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Title:
A SWASHPLATE ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2022/261720
Kind Code:
A1
Abstract:
A swashplate assembly for controlling rotor blades of a rotorcraft, comprising a rotating assembly, wherein the rotating assembly is arranged to rotate with the rotor blades about an axis and is coupled to the rotor blades; a base assembly, wherein the base assembly includes a first portion and a second portion, and the rotating assembly is guided by the base assembly to set blade angles of the rotor blades as the rotating assembly rotates with the rotor blades, wherein the second portion is arranged to induce a change in blade angle of the rotor blades.

Inventors:
BATTEN PETER JAMES (AU)
Application Number:
PCT/AU2022/050608
Publication Date:
December 22, 2022
Filing Date:
June 17, 2022
Export Citation:
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Assignee:
HYPER Q AEROSPACE HOLDINGS PTY LTD (AU)
International Classes:
B64C27/10; B64C27/605; B64C27/72; B64C27/80
Foreign References:
US10272998B22019-04-30
EP2851294A12015-03-25
EP2730505A12014-05-14
Attorney, Agent or Firm:
ALLENS PATENT & TRADE MARK ATTORNEYS (AU)
Download PDF:
Claims:
CLAIMS:

1. A swashplate assembly for controlling rotor blades of a rotorcraft, comprising: a rotating assembly, wherein the rotating assembly is arranged to rotate with the rotor blades about an axis; a base assembly, wherein the base assembly includes a first portion and a second portion, and the rotating assembly is guided by the base assembly to set blade angles of the rotor blades as the rotating assembly rotates with the rotor blades, wherein the second portion is arranged to induce a change in blade angle of the rotor blades.

2. The swashplate assembly of claim 1 , wherein the second portion is arranged to induce a change in blade angle of the rotor blades when they are in at least a portion of a retreating phase.

3. The swashplate assembly of claim 1 or 2, wherein the second portion is pivotable with respect to the first portion.

4. The swashplate assembly of claim 3, wherein the pivotal movement of the second portion with respect to the first portion causes a relative change in blade angle of the rotor blades.

5. The swashplate assembly of claim 3 or 4, wherein the pivotal movement of the second portion is controlled by an articulation motor.

6. The swashplate assembly of any one of claims 1 to 5, wherein the base assembly comprises: a circular section, wherein the circular section is divided into portions including, at least, the first portion and the second portion.

7. The swashplate assembly of claim 6, the circular section is divided into a plurality of portions, each portion arrangeable to provide a relative change in blade angle with respect to other portions.

8. The swashplate assembly of any one of claims 1 to 7, wherein the first portion and the second portion are each formed in an arc shape.

9. The swashplate assembly of any one of claims 1 to 8, wherein an angular orientation of the base assembly is adjustable, that is, the base assembly is movable in a direction generally perpendicular to a plane of the base assembly.

10. The swashplate assembly of claim 9, further including a slew motor, being configured to control an angular displacement of the base assembly in response to a direction of a wind.

11 . The swashplate assembly of any one of claims 1 to 10, wherein the rotating assembly comprises: a rotor shaft, one or more extension arms, which extend radially outwardly from the rotor shaft to the base assembly, wherein the rotor shaft is caused to rotate about a longitudinal axis of the rotor shaft, and rotations of the rotor shaft are translated into spinning motions of the rotor blades.

12. The swashplate assembly of claim 11 , wherein the rotor shaft is rotated by one or more electric motors or by way of a mechanical gearbox coupled to an engine of the rotorcraft.

13. The swashplate assembly of claim 11 or 12, wherein the extension arms are of an elongate configuration, preferably each of the extension arms is of a substantially trapezoid shape in plan view, meaning a width of the extension arms at one end is smaller than a width of the extension arms at the other end.

14. The swashplate assembly of any one of claims 11 to 13, wherein the one or more extension arms are connected to the rotor shaft at a first end, and movably coupled to the base assembly at a second end, such that rotations of the rotor shaft cause the one or more extension arms to also spin about the rotor shaft and rotate with respect to the base assembly.

15. The swashplate assembly of any one of claims 11 to 14, wherein the one or more extension arms are fixedly connected to the rotor shaft via a coupling ring at a first end.

