A propeller

Abstract

A propeller is described. The propeller comprises an aerofoil, a hub, a primary structural member, and a secondary structural member. The primary structural member and the secondary structural member are each configured to connect the aerofoil to the hub via a respective independent load path. Each structural member is configured to withstand a maximum centrifugal force exerted on the aerofoil in operation in the event that the other structural member fails.

Description

[0001] A PROPELLER

[0002] TECHNICAL FIELD

[0003] The present invention relates to a propeller.

[0004] BACKGROUND

[0005] Propellers are used on vehicles to generate thrust. There is a general desire to improve the performance and safety of propellers.

[0006] SUMMARY

[0007] According to a first aspect of the present invention there is provided a propeller comprising: an aerofoil; a hub; a primary structural member; and a secondary structural member, wherein: the primary structural member and the secondary structural member are each configured to connect the aerofoil to the hub via a respective independent load path; and each structural member is configured to withstand a maximum centrifugal force exerted on the aerofoil in operation in the event that the other structural member fails.

[0008] Each structural member provides a load path between the aerofoil and the hub. In this way, the two, independent structural members provide a redundant connection between the hub and the aerofoil such that the connection between the hub and the aerofoil is not subject to a single point of failure. This may improve the safety of the propeller by reducing the likelihood of the aerofoil being released from the hub during operation. A released blade could impact with, and damage, adjacent systems of an aircraft comprising the propeller. Additionally, or alternatively, out of balance forces resulting from the release of the blade could result in damage to structures supporting the propeller.

[0009] Optionally, the secondary structural member is configured to restrict a movement of the aerofoil away from the hub after a failure of the primary structural member to less than 10mm. The magnitude of the out of balance forces may be related, for instance proportional to, the distance the aerofoil moves away from the hub after a failure of one structural member. By limiting the radial movement to less than 10mm, the magnitude of potential out of balance forces that the structures supporting the propeller are required to withstand may be reduced. As a result, the supporting structures may be designed to withstand lesser out of balance forces and may thereby be lighter.

[0010] Optionally, the secondary structural member is configured to restrict the movement of the aerofoil away from the hub after the failure of the primary structural member to less than 1.75mm. At 1.75mm, the out of balance forces may be reduced sufficiently that the aircraft is able to complete its mission. This may provide greater operational flexibility to the pilot when compared with a greater movement, which may necessitate the immediate landing of the aircraft after the primary structural member has failed. Optionally, the secondary structural member is configured to restrict the movement of the aerofoil away from the hub after the failure of the primary structural member to less than 1.5mm, 1.25mm, or 1.1mm.

[0011] Optionally, the secondary structural member is configured to restrict a movement of the aerofoil away from the hub after the failure of the primary structural member to less than 0.1mm. As a result, the out of balance forces may be further reduced and the primary structural member may be retained in practically its original position.

[0012] Optionally, the secondary structural member is configured to allow the movement of the aerofoil away from the hub after the failure of the primary structural member. Allowing a movement, for instance of greater than 0.01mm, away from the hub after the failure of the primary structural member may beneficially allow the secondary structure second structure to be substantially unloaded in normal operation and hence substantially not degrade during normal operation.

[0013] Optionally, the secondary structural member is configured to allow a movement of the aerofoil away from the hub after the failure of the primary structural member of no less than 0.1mm, 0.5mm, 0.75mm, or 0.9 mm. In some examples, allowing a sufficiently large movement away from the hub after the failure of the primary structural member may result in a change in vibration of the propeller which is sufficiently large to be detectable in the output of the vibration sensor. Thereby the failure of the primary structural member may be more easily detectable.

[0014] The movement having a range of 0.25mm to 1.75mm (and more specifically, 0.9mm to 1.1mm) may provide a good balance between the competing requirements of allowing sufficient movement for detectability and restricting movement to keep the out of balance forces within an acceptable limit. Optionally, the propeller comprises a sensor configured to output a signal indicative of the failure of one of the structural members. Due to the secondary structural member restricting the movement of the aerofoil away from the hub, the failure of the primary structure may be difficult to detect on visual inspection. Therefore, without an alternative detection means, the failure of the primary structural member may be difficult to detect and thereby, the pilot may not be aware that there is no longer redundancy in the structural members. Thereby, providing the sensor, may make the failure of the primary structural member more easily detectable.

[0015] Optionally, the sensor is a vibration sensor which is configured to output a signal indicative of a change in the vibration of the propeller which is indicative of the failure of one of the structural members. In some examples, allowing a movement, for example of greater than 0.95mm, away from the hub after the failure of the primary structural member may result in a change in vibration of the propeller which is sufficiently large to be detectable in the output of the vibration sensor. Thereby the failure of the primary structural member may be more easily detectable.

[0016] Optionally, when the primary structural member has not failed, the primary structural member contacts the hub via a contact area; and the secondary structural member is configured to restrict the movement of the aerofoil away from the hub such that after a failure of the primary structural member, the primary structural member contacts the hub via at least a portion of the contact area. As a result, the primary structural member may still be supported by the contact area (which may be, for example, a bearing in which the primary structural member sits) after the failure. This may reduce the movement of the aerofoil, and associated risk of damage, compared with if the primary structural member were permitted to move free of the contact area (such as the primary structural member moving out of the bearing entirely). Optionally, the hub comprises a bearing, and the bearing defines at least part of the contact area. Optionally, the hub comprises an opening; the primary structural member extends through the opening; and the contact area is at least partially defined by a part of the hub which defines the opening.

[0017] Optionally, one of the primary structural member and the secondary structural member is located within the other of the primary structural member and the secondary structural member. Locating one of the structural members within the other may improve the manufacturability of the propeller and / or reduce the space needed to accommodate the structural members. For example, the hub may comprise a bearing within which the structural members sit to facilitate pitching of the aerofoil. The dimensional tolerances of the bearing may be relatively tight. It may be easier to manufacture a single outer surface of one of the structural members to fit within the bearing than it would be to manufacture the outer surfaces of two structural members to sit side by side and fit within the bearing.

[0018] Optionally, the structural members are concentric with one another. The behaviour of the aerofoil after a failure of the primary structural member may thereby be more similar, and thereby controllable, to its pre-failure behaviour than if the centres of the structural members were offset from one another.

[0019] Optionally, the secondary structural member is located closer to a longitudinal axis of the aerofoil than the primary structural member. In a propeller, the dominant loads driving the thickness of the primary structural member may be bending loads in a plane which extends along the longitudinal axis of the aerofoil. As material near the neutral axis (which, in the case of the aerofoil, is the longitudinal axis) provides significantly less bending stiffness than material further from the neutral axis, the centre (or inner region) of the primary structural member may be hollow. Thereby, the secondary structural member can be located within the hollow space close to neutral axis to provide a space efficient design.

[0020] Optionally, the movement is in a radial direction perpendicular to a rotational axis of the hub; and a total bending stiffness of the primary structural member and the secondary structural member, in a plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no greater than 50% of a total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. As mentioned above, the dominant loads driving the thickness of the structural members may be bending loads. By designed the primary structural member and the secondary structural member to provide less than 50% of the total stiffness before the failure, the secondary structural member can be made significantly thinner and thereby lighter whilst still providing sufficient tensile stiffness and strength in the radial direction to withstand the centrifugal load acting to move of the aerofoil away from the hub.

[0021] Additionally, in propellers in which the secondary structural member is located within the primary structural member, the thinner secondary structural member may be more easily accommodated within the primary structural member without increasing the outer dimensions of the primary structural member. This may be desirable in examples where the primary structural member sits in a bearing because increasing the outer dimensions of the primary structural member may result in an increase in the size of the bearing, which may drive significant extra weight into the propeller.

[0022] Furthermore, this may make the secondary structural member more compliant than the primary structural member. Thereby, the secondary structural member may be more able to withstand the loads induced by the flapping of the aerofoil after the primary structural members failure than if the secondary structural member were stiffer.

[0023] Optionally, the total bending stiffness of the primary structural member and the secondary structural member, in the plane which extends along the longitudinal axis of the aerofoil, after the failure of the primary structural member is no greater than 40%, 30%, 20%, or 10% of the total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. Further reducing the bending stiffness may reduce the weight of the secondary structural member whilst still providing sufficient tensile stiffness to resist the movement.

[0024] Optionally, the movement is in the radial direction perpendicular to the rotational axis of the hub; and the total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no greater than 50% of a torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. As a result, the secondary structural member may be made lighter whilst still providing sufficient stiffness and strength in the radial direction to withstand the centrifugal load acting to move of the aerofoil away from the hub.

