Field of the Invention The present invention relates to a system and apparatus for controlling the rotor head of a rotorcraft (eg. helicopter) . The system and apparatus relate particularly though not exclusively to the control of helicopter rotor blade angles by electrical stepper motors, and enable a real-time determination to be made of blade angles by software algorithms.

Background to the Invention Helicopters are unique aircraft in that they can fly forwards, backwards and sideways, as well as being able to hover. However, helicopters are by nature unstable machines and thus require constant pilot attention. There are many interdependent parameters involved in helicopter control, such that changing any one parameter typically requires a simultaneous adjustment to other parameters in order to maintain control of the helicopter. Unlike fixed wing aircraft, which can remain stable even when flown "hands off" for a time, to maintain helicopter stability continuous control adjustments must be made to most controls requiring a high pilot skill level. Helicopter flight modes are controlled, inter alia, by varying the pitch angle of each blade in the rotor assembly through each revolution of a mast (or drive shaft) to which the assembly is mounted. If the pitch angle of each blade remains constant throughout a mast revolution, then a symmetric downwash can be produced such that the helicopter will hover. Increasing the pitch angle of all blades by the same amount increases the downwash, and the helicopter will ascend. Decreasing the pitch angle of all blades by the same amount decreases the downwash and the helicopter will descend. When the pitch angle of each blade is varied throughout a mast revolution, then the downwash will no longer be symmetric. More lift is produced on one side of the rotor disk and less on the other side. Accordingly, the rotor disk tilts resulting in a horizontal lift component, or "thrust", and the helicopter moves horizontally or laterally, away from the area of higher lift and towards the area of lower lift. The pitch angle of each blade in the rotor assembly therefore has two components: a symmetric component which is the same for all blades, and an asymmetric component, whereby the pitch angle of each blade varies throughout the mast revolution. The pilot manages these two pitch angle components by two controls, the so-called "Collective" and "Cyclic" . The Collective controls the constant component and the Cyclic controls the variable component. Increasing the blade pitch by either of these two controls requires increased lift in order to maintain a constant altitude, and therefore engine power must be increased. This is accomplished either by a pilot- controlled engine throttle, typically implemented as a twist grip on the collective control, or in some cases, the collective itself can be coupled to the throttle so that changing the collective automatically induces a corresponding adjustment to engine power. Increasing or decreasing blade pitch and/or engine power affects mast rotation speed, which also needs to be constrained within limits. Mast revolution per minute (RPM) in turn defines the "coning angle" of the rotor system, which results from the balance between the centrifugal force on the spinning rotor and the rotor lift. Moreover, as the engine drives the mast in one direction, there is a tendency for the entire craft to rotate in the opposite direction. This effect is typically countered by a variable pitch tail rotor controlled by pilot foot pedals (so called yaw control) . The four primary pilot control inputs are, therefore, cyclic and collective controls, throttle and yaw. Accordingly, helicopter flight reduces to the control and implementation of these four key parameters - collective blade pitch, cyclic blade pitch, engine power, and anti-torque. To date, the implementation of these four parameters has been of a mechanical nature, with or without power assistance, which when employed is normally provided by hydraulics. By way of note, in fixed wing aircraft, computer technology has been successfully developed over many years to produce λVfly-by-wire" controls. In a fly-by-wire system, pilot input is not mechanically connected to aircraft control surfaces, but instead is treated as commands to an onboard computer, which produces control outputs which in turn move the control surfaces. This technology has matured and is now used in military fighters as well as large commercial aircraft. More recently, computer controlled systems in fixed wing aircraft have aided the development of remotely controlled, and in some cases, autonomously controlled unmanned aerial vehicles (UAVs) . However, to date, the development of rotary wing UAVs has been problematic. In helicopters, the method of controlling blade pitch angle has remained essentially unchanged, employing variations of a swash plate mechanism. The swash plate comprises two sections - a rotating section, and a fixed section. The rotating section is attached by a series of mechanical linkages to each blade and the fixed section is attached to the pilot-operated collective and cyclic controls. The swash plate can be moved vertically up and down the mast, and can tilt in any direction with respect to the aircraft body. When the pilot adjusts the collective control, the entire swash plate moves vertically up or down the mast and, in turn, the pitch of all blades increases or decreases by the same amount. When the pilot adjusts the cyclic control, the swash plate tilts. In this case, the blade pitch angle varies as a function of the angular position of the blade as it rotates around the mast, with respect to the fixed swash plate. This pitch angle function is sinusoidal in nature and its period is one mast revolution. Whilst for a mechanical system this is a natural relationship, since it harmonises accelerative forces and stresses on the components, the mechanical construction of the swash plate precludes periods which are both shorter and longer than a single revolution and so prevents the exploitation of alternative functions. Known swash plate mechanisms for driving blade pitch angle thus produce a simple mechanical periodic sinusoidal function which is the same for each blade in the assembly and the period is exactly one mast revolution. To date, alternative functions have not been exploited. Small helicopters rely on purely mechanical linkages to convey pilot collective and cyclic inputs to the swash plate. Larger machines, which require higher control forces, supplement pilot input with power assistance. Whether power assistance is included or not, the swash plate is essentially mechanical and is therefore subject to mechanical stresses and operational limitations. In addition, because the swash plate is connected to each blade by a mechanical system comprising links, levers, and bearings, the speed that the mast and rotor system can be rotated is limited by the inertia of this system. Typically, helicopter masts rotate this mechanical system at or below about 500 RPM. Factors such as weight, cost, and mechanical complexity of the swash plate system and linkages may sometimes mean that, for small to medium-sized helicopters, the number of blades that can be driven is small. Most light helicopters use only two blades, or sometimes three. Larger commercial and military machines can use a greater number of blades, but at the expense of heavy and complex rotor head systems. Where the number of blades is limited to two or three, and the mast RPM is limited then, in order to provide the necessary lift, the disk diameter swept by the blades needs to be large. This combination of a small number of blades, large rotor disk diameter, and low disk RPM leads to high noise levels both within the vehicle and on the ground and produces a distinctive "chopping" sound of a helicopter in flight. Since the pilot collective and cyclic controls are mechanically connected to the swash plate, which in turn is in direct contact with the rotating assembly, any vibrations or eccentricities in the system feed back directly to the pilot, which present in the cockpit as stick vibration. In summary, because existing helicopter blade pitch control is determined by a mechanical swash plate, which imposes a sinusoidal function on blade pitch angles, with a period of one mast rotation, it has not to date been feasible to evaluate and investigate flight characteristics where the above limitations can be altered or transcended. Definitions The following definitions will be referred to throughout the remainder of the specification.