16. The swashplate assembly of claim 14 and 15, wherein the base assembly comprises a channel or guide arranged to receive a second end of the one or more extension arms.

17. The swashplate assembly of claim 16, wherein the channel is configured to receive a low friction coupling which allow relative movement between the one or more extension arms and the base assembly.

18. The swashplate assembly of any one of claims 1 to 17, further including coupling means which mechanically couple the rotating assembly to the rotor blades, such that rotations of the rotating assembly are translated into spinning motions of the rotor blades.

19. The swashplate assembly of claim 18, wherein the coupling means comprise a plurality of pushrod arms, wherein each pushrod arm is fixedly attached to an extension arm at one end, and operatively coupled to a respective rotor blade, or actuation means of a respective rotor blade at the other end.

20. The swashplate assembly of any of claims 1 to 19, further comprising cyclic/collective actuators.

21 . The swashplate assembly of claim 20, wherein the cyclic/collective actuators perform the function of generating pitch, roll and climb effects: as the second portion is arranged to pivot/articulate with respect to the first portion to alter the blade angle of the rotor blades in the retreating phase, both the first and second portions may be arranged to move perpendicularly along the rotor shaft to impart a change in all blade angles at the same time, which is known as a collective input and is used to control a vertical component of the lift vector; by controlling the angle of the base assembly, cyclic input to the swashplate assembly will occur, which causes roll and pitch effects on rotorcraft.

22. The swashplate assembly of claim 1 or 2, wherein the second portion is in a substantially fixed relationship with the first portion, and the first portion and the second portion are arranged at an angle.

Description:
A SWASHPLATE ASSEMBLY

Technical Field

[001] The present technology relates to a system and a method for providing lift and thrust to a rotorcraft. More specifically, the present disclosure relates to a swashplate assembly.

Background

[002] Characteristics of rotorcraft or rotary-wing aircraft having rotor blades for producing lift and thrust are known. As the rotorcraft begins to move horizontally, a rotor blade travelling in the same direction as the rotorcraft is known as an advancing blade and a rotor blade travelling in the opposite direction to the rotorcraft is known as a retreating blade.

[003] During operation, rotorcraft rotor blades spin about a rotor shaft at a typically constant speed QR (usually measured in RPM). In coaxial helicopters, the rotor disks rotate in opposite directions about a central axis.

[004] In single rotor rotorcrafts, when the rotorcraft has no vertical velocity, lift is generated uniformly across the rotor disks as the blades pass through the air. If the rotorcraft blades are subject to a relative wind (usually induced by the forward velocity of the helicopter), more lift is generated on the advancing side of the rotor disk, and less lift is generated on the retreating side. This dissymmetry of lift shows a fundamental problem encountered by single rotor helicopters. Coaxial helicopters amend this issue by using counter-rotating disks.

[005] To address dissymmetry of lift, helicopter rotor blades may rotate about three axes. The lead-lagging angle (angular position on the plane of the blades), the flapping angle (between the blade and the rotational axis) and the feathering angle, also known as angle of attack (AOA), which is the angle between the oncoming air or relative wind and a respective rotor blade.

These angles are managed to decrease lift on the advancing blades and increase lift on the retreating blades. The primary limitations of these measures are found in retreating blade stall and supersonic blade airflow.

[006] The lift generated by a rotor blade is calculated by the lift equation:

L = C L * A * 0.5 * p * V 2 or more commonly written:

L = C L * ½ p V 2 S

Where: CL = Lift coefficient, which is dependent on the angle of attack (AOA) of a rotor blade (which is also referred to as the blade pitch angle, or blade angle);

A and S = surface area of the wing blade, or propeller (m 2 ); p = air density (kg/m 3 ); and

V = velocity of airflow over the rotor blade (m/s).

[007] The lift generated by a rotor blade of a rotorcraft can be broken down into two components, a vertical component that counters gravity and a horizontal component that causes the rotorcraft to move horizontally and accelerate in a horizontal direction. It will be appreciated that increasing the velocity of air flowing over the rotor blade will increase the amount of lift generated by the rotor blade.

