Method and apparatus for determining the airspeed of rotary wing aircraft

Abstract: A method and apparatus for determining the airspeed of a rotary wing aircraft having a sustaining rotor (23) with a plurality of radially extending rotor blades (23A-23D) for rotation about a substantially vertical axis (24) comprises the steps of measuring an airflow sideslip angle (β) established during rotation of each rotor blade (23A-23D) by the vector of a radial flow component (29) due to the relative wind vector and the rotational velocity (30) perpendicular to a blade feathering axis, producing a signal representative of the sideslip angle (β), comparing the signal in processing means containing information on a known relationship between the sideslip angle (β) and airspeed (VT) providing an output signal representative of the airspeed and wind direction to a display means (39) which may display both the airspeed and wind direction (43).

Description: Description of Invention This invention relates to a method and apparatus for determining the airspeed of a rotary wing aircraft and the wind direction, and is particularly concerned with providing such information at low airspeed. A means for accurately determining the airspeed of a helicopter and wind direction relative thereto in low speed flight is highly desirable especially during certain phases of operation such as during rejected take-offs and when operating from the decks of ships and oil rig platforms. Such instrumentation with sufficient accuracy has thus far proved difficult to achieve. Conventional fuselage mounted devices such as pitot and static pressure sensing devices are adversely affected especially at low airspeeds by the highly irregular flow close to the helicopter fuselage due to the interaction of many influences including the effects of main rotor wake. In US-A-3332282 there is a disclosed a modification of a pressure sensing device in which pitot sensors are provided on the tips of the rotor blades. An alternative prior approach has been to relate the rotor control positions to the airspeed and direction of the helicopter. This has proved difficult because the centre of gravity position must be known, the position of control means such as a swash plate must be very accurately monitored and allowances must be made for control inputs and accelerations so that the steady airspeed can be deduced. A main objective of this invention is to provide a method and apparatus for determining the airspeed of a rotary wing aircraft which overcomes or reduces the aforementioned problems. A further objective is to achieve the main objective using measurements of the airflow on the main sustaining rotor blades which can be related directly to airspeed and relative wind direction. Accordingly, in one aspect, this invention provides a method of detecting the airspeed of a rotary wing aircraft having a sustaining rotor with a plurality of radially extending rotor blades for rotation about a substantially vertical axis, characterised in that the method comprises the steps of continuously measuring on at least one of the rotor blades of the sustaining rotor, a blade airflow sideslip angle established on the surface of the blade by interaction between a radial flow caused by a relative wind vector and a velocity perpendicular to a blade feathering axis due to rotation of the blade, producing a signal representative of said sideslip angle, determining the phase and amplitude of the signal in processing means containing information on a known relationship between the phase of the signal and the relative wind direction and between the amplitude of the signal and the magnitude of the airspeed and providing output signals representative of the magnitude of the airspeed and the relative wind direction to a display means. In another aspect the invention provides an airspeed indicating system for a rotary wing aircraft having a sustaining rotor comprising a plurality of radially extending rotor blades for rotation about a generally vertical axis comprising sensing means on at least one of the rotor blades for sensing blade airflow sideslip angle established on the surface of the blade by interaction between a radial flow caused by a relative wind vector and a rotational velocity perpendicular to a blade feathering axis due to rotation of the blade, and for generating a signal representative of the blade airflow sideslip angle, transmission means for transmitting the signal to processing means on or in the aircraft fuselage, the processing means containing information on a known relationship between the phase of the signal and the relative wind direction and between the amplitude of the signal and the magnitude of the airspeed and the processing means being adapted to determine the phase and amplitude of the signal therefrom and to provide output signals representative of the magnitude of the airspeed and the relative wind direction, display means for displaying said output signals. The airflow sideslip angle β may be established by the equation: β = arctan{Vcos(Ψ+)/(x VT + Vsin(Ψ + ))} where: V is the velocity of the relative wind, (i.e. the airspeed),  is the direction of the relative wind vector, VT is the tip speed of the rotor blades, x is the radial location of the sideslip detectors on the blades, and Ψ is the rotor blade azimuth angle. The invention will now be described by way of example only