propellers

Abstract

A vertical thrust propeller [200] is described. The propeller [200] comprises at least a first propeller blade [B1], a second propeller blade [B2] and a third propeller blade [B3], each disposed in sequence, in fixed angular relation to the other propeller blades, around an axis of rotation [215] of the propeller [200]. An angle [α°] between the first propeller blade [B1] and the second propeller blade [B2] is smaller than an angle [β°] between the second propeller blade [B2] and the third propeller blade [B3]. The angles [α°, β°] are measured about the axis of rotation [215] and between longitudinal axes [220] of the respective propeller blades.

Description

[0001] PROPELLERS

[0002] FIELD

[0003] The present invention relates to a propeller for a vertical take-off and landing (VTOL) aircraft and to a VTOL aircraft incorporating such a propeller. The propeller finds particular, but not exclusive, application as a vertical thrust propeller, such as a vertical lift propeller, for an electric VTOL (eVTOL) aircraft.

[0004] BACKGROUND

[0005] Range capability is relevant to all vehicles, and especially aircraft. For aircraft, range and payload capability are typically the most critical parameters. Range can be increased in many ways, including by decreasing weight, increasing drive train efficiency, and minimising drag. Certain kinds of eVTOL aircraft have battery driven motors that drive propellers that are arranged for VTOL operation and for forward flight operation.

[0006] For example, EP 4192731 describes an eVTOL aircraft with front-mounted, tilt propellers, which tilt between VTOL and forward flight (or wing-borne) configurations, and rearmounted propellers that are fixed in a VTOL (i.e. upwardly facing, vertical thrust) configuration. When the aircraft of EP4192731 is in forward flight, the rear-mounted propellers are non-rotating and stowed. In the examples illustrated therein, the rear propellers comprise two opposing pairs of propeller blades that are stacked on top of one another. When in VTOL operation, the two pairs of propellers blades are deployed such that each blade is rotationally disposed by 90° with respect to each neighbouring propeller blade. When in forward flight operation, the propeller blades of each stacked pair are stowed by being substantially aligned with an axis that is parallel to a longitudinal or roll axis of the aircraft. The two pairs of propeller blades substantially overlie one another such that neighbouring propeller blades are aligned and there is no angle between them, to provide a relatively low drag arrangement. Other designs of eVTOL aircraft, for example WO2022159975, having front and rear propeller blades, are arranged with rear VTOL propellers that have only two opposing propeller blades. As described, “the blades 120 of the rotors 112 may be locked in a low drag position for aircraft cruising”. In this arrangement, the propeller blades are substantially aligned with an axis that is parallel to a longitudinal axis of the aircraft. However, to achieve a similar thrust as a four bladed design, the propeller blades need to have an increased size / surface area, or be operated at a higher rotational speed, or there needs to be an increased number of propellers. A two-bladed propeller has more adverse cycle oscillatory load characteristics than a four bladed design, which drives structural weight into the aircraft. All of these options may increase noise, which is undesirable in metropolitan environments, or may be otherwise detrimental to performance.

[0007] SUMMARY

[0008] According to a first aspect, the present invention provides a vertical thrust propeller, the propeller comprising at least a first propeller blade, a second propeller blade and a third propeller blade, each disposed in sequence, in fixed angular relation to the other propeller blades, around an axis of rotation of the propeller, an angle between the first propeller blade and the second propeller blade being smaller than an angle between the second propeller blade and the third propeller blade, the angles being measured about the axis of rotation and between longitudinal axes of the respective propeller blades.

[0009] A propeller according to present examples, which is intended to be mounted on a VTOL aircraft and arranged for vertical thrust operation, is referred to herein as a ‘vertical thrust propeller’. The vertical thrust propeller can be seen as a propeller providing thrust to generate vertical lift and may also be referred to as a vertical lift propeller. Such vertical thrust propellers come with several aerodynamic and operational challenges. Specifically, during vertical lift operations, the propeller must deliver high thrust efficiency while minimizing noise and oscillatory loads caused by edgewise airflow, conditions that arise due to the vehicle’s forward airspeed. Conversely, during forward flight, when lift is primarily generated by wings of the VTOL, the vertical thrust propeller must contribute minimal aerodynamic drag. The disclosed vertical thrust propeller, having a propeller blade arrangement with a non-uniform angular spacing between the propeller blades, is designed to optimize performance across these varying flight regimes by mitigating the adverse effects associated with conventional symmetric blade configurations.

[0010] Known propellers, such as known vertical thrust / lift propellers, which have equal angles between neighbouring propeller blades and are mounted for vertical thrust generation but are non-rotating during forward flight, present to the forward direction of travel a relatively large area, and a commensurately short component of length, which generates a commensurately large amount of drag. Examples of propellers herein can be orientated to have a reduced area presented, and an increased component of length parallel, to the forward direction of travel, and hence can produce less drag. Less drag means that the range or payload of the respective aircraft can be increased. In examples, the vertical thrust propeller is configured to deliver predominantly vertical thrust, such as vertical lift, during flight operations.

[0011] In examples, the plurality of propeller blades comprises an even number of propeller blades. Such a propeller may have a rotational symmetry of at least order 2. This ensures that the propeller is balanced about the axis of rotation. In examples, the order of symmetry is 2.

[0012] In examples, the vertical thrust propeller comprises only four propeller blades disposed in sequence around the axis of rotation, an angle between the third propeller blade and a fourth propeller blade being less than an angle between the fourth propeller blade and the first propeller blade. Then, the angle between the first propeller blade and the second propeller blade may equal the angle between the third propeller blade and the fourth propeller blade. In addition, or alternatively, the angle between the second propeller blade and the third propeller blade may equal the angle between the fourth propeller blade and the first propeller blade. Such a propeller has a rotational symmetry of order 2.

[0013] In examples, the vertical thrust propeller may have an angle between the first propeller blade and the second propeller blade that is less than 90°, less than 80° or less than 70°. By reducing the angle between the first propeller blade and the second propeller blade an increasingly reduced area can be presented to the forward direction of travel, and hence can produce less drag. In examples, the angle may be in the range 55° to 75° and may be about 60°. Accordingly, the angle between the second propeller blade and the third propeller blade may be greater than 90°, greater than 100°, or greater than 110°.

