Passive rotor alignment in free running state

Abstract

The invention relates to passive rotor alignment in a free-spin state. A rotor assembly for an aircraft is described, where one or more blades of the rotor assembly include a blade alignment device that orients the blade in response to airflow around the blade alignment device when the rotor assembly is free spinning. One embodiment includes a method of operating a rotor assembly for an aircraft. The method includes operating the rotor assembly in a free-spin state, where the rotor assembly includes a rotating hub that rotates about an axis and a plurality of blades that extend radially from the rotating hub. At least one of the plurality of blades includes a blade alignment device. The method further includes aligning the at least one of the plurality of blades in response to airflow around the blade alignment device when the rotor assembly is in the free-spin state.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of aircraft and, in particular, to aircraft utilizing rotors. BACKGROUND

[0002] Aircraft types capable of taking off, hovering and landing vertically are known as vertical takeoff and landing (VTOL) aircraft. VTOL aircraft have one or more rotors that generate vertical lift. Some VTOL aircraft also have fixed wings that generate lift when the aircraft is propelled forward by propellers, jet engines, etc. When these fixed wing aircraft transition from vertical flight to horizontal or wingborne flight, the rotors are not driven to rotate. One problem is that the rotors generate drag when they are free to turn, which hinders the efficiency of wingborne flight. SUMMARY

[0003] A rotor assembly for an aircraft is described, wherein one or more blades of the rotor assembly include a blade alignment device that aligns the blade in response to airflow around the blade alignment device when the rotor assembly is free to turn.

[0004] One embodiment includes a method of operating a rotor assembly for an aircraft. The method includes operating the rotor assembly in a free to turn state, wherein the rotor assembly includes a rotating hub that rotates about an axis and a plurality of blades that extend radially from the rotating hub. At least one of the plurality of blades includes a blade alignment device. The method further includes aligning the at least one of the plurality of blades in response to airflow around the blade alignment device when the rotor assembly is in the free to turn state.

[0005] Another embodiment includes a rotor assembly for an aircraft. The rotor assembly includes a rotating hub that rotates about an axis and a plurality of blades that extend radially from the rotating hub. At least one of the plurality of blades includes a blade alignment device that aligns the at least one of the plurality of blades in response to airflow around the blade alignment device when the rotor assembly is free to turn.

[0006] Another embodiment includes an aircraft having at least one rotor assembly that provides lift for the aircraft. The at least one rotor assembly includes a rotating hub that rotates about an axis and a plurality of blades that extend radially from the rotating hub. At least one of the plurality of blades includes a blade alignment device that aligns the at least one of the plurality of blades in response to airflow around the blade alignment device when the rotor assembly is free to turn.

[0007] The features, functions, and advantages that have been discussed can be implemented independently in various embodiments of the present disclosure or can be implemented in combination, further details of which can be known with reference to the following description and attached drawings. BRIEF DESCRIPTION OF DRAWINGS

[0008] Some embodiments will now be described by way of example only, and with reference to the accompanying drawings. The same reference numbers and symbols in different drawings represent the same elements or the same types of elements.

[0009] Figure 1 is a perspective view of an aircraft in a schematic embodiment.

[0010] Figure 2 is a perspective view of an aircraft in a schematic embodiment during a cruise phase. Figure 1

[0011] Figure 3 is a block diagram of a rotor assembly in a schematic embodiment.

[0012] Figure 4A is a perspective view of a rotor assembly in a schematic embodiment with blade alignment devices near blade tips and in a deployed position.

[0013] Figure 4B is a perspective view of a rotor assembly in a schematic embodiment Figure 4A with blade alignment devices near blade tips and in a stowed position.

[0014] Figure 5A is a perspective view of a rotor assembly in a schematic embodiment with blade alignment devices near midpoints in the blades and in a deployed position.

[0015] Figure 5B is a perspective view of a rotor assembly in a schematic embodiment Figure 5A with blade alignment devices near midpoints in the blades and in a stowed position.

[0016] Figures 6-9 is a top perspective view of a rotor assembly in a schematic embodiment Figures 4A-4B

[0017] Figures 10-12 is a perspective view of a rotor assembly in a schematic embodiment utilizing spanwise vanes as blade alignment devices.