[0025] Optionally, the total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no greater than 40%, 30%, 20%, or 10% of the torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. Further reducing the torsional stiffness may reduce the weight of the secondary structural member whilst still providing sufficient tensile stiffness to resist the movement.

[0026] Optionally, the secondary structural member comprises a composite material. As the loads on the secondary structural member are highly directional, the anisotropic properties of a composite material (such as a glass fibre reinforced plastic) may be exploited to provide a lighter design than may be possible with an isotropic material such as metal. For example, the secondary structural member may comprise a high proportion of unidirectional plies orientated in the radial direction to resist the movement of the aerofoil.

[0027] Optionally, primary structural member comprises a first material, and the secondary structural member comprises a second material which is different to the first material. This may result in a more volumetrically or weight optimised propeller than if the same material were used for both structural members due to the different desired mechanical properties of each structural member.

[0028] Optionally, an ultimate tensile strain of the second material is greater than an ultimate tensile strain of the first material. As a result, the secondary structural member may be better able to withstand the potential flapping motion of the aerofoil after a primary structural member failure than if the secondary structural member had a lower ultimate tensile strain. For example, the first material may be a carbon fibre reinforced plastic, whilst the second material may be a glass fibre reinforced plastic.

[0029] Optionally, the hub comprises an opening which has a maximum width measured in a plane which extends parallel to the rotational axis of the hub; and at least one of the primary structural member and the secondary structural member extend through the opening and comprise a flange, located within the hub, which has a width measured in the plane which extends parallel to the rotational axis of the hub greater than the maximum width of the opening. The flange provides a failsafe which retains the structural member within the hub in the event that a fastener connecting the structural member to the hub breaks. Optionally, the primary structural member and the secondary structural member extend through the opening and each comprise a respective flange, located within the hub, which has a width measured in the plane which extends parallel to the rotational axis of the hub greater than the maximum width of the opening.

[0030] Optionally, at least one of the primary structural member and the secondary structural member comprise a composite material; and the flange is bonded to the hub. Optionally, the flange is formed and cured with at least one of the primary structural member and the secondary structural member located within the hub. This may improve the manufacturability of the propeller and improve the reliability of the connection between the structural members and the hub.

[0031] Optionally, the primary structural member is a separate part to the secondary structural member; the primary structural member and the secondary structural member each comprise a respective flange; the propeller comprises: a first connection mechanism configured to connect the primary structural member to the aerofoil; and a second connection mechanism configured to connect the secondary structural member to the aerofoil. Having the aerofoil and structural members as three separate parts provides a means for the propeller to be assembled with the structural members each comprising a flange. For example, the structural members can be inserted through the opening from the inside of the hub, and then bolts inserted into bolt holes (an example of a connection mechanism) in each of the aerofoil and structural members to assemble the propeller. If the aerofoil and structural members were manufactured as a single piece (without the flanges being formed in situ), it may be challenging to insert the aerofoil and structural members through the opening in the hub.

[0032] Optionally, the first connection mechanism and the second connection mechanism connect to the aerofoil on opposite sides of a plane which extends along the longitudinal axis of the aerofoil, and a total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no less than 95% of the torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. As a result, the secondary structural member may be able to maintain the original performance of the propeller in the event of the primary structural member failing.

[0033] Optionally, the primary structural member and the secondary structural member form a monolithic structure; and the monolithic structure is configured to inhibit a crack from propagating between the primary structural member and the secondary structural member. A monolithic structure, such as a composite laminate manufactured in a single layup, may provide a more compact and mass efficient arrangement than if the structural members were physically separate to one another. This may be desirable to aid fitting the supporting structures within a propeller’s space constrains. Additionally, a monolithic structure may be more easily manufactured than two separate structural members which then need to be tolerance to fit together or into other space constrains such as a bearing.

[0034] If the monolithic structure were not configured to inhibit crack propagation between the primary structural member and the secondary structural member, then the structural members would not provide independent load paths and the associated benefits because a crack which caused the failure of the primary structural member could propagate to the second structural member. Thereby, configuring the monolithic structure to inhibit a crack from propagating between the primary structural member and the secondary structural member may enable the benefits of a monolithic structure to be achieved at the same time as achieving the benefits of the two independent load paths.

[0035] Optionally, the monolithic structure comprises an intermediate layer located between the primary structural member and the secondary structural member; and the intermediate layer has a lower strength and / or a lower modulus than a strength and modulus of the primary structural member and a strength and modulus of the secondary structural member. This may provide an effective means of inhibit a crack from propagating between the primary structural member and the secondary structural member.

[0036] Optionally, the intermediate layer has a plane strain fracture toughness which is greater than a plane strain fracture toughness of the primary structural member or the secondary structural member.

[0037] Optionally, a relative difference between a first property of the primary structural member and a second property of the secondary structural member is such that the monolithic structure is configured to inhibit a crack from propagating between the primary structural member and the secondary structural member. Using a relative difference in the properties of the structural members to provide the crack inhibiting properties rather than, for example, a dedicated intermediate layer, may reduce the size, weight and / or manufacturing complexity of the monolithic structure.

[0038] Optionally, the first property is a first fibre orientation of a first ply of the primary structural member which abuts a second ply of the secondary structural member; the second property is a second fibre orientation of the second ply of the secondary structural member; and the relative difference is a misalignment between the first fibre orientation and the second fibre orientation. Cracks may be inhibited from propagating between misaligned plies. This may provide a simple to manufacture means of inhibiting crack propagation. Optionally, the misalignment between the first fibre orientation and the second fibre orientation is no less than 10°, 20°, 40°, 45°, 60°, 80°, 85°, or 90°. Increasing the misalignment may further reduce the likelihood of crack propagation.

[0039] Optionally, the first property is a strain to failure of the primary structural member; and the second property is a strain to failure of the secondary structural member. A crack may be inhibited from propagating from a lower strain to failure material into a higher strain to failure material. This may provide a simple to manufacture means of inhibiting crack propagation. Optionally, the strain to failure of the primary structural member is less than the strain to failure of the secondary structural member.

[0040] Optionally, a total torsional stiffness of the primary structural member and the secondary structural member, about a longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 10% of a torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. As a result, the secondary structure has a significant mechanical property in a loading direction other than the tensile direction, which may improve the propeller’s behaviour after a failure of the primary structural member.

[0041] After failing, the primary structural member may no longer provide sufficient torsional stiffness to restrain the pitch of the aerofoil. As a result, the pitch may oscillate between increasing, resulting in the aerofoil moving upwards, and decreasing, resulting in the aerofoil moving downwards. This may result in the aerofoil undergoing large deflections up and down, which may be unpredictable and result in damage to the hub and structures supporting the propeller. By providing no less than 10% of the torsion stiffness, the torsional stiffness may be sufficient to restrain the pitch of the aerofoil such that the motion of the aerofoil is more predictable and manageable.

[0042] Optionally, the total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no less than 20%, 30%, or 40% of the torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. Further increasing the torsional stiffness may increase the restraint of the pitch and thereby further increase the predictability and manageability of the motion of the aerofoil.

[0043] Optionally, the total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no less than 30% and no greater than 40% of the torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. This range may provide a good balance between the competing needs of providing sufficient restraint of the pitch and reducing the weight of the secondary structural member. Optionally, a total bending stiffness of the primary structural member and the secondary structural member, in a plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 10% of a total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. As a result, even after a failure has occurred, the propeller may have significant mechanical property in a loading direction other than the tensile direction, which may improve the propeller’s behaviour after a failure of the primary structural member.

[0044] After failing, the primary structural member may no longer provide sufficient bending stiffness to resist bending of the aerofoil. Whilst the propeller is spinning, the centrifugal forces may be sufficient to resist the bending. However, when the propeller slows, the centrifugal forces will no longer resist the bending, and the released aerofoil may move and become tangled with and damage the structures supporting the propeller. For example, the centrifugal forces may be sufficient to resist gravity and keep the released aerofoil up. When the propeller slows, the centrifugal forces will no longer resist gravity, and the released aerofoil may drop and become tangled with the structures supporting the propeller. Providing no less than 10% of the total bending stiffness may provide sufficient bending stiffness to resist this dropping and reduce the likelihood of, or prevent, the aerofoil becoming tangled with the structures supporting the propeller.

[0045] Optionally, the total bending stiffness of the primary structural member and the secondary structural member, in the plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 20%, 30%, or 40% of the total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. Further increasing the stiffness may further reduce the likelihood of the aerofoil becoming tangled with the structures supporting the propeller.