[008] For any velocity of airflow over the rotor blade, there is a maximum AOA at which the rotor blade can be positioned before the rotor blade stalls and no longer produces any lift. This is known as the critical angle. It will be appreciated that the velocity of airflow over a rotor blade is greatest at the tip of the rotor blade and that the velocity of airflow over the rotor blade at any given point is dependent on the revolutions per minute (RPM) or angular velocity of the rotor blade and the distance from the centre of the rotor hub.

[009] When a rotorcraft moves horizontally, the velocity of airflow over an advancing rotor blade is greater than the velocity of airflow over a retreating rotor blade. So that the advancing rotor blade and the retreating rotor blade generate about the same lift, the AOA of the advancing rotor blade is usually decreased, and the AOA of the retreating rotor blade is usually increased.

[010] As the air velocity of the rotorcraft increases and the RPM of the rotor is maintained constant, the velocity of airflow over the retreating rotor blade decreases. The reduction of airflow over the retreating rotor blade will cause the retreating rotor blade to approach the critical angle. As the air velocity of the rotorcraft increases further and the RPM of the rotor is maintained constant, the retreating rotor blade will eventually exceed the critical angle and stall, thereby causing the rotorcraft to become unstable and possible loss of control of the rotorcraft may occur. This phenomenon is known as retreating blade stall (RBS). Accordingly, it will be appreciated that RBS is one factor that limits the maximum air velocity of the rotorcraft.

[011] Critical angle is directly influenced by rotor blade angle of attack. The higher the rotor blade angle of attack, the sooner the rotor blade will stall. Conversely, if the angle of attack is close to zero, RBS will not occur until a much higher speed. Furthermore, the resultant drag generated by a stalled aerofoil, even if it is only partly stalled will be significantly reduced.

[012] A swashplate is a device that translates input via aircraft flight controls into motion of the rotor blades. Swashplate design exists in the prior art in many forms. The basic principles remain the same whereby a static element imparts angular displacement to cause a change in rotor blade angle. The nature of existing swashplates is generally considered limited by the rigid nature of the rotating and static sections of the swashplate.

[013] Accordingly, there is a need for a swashplate which improves control of the rotor blades, or to at least provide an alternative choice.

Summary of invention

[014] In a first aspect, the present disclosure provides a swashplate assembly for controlling rotor blades of a rotorcraft, comprising: a rotating assembly, wherein the rotating assembly is arranged to rotate with the rotor blades about an axis; a base assembly, wherein the base assembly includes a first portion and a second portion, and the rotating assembly is guided by the base assembly to set blade angles of the rotor blades as the rotating assembly rotates with the rotor blades, wherein the second portion is arranged to induce a change in blade angle of the rotor blades.

[015] In one embodiment, the second portion is arranged to induce a change in blade angle of the rotor blades when they are in at least a portion of a retreating phase.

[016] In one embodiment, the second portion is pivotable with respect to the first portion.

[017] In one embodiment, the pivotal movement of the second portion with respect to the first portion causes a relative change in blade angle of the rotor blades.

[018] In one embodiment, the pivotal movement of the second portion is controlled by an articulation motor.

[019] In one embodiment, the base assembly comprises: a circular section, wherein the circular section is divided into portions including, at least, the first portion and the second portion.

[020] In one embodiment, the circular section is divided into a plurality of portions each portion arrangeable to provide a relative change in blade angle with respect to other portions.

[021] In one embodiment, the first portion and the second portion are each formed in an arc shape.

[022] In one embodiment, an angular orientation of the base assembly is adjustable, that is, the base assembly is movable in a direction generally perpendicular to a plane of the base assembly. [023] In one embodiment, the swashplate assembly further includes a slew motor, being configured to control an angular displacement of the base assembly in response to a direction of a wind.

[024] In one embodiment, the rotating assembly comprises: a rotor shaft, one or more extension arms, which extend radially outwardly from the rotor shaft to the base assembly, wherein the rotor shaft is caused to rotate about a longitudinal axis of the rotor shaft, and rotations of the rotor shaft are translated into spinning motions of the rotor blades.

[025] In one embodiment, the rotor shaft is rotated by one or more electric motors or by way of a mechanical gearbox coupled to an engine of the rotorcraft.