and with reference to the accompanying drawings in which, Figure 1 is a schematic plan view of a helicopter and shows velocity vectors used in the method and apparatus of this invention, Figure 2 is a graph illustrating a relationship between a blade airflow sideslip angle and airspeed, Figure 3 plots blade airflow sideslip angle against rotor azimuth for selected airspeeds above 50 knots, Figure 4 plots blade airflow sideslip angle against rotor azimuth for selected airspeeds below 50 knots, Figure 5 plots blade airflow sideslip angle against rotor azimuth and illustrates a change in phase due to a change in wind azimuth, Figure 6 illustrates desirable characteristics of a sideslip sensor, Figures 7 to 10 inclusive illustrate some suitable sensing devices and arrangements, and Figure 11 illustrates an instrument suitable for displaying airspeed and wind direction information. Referring now to Figure 1 a helicopter, illustrated schematically in plan, includes a fuselage 21 having a rearwardly extending tail boom 22 located symmetrically about a longitudinal centre line 26. The helicopter has a main sustaining rotor 23 comprising four rotor blades 23A, 23B, 23C and 23D extending radially from a rotor head (not shown) for rotation about a generally vertical axis 24 in a generally horizontal plane in the direction of arrow 24A. An anti-torque rotor 25 is located at the end of the tail boom 22. The azimuthal position of the rotor blades in the rotational disc are usually identified from 0° relative the longitudinal centreline at the rear of the tail boom 22 in the direction of rotation, and the 0°, 90°, 180° and 270° azimuth positions are identified in Figure 1. This identification arrangement will be retained throughout this specification. Normally, with the helicopter flying forward parallel to the longitudinal centreline 26, a rotor blade moving between 0° and 180° azimuth positions is termed the advancing blade and when moving between 180° and 360°, the retreating blade. In this specification however, these terms are applied in respect of rotation of the blades commencing at positions perpendicular to a relative wind vector (as hereinafter described). In contrast, wind azimuth angles are identified as 0° approaching the nose of the helicopter and run clockwise from that position. The method and apparatus of this invention is based on the realisation by the inventor that, as a helicopter translates in any direction relative to the wind, a cyclic airflow velocity component is generated along the radial dimension, i.e. the span, of the main rotor blades. The velocity vector diagrams concerned with this invention are illustrated in Figure 1. The vector diagram at the top of the Figure is representative of a helicopter velocity 27 illustrated as a forward velocity along the longitudinal centreline 26 and a natural wind direction 28 which is resolved into a relative wind vector 29 as actually felt by the helicopter. The relative wind vector 29 extends at an angle a from the 0° wind azimuth position and, for the purposes of description only, is 75° as shown in Figure 1. The vector diagrams shown in connection with rotor blades 23A and 23C are located at a predetermined radial location (as hereinafter discussed) and comprise a rotational velocity vector 30 located perpendicular to a blade feathering axis, and the relative wind vector 29. These vectors resolve into a vector 31 defining with vector 30, a blade airflow sideslip angle β. It will be understood that β is at a maximum in respect of diametrically opposed blades 23A and 23C which are parallel to the relative wind vector 29 (at blade azimuth positions as hereinbefore defined of 105° and 285°), and that β is zero (i.e. there is no blade airflow sideslip angle) on diametrically opposed blades 23B and 23D where the rotational velocity 30 and relative wind vector 29 are parallel (at blade azimuth positions of 15° and 195° respectively). Thus it will be clear that the actual azimuth position of maximum and zero blade airflow sideslip angles β will depend on the direction of the relative wind vector 29, and that they will always be approximately 90° apart in the rotor rotational disc, as determined by equation 1. Accordingly each of the rotor blades 23A, 23B, 23C and 23D experiences a variation in blade airflow sideslip angle β as the blades rotate around the rotor azimuth, and the inventor has found that the airspeed and the relative wind direction can be obtained from measurements of the blade airflow sideslip angle β. The relationship between the blade airflow sideslip angle β and airspeed in the direction of the relative wind vector is given by the equation: β = arctan {Vcos(Ψ + )/(xVT+Vsin(Ψ + ))} where: V is the velocity of the relative wind, (i.e. the airspeed)  is the direction of relative wind vector, VT is the tip speed of the rotor blades, x is the radial location of sideslip detectors on the blades, and Ψ is the rotor blade azimuth angle. Figure 2 is a graph plotting various maximum blade airflow sideslip angles (βMAX) against airspeed (knots) at a low end of the speed range, and clearly shows that the relationship is almost linear. Whilst not shown, this generally linear relationship was found to extend throughout the blade airflow sideslip angles of interest in a helicopter up to a maximum of about 34 degrees that is representative of an airspeed of about 200 knots. Figure 3 