GB2274338A

GB2274338A - Combined airspeed and direction detector, measurer and indicator

Info

Publication number: GB2274338A
Application number: GB9300850A
Authority: GB
Inventors: Donald Clive Webber; Iain Bruce Webber; Thomas George Barry
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-01-18
Filing date: 1993-01-18
Publication date: 1994-07-20
Also published as: GB9300850D0

Abstract

An airspeed and direction indicator for use on aircraft, sailing vessels, weather stations or in any situation where sensitive measurement of relative airflow is required. Referring to figure 1, a curved body (1) is placed in the airstream to be measured and holes (A, B and C), which are caused - automatically or manually - to face the airstream, are subject to aerodynamic pressures at their respective locations. Information about the relative strength and direction of the airstream is obtained from these pressures. A hole (D) in the end of the curved body (a cylinder) is used to derive static pressure. <IMAGE>

Description

COMBINED AIRSPEED AND DIRECTION DETECTOR, MEASURER AND INDICATOR.

This invention relates to a device for detection, measurement and indication of both airspeed and direction relative to the main body on which it is mounted.

Airspeed and windspeed indicators are found in aircraft, sailing vessels and static locations such as weather stations. Airspeed indicators on aircraft normally operate by measuring the pressure difference between static pressure and the dynamic pressure caused when the relative airflow comes to a stagnation point at, for instance, a flush mounted pitot. This system suffers from the following disadvantages. One is that it is often too insensitive at the low airspeeds at which a helicopter, for instance, needs to operate. A second is that if the airflow is not coming from directly ahead of the aircraft, errors can occur in the measurement. A third is that there is no indication of which direction the airflow is coming from if not from directly ahead.

Therefore, such a system does. not give both airspeed and direction information to a pilot when taxying or just before take-off to enable him or her to know the strength and direction of a cross wind. Nor does it give information about sideslip when in flight. It is of little use to the pilot of an aircraft, for instance a helicopter, when operating at very low airspeeds or when trying to establish a zero airspeed hover. When flying sideways or backwards the pilot would have no indication of speeds which may approach or exceed the design limits of the airframe.

Sailing vessels measure apparent windspeed and direction by two separate systems and, again, these tend to be insensitive at low speeds.

According to the present invention there is provided a combined airspeed and direction detector, measurer and indicator comprising a sensor body in the form of a cylinder or sphere (or any shape which, in the region where measurements are to be made, is curved in at least the desired plane of direction measurement and where the aerodynamic pressure distribution, as a result of the airflow adjacent to it, is reasonably well known or can be deduced by theory or experiment). Direction is measured indirectly, having taken into account the approximate magnitude or sign (or both) of the measured airspeed as necessary, by the difference in aerodynamic pressure on either side of a datum on the sensor body.Airspeed is measured indirectly, having taken into account by some means the approximate direction of the airflow, by the aerodynamic pressure at the datum or the sum of pressures at specific points on either side of the datum. Holes at the surface of the body are connected to pressure or flow sensors or both pressure and flow sensors. Using the magnitudes and signs (positive or negative) of the pressures, or by using flows caused by pressure differences, both airspeed and direction are deduced mechanically or electrically. The sensors may be contained inside the sensor body or in a remote container. If in a remote container, a means (such as tubes) would be required to transmit the pressures at the holes to the remote container.

Similarly, electrical circuitry may be contained inside the detector body or in a remote container. The remote container may be the main body on which the detector body is mounted, for instance an aircraft or a sailing vessel. Indication of airspeed and direction would normally be displayed on meters or a single combined display situated remotely from the detector body.

Electrical analogues of airspeed and direction can be available for other uses such as inputs into computers. The detector body may be mounted on the main body or be a suitably shaped part of the main body.

A specific embodiment of the invention will now be described by way of example with reference to the following drawings in which: Figure 1 shows the body of an airspeed and direction detector mounted on part of some other main body (e.g. an aircraft).

Figure 2 shows a cylinder in an airstream. It is oriented to the desired plane of angle measurement as shown.

Figure 3 shows the pressure distribution around the surface of the cylinder.

Figure 4 shows, in orthographic projection, the body and internal organisation of an airspeed and direction detector.

Figure 5 shows in the form of a block diagram the circuitry of an airspeed and direction detector, measurer and indicator.

Figure 6 shows various arrangements for deriving static or pseudo-static pressures.

Figure 7 shows an alternative arrangement of sensor body mounted on some part of a main body.

Figure 8 shows the top view of the sensor in figure 7 and the internal organisation.

Figure 9 shows the block diagram of the circuitry for the said alternative device.

A suitable sensor body is a cylinder (1) as shown in figure 1 where it is mounted on part of of a main body (2) such as an aircraft by a shaft (3). The sensor body has holes A, B, C and D for use in sensing pressures at their respective positions.

