METHOD AND APPARATUS FOR REDUCING VIBRATION

Abstract: Vibrations induced by a propeller (11) rotating in a medium to drive a body (10) supporting the propeller through the medium can be reduced by generating a timing signal (19) synchronised with the rotation of the propeller and using this timing signal to reduce the perturbations in propeller performance by varying the pitch of at least one blade (11a, 11b, 11c) of the propeller during each rotation of the latter, and/or by varying the pressure in at least one localised region of the medium through which the propeller turns. The success with perturbation reduction can be monitored by sensors (14, 15, 16) and the variations of pitch/pressure adapted (17) to optimise propeller performance.

Description: Method and apparatus for reducing vibration BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for the reduction of vibration induced by the propellers of ships or aeroplanes. 2. Description of the Prior Art As a propeller rotates through water, it passes through regions which have higher or lower water velocity relative to its own velocity and this results in cyclic variations in thrust as the propeller rotates. The varia¬ tions may be caused by variations in water density due .to temperature or salinity gradients, or to the differen¬ tial pressure causing cavitation to occur earlier when the blade is in shallower water, or because of some obstruction to the water f.low (e.g. the rudder or moulding of a ship's hull), or because of turbulence in the water. Problems of this general character can also arise in the case of propellers rotating in air, although clearly the rates of change and weights involved will be - very different. SUMMARY OF THE INVENTION According to the invention, there is provided a method of reducing the vibration induced by a propeller rotating in a medium to drive a body supporting the prop¬ eller through the medium, which method involves generating a timing signal synchronised with the rotation of the propeller and using this timing signal to reduce the per- turbations in propeller performance , said perturbation reduction being effected by at least one of a ) varying the pitch of at least one blade of the prop¬ eller during each rotation of the latter, and _ OMPI b) varying the pressure in at least one localised region of the medium through which the propeller turns. The perturbations which are to be reduced can be sensed, for example, by a motion-sensitive transducer on the body driven by the propeller, by a pressure-sensi¬ tive transducer disposed in the medium where it can moni¬ tor the pressure field generated by the propeller or by a force sensor on the drive shaft of the propeller to monitor changes in the dynamic thrust generated by the propeller. The pitch or angle of attack of the blades of the propeller can be varied cyclically as each blade passes through a specific part of each turn of the propeller or the angle of attack of each blade can be separately controlled. The pressure in the medium can be varied by periodic¬ ally injecting medium into the vicinity of the propeller or, additionally or alternatively in the case of the med¬ ium being water, by injecting air into the water in the vicinity of the propeller (e.g. from a supply ring surrounding the water in which the propeller turns or from apertures in at least one blade of the propeller) . The effective pitch of at least one blade can be influenced by a remotely adjustable spoiler or trimtab on one or more of the propeller blades. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows, purely schematically, how a ship- borne adaptive controller can be used to control a prop- - -REA - 3 - eller of the ship to eliminate energy-wasting vibrations, Figure 2 shows schematically how the blades of a propeller can have their angles of attack adjusted cyclic¬ ally one after the other in the performance of the method of the invention, Figure 3 shows schematically how one blade of a prop¬ eller can be independently adjusted, Figure 4 shows how the operating conditions of a propeller turning in water can be modified selectively by air injection, Figure 5 shows how air injection can be effected via apertures in a blade, and Figure 6 shows how a trim tab can be used • to modify the effective pitch of a propeller blade. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates the general situation in a mar- ine vessel 10 adapted to improve the performance of a propeller 11 driven via a shaft 12 from a prime mover 13. As the propeller 11 rotates in the water, the blades 11a, lib, lie thereof experience somewhat different water conditions as they traverse one turn around the axis of the shaft 12. These varying water conditions, as explained previously can be due to variations in water velocity, water density or water pressure and have the effect of causing the propeller thrust to vary cyclically as it rotates and/or of causing vibrations to develop in the hull of the vessel and/or of causing vibrations to be emitted by the propeller into the water. One or more of these effects may be so undesirable as to justify the installation of equipment to at least reduce it -and at best remove it, and Figure 1 shows such equipment. On the shaft 12 is a