[0018] Figures 13-14 is a perspective view of a rotor assembly in a schematic embodiment utilizing grid vanes as blade alignment devices.

[0019] Figure 15 is a perspective view of a grid vane in a schematic embodiment Figures 13-14

[0020] Figure 16 is a flowchart of a method of repositioning a rotor assembly in a schematic embodiment when the rotor assembly is free to rotate.

[0021] ​​​Figures 17-20 is a flowchart illustrating additional details of the method of Figure 16 DETAILED DESCRIPTION

[0022] The accompanying drawings and the following description illustrate specific example embodiments. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its scope. Furthermore, any example described herein is intended to help illustrate the principles of the disclosure and should not be construed as limiting. Accordingly, the disclosure is not limited to the specific embodiments described in the following examples, but only by the claims and their equivalents.

[0023] Figure 1 is a perspective view of an aircraft 100 in an illustrative embodiment. The aircraft 100 can be a manned aircraft piloted by one or more pilots, or can be an unmanned aircraft (e.g., a drone). The structure of the aircraft 100 is provided by way of example only, and the principles described herein are applicable to any aircraft. In this example, the aircraft 100 includes a fuselage 102 and wings 104-105 extending from opposite sides of the fuselage 102 so as to define a support plane for horizontal or wingborne flight. The aircraft 100 also includes a propeller 108 to provide thrust for wingborne flight, although other types of engines are used to generate thrust in other embodiments.

[0024] To provide vertical flight (i.e., takeoff, hovering, and landing), the aircraft 100 includes one or more rotor assemblies 110. Figure 1 The number and location of the rotor assemblies 110 shown in FIG. 1 is provided by way of example only and varies as desired. Further, while the rotor assemblies 110 are shown as each including two blades, the rotor assemblies 110 include different numbers of blades in other embodiments.

[0025] Because the rotor assemblies 110 spin via motors that can operate using mechanical, electrical, or a combination of mechanical and electrical power, the rotor assemblies 110 provide vertical lift so that the aircraft 100 can take off, hover, and land. Figure 2 is a perspective view of the aircraft 100 during a cruise phase in an illustrative embodiment. In this example, the aircraft 100 is in a wingborne flight configuration. Figure 2 ​In this embodiment, when the aircraft 100 changes from vertical flight to wing-borne flight, the blades of the rotor assembly 110 passively orient themselves to reduce drag. In the embodiment described herein, the rotor assembly 110 is enhanced because one or more blades include a blade alignment device 308 that passively orients the rotor assembly 110 in response to airflow around the blade alignment device 308 when the rotor assembly 110 is freely rotating (e.g., when the rotor assembly 110 is not driven to rotate). Because the rotor assembly 110 is passively oriented, drag is increased. Figure 2 The efficiency during the cruise phase is shown. Additionally, by passively aligning the thrusters of rotor assembly 110 with the local direction of the airflow, rotor assembly 110 is positioned at the point of minimum drag possible. If the thrusters were mechanically locked in the fore-and-aft position, variations in the airflow around each thruster would cause them to not always be precisely aligned with the airflow. By allowing them to freely and passively discover the direction of the airflow, drag is minimized. Although Figure 2 The blade alignment device 308 shown has a specific shape, although there are other embodiments of the blade alignment device 308.

[0026] Figure 3 This is a block diagram of a rotor assembly 110 in an illustrative embodiment. In this embodiment, the rotor assembly 110 includes a rotating hub 302 that rotates about an axis (not shown). The rotor assembly 110 further includes a plurality of blades 304-305 extending radially from the rotating hub 302. Although Figure 3 Only two of blades 304-305 are shown in the image, but as a design option, rotor assembly 110 may include any number of blades.