N denotes a plurality of blades which are attached to the hub. N may be in the range 2,3,... up to 9 or more.

n denotes an individual blade in the range 1..JM

θ denotes the angular position of a blade as it rotates though a revolution about the mast. By convention, when viewed from above, alignment with the longitudinal axis of the helicopter body occurs when the blade is at 0°, and θ has the range 0°...359°. The value of θ may be quantized to divide a complete rotation into 360 discrete steps, commonly called degrees. The division of a complete rotation into 360 steps is, however, completely arbitrary. Thus, in a given implementation, dividing the rotation into more or less steps can be determined by the implementer. In any case, the θ increment chosen does not affect the overall principle of the scheme.

φ denotes the angular pitch of a blade, which may have both positive and negative values, about the blade shaft axis at the blade root, with the convention that 0° is zero pitch. In principle the blade pitch angle φ can be any value, but in practice it will be sufficient to constrain φ, both electrically and by mechanical stops to lie within a range of a negative pitch of -45°, and a positive pitch of +45°.

t denotes the variable time. υ denotes the position vector in 3-space of a helicopter or vehicle to which the rotor head is attached. Commonly, the components of υ are expressed as latitude, longitude and altitude, and may be derived from GPS receivers or other means, υ' denotes the first derivative of υ (instantaneous velocity vector) and υ" denotes the second derivative (instantaneous acceleration vector) .

a denotes the attitude vector of a helicopter or vehicle to which the rotor head is attached. Commonly, the components of a are expressed as degrees of roll, pitch and yaw.

Summary of the Invention In a first aspect the present invention provides a rotor for mounting on a helicopter drive shaft, the rotor comprising a hub for location on the shaft and a plurality of blades mounted to and extending out from the hub, wherein a pitch angle of at least one of the blades is controllable with respect to each other blade by an electrical stepper motor mechanism arranged at the hub. Advantageously, by using an electrical stepper motor, changes to blade pitch angle can be made rapidly and precisely. Furthermore, non-sinusoidal blade pitch angle functions can be achieved for each rotating blade. This can lead to a diversity of helicopter control effects not previously achievable, including faster pitch actuation, the limiting of pitch change to produce a clipped plateau, and even the employment of negative blade angles for part or even all of a mast revolution. In a second aspect the present invention provides a method for determining a pitch angle of at least one blade of a helicopter main rotor, the method comprising the step of determining the pitch angle of the at least one blade according to a mathematical function independently of the pitch angle of each other blade of the rotor. Advantageously, by independently controlling the pitch angle of each blade with respect to each other blade the known mechanical swash plate system can be replaced. In a third aspect the present invention provides a computer program arranged to, when loaded onto a computing system, utilise an algorithm for determining blade pitch angle values for at least one blade of a helicopter main rotor, the values corresponding to angular positions of the at least one blade with respect to a centreline of a body of the helicopter, wherein each value for blade pitch angle is determined before the at least one blade reaches the angular position corresponding to that angle. Advantageously, a set of future blade angles can be determined in order to allow the helicopter to carry out a predetermined manoeuvre or mission. In a fourth aspect the present invention provides an alternator for providing power to at least one motor that controls the pitch of a helicopter blade, wherein the alternator is in-line with and located at a helicopter drive shaft . Advantageously, mechanical transmission of power from an external generator, into the assembly by slip rings or other means is avoided. In a fifth aspect the present invention provides a control method for implementation by a computer in controlling the pitch of at least one blade of a helicopter in real time, the control method comprising receiving at least a first input corresponding to current blade position, feeding the first input into an algorithm and thereby calculating an output being the required pitch of the at least one blade when the blade reaches one or more future positions.

Further Embodiments of the Invention Typically, at least one electrical stepper motor mechanism is provided for controlling the pitch of each blade and/or the pitch of at least one blade is independently controlled with respect to the pitch of the other blades of the rotor. Stepper motors provide precise and responsive control. Usually, a fixed end of the at least one blade is connected to an output shaft of the electrical stepper motor mechanism. Optionally, the stepper motor mechanism includes an internal gearing arrangement to provide a mechanical advantage. Usually, the electrical stepper motor mechanism is mounted to the hub wherein the mounting is adapted to either: - fix the stepper motor mechanism to the hub; or - allow movement of the stepper motor mechanism about the hub generally in a vertical plane with respect to the plane of rotation of the rotor in use. When this is the case, the movement of the stepper motor mechanism may advantageously be restricted in its range by predetermined limits or upper and lower mechanical stops. An end of the drive shaft can be modified to mount the stepper motor mechanism directly thereto. In this case, the term "hub" should be interpreted broadly. For redundancy and load sharing, each stepper motor mechanism may comprise two or more stepper motors arranged such that, should one of the stepper motors fail, the or each other stepper motor can maintain control over blade pitch angle. The two or more stepper motors may be arranged coaxially or radially about the blade root. The rotor may be retrofittable to a typical helicopter flight system.