plots blade airflow sideslip angle (β) against rotor azimuth (Ψ) with relative wind vector  = 0, rotor blade tip speed VT = 670 feet per sec. and sensor location x = 0.75, and shows resultant curves in respect of airspeeds of 50, 100 and 150 knots. The graph shows that maximum and minimum blade airflow sideslip angles (i.e. the sideslip angles of opposite sign) occur approximately midway between the points where the curves cross the axis at zero blade airflow sideslip angle. Since equation (1) involves division by a constant plus cyclic term, higher harmonics were found to be present in the blade airflow sideslip angle β and this was particularly evident as airspeeds increased above about 50 knots as shown in Figure 3. However, as shown in Figure 4 for the same design parameters the higher harmonics almost disappear and the blade airflow sideslip angle β tends to a simple cosine variation for speeds less than 50 knots. Figure 4 includes curves representative of airspeeds of 10, 20, 30, 40 and 50 knots and as would be expected with  (wind vector) = 0, the maximum blade airflow sideslip angles (β) are achieved at 0 and 180 degrees and zero blade airflow sideslip angles occur at the cross over points at 90 and 270 degrees azimuth. The change in phase of the blade airflow sideslip angle β due to a change in the relative wind vector  is shown in Figure 5 for a relative wind velocity V (or airspeed) of 10 knots. The full line is representative of the 10 knot airspeed curve of Figure 4 on a larger scale and the broken line represents the curve at the same airspeed but with the relative wind vector  = 30 degrees. As shown, maximum blade airflow sideslip angles now occur at 150 and 330 degrees rotor azimuth and zero blade airflow sideslip angles occur at 60 and 240 degrees rotor azimuth. Thus, there is no variation in amplitude in respect of the blade airflow sideslip angles only a change in phase. It will be noted that the maximum amplitude of blade airflow sideslip angle β in Figure 5 is almost 2 degrees which should be easily measurable by appropriate sensors and is much larger than any cyclic lag motion of the rotor blades themselves. It is anticipated that the blade airflow sideslip angle should be able to be resolved to better than 0.5 degrees, in which case the airspeed should be able to be resolved within about 2 knots. As will now be clear, this invention relies on the measurement of the blade airflow sideslip angle β resulting from radial and tangential airflow components effective on a rotating rotor blade, and this means that an appropriate blade airflow sideslip detector must be positioned on the blade so as to be insensitive to blade angle of attack and away from tip effects. For these reasons it may be preferable that the sensor is located on the lower surface of the rotor blade and in a region between about a 50 per cent blade chord and a 95 per cent blade chord where the local velocity is relatively insensitive to angle of attack, and the heat transfer coefficient is almost constant. The radial location of the blade airflow sideslip sensor will be a compromise between the need to maximise the resultant blade airflow sideslip angle by mounting the sensor inboard, and the need to minimise higher harmonics and avoid any interference effects of the fuselage by mounting it sufficiently far outboard. The sensor should therefore be located outboard of a 50 per cent radial station and preferably at about a 75 per cent radial station where the influence of the nose and tail boom should produce only a minimal disturbance to the blade airflow sideslip angle, and the effect of trailed tip vortices intersecting with the sensing rotor blade will be of a transient nature that can be removed by filtering the signal. Any suitable blade airflow sideslip angle sensor can be used although the design must be such that an electrical output (volts) is generally linearly proportional to the blade airflow sideslip angle β such as shown in the full line in Figure 6. Some reduction in sensitivity at large blade airflow sideslip angles may be acceptable and can be calibrated out, however, a loss of sensitivity at small blade airflow sideslip angles such as indicated by the broken line is unacceptable since it would reduce the accuracy to which the airspeed can be resolved. Several alternative instrumentation systems are suitable for use in detecting the blade airflow sideslip angle directly or, alternatively, the radial flow component from which the blade airflow sideslip angle can be calculated. Two embodiments of a pressure sensing yaw meter similar to that commonly used in wind tunnel testing and adapted to the measurement of blade airflow sideslip angle β are illustrated in Figure 7 in which the illustrated portion of blade 23 is shown inverted. Thus, either a small aerodynamically shaped blister 32 or a tapered vane 33 is attached chordwise to the blade 23 separating pressure ports 34 connected to a differential pressure transducer (not shown) for measuring the differential pressures due to blade airflow sideslip. A further alternative system for detecting the blade airflow sideslip angle which eliminates possible detrimental effects of centrifugal force is illustrated in Figure 8 and consists of the use of hot film gauges 35 positioned on opposed surfaces of an aerodynamic blister 36 to measure the differential heat transfer created by the airflow