force sensor 14 which monitors variations in the thrust generated by the propeller 11, in the hull of the vessel 10 is a vibration sensor 15 and in the water is a water-pressure sensor 16 capable of detecting vibration in the water generated by the prop¬ eller 11. One o more of these sensor 14-16 is used and is connected to an adaptive controller 17 which receives a synchronising signal on a line 18 from a sensor 19 on the prime mover 13. The adaptive controller 17 is used to control the performance of a performance improving means 20 which can be on the propeller 11 (as 20a) on its shaft 12 (as 20b) or in the water (as 20c). The performace improving me'ans 20 can take many forms and a few such will now be described by way of example. PERFORMANCE IMPROVEMENT BY VARYING ANGLE OF ATTACK OF THE PROPELLER BLADES The angle of attack of at least one blade 11a to lie of the propeller 11 (or propellers, the invention can be applied to the minimisation of the thrust differ¬ ences brought about by the mutual interaction of several synchronised propellers), is desirably adaptively altered to minimise the output from one or more of the sensors 14 to 16. This can be achieved using one or a combination of different ' improving means 20 as follows: The angle of attack of each blade lla-llc as each passes through the different parts of its 360° propulsion cycle can be cyclically varied using the- arrangement shown in Figure 2. A swash plate 21 rotates with the shaft 12, and by means of hydraulic actuators 22, 23 bear- -1TR£ OMPI ing on the plate 21 at the 12 o'clock and 3 o'clock posi¬ tions, the inclination of the plate 21 in phase and ampli¬ tude is adjusted. Push rods 24 (one for each blade) bear against the plate 21 and alter the blade pitch as they are moved axially of the shaft 12 by the swash plate 21. The arrangement shown in Figure 2 effectively injects a single frequency perturbation on the thrust of the prop¬ eller, which can be optimised via the controller 17 to eliminate thrust variation at that frequency. The actu- ators 22, 23 modify two parameters of the propeller effec¬ ting the amplitude and the phase of the cyclic varia¬ tion in pitch of the blades, and the extension of the actuators is varied to minimise the output on one or more of the sensors 14-16. The extensions of the actuators 22, 23 will be expected to vary with operating conditions. Figure 3 shows an improving means in which the indi¬ vidual blades of the propeller 11 are individually con¬ trolled with the revolution rate and individually phased during the cyclic passage of the respective blade by means of an actuator 26 for each blade which rotates with the shaft 12. Only one actuator has been shown in Figure 3. Each individual blade actuator 26 could be fed with its own time sequence of amplitudes, generated according to the adaptive method described in US Patent 4153815. PERFORMANCE IMPROVEMENT BY VARYING WATER PRESSURE Figures 4 and 5 show improving means designed to locally modify the density of water in which the propeller 11 rotates. Compressed air injected into water will act to momentarily reduce the thrust achievable by the prop- eller blade working in the aerated water.Using the Figure 4 arrangement, cyclic modulation of the propeller thrust can be achieved by surrounding the propeller 11 with a supply ring 30 having a ring of holes 31 therein. As an example, Figure 4 shows the ring of holes divided into O - 6 - three groups 31a, 31b and 31c, each occupying an arc sub¬ tending 120° at the axis of the propeller 11, and each fed with its own air supply via a solenoid-controlled valve 32a, 32b and 32c, respectively. Three separate adaptive controllers 17a, 17b and 17c are shown in Figure 4. Naturally more than, or less than, three separate supplies can be used. The ring 30 can be in advance of the propeller 11. The controllers 17a-17c can individually control the time waveform of the compressed air output supplied to the segments 31a, 31b, 31c surrounding the propeller 11. Figure 5 shows air apertures 40 in one blade (11a) of a propeller and the air emitted through these apertures can be modulated by any of the methods described pre¬ viously to smooth the performance of the propeller. The profile of the compressed air supply applied to the feed - for the rotating nozzles 40 on each blade of the propeller can be individually modulated via an adaptive controller 17 (not shown) . Water pressure actuators could be disposed around the propeller (e.g. in a supply ring like that shown at 30 in Figure 4) and driven with waveforms from an adaptive sunchronous waveform generator of the kind described in US Patent 4153815. The simplest form of such an actuator would consist of a piston in a cylinder, mounted in the bottom of the vessel 10 in such a manner that the bottom of the cylinder is in contact with the water. The cylin¬ der could be driven directly from a displacement actuator, or could be driven from a mass-spring actuator which would provide an oscillating output for a minimum of force input, or could be driven by .one or more out-of-balance vibration actuators, or even from a crank and shaft. The residual signal for controlling the adaptive OM controller 17 in Figure 1 could be taken from a force sensor