[0014] In examples, the vertical thrust propeller defines: a disc having a diameter D in a plane of rotation of the propeller and a height H, equal to a maximum height of the propeller blades in a frontal plane that is perpendicular to the plane of rotation and includes the axis of rotation; and a frontal area, in the frontal plane, having the height H of the disc and a width W that varies, according to a relative angular position between the propeller and the frontal plane, from a maximum width Wmax that is equal to the diameter D and a minimum width Wmin that is less than 0.7D.

[0015] When Wmin is less than 0.7D, the angle a between the first and second propeller blades is less than 90° and hence drag is reduced compared to a known propeller, e.g. a four bladed propeller, in which all propeller blades are separated by 90°.

[0016] In examples, the vertical thrust propeller defines: a disc having a diameter D in a plane of rotation of the propeller and a height H, equal to a maximum height of the propeller blades in a frontal plane that is perpendicular to the plane of rotation and includes the axis of rotation, the height H varying according to an axial pitch of the propeller blades, from a relatively lesser operating pitch and associated height during VTOL operation and a relatively larger operating pitch and associated height during forward flight operation.

[0017] In such examples, the relatively lesser operating pitch is when the propeller is non-rotating, and that pitch may be less than a normal operating pitch (or range of pitches) that are employed when in VTOL operation.

[0018] In examples, the propeller blades of the vertical thrust propeller, such as the first propeller blade, the second propeller blade, the third propeller blade and / or the fourth propeller blade, may be configured as a relatively flat root-section aerofoil having a low spanwise twist compared to a propeller blade for a front-mounted tilt propeller. A flat root-section aerofoil can herein be seen as the pitch angle of the aerofoil in relation to a plane of rotation of the propeller at the root being low, such as being equal to or below a threshold pitch angle. In one or more examples, the threshold pitch angle is 25°. In one or more examples, the threshold pitch angle is 20°. A relatively low spanwise twist can herein be seen as the blade’s, such as the aerofoils’, pitch angle being substantially constant along its length. In one or more examples, the propeller blades of the vertical thrust propeller have a spanwise twist less than 20°. In one or more examples, the propeller blades of the vertical thrust propeller have a maximum spanwise pitch equal to or less than 25°. In one or more examples, the propeller blades of the vertical thrust propeller have a maximum spanwise pitch equal to or less than 20°. This results in an overall flatter pitch profile of the blade, which reduces the propeller disc height H and the frontal area A presented in the forward direction of travel, thereby reducing aerodynamic drag when the propeller is in the stowed position.

[0019] In examples, the second propeller blade and / or the fourth propeller blade is / are arranged at a larger pitch angle relative to a plane of rotation of the propeller than the first propeller blade and the third propeller blade. Since the second propeller blade and the fourth propeller blade are arranged closer to the blade ahead than the first propeller blade and third propeller blades, the second propeller blade and the fourth propeller blade are more-negatively affected by downwash created by the first propeller blade and the third propeller blade, respectively. The downwash reduces the thrust produced by the second propeller blade and / or fourth propeller blade and leads to an uneven load distribution on the propeller. By increasing the pitch of the second propeller blade and / or fourth propeller blade in relation to the plane of rotation of the propeller, the effective inflow angle of the air on the second propeller blade and / or fourth propeller blade can be increased, which brings the thrust output of the second propeller blade and the fourth propeller blade closer to that of the first propeller blade and the third propeller blade, thereby improving the load balance across the propeller disc. By increasing the blade pitch of the second propeller blade and / or the fourth propeller blade, the thrust performance of the example propeller disclosed is comparable to the thrust performance of propellers having four propeller blades spaced at 90°.

[0020] In examples, the vertical thrust propeller is fixed in a VTOL vertical thrust configuration. In examples, the vertical thrust propeller is fixed relative to an airframe of the VTOL aircraft. In examples, the vertical thrust propeller is fixed relative to the airframe of the VTOL aircraft such that a plane of rotation of the vertical thrust propeller is at a fixed orientation relative to the airframe.

[0021] In examples, the propeller blades are arranged at a fixed pitch angle relative to a plane of rotation of the propeller. In other words, the pitch of the blades cannot be varied during flight operations.

[0022] According to a second aspect, the present invention provides a VTOL aircraft comprising at least one vertical thrust propeller according to any one of the preceding claims.

[0023] In examples, the vertical thrust propeller may be arranged to have a non-rotating mode during forward flight. Then, when in the non-rotating mode, the vertical thrust propeller may be rotationally aligned such that a plane that is perpendicular to the plane of rotation and includes the axis of rotation, and which is parallel to a forward direction of travel of the aircraft, is between the first propeller blade and the second propeller blade. In some examples, the plane substantially bisects the angle between the first propeller blade and second propeller blade.

[0024] In examples, the VTOL aircraft comprises a propeller drive unit to rotate the propeller, wherein, the propeller drive unit is arranged to rotationally align the propeller when in the non-rotating mode.

[0025] In examples, the VTOL aircraft comprises one or more tiltable thrust propellers and one or more vertical thrust propellers according to examples of the first aspect.

[0026] In examples, the one or more tiltable thrust propellers are adapted to pivot, in a plane parallel to a roll axis of the VTOL aircraft, between a forward flight mode, in which the propeller is orientated to generate forward thrust, and a VTOL mode, in which the propeller is orientated to generate vertical thrust.

[0027] In examples, the VTOL aircraft comprises a fuselage and a wing section extending on either side of the fuselage, wherein each wing section supports at least one tiltable propeller and at least one vertical thrust propeller. Then, each wing section may support at least two tiltable propellers and at least two vertical thrust propellers. More particularly, each wing section may support at least one pylon, and each pylon may support a tiltable propeller and a vertical thrust propeller.

[0028] In examples, the tiltable propeller and the vertical thrust propeller supported on a respective pylon are aligned with each other along a roll axis, such as a longitudinal axis, of the VTOL aircraft.

[0029] In examples, the tiltable propeller supported on a respective pylon is located forward of the wing, and the vertical thrust propeller supported on the respective pylon is located rearward of the wing.