An electronic drive controller for actuating the at least one stepper motor may be provided and may comprise a respective motor controller for the at least one stepper motor. A computer configured with software to control the electronic drive controller may be provided, the computer being either autonomously or pilot controlled.

With reference to the abovementioned method, the mathematical function may be a non-sinusoidal function and have a period which is either less than or greater than the period of one mast rotation. The pitch angle of the at least one blade is controlled by an electrical stepper motor, wherein the electrical stepper motor is controlled by an electronic drive controller. The electronic drive controller is controlled by a computer configured with software, the software utilising the mathematical function to produce an output which is transmitted to the electronic drive controller, and in turn to the electrical stepper motor.

A computer readable medium incorporating the abovementioned computer program may be provided. With reference to the computer program described above, the computer which implements the program may be an autonomous onboard computer. Optionally, the algorithm may be configured to receive pilot inputs. The algorithm may also be configured to receive inputs including a blade identifier, the angular position of the blade with respect to the centreline of the helicopter body, and environmental inputs. The algorithm may also be configured to receive inputs corresponding to mission objectives.

Utilising the abovementioned method a system may be provided for controlling the pitch angle of each blade n, the system comprising a controller for independently changing the pitch angle φ of each blade n responsive to a measured rotational angular position θ of the blade. Advantageously, a system that comprises a controller for independently changing the pitch angle φ of each blade responsive to a measured rotational angular position θ of the blade can remove the limitations that known mechanical systems impose on the determination of φThe system will enable control of blade angle by feeding control inputs into a software algorithm so that continual real-time calculations of the φ function can be made, and the φ result transmitted to the rotor head in real-time. The system also enables the evaluation of more complex blade movement functions, whereby all blades are not required to have the same pitch profile throughout each mast rotation. Therefore, the output may be non-sinusoidal which is not possible using normal swash plate pitch actuators, and the period of the function may be less than, equal to or greater than one mast rotation. Inputs to the calculation of the φ function, in addition to blade position may include control, environmental and/or mission related inputs. Control inputs originate via pilot manipulation of cyclic and collective controls, whether from within the helicopter, or remotely (eg from a remote ground control station) . Environmental inputs may include υ, υ' , υ" , the attitude vector a, and other parameters such as the average wind vector and air pressure, temperature and density. Mission related inputs may include a generally predetermined flight profile, which describes in advance the desired position and attitude of an aircraft at a certain point in time, expressed functionally as υ(t), and a(t) The software of the abovementioned computer may include the control methods described above. An electronic drive controller may be provided wherein the at least one motor controller selectively activates or deactivates the stepper motors responsive to the control output of the abovementioned computer. The alternator may comprise a static ring and a rotatable ring, the static ring being mounted to the helicopter body, concentric with the helicopter drive shaft and comprising a plurality of magnets, the rotatable ring being concentric with and fixedly mounted to the helicopter drive shaft and comprising a plurality of coiled portions so that, as the rotatable ring rotates within the static ring, an AC electrical current is generated within the coiled portions. The magnet may be an electromagnet . Advantageously, power does not need to be fed through the main helicopter drive shaft using a slip ring arrangement, which are generally prone to wear. The alternator may be used to power each stepper motor mechanism. A power conditioner may be provided between the alternator and stepper motor(s) for converting the AC current to a DC current and conditioning the current. Brief Description of the Drawings. Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a functional block diagram showing how the components of the system interact with each other, together with the overall flow of commands, data and status; Figure 2 shows a sectional view through a rotor head structure of a preferred rotor assembly; Figures 3 a & b show plan and elevation views respectively of a preferred rotor head assembly, being part of the rotor assembly of Figure 2, and comprising a six blade rigid rotor head; Figures 4 a & b show plan and elevation views respectively of a three blade semi-rigid rotor head of another preferred rotor head assembly; Figures 5 a & b show sectional plan and elevation views respectively of an alternator, main bearing and mount points of the rotor assembly, being part of the rotor assembly of Figure 2; Figure 6 shows a mechanism for storing and delivering a continual stream of φ values to the rotor head. It illustrates the manipulation of a double- buffered pair of tables for driving the system in look- ahead mode, and also illustrates indicative timing sequences for loading the tables and clocking the values out to motor controllers; Figure 7 shows a perspective view of an assembled rotor head assembly; and Figure 8 shows an axially exploded view of the rotor head assembly of Figure 7.

