This invention relates to rigid sails and in particular to monitoring wind pressure and/or flow conditions in the vicinity of such sails.

The type of sailset to which this invention is applicable generally comprises one or more rigid aerofoils, each of which is constructed in the form of a wing (as of an aircraft) althouqh the fabric from which it is made may be different to the fabric of aircraft wings. Usually the cross-section of the aerofoil will be symmetrical but asymmetric cross-sections are not excluded. The aerofoils are mounted to rotate about upright axes which may pass through the aerofoil or be remote from it and connected for example by a boom. Trimming of the sailset is achieved by rotatinq one or more of the aerofoils about its axis. Sailsets of this type are described in the published European Patent Applications 61291 and 77205.

During sailing there is a constant need to select the angle of attack for the aerofoils that qives the required thrust, which may be a varying percentage of the maximum available thrust, and to protect the sailset and vessel from the excessive forces of high winds. Additionally it may be desired~€o incorporate specialised systems that modulate the trimminq action, for example to maintain a constant angle of heel while sailing or, when the vessel is moored, to minimise warp tension, roll, send etc.

In order to provide an automatic or semi automatic system that responds to the prevailing conditions and the-input direction and

thrust demands, it is desirable to monitor the precise wind and flow conditions in the vicinity of the aerofoil.

Accordingly the invention provides a sailset comprising at least one rigid aerofoil and including at least one pair of pressure tappings arranged to give an indication of the state of airflow in the vicinity of the aerofoil surface.

The invention is now described by way of example with reference to the accompanying drawings in which:

Figure 1 illustrates schematically the flow conditions around an aerofoil sailset at the onset of stalling;

Figure 2 shows an aerofoil section with pressure tapping points in accordance with the invention,

Figure 3 shows schematically in section, an aerofoil sailset with an alternative arrangement of pressure tapping points in accordance with the invention, and

Figure 4 shows schematically a flow sensing and stall warninq device in accordance with the invention.

For the avoidance of dαubt, the terms lift, drag and thrust have the following relationship. Lift is the crosswind force, draq is the downwind force and thrust is the vector sum of lift and drag.

Referring to Figure 1, an aerofoil sailset is represented in sectional view. The sailset consists of a leadinq sail element 1 that will generally be pivoted about an upright axis passing through the aerofoil, and a trailing sail element 2 that will generally be pivoted about an upright axis that is carried by the leadinq sail element 1 and passes through the leading sail element towards its trailinq edge. The trailing element 2 is attached to its axis by booms which permit the trailing element to be swung from one side of the leading element 1 to the other in mirror image

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configurations. A slat or flap may be attached to the trailing edge of the leading element 1 in order to enhance the aerodynamic slot configuration formed between the two elements. The elements 1 and 2 constitute the principal sail of the sailset and the whole assembly may be rotated by the action of a tail vane (not shown).

In Figure la the wind, indicated by the arrow, is incident to the leading element 1 at an angle of -10° and the air flow around the aerofoils is smooth and 'attached' with the exception of a small separated wake 3a at the trailing edge of the trailing section 2, on its low pressure side. The presence to a significant degree of this separated wake signals the onset of stalling and as the deqree of stalling increases the separated wake moves further alonq the aerofoil towards the leading edge (i.e. the separation occurs earlier in the airflow) as represented in Fiαures lb and lc, respectively, by references 3b and 3c, where the wind is incident at n° and +lD ^α .

For a single aerofoil the onset of stallinq occurs in the same way: at a critical point the airflow becomes separated, first at the trailinq edqe on the low pressure side and then progressively alonq the low pressure side towards the leadinα edge . At the stall a point is reached at which the aerofoil no loπqer provides useful thrust.

Except for particular circumstances such as running downwind, it is generally the aim to avoid stalling the sailset. For a particular sailset and control system the preferred operatinq anqles of attack depend on the positions of the aerofoils in which maximum lift and maximum lift : drag ratio occur. The lift maxima may occur at positions in which- the sailset is close to stallinα, and as the wind can shift rapidly, perhaps by 40° per second, and the control system for rotating the aerofoils has a finite response time, the preferred operating positions are chosen to be a safe margin from the stallinq positions, for example 4° off the position where the respective maxima occurs.

In the present invention pressure tappiπq ^p oints are located at positions where, by adjusting the anqle of attack so that a difference value for the pressure tappings is zero, the sailset is located in the preferred operating position.