[026] In one embodiment, the extension arms are of an elongate configuration, preferably each of the extension arms is of a substantially trapezoid shape in plan view, meaning a width of the extension arms at one end is smaller than a width of the extension arms at the other end.

[027] In one embodiment, the one or more extension arms are connected to the rotor shaft at a first end, and movably coupled to the base assembly at a second end, such that rotations of the rotor shaft cause the one or more extension arms to also spin about the rotor shaft and rotate with respect to the base assembly.

[028] In one embodiment, the one or more extension arms are fixedly connected to the rotor shaft via a coupling ring at the first end.

[029] In one embodiment, the base assembly comprises a channel arranged to receive the second end of the one or more extension arms.

[030] In one embodiment, the channel is configured to receive a low friction coupling such as ball bearings, which allow relative movement between the one or more extension arms and the base assembly.

[031] In one embodiment, the swashplate assembly further includes coupling means which mechanically couple the rotating assembly to the rotor blades, such that rotations of the rotating assembly are translated into spinning motions of the rotor blades.

[032] In one embodiment, the coupling means comprise a plurality of pushrod arms, wherein each pushrod arm is fixedly attached to an extension arm at one end, and operatively coupled to a respective rotor blade, or actuation means of a respective rotor blade at the other end.

[033] In one embodiment, the coupling further includes a rotor head, which is positioned at or near the upper end of a rotor shaft and holds the rotor blades [034] In one embodiment, the swashplate assembly further includes cyclic/collective actuators.

[035] The cyclic/collective actuators perform the function of generating pitch, roll and climb effects: as the second portion is arranged to pivot/articulate with respect to the first portion to alter the blade angle of the rotor blades in the retreating phase, both the first and second portions may be arranged to move perpendicularly along the rotor shaft to impart a uniform change in all blade angles at the same time. This is a collective input and is used to control the vertical component of the lift vector; by controlling the angle of the base assembly, cyclic input to the swashplate assembly will occur. This will cause roll and pitch effects on rotorcraft.

[036] In an alternative embodiment, the second portion is in a substantially fixed relationship with the first portion, and the first portion and the second portion reside in different planes.

Brief Description of the Drawings

[037] Preferred embodiments of the present invention will be described, by way of examples only, with reference to accompanying figures, wherein:

Figures 1 is a schematic diagram of one embodiment of a swashplate assembly in accordance with the present disclosure;

Figure 2 is a schematic diagram of another embodiment of a swashplate assembly operatively coupled to rotor blades of an aircraft;

Figure 3 illustrates a schematic diagram of a further embodiment of a swashplate assembly operatively coupled to rotor blades of an aircraft, with the position of the second portion adjusted in response to direction 203 in which the aircraft is moving;

Figure 4 illustrates a schematic diagram of yet another embodiment of a swashplate assembly operatively coupled to rotor blades of an aircraft, having a base assembly displaced in an angular direction in response to direction 203 in which the aircraft is moving, and/or wind direction 403;

Figure 5 illustrates an example of a fixed pitch multi rotor VTOL rotorcraft;

Figure 6 illustrates another embodiment of a swashplate assembly in accordance with the present disclosure;

Figure 7 is a diagram demonstrating how the present disclosure reduces effects of drag.

Detailed Description of Embodiments [038] Referring to Figure 1 , there is shown a swashplate assembly 100 in accordance with one embodiment of the present disclosure. The swashplate assembly 100 is for use with rotor blades of a rotorcraft, such as a helicopter, and translates input via flight controls of the rotorcraft into motion of the rotor blades, particularly blade angle/pitch.

[039] The swashplate assembly 100 comprises at least a base assembly 101 and a rotating assembly 102. Other components of the swashplate assembly 100 are not shown in Figure 1 for ease of illustration. It will be appreciated that distance between the swashplate assembly 100 and the rotor blades, connected by way of coupling mechanisms such as pushrods and linkages, will ultimately determine the rotor blade pitch angle. This gives rise to collective and cyclic inputs of the rotor disk.

