A method of detecting positions of blades of a rotating helicopter rotor

Abstract: In a method of detecting the deflection of the blades (13) of a helicopter rotor as it rotates, a beam (14) of radiation from a transmitter/receiver (11) encounters the tips of the rotating blades which send reflected signals to the receiver. The phase difference due to deflection of the blade tip can be used to give a measure of the deflection of each blade, and that can be displayed graphically as shown in Figures 7-10 for the various blades at various speeds. In a related method of measuring deflections of the blades (111) for a tail rotor, a laser beam transmitter (16) in Figure 14 has its reflected beam (122, 123) from an undeflected blade (111) or a deflected blade (111′) received by a particular receiver in a linear array (119) of receivers so that the particular receiver gives an indication of the amount of deflection.

Description: This invention relates to methods of detecting deflections of parts of a rotating mass and was developed for particular application in detecting the degree of unbalance of a helicopter rotor although the method invented does have application to other rotating masses. When the method is applied to the detection of deflections of helicopter rotor blades, it is convenient if the results calculated from the test can be displayed in a manner which is very readily understood, and another aspect of the invention is the method of displaying the calculated information. According to the present invention in a method of detecting deflections of blades of a rotating helicopter rotor, a beam of laser, radar, or other, electromagnetic radiation is transmitted towards the rotating blades and received signals reflected from the blades are analysed to give a measure of blade position. It is important that a helicopter rotor should be balanced in terms of mass about its centre of rotation and equality of lift from each blade, whether it is a main lifting rotor or a tail rotor. A balanced rotor is one in which the blades are maintained equiangularly spaced around the axis of rotation, and in which the rotor tips rotate in the same horizontal plane with each other, and generate equal lift throughout the speed range. The plane will be different at different flight speeds but the important thing is that at a given speed all the tips are rotating in the same plane, and generating equal lift. It is known how much each blade is deflected in relation to the others, at different flight speeds, and in the past such deflections have been measured visually or by trial and error, and results have not only been unreliable but they have also taken a considerable time to evaluate. The present invention according to which the range of, for example, each rotor blade tip from a transmitter/receiver is measured as the blade concerned passes a particular point in the path of rotation, has been shown to give surprisingly accurate results which are available from a computer very shortly after received signals have been fed into the computer. One preferred type of transmitter/receiver is a double Doppler radar system with two closely spaced transmitter/receivers transmitting at frequencies slightly displaced from each other and with the phase difference of the reflected signals being a measure of the range of the surface from which they were deflected. It is also possible to use a single pulse type of transmitter/receiver using frequency modulated carrier wave pulses. Such systems can give an indication of the distance of the blade tips from the transmitter/receiver, so that if the transmitter/receiver is appropriately positioned, the measured range can be used as a measure of the deflection of the blades from a mean plane. The transmitter/receiver will in general have to be substantially displaced from that plane, typically at a 30° angle. By relating the time of detection of the received radiation to a cyclic reference pulse, any angular displacement of a blade in a horizontal plane about this axis of rotation can also be determined. In another method the transmitter can be a laser or other source of a collimated beam of radiation, and then the reflected beam can be displaced in dependence upon deflection of the individual rotor blades, so that if for example the reflected beam is directed towards an array of receivers, then the particular receiver in the array receiving the reflected beam will depend upon the amount of deflection and a signal representative of that deflection can be derived. Such a system is of particular value in the case of a tail rotor, where there is usually no scope for relative movement of the blades about the axis of rotation. Tests have shown that by feeding the received information into a computer which has been suitably programmed, the measured deflections can be available very shortly after the tests have been taken, and indeed it is possible to have a transmitter/receiver mounted on a helicopter for measuring deflections of main rotor blade tips, and a