Modes for Carrying out the Invention. Prior to describing preferred embodiments of the invention, a general overview of a preferred system in accordance with the invention will be given. In this regard, it will be noted that a helicopter rotor head provides a common mounting point for a number of rotor blades. In accordance with the invention, the pitch angle φ of each blade n is independently computed as a function of the angular position of the blade θ for each revolution of the rotor head in order to attain a required flight mode. This blade angle computation is typically performed in real-time by a computer system mounted in the vehicle body, and the results are transmitted into the rotor head. The computation determines a pitch angle profile for each blade for each revolution of the mast and is used to drive each blade angle independently of the others. Typically, each blade is driven by a direct electrical stepper motor mechanism, having the advantage of no mechanical linkages. Optimally, electrical power for the stepper motors and electronic control systems mounted in the rotating assembly is provided by an alternator which is integrated with the rotor head. Alternatively, electrical power for the stepper motors and electronic control systems mounted in the rotating assembly is generated externally and supplied to the assembly by slip rings or other means.

The preferred system in accordance with the invention can perform two functions: (i) compute the pitch angle φ of each blade n, as it moves through an angular position of θ by taking a wide variety of input variables, including but not limited to the previously mentioned environmental, control and mission related inputs.

(ii) once the required φ value for blade n at angular position θ is calculated , the system physically moves the blade n to the desired φ angle by the time the blade attains rotational angle θ. The total flight behaviour of a helicopter using this system is therefore described by the function φ, which is continually calculated .for each blade n at angle θ, from input variables. For each mast revolution, this φ function is determined, by a software algorithm and is calculated in real-time for each blade, typically by an onboard computer executing those algorithms. Even though this φ function can mimic conventional swash plate blade angle control, there is no requirement to constrain it as such and instead a full range of alternative functions may be used. Advantageously, a separate independent φ profile can be computed for and supplied to each blade, in addition to the simplest case in which all blades have the same profile. Moreover, the φ function is not limited to a simple periodic function of mast angle θ, but can include longer or shorter periods. This ability to: 1. define non-sinusoidal blade angles as a function of θ; 2. support part-rotation or extended multi-rotation periods; and 3. support an independent φ function for each blade; in a multi-blade system allows for complex flight characteristics that have not previously been feasible. Known swash plate mechanisms are a mechanical attempt to implement some of these functions. The swash plate accepts pilot control input and the mechanical design produces the appropriate φ function by physically moving the blades to the desired φ position. However, preferred embodiments of the present invention advantageously: (a) replace the mechanical swash plate system for moving the blades to the desired pitch angle with an electrical stepper motor mechanism, and thereby remove the limitations which the mechanical design imposes on the φ function; (b) replace the mechanical swash plate system for determining blade angle from control inputs with a software algorithm that provides continual real-time calculations of the φ function, and then feeds the φ function in real-time to the rotor head; (c) provide electrical power for the stepper motors and control electronics by an integrated alternator which rotates as part of the rotor head assembly. Referring now to Figure 1 the components of a preferred system according to the invention are depicted using a functional block diagram. In Figure 1 the following reference numerals denote the following components and functions: (a) is a stepper motor which drives the φ function for an attached blade. In Figures 2 to 4 a single separate stepper motor is shown driving each blade, but as described above, multiple stepper motors can be employed for each blade to provide for fail-safe operation. Typically each stepper motor assembly comprises two or more (eg. three) co-axially mounted and interconnected stepper motors that are arranged such that, should one of the stepper motors fail, the or each other stepper motor can maintain operation. ' In principle, each stepper motor can drive its respective blade to any desired angular position in the range 0° to 360° but, in practice, the blade angle is limited to a smaller range by electrical and mechanical stops. (b) is a mounting point for connecting the stepper motor body to the rotor head. As stated above, both rigid and semi-rigid mounting point designs can be employed. In the semi-rigid design, the entire motor and attached blade may rotate vertically about the mounting point, with mechanical stops limiting the total amount of travel. (c) is the blade which is connected directly to the rotating part of the stepper motor. In this way the blade becomes an extension of the main shaft of the motor. Thus, no gears, cams, linkages, or other mechanical devices are required between the stepper motor and the blade. The stepper motor drives the blade directly and is able to position the blade on command to any desired φ value, whilst the entire system rotates on the θ axis. (d) is a θ encoder which measures the current θ angle of blade n, where nominally n=l. (e) is an electronic drive controller for each stepper motor. Thus, controller n drives stepper motor n, which is attached to blade n. The controller accepts the two parameters φ and θ as inputs and its purpose is to move the stepper motor to the desired φ position at the point in time that the blade rotates to the supplied θ angle in the mast revolution. (f) is a bi-directional non-mechanical digital link for feeding control signals and data to/from the vehicle body, into and out of the rotating assembly. Commercially available high-speed digital links such as wireless, or optical coupling may be employed. This link is used to transmit the φ values into the rotating assembly, and to return θ and various status values back from the assembly. (g) is a computing system which is hereafter denoted the φ consumer. Its purpose is to receive φ input via the digital link (f) which has been produced by a φ generator (j), to process and store the data, and to forward it in real-time to the motor controllers (e) . The φ consumer also receives parametric information from the rotating system, and forwards it back through the digital link to the φ generator. (h) is a power conditioner. Its purpose is to provide stablised DC supply to the stepper motor controllers and to other electronics mounted in the rotating assembly. (i) is a mast mounted alternator. Its purpose is to generate raw AC power which is fed into the power conditioner. The mast-mounted alternator consists of a rotor part, containing coil windings, which rotates with the mast, and a stator part, which is attached to the vehicle body. The stator provides a magnetic field through a series of permanent or electro-magnets, in order to induce an electrical current in the mast-mounted rotor, and is described in greater below with reference to Figure 5. Feature (i) avoids the issue of generating power externally in the craft body and then feeding it into the rotating mechanism by mechanical or inductive slip rings or by other means. By integrating the power generation within the rotating structure, the mast-rotor head-blade system becomes autonomously self sufficient and, provided the mast continues to rotate, power will continue to be generated totally within the structure. (j) is the φ generator. It is a computer system typically mounted in the vehicle body, external to the rotating mechanism. Its purpose is to take a wide range of environmental, pilot, mission and other inputs, and to continuously calculate the φ values for delivery into the φ consumer via the digital link (f) . Two modes of providing φ values are supported - a real-time mode in which a single φ(θ,n) value is provided for the next value of θ for each blade n, and a look-ahead mode in which a table of φ(θ,n) values where θ = 0° to 359° is fed to the φ consumer in advance of the mast revolution to which the table applies, for each blade n. As well as driving the rotor head by the φ function, the φ generator provides two other outputs: it computes required engine power which it feeds to the main engine management system, and it computes the required settings of the tail rotor system to provide yaw control. Thus, the φ generator replaces the conventional direct pilot functions of collective and cyclic rotor control, engine control and yaw control. (k) is the main engine, controlled by a conventional digital engine management system. The required power settings are computed in real-time by the φ generator. (1) is an anti torque system, typically a variable pitch tail rotor, which is driven by output from the φ generator. Implementation of the Functional Blocks Stepper Motor The stepper motor assembly typically consists of one, two or three separate motors. In the case of- a three motor system, the assembly is configured so that in normal operation, all three motors are driven in parallel, sharing the total torque requirements. The system is configured such that, in the case of total failure of one of the three motors or its associated controller, the remaining two motors have sufficient torque to maintain full blade control for short periods.

