GB2159484A

GB2159484A - Method and apparatus for tracking a helicopter rotor or other rotating mass

Info

Publication number: GB2159484A
Application number: GB08413044A
Authority: GB
Inventors: Noel E Trigg
Original assignee: Micro Control Technology Ltd
Current assignee: Micro Control Technology Ltd
Priority date: 1984-05-22
Filing date: 1984-05-22
Publication date: 1985-12-04
Also published as: GB8413044D0

Abstract

A helicopter has means for giving a prominent display or graphical representation of the rate of revolution of the rotor or of the value of m/R where m is the number of rotor blades and R is the rotor cyclic time, while it is also possible to display measurements of the vibration amplitude at the frequency l /R, or m/R against %RRPM. This kind of display has not previously been thought necessary for helicopter operation but has been discovered to be most useful in correcting instrument readings in adjusting helicopter rotors for any lack of balance, and in adjusting balancing weights so that there will be a minimum vibration at the rotor speed at which it is intended to fly the helicopter.

Description

SPECIFICATION Method and apparatus for tracking a helicopter rotor or other rotating mass This invention relates to an apparatus and a method for tracking a helicopter rotor or other rotating mass, and is a development of the invention the subject of European Patent Publication No 0089228(Application No.83.301422.8).

That specification and also European Patent Specification No.83306842.(Publication No.0112031) teach methods of measuring and using signals representing the deflection of rotating helicopter rotor blade tips for providing information enabling the rotor to be balanced.

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 generally 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, perpendicular to the axis of rotation, and generating equal lift.For balancing it is necessary to know 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.

In general Publication No.89228 discloses some of the displays that are useful in tracking a helicopter rotor, as shown in FIGURES 7,8,9, 10, 11, and 12 of that publication. In general, Specification No.

83306842.2 discloses methods of obtaining information about the horizontal and vertical position of each rotor blade tip as it rotates past a target position.

An object of the present invention is to provide alternative or additional useful displays that can be derived from the same information, but which enable tracking, adjusting, and control, of a helicopter rotor to be performed more efficiently.

According to one aspect of the present invention, a method and apparatus for displaying information about a helicopter rotor and a method of testing and adjusting a helicopter rotor are characterised by the prominent display of a figure or graphical representation of the rate of revolution of the rotor. Although the rate of revolution of the rotor could be in various dimensions, or could be represented as an analogue quantity, for convenience in this specification the quantity will be called RRPM (rotor revs per minute).

Thus, in all the displays of the kind described in Publication No.89228 the RRPM could be prominently displayed at all times.

Once it has been discovered that such a display is useful for many applications, a display which was not previously thought to be of great importance, it is not difficult to derive the value to be displayed, for example, by use of continuous signals derived from an electromagnetic or other pickoff on the rotor shaft or hub and possibly even from one of the rotor blades, although for many applications that is not desirable because blades can move angularly as they rotate.

One method of deriving the figure is to display the count of an accurate oscillator over a period beginning at a certain instant in one rotor cycle, and ending at the corresponding instant in the next cycle.

That time over which oscillator cycle can be counted - or the rotor cyclic time - is frequently given the symbol R, so that R is proportional to the reciprocal of RRPM.

Many helicopter instruments have readings which vary with RRPM and they are provided with calibration charts so that if the instantaneous RRPM is always displayed, it is a fairly simple matter to correct the instrument reading. Thus, even in normal use of a helicopter instantaneous instrument readings can be corrected in dependence on the actual RRPM.

It will be appreciated that the rotor frequency liR will tend to be a fundamental vibration frequency experienced by all components of the helicopter, and so will be a factor to be considered in many circumstances and in particular when tracking and balancing the rotor.

Another fundamental vibration frequency will be m/R where the rotor has m blades and it has been discovered that the m/R frequency is surprisingly important in some applications, and according to another aspect ofthe invention, a method and apparatus for displaying information about a helicopter rotor and a method of testing and adjusting a helicopter rotor, are characterised by a display of the value of miR.

Of particular significance is the amplitude of vibration at the frequency liR or m:R and it has been found that even at more-or-less constant RRPM, the amplitude of vibrations at those frequencies varies over the speed range.

Figure 1 of the accompanying drawings is a typical graph showing for each of a number of air speeds the amplitude of vibration, for example obtained from an accelerometer mounted on the rotor hub bearing housing. The points along the ordinate indicated as G, H, 1, 2, 3, are respectively with the helicopter on the ground, with the helicopter hovering at zero air speed, and at air speeds of 60, 100, and 140 knots. The abscissa values represent the amplitude of the component of the output signal from the accelerometer at the frequency liR or miR. R, the rotor cyclic time, is constant.

It will be seen that the curve of Figure 1 is a reasonably smooth curve, and it may be inferred that at different air speeds the amplitude of vibration at liR will be approximately as determined by that smooth curve, and accordingly, the preferred display on the tracking equipment will be the graph of FIGURE 1 which can be derived from a series of tests at the various air speeds indicated by analysing the output from the accelerometer in a manner to select the component at the frequency mlR or 11R.

According to another aspect of the invention, measurements derived from the vibration amplitude at the frequency 11R or even m/R can be displayed against %RRPM, for example as shown in Figure 2 of the accompanying drawings.

A helicopter is designed to operate at a certain nominal RRPM which is at the value 100 along the ordinate in Figure 2, and analyses of the output of an accelerometer in tests at rotor speeds above and below the nominal 100% can give a curve as indicated at 'X' showing that the minimum amplitude of vibration at 1/R occurs at say, 99% of nominal RRPM.