laser beam transmitter and receiver mounted on the tail boom for measuring tail rotor blade deflections, and a computer in the cabin receiving signals from both systems and to have the complete results of tests made at different flight speeds available in flight. When the helicopter lands after the tests the fitters can immediately make the necessary adjustments to the blade mountings. The invention includes such apparatus as well as the method of using it. A related invention is the method of displaying the results achieved by one of the above methods, where the method is applied to the deflection of the individual blades of a multi-bladed helicopter rotor, and according to that related invention, for one or each of the blades, the deflections in the cycle of revolution are displayed graphically for one or for each of a number of flight speeds. In particular the position of mean deflection of all the blades at a particular speed can be determined and the display can be related to that mean deflection graphically. A further possibility is to show the deflections of all the blades both angularly in relation to each other, and perpendicular to the plane of rotation in relation to the mean deflection for a given flight speed. A trend chart can be displayed showing a curve for each blade which reveals its deflection at each of a number of speeds in a range, and it is possible for such a display to have curves for all the blades spaced apart along the display, or to display a curve for a single blade. By detecting time anomalies in the passage of individual rotor blades past a given point - as related to the one per revolution marker - it is possible to detect and identify a defective inter-blade damper and to display the result to the operator. Hitherto, this has not been possible with any degree of certainty. Brody's US patent number 2,913,700 discloses an earlier proposal for a method of detecting misalignment of helicopter blades. The method involves a transmitter 12 of supersonic vibrations, for example, at 20 kilocycles per second using a crystal for converting the driving electrical energy to the sound energy directed against the rotating rotor blades by a reflector 28. After reflection from a reflector on each blade the sound oscillations are received in a receiver and after electrical analysis provide a reading on a DC meter which is an indication of the degree of out of alignment of a particular blade. It may be noted that meter readings are taken but that otherwise there is no display of the results of the test either in flight during the test or at the end of the test. Moreover the readings are derived from sonic vibrations and not by the radiation of the present invention and also the sonic transmitter receiver is mounted on the tail boom as seen in Figure 4 of US-A-2913700. It has been found that the Brody proposal is quite impractical, partly because with all the noise accompanying operation of a helicopter the supersonic signal gets distorted and also because the boom where the transmitter receiver is mounted is itself subject to very substantial vibration. Also the time for sonic waves to travel from the transmitter to the blade and back to the receiver is far to long for worthwhile results to be obtained. Ideally to get worthwhile results it is necessary to have several reflections during the passage of each blade tip through the field of view of the receiver. That can be achieved using the laser, radar, or other, radiation of the present invention which of course travels at the speed of electromagnetic radiation 3X10¹⁰ cm per second. The present invention is limited to systems using such radiation, and systems using sonic transmitted and received beams are specifically disclaimed. The invention may be carried into practice in various ways, and one embodiment will now be described by way of example, with reference to the accompanying drawings, in which:- FIGURE 1 is a side elevation of a helicopter fitted with track and unbalance detectors in accordance with the invention; FIGURE 2 is a plan view corresponding to FIGURE 1; FIGURES 3, 4 and 5 are diagrams useful in explaining the operation of the track and unbalance detector; and FIGURE 6 shows signal characteristics used in the system; FIGURES 7-12 are various displays of track and unbalance information determined in tests; FIGURE 13 is an elevation of the tail of a helicopter showing the tail rotor; and FIGURE 14 is a view seen in the direction of the arrow II in FIGURE 13. The rotor of a helicopter requires balancing both when manufactured and from time to time, so that the rotor blades are equally angularly spaced when seen in plan view, and so that the rotor blade tips all move in the same plane substantially perpendicular to the axis of rotation. The method of adjusting the mounting of the blades to achieve that balance is well understood provided the amount of unbalance of each blade is known, but until now no satisfactory method has been found for accurately detecting the amounts of individual