Even though the motor and controller could in principle drive the blade to any φ angle, it is unlikely that this would be required. In practice, limiting the blade angle to a range of -45° to +45° provides sufficient flexibility to explore currently unmapped flight envelopes. For example, negative blade angles for part of the θ cycle produce an upwash which, when combined with a downwash on the other part of the cycle, would lead to very high roll rates, and achieve aircraft behaviour not currently possible. Fine control over the number of steps induced by the stepper motor is desirable. Since both positive and negative changes to blade angle are required, the stepper motor drives in both clockwise and counter clockwise directions. The stepper motor may include an internal gearing mechanism to increase torque at the output shaft.

When the controller sends a command to the motor to drive to a required angle, this command will not be fulfilled immediately. Rather, there will be a certain and predictable slight time lag for the blade to accelerate, move and then decelerate to stop in the commanded position. This inertial effect is desirably- compensated for by the timing of the controller in issuing the command to move the blade. In other words, if a blade has to attain a required φ pitch angle by a certain θ angle, then the controller uses the current φ and θ co-ordinates of the blade to issue the command with sufficient look-ahead so that the target angle will be met within the rotational timeframes. Even though it is driven electrically, the motor and blade assembly remains a mechanical system and is therefore subject to normal forces, associated with accelerating and stopping of an object. The optimal system is therefore not configured such that the blade is commanded to make wild or wide swings of the φ angle in short θ transitions. For example, in the case of but not limited to, a sinusoidal function, the blade will make small continual incremental changes. In practice, even though the stepper motor(s) will drive the blade in a series of incremental steps, the blade will not stop moving and, at the top and bottom of a curve, will decelerate, stop, change direction and accelerate gracefully. Figures 7 and 8 generally depict a rotor head assembly 10 in an assembled and exploded configuration, respectively. The assembly includes blades 16 which extend out from a rotating part of the hub housing 23. Four mounting points 26 are provided and are integral with and disposed around the outside of the peripheral wall of a non-rotating part 28 of the housing 23. Figure 8 shows, in particular, the location of the Phi consumer (g) , motor driver controllers (e) and power conditioner (h) . Six stepper motors 18 are located within the rotor head housing 23. An alternator is also provided concentric with an upper mast portion 32 and comprises coil windings 66 mounted on the rotating part of the housing 23 and permanent magnets 72 mounted on the non- rotating part 28 of the housing 23. Having now described the assembly 10 generally, the respective components of the assembly will now be described in greater detail. Referring now to Figures 2 and 4, where like reference numerals denote similar or like parts, two alternative implementations of a preferred rotor head assembly 10' will now be described, namely: - a rigid head, in which the stepper motors are rigidly mounted within the rotor head, and - a semi-rigid head, in which the stepper motors themselves provide a bearing so that the whole motor and blade assembly pivots about a vertical plane. In both cases, the rotor head is a single integrated unit which consists of three functional segments, mounted vertically on top of each other. Figure 2 shows the rigid rotor head assembly 10' in an assembled format and having, for ease of description, three segments (A) , (B) , and (C) . The top segment (A) provides a blade mounting arrangement and houses in a housing 23 the stepper motors 18, which drive the respective blades 16. Segment (A) is shown in clearer detail in Figure 3. The middle segment (B) houses the power conditioner (h) , the stepper motor controllers and processors (e) , and the φ consumer (g) . Since the entire rotor head assembly 10' can rotate at speeds between 500 and 2000 RPM, all components in this segment are mounted as close as possible to the axis of rotation to minimize centrifugal forces, and are designed and engineered symmetrically to maintain overall rotational balance. The bottom segment (C) comprises the mast-mounted alternator (i) , and includes the load carrying main bearing 24 and the mounting points 26 for mounting the entire assembly 10' to the vehicle body. These mounting points in turn carry the entire load of the vehicle. Segment (C) thus comprises a part which rotates (the rotor of alternator (i) ) and the non-rotating part 28 which is fixed to the vehicle at 26 via bolts 30. Segment (C) is shown in clearer detail in Figure 5. The upper mast portion 32 is hollow 34 to provide a conduit for electrical power leads and cables. The upper mast portion 32 is attached (eg. keyed or splined) to a drive shaft 36 which extends down to the main rotor gearbox of the vehicle. The drive shaft provides rotational drive to the head, but no vertical lifting forces are transmitted through this part. Referring now to Figures 3a&b, where like reference numerals denote similar or like parts, a rigid rotor head configuration 40 for a six blade system will now be described. Each stepper motor 18 is integrated within the rotor head housing 23. Within housing 23, front 42 and rear 43 bearing mounts provide support, dissipation of heat as well as insulation from contamination for each stepper motor. The stepper motor rotor shaft 20 is connected directly to the blade 16 as shown. Electrical drive current for the stepper motors, as well as stepper motor parametric feedback, is transmitted through a hole 45 in the floor of the head to/from the middle segment (B) located immediately below rotor head 40. Referring now to Figures 4a&b, where like reference numerals denote similar or like parts, a semi-rigid rotor head configuration 50 for a three blade system will now be described. The semi-rigid rotor head configuration is adapted to allow each blade to swivel in a vertical plane about a horizontal axis A. In this regard, each stepper motor 18 and respective blade 16 swivels vertically about a rear motor mount pin 53, the pin extending through a bracket 54 formed integrally with the stepper motor 18. The bracket 54 straddles a lug 56, which