WO2001098745A1

WO2001098745A1 - Adaptive balancing arrangement for a rotating mass

Info

Publication number: WO2001098745A1
Application number: PCT/GB2000/002367
Authority: WO
Inventors: Colin Frazer Ross; Ian Stothers
Original assignee: Ultra Electronics Ltd
Current assignee: Ultra Electronics Ltd
Priority date: 2000-06-19
Filing date: 2000-06-19
Publication date: 2001-12-27
Anticipated expiration: 2002-12-19
Also published as: EP1292811A1; AU2000255480A1; JP2005502026A; CA2412809A1

Abstract

A method of adaptively balancing a rotating mass comprises using a mass arranged in at least one circumferential region about the axis of rotation. Vibrations generated by the rotating mass are detected and used to generate control signals. The control signals are used to redistribute the mass circumferentially using actuator means to reduce the detected vibrations.

Description

ADAPTIVE BALANCING ARRANGEMENT FOR A ROTATING MASS

The present invention generally relates to an adaptive balancing arrangement for correcting for the imbalance of a mass rotating about an axis.

When a mass rotates about an axis, if the center of mass is not coincident with the axis, significant forces are exerted which can cause structural vibrations in the support structure of the rotating mass. The rotating mass can comprise a rotor such as a drive shaft and any imbalance, in addition to generating undesirable noises, can result in fatigue in the rotating machinery or in any structure to which the rotor is coupled.

Conventionally rotors are balanced during their manufacture. However, during use rotors can become unbalanced due to for example wear. Such an unbalanced rotor must be rebalanced in order to prevent structural vibrations and possible fatigue in a structure. Rebalancing can be carried out manually but is time consuming and costly.

Structures for automatically balancing rotors have been developed in the prior art. Passive means have been developed such as in US patent No. 4075909, 3696688 and 5860865 wherein weights are provided in an annulus about the axis of rotation which automatically reposition themselves to counteract imbalance in the rotation of the rotor. Such techniques however do not provide for any adaptive control. In the techniques disclosed in US 4075909 and US 3696688 high precision manufacturing is required to provide any level of accurate balancing. Even so, the effects of temperature on differential thermal expansion will cause the response of the balancer to vary. Thus they cannot be assured of maintaining good balance through all rotational speeds and all environmental conditions. In the arrangement of US 5860865, a pressure driven mechanism moves the masses circumferentially over limited ranges. This mechanism is however complex with many moving parts. Further, the mechanism needs to be placed in a position to sense the out of balance through flexure. This therefore limits the positioning of the arrangement. The position of monitoring of out of balance and the position of correction of the out of balance cannot be decoupled in this arrangement.

US patent No. 5197010 discloses an arrangement for continuously and actively balancing rotors by controlling the distribution of a liquid mass in the rotor. Such a system for controlling mass distribution in the rotor is however complex requiring a supply of hydraulic pressure.

US patent no. 3769854 also discloses an active balancing arrangement in which imbalance is detected and masses are moved radially to minimise the imbalance. The use of radially movable masses however increases the size of the arrangement. Also strong actuators are required to act against centrifugal force. Further, if an actuator were to fail, there is a possibility that the mass being actuated will lie greatly out of balance.

It is an object of the present invention to provide an improved arrangement for actively balancing a mass rotating about an axis.

In accordance with one aspect of the present invention a mass rotating about an axis is balanced by arranging mass in at least one circumferential region about the axis and actively redistributing the mass circumferentially to reduce vibrations generated by the rotating mass.

In accordance with the present invention, there is no movement of mass in a radial direction: only in a circumferential direction. Further, no mass is added to or removed from the arrangement: the mass is simply redistributed about one or more circumferential regions. The redistribution is carried out on an active basis in dependence upon detected vibrations. Thus there is some intelligent control of the way in which the mass is redistributed in order to reduce the vibrations generated by the rotating mass and the detection need not take place at the same position as the mass arrangement for reducing vibrations. Because there is no radial movement in order to rebalance the rotating mass, the work which is required in order to move the mass is minimised because the mass is never moved against the centrifugal force experienced by the mass during rotation. At high rotation speeds, the centrifugal force can be extremely high.