[040] In this embodiment, the base assembly 101 comprises two semi-circular portions: a first portion 101 a, and a second portion 101b, which is movable with respect to a first portion 101 a of the base assembly 101. The movement of the second portion 101b is arranged to induce a change in blade angle of the rotor blades when they are in at least a portion of a retreating phase. In this way, a differentiation in blade angles for advancing blades and retreating blades can be introduced, which is a more effective method of controlling the retreating blade angle of attack and managing rotor blade stall (also known as RBS). In general, as a rotor blade rotates such that it is considered a retreating blade, it can be considered to be in the retreating phase. That is, the retreating phase corresponds to the rotational range of the rotor blade arc where a portion of the rotor blade is moving in the opposite direction to the rotor craft.

[041 ] In this example, the second portion 101 b and the first portion 101 a of the base assembly 101 are each formed in an arc shape, so that the two portions jointly form a closed, substantially circular section, in which the second portion 101 b is in the same plane as the first portion 101a. In other embodiments, the circular section may be divided into a plurality of such portions, to allow more accurate and effective control of the blade angles for each individual rotor blade at particular angular positions. Two articulation joints 104a and 104b are provided at ends of the second portion 101b where it meets the first portion 101 a, to enable the second portion 101 b to be pivoted/articulated as required. In other embodiments, more articulation points may be provided along the circular section such that the blade angles of the relevant rotor blades may be varied in a non-linear manner.

[042] Figure 2 shows a more detailed schematic diagram of a swashplate assembly 100 operatively coupled to rotor blades 205 of a rotorcraft (not shown). The rotor blades 205 are spun by a rotor shaft 202 to provide lift and thrust to the rotorcraft. Pushrods and linkages 204 mechanically couple the rotor blades 205 to the extension arms 102a, 102b, 102c, 102d to control blade angle of the rotor blades 205. Depending on its main functional purpose, the swashplate assembly 100 can be broadly divided into: a. a rotating assembly 102 which includes components that are caused to rotate about an axis of the main rotor shaft 202, wherein rotations of these components reflect the rotation of the rotor blades 205 about the rotor shaft 202. b. a base assembly 101 which is configured to control blade angles as cyclic and collective inputs of the rotor blades 205, and is capable of imparting a relative change in the blade angle of at least one rotor blade compared to the other rotor blades. c. mechanical coupling means which assist in translating movement of the base assembly 101 and the rotating assembly 102 into corresponding motions, preferably rotational motions of the rotor blades 205.

[043] The swashplate assembly 100 also includes driving means, such as various motors and rotors, to provide actuation to various components of the base assembly 101 and the rotating assembly 102.

[044] Similar to the embodiment shown in Figure 1 , the base assembly 101 comprises a first portion 101 a, and a second portion 101b, wherein the second portion 101 b is pivotable/articulatable relative to the first portion 101a.

[045] The rotating assembly 102 of the swashplate assembly 200 includes a plurality of extension arms 102a, 102b, 102c and 102d, which extend radially outwardly from the rotor shaft 202 to the base assembly 101. Rotations of the rotor shaft 202 are mechanically translated to the extension arms 102a, 102b, 102c and 102d, which are coupled to the rotor shaft 202 at a first end, and are movably connected to the base assembly 101 at a second end. The base assembly 101 provides a guide to the extension arms 102a, 102b, 102c and 102d of the rotating assembly 102 and such guide is determined by the relative positioning of the first and second portions of the base assembly 101.

[046] In one embodiment, the mechanical coupling is achieved via a plurality of pushrod arms 204, with each pushrod arm 204 acting as the linkage between a corresponding rotor blade 205 and a corresponding extension arm 102a, 102b, 102c or 102d . Preferably, each pushrod arm 204 is fixedly attached to an extension arm 102a, 102b, 102c or 102d at one end, and operatively connected to actuation means of a corresponding rotor blade at the other end which controls blade angle. As the extension arms 102a, 102b, 102c or 102d are rotated, or tilted as they continuously move between the first portion 101 a and the second portion 101 b, corresponding movement and blade angle changes are introduced to the rotor blades 205. [047] In this embodiment the rotor blades 205 are supported by a rotor hub 211 , which is placed at or near a top end of the rotor shaft 202. In this example, four rotor blades 205 are evenly spaced apart and extend outwardly from an external surface of the rotor hub 211. In other embodiments, a different number of rotor blades 205 could be chosen depending on the type of the aircraft. Other embodiments may locate the swashplate above the rotor hub 211 or position the rotor hub 211 part way up the mast.

