Abstract:
The method of the invention includes forming a difference between left and right braking commands, by converting the difference into an additional control command for rudder and for a steerable nose gear, and applying the additional control command to the rudder and steerable nose gear according to both of the following conditions: the difference is greater than a first threshold and the combined control command transmitted to the rudder and steerable nose gear by a rudder bar is less than a second threshold.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is aimed at a method and a device for driving an aircraft during its run over the ground, as well as an aircraft equipped with such a device. 
     BACKGROUND OF THE INVENTION 
     It is known that the driving of an aircraft on the ground, that is to say the yaw control of said aircraft, is chiefly effected by the rudder and the steerable nose gear, disposed near the nose of the latter (commonly called the “nosewheel”). For this purpose, the rudder and the steerable nose gear are controlled from a rudder bar, at the disposal of the pilot. A depression to the right, for example, of the rudder bar conveys the desire of the pilot to produce a yawing moment tending to move the nose of the aircraft toward the right, this yawing moment being obtained by a rightward deflection of the rudder and of the steerable nose gear. 
     It is known moreover that ground braking of aircraft is ensured by brakes installed on the wheels of the undercarriage legs, as well as by spoiler flaps (airbrakes) capable of increasing the drag of the aircraft and of holding it hard down on the ground so as to increase the effectiveness of the brakes of the wheels and/or by thrust reversers, the wheel brakes being controlled by a system capable of taking into account commands coming from the pilot or from an automatic device. To brake the aircraft on the ground, the pilot has two pedals mounted on the rudder bar and associated respectively with the brakes of the wheels disposed on either side of the longitudinal axis of the aircraft: thus, the right pedal can control the brakes disposed on the right of the aircraft and the left pedal can control the brakes disposed on the left. If he exerts different braking actions on the two brake pedals, the pilot produces differential braking between the wheel sets disposed on either side of the longitudinal axis of the aircraft, this differential braking producing a yawing moment for the aircraft. 
     The driving of an aircraft on the ground can therefore also be obtained by such differential braking. Thus, when the pilot of the aircraft wishes to correct the lateral path of the aircraft on the ground, he can act on the rudder bar, to involve the rudder and the steerable nose gear, and/or on the brake pedals, to produce differential braking. 
     It should however be noted that action on the brake pedals alone may, under certain conditions such as strong sidewind, engine failures, etc., be insufficient to control the lateral path of the aircraft and lead the latter to go off the taxiway. 
     Now, such a situation can occur in the event of failure of the rudder bar. Specifically, in this case, only the brake pedals are available for yaw control of the aircraft during its ground run. 
     SUMMARY OF THE INVENTION 
     The present invention is aimed at remedying this drawback and at making it possible to increase the yawing moment produced on the aircraft by dissymmetric action alone on the brake pedals. 
     To this end, according to the invention, the method for driving an aircraft during its run over the ground, said aircraft comprising: 
     a steerable nose gear; 
     a rudder, disposed at the rear of said aircraft; 
     a rudder bar at the disposal of the pilot of the aircraft, making it possible to address a combined control command to said steerable nose gear and to said rudder, to control said aircraft in yaw; 
     at least two undercarriage legs, symmetric with one another with respect to the longitudinal vertical mid-plane of the aircraft, the wheels of said undercarriage legs being equipped with brakes; and 
     two braking control members at the disposal of said pilot, respectively associated with said undercarriage legs and each producing a braking command to control the wheel brakes of the associated undercarriage leg, 
     is noteworthy in that: 
     the difference between said braking commands is formed; 
     said difference in the braking commands is transformed into an additional control command for said rudder and for said steerable nose gear; and 
     said additional control command is applied to said rudder and to said steerable nose gear, on the dual condition that: 
     said difference in the braking commands is greater than a first threshold; and 
     said combined control command addressed by said rudder bar to said rudder and to said steerable nose gear is less than a second threshold. 
     Thus, by virtue of the present invention, in the case where the rudder bar has failed with its two levers locked in the vicinity of the neutral position, a differential braking action on the part of the pilot will be able to bring about a rotation in the appropriate sense of the rudder and of the steerable nose gear, allowing the yaw control of the path of the airplane on the ground. On the other hand, if the aircraft on the ground is subjected to a strong crosswind compelling the pilot to control the path of the aircraft with the rudder bar, and possibly with a differential braking action, the latter will not be able to exert a complementary effect either on the position of the rudder, or on that of the steerable nose gear. 