blade unbalance. In accordance with the present invention, a double Doppler transmitter/receiver is mounted as indicated at 11 to one side of the cabin just below the door 12 with its central axis of transmission inclined upwardly at an angle of about 35° so as to be intercepted by the tips of the rotor blades 13 as they rotate. FIGURE 2 shows that the central axis of transmission 14 is not only inclined upwardly but is inclined inwardly in a preferred arrangement, but FIGURE 4 which is a diagrammatic plan view, shows another possible arrangement in which the cone 15 around the central axis of transmission is directed at an angle to the fore-and-aft axis of the helicopter as well as at an angle upwards. The precise position may not matter provided the tips of the rotating blades can give reflected signals to the Doppler receiver. Another possible disposition is shown at 11' in FIGURE 4. The tips of the rotor blades have small metal reflecting surfaces fixed to them for giving a good reflected signal. FIGURE 1 shows how the rotor blades 13 droop when the rotor is not rotating, and it will be appreciated that as rotation starts, the blades will lift and tend to extend horizontally outwardly and when they start to lift the helicopter they will start to cone upwards as indicated diagrammatically at 16 in FIGURE 5. FIGURE 5 is a diagrammatic view in a vertical plane containing the rotor axis, and containing the transmitter/receiver 11. Extreme positions of a blade in that plane respectively when the rotor is stationary and when it is giving maximum lift, are shown at "S" and "L" and FIGURE 5 shows how because of the displacement of the transmitter receiver 11 below the horizontal plane through the hub 17, movement of a blade tip between "S" and "L" effects a notable difference in the distance R s or R l from the transmitter/receiver to the blade tip. The transmitter/receiver uses a multiple frequency carrier wave radar system of the kind described by Skolnik in "Introduction to Radar Systems", page 106, according to which two transmitter receivers as indicated at 18 and 19 in FIGURE 3, continuously transmit at frequencies f₁ and f₂. The reflected signals from a blade tip to the receivers are at f₁ + fd₁ and f₂ + fd₂ where fd₁ and fd₂ are Doppler frequency components derived from the component of velocity of the blade tip towards the transmitter/receiver. The phase difference between the reflected signals is a direct measurement of the range of the blade tip from the transmitter/receiver, and so is approximately a direct measure of the out of track condition. Each blade tip will give a reflected signal to each receiver, so that there will be three pairs of reflected signals in each rotor revolution, as indicated generally at 22 in FIGURE 6. If the difference between the frequencies f₁ and f₂ is very small compared with f₁, then fd₁ can be considered to be equal to fd₂ and the quotient t/T is directly proportional to the range of the blade tip where t is the phase difference between the reflected signals, as shown in FIGURE 6, and T is the frequency difference between the two signals, both differences being expressed in terms of time. This method of measuring the distance of the rotating blade tips from the transmitter/receiver 11 as they rotate has been found to be surprisingly accurate, and it is possible from an analysis of the received signals to know fairly accurately just how much each blade tip is above or below the average plane of rotation of all the rotor blade tips at the position in a revolution where the cone 15 from the transmitter/receiver is encountered. Moreover, examination of FIGURE 6 shows how the centres of the reflected bursts of radiation can be related to cyclic reference pulses 21 so that analysis will reveal whether the blades are equally angularly spaced horizontally or whether the gap between one blade and its leading neighbour is more or less than the gap between that blade and its trailing neighbour. Once that information is known it is well understood how to adjust the mountings of the blades to correct for any out of track condition or lead/lag error. The invention contemplates having electronic recording and computing equipment onboard the helicopter for recording the results of tests performed when the rotor is rotating on the ground, and when the helicopter is hovering or flying at different speeds because the performance of the blades may well be different at different rotor and flight speeds. The computer in the recording equipment can be programmed to display the information in a manner which is easily readable. For example, FIGURE 7 shows three VDU displays for a particular four bladed helicopter rotor respectively at speeds of 149 KM/hr (100 knots), 179 KM/hr (120 knots) and 210 KM/hr (140 knots). The horizontal line at 24 is calculated as the mean height of the rotor blade tips where they intersect the cone of transmission and the height of each blade tip is displayed in relation to that means. The ordinate is calibrated in cms as