[0046] Optionally, the total bending stiffness of the primary structural member and the secondary structural member, in the plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 40% and no greater than 50% of the total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member. This range may provide a good balance between the competing needs of providing sufficient bending stiffness to resist the dropping of the aerofoil and reducing the weight of the secondary structural member. Optionally, the propeller comprises a blade which comprises the aerofoil. Optionally, at least one of the primary structural member and the secondary structural member are integrally formed with the blade. Optionally, the blade comprises a spar. Optionally, at least one of the primary structural member and the secondary structural member are integrally formed with the spar. This may improve the robustness of the propeller by reducing the likelihood of the blade becoming detached from the structural members when compared with, for example, if the structural members were distinct components from the blade and connected by bolts. Optionally, the spar is integrally formed with the aerofoil. Optionally, at least one of the primary structural member and the secondary structural member are integrally formed with the spar such that continuous fibres extend between the at least one of the primary structural member and the secondary structural member and the spar.

[0047] Optionally, a tensile stiffness of the secondary structural member along a longitudinal axis of the aerofoil is less than a tensile stiffness of the primary structural member along a longitudinal axis of the aerofoil. This may provide a convenient means for allowing the movement of the aerofoil away from the hub after a failure of the primary structural member and / or allowing the secondary structural member to be unloaded prior to a failure of the primary structural member. This may provide a means for allowing significant movement to occur (for example, up to 10 mm), without significantly increasing the size and / or complexity of the propeller. This may provide a means for allowing the movement to occur without introducing a significant mechanical shock which could damage the secondary structural member. For example, as may occur if the secondary structural member were required to move into contact with a stop to allow the movement.

[0048] According to a second aspect of the present invention there is provided a vertical take-off and landing aircraft comprising the propeller, wherein the longitudinal axis of the aerofoil extends horizontally during normal operation of the vertical take-off and landing aircraft. In a horizontal take-off and landing aircraft, the aerofoil may extend vertically from the hub and the structures supporting the propeller may extend horizontally from the propeller hub. Thereby, the released aerofoil can drop without contacting the supporting structures. However, in a vertical take-off and landing (VTOL) aircraft, during landing and take-off, the aerofoils extend horizontally from the hub, and the supporting structures extend vertically beneath the propeller hub. As a result, in a VTOL aircraft which is landing after a failure has occurred, the released aerofoil cannot drop without contacting the supporting structure. Thereby, providing no less than 10% of the bending stiffness may be especially beneficial in a VTOL aircraft. Optionally, the vertical take-off and landing aircraft is an electric vertical take-off and landing (eVTOL) aircraft.

[0049] According to a third aspect of the present invention there is provided a vertical take-off and landing aircraft comprising the propeller, wherein the vertical take-off and landing aircraft comprises a failure detection system configured to, based on the signal output by the sensor, output an alert signal in response to detecting the failure of the primary structural member.

[0050] Optionally, the failure detection system configured to, based on the signal output by the vibration sensor, output the alert signal in response to detecting the change in the vibration of the propeller which is indicative of the failure of the primary structural member.

[0051] Optionally, the alert signal is output to an alert device in a cockpit of the vertical take-off and landing aircraft; and the alert device is configured to output an alert in response to receiving the alert signal. Optionally, the failure detection system is comprised by the propeller.

[0052] According to a fourth aspect of the present invention there is provided a propeller comprising: an aerofoil; a hub; a sensor; and a structural member configured to connect the aerofoil to the hub, wherein: the propeller is configured to restrict a movement of the aerofoil away from the hub after a failure of the structural member, and allow the movement of the aerofoil away from the hub after the failure of the structural member; and the sensor is configured to output a signal indicative of the failure of the structural member.

[0053] Optionally, the propeller is configured to allow the movement of the aerofoil away from the hub after the failure of the structural member to be greater than a lower limit, 0.01mm, 0.25mm, 0.5mm, 0.75mm, or 0.9mm. Optionally, the propeller is configured to restrict the movement of the aerofoil away from the hub after the failure of the primary structural member to less than an upper limit, 10mm, 1.75mm, 1.5mm, 1.25mm, or 1.1mm. Optionally, the sensor is a vibration sensor. Optionally, the signal is indicative of a change in the vibration of the propeller which is indicative of the failure of the structural member.

[0054] According to a fifth aspect of the present invention there is provided an electric vertical takeoff and landing aircraft comprising: the propeller of the fourth aspect of the present invention; and a failure detection system configured to, based on the signal output by the sensor, output an alert signal in response to detecting the failure of the structural member.

[0055] Optionally, the failure detection system is configured to, based on the signal output by the vibration sensor, output the alert signal in response to detecting the change in the vibration of the propeller which is indicative of the failure of the structural member. Optionally, the alert signal is output to an alert device in a cockpit of the vertical take-off and landing aircraft; and the alert device is configured to output an alert in response to receiving the alert signal. Optionally the alert signal is output to a maintenance system of the vertical take-off and landing aircraft; and the maintenance system is configured to prompt a maintenance action in response to receiving the alert signal. Optionally, the failure detection system is comprised by the propeller.

[0056] According to a sixth aspect of the present invention there is provided a method comprising: providing an aerofoil comprising an aerofoil, a hub, and a structural member configured to connect the aerofoil to the hub; restricting a movement of the aerofoil away from the hub after a failure of the structural member to a range of 0.01mm to 10mm; and outputting a signal indicative of the failure of the structural member.

[0057] Optionally, the range is between 0.1mm and 10mm, 0.25mm and 1.75mm, 0.5mm and 1.5mm, 0.75mm and 1.25mm, or 0.9mm and 1.1mm.

[0058] Optionally, the signal is indicative of a change in the vibration of the propeller which is indicative of the failure of the structural member. Optionally, the method comprises: receiving the signal indicative of the change in the vibration of the propeller which is indicative of the failure of the primary structural member; based on the signal, detecting the change in the vibration of the propeller which is indicative of the failure of the primary structural member; and in response to the detecting the change in the vibration of the propeller which is indicative of the failure of the primary structural member, outputting an alert signal.

[0059] Optionally, the method comprises outputting an alert to a pilot in response to the outputting the alert signal. Optionally, the method comprises prompting a maintenance action in response to the outputting the alert signal. Optionally, the method comprises taking corrective action in response to the outputting the alert signal. Optionally, the method comprises providing the propeller of the first aspect of the present invention.

[0060] According to a seventh aspect of the present invention, there is provided an electric vertical take-off and landing aircraft comprising the propeller of any one of the preceding aspects of the present invention.

[0061] Optional features of aspects may be equally applied to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure l is a schematic perspective view of an example propeller;

[0063] Figure 2 is a schematic section side view of the propeller;

[0064] Figure 3 is a schematic section side view of a hub of the propeller;

[0065] Figure 4 is a schematic side view of the hub;

[0066] Figure 5 is a perspective view of a blade of the propeller;

[0067] Figure 6 is a perspective view of the blade with components of the blade shown;

[0068] Figure 7 is a schematic section side view of a blade root of the blade;

[0069] Figure 8 is a schematic section side view of a primary structural member of the blade root;

[0070] Figure 9 is a schematic section side view of a secondary structural member of the blade root;

[0071] Figure 10 is a schematic section side view of an intermediate layer of the blade root;

[0072] Figure 11 is a schematic section side view of a bearing of the blade;

[0073] Figure 12 is a schematic section side view of the blade root when assembled;

[0074] Figure 13 is a perspective view of the blade when assembled;

[0075] Figure 14 is a schematic section side view of the propeller when assembled;

[0076] Figure 15 is an enlarged schematic section side view of the blade root, the hub, and the bearing;

[0077] Figure 16 is a schematic section side view of a pitching mechanism of the propeller;

[0078] Figure 17 is a perspective view of the blade showing loads experienced during operation;

[0079] Figure 18 is an enlarged schematic section side view of the propeller after a failure of the primary structural member;

[0080] Figure 19 is a perspective view of an example electric vertical take-off and landing aircraft which comprises the propeller; Figure 20 is a perspective view of the example electric vertical take-off and landing aircraft with axes of the aircraft shown;

[0081] Figure 21 is a schematic of a vibration sensor, a failure detection system, and an alert device of the example aircraft;

[0082] Figure 22 is a flow chart of steps taken by the vibration sensor, the failure detection system, and the alert device after a failure has occurred;

[0083] Figure 23 is a schematic section side view of an example second blade;

[0084] Figure 24 is a schematic section top view of the second blade;

[0085] Figure 25 is a schematic section side view of an aerofoil of the second blade;

[0086] Figure 26 is a schematic section top view of the aerofoil of the second blade;

[0087] Figure 27 is a perspective view of a blade root of the second blade and a hub; and

[0088] Figure 28 is perspective section view of the blade root of the second blade and the hub.