[0030] In examples, the VTOL aircraft is configured such that the roll axis of the VTOL aircraft remains substantially horizontal during flight operations, such as during take-off and landing.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Examples of the invention will now be illustrated by the accompanying drawings, of which:

[0033] Figures la and lb are schematic diagrams that illustrate a prior art propeller;

[0034] Figure 2 is a schematic diagram that illustrates a vertical thrust propeller according to an example of the invention;

[0035] Figures 3a and 3b are schematic diagrams that illustrate, side-by-side, respectively, the prior art propeller of Figure 1 and the propeller according to the example of the invention in Figure 2;

[0036] Figure 4 is a graph that illustrates a variation in drag components as a function of a stagger angle of a propeller according to an example of the invention;

[0037] Figure 5 is a graph that illustrates a variation in limit aerodynamic loads on the blades as a function of the stagger angle of the propeller according to an example of the invention; Figures 6a and 6b are schematic diagrams that illustrate computational fluid dynamics (CFD) simulations of a drag contribution on two propellers configured with different stagger angles according to an example of the invention;

[0038] Figures 7a and 7b are schematic diagrams illustrating the impact of a blade geometry on the drag contribution of the propellers of Figures 6a and 6b;

[0039] Figures 8a, 8b and 8c are schematic diagrams that illustrate, side-by-side, respectively, the prior art propeller with a maximum width presented to a forward direction of travel, the prior art propeller with a minimum width presented to the forward direction of travel, and the propeller according to an example of the invention with a minimum width presented to the forward direction of travel;

[0040] Figure 8d is a schematic diagram that illustrates another example of the invention, with variable pitch propeller blades;

[0041] Figure 9 is a graph illustrating comparatively a pitch distribution of a fixed pitch blade for a vertical thrust propeller and a blade for a fore-mounted tilt propeller according to an example of the invention;

[0042] Figure 10 is a graph illustrating comparatively a spanwise thrust distribution on the blades of a vertical thrust propeller having unevenly spaced blades according to an example of the invention;

[0043] Figure 11 is a schematic diagram of an overhead view of an aircraft according to an example of the invention;

[0044] Figures 12a and 12b are schematic diagrams of side elevations of the aircraft of Figure 11; Figure 13 is a schematic block diagram of a control system arrangement for controlling one of the propellers of the aircraft in Figures 11 and 12a and 12b;

[0045] Figure 14 is a schematic diagram of a further example of a vertical thrust propeller according to the present invention; and

[0046] Figure 15 is a graph illustrating comparatively a drag coefficient of a plurality of vertical thrust propellers arranged on an aircraft for different cant angles according to an example of the present invention.

[0047] DETAILED DESCRIPTION Figure la illustrates an overhead view and a side view 100a of a propeller 100 having four propeller blades 105, as is generally known in the prior art. The propeller blades 105 each have a longitudinal axis, a, and are disposed in sequence around a hub 110, and axis of rotation 115, separated by 90° with respect to neighbouring propeller blades. The propeller 100 produces a disc with a diameter D and a maximum height Hi, which is a maximum height of the propeller blades, due to a combination of a twist and a pitch of each blade, when viewed from the side (i.e. perpendicular to the plane of the disc). Different designs of blade will of course exhibit different amounts of twist and may be operated at different pitches, leading to a variety of different possible heights Hi.

[0048] As shown by the side view 100a in Figure la, an area Ao of the propeller is illustrated as a rectangle with a width being the width of the propeller and a height being the maximum height Hi of the propeller blades. When the propeller 100 is mounted on a VTOL aircraft to generate vertical thrust, the area Ao of the propeller is in a plane that is generally parallel to yaw and pitch axes of the aircraft. In this way, for reasons which will be explained in more detail below, the area Ao of the propeller is related to the amount of drag caused by the propeller when the aircraft is operating in a forward cruise mode, in a forward direction of travel Dt. The forward direction of travel Dt is of course generally the opposite of the direction of airflow -Dt towards and impinging on the propeller. In the forward direction of travel Dt, the area Ao of the propeller will be referred to herein as the frontal area Ao of the propeller, which is on a frontal plane, which can be thought of as being perpendicular to the plane of rotation and including the axis of rotation. As will be explained, the greater the frontal area Ao is for a given propeller, the greater the amount of drag there may be when moving in the forward direction of travel Dt and encountering the impinging airflow -Dt.

[0049] When orientated as shown in Figure la so that two propeller blades are parallel to the forward direction of travel Dt and two propeller blades are perpendicular to the forward direction of travel Dt, the width is a maximum width Wo, which is equal to the disc diameter D, and the drag will be at or around its maximum. For the sake of simplicity, it will be taken herein that a length of a propeller is approximately Wo / 2. It can therefore be seen that a length Wo / 2 of each of the perpendicular propeller blades is perpendicular to the direction of airflow -Dt. Figure lb illustrates the same prior art propeller as in Figure la, but this time rotated by 45° relative to the forward direction of travel Dt. As can be seen, this time, a frontal area Ai of this propeller that is presented when moving in the forward direction of travel Dt is reduced to a minimum, and width Wi for this propeller is also at a minimum. It can also be seen that a component of length Li of each propeller blade is now parallel to the direction of airflow - Dt. As the orientation of the propeller is 45°, it will be appreciated that the component of length Li equals the component of length Wi / 2 of each propeller blade that is perpendicular to the direction of airflow -Dt.

[0050] Figure 2 illustrates an overhead view 200 and a frontal view 200a of a propeller 200 having four propeller blades 205 arranged according to an example of the present invention. This propeller is adapted to be employed as a vertical thrust propeller, which is operated to generate vertical thrust when mounted on a VTOL aircraft.

[0051] As can be seen, in Figure 2, a first opposing pair of propeller blades B1,B3 is arranged to have an angle a of less than 90° between it and a second pair of opposing propeller blades B2,B4. The angles are measured about an axis of rotation 215 and between similar longitudinal axes 220 of the respective propeller blades 205. Put another way, the propeller comprises a first propeller blade Bl, a second propeller blade B2, a third propeller blade B3 and a fourth propeller blade B4, each disposed in sequence, in fixed angular relation to the other propeller blades, around an axis of rotation 215 of the propeller 200. In other words, each propeller blade is arranged in a permanent, non-variable angular position with respect to the other propeller blades around the hub and / or rotation axis. An angle a between the first propeller blade Bl and the second propeller blade B2 is smaller than an angle P between the second propeller blade and the third propeller blade. An angle between the third propeller blade B3 and the fourth propeller blade B4 is the same as the angle a between the first propeller blade Bl and the second propeller blade B2, and the angle between the fourth propeller blade B4 and the first propeller blade Bl is the same as the angle P between the second propeller blade B2 and the third propeller blade B3. The angle a between the closest blades, such as between the first propeller blade Bl and the second propeller blade B2 or between the third propeller blade B3 and the fourth propeller blade B4, may herein be referred to as a stagger angle. As will be appreciated, due to the difference between angles a and P, the exemplary propeller 200 in Figure 2 has a rotational symmetry of 2 about its axis of rotation 215.

