Abstract:
A roll steering method subdivides the direction of an aircraft control surface into two elements and, during a roll control operation using ailerons, steers an upper element of the control surface in the roll direction and a lower element in the opposite direction.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for improving the roll steering of an aircraft, as well as to an aircraft implementing this method. 
     BACKGROUND OF THE RELATED ART 
     It is known that the wings of an aircraft are provided with controllable aerodynamic surfaces—principally ailerons and subsidiarily spoiler flaps—making it possible to steer said aircraft roll-wise about its longitudinal axis. It is also known that, in particular for aircraft of large dimensions, said wings are flexible and deformable so that, in certain flight situations (high speed, high Mach number, high dynamic pressure), the deflection of said aerodynamic roll control surfaces results in the twisting of said wings, thereby causing the latter to take up a local angle of incidence opposing the aerodynamic roll effects of said aerodynamic surfaces and greatly reducing their effectiveness. The roll response of the aircraft does not therefore correspond to the roll instructed by said aerodynamic surfaces. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to remedy this drawback. 
     Accordingly, according to the invention, the method for improving the roll steering of an aircraft comprising:
         a fuselage,   two wings, symmetric with respect to said fuselage, provided with controllable aerodynamic surfaces able to produce a roll movement for the aircraft, and   a vertical fin projecting with respect to the rear part of said fuselage and provided with a rudder extending along the rear edge of said fin and able to turn, with respect to the latter, about an axis of rotation,
 
is noteworthy in that:
   said rudder is divided, transversely to said axis of rotation, into at least two rudder elements disposed successively along said axis of rotation and being independently controllable in rotation about the latter; and   when said wing aerodynamic surfaces are deflected so as to communicate a roll movement to said aircraft in a determined direction:
           at least one of said rudder elements disposed on the side of the end of said vertical fin opposite to said fuselage is deflected in said determined direction of roll, and   simultaneously, at least one of said rudder elements disposed on the side of the end of said vertical fin neighboring said fuselage is deflected in the opposite direction.   
               

     Thus, said rudder elements produce antagonistic lifting forces transverse to said aircraft, which exert on the latter roll moments, likewise antagonistic, with respect to the longitudinal axis of said aircraft. However, on account of the fact that the rudder elements which exert a roll moment in the same direction as the movement due to said aerodynamic surfaces of the wings are further from said axis than said rudder elements which exert a roll moment in the opposite direction, the resultant moment exerted roll-wise by said rudder elements therefore enhances the roll movement produced by said aerodynamic surfaces of the wings. 
     Preferably, the number, the surface area, the disposition, etc., of said rudder elements is chosen in such a way that the antagonistic yaw effects, produced by said rudder elements deflected in opposite directions, balance one another at least approximately, the resultant yaw effect being practically zero. 
     In an advantageous mode of implementation of the present invention, said rudder comprises just two rudder elements, namely a lower rudder element and an upper rudder element, and, during a roll movement produced by said aerodynamic surfaces of the wings, said upper rudder element is deflected in the direction of the roll movement and said lower rudder element is deflected in the opposite direction simultaneously. In this case, the surface areas of said lower and upper rudder elements are at least approximately equal and said lower and upper rudder elements are deflected symmetrically about said axis of rotation of the rudder. Thus, no yaw effect results therefrom. 
     Regardless of the number of said rudder elements, it is preferable for the extra roll control afforded by said rudder elements to take place only when the aircraft is in a flight situation in which said aerodynamic surfaces of the wings exhibit a loss of roll effectiveness. Generally, in such a situation, the speed, the Mach number or the dynamic pressure of the aircraft are very high. So, in order to determine such a situation, it is possible to measure at least one of the three quantities hereinabove in the guise of parameter and to compare the measurement of said parameter with a threshold, for example determined experimentally, beyond which said situation occurs. Thus, as long as the measurement of the parameter is below said threshold, the roll effectiveness of the aerodynamic surfaces of the wings is satisfactory and it is not necessary to involve the rudder elements. On the other hand, when the measurement of the parameter is above said threshold, the roll effectiveness of the aerodynamic surfaces of the wings is no longer satisfactory and the method according to the invention is implemented. 
     The present invention relates moreover to an aircraft implementing the above-described method of the invention. Such an aircraft, comprising:
         a fuselage;   two wings, symmetric with respect to said fuselage, provided with controllable aerodynamic surfaces able to produce a roll movement for the aircraft;   means of roll steering of said aircraft able to control said controllable aerodynamic surfaces;   a vertical fin projecting with respect to the rear part of said fuselage and provided with a rudder extending along the rear edge of said fin and able to turn, with respect to the latter, about an axis of rotation; and   means of yaw steering of said aircraft able to control said rudder,
 