Figure 2 shows such a cylinder (1) in an airstream (4) where the desired plane of angle measurement is about the axis of rotation (5). The top t6) and bottom surfaces of the cylinder, being parallel to the airflow, do not substantially disturb it and they remain approximately at static pressure. Aerodynamic forces around the curved surface of the cylinder cause a pressure distribution as illustrated in figure 3. Positive pressures are shown as arrows pointing inwards and negative pressures as arrows pointing outwards. The spacing between arrows in this diagram is 10 degrees although, in reality, the distribution is continuous as shown by the dotted line.The length of any arrow indicates the magnitude of the pressure, which is proportional to the square of airspeed, and the maximum positive pressure occurs in line with the airstream (4) as shown. Referring to figure 4, the detector body comprises a cylinder (1) (in this case it is hollow) with holes A, B, C and D as described above. Hole D is used, in this variant, to derive a static pressure inside the sensor body. Hole B is at the datum (7) and when it is pointing into the airstream, (defined here as zero degrees of error between sensor datum and airstream direction), it is subjected to a pressure, above static, proportional to the square of the airspeed.Hole B is connected by some means such as a tube to one side of a flow sensor (8) whose other side is open to static, this sensor gives an electrical output in proportion to the pressure difference and thus the square of airspeed. (Flow sensors are more sensitive than pressure sensors at low airspeeds). Holes A and C are offset one on each side of hole B by equal angles. When Hole B is pointing directly into the airstream, holes A and C are subject to equal pressures and therefore the difference is zero. When hole B is pointing slightly off to one side, say + 5 degrees error, one of holes A and C comes closer to being in line with the airstream and its pressure increases slightly. The other hole moves further away and its pressure decreases slightly. Thus there will be a difference in pressure between these two holes.This difference in pressure changes polarity when hole B is pointing off slightly to the other side of the airstream, say - 5 degrees error. For a limited but nevertheless significant range in error angle either side of zero, this pressure difference between holes A and C increases in magnitude with increasing angle. Holes A and C are connected by some means such as tubes to each side of a flow sensor (9) which gives an electrical analogue of the error angle.

This error signal can be used, as described below, as the input to a servo to drive the cylinder to zero error angle and keep hole B pointing into the airstream.

Figure 5 shows, in block diagram form, the essence of the electrical circuitry needed to display airspeed and direction relative to the main body. The output of the sensor between holes A and C which generates the error signal is fed to the input of a servo circuit. This drives the motor and gearbox in a direction dependant on the polarity of the error signal. The gearbox is connected via a shaft to the sensor body which is driven until the error signal reduces to zero and the servo settles with hole B pointing into the airstream. The shaft is also connected to an angle transducer which gives an electrical analogue of the angle between the sensor body datum and a datum on the main body.

This is fed to a suitably calibrated display for direction. The output of the sensor between holes B and D is connected to an amplifier and thence to a suitably calibrated display for airspeed. It is also fed to the servo circuit where it is used in such a way that the servo may only settle when the airspeed signal is of the correct polarity or above a set threshold. This ensures that the servo cannot settle when the datum is pointing 180 degrees away from the correct direction (where the error signal becomes ambiguously zero). It is also used to adjust the amplification of the servo amplifier giving the servo more sensitivity at lower airspeeds. The shaft between the sensor body and the main body is also used to carry any means, such as wires or slip rings, needed to communicate between the said bodies.The servo may be analogue, digital or computer circuitry, or a hybrid of any of these, or a human operator may adjust the angle of the sensor body to the main body by interpreting the angle error and speed signals. In the latter case the human is the servo.

The angular spacing between holes A and C (measured from their centres) is not critical although optimal performance is in the region from around 50 degrees to 100 degrees. Referring now to figure 6, this shows, as alternatives to hole D, arrangements for deriving static or pseudo-static inside the sensor body. Pseudostatic is defined here as a pressure which varies with airspeed or both airspeed and direction. Figure 6a shows a large number of small holes all round the sensor body. This sets a pseudostatic pressure inside the sensor body which becomes increasingly negative in proportion with the square of airspeed and, as a consequence, improves sensitivity of the desired measurements.

Figure 6b shows an arrangement whereby static or pseudo-static derived elsewhere (for instance from an aircraft's static system) is fed to the sensor body by some means such as a tube through the shaft (3). Figure 6c shows a hole at the back of the sensor body on the opposite side of the datum (7). When the datum is pointing approximately into the airstream this pressure, as can be seen from reference to figure 3, is negative in proportion with the square of airspeed and, again, sensitivity is increased as a consequence. A similar effect may also be achieved by more than one hole spaced symetrically about a line diametrically opposite the datum (7) as illustrated in the example in figure 6d. These may both be open to a hollow cylinder or they may be connected individually to flow or pressure sensors.