[0027] In this embodiment, blade 304 includes surfaces 306-307 forming the airfoil of blade 304. Blade 304 includes blade alignment means 308 that conforms to surfaces 306 and / or 307 of blade 304 in a retracted position and protrudes from surfaces 306 and / or 307 of blade 304 in an deployed position. Generally, when rotor assembly 110 is freely rotating (e.g., when rotor hub 302 is not driven to rotate), blade alignment means 308 in the deployed position aligns blade 304 in response to airflow around blade alignment means 308. When rotor assembly 110 is in operation (e.g., when rotor hub 302 is driven to rotate), blade alignment means 308 is in a retracted position to ensure that blade alignment means 308 does not interfere with chordal airflow across surfaces 306 and / or 307 of blade 304, which would otherwise reduce the lift of blade 304. Although in this embodiment the blade alignment device 308 is shown on the blade 304, in other embodiments, as a design choice, the blade alignment device 308 is implemented on any number of blades.

[0028] Figure 4A is a perspective view of a rotor assembly in an illustrative embodiment with blade alignment devices 308 proximate to tips 410 of blades 304 and in a deployed position. In this embodiment, rotor assembly 110 includes a rotating hub 302 that is mechanically driven to rotate about an axis 402 in the direction of arrow 404 to provide lift to aircraft 100. In this embodiment, blades 304-305 extend radially from rotating hub 302. In particular, blades 304-305 extend radially from rotating hub 302 along a longitudinal direction 406 that is substantially perpendicular to axis 402. Rotor assembly 110 in this embodiment further includes a spanwise flap 308-1 that operates similar to previously described blade alignment device 308 to passively orient rotor assembly 110 as shown. Figure 2

[0029] In this embodiment, spanwise flap 308-1 (i.e., an embodiment of blade alignment device 308) protrudes from surface 306 of blade 304 in a deployed position when rotor assembly 110 is freewheeling. Spanwise flap 308-1 conforms to surface 306 of blade 304 in a stowed position when rotor assembly 110 is driven to rotate. In general, spanwise flap 308-1 is positioned along a span of an airfoil of blade 304. Although spanwise flap 308-1 is shown protruding from surface 306, in other embodiments spanwise flap 308-1 is alternatively or additionally implemented to protrude from surface 307.

[0030] Figure 4B is a perspective view of rotor assembly 110 in an illustrative embodiment with blade alignment devices 308 proximate to tips 410 of blades 304 and in a stowed position. In this embodiment, spanwise flap 308-1 is in a stowed position when rotating hub 302 is mechanically driven to provide lift to aircraft 100. In the stowed position, spanwise flap 308-1 is held within a cutout 408 within blade 304 that allows spanwise flap 308-1 to conform to surface 306 and surface 307 of blade 304 when rotating hub 302 is mechanically driven to rotate. Although cutout 408 is shown proximate to tip 410 of blade 304, cutout 408 is located at other positions on blade 304 between tip 410 and rotating hub 302 as a design choice in other embodiments. Figure 4B

[0031] In some embodiments, spanwise flap 308-1 protrudes from surface 306 of blade 304 with a spring (not shown) or other mechanical actuator (not shown) when rotor assembly 110 is freewheeling (see Figure 4A ​​). For example, the spanwise vane 308-1 is rotationally coupled to the short edge 412 of the cutout 408 and pivots out of the cutout 408 about the pivot axis 409 in response to a trigger. In some embodiments, the trigger is a release of a holding device (not shown) or other mechanical feature that normally holds the spanwise vane 308-1 within the cutout 408 until activation.

[0032] In some embodiments, the spanwise vane 308-1 passively transitions to the stowed position and conforms to the surface 306 of the blade 304 and passively transitions to the deployed position and protrudes from the surface 306 of the blade 304 in response to a change in the rotational rate of the rotational hub 302. For example, the spanwise vane 308-1 passively transitions to the stowed position and conforms to the surface 306 in response to the rotational rate being greater than a first rotational rate (e.g., the spanwise vane 308-1 pivots into the cutout 408 at the short edge 412) and passively transitions to the deployed position and protrudes from the surface 306 in response to the rotational rate being less than a second rotational rate (e.g., the spanwise vane 308-1 pivots out of the cutout 408 at the short edge 412). In some embodiments, the first rotational rate is greater than the second rotational rate.