An advantage of the present invention is that since mass may be moved all the way from one side of the centre of rotation to the other, the amount of mass required is less than that required by radially varying mass balancers of the same diameter.

A further advantage of the present invention is that because the mass is only redistributed circumferentially, in the event of a failure of the actuator means for redistributing the mass, the effect on the system is limited. Thus the system "fails safe".

The mass redistribution can take place using any form of actuation. In one embodiment the actuation is achieved hydraulically or pneumatically. The hydraulic or pneumatic redistribution can be of a solid or of a liquid.

The mass can alternatively comprise a combination of two materials of different densities and the redistribution of the mass comprises differentially moving the two materials circumferentially. The two materials can comprise two immiscible fluids of different densities. Alternatively the two materials can comprise a liquid carrying an ionic salt such as a mercury salt and the redistribution is achieved by moving the salt in the liquid under the influence of an electric field.

In another embodiment of the present invention, the redistribution is achieved mechanically. The mass can comprise a plurality of mass members arranged around one or more circumferential regions and these mass members are relatively moved to redistribute the mass. In one convenient arrangement, the mass members are arranged on one or more circumferential tracks for relative movement. In another embodiment, the mass members are arranged on arms which are relatively pivotable about the axis of rotation of the rotating mass.

In one embodiment of the present invention, since the detection of vibrations are carried out on a static structure on which the rotating mass is mounted, control signals generated by a controller to control the redistribution of the mass to rebalance the rotating mass are comutated across to the rotating mass. An inductive transmission means or slip rings can be used. When the rotating mass has one or more electrically powered devices on it which require power and control, the control signals for balancing can conveniently be communicated in combination with the control signals for the electrically powered devices. Thus in this embodiment of the present invention, the control signals can be communicated using the technique disclosed in UK patent No. 2293522 in the name of the current proprietors. The content of UK patent No. 2293522 is hereby incorporated by reference.

The present invention is applicable to any rotating mass and has particular application to the balancing of an aircraft propeller shaft on which a propeller is mounted. Imbalances in rotating propellers of propeller aircraft and turbo props can cause significant vibrations or noise in the aircraft cabin. In order to reduce these vibrations within the cabin, vibrations or sound within the cabin can be detected and correlated to the rotation of the shaft using a tachometer and these detections can be used to try to reduce the vibrations experienced within the cabin by rebalancing the shaft. Alternatively, the vibrations or sound can be detected at the engine and used to control the adaptive balancing of the shaft.

The present invention is applicable to the detection of any form of acoustic vibration whether it is solid borne or airborne vibration e.g. mechanical vibrations or sound.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a fluid balancing arrangement of a first embodiment of the present invention,

Figure 2 is a schematic diagram of a fluid balancing arrangement of a second embodiment of the present invention,

Figure 3 is a schematic diagram of a fluid driven mass redistribution arrangement of a third embodiment of the present invention,

Figure 4a is a schematic plan view of a heavy salt mass redistribution arrangement of a fourth embodiment of the present invention,

Figure 4b is a cross-section through the arrangement of Figure 4a,

Figure 5 is a schematic diagram of a heavy salt redistribution system of a fifth embodiment of the present invention,

Figure 6a is a plan view of a mechanical weight redistribution arrangement in which balance weights move round a linear track in accordance with a sixth embodiment of the present invention,

Figure 6b is a partial cross-sectional view through the arrangement of Figure 6a,

Figure 7a is a plan view of a mechanical redistribution arrangement in which balance weights are arranged on arms in accordance with the seventh embodiment of the present invention,

Figure 7b is a side view of the arrangement of Figure 7a,

Figure 8 is a schematic diagram of a complete propeller shaft balancing system in accordance with an embodiment of the present invention, Figure 9 is a diagram illustrating the placement of vibration detectors in accordance with an embodiment of the present invention,

Figure 10 is schematic diagram illustrating the placement of pick-up coils for the transmission of control signals to actuators on the rotating member in accordance with an embodiment of the present invention, and

Figure 11 is a schematic diagram of a transmission system used for controlling and powering electrical devices and for controlling the adaptive balancing in accordance with an embodiment of the present invention.