Figure 7 shows an alternative arrangement of the sensor body (1) which is mounted on some part of the main body (2) by a shaft (3). In this case the sensor body (1) has only holes A and C equally spaced on either side of the datum (7) and, in this variant, hole D which sets static pressure inside of the hollow cylinder. Figure 8 shows the top view of the cylinder and the internal organisation. Holes A and C are each connected by some means such as tubes to flow sensors (10), the other ends of which are open to static pressure. The angle error signal is derived from the difference between the two flows. The speed signal is derived from the sum of the two flows which gives a result proportional to the square of airspeed. Referring now to figure 9, this shows in essence the block diagram of the circuitry needed to display airspeed and direction relative to the main body.Each of the flow sensors' outputs are connected to a subtracter to derive the angle error signal and to an adder to derive the speed signal. The remainder of the block diagram is identical to figure 5 previously described. The angle between the datum and holes A and C should not be close to 45 degrees otherwise, as can be seen from reference to figure 3, when the angle error is zero there will be little or no pressure at holes A and C to derive airspeed. Static or pseudo-static can also be derived as previously described from arrangements shown in figure 6.

If only a limited range of angle measurement is required either side of some datum on the main body, as for example in a combined airspeed and slip indicator for a fixed wing aircraft, the sensor body need not be made to rotate but may be rigidly fixed to, or be a suitably shaped part of, the main body. The angle error and speed signals, however derived, may be amplified as necessary and used as required for display or as inputs into a computer. As the angle error signal (for any given angle within the said limited range) increases in amplitude with increasing airspeed, the airspeed signal may be used in the angle error ciuitry to adjust the amplification to compensate.

Claims

CLAIM 1 A combined airspeed and direction detector, measurer and indicator comprising a body with a curved surface where the aerodynamic pressure distribution around that body, as a result of airflow past it, is known or can be deduced. The said body may be mounted on, or be part of, some other main body and components may be distributed between these bodies. On or in the said curved body is provided a means of detecting, either directly or indirectly, aerodynamic pressures at specific points on the surface of that body, by which means the relative airspeed may be deduced by mechanical or electrical means.On or in the said curved body is also provided a means of detecting, either directly or indirectly, aerodynamic pressure differentials between specific points on the surface of that body, by which means the angle of the airstream relative to the main body may be deduced by mechanical or electrical means.

CLAIM 2 A combined airspeed and direction detector, measurer and indicator, as claimed in claim 1, using a circular cylinder as the curved body.

CLAIM 3 A combined airspeed and direction detector, measurer and indicator as claimed in claims 1 and 2 using pressure sensors to measure aerodynamic pressures or aerodynamic pressure differentials.

CLAIM 4 A combined airspeed and direction detector, measurer and indicator as claimed in claims 1 and 2 using flow sensors to measure aerodynamic pressures or aerodynamic pressure differentials indirectly.

CLAIM 5 A combined airspeed and direction detector, measurer and indicator as claimed in any of the preceding claims whereby the direction is measured by causing. the cylinder or other curved body to rotate until there is no aerodynamic pressure differential between specific points on either side of a datum on the said body and whereby the airspeed is measured, directly or indirectly, by the aerodynamic pressure at the said datum.

CLAIM 6 A combined airspeed and direction detector, measurer and indicator as claimed in claims 1, 2, 3 and 4, whereby the direction is measured by causing the cylinder or other curved body to rotate until there is no pressure differential between specific points on either side of a datum on the said body and whereby the airspeed is measured, directly or indirectly, by the sum of pressures at specific points on either side of the said datum.

CLAIM 7 A combined airspeed and direction detector, measurer and indicator as claimed in claims 1, 2, 3 and 4 wherein the cylinder or other curved body is fixed rigidly to the main body and the direction measurement, within a limited range, is achieved by direct or indirect measurement of pressure differential between specific points on either side of a datum and whereby airspeed is measured by the aerodynamic pressure at the datum or by the sum of aerodynamic pressures at specific points on either side of the datum.

CLAIM 8 A combined airspeed and direction detector, measurer and indicator as claimed in any of the preceding claims wherein, instead of using static pressure wherever required, use is made of another pressure which is a function of airspeed and independent of direction of airflow relative to the datum.

CLAIM 9 A combined airspeed and direction detector, measurer and indicator as claimed in claims 1, 2, 3, 4, 5, 6 and 7 wherein, instead of using static pressure wherever required, use is made of another pressure (or other pressures) which is (or are) a function of both airspeed and direction of airflow.

CLAIM 10 A combined airspeed and direction detector, measurer and indicator as claimed in any previous claim wherein any combination of the means in any previous claim may be used.

CLAIM 11 A combined airspeed and direction detector, measurer and indicator substantially as described herein with reference to figures 1 to 9