[0052] As shown in Figure 2, by orientating the propeller 200 such that an axis 225 (or plane into the page) extending from the axis of rotation and substantially bisecting the relatively smaller angle a is parallel to a notional forward direction of travel Dt, a frontal area A2 of the propeller is reduced to a minimum, which is less than the minimum frontal area Ai that is achievable with the prior art propeller in Figure lb, by virtue of a width W2 being a minimum and even less than Wi. Likewise, a component of length L2 of each propeller blade that is parallel to the direction of airflow -Dt is greater than the corresponding components in Figures la and lb. ‘Substantially bisecting’, as used herein, accounts for the fact that most propeller blades are not symmetrical about any longitudinal axis. Due to this, it may be that, to minimise drag (and minimise A2 and maximise L2 across two propeller blades), the propeller may be slightly rotated relative to the axis 225, such that slightly more of the angle a may be on one side of the axis 225 than the other. For example, when the angle a is 60°, that may be split 29°:31° or even 28°:32°, and still be approximately or substantially bisected. In other examples, the angle a may be near enough or exactly bisected.

[0053] Figure 3a reproduces Figure lb and Figure 3b reproduces Figure 2. Being side-by-side, Figures 3 a and 3b clearly illustrate the impact width, W2 < Wi, has on the frontal area A of the propeller that is presented to the forward direction of travel Dt and on the respective components of length L that are parallel to the direction of airflow -Dt.

[0054] In examples, the relatively smaller angle a is less than 80°, or even less than 70°. In one or more examples, the angle a is in the range of 55° to 70°. In a particular example, the angle a is 60° or thereabouts.

[0055] It will be appreciated that, for the prior art propeller 100 of Figure lb,

[0056] W1 = 2(Wo / 2 sin (a° / 2)) = Wo sin 45° = O.71Wo, and

[0057] Li = (Wo / 2 cos (a° / 2)) = Wo cos 45° = O.71Wo / 2 = Wi / 2 whereby Wi is about 0.7 times the width Wo and Ai is about 0.7 times the area of Ao.

[0058] For the propeller 200 in Figure 2, when the angle a is 60°, W2= Wo sin 30° = O.5Wo, and

[0059] L2= Wo cos 30° = O.87Wo / 2 whereby W2and A2 are about 0.5 times the width Wo, and L2is nearly 0.9 times the width Wo / 2.

[0060] The angle a being 60° rather than 90° represents a significant decrease in frontal area A and a significant increase in length L that is achievable, compared to the prior art propeller of Figure 1, and a commensurate decrease in drag generated by the propeller 200 when the aircraft is operating in a forward cruise mode.

[0061] The impact that varying the angle a has on the width W and the component of length L is illustrated in the following table: Change in drag is not directly proportional to change in the width of the frontal area, due to complex relationships between airflow around static propeller blades at different angles and associated vorticity flow formation. However, the frontal area, and in particular a minimum attainable frontal area, is found to be a good proxy for drag, and, for a given configuration of propeller, a reduced frontal area will typically lead to reduced drag. When a vertical thrust propeller is spinning in VTOL operation, airflow separates over / under the propeller blades to generate lift, without generating significant drag, in a known way. Figure 4 illustrates a variation in drag components acting on a representative vertical thrust propeller at an aircraft level as a function of the blade stagger angle, such as angle a°. The total drag of the propeller is composed of induced drag, interference drag, and viscous drag. As shown in the figure, the total drag does not decrease linearly with the frontal area. The induced drag and viscous drag components increase with increasing stagger angle, while the interference drag decreases with an increasing stagger angle. For the given propeller blade geometry, the total drag decreases substantially when changing from a stagger angle of 90° to a stagger angle equal to or below approximately 65°. For stagger angles below 65° the total drag remains relatively constant. This demonstrates that it may be beneficial to provide the propeller with a stagger angle below 65° in order to minimize drag while achieving good performance, loads and noise characteristics when the propeller is active / spinning.

[0062] A vertical thrust propeller, however, is not optimised to be stationary when the associated aircraft is flying forwards in wing-borne operation. Consequently, when a leading edge of a stationary propeller blade is substantially perpendicular to the direction of airflow (e.g. - Dt in Figure la), the propeller blade acts like an inefficient wing and can generate increased drag. As a propeller blade of a vertical thrust propeller is angled away from being substantially perpendicular to the direction of airflow (i.e. as L increases), the leading edge of the propeller blade becomes more swept, either backwards or forwards, relative to the roll axis of the aircraft (e.g. as in Figure 2). ‘Swept’ in this context is analogous to swept wings of high-speed aircraft, such as fighter aircraft, which have a greater longitudinal component of wing length parallel to a roll axis of the respective aircraft.

[0063] As a propeller blade becomes more swept, a threshold pitch which leads to increased drag also increases. Put another way, for a given propeller blade and pitch, increasing the sweep of the propeller blade reduces the drag when the threshold pitch has increased to above that of the propeller blade.

[0064] In practice, reducing the angle a° will tend to reduce drag, as is illustrated in Figure 4. However, reducing the angle a° is also associated with increasing levels of cyclic loading oscillations of the propeller, as illustrated in Figure 5. It has been found that, for many propeller blades, angles a° in the range 55°-75° provide a good compromise between reduction in drag without introducing unduly high levels of cyclic loading oscillations. In one or more examples, the angle a° is 60° or thereabouts.

