Abstract:
A vane designed to get oriented in the axis of an ambient airflow. The vane allows for the intake of differential pressure presenting the aerodynamic incidence α of the vane. According to the invention, the differential pressure intake is balanced when the vane is oriented naturally in the axis of the airflow.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to a vane designed to get oriented in the axis of an ambient airflow. 
     Such a vane is advantageously used in an aircraft probe designed to measure aerodynamic parameters of the ambient airflow of the aircraft. 
     2. Discussion of the Background 
     The piloting of any aircraft implies knowledge of the relative speed of the aircraft with respect to the ambient air, namely the relative wind. This speed is determined by means of sensors of static pressure po, the total pressure pt and the angle of incidence α. The angle α gives the direction of the speed vector in a reference system related to the aircraft and pt−po gives the modulus of this speed vector. The three aerodynamic parameters are therefore used to determine the speed vector of an aircraft and subsidiarily a convertible or tilt-rotor aircraft. 
     One vane described in French patent FR 2 665 539 shows the utility of making the probe get oriented in the axis of the ambient airflow in order to facilitate the measurement of the angle of incidence. To overcome the frictional forces in the axis of rotation of the vane, that patent describes the use of a mechanical control loop system to cancel the aerodynamic incidence of the vane. That control loop system is useful above all at the low speeds of ambient air flow because, the lower the speed, the weaker are the aerodynamic forces of air on the vane, which do not suffice to overcome the mechanical frictional forces to orient the vane accurately in the axis of the ambient airflow. The use of the control loop system based on the cancelling of the aerodynamic incidence of the vane, however, has one drawback. In practice, the inevitable imperfections in the making of the vane create an angular divergence between the orientation of the vane due to the aerodynamic forces at high airflow speeds and the orientation of the vane due to the control loop at the low airflow speeds. Furthermore, at high airflow speeds, the control loop system may attempt to modify the orientation of the vane without being able to do so because of the size of the aerodynamic forces. This results in unnecessary consumption of electrical power by the control loop system. 
     SUMMARY OF THE INVENTION 
     The invention is aimed at overcoming these drawbacks by proposing a device that improves the consistency of the orientation of the vane whatever the speed of the ambient airflow. 
     To achieve this goal, an object of the invention is a vane device designed to get oriented in the axis of an ambient airflow and provided with means for the intake of differential pressure representing the aerodynamic incidence of the vane, characterized in that the intake of differential pressure is balanced when the vane is oriented naturally in the axis of the flow. 
     One advantage related to the invention is that it limits the cost of making the vane by preventing the narrowing of the tolerance values of shape, position, and dimension in the definition of the profile of the vane. 
     Another advantage related to the invention is that it prevents the introduction of corrections into the control loop parameters. Such corrections would hamper the interchangeability of the mobile blade of the vane alone, without its control loop means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more clearly and other advantages shall appear from the following detailed description of an embodiment illustrated by the appended drawings, of which: 
     FIG. 1 shows a vane according to the invention; and 
     FIG. 2 shows the interior of a blade of the vane. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The vane  1  shown in FIG. 1 is rotationally mobile about an axis  2 . It has for example a shaft  3  with an axis  2  that penetrates the skin  4  of an aircraft. The shaft  3  is rotationally mobile with respect to the aircraft, for example by means of a roller bearing  5 . The shaft  3  is rotationally driven by means of a motor  6  designed to orient the vane  1  in a precise angular position about the axis  2 . The shaft  3  is fixedly joined to means  7  for measuring this angular position. These means  7  comprise, for example, an optical encoder. The angular position of the vane  1 , available at output of the means  7 , defines the angle of incidence a of the aircraft. The shaft  3  is also fixedly joined to a pneumatic seal  8  used to transmit the pneumatic information, picked up by the vane  1 , to the processing means  9 . 
     The vane  1  may comprise means for the intake of the total pressure comprising a tube  10  open at one of its ends  11 . The tube  10  is substantially oriented in the axis of ambient airflow  12  when the vane is oriented in this axis of airflow  12 . 
     The vane  1  may also comprise means for the intake of the static pressure po located on the side of the tube  10 . These means are not shown in FIG.  1 . 
     The vane  1  has a blade  15 , for example with the shape of a delta half-wing. The blade  15  is symmetrical with respect to the plane of FIG.  1 . The blade has a leading edge  16 . On its lower and upper faces, in the vicinity of the leading edge  16 , the blade  15  has differential pressure intake means. On at least one face of the blade  15 , for example the lower face, these means have two holes I 1  and I 2  respectively located at a distance e 1  and e 2  from the leading edge  16 . The holes I 1  and I 2  both communicate with a chamber  20  which is better defined in FIG.  2 . 