is noteworthy in that:
   said rudder consists of at least two rudder elements disposed successively along said axis of rotation;   said means of yaw steering are able to produce first individual deflection orders for each of said rudder elements;   said means of roll steering are able to produce, in addition to deflection orders for said aerodynamic surfaces, second individual deflection orders for said rudder elements, said second deflection orders being such that the resultant yaw action is at least approximately zero; and   means of addition are provided for adding, for each of said rudder elements, the second individual deflection order to the corresponding first individual deflection order, when the measurement of a parameter representative of a particular flight situation exceeds a preset threshold.       

     In a preferred embodiment, said rudder consists of a lower rudder element and of an upper rudder element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will elucidate the manner in which the invention may be embodied. In these figures, identical references designate similar elements. 
         FIG. 1  is a perspective view, from above and from the rear, of a wide-bodied civil aircraft, whose rudder is, according to an exemplary implementation of the present invention, divided into an upper rudder element and into a lower rudder element. 
         FIG. 2  is a diagrammatic view of the front of the aircraft of  FIG. 1 , illustrating the situation in which the roll is controlled by the ailerons alone of said aircraft. 
         FIG. 3  is a diagrammatic view of the front of the aircraft of  FIG. 1 , comparable to  FIG. 2 , illustrating the implementation of the method in accordance with the present invention. 
         FIG. 4  is a diagrammatic view from above of the rear part of said aircraft, with the rudder elements in the configuration of  FIG. 3 . 
         FIG. 5  shows the schematic diagram of a roll and yaw control device for the implementation of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The wide-bodied civil aircraft  1 , shown in  FIG. 1 , comprises, in a known manner, a fuselage  2  exhibiting a longitudinal axis L-L and provided with two wings  3 G and  3 D symmetric with respect to said fuselage  2  as well as a vertical fin  4 , projecting upwards with respect to the rear part  2 R of the fuselage  2 . Moreover, likewise in a known manner, on the one hand, said wings  3 G and  3 D are each provided with at least one aileron  5 G or  5 D, said ailerons  5 G and  5 D being symmetric with respect to the fuselage  2  and being able to produce a roll movement for the aircraft and, on the other hand, the vertical fin  4  is provided with a rudder  6  extending along the rear edge  7  of said fin  4  and being able to turn, with respect to the latter, about an axis of rotation z-z. 
     Moreover, said wings  3 D and  3 G are respectively provided with spoiler flaps  8 D and  8 G, pair-wise symmetric with respect to the fuselage  2 , said spoiler flaps  8 D and  8 G being usable, in a known manner, for the roll control of the aircraft  1 , to enhance the action of the ailerons  5 D and  5 G. 
     According to a first particular feature of the exemplary implementation of the present invention, represented in  FIG. 1 , said rudder  6  is divided, transversely to said axis of rotation z-z, into two rudder elements  6 S and  6 I, of aerodynamically equivalent service area, disposed one following the other, so that one,  6 I, is near the rear part  2 R of the fuselage  2  and occupies a lower position, while the other,  6 S, is near the upper end  4 S of the vertical fin  4 , opposite from said rear part  2 R, and therefore occupies an upper position. 
     The upper  6 S and lower  6 I rudder elements may be controlled jointly in rotation so that the rudder  6  behaves as if it were monolithic. The rudder elements  6 S and  6 I may also be controlled individually and, as the case may be, in opposite directions. 
     When, as is illustrated in  FIG. 2 , a roll movement is controlled in a standard fashion with the assistance of said ailerons  5 G and  5 D (the action of which is optionally enhanced by the spoiler flaps  8 G,  8 D, not represented in  FIG. 2 ), it may happen that in certain flight situations where the speed is high, the deflection of the ailerons  5 G,  5 D—and possibly of the spoiler flaps  8 G,  8 D—results in the twisting of the wings  3 G,  3 D with respect to their point of anchoring in the fuselage  2 . This results in adoptions of local angle of incidence of said wings, symbolized by the arrows f in  FIG. 2 , opposing the aerodynamic effects of the ailerons  5 G,  5 D and, possibly, of the spoiler flaps  8 G,  8 D and greatly reducing the roll effectiveness of said ailerons  5 G,  5 D and spoiler flaps  8 G,  8 D. The aircraft  1  therefore no longer has a roll response tailored to the request of the pilot. 
