system according to claim 20, wherein, during operation of the lift 10 generating system, each of the first outboard section and the second outboard section are movable independently of one another between a first position having a first angle of attack relative to the airflow presented by the duct and a second position having a second angle of attack relative to the airflow presented by the duct, wherein the second angle of attack is different to the first angle of attack.1522. An aerial vehicle comprising the lift generating system according to any preceding claim.

23. An aerial vehicle according to claim 22, wherein the aerofoil is the primary lift generator of the aerial vehicle during all stages of flight.2024. An aerial vehicle to according to claim 22 or 23 comprising one or more propulsion units for providing one or more of: yaw control of the aerial vehicle and counter-thrust to counter any thrust generated by airflow exhausted from the duct outlet.25 25. A kit of parts for assembling the lift generating system according to any one of claims 1 to21, the kit of parts comprising the airflow generator and the duct.

Images