[0033] In other embodiments, the spanwise vane 308-1 passively transitions to the stowed position and conforms to the surface 306 of the blade 304 and passively transitions to the deployed position and protrudes from the surface 306 of the blade 304 in response to a change in the centrifugal force applied to the spanwise vane 308-1. For example, the spanwise vane 308-1 passively transitions to the stowed position and conforms to the surface 306 in response to the centrifugal force applied to the spanwise vane 308-1 being greater than a first centrifugal force value (e.g., the spanwise vane 308-1 pivots into the cutout 408 at the short edge 412) and passively transitions to the deployed position and protrudes from the surface 306 in response to the centrifugal force applied to the spanwise vane 308-1 being less than a second centrifugal force value (e.g., the spanwise vane 308-1 pivots out of the cutout 408 at the short edge 412). In these embodiments, the first centrifugal force value is greater than the second centrifugal force value.

[0034] Figure 5A is a perspective view of a rotor assembly in an illustrative embodiment with the blade alignment device 308-1 proximate a midpoint in the blade 304 and in a deployed position, and Figure 5B is a perspective view of a rotor assembly in an illustrative embodiment with the blade alignment device 308-1 proximate a midpoint in the blade 304 and in a stowed position. Figure 5B is a perspective view of a rotor assembly in an illustrative embodiment with the blade alignment device 308-1 proximate a midpoint in the blade 304 and in a stowed position.

[0035] As previously discussed, the spanwise vane 308-1 can be located at any position between the tip 410 of the blade 304 and the rotational hub 302. In some embodiments, the spanwise vane 308-1 is located at a position that is proximate a midpoint in the blade 304. Figures 5A-5BIn some embodiments, the cutout 408 is proximate to a midpoint of the blade 304 between the tip 410 and the hub 302. This is desirable in some embodiments because the thickness of the blade 304 generally increases from the tip 410 and the hub 302.

[0036] Figures 6-9 is a top perspective view of the rotor assembly 110 in an illustrative embodiment. Specifically, Figures 6-9 is shown how the spanwise vane 308-1 aligns the blade 304 when the aircraft 100 is in forward flight and the rotor assembly 110 is free to turn (e.g., the hub 302 is free to rotate about the axis 402 and is not being driven to rotate). While the discussion will refer to the spanwise vane 308-1, the principles shown in Figures 6-9 apply, however, Figures 6-10 to other embodiments of the blade alignment device 308, some of which will be discussed later.

[0037] Figure 6 is shown a situation in which the longitudinal direction 406 of the rotor assembly 110 is oriented substantially perpendicular to the airflow 602. This orientation can produce the most drag on the aircraft 100 and can represent the natural orientation of the rotor assembly 110 before the spanwise vane 308-1 aligns the blade 304. As the airflow 602 flows across the spanwise vane 308-1, a torque 604 is generated on the hub 302 that operates to cause the hub 302 to rotate in a counterclockwise direction. This process is shown in Figures 7-9 As the rotor assembly 110 rotates, the airflow 602 across the spanwise vane 308-1 orients the blade 304 in a downstream position relative to the airflow 602 and positions the blade 305 in an upstream position relative to the airflow 602. Figure 9 The result of the orientation shown is that the blade 304 is oriented to produce less drag on the aircraft 100 than Figure 6 the orientation shown.

[0038] Because the spanwise vane 308-1 operates to passively align the blade 304 in response to the airflow 602, sensors used to determine the orientation of the hub 302 and mechanical power used to maintain the hub 302 in Figure 7 the orientation shown are a technical benefit over the prior art.

[0039] Figures 10-12 is a perspective view of the rotor assembly 110 in an illustrative embodiment utilizing a spanwise vane 308-2 as the blade alignment device 308. In this embodiment, the spanwise vane 308-2 is pivotably coupled to the surface 306 of the blade 304 along the longitudinal direction 406. This is further shown in Figure 11 is shown, Figure 11 is shown a perspective view of the surface 306 of the rotor assembly 110.