on the thrust block to which the propeller is attached, but it could also or additionally be taken from the acceleration of the vessel, typically in the longitud- inal direction, or from the acceleration of the ship's hull where this is -the most sensitive parameter which must be minimised, or the cavitation noise or other physi¬ cal effect associated therewith or even, in the case of merchant vessels where economy is of prime importance, from a calculated .efficiency based on the propeller torque and engine speed at the particular water speed demanded. An additional sensor could be used to detect turbulence in the slipstream where it is important to minimise the vortex shedding from the blades. The algorithm for opti- misation could be a direct trial and error approach or could be based on the harmonic components individually processed, but taking account of any frequency shift which occurs as a result of the modulation of the cyclic rate by the signal applied to each individual blade. The invention is not limited to screw propellers . since the same principles could be applied to a non-screw propeller, such as one of the Schroeder type, in which case the actuation could be simpler. Particularly in the case of an aeroplane propeller, it should be possible to reduce the actuation power re¬ quired by making use of spoilers or trimtabs on the indi¬ vidual blades to act as amplifiers drawing their power from the main shaft rotation itself. Figure 6 diagrammat- ically illustrates such a case, a pivotally mounted blade 11a being provided with a trimtab 50 connected via hinges 51 to the main body of the propeller blade. An internal actuator 52 is controlled from the controller 17.

Claims: CLAIMS 1. A method of reducing the vibration induced by a propeller rotating in a medium to drive a body support¬ ing the propeller through the medium, which method involves generating a timing signal synchronised with the rotation of the propeller and using this timing signal to reduce the perturbations in propeller performance, said perturbation reduction being effected by at least one of a) varying the pitch of at least one blade of the propeller during each rotation of the latter, and b) varying the pressure in at least one localised region of the medium through which the propeller turns. 2. A method as claimed in claim 1, in which the effect of the variation of at least one of a) and b) is sensed and subsequent variations thereof modified to further reduce at least one of the following effects; ' A) the vibration fed from the propeller into the medium; B) the vibration conveyed from the propeller to the body; and C) the dynamic thrust variations produced by the prop- eller in each turn thereof. 3. A method as claimed in claim 2, in which the residual of the at least one effect A) to C) produced by the propeller is sensed by one of i) a motion sensitive transducer on the body driven by the propeller, ii) a pressure-sensitive transducer disposed in the medium where it can monitor the pressure field generated by the propeller, and ' iii) a force sensor on the drive shaft of the propeller. 4. A method as claimed in any preceding claim, in which the angle of attack of the individual blades of the propeller is varied cyclically as each blade passes through a specific part of each turn of the propeller. 5. A method as claimed in any of claims 1 to 3, in which the angle of attack of each blade is individually controlled during each turn of the propeller. 6. A method as claimed in any of claims 1 to 3, in which the pressure in water in which the propeller rotates is varied over a localised region by the localised injection of water into the water. 7. A method as claimed in any of claims 1 to 3 or 6, in which the pressure in water in which the propeller rotates is varied over a localised region by the localised injection of air into the water in which the propeller turns. 8. A method as claimed in claim 7, in which the air is injected from at least one of a supply ring surrounding the water in which the propeller turns, and ' apertures in the blades of the propeller. 9. A method as claimed in claim 8, in which a supply ring is used and segments of the supply ring are separ¬ ately supplied with air whereby the air supply to the different segments can be separately controlled. 10. A method as claimed in any of claims 1 to 3, in which the effective pitch of at least one blade of the propeller is modified cyclically in each turn thereof by means of at least one remotely adjustable trimtab on said at least one blade. 11. A method as claimed in t.ny preceding claim, \ m WIPO in which the timing signal is fed to an adaptive con¬ troller for the means varying at least one of the a) and b) variations defined in claim 1, the said controller being influenced by the output from at least one of the sensors i), ii) and iii) defined in claim 3. 12. A method as claimed in claim 11, in which the timing signal is derived from an engine driving the prop¬ eller shaft.

Inventors: CHAPLIN GEORGE BRIAN BARRIE, SMITH RODERICK ALAN, CHAPLIN, GEORGE, BRIAN, BARRIE, SMITH, RODERICK, ALAN
Applicants: SOUND ATTENUATORS LTD, SOUND ATTENUATORS LIMITED
Application year: 1984
Main classification: B63H3
IPC codes: B63H1, B63H3