extends integrally from a modified end of upper mast portion 32', the pin 53 also extending through lug 56. Typically, vertical limits are imposed on the degree of swivel by simple mechanical stops. This arrangement considerably simplifies known hub arrangements. Power to each motor as well as parametric feedback from the motor is provided by a flexible electrical cable 58, which is routed down the hollow 34 in mast portion 32' to the middle segment (B) . Again, the stepper motor is typically sealed within a housing to protect against contamination. In operation, the semi-rigid design allows the rotor disk to attain a natural cone angle. This cone angle is defined by a balance of blade pitch, which tends to increase angle by a reactive force to downwash, and rotor RPM which tends to decrease angle through centrifugal force. For a semi-rigid head, the actual instantaneous cone angle parameter is fed in real-time to the φ generator as one of its inputs. The φ generator has direct control of both blade pitch and rotor RPM, and therefore can determine and adjust an appropriate cone angle for a particular flight mode. In addition, in both the assembly 40 and the assembly 50, up to three co-axial, co-operating stepper sub-motors can be provided for each blade within the housing. The middle segment (B) is attachable to and rotatable with either of the assemblies 40 or 50. The middle segment has sufficient space to mount power conditioners (h) , motor drivers (e) and the φ consumer (g) . The power conditioners (h) take raw variable frequency AC power from the alternator and convert it to stabilized ripple-free smooth DC power. Output from the power conditioners is used to drive the stepper motors via their controllers, and and other head-mounted electronics. Referring now to Figures 5a&b, where like reference numerals denote similar or like parts, the bottom segment (C) will now be described. The bottom segment houses three functions, each of which will be described in turn. Firstly, the segment comprises an alternator (i) , which acts as the power generator for the stepper motors and associated electronic control components. The alternator is mounted to the upper mast portion 32 via an annular plate 60 and further comprises a plurality of fingers 62 radiating out from and spaced evenly around the plate 60. To a free end of each finger a spool 64 is fastened, and the coil windings 66 of conductive wire are wound around each spool . The alternator further comprises a fixed ring 68, which forms part of a mounting plate 70. The mounting plate 70 is attached via bolts 30 to the non-rotating part 28. The series of permanent magnets 72 is mounted within and spaced evenly around the ring 68 to face inwardly and provide a magnetic field for generating current in the moving coil windings 66. As the mast portion 32 and annular plate 60 rotate, the coil windings 66 are moved through the magnetic field so that each winding generates an alternating current AC. The AC is fed by cables 74 into the hollow 34 of mast portion 32, where it is delivered into the power conditioners (h) of middle segment (B) . Because the rotational mast speed is relatively low (between 500 and 2000 RPM) , the diameter of the alternator is quite large, in order to achieve a high linear speed of the coils past the magnets. This diameter in turn determines the overall construction and dimensions of the middle and top segments. The alternator shown in Figure 5 is a permanent magnet design, although electromagnets for providing the field may also be used. The alternator provides AC power for consumption within the head. Raw AC power is conditioned by the middle segment power conditioners for use by stepper motor drivers, associated electronics and processors in the middle segment, and by the stepper motors in the top segment. Thus, providing the mast rotates, the alternator generates power and the entire rotor head is self sufficient in electrical operation. Advantageously, this inline method of generating power completely eliminates the need to generate power in the vehicle body and to then transfer that power into the rotating assembly by mechanical slip rings or other means, which are considered wear-prone and maintenance-intensive items. The self sufficiency of rotor head power also provides a built-in safety mechanism. Even with total loss of engine power, as long as the mast continues to rotate (and assuming that the vehicle mounted control inputs and φ generator can run on battery power) full control over blade angles is maintained in an auto- rotation landing. The bottom segment (C) also comprises the main mast bearing 24 in the form of upper and lower roller bearings 80, and attachment points 70 for bolts 30, for attaching the non-rotating outer casing to the vehicle body. All vertical lifting load is carried through this bearing and the attachment points. Furthermore, because the bottom segment (C) provides a close non-contact interface between rotating and non- rotating parts, this is an appropriate point at which data can be transferred in either direction between the vehicle body and rotating assembly. A bidirectional link is employed to provide this function. All data transfer is digital. The close non-contact interface allows for real-time transmission of the φ data from the vehicle mounted φ generator, into the rotor head, and for the return of current status parameters from the rotor head to the vehicle. Optical or wireless transmission protocols can be employed as appropriate. The φ Generator Turning now to Figure 6, the method of generating and delivering a stream of φ values is now described, and a typical timing sequence for the generation and consumption for data for each mast rotation is supplied. Whereas in a mechanical swash plate design, the swash plate combines the two functions of determining blade angle and driving the blade to the correct angle, in the present embodiments these two functions are separated. The stepper motor controllers and motors are responsible for driving the blade. The function of determining the blade angle falls to a separate system, which is called the φ generator. The φ generator computer system is mounted in the main vehicle body. Its purpose is to accept a set of input parameters which are supplied by a number of external sources and to determine the values of φ to supply to the rotor head, for each blade, for a short time period into the future. At the same time, the φ generator determines and controls engine power, and the amount of tail rotor torque that is required.

