Rotary blades used in rotorcraft such as helicopters or autogyros, generally comprise blade aerofoils or rotary"wings" of the desired cross-sectional shape, which defines an aerofoil section. They rotate about a central rotor hub. The primary task of these blades is to generate and transmit the aerodynamic lift (thrust) to the rotor or propeller hub, to which they are attached. They may be powered (as in the case of a helicopter) or un-powered (as in an autogyro) or a combination of these (as in the CarterCopter which is being developed in the USA).

In a wind turbine, the blades are used to generate and transmit torque, rather than lift.

In order to control the rotor, it is desirable to be able to change the aerodynamic angle-of-attack or pitch of the blades relative to the hub. This is usually achieved by a pitch control mechanism at the blade root end that can apply a variable angle of attack to the blade In the case of existing helicopter or variable-pitch propeller designs, the blade pitch change is usually facilitated by a "feathering"bearing, positioned between the blade and the hub.

This bearing is usually a roller-thrust bearing, a combined tie- bar/roller bearing assembly or an elastomeric bearing. These bearings or bearing assemblies must be able to rotate freely and smoothly, but must also carry the very high centrifugal loads associated with rotating blades.

In addition, they must be strong enough to transmit the flap (lift) and drag forces and bending moments from the blades to

the hub. Helicopters and autogyros may also require the provision of flapping and drag hinges, and possibly dampers. As a result, rotor assemblies are generally mechanically complex, heavy, expensive and maintenance intensive.

Flexbeam and blade systems with matched stiffness are described in US Patent No 4,332, 525. In that case, the flexbeam is fixed to the blade along the majority of the blade length, so that the length of the torsionally flexible part of the flexbeam is a relatively small proportion of the blade length.

Variable pitch aircraft propellers which use twistable spars and blades which twist the spar for pitch changes are described for example in US Patent No. 6,155, 784. These devices however utilise a bifurcated spar, which separates at the rotor hub and are joined together at the tips in order to achieve the desired torsional flexibility.

The applicants have found a more structurally efficient way of achieving the above requirements.

According to the present invention there is provided a rotary blade comprising a hollow elongate blade envelope having a desired aerodynamic cross section, said blade envelope accommodating a single unitary elongate blade spar which is of an axially strong material having some torsional flexibility and which is fixed to the blade envelope at an outboard region thereof, and rotatable relative to the blade envelope over a substantial portion of the blade length, said blade spar being fixable to a rotor hub.

In this arrangement, all the centrifugal, aerodynamic and inertia loads on the blade are transferred through the spar via the outboard attachment to the rotor hub. When pitch is applied to the blade, the outboard end of the spar, which is fixed to the blade envelope, rotates in pitch together with the blade.

However, the root end of the spar is fixed to the hub, and does not rotate in pitch. Therefore, the spar must twist between its two fixed ends according to the pitch angle applied to the blade. The spar in this region is free to twist relative to the blade envelope, and is torsionally flexible so that the forces required to apply pitch control to the blade envelope are reasonably small.

Suitably the spar is shaped to provide a flexure region at the root end thereof. For instance, an area of reduced section just outboard of the root end of the spar may form a flexure region.

This is suitably shaped to give the spar the desired flapwise and lagwise flexibilities or stiffness for optimum rotor, or propeller dynamic characteristics.

In effect, by using a rotary blade of this type, the flapwise, lagwise and pitch rotation freedoms and/or structural flexibilities required may be incorporated into the blade structure as a"stand-alone"unit. This means that complex built-in hinges or flexures may be eliminated from the rotor hub.

The blade envelope is made of a material, which is relatively torsionally stiff as compared to the blade spar, in order to ensure that the pitch of the blade is transferred from root to tip along its entire length. Typically, the blade envelope is constructed of a fibre reinforced composite material such as glass or carbon fibre reinforced epoxy resin (CFRE), for instance, T-300"m (available from Toray, or Grafil XASm (available from Hysol Grafil). Generally the blade envelope is constructed so that most of the fibres of the material are orientated at + 45 degrees to the longitudinal axis of the blade.

Generally the cross section of the blade envelope will vary along its length, being of a relatively thin aerofoil section over most of its length outboard, corresponding with the

diameter of the spar circular section. The envelope may then change shape to a deeper section near the root end, in order to accommodate the relative twisting of the non-circular, flexure region of the spar in this region.

The spar is of a material which is suitably rigid in the sense that it is resistant to bending, but is relatively flexible in a shearing sense, and hence in torsion.

Examples of suitable materials for use in the construction of the blade spar include highly anisotropic materials such as the composites including unidirectional carbon fibre or glass reinforced resin as described above, but in this case, they will be orientated differently, for instance with the fibres in an axial direction relative to the spar length.