In one embodiment of the invention, with the sailset arrangement as generally illustrated in Figure 3 pressure tapping points 10, 11 and 12 are provided around the leading edge of the leadinq section, signals from the three pressure tappings being supplied to a computing means that adjusts the angle of the sailset in accordance with the criteria demanded. For optimum thrust adjustment is made until the followiπq relationship is satisfied

where P,.-, is the pressure at tappinπ 10 P,, is the pressure at tappinα 11 P-,2 is the pressure at tanning 12

Tapping 10 is located on the centre line of the leadinq aerofoil element. The positions of tappings 11 and 12 determine the precise angle of incidence of the wind to which the sailset is 'tuned'. It has been determined that a preferred location for tanpinαs 11 and 12 is at a distance of 5?ό of the total chord around the perimeter from the leading edge and tappinq ID. With these pressure tapninq locations the above relationship is satisfied when the anqle of incidence to the wind is zero.

By placing the tappings closer to the leadinq edαe the relationship is satisfied when the angle of incidence is neqative, for example if the tappings are located at a distance around the perimeter of 2_% of the total chord then the angle of incidence for which the relationship is satisfied is -1°. This will be further from the stalling position than when the tappinqs are at the 5% points, and also the tuninq is less sensitive as the rate of change of the value of the relationship close to the zero point is lower.

If the tappings are located further away from the leading edge, for example at the 10% points, then the relationship is satified by a positive angle of incidence (l_-° for the 10?ό points) and the sailset is operating closer to the stalling position. The problem with tappings at these latter points is that the value of the relationship and hence the sensitivity may suffer due to the pressure versus angle of incidence curves exhibiting a hysteresis between moving into and moving out of the stalled state.

In the event of a deviation of the value of 2R. _n -E -P from zero, the control system or computing means will rotate the sailset until the zero value is regained. On a given tack a positive deviation will indicate that rotation in one direction is required, a negative deviation indicating the reverse rotation. The sense of rotation required for a given deviation changes with a change in tack, and therefore the computing means is also supplied with information regarding which tack is being sailed. This could be incorporated by way of a switching system linked to the movement of the trailing aerofoil element or taken from a wind vane, or various other means.

In order to provide optimum lift : drag ratio the relationship that is to be satisfied is __ι_ ~ __2 -^ ' With the tappings at the 5% point a zero is established when the angle of incidence of the wind is -16.5°. When the tack is changed the sign of the difference changes and so, for example, a positive is clockwise, negative is anti-clockwise convention may be adopted. (Positive is clockwise means that tapping 11 is on the starboard side). If a hiqher lift : drag ratio is desired the tappings may be placed further around the perimeter from the leading edge.

In storm conditions when it is desired to protect the sailset from excessive forces, the trailing section is aligned with the leadinq section and the sailset is maintained at zero anqle of incidence. For this purpose the difference between pressure tappings 11 and 12 is also held at zero.

Iπ all cases the figures given are approximate, and for a given sailset the precise location of the tappings should be determined experimentally, for example by wind tunnel experiments.

Referring now to Figure 2, a single aerofoil section is shown on which the points marked 10, 11, 12, 13, 14, 15 and 16 represent pressure tapping points according to the invention. All the pressure tappinq points marked need not be incorporated in every embodiment of the invention, or in some embodiments there may be tappings all along one or both sides of the aerofoil. Symmetrical arrangements of tappings are generally preferred as in most circumstances the aerofoils are symmetrical and it is desired that the sailset function similarly in each of the mirror image configurations, that is function similarly on both port and starboard tac .

As illustrated the tappings are located symmetrically in pairs on opposite sides of the aerofoil, with the exception of a lone tappinq on the centre line of the leadinπ edge. Each of the tappings is arranged to qive a siqnal, such as via valves and pressure transducers, to a computing means that then determines the optimum sailinα conditions and/or alerts for stallinq conditions.

The pressures at the tappings may be compared in either or both of the following ways in order to qive an indication of the state of flow.

Firstly the difference between a pair of tappinσs on the low pressure side of the aerofoil may be monitored. That is the difference between tappings 11 and 13, ^" or between tappinqs 12 and 14 depending on the tack. When smooth flow exists the pressure difference is small, but at the onset of stall the pressure difference increases as the separated wake has a lower velocity and produces a higher pressure (which leads to reduced lift). When - monitoring for stalling conditions in this way the second tappinq of the pair (or a further pair) should be placed at the point on the aerofoil at which the extent of the separation of the airflow

is tolerable but a warning is desired. Of course by using many pairs of tappings the exact progression of the separated airflow may be ascertained.

Alternatively the pressure difference between tappings on opposite sides of the aerofoil may be monitored. In this instance each one of the pairs 11 and 12, 13 and 14 and 15 and 16 are monitored. When smooth airflow giving thrust exists, there is a pressure difference between the high and low pressure side of the aerofoil. In stalling conditions the pressure on the low pressure side increases due to the separation of the airflow and thus the pressure difference decreases. In the instance of tappings located as in Figure 2, the pressure difference will fall first between pair 15 and 16, then between pair 13 and 14 and finally between pair 11 and 12. Clearly by putting further pairs between the pair 15 and 16 and the pair 13 and 14 a closer monitorinq of the onset of stall would be obtained.