Input parameters The φ generator accepts blade number n and its current angular position θ as parameters. Additional parameters may include control, environmental and/or mission related inputs. Control inputs originate via pilot manipulation of cyclic and collective controls, whether from within the helicopter, or remotely (eg from a remote ground control station) . Environmental inputs may include υ, υ' , υ" , the attitude vector a, and other parameters such as the average wind vector and air pressure, temperature and density. Mission related inputs may include a generally predetermined flight profile, which describes in advance the desired position and attitude of an aircraft at a certain point in time, expressed functionally as υ(t) , and a(t) φ generation The detailed algorithms which determine the φ values from these inputs are noted to be dependent on a particular helicopter configuration and the flight characteristics which are intended. In its simplest case, the φ generator is able to emulate the mechanical swash plate if a) the same φ values are calculated for each blade and b) the calculations produce a sinusoidal response with a period of one mast revolution. The overall principles involve two modes of generating φ - a real-time mode and a look-ahead mode. In both cases, an independent and separate set of φ values is calculated and supplied to each blade. This means that individual blades can have different φ profiles throughout each mast revolution. Real-time mode The real-time mode supplies a continual stream of φ values to the rotor head as the rotor spins. For example if the reference blade is currently at position θ, then the φ generator calculates the values of φ for θ + 1, or possibly θ + 3 or θ + 5, or some other short time into the future, consistent with the inertial ability of the motor to drive the blade to the required φ position at the specified θ angle. For example in an N blade system where pitch is constrained to be within +-45° the φ values for all blades N are described by a set of integers in the range -45° to +45°, where integer n represents the calculated angle for blade n at the next θ position. Clearly, in order provide a continual stream of φ values to the head, the generator has to compute the next value for each blade within the time that it takes the mast to perform l/θ of a revolution. Look-ahead mode The look-ahead mode calculates in advance a set of φ values, which can be represented as a table of N columns wide by θ rows deep, where N is the number of blades in the system. This N x θ table specifies the complete set of pitch angles for each blade for the entire upcoming mast revolution. Each column n in the table is the φ profile of a specific blade n - that is -the set of numbers (360 in this example) which specify what the blade pitch angles for that blade must be for each increment of the blade revolution. In order to maintain a continual supply of tables to the head, the φ generator has to compute and deliver the next table within the time of one mast revolution. Receipt of φ by the φ consumer In the real-time mode, the rotor head φ consumer reads each set of N φ values as they are delivered, and immediately forwards these to the motor drivers, as immediate commands to drive the blades. In the look-ahead mode, the φ consumer reads an entire N x θ table at a time. This table is transmitted into the consumer by the generator as serial data and takes a finite amount of time to deliver. There are three separate phases in the life of any particular table. During the life of the current mast rotation, the table must: (1) be calculated by the φ generator and (2) transmitted into the rotating assembly for reception by the φ consumer. (3) during the next rotation, each row of the table is delivered in real-time by the consumer to the stepper motor drivers. This requirement for the generator to calculate and deliver the table during rotation m, for enactment during rotation m+1, at the same time as the consumer is enacting the table for rotation m, leads to a simple double buffer scheme. This double buffering is described as follows: The φ consumer maintains two storage areas within memory: the "current" table, which holds data being supplied to the motor drivers during the current revolution, and the "next" table which holds data to be used during the next revolution. A simple flip/flop register identifies which table is in use. The flip/flop is fired by the reference blade going through the θ=0° position of the mast rotation. During any one mast rotation, the consumer is clocking values out to the motor drivers, one row at a time from the current table, and in the same timeframe is either waiting for data to arrive, or is receiving data and populating rows within the next table. Fail-safe and missed calculations The look ahead mode has a built in fail-safe mechanism which maintains a high degree of stabilized flight in the case that the φ generator fails to deliver one or more tables within the required time of one or more mast rotations. In the case that the φ consumer does not receive a new table within the required timeframe, it simply ignores the flip/flop command and re-uses the last valid table that was received. This fall back typically occurs if the φ generator missed one or even two mast rotations, and would have minimal effect on the vehicle behaviour. Even in the case of total failure of the φ generator, the vehicle maintains some degree of stable flight for several hundreds or even thousands of mast rotations, before a failure recovery mechanism takes over and restores φ delivery. Whilst the invention has been described with reference to a number of preferred embodiments, it will be appreciated that it can be embodied in many other forms.