Now for a certain flight, in dependence on the loading and conditions, the pilot may decide that he is going to fly at an RRPM different from the 99% figure which is where vibration at the fundamental frequency would be a minimum.

By adjustment of balancing weights, for example, by loading or adjusting the position of the engine battery, or even by adjusting the position of a specially mounted balancing weight, the position of the curve 'X' in Figure 2 can be adjusted so that the minimum amplitude of vibration at the rotor frequncy occurs at the particular speed the pilot intends to drive the rotor, and the curve 'X' can effectively be displaced horizontally to be as indicated at 'Y' in FIGURE 2 if the rotor RRPM is to be at 102% From the curves obtained during preliminary tests, it is possible to derive information indicating how the balance weight should be adjusted for flight at any selected percentage RRPM, and the method of using information derived from preliminary tests for this purpose is a further aspect of the present invention.The method of determining the amplitude of a component at a particular frequency from the output wave of the accelerometer can be as described in a co-pending Patent Application 8413045 filed on the same day as the present application (Case 8).

In that method described with reference to Figure 3 of the accompanying drawings, a signal from an accelerometer 11 mounted on a rotor bearing housing is fed to a filter and shaper 12 to produce at 13 a continuous analogue signal which will in general be of irregular shape, and will require analysis if the component at the fundamental rotor frequency is to be extracted.

That signal is fed as input to an analogue-to-digital converter 14 so-that the analogue signal is sampled repeatedly to give digital values at each sampling time generally as indicated at 15.

The rate of sampling is controlled by a signal at 16 which is derived from a pick-off 17 mounted adjacent a rotating part of the rotor, for example the rotor hub, to produce one pulse per cycle, so that the interval between pulses is the cyclic time of the rotor R as indicated at 18. By use of a divider circuit 19, sampling pulses can be fed at 16 to the converter 14 to ensure that the sampling intervals are evenly divided over the rotor cycle time, and a preset number of samples, for example 1 2 8, are taken per cycle.

The digital samples are fed as input to a fast Fourier transform unit~21 which is controlled by an input at 22 set to a particular frequency f equal to the rotor rotation frequency. The transform 21 gives two outputs by means of the Guertzel Algorithm, the outputs being respectively indicative of the amplitude and phase of the component at frequency f of the input signal to the transform.

The signal f can of course be derived accurately from the rotor pick-off 17 so that the amplitude and phase at that precise frequency can be derived.

In a series of tests on a helicopter rotor, the amplitude and phase at a number of different rotor frequencies can be measured, and a complete pattern can be built up and displayed graphically.

In general helicopters are dsigned for operation at a predetermined rotor speed which will be more-orless constant for all air speeds including ground speed and hover. The helicopter will be designed to have as few resonances as possible at that set rotor speed, or indeed to have resonance frequencies which are as far removed as possible from the basic rotorfrquency, and indeed from multiples of that frequency, and in particular the m muitiple, where m is the number of rotor blades.

However it often happens that the pilot wishes to fly using a slightly different rotor speed which he will determine in dependence on the loading and conditions of a particular flight. If he is to fly at a rotor speed different from the manufacturer's set rotor speed, perhaps within the range 96% to 104% of that speed, it is clearly advantageous if he can make adjustments to the helicopter before take-off to ensure that there will be little vibration at the rotor speed he is going to fly at.

Claims

1. A method for displaying information about a helicopter rotor characterised by the use of a prominent display of a numerical or graphical representation of the rate of revolution of the rotor.

2. Apparatus for displaying information about a helicopter rotor characterised by a unit arranged to give a prominent display of a numerical or graphical representation of the rate of revolution of the rotor.

3. A method of testing and adjusting a helicopter rotor in which use is made of a prominent display of a numerical or graphical representation of the rate of revolution of the rotor.

4. A method or apparatus as claimed in any of the preceding claims in which the display is a achieved by use of repeated signals derived from an electromagnetic or other pick-off on the rotor shaft or hub or from one of the rotor blades.

5. A method or apparatus as claimed in any of the preceding claims in which the display is that of the count of an accurate oscillator over a period beginning at a certain instant in one rotor cycle and ending at the corresponding instant in the next cycle.

6. A method of operating a helicopter in which an instrument reading is corrected using a prominent display of a numerical or grpahical representation of the rate of revolution of the rotor.

7. A method for displaying information about a helicopter rotor characterised by the use of a display of a numerical or graphical representation of the value of mlR, where m is the number of rotor blades and R is the rotor cyclic time.

8. Apparatus for displaying information about a helicopter rotor characterised by means arranged to display a numerical or graphical representation of the value of mlR, where m is the number of rotor blades and R is the rotor cyclic time.

9. A method of testing and adjusting a helicopter rotor by using a display of a numerical or graphical representation of the value of m/R where m is the number of rotor blades and R is the rotor cyclic time.

10. A method of operating a helicopter in which vibration amplitudes at the frequency 1/R or m/R are displayed against %RRPM.

11. A method of operating a helicopter in which balancing weights are adjusted so that the minimum amplitude of vibration at the rotor frequency occurs at the particular speed at which the pilot intends to drive the rotor.

12. a method as claimed in Claim 11 using the display defined in Claim 10.

13. A method for displaying information about a helicopter rotor performed substantially as herein specifically described with reference to the accompanying drawings.

14. Apparatus for displaying informationabout a helicopter rotor constructed and arranged substantially as herein specifically described with reference to the accompanying drawings.

15. A method oftesting and adjusting a helicopter rotor performed substantially as herein specifically described with reference to the accompanying drawings.