Referring now to figure 1 , in a first embodiment a hub 1 is provided for attachment to a rotating shaft 2. The hub 1 carries four fluid reservoirs 3 in substantially diametrically opposed positions near the circumference of the hub 1. Each fluid reservoir 3 is interconnected with an adjacent fluid reservoir 3 via two conduits 4a and 4b. Conduits 4a and 4b allow the transmission of fluid in opposite directions between the fluid reservoirs 3. Each conduit 4a and 4b allows for the removal of fluid from an outer radial portion of a fluid reservoir 3 to an inner radial portion of an adjacent reservoir 3. This arrangement is provided since when the hub 1 is rotating at high speeds, the centrifugal force will tend to force the fluid in the reservoirs 3 outwards. Pumps 5 provided in the fluid reservoirs 3 at the end of the conduits 4a and 4b are thus immersed in fluid. Non- return valves are also arranged but not shown in conjunction with the pumps to ensure that the conduits 4a and 4b only exchange fluid between the fluid reservoirs 3 when the pumps are controlled to do so.

Thus in this embodiment of the present invention, the eight pumps provided in the fluid reservoirs 3 at the outlets are controlled to redistribute the fluid between the fluid reservoirs 3. Thus the fluid system comprises a closed system which does not require any external pressure source. In order to correct for imbalance in the shaft, the center of mass of the hub 1 is adjusted simply by redistributing the mass of fluid around the hub circumference in the fluid reservoirs 3.

In this embodiment, the fluid reservoirs can be evacuated and provided with a limited amount of liquid. Thus, when liquid is exchanged from one reservoir 3 to another, there is no displacement of any other fluid in the receiving reservoir.

In an alternative arrangement, the fluid reservoirs can be filled with two immiscible fluids e.g. oil and an inert gas. The oil has a higher density than the inert gas and this will therefore tend to reside in the outer radial portions of the reservoirs 3 during rotation of the hub 1. Thus the oil will be pumped along the conduits 4a and 4b. The receiving reservoir 3 will thus receive fluid leading to the pressurisation of the gas in the reservoir. This will act against the pump but as along as the relative displacement is small, the pressure will not rise significantly.

In this arrangement at least three evenly distributed reservoirs can be used which need not be arranged diametrically opposed.

Figure 2 illustrates a second embodiment which is an alternative arrangement to the arrangement of Figure 1 in which less pumps and valves are required. In this embodiment the hub 1 is arranged for mounting on a shaft 2. As in the embodiment of Figure 1, four fluid reservoirs 6 are arranged in substantially diametrically opposed positions in a circumferential region of the hub 1. Each fluid reservoir 6 has a conduit 7 to a diametrically opposed fluid reservoir 6. In this way fluid can be exchanged circumferentially only between diametrically opposed fluid reservoirs 6. This reduces the number of pumps 8 and non-return valves required. In the embodiment of Figure 2 only four valves and pumps are required: one for each fluid reservoir.

In this arrangement any even number of reservoirs can be used which are substantially diametrically opposed. Figure 3 is a schematic illustration of a third embodiment of the present invention in which a plurality of mass members 13 are arranged in a respective plurality of fluid chambers 12 which are distributed around a circumferential region of a hub 10 for mounting on a shaft 11. Each chamber 12 contains a mass member 13 in the form of a piston. Either side of the piston in the chamber 12 is filled with fluid. The fluid either side of the piston 13 can be exchanged via a fluid transfer 14. A pump (not shown) enables the fluid pressure on either side of the piston to be increased in order to displace the piston 13. In this way the pistons 13 of each chamber 12 can be controllably positioned around the circumferential portion of the hub 10. This provides for the redistribution of the mass around the hub 10.