about the blister 36 on the surface of blade 23. The gauges may be manufactured integral with the aerodynamic blister 36 and coated with a protective layer to prevent contamination. In an unillustrated embodiment, temperature sensitive transistors could be used in similar locations to the gauges 35 of Figure 8, to detect the difference in heat transfer on each side of the aerodynamic blister 36 from which the sideslip angle b can be calculated. The aerodynamic blister 36 and hot film gauges 35 or temperature sensitive transistors and necessary electrical wiring can be retro-fitted to existing rotor blades or incorporated during manufacture of new blades. Another type of hot film device capable of measuring both the direction and magnitude of the flow over a surface consists of twin hot film filaments 37 having a length to width ratio of about 100 and mounted in the shape of an inverted 'V' as shown in Figure 9. Such a gauge detects the flow direction over a smooth surface by measuring differences in the thermal boundary layer, and the heat transfer from the gauges and hence their output follows a cosine relationship to the incident flow (with the cosine raised to a fractional power), from which a differential signal can be obtained. Such a hot film 'V' gauge is considered particularly well suited for use in this invention since the twin gauges 37 can either be mounted directly on the blade surface, making retro-fit possible, or ultimately formed during the manufacture of the blade. An analysis of the characteristics of such devices suggests that an optimum configuration can be achieved which will both maximise sensitivity in the range of blade airflow sideslip angles of interest and that the output will be linear within that range. Another method for detecting the radial airflow along a rotor blade from which the blade airflow sideslip angle β can easily be calculated is the use of ultra-sonics. A possible installation is shown in Figure 10, in which the rotor blade 23 is again shown inverted, as comprising two ultra-sonic transducers 38 (shown exaggerated in size for clarity) spaced-apart chordwise on the lower surface. This method has the advantage of not being affected by either centrifugal forces or contamination or bridge balancing requirements, and would be very robust. Since the transducers 38 are quite small, the system may be self contained with the radio transmission electronics included in a detector housing. Another possibility for detecting the direction of the airflow is the use of laser light. Small laser diodes could also be arranged to detect the radial flow velocity. The electrical signal from the blade airflow sideslip detectors (of whatever form) is transmitted from the rotating blades to a signal processing unit P in the aircraft fuselage. This can be accomplished using slip rings but a system based on non-contact transmission using radio or other means is preferred. The transmission system can be small, lightweight, robust and inexpensive. The necessary electrical wiring can be built into new blades during manufacture, and sensor and transmission electronics could be integrated. The signal processing unit will contain information in suitable form concerning the relationship between blade airflow sideslip angle β and airspeed. Signal processing is straightforward since it is only necessary to filter the signal from the sideslip detectors and determine its phase and amplitude. At low speeds (as illustrated in Figure 3) the signal closely follows a cosine wave with its phase and amplitude related directly to the relative wind vector and airspeed respectively. The phase angle can be established by detecting the zero sideslip angle crossing points in the rotor azimuth so as to minimise the effects of any higher harmonics present at high airspeeds. Since the amplitude and phase of the harmonics depends only upon the advance ratio, the processed signal can be readily calibrated for use also at high forward speeds. The resultant processed signals relating to both airspeed and relative wind vector can be incorporated into existing instrumentation in a cockpit for display to a pilot. Alternatively, a dedicated instrument as illustrated in Figure 11 could be used. The instrument 39 includes digital displays 40 and 41 for displaying the relative wind vector and airspeed, and a central circular display screen 42 including a movable bar 43 showing the direction of the relative wind vector from the longitudinal centreline of the helicopter as represented at 26. The length of the bar 43 varies with airspeed so as to provide, in combination with concentric rings 44 representative of different airspeeds, a visual indication of airspeed to the pilot. While several embodiments have been described and illustrated it will be understood that many modifications may be made without departing from the scope of the invention. For example, the method and apparatus can be applied to rotary wing aircraft having any number of rotor blades. Any suitable detection means can be used and a plurality of sensors could be used on each blade to extend or optimise the range and sensitivity of the system. In such an arrangement, automatic switching between the plurality of sensors could be incorporated to cater for different phases of operation. In some installations, the or some of, the sensors may be located on an upper surface of the rotor blade.