[0065] To determine an appropriate stagger angle, a limit aerodynamic load on the blades may be taken into consideration. The limit aerodynamic load can be seen as the load in a worst-case scenario operating condition. The appropriate stagger angle may be determined by balancing cruise drag and the limit aerodynamic loads. The effect of the stagger angle on the limit aerodynamic loads on the blades is illustrated in Figure 5. A stagger angle of 90° corresponds to a four bladed propeller, in Figure 5 shown having a normalized limit aerodynamic load of 1. A propeller with a 0° stagger angle is equivalent to a two-bladed propeller, which experiences twice the peak loads of the four-bladed propeller. Staggering a 4-bladed propeller at an angle different to 90° provides a way to balance cruise drag and limit aerodynamic loads. By comparing the total drag for the different stagger angles in Figure 4 with the limit aerodynamic loads of Figure 5, it can be seen that a balanced stagger angle may be in the range of 55° to 70°, such as 60°.

[0066] Figures 6a and 6b depict computational fluid dynamics (CFD) simulations of a drag contribution on two propellers configured with different stagger angles and different blade, such as aerofoil pitch. Figure 6a illustrates a propeller having a stagger angle of 90°, while Figure 6b illustrates a propeller having a stagger angle of 60°. In addition to having a different stagger angle, the blades of the propeller illustrated in Figure 6b have a smaller pitch, such as aerofoil pitch, at the root-section than the blades of the propeller illustrated in Figure 6a. In both figures, the propellers are shown in a stationary state relative to the aircraft, with the direction of airflow towards and impinging on the propeller indicated as - Dt. Accordingly, the airflow over the propellers is oriented opposite to the direction of travel, i.e., from left to right in the figures. Regions where the surface pressure result in high drag contribution, herein also referred to as regions of high drag contribution, (in Figures 6a and 6b denoted as RHD) are visually represented by darker shading, with regions of lower drag contribution (in Figures 6a and 6b denoted as RLD) being visually represented by lighter shading. In Figures 6a and 6b, the regions of high drag contribution are concentrated in regions where the blade pitch, such as an aerofoil pitch, is high. The aerofoil pitch in the regions of high drag contribution, such as in the cross-sections A-A and B-B of Figure 6a and cross sections C-C and D-D of Figure 6b, and the corresponding airflow around the aerofoils are illustrated in Figs. 7a and 7b, respectively.

[0067] As can be seen in Figure 6a, the propeller having a stagger angle of 90° has high drag contributions RHD concentrated in regions of the blade where the aerofoil pitch is high compared to other regions of the blade, such as along the trailing edges and central parts of the blades. These regions are prominently highlighted by the dark shadings, while the remainder of the blade surfaces exhibit lower drag levels, represented by the lighter shading. This distribution indicates significant aerodynamic penalties at high stagger angles and at regions of the blade having a high aerofoil pitch, particularly at the central parts of the blade, such as near the blade roots.

[0068] As can be seen in Figure 6b, for the propeller having a stagger angle of 60°, the propeller blades are predominantly shaded in lighter shades, with localized areas of darker shading indicative of high drag contributions RHD. These higher-drag regions are arranged on the central parts of the blades but are less extensive than those observed for the blades in the 90° configuration of Figure 6a. These simulations thus confirm the reduction in overall drag and improved aerodynamic performance for the blades in the 60° stagger angle configuration and thereabouts, such as in the range of 55°-75°.

[0069] Figures 7a and 7b illustrate how blade geometry, specifically the pitch angle at the root section, and the stagger angle affects the airflow around the blades of the example propellers shown in Figures 6a and 6b. As illustrated in section views A-A and B-B of Figure 7a, the blades of the propeller in Figure 6a exhibit a more pronounced curvature, including a steeper pitch angle, compared to the blades of the propeller in Figure 6b, shown in section views C- C and D-D of Figure 7b. This increased curvature in Figure 6a results in a steeper surface gradient on the upper side of the blades, leading to airflow separation (S) and increased drag. Conversely, the blades in Figure 6b, with their gentler curvature and lower pitch angle, experience reduced airflow separation (S), thereby minimizing drag.

[0070] Figures 4, 5, 6a and 6b collectively demonstrate the impact of the stagger angle on the drag distribution across the propeller blades and support the selection of lower stagger angles to minimize drag and enhance efficiency of the vertical thrust propeller during forward flight operations.

[0071] For example, propeller blades having a fixed angular relation in both VTOL and forward flight operation greatly reduces the complexity of the arrangement. As compared to designs such as in EP4192731, there do not need to be two pairs of opposing propeller blades that can be moved independently, via appropriate mechanisms, to be stowed and deployed. This represents a reduction in mechanical complexity and weight. This can be significant when multiplied by two, four, six, eight, or any number of, such propellers. Reduced weight, of course, affords increased range or payload.

[0072] In addition, the propeller blades according to examples can in principle be manufactured as a single piece or plural pieces that are bonded together. In principle, such a propeller blade arrangement could be coupled directly to a drive shaft, potentially without a hub as such. Alternatively, the propeller blades may each be coupled to a hub. Such a hub can be relatively simple, particularly if the propeller blades maintain a fixed pitch, or contain a relatively simple pitch varying mechanism, if propeller blade pitch needs to be varied. All such variants, again, can exhibit greatly reduced weight characteristics as compared to designs such as in EP4192731.

[0073] Moreover, the present inventors have determined that examples do not exhibit a significantly reduced thrust performance as compared to propellers with four, 90° spaced propeller blades. In addition, the thrust performance and oscillating loading characteristics of examples herein remain significantly better than a propeller arrangement with only two opposing propeller blades of similar design. The different propeller widths are again illustrated, side-by-side, in Figures 8a, 8b and 8c. This clearly illustrates the benefit of examples herein. Figure 8d illustrates an alternative arrangement in which propeller blade pitch can be varied. In particular, the pitch of the propeller blades is flattened, or horizontally feathered (i.e. to a minimum drag pitch), to a pitch that is less than an operating pitch, when in a stowed orientation, to achieve a relatively reduced disc height H2. It can be seen that a combination of modified angular displacements, as illustrated in Figure 2, with flattened pitch, as in Figure 8d, reduces the disc height H2 and further reduces the total frontal area A3 of the propeller that can be presented to the forward direction of travel Dt, to further reduce drag.

[0074] More generally, it will be appreciated that flattening or feathering the pitch of the propeller blades can act to reduce drag in any angular arrangement of propeller blades. For example, even if an arrangement as illustrated in Figure la is adopted, flattening the pitch of the blades to less than an operating pitch serves to reduce the frontal area of the propeller when in forward flight and hence reduces drag. Indeed, reduced drag may be achieved by flattening the pitch of the two opposing propeller blades that are orientated perpendicularly with respect to the forward direction of travel Dt. A pitch of the propeller blades that are parallel to the forward direction of travel Dt has relatively less impact on the drag, so may not need to be flattened.