     To remedy this drawback, according to the invention, at the same time as the ailerons  5 G,  5 D (and possibly the spoiler flaps  8 G,  8 D) are deflected to obtain a roll movement of the aircraft  1  about the longitudinal axis L-L, the rudder elements  6 S and  6 I are deflected in a symmetric manner with respect to the fin  4  of the aircraft (see  FIGS. 3 and 4 ), the upper rudder element  6 S being deflected in the direction of the roll instructed, while the lower rudder element  6 I is deflected in the opposite direction. 
     Under these conditions, the upper and lower rudder elements produce respectively lateral lifting forces FS and FI, of equal moduli, but of opposite directions. In their turn, these forces FS and FI respectively produce, and with respect to the longitudinal axis L-L of the aircraft  1 , a moment in the direction of the roll movement instructed and a moment antagonistic to said roll movement. Since the lever arm of the force FS is larger than that of the force FI, the moment in the direction of roll is greater than the moment in the antagonistic direction and the resultant moment of these two moments therefore acts in the direction of the roll movement instructed. 
     Thus, the rudder elements  6 S and  6 I assist the ailerons  5 G,  5 D (and possibly the spoiler flaps  8 G,  8 D) in the achieving of said instructed roll movement. 
     Moreover, it will be noted that, since the rudder elements  6 S and  6 I exhibit almost identical surface areas and are deflected symmetrically with respect to the fin  4 , their deflections do not cause any yaw effect. 
     The device for the implementation of the method described above, represented diagrammatically in  FIG. 5 , comprises:
         a stick system  11 , able to produce, among other things, roll control orders for the ailerons  5 G,  5 D and, possibly, for the spoiler flaps  8 G,  8 D;   a rudder bar system  12 , able to produce control orders for the rudder elements  6 S and  6 I;   a computer  13  receiving said control orders originating from the stick system  11  and from the rudder bar system  12  and delivering, to its outputs and as a function of the electric flight control laws that it possesses in memory, respectively, a moment order instructed yaw-wise and a moment order instructed roll-wise;   a computer  14  receiving from said computer  13  the yaw-wise instructed moment order and formulating respective control orders for the rudder elements  6 S and  6 I, which orders are addressed to the actuators of the latter, respectively by lines  15  and  16 ;   an adder  17  interposed on the control line  15  for the upper rudder element  6 S;   an adder  18  interposed on the control line  16  for the lower rudder element  6 I;   a computer  19  receiving from said computer  13  the roll-wise instructed moment order and formulating respective roll-wise control orders:
           for the ailerons  5 G,  5 D and possibly for the spoiler flaps  8 G,  8 D, said corresponding orders being addressed to the actuators of the latter by a line  20 ,   for the upper rudder element  6 S, said corresponding orders being available on the working contact of a switch  21 , whose resting contact r is connected to a zero potential and whose common contact c is connected to the adder  17  by a line  22 , and   for the lower rudder element  6 I, said corresponding orders being available on the working contact of a switch  23 , whose resting contact r is connected to a zero potential and whose common contact c is connected to the adder  18  by a line  24 ; and   
           a comparator  25  receiving, from a terminal  26 , the measurement of a parameter P, such as the speed of the aircraft, the Mach number, the dynamic pressure, etc., and comparing this measurement with a preset threshold Po representative of a flight situation beyond which the roll control by the ailerons  5 G,  5 D assisted possibly by the spoiler flaps  8 G,  8 D, is no longer satisfactory, said comparator  25  being able to control said switches  21  and  23  by an action line  27 .       

     Thus, when the aircraft  1  is not in a flight situation for which the roll action of the ailerons  5 G,  5 D (and possibly that of the spoiler flaps  8 G,  8 D) is lessened, the rudder elements  6 S and  6 I are controlled by the rudder bar system  12 , through the computers  13  and  14  and the lines  15  and  16 . 
     On the other hand, when such a situation occurs, it is detected by the measurement of the parameter P which becomes greater than the threshold Po and the comparator  25  toggles the switches  21  and  23 , from their resting positions r to their working positions t, so that the roll orders formulated by the computer  19  respectively for the upper rudder element  6 S and for the lower rudder element  6 I are transmitted to the adders  17  and  18 , respectively by the lines  22  and  24 . In this case, the orders addressed to the rudder elements  6 S and  6 I comprise, on the one hand, yaw orders instructed by the rudder bar system  12  and, on the other hand, roll-assist orders originating from the computer  14 .