The spar cross-section may also be shaped to assist in the carrying out of the function. In particular, the section of the spar which extends inside the envelope may be of relatively small cross section, for example of circular cross section, having a diameter chosen according to the application and size of the rotor. For use in autogyros or helicopters, the diameter may typically be from 15 to 150mm diameter, and preferably about 26mm diameter. In large wind turbines however, this could be much greater. Model helicopters or autogyros may have much smaller spars for, example of 5mm or less in diameter.

For a given cross-sectional area and overall length, circular section spars are stiffer in torsion compared with previously used flat section spars. However, the spar of the present invention is allowed to twist over a much greater length than previous flat spar designs in order to achieve the required torsional flexibility.

An advantage of circular section spars is that they require only simple, circular support bearings in order to maintain bending

conformity with the blade envelope. By contrast, other cross- sections, e. g. flat sections require more complex support bearings or mechanisms to achieve this function.

The outboard end region of the spar is fixed to the blade envelope, for example by permanent bonding, by being integrally layered in a composite fabrication, or by means of a mechanical joint such as bolts or pins. In the latter case, separation is possible for maintenance or replacement.

As used herein, the term"substantial portion"refers to a portion which constitutes at least 50%, more suitably 60% and preferably 70%, and more preferably over 80% of the total.

The length of the said outboard region of the blade spar, which is fixed to the blade envelope, need only be sufficient to transmit the centrifugal, aerodynamic and inertial loads of the blade to the hub, as mentioned above. This will vary depending upon factors including the type of joint between the spar and the blade envelope, the length of the blade and the materials used in the construction of the spar and the envelope.

Typically, the fixed outboard region comprises up to approximately one quarter of the total length of the blade.

The spar is suitably provided with fixing means at the root end thereof, to allow it to be fixed to a rotor hub. For example, it may be provided with a root end joint, which is fixable to the rotor hub and located at the inboard end or region of the spar. The joint may suitably extend outside the blade envelope so as to facilitate attachment to the rotor hub.

The joint suitably comprises holes, which may accommodate attachment bolts or pins, which may be detached from the hub to release the blades for maintenance or replacement, and/or for blade folding. However, any suitable fixing means or joints may be used.

In a particular embodiment, the joint has an increased cross sectional area as compared with the remainder of the spar. This reinforces the strength of the spar at this section, which may have holes, through it to take attachment bolts. Such a joint may also be fabricated by, for example, wrapping the composite fibres of the spar around attachment lugs.

In addition, as discussed above, an area of reduced section just outboard of the root end of the spar may form a flexure region.

This is suitably shaped to give the spar the desired flapwise and lagwise flexibilities or stiffness for optimum rotor, or propeller dynamic characteristics.

The region of the spar intermediate the fixed outboard region and the root end joint is free to twist relative to the blade envelope. Depending upon the length of the blade, one or more shear support bearings may be provided at intervals along the blade envelope in order to ensure conformity of bending between the blade envelope and the spar. These support bearings may comprise low-friction materials, such as polytetrafluoroethylene (PTFE), or alternatively, low-profile needle-roller bearings, that offer minimal restraint against the rotation of the spar relative to the blade envelope during blade pitch changes.

The pitch of the blade envelope is suitably variable using a pitch control mechanism, such as those known in the art. The blade envelope may therefore be provided with connection means such as a pitch control rod attachment. This will suitably be located at the inboard end of the blade envelope and offset from the spar axis in order to provide a twisting torque to the blade. For instance, a conventional pitch control rod can be attached directly to some point at the root end of the blade offset from the feathering (twisting) axis. The radial position of this point relative to the rotor axis is largely determined by the bending characteristics of the spar/blade envelope

assembly and therefore the choice of this position is somewhat constrained.

Preferably, however, the blade pitch is controlled by means of a torsion member, extensible by means of a guide-rod sliding inside the outer torsion member. The guide-rod at its inboard end is connected to the rotor hub by a spherical joint, whilst the outer torsion member at its outboard end is connected to the blade envelope using a universal joint. The above assembly is designed to accommodate all relative movements between the hub and blade envelope except torsion (twist). The pitch control rod (as known in the art) is connected to a horn near the inboard end of the outer torsion member, offset from the axis of the member, so that control-twisting rotations can be transmitted to the blade envelope via the universal joint. This arrangement allows more choice of position for the pitch control rod and hence also for other components in the rotor control system.

Essentially, it allows the pitch rods to be positioned closer to the rotor hub, resulting in a more compact and lower-drag assembly. In addition, damping can be incorporated in the sliding-function of the torsion member, thus eliminating the requirement for any additional damping mechanisms.

Spar/blade envelope assemblies may be combined together on a hub to form a rotor of any desired number of blades. In particular, three blades may be combined to form a three-bladed rotor.

In an alternative embodiment, a single spar carries two blade envelopes at either end, and thus forms a pair of opposed rotary blades. In this instance, the joint for the hub is provided in a central region of the spar, intermediate the two envelopes.