Claims: A method of detecting the airspeed of a rotary wing aircraft (10) having a sustaining rotor (23) with a plurality of radially extending rotor blades (23A-23D) for rotation about a substantially vertical axis (26), characterised in that the method comprises the steps of continuously measuring on at least one of the rotor blades (23A-23D) of the sustaining rotor (23), a blade airflow sideslip angle (β) established on the surface of the blade by interaction between a radial flow caused by a relative wind vector (29) and a velocity (30) prependicular to a blade feathering axis due to rotation of the blade, producing a signal representative of said sideslip angle (β), determining the phase and amplitude of the signal in processing means containing information on a known relationship between the phase of the signal and the relative wind direction and between the amplitude of the signal and the magnitude of the airspeed (V) and providing output signals representative of the magnitude of the airspeed (V) and the relative wind direction to a display means (39). A method according to Claim 1 characterised in that the blade airflow sideslip angle (β) is established by the equation: β = arctan{Vcos (Ψ+ )/(x VT + Vsin(Ψ + ))} where: V is the velocity of relative wind, (i.e. the airspeed)  is the direction of the relative wind vector, VT is the tip speed of the rotor blades, x is the radial location of the sideslip detectors on the blades, and Ψ is the rotor blade azimuth angle. An airspeed indicating system for a rotary wing aircraft (10) having a sustaining rotor (23) comprising a plurality of radially extending rotor blades (23A-23D) for rotation about a generally vertical axis (24) comprising sensing means (32/33;34;35;36;37;38) on at least one of the rotor blades (23) for sensing a blade airflow sideslip angle (β) established on the surface of the blade by interaction between a radial flow caused by a relative wind vector (29) and a rotational velocity (30) perpendicular to a blade feathering axis due to rotation of the blade (23), and for generating a signal representative of the blade airflow sideslip angle (β), transmission means for transmitting the signal to processing means (P) on or in the aircraft fuselage, the processing means (P) containing information on a known relationship between the phase of the signal and the relative wind direction and between the amplitude of the signal and the magnitude of the airspeed (V) and the processing means (P) being adapted to determine the phase and amplitude of the signal therefrom and to provide output signals representative of the magnitude of the airspeed (V) and the relative wind direction, display means (39) for displaying said output signals. A system according to Claim 3 characterised in that the indicating means (39) is adapted for displaying said airspeed (41) and relative wind direction (40). A system according to Claim 3 or Claim 4 characterised in that the system includes means for transmitting by a radio or the like, the signal from the sensing means (32(32/33;34;35;36;37;38) to the processing means (P). A system according to any one of Claims 3 to 5 characterised in that the sensing means (32/33;34;35;36;37;38) is located on the lower surface of the rotor blade (23) and in a region between about a 50% blade chord and a 95% blade chord. A system according to any one of Claims 3 to 6 characterised in that the sensing means (32/33;34;35;36;37;38) is located outboard of a 50% radial station. A system according to claim 7 characterised in that the sensing means (32/33;34;35;36;37;38) is located at about a 75% radial station. A system according to any one of Claim 3 to 8 characterised in that the sensing means (32(32/33;34;35;36;37;38) produces an electrical signal indicative of the sideslip angle (β). A system according to any one of Claims 3 to 9 characterised in that the sensing means comprises at least one of a) a pressure sensing yaw meter (32/33;34) which includes a pair of separated pressure ports (34) connected to a differential pressure transducer which measures differential pressures on the rotor blade (23) due to sideslip; b) a pair of hot film gauges (35) mounted to measure the differential heat transfer created by the airflow on the surface of the blade (23); c) a hot film V gauge (37) which detects the flow direction over the surface of the blade (23) by measuring differences in the thermal boundary layer and the heat transfer from the gauges (37); d) a pair of ultrasonic transducers (38) spaced apart chordwise on the blade (23); e) one or more laser diodes arranged to detect radial flow velocity. A system according to any one of Claims 3 to 10 which incorporates more than one different kind of sensing means (32/33;34;35;36;37;38) to detect the radial flow velocity and to provide a signal to the processing means.

Inventors: BROCKLEHURST ALAN, BROCKLEHURST, ALAN
Applicants: GKN WESTLAND HELICOPTERS LTD, GKN WESTLAND HELICOPTERS LIMITED
Application year: 1996
Main classification: G01P5
IPC codes: G01C21, B64C27, B64D43, G01P13, G01P5