[0089] DETAILED DESCRIPTION

[0090] Figures 1 and 2 show a propeller 11 comprising a hub 13, four blades 15, and a pitching mechanism 17. Other propellers may have a different number of blades, e.g., five blades.

[0091] The hub 13, shown in isolation in Figures 3 and 4, comprises a nose cone 19, a main body 21, a rotational axis 22, a radial direction 24, and four blade openings 26 (one blade opening 26 for each of the blades 15).

[0092] The nose cone 19 has a hollow conical shape and deflects airflow around the hub 13 in operation. The main body 21 has a cylindrical tubular shape which defines an inside 23 of the hub 13, and an outside 25 of the hub 13. The main body 21 in this example has a wall thickness 27 of 2mm. In other propellers, the wall thickness 27 could be a different value.

[0093] The rotational axis 22 extends through a centre of the main body 21 and is the axis about which the propeller 11 rotates during operation. The radial direction 24 of the hub 13 extends in a direction perpendicular to the rotational axis 22 of the hub 13. The four blade openings 26 are equally spaced around a circumference of the main body 21 of the hub 13 and extend through the main body 21 to provide access to the inside 23 of the main body 21. The four blade openings 26 are circular. Each of the blade openings 26 has a diameter 31 (which, in this example, is the maximum width of each blade opening 26) measured in a plane 33 which extends in a direction parallel to the rotational axis 22 of the hub 13. The diameter 31 and the plane 33 are shown in Figure 4. In Figure 4, the hub 13 is rotated by 90° about the rotational axis 22 from the view of the hub 13 in Figure 3.

[0094] The four blades 15 are identical, so only one blade 15 will be described hereafter for brevity. The blade 15, shown in isolation in Figure 5, comprises an aerofoil 35, a blade root 37, and a bearing 39. The blade 15 is a single, monolithic, structure. However, in other blades, the blade 15 may comprise multiple, distinct components.

[0095] The aerofoil 35, shown in Figure 6, comprises a spar 41, a longitudinal axis 43, and a trailing edge 45. The aerofoil 35 is the surface of the blade 15 which interacts with an airflow in use to generate lift.

[0096] The spar 41 has an elongate shape with a proximal end 47 which adjoins the blade root 37, and a distal end 49 which constitutes the tip of the aerofoil 35. The spar 41 constitutes the primary structural element of the aerofoil 35. The spar 41 comprises a carbon fibre reinforced plastic material.

[0097] The longitudinal axis 43 of the aerofoil 35 extends approximately down the centre of the spar 41 between the proximal end 47 and the distal end 49 of the spar 41.

[0098] The trailing edge 45 extends from the spar 41 and provides an additional lifting surface for the aerofoil 35.

[0099] The blade root 37, shown in isolation in Figure 7, comprises a primary structural member 51, a secondary structural member 53, and an intermediate layer 55. The blade root 37 is a single piece, monolithic, composite laminate, which is manufactured in a single layup.

[0100] The primary structural member 51, shown in isolation in Figure 8, comprises a central longitudinal axis 60, main body 61, a flange 63, and a pitching horn 65. The primary structural member 51 comprises a carbon fibre reinforced plastic material, which has unidirectional plies which are orientated to align with the central longitudinal axis 60. The main body 61 of the primary structural member 51 has a cylindrical shape. The main body is hollow such that there is a hollow space 67 around the central longitudinal axis 60 of the primary structural member 51. The hollow space 67 has a cylindrical shape. The main body 61 has an outer diameter 69 , which is measured in a direction perpendicular to the longitudinal axis 60. The hollow space 67 has an outer diameter 71, which is measured in a direction perpendicular to the longitudinal axis 60. The main body 61 has a length 73, which is measured in a direction parallel to the longitudinal axis 60. The main body has a first end 75 and a second end 77.

[0101] The flange 63 of the primary structural member 51 projects out from a circumference of the first end 75 of the main body 61 of the primary structural member 61 and has an annular shape. The flange 63 has a maximum diameter 77 (which, in this flange 63, is equal to the maximum width) measured in a direction perpendicular to the central longitudinal axis 60 of the primary structural member 51.

[0102] The pitching horn 65 of the primary structural member 51 comprises a first part which extends away from a circumference of the flange 63 of the primary structural member 63, and a second part which extends in a direction perpendicular to the first part towards the central longitudinal axis 60 of the primary structural member 51. A hole (not shown) is located in the second part.

[0103] The secondary structural member 53, shown in isolation in Figure 9, comprises a central longitudinal axis 80, a main body 81, a flange 83, and a pitching horn 85. The secondary structural member 53 comprises a glass fibre reinforced plastic material. Thereby, the primary structural member 51 and the secondary structural member 53 comprise different materials, and the secondary structural member 53 comprises a material having a greater ultimate tensile strain than the material of the primary structural member 51. The secondary structural member 53 comprises unidirectional plies which are orientated to align with the central longitudinal axis 80 of the secondary structural member 53. The secondary structural member 53 has a higher proportion of unidirectional plies orientated to align with the central longitudinal axis 80, than the primary structural member 51 has to align with its central longitudinal axis 60.

[0104] The main body 81 of the secondary structural member 53 has a cylindrical shape. An outer diameter 87 of the main body 81, measured in a direction perpendicular to the central longitudinal axis 80 of the secondary structural member 53, is less than the outer diameter 71 of the hollow space 67. The length 89 of the main body 81, measured in a direction parallel to the central longitudinal axis 80 of the secondary structural member 53, is greater that the length 73 of the main body 61 of the primary structural member 51. The main body 81 has a first end 91 and a second end 93.

[0105] The flange 83 of the secondary structural member 53 projects out from a circumference of the first end 91 of the main body 81 of the secondary structural member 53 and has an annular shape. The flange 83 has a maximum diameter 95 (which, in this flange, is the maximum width) measured in a direction perpendicular to the central longitudinal axis 80 of the secondary structural member 53 which is less than the maximum diameter 77 of the flange 63 of the primary structural member 51.

[0106] The pitching horn 85 of the secondary structural member 53 comprises a first part which extends away from a circumference of the flange 85 of the secondary structural member 53, and a second part which extends in a direction perpendicular to the first part towards the central longitudinal axis 80 of the secondary structural member 53. A hole (not shown) is located in the second part.

[0107] The intermediate layer 55, shown in isolation in Figure 10, comprises a central longitudinal axis 90, a main body 91, and a flange 93. The main body 91 has the shape of a hollow cylinder. The main body 91 has an outer diameter 95, measured in a direction perpendicular to the longitudinal axis 90 of the intermediate layer 55, which is less than the outer diameter 71 of the hollow space 67. The main body 91 has an inner diameter 97, measured in a direction perpendicular to the longitudinal axis 90 of the intermediate layer 55 which is greater than the outer diameter 87 of the main body 81 of the secondary structural member 53. The intermediate layer 55 is formed from a material which has a lower strength and a lower modulus than a strength and modulus of the primary structural member 51 and a strength and modulus of the secondary structural member 53. The main body 91 has a first end 99 and a second end 101.

[0108] The flange 93 of the intermediate layer 55 projects out from a circumference of the first end 99 of the main body 91 of the intermediate layer 55 and has an annular shape. The flange 93 has a maximum diameter 103 (which in this flange 93, is the maximum width) measured in a direction perpendicular to the central longitudinal axis 90 of the intermediate layer 53.

[0109] The bearing 39, shown in isolation in Figure 11, is a bearing which has an inner diameter 105 which is larger than the outer diameter 69 of the main body 61 of the primary structural member 51, and an outer diameter 107 which is less that the diameter 31 of the blade openings 26. The pitching mechanism 17 comprises four linkages 70 (one linkage is shown by way of example in Figure 16) which are actuated in use to change the pitch of the blades 15.

[0110] Propeller assembly

[0111] The assembly of the propeller 11 will now be described.

[0112] Figure 12 shows the assembled blade root 37. The primary structural member 51, the secondary structural member 53, and the intermediate layer 55 are concentric with each other such that the central longitudinal axis 60 of the primary structural member 51, the central longitudinal axis 80 of the secondary structural member 53, and the central longitudinal axis 90 of the intermediate layer 55 are aligned with one another. The intermediate layer 55 and the secondary structural member 53 are located within the hollow space 67 of the primary structural member 51 with the intermediate layer 55 located between the primary structural member 51 and the secondary structural member 53. Thereby, the secondary structural member 53 is located closer to the longitudinal axes 60,80 of the structural members 51,53 than the primary structural member 51.