In practice it is not necessary to have pressure tappings all along the aerofoils: a set of three pressure tappings around the leading edge is sufficient for alionment pumoses and/or a pair close to the trailiπq edqe as a stall warnino system. Figure 3 illustrates this combination adapted for a sailset with leadinπ and trailing elements, the stall warniπq tappings being located on the trailing element. Due to the variation in conditions at different heights it is preferable to average the signals (in the computing means) from several sets of tappings spaced apart in the spanwise (upright) direction.

For protection aqainst the inqress of weather the tappings may be closed by an impermeable membrane of low modulus of flexure to transmit pressure signals to the interior so nearly as possible unmodified by the tension in the membrane. To protect against ice local heating or low freezing point spray devices, or other suitable means are included.

A limitation that can occur using a system with, say, three pressure tappinqs ^' at a given level at or near the leading edge of an aerofoil is that small errors in location of the tappings causes a significant error in the angular adjustment of the aerofoil. Also gradual deterioration of the smoothness and symmetry of the aerofoils due to wear would not be accounted for by the control system which simply relies upon local pressure information.

In order to remove this limitation, in a preferred embodiment of the invention the pressure tappings are modified to provide a device that monitors the flow state for varying wiπ'd speeds. Referring to Figure 4, the device comprises a box 17 recessed in a cavity in the aerofoil structure, the outer surface of the box comprisiπq a flap 18 that is hinged or otherwise pivoted at 19 along the edge closest to the leadinq edαe of the aerofoil. The flap 19 is arranged so that its hinged edqe is generally transverse to the direction of airflow alonq the surface of the aerofoil, and when in the closed position the flap lies flush with the surface of the aerofoil. The eriαes of the flap I are sealed to the sides of the box by a flexible diaphraσm 2D. The flap is lightly biased, such as by spring loading outwards. The interior of the box is supplied with a pressure representative of the external wind speed by means of a pitot tube 21, the probe end 22 of the tube being mounted at or in front of the leadinα edqe of the aerofoil. Thus the combination of the pitot pressure and the biassing tends to push the flap 18 outwards. When smooth flow over the surface of the aerofoil exists the kinetic energy of that flow prevents the flap from opening and opposes the outward biassing and pitot pressure. In stallinq conditions the kinetic enerqy is small, or even reversed and the pressure is insufficient to prevent the flap from moving outward, to the extent permitted by the diaphraqm 20, and separating contact points 23, which initiates a stall warning indicator. (Of course the contacts could be arranged so as to make contact as the flap moves outward).

This device has the advantaqe that it is triggered by a change in flow state away from smooth high kinetic enerqy flow, and operates

independently of wind speed, as the pitot probe transmits a compensatinq pressure. Depending on the positions of these devices, they are triggered at various lift : drag ratios, those towards the trailing edge being influenced by the stalling wake before those located closer to the leading edge.

In another embodiment of the invention a pair, or spanwise separated pairs, of pressure tappings located approximately at the maximum diameter of the aerofoil (i.e. the maximum thickness perpendicular to the chord) are monitored for pressure difference ΔP . The lift coefficient C^ is the integral of the pressure coefficients C _p over the whole aerofoil surface.

Pressure on aerofoil = C _Ό X _p< * where, C _D is the coefficient of pressure, v is the stream velocity, and ρ> is the density of fluid io is called the kinetic head (which may be measured by a pitot-static assembly). ^'

Thus pressure/iύv* ^" = C

This relationship gives C from two pressure tappings and a pitot-static assembly.

Alternatively an anemometer could be used to calculate the value of the kinetic head, the anemometer yieldinq an output proportional to the velocity, and p is a constant.

Having the capacity to establish C at any particular point enables the value of C to be plotted for all angles of the sailset. Thus, in a sailset as illustrated in Figure -the sailset may be set to rotate, say at an angular rate 0 , thus causinσ the leadinq section to rotate and the angle of incidence to change. Ry monitorino the value of Cp, a curve of C versus anqle of incidence may be plotted for the current prevailinq conditions. At the stallinq point the rate of increase of C _p with respect to θ (or other monitored variable related to the anqle of incidence) will fall,

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eventually becoming negative. Having established the current C _D - versus 9 curve and established the current CL maximum the sailset -_ may then be set at the desired operating C which will be a safe level ΔC below the maximum C value.

Periodically the C versus $ curve will be redrawn and the sailset adjusted accordingly. It is envisaged that the sailset may perform a '9 sweep' excursion every three to five minutes. Typically it may be expected to find C maximum at about +5 ^α to +10°, and lift : drag maximum at about -15° to -20°.

In all embodiments it may be advantageous to utilise a wind vane upstream of the sailset to provide the control system with approximate values for the angle of incidence, the pressure tappings beinq used for fine adjustment.

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