[048] The arrow 203 in Figure 2 indicates a direction in which the rotorcraft is travelling. In this example, there is no wind effect. If the first portion 101 a and second portion 101 b of the swashplate assembly 100 reside in the same plane and that plane is horizontal, then all the rotor blades 205 will have the same blade angle, which is typically required for a rotorcraft to hover, climb, or descend. If, as shown in Figure 2, the first portion 101a and second portion 101b are in the same plane but that plane is not horizontal, then the rotor blades 205 will have their blade angle modified as they rotate around the swashplate assembly 200 and generate a thrust vector to accelerate the aircraft.

[049] As the rotorcraft accelerates, the effects of RBS increase. If the second portion 101b remains in the same plane as the first portion 101a, the effects of RBS governed by the swashplate are identical to a conventional swashplate. Eventually the rotorcraft will reach a velocity where the retreating blade is sufficiently stalled to cause a loss of control.

[050] Alternatively, as the rotorcraft accelerates, the second portion 101 b is articulated with respect to the plane of the first portion 101 a in a way that reduces the retreating blade angle of attack as illustrated in Figure 3. In Figure 3, arrows 310 indicate the pivoting motions of the second portion 101b. The articulation angle is controlled by articulation motors 201 connected to the base assembly 101. In this embodiment, the angle of attack on the advancing blades is unaffected, however, for the retreating blade 205a, RBS will be mitigated or eliminated enabling greater speed for the rotorcraft.

[051] In this example, it can be seen that cyclic and collective control inputs are unaffected by the present invention. Some adjustment of collective input may be required as the increase of the articulation angle will cause a decrease of overall lift generated by the rotor blades 205 as the rotorcraft accelerates. There will be some natural compensation due to the increase in lift on the advancing blades as the aircraft accelerates.

[052] As mentioned, it is advantageous to control blade angle of attack of the retreating blade, that is, a rotor blade which is in at least a portion of a retreating phase, individually and differently from the other rotor blades, in order to manage retreating blade stall more effectively, and to increase the maximum airspeed that the aircraft is able to achieve. In Figure 3, the retreating blade is rotor blade 205a, provided that the rotor blades are rotating in an anti- clockwise direction. Therefore, the orientation of the base assembly 101 is configured such that the second portion 101b is placed directly beneath the retreating blade 205a. This will ensure that the blade angle of the retreating blade 205a is adjusted as it travels past the second portion 101 b, and as soon as it rotates back to the first portion 101 a of the base assembly, it will revert to blade angles induced or set by the plane of the first portion 101 a. In another example, this relative positioning of the second portion part 101b may need to be rotated about the rotor shaft 202 to allow for phase lag.

[053] The blade angle at which a blade will stall is related to the shape of the blade, namely its chord and length, the velocity of air flowing over the blade and air density. A library of the profile data of the blade matched against the true airspeed and current air density will enable a computer to calculate the blade stall speed. Given this data, a blade angle reduction algorithm, which provides adequate margin from the stall, can be created. The result is then applied to the articulation motors to reduce the blade angle of attack of the second portion 101b which is directly proportional to incident aircraft velocity measured as true airspeed.

[054] To be effective throughout the flight envelope, embodiments of the present disclosure also consider the effect of wind on the airframe.