[0113] The flange 63 of the primary structural member 51 overlies the flange 83 of the secondary structural member 53, and the flange 93 of the intermediate layer 55 is located between the flanges 63,83 of the structural members 51,53. The pitching horn 65 of the primary structural member 51 overlies the pitching horn 85 of the secondary structural member 53 such that the holes in the pitching horns 65,85 align with one another.

[0114] Turning now to Figure 13, the bearing 39 is located around the outside of the main body 61 of the primary structural member 51 of the blade root 37.

[0115] The second ends 51,53,101 of the structural members 51,53, and the intermediate layer 55, adjoin the proximal end 47 of the spar 41. Although described here as two distinct entities, in this blade 15, the spar 41 and the blade root 37 are a single part with continuous fibres extending between the spar 41 and the blade root 37.

[0116] The spar 41 and the blade root 37 are orientated such that the longitudinal axis 43 of the aerofoil is aligned with the central longitudinal axes 60,80,90 of the structural members 51,53 and the intermediate layer 55. Thereby, the central longitudinal axes 60,80,90 of the structural members 51,53 and the intermediate layer 55 are equivalent to the longitudinal axis 43 of the aerofoil 35 in this blade 15.

[0117] Turning now to Figure 14, the blade 15 is orientated relative to the hub 13 such that the longitudinal axis 43 of the aerofoil 35 is aligned with the radial direction 24 of the hub 13. Thereby, the radial direction 24 of the hub 13, the central longitudinal axes 60,80,90 of the structural members 51,53 and the intermediate layer 55, and the longitudinal axis 43 of the aerofoil 35 are all synonymous with one another. Additionally, the diameters 77,95,103 of the flanges 63,83,93 are thereby measured in a direction perpendicular to the rotational axis 22 of the hub 13 and are greater than the diameters 31 of the blade openings 31. The nose cone 19 connects to a first end 29 of the main body 21.

[0118] As shown in Figure 14, and the enlarged view of Figure 15, the blade root 37 extends through one of the blade openings 26 such that the second ends 77,93,101 of the structural members 77, 93 and intermediate layer 55, are on the outside 25 of the hub 13, and the flanges 63,83,93 are on the inside 23 of the hub 13. The flanges 63,83,93 are connected to the hub 13 via the bearing 39. To locate the flanges 63,83,93 within the hub 13, during manufacture, the blade root 37 is inserted through the blade opening 26 prior to forming the flanges 63,83,93, forming the flanges 63,83,93 inside the hub 13, and then curing the blade 15 with the flanges 63,83,93 in situ inside 23 the hub 13. In other propellers the flanges 63,83,93 may be omitted and the structural members 77.93 and intermediate layer 55 bolted directly to the hub 13.

[0119] By connecting the flanges 63,83,93 to the hub 13, the blade root 37, and specifically the primary structural member 51 and the secondary structural member 53, connect the aerofoil 35 to the hub 13. As mentioned previously, the intermediate layer 55 is located between the primary structural member 51 and the secondary structural member 53. The intermediate layer 55 provides independence between the two structural members: the intermediate layer 55 may inhibit a crack which caused the failure of the primary structural member 51 from propagating to the secondary structural member 53. Thereby, each structural member 51,53 provides an independent load path between the aerofoil 35 and the hub 13, and thereby independently connects the aerofoil 35 to the hub 13.

[0120] As shown in Figure 15, the bearing 39 is located within the blade opening 26. This facilitates rotation of the blade 15 about the longitudinal axis 43 of the aerofoil 35 relative to the hub 13, which changes the pitch of the blade 15. The cylindrical shape of the blade root 37 enables the bearing 39 to be received within the blade opening 26 and permit rotation.

[0121] The blade root 37 (and specifically, the primary structural member 51) contacts the hub 13 via a contact area 111 (denoted by the dashed rectangles in Figure 15) which comprises the area of the flanges 63,83,93 which is in contact with the inside surface of the hub 13, and the area of the blade root 37 which is in contact with the hub 13 via the bearing 39.

[0122] As shown in Figure 16, one of the linkages 70 of the pitching mechanism 17 is connected to the pitching horns 65,85 by a bolt (not shown) which extends from the linkage 70 and through the holes in the pitching horns 65,85.

[0123] Propeller Loading and Operation

[0124] The loading experienced by the propeller 11 during operation (both normal, non-failure, operation and after a failure has occurred) will now be described to aid in the subsequent discussion of the mechanical properties of the propeller 11. As the blades 15 are each loaded in a corresponding manner, the loading of only a single blade 15 will be discussed.

[0125] During operation, the hub 13 rotates around the rotational axis 22 of the hub 13. For example, the hub 13 could be driven by the shaft of an electric motor. The hub 13 transmits this rotation to the blade 15 such that the blade 15 also rotates around the rotational axis 22 of the hub 13. As shown in Figure 17, this generates loads in four loading regimes.

[0126] Firstly, the rotation generates a centrifugal force 121 which acts along the longitudinal axis 43 of the aerofoil 35. This centrifugal force 121 acts to try and move the aerofoil 35 away from the hub 13 in the radial direction 24 of the hub 13.

[0127] Secondly, the interaction of the aerofoil 35 with the air generates a flapping bending load 123 in a plane 125 which extends along the longitudinal axis 43 of the aerofoil 35 and the rotational axis 22 of the hub 13. This flapping bending load 123 seeks to bend the blade 15 up and down as the blade 15 rotates.

[0128] Thirdly, cyclical loading due to the alternating advancing and retreating of the blade 15 into and out of the predominant airflow direction generates a lead / lag bending load 127 in a plane 129 which extends along the longitudinal axis 43 of the aerofoil 35 and in a direction perpendicular to the rotational axis 22 of the hub 13 and the longitudinal axis 43 of the aerofoil. This lead / lag bending load 127 seeks to bend the blade 15 backwards and forwards as the blade 15 rotates.

[0129] Fourthly, the interaction of the aerofoil 35 with the air generates torsional loading 131 about the longitudinal axis 43 of the aerofoil 35 as the airflow seeks to change the pitch of the blade 15.

[0130] During non-failure operation (i.e., before any part of the blade 15 has failed), the loads 121,123,127,131 are transmitted from the aerofoil 35 to the hub 13 via the blade root 37, in particular, the primary structural member 51. In this propeller 11, the secondary structural member 53 carries zero or minimal load during non-failure operation. However, another example blade will be described below in which the secondary structural member 53 carries considerable load during non-failure operation.

[0131] During operation, the primary structural member 51 may fail, as shown in Figure 18. For example, a piece of debris may impact the blade root 37 and propagate a crack 141 through the primary structural member 51 (the size of the crack 141 is exaggerated in Figure 18 for clarity). The propagation of the crack 141 may be inhibited by the intermediate layer 55, such that the secondary structural member 53 remains intact. After failure of the primary structural member 51, the primary structural member 51 unloads, and the secondary structural member 53 withstands the entire centrifugal force 121 that continues to act on the aerofoil 35. The unloading of the primary structural member 51 and the loading of the secondary structural member 53 may result in a certain amount of radial movement of the blade 15 (such a movement is shown by arrows 122 in Figure 18). If the secondary structural member 53 were not present, this movement 122 may result in the blade 15 being released from the hub 13 entirely. However, the secondary structural member 53 resists the centrifugal force 121 exerted on the aerofoil 35 and restricts the movement 122 of the aerofoil 35, subject potentially to a certain degree of movement 122, away from the hub 13 to maintain the connection between the aerofoil 35 and the hub 13.

[0132] Mechanical properties of the propeller

[0133] The primary structural member 51 has a tensile strength along the longitudinal axis 43 of the aerofoil 35 and a cross sectional area in a direction perpendicular to the longitudinal axis 43 such that the primary structural member 51 is sufficiently strong to withstand (i.e., transmit the force without failing) a maximum centrifugal force 121 exerted on the aerofoil 35 in operation in the event that the secondary structural member 53 fails. The maximum centrifugal force 121 will be dependent on the mass of the blade 15 and the rotational speed of the blade 15 and will therefore be different depending on the specific blade 15 design. Therefore, different blade 15 designs may require different tensile strengths and cross-sectional areas in order to withstand their respective maximum centrifugal forces 121. It will be appreciated that the other loads 123,127,131 will also depend on the specific blade 15 design, so will also vary between blades 15 and thereby each different blade design will require different mechanical properties to withstand these loads 123,127,131.

