Propellers

Abstract

A vertical thrust propeller 200 is described comprising at least first, second and third propeller blades B1, B2 and B3 each disposed in sequence, in fixed angular relation to each other around an axi

Description

FIELD 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 for an electric VTOL (eVTOL) aircraft. BACKGROUND Range capability is relevant to all vehicles, and especially aircraft. 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. 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. All of these options may increase noise, which is undesirable in metropolitan environments, or may be otherwise detrimental to performance. SUMMARY 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. 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’. Known 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 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. 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. 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°. 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. 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°. 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. 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. 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. 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. 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. 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. 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. 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 the or each pylon may support a tiltable propeller and a vertical thrust propeller. BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention will now be illustrated by the accompanying drawings, of which: Figures la and lb are schematic diagrams that illustrate a prior art propeller; Figure 2 is a schematic diagram that illustrates a vertical thrust propeller according to an example of the invention; 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; Figures 4a, 4b and 4c 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; Figure 4d is a schematic diagram that illustrates another example of the invention, with variable pitch propeller blades; Figure 5 is a schematic diagram of an overhead view of an aircraft according to an example of the invention; Figures 6a and 6b are schematic diagrams of side elevations of the aircraft of Figure 5; Figure 7 is a schematic block diagram of a control system arrangement for controlling one of the propellers of the aircraft in Figures 5 and 6; and Figure 8 is a schematic diagram of a further example of a vertical thrust propeller according to the present invention. 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. 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. 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. 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. 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. 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. 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. 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 Az 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. Figure 3a reproduces Figure lb and Figure 3b reproduces Figure 2. Being side-by-side, Figures 3a 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. In examples, the relatively smaller angle a is less than 80°, or even less than 70°. In a particular example, the angle a is 60° or thereabouts. It will be appreciated that, for the prior art propeller 100 of Figure lb, Wi = 2(W0 / 2 sin (a72)) = Wo sin 45° = 0.71 Wo, and 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. For the propeller 200 in Figure 2, when the angle a is 60°, W2 = Wosin 30° = O.5Wo, and 5 L2 = Wocos 30° = O.87Wo / 2 whereby W2 and A2are about 0.5 times the width Wo, and L2 is nearly 0.9 times the width Wo / 2. 10 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 degrease in drag generated by the propeller 200 when the aircraft is operating in a forward cruise mode. 15 The impact that varying the angle a has on the width W and the component of length L is illustrated in the following table: Angle a° Angle P° Min Width W2 Length L2 40 140 0.34 Wo 0.94 Wo / 2 45 135 0.38 Wo 0.92 Wo / 2 50 130 0.42 Wo 0.91 Wo / 2 55 125 0.46 Wo 0.89 Wo / 2 60 120 0.50 Wo 0.87 Wo / 2 65 115 0.54 Wo 0.84 Wo / 2 70 110 0.57 Wo 0.82 Wo / 2 75 105 0.61 Wo 0.79 Wo / 2 80 100 0.64 Wo 0.77 Wo / 2 85 95 0.68 Wo 0.74 Wo / 2 Prior Art Prior Art Prior Art Prior Art 90 90 0.71 Wo 0.71 Wo / 2 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 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. 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 (eg. 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. 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. This effect is due to increased vortical flow formations generated along a leading edge of the propeller blade, as the propeller blade becomes more swept, which assists in preventing airflow separation over / under the propeller blade, and which consequently decreases drag. In practice, reducing the angle a° will tend to reduce drag. However, reducing the angle a° is also associated with increasing levels of cyclic loading oscillations of the propeller. 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. Although examples herein present an increased total frontal area when in forward flight compared to folly stacked and stowed arrangements, such as in EP4192731, other benefits of examples herein have been found to outweigh the associated increased drag. 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. 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. 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 4a, 4b and 4c. This clearly illustrates the benefit of examples herein. Figure 4d 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 4d, 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. 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. Figure 5 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. As shown in Figure 5, the aft propellers 510 are locked 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, thereby presenting a relatively small total frontal area A2 to the forward direction Dt and an increased component of length L2. Figure 6a illustrates, a side elevation of the eVTOL aircraft of Figure 5 when the fore propellers 505 are in the forward flight configuration and the direction of travel Dt is forwards. Figure 6b 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 6a and 6b, the aft propellers are mounted for vertical thrust and are in accord with the invention. Figure 6b 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 7. As shown in Figure 7, 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 I / O ports 780 for communicating with and controlling the electric motor 740, the position detector 760 and the locking device 770. 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. 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. 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 8. In this example, the rotational symmetry remains of order 2. 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. 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.

Claims

1. 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; anda 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 lesser operating pitch and associated height during VTOL operation and a relatively larger operating pitch and associated height during forward flight operation.

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

12. A VTOL aircraft according to claim 11, wherein the vertical thrust propeller is arranged to have a non-rotating mode during forward flight.

13. A VTOL aircraft according to claim 12, 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.

14. A VTOL aircraft according to claim 13, wherein, the plane substantially bisects the angle between the first propeller blade and second propeller blade.

15. A VTOL aircraft according to any one of claims 11 to 14, 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.

16. A VTOL aircraft according to any one of claims 11 to 15, comprising one or more tiltable thrust propellers and one or more vertical thrust propellers according to any one of claims 1 to 10.

17. A VTOL aircraft according to claim 16, 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.

18. A VTOL aircraft according to any one of claims 11 to 17, 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.

19. A VTOL aircraft according to claim 18, wherein each wing section supports at least two tiltable propellers and at least two vertical thrust propellers.

20. A VTOL aircraft according to claim 18 or claim 19, wherein each wing section supports at least one pylon, and the or each pylon supports a tiltable propeller and a vertical thrust propeller.

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