[055] Figure 4 depicts an embodiment of a swashplate assembly 100 with the rotorcraft travelling in direction 203 and encountering a wind from a different direction 403. In some wind conditions, the swashplate assembly 100 is aligned with the fore/aft axis of the aircraft, meaning the second portion 101b is generally placed at the retreating side of the aircraft, and the first portion 101a is generally placed at the advancing side of the aircraft. However, in certain wind conditions, the maximum airflow over the airframe may not be aligned with the fore/aft axis of the aircraft and therefore the full effect of RBS mitigation may not occur. In this embodiment, the swashplate assembly 100 further includes slew control motors 404, 405 and 406, which are utilised to displace the base assembly 101 in an angular direction about the rotor shaft 202, as indicated by arrows 410. In this example, the base assembly 101 has been rotated in a clockwise direction. Referring to the wind triangle 400, it can be seen that heading, namely the fore/aft axis of the aircraft, is adjusted into wind to maintain track when wind is encountered. To determine the direction of the highest air velocity, namely the direction and speed components of airflow over the aircraft, the effects of incident airflow plus airflow effects from wind must also be taken into consideration. A wind vane or arithmetic calculation is used to sense or determine any angular difference between the fore/aft axis and maximum incident wind. This angle is the slew angle. Slew motors and gears 405 will drive the assembly to the slew angle. In this way, the full effect of altering the pitch of retreating rotor blades can be achieved and efficiency and speed can be maximised to mitigate RBS. [056] Various fixed pitch multi rotor VTOL rotorcrafts are being developed globally. As shown in Figure 5, these rotorcrafts control lift by changing the velocity of the rotor blades 502 using electric motors 501. The blades 502 are set at a fixed pitch adequate to provide sufficient lift throughout the design flight envelope. From an aerodynamic perspective, these blades will also suffer from the same Retreating Blade Stall (RBS) problems.

[057] Accordingly, in a further embodiment and with reference to Figure 6, which shows an enlarged view of rotor blades 602a and 602b and the electric motor 501 , the second portion 101 b is in a substantially fixed relationship with respect to the first portion 101 a. In addition, the first and the second portions 101a and 101 b are arranged at an angle so that they do not reside in the same plane. Although articulation motors 206 could be used to vary the angle of part 101 b as shown at 310, in this embodiment, there is a fixed angular deflection between the first and second portions 101a and 101b, as the retreating blade 602b rotates past the second portion 101b, a relative change in blade angle is induced, which also achieves blade flattening on the retreating side. The change in blade angle is preferably translated to the retreating blade 602b by a simple pushrod 204 which mechanically connects the extension arm 102a to the retreating blade 602b. It is envisaged that in this embodiment the rotor blades 602a and 602b may be required to spin faster to maintain the same lift. These embodiments would require that the rotor blade no longer be fixed and rigid in relation to the two halves 602a and 602b but be free to rotate about the longitudinal axis of the blade as indicated by arrows 610

Advantages

[058] Embodiments of the invention allow the blade angle of a rotor blade to be reduced partially, completely reduced to zero degree AOA, or have negative values. Negative blade angles may be required where twist on a blade may not use zero degree as the base line for the rigging of the blade angle. Negative values may be used to achieve a zero average blade angle resulting from blade twist. By achieving a near zero angle of attack, no lift occurs at that point of blade revolution and consequently the blade is not be stalled. This process minimises or eliminates RBS.

[059] Embodiments of the system and method disclosed herein aim to mitigate retreating blade stall (RBS) and permit rotorcraft operation at higher velocities than conventional rotorcrafts. In the same way, control of the RBS aerodynamic limitation allows the rotorcraft to take off at higher weight as a greater RPM can be applied to the rotor head when operating in the hover. Retreating blade flattening (RBF) will also reduce drag allowing the rotor blades to fly more efficiently as shown in Figure 7.

[060] Figure 7 shows a plot of the lower rotor disk flying toward the left side of the page trimmed at 250 kts. On the left plot there is no RBF, and RBF is engaged on the right plot. The lower half of each plot is the retreating blade hemisphere. Using the gradient scale shown on the right it can be seen that higher drag is indicated by darker shading. The plot demonstrates a measurable drop in drag when RBF is engaged.

[061] The improvement in aircraft flight performance of the invention can be achieved using a coaxial rotor head design. In this way, embodiments of the invention incorporating lift, drag, thrust and weight vectors can be applied equally and oppositely across the two rotor disks and provide effective management of balanced flight throughout the entire flight envelope.

[062] Although the invention has been described with reference to various preferred embodiments, it will be appreciated by persons skilled in the art that the invention may be embodied in many other forms. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the technology as shown in the specific embodiments without departing from the spirit or scope of technology as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

[063] Throughout this specification, unless the context clearly requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[064] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this specification.