[0040] InFigures 10-11 In the illustrated embodiment, the spanwise vane 308-2 projects from the surface 306 of the blade 304 in the stowed position when the rotor assembly 110 is idling or free to rotate. Figure 12 The spanwise vane 308-2 is shown in the deployed position when the rotor assembly 110 is in operation (e.g., the rotor assembly 110 is mechanically driven to rotate). In the deployed position, the spanwise vane 308-2 is pivoted at the surface 306 along an edge that is substantially parallel to the longitudinal direction 406, which allows the spanwise vane 308-2 to lie flat against the surface 306. Although the spanwise vane 308-2 is shown near the tip 410 of the blade 304, in other embodiments the spanwise vane 308-2 is located at other positions along the blade 304 as a design choice.

[0041] In some embodiments, the spanwise vane 308-2 projects from the surface 306 of the blade 304 with a spring (not shown) or other mechanical actuator (not shown) when the rotor assembly 110 is not driven to rotate (see Figures 10-11 ). For example, the spanwise vane 308-2 is held within a surface relief (not shown) of the surface 306 and pivots out of the surface relief in response to a trigger. In some embodiments, the trigger is due to a release of a holding device (not shown) or other mechanical feature that normally holds the spanwise vane 308-2 close to the surface 306 until activation.

[0042] In some embodiments, the spanwise vane 308-2 passively transitions to the stowed position and conforms to the surface 306 of the blade 304 and projects therefrom in response to a change in the rotational rate of the rotor assembly 110. For example, the spanwise vane 308-2 passively transitions to the stowed position and conforms to the surface 306 in response to the rotational rate being greater than a first rotational rate (see Figure 12 ) and passively transitions to the deployed position and projects from the surface 306 in response to the rotational rate being less than a second rotational rate (see Figure 11 ). In some embodiments, the first rotational rate is greater than the second rotational rate.

[0043] In other embodiments, the spanwise vane 308-2 passively transitions to the stowed position and conforms to the surface 306 of the blade 304 and passively transitions to the deployed position and projects from the surface 306 of the blade 304 in response to a change in the chordwise airflow 1102 applied to the spanwise vane 308-2. The chordwise airflow 1102 is an airflow across the blade 304 from a leading edge 1104 to a trailing edge 1106 of the blade 304 as the rotor assembly 110 rotates. For example, the spanwise vane 308-2 passively transitions to the stowed position and conforms to the surface 306 in response to the chordwise airflow 1102 of the blade 304 being greater than a first airflow (see Figure 12) and passively transition to the deployed position and protrude from the surface 306 of the blade 304 (see FIG. 11B) in response to the chordwise airflow 1102 being less than the second airflow rate. Figure 11 In some embodiments, the first airflow rate is greater than the second airflow rate.

[0044] Figures 13-14 is a perspective view of the rotor assembly 110 utilizing a grid fin 1302 as the blade alignment device 308 in another illustrative embodiment. In this embodiment, the grid fin 1302 protrudes from the surface 307 of the blade 304 when in the deployed position, where the chordwise orientation of the grid fin 1302 is substantially parallel to the longitudinal direction 406. Generally, a grid fin is a lattice of small aerodynamic surfaces arranged within a box, where the chord of the aerodynamic surfaces is shorter relative to the other dimensions. In Figure 13 In the illustrated embodiment, the grid fin 1302 protrudes from the surface 307 of the blade 304. In other embodiments, the grid fin 1302 protrudes from the surface 306 of the blade 304. In some embodiments, the grid fin 1302 is positioned at any location between the hub 302 and the tip 410 of the blade 304.

[0045] Figure 14 is a perspective view of the rotor assembly 110 in another illustrative embodiment. In this embodiment, the grid fin 1302 conforms to the surface 307 of the blade 304 when in the stowed position. Figure 15 is a perspective view of the grid fin 1302 in an illustrative embodiment. In this embodiment, the grid fin 1302 includes a plurality of fins 1502 configured as a lattice, which are included within a box 1506. The fins 1502 include a leading edge 1508 and a trailing edge 1510 of the grid fin 1302 that define a chord 1512. As previously described, the chord 1512 of the grid fin 1302 is generally less than a height 1514 of the grid fin 1302 or a width 1516 of the grid fin 1302. As previously described with respect to the blade alignment device 308, the grid fin 1302 aligns the blade 304 in a similar manner when the aircraft 100 is in forward flight generating the airflow 602.