     FIG. 2 shows the blade  15  in a sectional view along the plane perpendicular to the plane of FIG.  1  and to the leading edge  16 . The profile of the blade  15  shown in FIG. 2 is symmetrical with respect to a plane  21  perpendicular to FIG.  2 . 
     The profile is for example that of an aircraft wing. On the lower surface, the two holes  11  and  12  communicate with the chamber  20  located inside the blade  15 . On the upper face, two holes E 1  and E 2  communicate with a chamber  22  also located inside the blade  15 . The chamber  20  communicates by means of a tubular hole  23  with a pneumatic seal  8  shown in FIG.  1 . Similarly, the chamber  22  communicates by means of a tubular hole  24  with the pneumatic seal  8 . 
     The pneumatic seal delivers the pressure of the chamber  20  and that of the chamber  22  to processing means  9 . The processing means  9  compare these pressure values and generate a control signal c for the motor  6  so as to orient the blade  15  in such a way that the pressure in the chamber  20  is equal to the pressure in the chamber  22 . The processing means  9  may have a flowmeter detecting a divergence from a zero flowrate between the two chambers  20  and  22 . 
     To give the vane  1  the right orientation, whatever the speed of flow  12 , and to mitigate the defects of symmetry of position of the holes E 1 , I 1  on the one hand and E 2 , I 2  on the other hand and also to make up for the differences in dimensions between these holes, it is planned to balance the differential pressure between each chamber  20  and  22  by matching the dimensions of at least one of the holes when the vane  1  is naturally oriented in the axis of the flow  12 . 
     It is noted for example that, when the holes I 1  and I 2  have the same dimensions, the pressure PI within the chamber  20  is equal to:        PI   =         PI1   +   PI2     2     .                            
     PI 1  is the air pressure at the hole I 1 , and P 12  is the air pressure at the hole  12 . 
     The pressure PI 1  is greater than the pressure PI 2 . This difference in pressure is due to the difference between the distances e 1  and e 2 . The closer the hole, in this case I 1 , to the leading edge  16 , the greater is the to pressure PI 1  therein. 
     Consequently, when the vane  1  is naturally oriented in the axis of the flow  12  and if a difference in pressure is observed between the chambers  20  and  22 , it is possible in order to balance the pressures in the two chambers  20  and  22 , for example to increase the pressure PI present in the  15  chamber  20  by increasing a dimension, for example the diameter, of the hole I 1 . It is equally well possible to reduce the pressure PI present in the chamber  20  by increasing a dimension of the hole I 2 . Similarly, it is possible to modify the pressure PE prevailing in the chamber  22  by modifying a dimension of one of the holes E 1  or E 2 . 
     In practice, the vane  1  can be placed in a wind tunnel with a high-speed airflow  12  so that the blade  15  gets naturally oriented in the axis of airflow without any control by the processing means  9  over the motor  6 . If a difference in pressure is observed between the chambers  20  and  22 , one dimension of one of the holes, for example I 1  or I 2 , is modified so as to modify the pressure of one of the chambers so as to substantially cancel out the difference in pressure between the two chambers  20  and  22 . 
     To implement the invention, it is enough that at least one of the chambers  20  and  22  should have means to modify the pressure prevailing therein, and the other chamber may comprise only one orifice. However, for reasons of symmetry, it is preferable that both chambers  20  and  22  should both comprise holes that are symmetrical with respect to the plane  21 . 
     In the embodiment described by means of FIGS. 1 and 2, the holes I 1 , I 2 , E 1  and E 2  are single. However, it may be planned that there may be a plurality of one (or more) of these holes. This may be done to obtain a minimum airflow in the conduits  23  and  24  so that the flowmeter located in the processing means  9  can work appropriately without the dimensions of the hole considered being excessive to the point where they modify the flow  12  in its vicinity. 
     It is also possible to envisage a case where the different holes have different dimensions. For example, the surface of the hole I 1  may be smaller than that of the hole I 2 . The pressure PI prevailing within the chamber will then have the form:        PI   =           a1   ·   PI1     +     a2   ·   PI2       2     .                            
     a 1  and a 2  are coefficients depending on the surfaces of the holes I 1  and I 2 . In the example referred to here above, we will have a 1 &lt;1&lt;a 2 . In this example or in the inverse example (a 1 &gt;1&gt;a 2 ), the invention can equally well be implemented.