[0134] The primary structural member 51 has a bending stiffness (also referred to as flexural rigidity) in the plane 125 which extends along the longitudinal axis 43 of the aerofoil 35 and the rotational axis 22 of the hub 13 which is sufficiently large to accommodate the flapping bending load 123 generated in operation.

[0135] The primary structural member 51 has a bending stiffness in the plane 129 which extends along the longitudinal axis 43 of the aerofoil 35 and perpendicular to the rotational axis 22 of the hub 13 and the longitudinal axis 43 of the aerofoil which is sufficiently large to accommodate the lead / lag bending load 127 generated in operation.

[0136] The primary structural member 51 has a torsional stiffness about the longitudinal axis 43 of the aerofoil 35 which is sufficiently large to withstand the torsional loading 131.

[0137] The secondary structural member 53 has a tensile strength along the longitudinal axis 43 of the aerofoil 35 and a cross sectional area in a direction perpendicular to the longitudinal axis 43 such that the secondary structural member 53 is sufficiently strong to withstand the maximum centrifugal force 121 exerted on the aerofoil 35 in operation in the event that the primary structural member 51 fails.

[0138] The secondary structural member 53 has tensile stiffness along the longitudinal axis 43 of the aerofoil 35 such that for the maximum centrifugal force 121 experience by the aerofoil 35 in operation, the tensile stiffness results in the secondary structural member 53 restricting the movement 122 of the aerofoil 35 away from the hub 13, after a failure of the primary structural member 51, to less than 1.1mm. The tensile stiffness also results in the secondary structural member 53 allowing movement 122 of the aerofoil 35 away from the hub 13 after a failure of the primary structural member 51, of no less than 0.9mm. Thereby, the secondary structural member 53 restricts the movement 122 to a range of 0.9mm to 1.1mm. The tensile stiffness of the secondary structural member is less than a tensile stiffness of the primary structural member 51 along the longitudinal axis 43 of the aerofoil 35. This difference may provide a means for enabling the movement 122 to occur.

[0139] The wall thickness 27 of the main body 21 of the hub 13 is greater than the movement 122 such that the primary structural member 51, via the bearing 39, will remain in contact with the part of the hub 13 which defines the blade opening 26 and thereby remain in contact with a portion 140 (denoted by the dashed rectangles in Figure 18) of the contact area 111 that the primary structural member 51 contacted prior to the failure. As indicated previously, in Figure 18, the size of the crack 141 has been exaggerated for clarity, and the size of the portion 140 has been correspondingly reduced. In other examples, the tensile stiffness of the secondary structural member 53 may be different. As a result, the secondary structural member 53 restricting the movement 122 of the aerofoil 35 away from the hub 13 to a different degree. For example, such that the range is between 0.01mm and 10mm, 0.1mm and 10mm, 0.25mm and 1.75mm, 0.5mm and 1.5mm, 0.75mm and 1.25mm, 0.9mm and 1.1mm, or less than 0.1mm.

[0140] In this propeller 11, the secondary structural member 53 acts as a fail safe for the primary structural member 51. The key purpose of the secondary structural member 53 is to retain the blade 15 after the failure. It may be acceptable for the torsional and bending performance of the blade 15 to be lower after the failure, and thereby the bending and torsional properties of the secondary structural member 53 to be less than those of the primary structural member 51, in order to reduce the weight of the secondary structural member 53 and fit the secondary structural member 53 within the hollow space 67 of the primary structural member 51.

[0141] The secondary structural member 53 has a bending stiffness in the plane 125 which extends along the longitudinal axis 43 of the aerofoil 35 and the rotational axis 22 of the hub 13 which is of a value such that a total bending stiffness of the primary structural member 51 and the secondary structural member 53, in the plane 125, after the failure of the primary structural member 51 is no less than 40% and no greater than 50% of a total bending stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member.. In other examples, the total bending stiffness of the primary structural member 51 and the secondary structural member 53 in the plane 125 after the failure of the primary structural member 51 may be no less than 40%, 30%, 20%, or 10% or no greater than no greater than 40%, 30%, 20%, or 10% of the total bending stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member 51.

[0142] The secondary structural member 53 has a bending stiffness in the plane 129 which extends along the longitudinal axis 43 of the aerofoil 35 and in a direction perpendicular to the rotational axis 22 of the hub 13 and the longitudinal axis 43 of the aerofoil which is of a value such that a total bending stiffness of the primary structural member 51 and the secondary structural member 53, in the plane 123, after the failure of the primary structural member 51 is no less than 40% and no greater than 50% of a total bending stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member. In other examples, the total bending stiffness of the primary structural member 51 and the secondary structural member 53 in the plane 123 after the failure of the primary structural member 51 may be no less than 40%, 30%, 20%, or 10% or no greater than no greater than 40%, 30%, 20%, or 10% of the total bending stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member 51.

[0143] The secondary structural member 53 has a torsional stiffness about the longitudinal axis 43 of the aerofoil 35 which is of a value such that the total torsional stiffness of the primary structural member 51 and the secondary structural member 53, about the longitudinal axis 43 of the aerofoil 35, after the failure of the primary structural member 51 is no less than 30% and no greater than 40% of the torsional stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member 51.. In other example propellers, the total torsional stiffness of the primary structural member 51 and the secondary structural member 53, about the longitudinal axis 43 of the aerofoil 35, after the failure of the primary structural member 51 is no less than no greater than 40%, 30%, 20%, or 10% or no less than 10%, 20%, 30%, or 40% of the torsional stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member 51. eVTOL

[0144] Figures 19 and 20 show an electric vertical take-off and landing aircraft (eVTOL) 201 which comprises the propeller 11 described above with reference to Figures 1 to 18. The eVTOL aircraft 201 comprises a centre of gravity 203, a pitch axis 205, a roll axis 207, a yaw axis 209, a left / right plane 208, a fore / aft plane 210, an airframe 211, batteries 213, eight motors 215, eight propellers 11, and four tilting mechanisms 217.

[0145] The centre of gravity 203, the pitch axis 205, the roll axis 207, and the yaw axis 209, the left / right plane 208, and the fore / aft plane 210 are shown in Figure 20. The left / right plane 208 extends in a direction parallel to the roll axis 207, perpendicular to the pitch axis 205 and the yaw axis 209, and through the centre of gravity 203. The fore / aft plane 210 extends in a direction parallel to the pitch axis 205 and the yaw axis 209, perpendicular to the roll axis 207, and through the centre of gravity 203.

[0146] The airframe 211 comprises a fuselage 221, a pair of wings 223, and four pylons 225. The fuselage 221 comprises a cockpit and a passenger area. The pylons 225 each have a generally cuboid shape. The length of each pylon 225 in a direction parallel to the roll axis 207 is greater than a width of the wings 223 in the direction parallel to the roll axis 207.

[0147] The wings 223 are fixed to an upper side of the fuselage 221 approximately mid-way along the fuselage 221 and extend in a direction perpendicular to the fuselage 221. The pylons 225 are fixed to undersides of the wings 223 and spaced apart from one another along the wings 223 in a direction parallel to the pitch axis 205 such that there are two pylons 225 on either side of the left / right plane 208. A fore pylon portion 231 of each pylon 225 projects from the wings 223 on a fore side of the fore / aft plane 210. An aft pylon portion 233 of each pylon 225 projects from the wings 223 on the aft side of the fore / aft plane 210.

[0148] The batteries 213 each comprise a plurality of individual battery cells enclosed within a housing. The batteries 213 are located within the fuselage 221 and provide power to the electric components of the eVTOL aircraft 201.

[0149] The eight motors 215 comprise four tilting motors 241 mounted fore of the wings 223 and four fixed motors 243 mounted aft of the wings 223. Each of the tilting motors 241 is fixed to a respective fore pylon portion 231. Each fixed motor 243 is fixed to a respective aft pylon portion 233. Each motor 215 is connected to a respective one of the eight propellers 11.

[0150] Each tilting mechanism 217 is operable to tilt the propeller 11 connected to the respective tilting motor 241 relative to the pylon 225 (and thereby the airframe) between a vertical flight mode in which the propeller 11 deliver thrust which is parallel to the yaw axis 209 and a convention flight mode in which the propeller 11 delivers thrust which is parallel to the roll axis 207 of the eVTOL aircraft 201.

[0151] The fixed motors 243 are fixed relative to the pylons 225 (and thereby the airframe) such that the propellers 11 connected to the fixed motors 243 deliver thrust which is parallel to the yaw axis 209 of the eVTOL aircraft 201 only.