[0046] Figure 16 is a flowchart of a method 1600 of operating a rotor assembly for an aircraft, and Figures 17-20 is a flowchart illustrating additional details of the method 1600 in various illustrative embodiments. The steps of the method 1600 will be discussed with respect to the rotor assembly 110, although the method 1600 can be applied to other rotor assemblies not shown. The steps of the method 1600 do not include all steps, and can include other steps not shown. Further, the steps can be performed in alternative orders.

[0047] Figure 16Step 1602 includes operating the rotor assembly 110 in a free-spinning state. This can occur, for example, when the aircraft 100 is in forward flight and the rotor assembly 110 is not being driven to rotate. Step 1604 includes aligning a blade (e.g., blade 304) that includes the blade alignment device 308 in response to airflow around the blade alignment device 308. For example, the longitudinal direction 406 of the rotor assembly 110 is aligned with the airflow direction 602 to maximize drag reduction. In one embodiment, the blade 304 of the rotor assembly 110 is oriented in a downstream position in the airflow 602 and the blade 305 of the rotor assembly 110 is oriented in an upstream position in the airflow 602.

[0048] Figure 17 Step 1702 includes operating the blade alignment device 308 in a deployed position when the rotor assembly 110 is free spinning (e.g., the blade alignment device 308 protrudes from the surface 306 of the blade 304). Step 1704 includes operating the blade alignment device 308 in a stowed position when the rotor assembly 110 is being driven to rotate (e.g., the blade alignment device 308 conforms to the surface 306 of the blade 304 during a portion of the span). For example, during takeoff, hover, and landing of the aircraft 100, the rotating hub 302 is driven to rotate by a mechanical power source to provide lift to the aircraft 100. During this phase of operation, the blade alignment device 308 is operated in the stowed position and conforms to the surface 306 of the blade 304.

[0049] In some embodiments, the blade alignment device 308 is passively transitioned between the stowed position and the deployed position based on conditions acting on the rotor assembly 110. In one embodiment, the blade alignment device 308 is passively transitioned to the stowed position when the rotational rate of the rotating hub 302 is greater than a first rotational rate (see Figure 18 , step 1802), and is passively transitioned to the deployed position when the rotational rate of the rotating hub 302 is less than a second rotational rate (see Figure 18 , step 1804).

[0050] In another embodiment, the blade alignment device 308 is passively transitioned to the stowed position when the chordwise airflow 1102 of the blade 304 is greater than a first airflow rate (see Figure 19 , step 1902), and is passively transitioned to the deployed position when the chordwise airflow 1102 of the blade 304 is less than a second airflow rate (see Figure 19 , step 1904).

[0051] In yet another embodiment, the blade alignment device 308 is passively transitioned to the stowed position when the centrifugal force exerted on the blade alignment device 308 is greater than a first centrifugal force value (see Figure 20and passively transition to the deployed position (see FIG. 20B) in response to the centrifugal force exerted on the blade alignment device 308 being less than a second centrifugal force value. Figure 20

[0052] The use of the blade alignment device 308 for the rotor assembly 110 allows for passive orientation of the rotor assembly 110 during a cruise phase of flight of the aircraft 100, thereby reducing drag exerted on the aircraft 100 for a freely rotating rotor assembly 110. Because the blade alignment device 308 passively orients the rotor assembly 110 during the cruise phase, mechanical power and / or complex positioning sensors are not required, thereby reducing the complexity of the aircraft 100.

[0053] Clause 1. A method of operating a rotor assembly for an aircraft, comprising: operating the rotor assembly in a freewheeling state, the rotor assembly comprising a rotating hub configured to rotate about an axis, a plurality of blades extending radially from the rotating hub, wherein at least one of the plurality of blades comprises a blade alignment device; and aligning the at least one of the plurality of blades in response to airflow about the blade alignment device when the rotor assembly is in the freewheeling state.

[0054] Clause 2. The method of clause 1, wherein: the method further comprises: operating the blade alignment device in a stowed position by conforming to a surface of the at least one of the plurality of blades during at least a portion of a span when the rotor assembly is driven to rotate; and aligning the at least one of the plurality of blades further comprises: operating the blade alignment device in a deployed position by protruding from the surface of the at least one of the plurality of blades when the rotor assembly is in the freewheeling state.