[0075] In one or more examples, the blade pitch of the vertical thrust propeller is fixed. In other words, the pitch of the propeller blade in relation to the hub cannot be changed. In one or more examples, the propeller blades of the vertical thrust propeller may be configured as a relatively flat root-section aerofoil having a relatively low spanwise twist compared to a blade of a fore-mounted tilt propeller. As used herein, twist refers to the variation in the blades’ internal pitch angle along the spanwise direction of the blade. In other words, the difference between the pitch at the root and the pitch at the tip of the blade is relatively low. A flat root-section aerofoil is understood to be a blade having a cross-section at the root with a reduced pitch angle, such as a pitch angle equal to or less than a defined threshold. A low spanwise twist denotes a configuration in which the pitch angle remains substantially constant along a significant portion of the blade’s length. In one or more examples, the spanwise twist of the blades of the vertical thrust propellers is less than 20°, such as less than 18°. In one or more examples, a maximum pitch of the blade along the span of the blade is less than 25°, such as less than 20°.

[0076] Figure 9 illustrates a comparison of the spanwise pitch distribution of an example propeller blade for the vertical thrust propeller (in Figure 9 denoted as VTP), such as an aft-mounted lift, vertical thrust propeller, and that of an example tilt propeller (in Figure 9 denoted as TP), such as a fore-mounted tilt propeller, over a length of the propeller blades. In the illustrated example, the tilt propeller blade exhibits a root pitch of about 44°, which decreases steeply along the span of the blade to approximately 0° at the blade tip. By contrast, the example blade for the vertical thrust propeller has a flatter root section with a root pitch of less than 25°, such as about 20°, and a low twist profile where the pitch remains substantially constant, such as around 20°, along the majority of the blade span, such as from the root to approximately 70% of the length of the blade. In one or more examples, the low twist profile may have a pitch that remains substantially constant at around 18-19°. Beyond this spanwise location, the pitch gradually decreases, such as to a pitch angle smaller than 10°, such as about 5°, at the tip of the blade. In some examples, the pitch distribution of the example propeller blade for the aft mounted vertical thrust propeller may vary by + / -3° from the distribution illustrated in Figure 9. As can be seen in Figure 9, the example blade of the vertical thrust propeller has both a flatter root section and a lower spanwise twist than the propeller blade of the fore-mounted tilt propeller. This results in an overall flatter pitch profile of the blade, contributing to a reduced propeller disc height H2 and a smaller frontal area A3 presented in the forward direction of travel Dt in accordance with Figure 6d, thereby further reducing aerodynamic drag when the propeller is in the stowed position.

[0077] Figure 10 is a diagram illustrating a spanwise thrust distribution on the propeller blades of a propeller having unevenly spaced blades when the propeller is in hover, such when the blades see no airspeed. In propellers having evenly spaced blades, such as e.g. 90° spacing for a four bladed propeller, all blades of the propeller are aerodynamically equivalent and experience identical aerodynamic forces and moments when in operation during axisymmetric conditions, such as hover. This results in a balanced and symmetric configuration. However, for the example propeller according to the present invention having unevenly spaced blades, the blades may experience different aerodynamic forces and moments when in operation. During operation, the blades in each pair of propeller blades, such as the two blades arranged closest to each other and forming the angle a°, form a leading blade and a trailing blade. In this example the leading blade is the blade in the pair having the larger distance to a blade ahead. The trailing blade is the blade in the pair having the smaller distance to a blade ahead. In one or more examples, the leading blades are positioned 180-a°, behind the blade ahead, while the trailing blades are only a° behind. For example, for the a° range of 55-75° the leading blade would be in the range of 125°-105° behind the blade ahead in the direction of rotation. In the examples disclosed herein, the first propeller blade and the third propeller blade are leading blades, while the second propeller blade and the fourth propeller blade are trailing blades. Due to the closer spacing of the trailing blades, the trailing blades are more affected by downwash generated by their respective leading blades. Downwash refers to a deflected airflow directed downward by the rotating blades as they generate lift or thrust. Due to the increased downwash on the trailing blades, the effective inflow angle of the air on the trailing blade is reduced which, consequently, reduces the thrust produced by the trailing blades in each pair. This is illustrated in Figure 10, where the round dotted line illustrates spanwise thrust distribution of the leading blade and the solid line illustrates the spanwise thrust distribution of the trailing blade in a baseline configuration where the leading blade and the trailing blade have the same pitch in relation to a plane of rotation of the propeller. As can be seen from Figure 10, the spanwise thrust distribution over the trailing blade is substantially lower than the spanwise thrust distribution over the leading blade as a result of the downwash. This may lead to an uneven load distribution over the propeller blades. In one or more examples, the pitch, such as the pitch angle, of the trailing blade is increased to compensate for the additional downwash it experiences. In one or more examples, the pitch angle of the trailing blades in relation to the plane of rotation of the propeller is increased by in the range of 0.5°-2° over the pitch angle of the leading blade. The pitch angle can herein be seen as the angle that an aerofoil of a given blade section, such as a slice of the blade at a fixed spanwise position, makes with the plane of rotation of the propeller. In other words, the trailing blades are arranged such that any given blade section is arranged at a higher pitch angle than the corresponding blade section of the leading blades. The pitch angle of the blade is thus increased equally along the entire span of the blade. By increasing the pitch angle of the trailing blade in relation to the plane of rotation of the propeller, the effective inflow angle of the air on the trailing blade can be increased, which brings the thrust output of the trailing blades closer to that of the leading blades. This is illustrated by the dash-dotted line in Figure 10, which shows the spanwise thrust distribution of a trailing blade having an increased pitch of 1° compared to the leading blade. As can be seen, the thrust distribution of the trailing blade with increased pitch is substantially higher, such as about 20N higher, than the trailing blade having the same pitch as the leading blade. As is illustrated by the long-dashed line, which is illustrative of the thrust distribution of the leading blade when the pitch of the training blade is increased, changing the pitch angle of the trailing blade does not have a negative effect on the leading blade. The thrust distribution of the leading blade remains substantially the same for both cases. By bringing the thrust distribution of the trailing blade closer to the thrust distribution of the leading blade, the load balance across the respective propeller disc is improved. Furthermore, the increased thrust distribution of the trailing blade improves hover efficiency of the propeller and allows for smaller and lighter blade designs. By increasing the pitch angle of the trailing blades some of the thrust lost due to the downwash can be recovered, thereby avoiding having to either spin the blades faster (which negatively affects noise) or increase the chord of the blades (which increases the mass of the propeller).