[0152] The propellers 11 comprise four bladed propellers 251, and five bladed propellers 253. The four bladed propellers 251 are identical to the propeller 11 described above with reference to Figures 1 to 17. The five bladed propellers 253 are similar to the propeller 11 described above with reference to Figures 1 to 17, except that they comprise five rather than four blades 15 and blade openings 26.

[0153] During vertical take-off and landing of the eVTOL aircraft 201, the propellers 11 connected to the tilting motors 215 are in their vertical flight mode. This results in a respective pylon 225 and motor 215 being located underneath each propeller 11. If the primary structural member 51 of a blade 15 were to fail at this point, gravity would pull a released blade 15 down towards the pylon 225 and the motor 215, and they could collide. However, the primary structural member 51 and the secondary structural member 53 of each blade 15 have a total bending stiffness in the plane 125 which extends along the longitudinal axis 43 of the aerofoil 35 and the rotational axis 22 of the hub 13 of no less than 10% of the bending stiffness of the total bending stiffness of the primary structural member 51 and the secondary structural member 53 before the failure of the primary structural member, which may provide sufficient stiffness to resist this dropping.

[0154] Failure Detection System

[0155] As described previously, the secondary structural member 53 has the tensile stiffness along the longitudinal axis 43 of the aerofoil 35 which allows movement 122 of the aerofoil 35 away from the hub 13, after a failure of the primary structural member 51, of greater than 0.9mm. This movement 122 results in out of balances forces on the propeller 11, which generate a sufficiently large change in vibration of the propeller 11 that the change can be used to detect that the primary structural member 51 has failed. Accordingly, the eVTOL aircraft 201 has systems used to detect the failure of the primary structural member 51 and take corrective action, as described below with reference to Figure 21 and Figure 22 (which shows a flow chart of the response to the failure). As shown in Figure 21, the eVTOL aircraft 201 has a failure detection system 271, which is connected to a vibration sensor 271 of the propeller 11 and an alert device 275 of the eVTOL aircraft 201. In this example, the failure detection system 271 is located within the fuselage 221 of the eVTOL aircraft 201. However, it is also envisaged that the failure detection system 271 could be provided as part of the propeller 11. The vibration sensor 273 is located within the hub 13 of the propeller 11. The alert device 275 is located in the cockpit of the eVTOL aircraft 201. In this example the alert device 275 is a display. Other alert devices 275 are also envisaged such as a speaker.

[0156] In the event of a failure of the primary structural member 51, the secondary structural member 53 restricts 276 the movement 122 of the aerofoil 35 away from the hub 13 to a range of 0.9mm to 1.1mm (i.e., the second structural member 53 restricts the movement 122 from being greater than 1.1mm, whilst allowing the movement 122 to be greater than 0.9mm). The movement 122 results in out of balance forces on the propeller 11 which cause a change in the vibration the propeller 11. This change in vibration (which may be a step change) is sensed by the vibration sensor 273. The vibration sensor 273 outputs 277 a signal indicative of the vibration of the propeller 11, and thereby indicative of the change in vibration of the propeller 11. Thereby, the vibration sensor 273 outputs 255 a signal which is indicative of the failure of the primary structural member 51.

[0157] The failure detection system 271 receives 275 the signal from the vibration sensor 273. The failure detection system 271 then detects 279 the change in the vibration of the propeller 11 which is indicative of the failure of the primary structural member 51 from the signal. For example, the failure detection system could detect a change in a characteristic of the vibration of the propeller 11 (such as frequency of the vibration) which is above a predetermined threshold to detect that the primary structural member 51 has failed. The threshold could be determined based on factors such as the geometry of the propeller 11, the mass of the propeller 11, and the rotational speed of the propeller 11.

[0158] In response to detecting the change in the vibration of the propeller 11 which is indicative of the failure of the primary structural member, the failure detection system outputs 281 an alert signal to the alert device 275. In response to receiving the alert signal, the alert device 275 outputs 283 an alert which alerts the pilot to the failure of the primary structural member 51. In this example, the alert is a visual message. In other examples, the alert could be a flashing light or audible tone. The pilot can then take corrective action, such as to power down the propeller 11. Multiple other uses for the alert signal are envisaged. For example, the alert signal could be output to a maintenance system (which could be external to the eVTOL aircraft 201) and used to prompt a maintenance action.

[0159] Alternatives

[0160] In the above example, the blade root 37 comprises an intermediate layer 55 which has a lower strength and a lower modulus than a strength and modulus of the primary structural member 51 and a strength and modulus of the secondary structural member 53. In other examples, the intermediate layer 55 may have a plane strain fracture toughness which is greater than a plane strain fracture toughness of the primary structural member 51 or the secondary structural member 53.

[0161] The intermediate layer 55 is an example of how the monolithic structure can be configured to inhibit a crack from propagating between the primary structural member 51 and the secondary structural member 53. It is also envisaged that this can be achieved by there being a relative difference between a first property of the primary structural member 51 and a second property of the secondary structural member 53.

[0162] In one example, the first property is a first fibre orientation of a first ply of the primary structural member 51 which abuts a second ply of the secondary structural member 53, the second property is a second fibre orientation of the second ply of the secondary structural member 53, and the relative difference is a misalignment of 90° between the first fibre orientation and the second fibre orientation. It is also envisaged that the misalignment could be no less than 10°, 20°, 40°, 45°, 60°, 80°, or 85°.

[0163] In another example, the first property is a strain to failure of the primary structural member 51, and the second property is a strain to failure of the secondary structural member 53.

[0164] In the above example, the propeller 11 has a vibration sensor 271 which outputs 277 a signal indicative of the change in vibration of the propeller 11 and thereby the failure of the primary structural member 51. Other types of sensors are also envisaged, such as other types of transducers. For example, a stress sensor could be used to output a signal indicative of the failure of the primary structural member 51. Second blade

[0165] In the blade 15 described above, the secondary structural member 53 has lesser mechanical properties than the primary structural member 51. However, in other blades, the primary structural member and the secondary structural member have equal mechanical properties and are each equally capable of supporting the in-service loads 121,123,127,131. The primary structural member may therefore be referred to as a first structural member, and the secondary structural member may be referred to as a second structural member. Such a blade is described hereafter.

[0166] Figures 23 and 24 show a second blade 301 comprising an aerofoil 335, and a blade root 337. Unlike the aforementioned blade 15, the aerofoil 335 is a separate part to the blade root 337.

[0167] The aerofoil 335, shown in isolation in Figures 25 and 26, is identical to the aerofoil 35 described above with reference to Figure 6, except that it additionally comprises a first set of lugs 341 and a second set of lugs 343.

[0168] The first set of lugs 341 comprises three lugs 345. The three lugs 345 each have the shape of a flat plate and are spaced apart from one another along an axis 344 which is perpendicular to the longitudinal axis 43 of the aerofoil 335. Each lug 345 comprises a hole 347. The holes 347 are aligned with one another along the axis 344 which is perpendicular to the longitudinal axis 43 of the aerofoil 335. The lugs 345 project from the proximal end 47 of the spar 41. The proximal end 47 of the spar 41 has a first side 361 and a second side 363. The second side 363 is located closer to the trailing edge 45 than the first side 361.

[0169] The second set of lugs 343 are identical to the first set of lugs 341, except that the first set of lugs 341 are located on the first side 361 of the proximal end 47 of the spar 41, while the second set of lugs 343 are located on the second side 343 of the proximal end 47 of the spar 41.

[0170] The blade root 337, shown located within a hub 13 in Figures 27 and 28, comprises a first structural member 351, and a second structural member 353. The first and second structural members 351,353 are separate to one another.

[0171] The first structural member 351 comprises: a main body 365, a central longitudinal axis 367, a lug portion 369, a flange 371, and a pitching horn (not shown). The main body 365 has a hollow cylindrical shape. The main body 365 has an inner diameter 373 measured in a direction perpendicular to the central longitudinal axis 367. The main body has a length 375 measured in a direction parallel to the central longitudinal axis 367. The central longitudinal axis 367 extends through the centre of the main body 365. A midway plane 377 extends along the central longitudinal axis 367 and in a direction perpendicular to the longitudinal axis 367. A first side 379 and a second side 381 are each on opposite sides of the midway plane 377 to one another.

[0172] The lug portion 369 comprises four lugs 383 and an end portion 385. The four lugs 383 each have the shape of a flat plate and are spaced apart from one another along an axis (not shown) which is perpendicular to the central longitudinal axis 367 of the first structural member 351. Each lug 383 comprises a hole 387. The holes 387 are aligned with one another along the axis (not shown) which is perpendicular to the central longitudinal axis 43 of the first structural member 351. The lugs 383 project from the end portion 385 away from the main body 365 of the first structural member 351. The lug portion 385 has a heigh 389, measured between the two lugs 383 which are furthest apart from one another, and a width 391. The four lugs 383 are located on the first side 379 of the midway plane 377.