In this embodiment, the fluid used in the chamber 12 can comprise any simple fluid such as a gas or a fluid e.g. oil.

In this arrangement at least three mass members 13 and chambers 12 can be used which are substantially evenly distributed circumferentially.

Figures 4a and 4b schematically illustrate a fourth embodiment of the present invention which uses an electric field to redistribute heavy salt ions in a liquid.

In this embodiment a hub 20 is provided for attachment to a shaft 23. In an outer circumferential region there is provided a channel 21 containing a liquid e.g. water carrying mercury salt. At four equally spaced positions around the channel 21 there are provided four porous plugs 22 in order to provide a resistance to movement of the mercury salt. At the outer circumference of the hub 20 there are provided four electrodes 24 at circumferential positions intermediate the porous plugs 22.

In order to redistribute the mass around the circumference, voltages are selectively applied to the electrodes 24. The electrodes generate respective fields within the channel 21 thus attracting the mercury salt correspondingly. Thus redistribution of the mercury salt within the channel 21 causes the redistribution of mass circumferentially. In this arrangement, at least three electrodes can be used which are substantially evenly distributed circumferentially.

Figure 5 illustrates a fifth embodiment of the present invention similar to the fourth embodiment of the present invention except that eight electrodes 34 are provided equally spaced around the circumference of the hub 30 for mounting on a shaft 31. The channel 32 contains a liquid carrying mercury salt as in the fourth embodiment of the present invention. However, in this embodiment the porous plugs are far more extensive along the channel 32 to provide restrictive regions 33 adjacent the electrodes 34 to which the mercury salt is attracted when a suitable voltage is applied.

In this arrangement, at least three electrodes can be used which are substantially evenly distributed circumferentially.

Figures 6a and 6b illustrate a sixth embodiment of the present invention in which mass distribution takes place by a mechanical means.

A hub 40 is provided for mounting on a shaft 41. Mounted on the hub 40 is a curved linear track 42. Mounted on the curved linear track 42 are balance weights 43a and 43b. The curved linear track 42 comprises a linear motor suitable for independently moving the balance weights 43a and 43b. Thus, when suitable control signals are received, the linear motor forming the curved linear track 42 can independently move the balance weights 43a and 43b circumferentially to redistribute the mass. Although two balance weights are shown in this embodiment more could be used.

Figures 7a and 7b schematically illustrate a seventh embodiment of the present invention in which a mass is circumferentially redistributed mechanically.

In this embodiment of the present invention, balance weights 53a and 53b are provided on respective arms 51 and 50 provided for independent pivoting about the shaft 52. The arms 51 and 50 can be rotated relative to one another and relative to the shaft. This is achieved by providing a hub 56 fixed to the shaft 52 and to rotary actuators 54 and 55.

It can thus be seen that in this embodiment, simply by relatively rotating the arms using the actuators 54 and 55, the balance weights 53a and 53b can be relatively moved thus causing the relative redistribution of the mass around a circumferential region about the shaft 52.

Figure 8 illustrates a system for adaptively balancing a propeller shaft mounted on the air frame of an aircraft. In this embodiment the propeller shaft 61 has a propeller 60 mounted thereon and on either side of the propeller 60 there is mounted a balance mechanism 62 and 63 as illustrated in more detail in figures 7a and 7b. Although in this embodiment two balance mechanisms 62 and 63 are provided, propeller shaft imbalance can be corrected for using only one. Use of two however, avoids torsional forces being experienced by the shaft when imbalance in the propeller 60 is corrected by the balance mechanism 62 or 63 at some position along the shaft 61 or where 2 plane balancing is required.

A controller 65 is provided in the aircraft to generate control signals to each of the balance mechanisms 62 and 63. One or more detectors 66 are provided on the air frame of the aircraft to detect vibrations such as mechanical vibrations or noise. Also, a tachometer 64 is provided to provide the controller 65 with rotational speed information. This rotational speed information can be used to correlate vibrations detected by the or each detector 66 in order to determine which vibrations have originated from imbalance in the propeller shaft 61.