     In a known manner, the maximum travel of each of said braking control members lies between a neutral position and a maximum braking position and, in particular, this maximum travel corresponds to a rotation of said braking control members between an angle of rotation value equal to zero (in said neutral position) and a maximum angle of rotation value (in said maximum braking position). 
     Advantageously, said first threshold corresponds to a fraction of said maximum travel of the braking control members lying between one third and two-thirds and, preferably, corresponds at least approximately to half of said maximum travel, that is to say said first threshold is then equal to half of said maximum angle of rotation value. 
     In a preferred mode of implementation of the present invention, to take into account said first threshold, prior to said transformation into an additional control command for the rudder, said difference in the braking commands is converted into a first function taking the value zero up to said first threshold and increasing, preferably linearly, onward of said first threshold, up to a maximum value attained for the maximum value (equal to the maximum angle of rotation value) of said difference in the braking commands. Said maximum value of said first function is equal to said maximum value of said difference in the braking commands. 
     In a manner similar to that recalled above for said braking control members, the maximum rotational travel of each of the levers of the rudder bar lies between a neutral position (corresponding to a zero angle of rotation) and a position corresponding to the maximum deflection of the rudder (corresponding to a maximum value of angle). 
     Preferably, the transformation of said difference in the braking commands into an additional control command for the rudder is obtained by multiplying said first function by a coefficient equal to the ratio of said maximum travel of the levers of the rudder bar to said maximum travel of said braking control members. 
     In order to take into account said second threshold, said additional control command thus obtained is subjected, before addition to said combined control command, to a limitation defining, with the aid of said second threshold, a domain outside which said additional control command is zero and inside which said additional control command has a limited authority on said rudder and on said steerable nose gear. The contour of said domain corresponds to a function which is zero when said combined control command is equal to said second threshold and which is equal to said second threshold when said combined control command is zero and varies linearly between these values. 
     Said second threshold corresponds to a fraction, for example two-thirds, of said maximum travel of the levers of the rudder bar. 
     As a precaution, each of said braking commands is limited before formation of their difference. Likewise, it is advantageous that the sum of said combined control command and of said limited additional control command be subjected to a limitation before application to said rudder and to said steerable nose gear. 
     The present invention relates moreover to a device for driving an aircraft during its run over the ground, said aircraft comprising: 
     a steerable nose gear; 
     a rudder, disposed at the rear of said aircraft; 
     a rudder bar at the disposal of the pilot of the aircraft, making it possible to address a combined control command to said steerable nose gear and to said rudder, to control said aircraft in yaw; 
     at least two undercarriage legs, symmetric with one another with respect to the longitudinal vertical mid-plane of the aircraft, the wheels of said undercarriage legs being equipped with brakes; and 
     two braking control members at the disposal of said pilot, respectively associated with said undercarriage legs and each producing a braking command to control the wheel brakes of the associated undercarriage leg. 
     According to a preferred embodiment, the device of the invention is noteworthy in that it comprises: 
     means for forming the difference between said braking commands; 
     a function generator transforming said difference into a function taking the value zero up to a first threshold and increasing, onward of said first threshold, up to a maximum value attained for the maximum value of said difference in the braking commands; 
     means for transforming said function into an additional control command for said rudder and for said steerable nose gear; 
     limitation means able to limit said additional control command and defining, with the aid of a second threshold, a domain outside which said additional control command is zero and inside which the authority of said additional control command has a limited authority on said rudder and on said steerable nose gear; 
     means for forming the sum of said combined control command and of said additional control command limited by said second function generator; and 
     means for applying said sum to said rudder and to said steerable nose gear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will elucidate the manner in which the invention may be carried out. In these figures, identical references designate similar elements. 
         FIG. 1  is an end-on view of a wide-bodied civil airplane to which the present invention can apply. 
         FIG. 2  is a lateral view of the airplane of  FIG. 1 . 