[0078] Figure 11 is an overhead view of an eVTOL aircraft 500 according to an example, comprising four fore propellers 505, that can be tilted between forward and VTOL flight configurations, and four aft propellers 510, that are fixed in a VTOL (i.e. upwards) vertical thrust configuration. The aft propellers 510 are similar to those illustrated in Figure 2, while the fore propellers 505 are similar to those illustrated in Figure 1. Pairs of fore and aft propellers 505,510 are mounted on pylons 515 that are attached to the underside of port 520 and starboard 525 wing sections. The fore and aft propellers 505,510 in each pair may be aligned, such as arranged on an imaginary axis being parallel to a roll axis of the VTOL. The roll axis of the VTOL remains substantially horizontal during flight operations. During forward flight operations, the roll axis is substantially parallel to the direction of travel Dt.

[0079] As shown in Figure 11, the aft propellers 510 are locked or held in a stowed orientation, in which a notional axis 530 (or plane into the page) that substantially bisects the smaller angle a of each aft propeller is parallel to the forward direction of travel Dt, such as parallel to the impinging direction of airflow -Dt, thereby presenting a relatively small total frontal area A2 to the forward direction Dt and an increased component of length L2.

[0080] Figure 12a illustrates, a side elevation of the eVTOL aircraft of Figure 11 when the fore propellers 505 are in the forward flight configuration and the direction of travel Dt is forwards. Figure 12b illustrates the same aircraft when the fore propellers 505 are tilted upwardly to VTOL configuration. In each case, only one pylon 515 and pair of propellers 505,510 are shown, for reasons of simplicity only. In each of Figures 12a and 12b, the aft propellers are mounted for vertical thrust and are in accord with the invention.

[0081] Figure 12b also illustrates an aircraft controller 600 and an electric propulsion unit (EPU) 620 for controlling the aft propeller 510. The aircraft controller 600 and the EPU 620 are illustrated in more detail in Figure 13.

[0082] As shown in Figure 13, the aircraft controller 600 comprises a central processing unit 700, memory 710 containing instructions including to control the EPU 620, and an I / O port 720 connecting the controller to a communications bus 730. The EPU 620 includes an EPU controller 735 to control the operation of the EPU 620 according to instructions from the aircraft controller 600, an electric motor 740 to drive the propeller 510 via a drive shaft 750 passing through windings (not shown) of the electric motor 740, a position detector 760 to detect the angular orientation of the drive shaft 750 and propeller 510, and a locking device 770 to lock the drive shaft 750, in a stowed orientation, to prevent rotation of the propeller 510 when stowed due to external (e.g. airflow) influences. The EPU controller 735 has an I / O port 775 for communicating with the aircraft controller 600 via the communications bus 730 and secondary VO ports 780 for communicating with and controlling the electric motor 740, the position detector 760 and the locking device 770.

[0083] In VTOL operation, the aircraft controller 600 controls the EPU 620 to drive the axle 750 and propeller 510 to generate vertical thrust. When in forward cruise mode, the aircraft controller 600 controls the EPU 620 to lock the axle 750 and propeller 510 in the stowed orientation. In particular, a signal from the aircraft controller 600 is received by the EPU controller 735, and the EPU controller 735 uses signals from the position detector 760 to determine when to control the locking device to lock the axle 750 and propeller 510 in the correct, stowed orientation.

[0084] The position detector 760 can be any appropriate kind that is suitable for detecting an angular orientation of a drive shaft. For example, the position detector 760 may be optical, magnetic or electrical. It may be separate from or integrated into the electric motor 740. The locking device 770 may be any appropriate kind. For example, the locking device 770 may be mechanical and lock the drive shaft mechanically through contact. In alternative examples, the motor 740 may be commended to actively maintain the desired propeller orientation, obviating the locking device 770. The electric motor 740 may be an AC electric motor driven by one or more batteries (not shown) via an invertor (not shown). Other kinds of electric motor and power arrangements are known.

[0085] While examples herein employ four propeller blades, it is conceivable that more propeller blades per propeller may be employed. For example, six propeller blades may be employed, as illustrated in Figure 14. In this example, the rotational symmetry remains of order 2.

[0086] While examples herein illustrate vertical thrust propellers on the rear of a pylon, there is of course no restriction on where such propellers may be mounted. For example, such propellers, in other examples, may be mounted at the front of a pylon or even on top of a fuselage of an aircraft, or on rear wing sections, or, more generally, wherever it would be practical to mount a vertical thrust propeller.

[0087] Although in the examples herein, the vertical thrust propellers are mounted with an axis of rotation that is parallel to a yaw axis, in other examples, such propellers may be mounted with a slight cant angle away from the yaw axis. By balancing such angles across multiple vertical thrust propellers, it may be possible to manoeuvre the respective aircraft, for example about the yaw axis, by varying the levels of thrust that are applied to individual propellers. Indeed, in some examples, the vertical thrust propellers may be actively manoeuvrable within bounds to permit a degree of thrust vectoring, which may assist with stabilising the respective aircraft during landing or take off. In all such cases, the frontal area of the propeller can be reduced by increasing the angle a. According to the example propeller configuration shown in Fig. 11, the aft mounted vertical thrust propeller is positioned behind both the forward propeller and the wing of the aircraft, which causes the airflow received by the vertical thrust propeller to be far from ideal. In cruise, a local flow field on the propeller includes a downwash component generated by the wing. This causes the airflow to be deflected slightly downwards over the rear half of the wing and behind it. This disturbed inflow of air to the vertical thrust propeller is expected to increase drag compared to drag estimates based on an isolated propeller, such as a propeller not mounted to an aircraft.