Example 1 A comparison of an embodiment of the invention to a conventional helicopter design is made. In this regard, the replacement of the conventional mechanical swash plate by a computer controlled stepper motor arrangement allowed a number of existing helicopter design parameters to be adjusted: Number of blades In accordance with the invention much of the complex mechanical linkage is eliminated, and replaced by a stepper motor assembly which is attached directly to the blade root. Thus, the limitation on blade count is lifted. Rotors consisting of multiple blades of up to nine or even higher are feasible. Blade diameter In accordance with the invention more blades means that the same lift can be achieved by smaller blade diameter, which in turn reduces in-craft and on-ground noise. A smaller blade diameter reduces overall inertia and reduces the torque and the time required to move the blade to a desired φ angle. This in turn reduces the physical size and weight of each stepper motor and reduces the power requirements of each motor. Whereas the total weight and power requirements of all motors in the system may not be reduced, the stepper motor reduction in size and weight brings the electrical and mechanical specifications of each stepper motor to within current engineering capabilities. Rotor RPM Embodiments of the invention that utilize smaller blades and eliminate mechanical complexity, can have a rotor that spins more quickly providing the advantage described under the heading '"Maximum Forward Speed" . Cyclic thrust pattern The strictly sinusoidal pattern of the swash plate, with a strictly once-per-mast-revolution period is, in accordance with the invention, replaced by an arbitrary φ function, with the possibility of both longer and shorter periods. Flight controls In accordance with the invention a full "fly by- wire" control system can be achieved. Compared to conventional craft, the smaller rotor disk, higher rotor RPM and lower overall inertia of the rotating system, may actually necessitate computer control, rather than direct pilot input. In addition, pilot controls are not directly connected to any primary surfaces, but instead are fed into the φ generator as one of many input parameters. The φ generator uses these pilot parameters for a certain flight mode, in conjunction with a range of other inputs, to calculate the φ function. Additionally, elimination of the mechanical connection between the flight controls and the rotor system will help eliminate undesirable mechanical feedback (which manifests as control stick vibrations) . Rather than requiring the pilot to make small continual corrections by hand to control each micro-change in flight direction, the pilot controls broad instructions to the φ generator, for example "bank 30°" or "climb at 500 ft per minute" . The φ generator then takes these broad control inputs and combines them in real-time with instantaneous environmental parameters to determine instantaneous blade pitch angles. Blade pitch The mechanical swash plate typically limits blade pitch angle to 0° to 20°. In accordance with the invention there is no limitation at all on pitch angle, including negative angles. Maximum forward speed The maximum forward speed of known helicopters is limited by the well known effect of blade stall of the retreating blade. In accordance with the invention, by allowing the main rotor to spin at substantially higher RPM, increased maximum forward speed of the vehicle automatically follows. Aircraft and ground noise level and vibration In accordance with the invention, noise levels due to the main rotor, both within the craft, and on the ground reduce. In particular, ability to generate highly optimized φ functions independently for each blade, reduces vibrations within the craft.

Example 2 φ profile The φ profile for any blade n is the plot of φ(θ,n) for one or more mast revolutions. It can be drawn as a smooth curve, but of course the actual φ values are quantized outputs of the curve at quantized sampling points of the θ axis. In accordance with the invention, the computer-generated φ profile allows a break from the mechanically constrained profiles of a swash plate. Several representative φ profiles are shown below. These profiles have not been confirmed in flight tests, but are included to show how blade pitch angle settings can be extended beyond current limitations.

Swash plate emulator

50 100 150 200 250 300 350 400 Mastangletheta

Profile 1 - This behaviour emulates a normal mechanical

swash plate

Linear rise/fall

50 100 150 200 250 300 350 400 Mastangletheta

Profile 2 - The blades rise and fall in a linear ramp as

a function of rotational angle. It is not yet apparent

whether profile 2 can be easily implemented as there is a discontinuity in the rate of pitch change at 90° and 270°. hli Bldge p ia ate However, the profile serves to demonstrate that the φ

generator is capable of producing any type of function.

Clipped

C) 50 100 150 200 250 300 350 400 Mastangletheta

Profile 3 - A sinusoidal function that has been clipped.

Flight characteristics of a vehicle using this curve are

yet to be explored. Negative blade angles

Mast angle theta Profile 4 - The φ generator can easily drive blade angles negatively, leading to rapid roll rates of the aircraft. Inverted flight

Mast angle theta Profile 5 - negative blade angles throughout the entire mast rotation lead to extremely rapid descent, or are applicable to inverted flight. Multiphased blades -♦-Blade 1 -■- Blade 2 — —Blade 3 — V- Blade 4 -3K- Blade 5

50 100 150 200 250 300 350 400 Mast angle theta

Profile 6 - Each blade has a 10° phase shift with respect

to the preceding blade This profile cannot be attained in

a conventional swash plate system but is easily produced

by the φ generator. period pictch

Profile 7 - Complex blade control . It is not clear that this profile would provide any sense of controlled flight, but it serves to demonstrate the essential characteristics of the φ generator in providing arbitrarily complex functions for each blade. Each of the three blades has a φ profile which is independent of the others. Blade 1 imposes a higher harmonic on a conventional swash plate emulator to demonstrate that periods of less than one mast rotation are possible. Blade 2 has an extended period, in this case two mast rotations. Blade 3 demonstrates the degenerate case of constant pitch. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.