As can be seen in Figure 9, detectors 71a-71f can be arranged at many different locations on the air frame of an aircraft 70. The detective vibrations can be vibrations generated by either engine of the aircraft. The tachometer reading however enables the correlation of the detected vibrations to the respective engine. In the embodiments of the present invention described herein above, since the mass is redistributed circumferentially, if any of the actuators fail, they will fail in a safe position: although the rotor will be unbalanced.

The control carried out on the actuators can operate an "out of control authority" mechanism. If the controller determines that it is necessary to operate an actuator in order to try to balance the rotor, and no effect is detected in response to the control, this can indicate either that the actuator is malfunctioning, or it is unable to perform any further actuating e.g. it has reached its limit of actuation. If this is detected, a warning can be generated to allow an operator to check whether the actuator has failed or to manually add a mass onto the rotor to counteract the large imbalance. If a mass is added to the rotor, this will enable the actuator to operate within its range during rotation of the rotor. Thus rebalancing can take place automatically and actively once the large imbalance is manually corrected.

The out of control authority warning can indicate to an operator a possible fault with the rotor e.g. in an aircraft, there may be damage to a propeller blade. The warning thus enables an operator to investigate why the automatic active rebalancing cannot cope with the large imbalance.

As illustrated in Figures 8 and 9, the detectors and controller are positioned on the air frame and not on the rotor. Thus control signals must be transmitted from the static air frame to the rotating rotor.

Figure 10 illustrates one technique for doing this wherein control signals are inductively transmitted via a coil or magnet 82 to a receiving coil or magnet 81 on the rotor 80. A number of receiving coils can be arranged around the circumference of the rotor 80 in order to pick up not only the control signals but also power necessary for driving the actuators. By phasing the power or signals applied to coil 82 selective coupling to different coils 81 can be provided. In another embodiment, the system illustrated in UK patent No. 2293522 can be used for transmitting power and control to the actuators.

Figure 11 illustrates schematically the arrangement disclosed in GB 2293522 for controlling the application of power to heaters on propeller blades. The same system can be used for transmitting power to the rebalancing actuators. In a preferred embodiment, the two systems can be combined.

In Figure 11 an array of permanent magnets 90 are arranged on the air frame. An auxiliary three phase winding 91 on the rotor inductively generates power to pass through switching units 92 to respective blades. In order to control the power supplied to the blades, a computer 94 generates control signals which are passed through a coil 95 on the magnets 90. The high frequency signal generated by the coil 95 is inductively picked up by a main winding 93. The main winding 93 also inductively generates power for the switching units 92. The inductively picked up signals are passed from the main winding 93 to control the switching units 92 to thereby control the supply of power to the blades.

Thus, in an aircraft already provided with a de-icing capability as disclosed in UK patent No. 2293522, the present invention can be combined to provide automatic and adaptive rebalancing of the propeller shaft. The same computer can be used and different windings can be used to generate the power for the actuators and to pick up control signals for the actuators.

Although the present invention has been described herein and above with reference to specific embodiments, it would be apparent to a skilled person in the art that the present invention is not limited to such embodiments and modifications will be apparent to a skilled person in the art.

Although in the embodiments the masses are redistributed about only one circumferential region, the present invention is applicable to the redistribution about more than one circumferential region. For example, in the embodiment of Figure 6a and 6b, the balance weights can be arranged in parallel tracks or in the embodiment of Figures 7a and 7b, the balance weights can be arranged on different length arms. Where the weights are arranged at different radii, the weight arranged at a shorter radius needs to be heavier than the weight arranged at a larger radius in order to enable the mass moment of the balance weights to be cancelled. Thus the present invention encompasses the circumferential movement of weights where there is no radial change.

The present invention encompasses the use of any number of mass positions around a circumferential region. For example, in the embodiments of Figures 4a, 4b and 5, the mass redistribution can be gradual and not a movement of discrete masses as in the mechanical embodiments.