[0088] Figure 15 illustrates how the drag coefficient of all four aft mounted vertical thrust propellers in the example configuration of Fig. 11 varies as function of the aircraft angle of attack. The solid line represents the drag of an isolated propeller with an a° of 60°, showing a sharp increase in drag as the angle of attack of the aircraft rises. However, when the same propeller is installed on the aircraft with an axis of rotation that is substantially parallel to a yaw axis, the drag becomes almost constant across the different angles of attack of the aircraft, as shown by the long-dashed line. Despite the drag being stable over the different angles of attack, there is a significant increase in drag at 0° angle of attack of the aircraft relative to the isolated propeller.

[0089] To address this, the aft mounted vertical thrust propellers may be canted, such as installed with the slight cant angle away from the yaw axis, such as around the pitch axis, to better align the vertical thrust propellers with the disturbed flow coming off the wing. Installed with the cant angle can herein be seen as the axis of rotation of the propeller being arranged at an angle to the yaw axis. In other words, the axis of rotation of the propeller is not parallel with the yaw axis. The propeller plane of rotation may be angled slightly rearward, such as by the cant angle, to better align the propeller with a freestream airflow downwash angle behind the wing. The cant angle may be in the range of 1° to 5°, such as in the range of 2° to 4°, such as 3°. The drag coefficient for the vertical thrust propellers arranged at an example cant angle of 3° is illustrated by the short-dashed line in Figure 15. As can be seen, a modest adjustment of the cant angle of just 3° produced a notable improvement of about 25 de reduction in drag at 0° angle of attack of the aircraft compared to a 0° cant angle, while still maintaining the relatively flat drag profile across the angle of attack range.

Claims

CLAIMS1. A vertical thrust propeller, the propeller comprising at least a first propeller blade, a second propeller blade and a third propeller blade, each disposed in sequence, in fixed angular relation to the other propeller blades, around an axis of rotation of the propeller, an angle between the first propeller blade and the second propeller blade being smaller than an angle between the second propeller blade and the third propeller blade, the angles being measured about the axis of rotation and between longitudinal axes of the respective propeller blades.

2. A vertical thrust propeller according to claim 1, wherein the plurality of propeller blades comprises an even number of propeller blades.

3. A vertical thrust propeller according to claim 1 or claim 2, wherein the propeller has a rotational symmetry of at least order 2.

4. A vertical thrust propeller according to any preceding claim, comprising only four propeller blades disposed in sequence around the axis of rotation, an angle between the third propeller blade and a fourth propeller blade being less than an angle between the fourth propeller blade and the first propeller blade.

5. A vertical thrust propeller according to claim 4, wherein the angle between the first propeller blade and the second propeller blade equals the angle between the third propeller blade and the fourth propeller blade.

6. A vertical thrust propeller according to claim 4 or claim 5, wherein the angle between the second propeller blade and the third propeller blade equals the angle between the fourth propeller blade and the first propeller blade.

7. A vertical thrust propeller according to any one claims 4 to 6, wherein the angle between the first propeller blade and the second propeller blade is less than 90°, less than 80° or less than 70°.

8. A vertical thrust propeller according to any one claims 4 to 7, wherein the angle between the second propeller blade and the third propeller blade is greater than 90°, greater than 100°, or greater than 110°.

9. A vertical thrust propeller according to any one of the preceding claims, wherein the propeller defines: a disc having a diameter D in a plane of rotation of the propeller and a height H, equal to a maximum height of the propeller blades in a frontal plane that is perpendicular to the plane of rotation and includes the axis of rotation; and a frontal area, in the frontal plane, having the height H of the disc and a width W that varies, according to a relative angular position between the propeller and the frontal plane, from a maximum width Wmax that is equal to the diameter D and a minimum width Wmin that is less than 0.7D.

10. A vertical thrust propeller according to any one of the preceding claims, wherein the propeller defines: a disc having a diameter D in a plane of rotation of the propeller and a height H, equal to a maximum height of the propeller blades in a frontal plane that is perpendicular to the plane of rotation and includes the axis of rotation, the height H varying according to an axial pitch of the propeller blades, from a relatively larger operating pitch and associated height during VTOL operation and a relatively lesser operating pitch and associated height during forward flight operation.

11. A vertical thrust propeller according to any one of the preceding claims, wherein the at least the first propeller blade, the second propeller blade and the third propeller blade have a spanwise twist less than 20°.

12. A vertical thrust propeller according to any one of the preceding claims, wherein the at least the first propeller blade, the second propeller blade and the third propeller blade have a maximum spanwise pitch less than 25°.

13. A vertical thrust propeller according any one of the preceding claims, wherein the second propeller blade is arranged at a larger pitch angle relative to a plane of rotation of the propeller blade than the first propeller blade and the third propeller blade.

14. A VTOL aircraft comprising at least one vertical thrust propeller according to any one of the preceding claims.

15. A VTOL aircraft according to claim 14, wherein the vertical thrust propeller is arranged to have a non-rotating mode during forward flight.

16. A VTOL aircraft according to claim 15, wherein, when in the non-rotating mode, the vertical thrust propeller is rotationally aligned such that a plane that is perpendicular to the plane of rotation and includes the axis of rotation, and which is parallel to a forward direction of travel of the aircraft, is between the first propeller blade and the second propeller blade.

17. A VTOL aircraft according to claim 16, wherein, the plane substantially bisects the angle between the first propeller blade and second propeller blade.

18. A VTOL aircraft according to any one of claims 15 to 17, comprising a propeller drive unit to rotate the propeller, wherein, the propeller drive unit is arranged to rotationally align the propeller when in the non-rotating mode.

19. A VTOL aircraft according to any one of claims 14 to 18, comprising one or more tiltable thrust propellers and one or more vertical thrust propellers according to any one of claims 1 to 13.

20. A VTOL aircraft according to claim 19, wherein the one or more tiltable thrust propellers are adapted to pivot, in a plane parallel to a roll axis of the VTOL aircraft, between a forward flight mode, in which the propeller is orientated to generate forward thrust, and a VTOL mode, in which the propeller is orientated to generate vertical thrust.

21. A VTOL aircraft according to any one of claims 14 to 20, comprising a fuselage and a wing section extending on either side of the fuselage, wherein each wing section supports at least one tiltable propeller and at least one vertical thrust propeller.

22. A VTOL aircraft according to claim 21, wherein each wing section supports at least two tiltable propellers and at least two vertical thrust propellers.

23. A VTOL aircraft according to claim 21 or claim 22, wherein each wing section supports at least one pylon, and each pylon supports a tiltable propeller and a vertical thrust propeller.

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