         FIG. 3  is a partial view from above of the civil airplane of  FIGS. 1 and 2 , only the contour of said airplane being represented so as to show the location of the various wheel trains with their braking device and the ground-borne yaw control device. 
         FIG. 4  gives the schematic diagram of an exemplary embodiment of the ground-borne yaw control device in accordance with the present invention. 
         FIGS. 5 and 6  are charts illustrating diagrammatically and partially the operation of the exemplary embodiment of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The airplane  1 , shown diagrammatically in  FIGS. 1 and 2  and taxiing on the ground S, comprises two pairs of wheel trains  2 G,  2 D and  3 G,  3 D, respectively, as well as a steerable nose gear  4 , disposed near the nose of the airplane  1  (commonly called a “nosewheel”). 
     The two wheel trains  2 G and  2 D, disposed respectively on the left and on the right of the airplane  1 , are symmetric with one another with respect to the longitudinal vertical mid-plane V-V of the airplane  1 . Likewise, the two wheel trains  3 G and  3 D, also disposed respectively on the left and on the right of the airplane  1 , are symmetric with one another with respect to said plane V-V. On the other hand, the two wheel trains  2 G and  2 D are closer to said plane V-V (and therefore to one another) than the wheel trains  3 G and  3 D. 
     Each wheel  5  of the close trains  2 G and  2 D is equipped with an individual brake (shown diagrammatically under the reference  9  in  FIG. 4 ) and the individual brakes of each of the trains  2 G or  2 D are controlled by a control device  6 G or  6 D, respectively. 
     Likewise, each wheel  7  of the far trains  3 G or  3 D is equipped with an individual brake (shown diagrammatically under the reference  10  in  FIG. 4 ) and the individual brakes of each of the trains  3 G or  3 D are controlled by a control device  8 G or  8 D, respectively. 
     The control devices  6 G,  6 D,  8 G and  8 D are themselves controlled by a braking distribution device  11 , receiving, respectively via lines  15 G and  15 D, commands for left FG and right FD braking for two transducers  14 G and  14 D respectively associated with two members left  12 G and right  12 D, at the disposal of the pilot. 
     In a known manner, the left braking command FG is most especially used to brake the wheels  7  of the left far train  3 G and can be used to brake the wheels  5  of the left close train  2 G. Likewise, the right braking command FD is most especially used to brake the wheels  7  of the right far train  3 D and can be used to brake the wheels  5  of the right close train  2 D. 
     Other braking members (not represented) are preferably placed at the disposal of a copilot of the airplane  1 . 
     As shown diagrammatically by  FIG. 4 , said braking members  12 G and  12 D can consist of rotary pedals, articulated respectively to the free ends of the levers  13 G and  13 D of the rudder bar  13  of the airplane  1 . 
     When the pilot, with his left (or right) foot rotates the left pedal  12 G (or right pedal  12 D), the rotation of said pedal is detected by the left transducer  14 G (or by the right transducer  14 D), which produces the corresponding left braking command FG (or right braking command FD), addressed to said braking distribution device  11 . The angle of rotation α of each pedal  12 G or  12 D lies between 0 (pedal at rest) and αmax (maximum rotation) and the corresponding braking command FG or FD is dependent on the value of said angle of rotation α. 
     In a known manner, the rudder bar  13  is intended, when the airplane is taxiing on the ground, to control the rudder  16  of the airplane  1  (see  FIG. 2 ) and the orientation of the steerable nose gear  4  of the airplane  1 . To this end, two transducers  17 G and  17 D are respectively associated with the two levers  13 G and  13 D of the rudder bar  13 , so as to produce combined leftward LGC and rightward LDC yaw control commands, respectively. The angle of rotation β of each lever  13 G or  13 D of the rudder bar  13  lies between 0 (lever at rest) and βmax (maximum rotation) and the combined control commands LGC and LDC are applied to said rudder  16  and to said steerable nose gear  4  respectively by way of actuation devices  18  and  19 . 
     In accordance with the present invention, the yaw control commands LGC and LDC, arising respectively from the transducers  17 G and  17 D, as well as the braking commands FG and FD, arising respectively from the transducers  14 G and  14 D, are transmitted to a processing device  20  able to produce an additional control command D 2   l  for the rudder  16  and for the steerable nose gear  4 , in the case where the differential braking is significant, while the command LGC or LDC is weak. 