The present invention provides a sealed system in which the mass is kept constant on the rotor: the mass is just redistributed. Because the mass is fixed, there is a control limit forced upon the control system thus making the system fail safe.

In mechanical embodiments of the present invention, instead of considering moving masses around a circumferential region, in accordance with the present invention, a disk which is centered on the axis and has holes at circumferential regions can instead be rotated. For example, in the embodiment of Figures 7a and 7b, instead of the balance weights 53a and 53b residing on arms, the two arms can be replaced by independently rotatable disks having holes therein. The relative rotation of the holes will cause the circumferential redistribution of mass.

The adaptive rebalancing system of the present invention can be mounted on the rotating mass (rotor) or in mechanical series with it connected by a shaft.

Claims

CLAIMS:

1. A method of adaptively balancing a first mass rotating about an axis using second mass arranged in at least one circumferential region about said axis, the method comprising: detecting vibrations generated by the first mass; generating control signals using the detected vibrations; and redistributing said second mass circumferentially using actuator means controlled by said control signals to reduce the detected vibrations.

2. A method as claimed in claim 1, wherein the redistribution step comprises hydraulically or pneumatically redistributing said second mass.

3. A method according to claim 2, wherein said second mass is liquid and said redistribution step comprises hydraulically redistributing said liquid circumferentially.

4. A method according to claim 2, wherein said second mass is a solid and said redistribution step comprises hydraulically or pneumatically moving said solid circumferentially.

5. A method according to claim 1, wherein said second mass comprises a combination of two materials of different densities and the redistribution step comprises differentially moving the two materials circumferentially.

6. A method according to claim 5, wherein the two materials comprise two immiscible fluids of different densities and the redistribution step comprises differentially moving said fluids circumferentially.

7. A method according to claim 5, wherein the two materials comprise a liquid carrying a salt and the redistribution step comprises differentially moving the salt in the liquid using said actuator means comprising circumferentially spaced electrodes.

8. A method according to claim 1, wherein the redistribution step comprises mechanically redistributing said second mass.

9. A method according to claim 8, wherein said second mass comprises a plurality of mass members arranged around said at least one circumferential region and the redistribution step comprises relatively moving said mass members circumferentially.

10. A method according to claim 9, wherein said mass members are mounted on at least one circumferential track and are relatively moved around said track.

11. A method according to claim 9, wherein said mass members are mounted on respective arms pivotable about said axis and the redistribution step comprises relatively pivoting said arms about said axis.

12. A method according to any preceding claim, wherein said first mass carries inductive windings, said first mass is mounted on a structure having magnets for cooperation with said inductive windings on said first mass to generate power for said actuator means, and said control signal is transmitted over a contactless inductive link formed between said inductive windings and said magnets to said actuator means.

13. A method of reducing vibrations generated by an aircraft propeller using the method of any preceding claim, wherein said first mass comprises a propeller shaft.

14. A method according to claim 13, wherein the detecting step comprises detecting vibrations in an aircraft cabin to reduce vibrations in said cabin.

15. A method according to claim 13, wherein the detecting step comprises detecting vibrations on a housing of said propeller shaft.

16. A structure for adaptively balancing a first mass rotating about an axis of rotation, the structure comprising: second mass arranged in at least one circumferential region about a central axis for coupling to said first mass to align said central axis and said axis of rotation; and actuator means for redistributing said mass circumferentially in dependence upon received control signals to adjust a position of a combined centre of mass of the first and second masses to the axis of rotation to reduce the detected vibrations.

17. A structure according to claim 16, wherein said actuator means is adapted to redistribute said second mass hydraulically or pneumatically.

18. A structure according to claim 17, wherein said second mass is a liquid and said actuator means is adapted to hydraulically redistribute said liquid circumferentially.

19. A structure according to claim 18, including a plurality of chambers arranged around said at least one circumferential region and interconnected by conduits arranged cicumferentially, wherein said actuator means is adapted to cause said liquid to be exchanged between said chambers to redistribute said liquid circumferentially.