In this case, both outboard regions of the single blade spar are fixed to the respective blade envelope.

This embodiment is suitable for use in any propellers or rotors which have an even numbers of blades. It is advantageous in

that the large centrifugal forces and bending moments are transmitted directly across the central spar region between the opposed pair of blades. This eliminates the need for strong joints to transmit the large centrifugal and bending loads between the blades and the hub, which simplifies and reduces the cost and weight of the rotor or propeller still further.

Where the rotor has more than two blades, each pair of blades may be axially offset from each other to allow clearance between the spars at the hub centre.

This embodiment may be particularly suitable for use in variable pitch propellers, which may require total feathering, i. e. where it is necessary to align the mean chord of all the blades with the axis of rotation. In this case, a considerable amount of blade rotation could be achieved by rotating the common spar of each pair of blades about a bearing, arranged at the centre of each blade pair. In this way, both blades can be rotated together, in the same direction (for example through up to 90 degrees). These rotations may be supplemented by the twisting of the spar, but the requirement to twist the spar through such large angles is thereby eliminated.

The said central bearing for each blade pair may be compact and lightweight since it would only have to carry blade shear forces (thrust) and negligible blade bending or centrifugal loads. In some cases, it may comprise a simple"dry"plain bearing, lined with low-friction material such as PTFE, or it may comprise a low-profile needle-roller bearing.

It should be noted that a consequence of feathering using this embodiment of the invention is that the leading edge of a blade is approximately aligned with the trailing edge of the opposed blade of the pair. This differs from a conventional feathered propeller, where all the blade leading edges are aligned together. However, as the aim of feathering is simply to stop

rotation and reduce, drag of a propeller or rotor, this is not believed to be disadvantageous.

Blades of the invention may be applied to rotorcraft, such as autogyros or helicopters, as well as variable pitch propellers such as constant speed propellers, or ground-based wind turbines. Such devices form a further aspect of the invention.

In particular however, the blades of the invention are applied to autogyros.

Using the rotary blades of the invention is advantageous in that, as described above, it eliminates the need for a complex rotor hub. This results is a relatively simple overall rotor compared with existing designs, which has fewer parts and may be manufactured more rapidly, at lower cost. The complete rotor blade may therefore be structurally very efficient in terms of strength to weight ratio.

As a result of the simplicity and the lack of bulky joints and bearings, the rotary blades of the invention may produce less drag. They are therefore aerodynamically highly efficient and produce improved performance.

The reduced number of joints and the use of fibre reinforced resin composite materials results in a better reliability, durability and fatigue life and therefore low maintenance and life-cycle costs.

The invention will now be particularly described by way of example with reference to the accompanying drawings in which: Figure 1 is an edge view of a rotary blade of the invention ; Figure 2 is a plan view of the blade of Figure 1; Figure 3 is an enlarged view of an end portion of Figure 1,

Figure 4 is an enlarged view of an end portion of Figure 2, Figure 5 is a section on line W-W of Figure 2 ; Figure 6 is a section on line X-X of Figure 2; Figure 7 is a section on line Y-Y of Figure 2 ; Figure 8 is a section on line Z-Z of Figure 2, Figure 9 is a diagram showing an autogyro comprising a rotary blade of the invention, Figure 10 is a graph showing control power as a function of thrust level and flapping hinge offset in relation to a range of rotorcraft; Figure 11 is a downward view of an alternative form of an autogyro rotor, including the invention, in which the pitch links are provided above the rotor; Figure 12 is a schematic section through the embodiment of Figure 11; and Figure 13 illustrates a three-bladed rotor incorporating the invention.

The rotary blade of Figure 1 comprises an elongate blade spar (1) of circular cross-section throughout most of its length, and made of a unidirectional carbon composite material. The spar (1) is located inside a hollow torsionally stiff blade envelope (2). The blade spar (1) extends outside the envelope (2) at one end. The cross-section of the spar (1) at this end is different, in that it includes a thinner section, which is

approximately rectangular, in order to form a flapping flexure region (3).

The inboard end region (8) of the spar (1) is also rectangular in section but thickened (Figure 5) in order to reinforce attachment bolts holes (9) (Figure 3), which are provided in this region.

Attachment bolts (4) (Figures 2) are provided at this end of the spar (1) in order to attach the blade to a rotor hub (10) of an autogyro (11) (Figure 9).

The blade spar (1) is fixed relative to the blade envelope (2) in the region shown as B in Figure 2. It is bonded to the structure at this point, although other forms of fixing such as bolts or pins may be used to achieve this. In the illustrated case, the length of the blade envelope is approximately 4 metres, and the region designated B is approximately 1 metre in length.