[0173] The end portion 385 extends from the circumference of one end of the main body 365 of the first structural member 351 to the lug portion 369. The end portion 385 comprises an opening 393 which is located on the second side 381 of the midway plane 377. The opening 393 has a height 395, which is greater than the height 389 of the lug portion 385, and a width 397, which is greater than the width 391 of the lug portion 385.

[0174] The flange 371 and pitching horn (not shown) of the first structural member 351 are identical to the flange 63 and pitching horn 65 of the primary structural member 51.

[0175] The second structural member 353 is similar to the first structural member 351, with the following differences. The outer diameter 397 of the main body 399 of the second structural member 353 is smaller than the inner diameter 373 of the main body 365 of the first structural member 351. The length 401 of the main body 399 of the second structural member 353 is greater than the length 375 of the main body 365 of the first structural member 351. The end portion 403 of the second structural member 353 is solid, so that it does not have an opening.

[0176] When the blade root 337 is assembled, the central longitudinal axis 367 of each of the first and the second structural members 351,353 are aligned with one another. The main body 399 of the second structural member 353 is inside the main body 365 of the first structural member 351. The lug portion 405 of the second structural member 353 projects through the opening 393 in the end portion 385 of the first structural member 351 such that the lugs 383 of the first structural member 351 are on the first side 379 of the midway plane 377, and the lugs 407 of the second structural member 353 are on the second side 381 of the midway plane 377.

[0177] When the second blade 301 is assembled (as shown in Figures 23 and 24), the three lugs 345 of the first set of lugs 341 are interspersed with the four lugs 383 of the first structural member 351 such that the holes 347 of the first set of lugs 341 and the holes 387 in the four lugs 383 of the first structural member 351 align with one another to form a first set of aligned holes. Similarly, the three lugs 345 of the second set of lugs 343 are interspersed with the four lugs 383 of the second structural member 353 such that the holes 347 of the second set of lugs 343 and the holes 387 of the four lugs 407 of the second structural member align with one another to form a second set of aligned holes. A respective bolt (not shown) is inserted through each of the aligned sets of holes to connect the aerofoil 335 to the blade root 337. In this way, the first set of lugs 341, the four lugs 383 of the first structural member 351, and the bolt constitute a first connection mechanism. Similarly, the second set of lugs 343, the four lugs 383 of the second structural member 353, and the other bolt constitute a second connection mechanism.

[0178] The first and the second structural members 351,353 each have the same mechanical properties as the primary structural member 51.

[0179] Whilst particular examples have been described, it should be understood that these are illustrative examples only and that various modifications may be made without departing from the scope of the invention as defined by the claims.

Claims

CLAIMS1. A propeller comprising: an aerofoil; a hub; a primary structural member; and a secondary structural member, wherein: the primary structural member and the secondary structural member are each configured to connect the aerofoil to the hub via a respective independent load path; and each structural member is configured to withstand a maximum centrifugal force exerted on the aerofoil in operation in the event that the other structural member fails.

2. The propeller of claim 1, wherein the secondary structural member is configured to restrict a movement of the aerofoil away from the hub after a failure of the primary structural member to less than 10mm.

3. The propeller of claim 1 or claim 2, wherein the secondary structural member is configured to restrict the movement of the aerofoil away from the hub after the failure of the primary structural member to less than 1.75mm.

4. The propeller of any one of claim 2 or claim 3, wherein the secondary structural member is configured to allow the movement of the aerofoil away from the hub after the failure of the primary structural member.

5. The propeller of any one of the preceding claims, wherein the propeller comprises a sensor configured to output a signal indicative of the failure of one of the structural members.

6. The propeller of any one of the preceding claims, wherein:when the primary structural member has not failed, the primary structural member contacts the hub via a contact area; and the secondary structural member is configured to restrict the movement of the aerofoil away from the hub such that after a failure of the primary structural member, the primary structural member contacts the hub via at least a portion of the contact area.

7. The propeller of any one of the preceding claims, wherein one of the primary structural member and the secondary structural member is located within the other of the primary structural member and the secondary structural member.

8. The propeller of claim 7, wherein the structural members are concentric with one another.

9. The propeller of claim 7 or claim 8, wherein the secondary structural member is located closer to a longitudinal axis of the aerofoil than the primary structural member.

10. The propeller of any one of the preceding claims, wherein: the movement is in a radial direction perpendicular to a rotational axis of the hub; and a total bending stiffness of the primary structural member and the secondary structural member, in a plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no greater than 50% of a total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member.

11. The propeller of any one of the preceding claims, wherein: the movement is in the radial direction perpendicular to the rotational axis of the hub; andthe total torsional stiffness of the primary structural member and the secondary structural member, about the longitudinal axis of the aerofoil, after the failure of the primary structural member is no greater than 50% of a torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member.

12. The propeller of claim 10 or claim 11, wherein the secondary structural member comprises a composite material.

13. The propeller of any one of claims 10 to 12, wherein the primary structural member comprises a first material, and the secondary structural member comprises a second material which is different to the first material.

14. The propeller of claim 13, wherein an ultimate tensile strain of the second material is greater than an ultimate tensile strain of the first material.

15. The propeller of any one of the preceding claims, wherein: the hub comprises an opening which has a maximum width measured in a plane which extends parallel to the rotational axis of the hub; and at least one of the primary structural member and the secondary structural member extend through the opening and comprise a flange, located within the hub, which has a width measured in the plane which extends parallel to the rotational axis of the hub greater than the maximum width of the opening.

16. The propeller of claim 15, wherein: the primary structural member is a separate part to the secondary structural member; the primary structural member and the secondary structural member each comprise a respective flange;the propeller comprises: a first connection mechanism configured to connect the primary structural member to the aerofoil; and a second connection mechanism configured to connect the secondary structural member to the aerofoil.

17. The propeller of any one of the preceding claims, wherein: the primary structural member and the secondary structural member form a monolithic structure; and the monolithic structure is configured to inhibit a crack from propagating between the primary structural member and the secondary structural member.

18. The propeller of any one of the preceding claims, wherein a total torsional stiffness of the primary structural member and the secondary structural member, about a longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 10% of a torsional stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member.

19. The propeller of any one of the preceding claims, wherein: the propeller comprises a blade which comprises the aerofoil; at least one of the primary structural member and the secondary structural member are integrally formed with the blade.

20. The propeller of claim 19, wherein: the blade comprises a spar; and at least one of the primary structural member and the secondary structural member are integrally formed with the spar.

21. The propeller of claim 20, wherein at least one of the primary structural member and the secondary structural member are integrally formed with the spar such that continuous fibres extend between the at least one of the primary structural member and the secondary structural member and the spar.

22. The propeller of any one of the preceding claims, wherein a tensile stiffness of the secondary structural member along a longitudinal axis of the aerofoil is less than a tensile stiffness of the primary structural member along a longitudinal axis of the aerofoil.

23. The propeller of any one of the preceding claims, a total bending stiffness of the primary structural member and the secondary structural member, in a plane which extends along the longitudinal axis of the aerofoil, after a failure of the primary structural member is no less than 10% of a total bending stiffness of the primary structural member and the secondary structural member before the failure of the primary structural member.

24. A vertical take-off and landing aircraft comprising the propeller of claim 23, wherein the longitudinal axis of the aerofoil extends horizontally during normal operation of the vertical take-off and landing aircraft.

25. A vertical take-off and landing aircraft comprising the propeller of claim 5, wherein the vertical take-off and landing aircraft comprises a failure detection system configured to, based on the signal output by the sensor, output an alert signal in response to detecting the failure of the primary structural member.

26. A propeller comprising: an aerofoil; a hub;a sensor; and a structural member configured to connect the aerofoil to the hub, wherein: the propeller is configured to restrict a movement of the aerofoil away from the hub after a failure of the structural member, and allow the movement of the aerofoil away from the hub after the failure of the structural member; and the sensor is configured to output a signal indicative of the failure of the structural member.

27. An electric vertical take-off and landing aircraft comprising: the propeller of claim 26; and a failure detection system configured to, based on the signal output by the sensor, output an alert signal in response to detecting the failure of the structural member.

28. A method comprising: providing an aerofoil comprising an aerofoil, a hub, and a structural member configured to connect the aerofoil to the hub; restricting a movement of the aerofoil away from the hub after a failure of the structural member to a range of 0.01mm to 10mm; and outputting a signal indicative of the failure of the structural member.

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