[0055] Clause 3. The method of clause 2, wherein: operating the blade alignment device in the stowed position comprises passively transitioning the blade alignment device to the stowed position when a rotational rate is greater than a first rotational rate, and operating the blade alignment device in the deployed position comprises passively transitioning the blade alignment device to the deployed position when the rotational rate is less than a second rotational rate.

[0056] Clause 4. The method of clause 2, wherein: operating the blade alignment device in the stowed position comprises passively transitioning the blade alignment device to the stowed position when a chordwise airflow of the at least one of the plurality of blades is greater than a first airflow rate, and operating the blade alignment device in the deployed position comprises passively transitioning the blade alignment device to the deployed position when the chordwise airflow is less than a second airflow rate.

[0057] ​Clause 5. The method of clause 2, wherein: operating the vane alignment device in the stowed position comprises passively transitioning the vane alignment device to the stowed position when a centrifugal force exerted on the vane alignment device is greater than a first centrifugal force value, and operating the vane alignment device in the deployed position comprises passively transitioning the vane alignment device to the deployed position when a centrifugal force exerted on the vane alignment device is less than a second centrifugal force value.

[0058] Clause 6. A rotor assembly for an aircraft, the rotor assembly comprising: a rotating hub configured to rotate about an axis; and a plurality of vanes extending radially from the rotating hub, wherein at least one of the plurality of vanes comprises a vane alignment device configured to align the at least one of the plurality of vanes in response to airflow around the vane alignment device when the rotor assembly is freewheeling.

[0059] Clause 7. The rotor assembly of clause 6, wherein: the vane alignment device is configured to conform to a surface of the at least one of the plurality of vanes in the stowed position during at least a portion of a span when the rotor assembly is driven to rotate; and the vane alignment device is configured to protrude from the surface of the at least one of the plurality of vanes in the deployed position when the rotor assembly is freewheeling.

[0060] Clause 8. The rotor assembly of clause 7, wherein: the vane alignment device is configured to passively transition to the stowed position when a rotational rate is greater than a first rotational rate, and the vane alignment device is configured to passively transition to the deployed position when the rotational rate is less than a second rotational rate.

[0061] Clause 9. The rotor assembly of clause 7, wherein: the vane alignment device is configured to passively transition to the stowed position when a chordwise airflow of the at least one of the plurality of vanes is greater than a first airflow rate, and the vane alignment device is configured to passively transition to the deployed position when the chordwise airflow is less than a second airflow rate.

[0062] Clause 10. The rotor assembly of clause 7, wherein: the vane alignment device is configured to passively transition to the stowed position when a centrifugal force exerted on the vane alignment device is greater than a first centrifugal force value, and the vane alignment device is configured to passively transition to the deployed position when the centrifugal force exerted on the vane alignment device is less than a second centrifugal force value.

[0063] Clause 11. The rotor assembly of clause 7, wherein: the vane alignment device comprises a spanwise flap pivotably coupled to a surface of the at least one of the plurality of vanes and substantially parallel to a longitudinal direction of the at least one of the plurality of vanes.

[0064] Clause 12. The rotor assembly of clause 7, wherein the at least one of the plurality of blades includes a cutout disposed between a tip of the at least one of the plurality of blades and the rotating hub, and the blade alignment device includes a spanwise tab pivotably coupled to an edge of the cutout.

[0065] Clause 13. The rotor assembly of clause 7, wherein: the blade alignment device includes a grid tab pivotably coupled to a surface of the at least one of the plurality of blades, wherein a chord of the grid tab is substantially parallel to a longitudinal direction of the at least one of the plurality of blades when in the deployed position.

[0066] Clause 14. An aircraft, comprising: at least one rotor assembly configured to provide lift for the aircraft, the at least one rotor assembly comprising: a rotating hub configured to rotate about an axis; and a plurality of blades extending radially from the rotating hub, wherein at least one of the plurality of blades includes a blade alignment device configured to align the at least one of the plurality of blades in response to airflow around the blade alignment device when the rotor assembly is freewheeling.