     The processing device  20  comprises a subtracter  21  to which the braking commands FG and FD are fed, using the lines  15 G and  15 D, by way of respective limiters  22 G and  22 D intended to avoid introducing completely erroneous input data into the subtracter  21 . For example, the limiters  22 G and  22 D require that FE and FD be limited to between 0 and αmax. 
     Thus, at its output, the subtracter  21  delivers a differential braking command D 1 , for example considered to be positive if FG is larger than FD and negative in the converse case. The differential braking command D 1  is addressed to a function generator  23 , able to transform the differential braking command D 1  into a function F(D 1 ), an example of which is shown by  FIG. 5 . In this example, the function F(D 1 ) is zero below half (αmax/2) the maximum travel of the pedals  12 G and  12 D and is a linearly increasing function of D 1  between said half maximum travel αmax/2 and the maximum travel αmax. For D 1  equal to αmax, F(D 1 ) is also at αmax. 
     Thus, the function F(D 1 ) is limited at high differential braking commands above the threshold αmax/2. It is transmitted to a converter  24  able to transform it into a command for the rudder  16 . For example, said converter  24  multiplies the function F(D 1 ) by a coefficient K equal to the ratio of the maximum deflection βmax of the levers of the rudder bar  13  to the maximum rotation αmax of the brake pedals  12 G and  12 D. 
     At the output of the converter  24 , an additional deflection command D 2  for the rudder  16  and the steerable nose gear  4  is therefore obtained. This additional deflection command D 2  is addressed to a limiter  25  which receives the control commands LGC and LDC arising from the transducers  17 G and  17 D tied to the rudder bar  13  and produces a function LimD 2  able to limit the domain of action of the command D 2  to weak rudder deflection commands  16  and to limit the authority of the brake pedals  12 G and  12 D on the rudder  16  and on the steerable nose gear  4 . 
     In  FIG. 6  is represented an exemplary limitation domain  26  produced by the limiter  25 . The domain  26  is bounded by a contour  27  satisfying a function which is zero when the combined control command LGC or LGD is equal to a threshold 2·βmax/3 equal to two-thirds of the maximum value βmax of the angle of deflection β of the levers  13 G and  13 D of the rudder bar  13 , and which is equal to said threshold 2·βmax/3 when said combined control command is zero. Between these two points, the variation of the contour  27  can be linear. 
     Outside the limitation domain  26 , the limiter  25  zeros the additional deflection command D 2 , while inside said domain the latter is compelled to vary inversely to the combined control command LGC or LGD. 
     Thus, at its output, the limiter  25  delivers a limited additional deflection command D 2   l , which is added to the appropriate combined control command LGC or LGD in a summator  28 . 
     The sum thus obtained is addressed to a limiter  29 , for example limiting it to the domain −βmax, +βmax, after which it is transmitted to the actuation devices  18  and  19  of the rudder  16  and of the steerable nose gear  4 . 
     Optionally, the command D 2   l  can also be addressed to aerodynamic surfaces of the airplane  1  (for example spoiler flaps, not represented) able to increase the yawing moment while moving over the ground. 
     Thus, in a strong crosswind, the pilot of the airplane  1  controls the path of movement of the airplane  1  with the rudder bar  13  and, if necessary, with a differential braking action on the pedals  12 G,  12 D. The rudder bar  13  being highly deflected, the differential braking has no complementary effect, either on the position of the rudder  16 , or on the steerable nose gear  4 . 
     Under the same conditions, if the levers  13 G and  13 D are locked, they are close to their neutral position, so that a differential braking action will act on the brakes and on the positions of the rudder  16  and of the steerable nose gear  4 , thus allowing the path of the airplane  1  to be controlled. 
     It will be noted that, by virtue of the present invention, the additional command D 2   l  is limited in a continuous and progressive manner as a function of the command LGC or LGD originating from the rudder bar  13 , in such a manner that this additional command D 2   l  actually equals zero when the command LGC or LGD reaches a certain threshold (2·βmax/3), always giving priority to the LGC or LGD command.