indicated at 25, and the indentification of the four blades is indicated at 26. The display for 179 KM/hr (120 knots) shows that blade number 2 is high at that speed, whereas it is low at 149 KM/hr (100 knots). In the displays of FIGURE 7, all the blades are correctly angularly spaced, but in an alternative display at 90 KM/hr (60 knots), shown in FIGURE 8, although the second blade is correctly positioned angularly, it can be seen that blades 1 and 4 are leading from their correct angular position, whereas blade 3 is lagging. The fitter studying that display can easily make an appropriate adjustment to the blade mountings. The computer is also programmed to collect the results for all four blades at various flight speeds, and display them in a summary display indicated in FIGURE 9. For each of the four blades, readings at each of the test speeds are displayed along a different vertical line being on the line, or to one or other side of the line, according as the blade tip is at the mean level or is above or below that level during the corresponding test. For example FIGURE 9 shows how blade number 1 is low during hovering, but gradually comes up towards the mean level as the flight speed increases until it is consistently just below the mean level in the range 179 to 210 KM/hr (120 to 140 knots). Figure 10 is a display corresponding to FIGURE 9 of a selected blade -in that case blade number 4 - and since a "trend" curve for only one blade is showing it is possible to calibrate the ordinate in terms of cms of deflection, as indicated at 28. During a test there will be many revolutions of the rotor and the readings during each revolution are recorded and stored and mean readings are accumulated. It is possible, for each of a number of different helicopter designs, to perform tests to achieve a characteristic of that helicopter design revealing a compound signature of the rotating masses. Thus, a typical characteristic vibration display at a certain air speed for a certain helicopter design may be as shown in FIGURE 11. That characteristic may be recorded and fed into the computer prior to a display of measurements made on a particular helicopter, and then the vibrations actually measured on that particular helicopter may be displayed as shown in FIGURE 12. It would be possible by programming-in flashes, as for example at 28, showing the calculated maximum permissible amplitude of vibration of each component, to enable the user seeing the measured characteristic to decide straight away that the rotor or any other component so labelled was, or was not, satisfactory. For example the vibration peak at 29 extends beyond the flash 28 and that would not be acceptable, whereas the peak 31 does not extend beyond its flash 28 indicating an acceptable level of vibration amplitude for the main rotor blades. In order to be able to balance the tail rotor of a helicopter, it is necessary to perform a test at one or more flight speeds to measure the deflection of each blade in relation to the mean plane of rotation of the blade tips. In FIGURE 13 the rotor is shown as having two blades 111 and in common with most helicopter tail rotors, the blades have no freedom of movement about the axis of rotation 112 so that any out of track in the rotor will be represented merely as a deflection of the tip of a blade towards or away from the tail boom 113. In accordance with this invention the blade tip deflection is measured as the rotor rotates, by means of a laser beam and a photo diode array. These are contained in a light box 114 which is mounted above the boom, so that a laser beam 115 from a laser diode 116 is interrupted by the blades as they rotate as shown diagrammatically at 117 in FIGURE 14. Each blade 111 has a reflective strip 118 near its tip which might be an applied strip of aluminium, or an area of reflective glass beads for example. Light reflected from the area 118 is received by one of an array 119 of photo-diodes mounted in the light box, the array extending within the range defined by the two arrows 121. An undeflected rotor blade 111, that is to say one rotating in the correct theoretical plane, is shown in solid lines in FIGURE 14, and it can be seen that the reflected laser beam 122 is received by a photo-diode at the centre of the array. If on the other hand the blade is deflected towards the boom 113, by a distance 'd' as shown in chain lines at 111', the reflected laser beam 123 will be received by a photo diode nearer one edge of the array 119. That may be merely because the point of reflection 124 is nearer to the light box 114 than the point of reflection 125 for the undeflected blade 111, or it may also be because the deflection 'd' is due to angular deflection of the blade about the hub axis 112, so that the plane of the reflecting surface 118 is at an angle to the plane of the undeflected blade. In the example shown in FIGURE 14, some specimen dimensions are given. The rotor blade tips are at a radius of 185 cms from the hub axis 112, and the undeflected