[0067] Clause 15. The aircraft of clause 14, wherein: the blade alignment device is configured to conform to a surface of the at least one of the plurality of blades in a stowed position during at least a portion of a span when the rotor assembly is driven to rotate, and to protrude from the surface of the at least one of the plurality of blades in a deployed position when the rotor assembly is freewheeling.

[0068] Clause 16. The aircraft of clause 15, wherein: the blade alignment device includes a grid tab pivotably coupled to the surface of the at least one of the plurality of blades, wherein a chord of the grid tab is substantially parallel to a longitudinal direction of the at least one of the plurality of blades when in the deployed position.

[0069] Clause 17. The aircraft of clause 15, wherein: the blade alignment device is configured to passively transition to the stowed position when a rotational rate is greater than a first rotational rate, and to passively transition to the deployed position when the rotational rate is less than a second rotational rate.

[0070] Clause 18. The aircraft of clause 15, wherein: the blade alignment device is configured to passively transition to the stowed position when a chord-wise airflow of the at least one of the plurality of blades is greater than a first airflow rate, and to passively transition to the deployed position when the chord-wise airflow is less than a second airflow rate.

[0071] Clause 19. The aircraft of Clause 15, wherein: the blade alignment device is configured to passively transition to the stowed position when a centrifugal force exerted on the blade alignment device is greater than a first centrifugal force value, and passively transition to the deployed position when a centrifugal force exerted on the blade alignment device is less than a second centrifugal force value.

[0072] Clause 20. The aircraft of Clause 14, wherein: the blade alignment device includes a spanwise flap pivotably coupled to a surface of the at least one of the plurality of blades and substantially parallel to a longitudinal direction of the at least one of the plurality of blades.

[0073] Clause 21. The aircraft of Clause 14, wherein: the at least one of the plurality of blades includes a cutout disposed between a tip of the at least one of the plurality of blades and a hub, and the blade alignment device includes a spanwise flap pivotably coupled to an edge of the cutout.

[0074] While specific embodiments are described herein, the scope should not be limited to these specific embodiments. Rather, the scope is defined by the appended claims and their equivalents.

Claims

1. A rotor assembly for an aircraft, the rotor assembly comprising: a rotating hub configured to rotate about an axis; and a plurality of blades extending radially from the rotating hub, wherein at least one of the plurality of blades includes a blade alignment device configured to align the at least one of the plurality of blades in response to airflow about the blade alignment device when the rotor assembly is freewheeling, wherein the blade alignment device is configured to conform to a surface of the at least one of the plurality of blades in a stowed position during at least a portion of a span when the rotor assembly is driven to rotate; and to protrude from the surface of the at least one of the plurality of blades in a deployed position when the rotor assembly is freewheeling, and the blade alignment device is configured to passively transition to the stowed position when a centrifugal force exerted on the blade alignment device is greater than a first centrifugal force value, and to passively transition to the deployed position when the centrifugal force exerted on the blade alignment device is less than a second centrifugal force value.

2. The rotor assembly of claim 1, wherein: the blade alignment device is configured to passively transition to the stowed position when a rotational rate is greater than a first rotational rate, and to passively transition to the deployed position when the rotational rate is less than a second rotational rate.

3. The rotor assembly of claim 1, wherein: the blade alignment device is configured to passively transition to the stowed position when a chord-wise airflow of the at least one of the plurality of blades is greater than a first airflow rate, and to passively transition to the deployed position when the chord-wise airflow is less than a second airflow rate.

4. The rotor assembly of claim 1, wherein: the blade alignment device includes a span-wise flap pivotably coupled to the surface of the at least one of the plurality of blades and substantially parallel to a longitudinal direction of the at least one of the plurality of blades.

5. The rotor assembly of claim 1, wherein: the at least one of the plurality of blades includes a cutout disposed between a tip of the at least one of the plurality of blades and the rotating hub, and the blade alignment device includes a span-wise flap pivotably coupled to an edge of the cutout.

6. The rotor assembly of claim 1, wherein: the blade alignment device includes a grid flap pivotably coupled to the surface of the at least one of the plurality of blades, wherein a chord of the grid flap is substantially parallel to a longitudinal direction of the at least one of the plurality of blades when in the deployed position.

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