Along the remainder of the envelope (2), in the region marked A, the spar (1) is free to rotate with respect to the blade envelope (2). A series of intermediate support bearings (5) are provided in order to support the spar (1) relative to the envelope (2) but the spar (1) is rotatable within these. An elastomeric lag damper (6) may be included in these supports, in particular in the most inboard support. It is suitably arranged between a first plate (12) attached to the blade envelope (2) but provided with an elongated hole to accommodate lag motion and twisting rotation of the spar (1), and a second plate (13) which is attached to the spar such that twisting rotation only of the spar is allowed.

The blade envelope (2) is shaped as an aerofoil with a relatively thicker leading edge and a thinner trailing edge (see Figures 6-8). It may become narrower towards the outboard region

(Figure 1). In the outboard blade section, the blade envelope (2) narrows so that it may contact the sides of the spar (1) (Figure 8) and is fixed thereto. The cross section of the spar (1) in the flexure region (3) is broadly rectangular (Figure 5) in order to provide the flexure function.

A pitch control rod attachment (7) is attached to the blade envelope (2) at the inboard end at the trailing edge. This can be connected to a pitch control mechanism to allow the pitch of the blade to be changed as required, and under the control of the pilot.

When this is activated in order to change the pitch of the blade, the blade envelope (2) is rotated through the desired angle. The rotation is translated directly to the outboard region of the blade envelope (2) as a result of the rigidity of the structure of the envelope (2). The resultant rotational motion is transferred to the spar (1) in the region B. This results in the twisting of the spar (1) within the blade envelope in the region (A). In this way, the need for conventional feathering bearings is eliminated.

These embodiments provide simple and aerodynamically efficient rotary blades.

Figure 10 indicates the properties of various blades including the blade of the invention when applied to autogyros.

It is a graph of control power, i. e. effectiveness of the rotor to control the aircraft, against normal acceleration, or"g" loading. The horizontal axis represents the part of the"flight envelope"loading on the aircraft between 0 and lg. (For a typical private aircraft, the total envelope is usually required to range between-lg and +3g, with lg representing normal level flight).

A range of lines 1 to 3 characterizing some existing rotorcraft is shown. Line 1 is for a state of the art high performance helicopter. It can be clearly seen that this rotor provides relatively large control power between 0 and lg loading and indeed outside this range as well. However, it has a drawback in that it requires an automatic flight control system at high speed because of perhaps a slightly too-large"hinge-offset"and hence too-great a control power.

Line 2 represents an earlier design helicopter with an articulated rotor, which has been found to be adequate and safe, but cannot be described as very responsive or agile.

Line 3 represents the majority of existing autogyros which have a two-bladed teetering rotor. By comparison, these have much reduced control power below lg flight and indeed have none at all in zero or negative g conditions. Thus it has been found that flying these autogyros in this part of the envelope is dangerous and should be avoided. However, considering the need to manoeuvre an aircraft and fly through turbulence etc. low or zero g conditions are often encountered in practice and indeed, a disproportionately large number of autogyros of this type have had"unexplainable accidents"with fatal results.

The invention proposes to rectify this situation for autogyros by fitting a rotor of optimized"hinge-offset"and therefore control power, thus eliminating any dangerous areas of the flight envelope. This rotor shown in Line 4, will have a slightly lower hinge offset than that in line 1. This will impart excellent handling qualities throughout the flight envelope, whilst not requiring any automatic stabilization.

Alternative rotors including the invention are illustrated in Figures 11 and 12 and Figure 13. Figures 11 and 12 show a four- blade rotor, each blade comprising a blade of the invention with a spar (1) and blade envelope (2). Each spar (1) is attached

to a rotor hub (10) by way of attachment bolts (4). In this case however, a combination unit comprising a combined control torsion-member (12) and lag damper (11) is provided and attached to the rotor hub (10) by a spherical joint at one end and to the blade envelope (2) at the other end by way of universal joint (13). The universal joints are fixed just inside each blade envelope (2) to reduce drag.

Blade pitch control rods (14) are attached by spherical joints to horns on the torsion members (12). In order to apply the control cyclic and collective pitch to the blades, the pitch rods (14) are attached, again by spherical joints, to a control "spider"mechanism (15) arranged, in this view, above the rotor hub (10) (See Figure 12). An alternative arrangement would be to have the control-spider (15) below the rotor blades (1,2), with the spider arms protruding through apertures in the rotor shaft (16) (see Figure 12).

Yet another control arrangement would be to use a conventional helicopter swashplate like those known in the art (not shown), instead of using the spider mechanism (15, shown). The former arrangement would enable the use of a rotor shaft (16) of much smaller diameter and possibly of solid cross-section.

An alternative, three-bladed rotor is illustrated in Figure 13.

Again, each spar (1) carrying a blade envelope (2) is attached to the rotor hub (10).

NB. A possible position for the spar inboard support bearing (17) for each blade, is shown in this view.