blade plane is at a distance of 300 cms from the face of the light box 114 and perhaps 10 cms from the side of the boom 113. The sort of deflection that may be experienced is unlikely to be more than three or four cms but the optical arrangement can be such that the particular photodiode in the array 119 that receives the reflected beam 122 or 123 can give an accurate indication of the amount of deflection of the blade. A signal can be obtained by reflection from a black patch or other reflecting surface on the rotor to act as a reference signal defining each rotor rotation, so that the output from the photodiode array can be related to those reference signals and identification of the blades giving the signals can be made. An accelerometer 127 is mounted on the hub bearing, and the output from that,after filtering to exclude components above and below the fundamental frequency of rotation, can be used to provide a reference sine wave which can be related to the cyclic pulse signals from the black patch or other reference surface, and then examination of the phase angle between the sine wave peaks and the black patch signals can give an indication of the degree of dynamic out-of-balance of the tail rotor by well understood methods. This apparatus for measuring the degree of out-of-balance of a tail rotor can be used in combination with the apparatus, the subject of FIGURE 1-6, for measuring the out-of-balance of a main helicopter rotor and for displaying the results visually. The computer described can thus receive inputs from the light box 114 and the accelerometer 127. Although the invention has been described as applied to measurement of deflection of blade tips on a rotor, it will be appreciated that it is applicable to measuring deflections at different points in any rotating mass. For example the signals can be received from the blades of a propeller, fan or a rotating turbine rotor, or could be obtained from positions along a rotating shaft subject to whirling and critical speed deflections. Again the invention has been described in FIGURES 3 and 6 as using a double Doppler method of obtaining a phase difference signal representing the range of the component giving the reflections, but it would also be possible to use a pulsed frequency modulated carrier wave signal in which in each pulse the frequency was modulated from the beginning to the end of the pulse, and the amplitude was also modulated. By using signals selected from reflected pulses at the beginning and the end of the pulses, range can also be determined.

Claims: A method of detecting deflections of blades (13) of a rotating helicopter rotor, in which in a test performed onboard the helicopter while it is in flight signals received by a receiver (11) after reflection from the blades (13) are analysed to give a measure of blade position; characterised in that the signals are reflections from the blades of a beam of electromagnetic radiation which has been transmitted towards the rotating blades from the body of the helicopter. A method as claimed in claim 1, in which the reflected radiation provides input to a computer for computation of blade positions during the test. A method as claimed in either of claims 1 and 2, using a Doppler radar system. A method as claimed in either of claims 1 and 2, using a laser or other collimated beam of radiation. A method as claimed in any of claims 1 to 4, in which the results of the analysis are displayed visually in the helicopter while the test is being performed in the form of a single display (Figure 7) showing deflection of all the blades, or a single display (Figure 19) showing the deflections of at least one blade at different airspeeds. A method as claimed in claim 5 in which the results are recorded so that the visual display is available after the flight for use in adjusting the rotor on the ground. A method as claimed in claim 5 or claim 6, in which the displayed positions are mean positions of individual blades over a number of body revolutions. A method as claimed in any of claims 5 to 7, in which the information relating to the helicopter is stored before the beginning of the test, and used in the analysis. Apparatus for testing a helicopter rotor including a receiver (11) of radiation which has been reflected from the rotor blades (13) and a computer arranged to compute from the received radiation deflections of the individual blades during rotation characterised by a transmitter of a beam of the electromagnetic radiation from the body of the helicopter towards the blades for reflection from the blades. Apparatus as claimed in claim 9 characterised by a visual display screen controlled by the computer to display the deflections of all the blades in a single display (Figure 7) or the deflections of at least one blade at different airspeeds in a single display (Figure 10).

Inventors: CLEARWATER, WALTER ALLAN, MOIR, CHRISTPHER IAN
Applicants: MICRO CONTROL TECHNOLOGY LIMITED
Application year: 1983
Main classification: G01M5
IPC codes: G01B17, G01B11