20. A structure according to claim 19, wherein said actuator means is adapted to pump said liquid between said chambers to cause said liquid to be exchanged between said chambers.

21. A structure according to claim 16, wherein said second mass is a solid and said actuator means is adapted to hydraulically or pneumatically move said solid circumferentially.

22. A structure according to claim 21 , including a plurality of chambers arranged to extend substantially circumferentally, each said chamber containing a piston, wherein said mass comprises the pistons in said chambers, and said actuator means comprises means for hydraulically or pneumatically displacing said pistons along said chambers in response to said control signals.

23. A structure according to claim 16, wherein said second mass comprises a combination of two materials of different densities, and said actuator means is adapted to differentially move said materials circumferentially.

24. A structure according to claim 23, wherein said two materials comprise two immiscible fluids of different densities and said actuator means is adapted to differentially move said fluids circumferentially.

25. A structure according to claim 23, wherein said two materials comprise a liquid carrying a salt and said actuator means comprises electrode means for differentially moving said salt in said liquid.

26. A structure according to claim 25, including an annular channel containing said liquid and having a plurality of spaced porous plugs dividing said channel, wherein said actuator means comprises a plurality of electrodes, each electrode being arranged adjacent said annular channel and between two said porous plugs.

27. A structure according to claim 16, wherein said actuator means is adapted to mechanically redistribute said second mass.

28. A structure according to claim 27, wherein said second mass comprises a plurality of mass members arranged around said at least one circumferential region and said actuator means is adapted to relatively move said plurality of mass members.

29. A structure according to claim 28, including at least one annular track, wherein said mass members are mounted on said at least one annular track, and said actuator means is adapted to relatively move said mass members around said at least one annular track.

30. A structure according to claim 28, including a plurality of arms pivotable about said central axis, wherein said mass members are mounted on said arms and said actuator means is adapted to relatively rotate said arms about said central axis.

31. A structure according to any preceding claim, including inductive windings, a mounting structure having magnets for co-operation with said inductive windings to generate power for said actuator means, wherein said second mass, said actuator means and said inductive windings are rotatable relative to said mounting structure, and said inductive winding and said magnets are adapted to transmit said control signal over a contactless inductive link formed between said inductive windings and said magnets to said actuator means.

32. A rotary apparatus comprising a rotating mass mounted on a mounting structure, and a structure according to any one of claims 16 to 30, wherein said rotating mass includes at least one electrically powered device and carries inductive windings, said mounting structure having magnets for co-operation with said inductive windings on said rotating mass to generate power for said at least one electrically powered device, switching means is provided on said rotating mass for switchably controlling said electrically powered device, said switching means being adapted to respond to a switching control signal transmitted over a contactless inductive link formed between said inductive windings and said magnets, and said actuator means is adapted to respond to said control signal transmitted to said actuator means using said contactless inductive link.

33. Apparatus for reducing vibrations generated by an aircraft propeller comprising: a structure according to any one of claims 16 to 31 for adaptively balancing a propeller shaft; detector means for detecting vibrations generated by said propeller shaft; and control means for generating control signals for said actuator means using said detected vibrations.

34. Apparatus according to claim 33 wherein said detector means is adapted to detect vibrations in an aircraft cabin to reduce vibrations in said cabin.

35 Apparatus according to claim 33 wherein said detector means is adapted to detect vibrations on a housing of said propeller.

36 Apparatus for reducing vibrations generated by an aircraft propeller comprising: a rotary apparatus according to claim 32 for adaptively balancing a propeller shaft; detector means for detecting vibrations generated by said propeller shaft; and control means for generating control signals for said actuator means using said detected vibrations.

37 Apparatus according to claim 36 wherein said detector means is adapted to detect vibrations in an aircraft cabin to reduce vibrations in said cabin.

38 Apparatus according to claim 36 wherein said detector means is adapted to detect vibrations on a housing of said propeller.