Abstract:
A method for enhancing aerodynamic efficiency of a vertical tail includes varying the ratio between the control surface local chord and the vertical stabilizer local chord along the height of the vertical tail. This variation is configured to adapt the local value of the coefficient of the side lift applied to the vertical tail to a maximum acceptable value of the side lift coefficient. As a result, the aerodynamic efficiency is maximized by applying a coefficient approaching the maximum acceptable side lift coefficient.

Description:
This application claims priority, under Section 371 and/or as a continuation under Section 120, to PCT Application No. PCT/FR2010/050943, filed on May 17, 2010, which claims priority to French Application No. 0902466, filed on May 20, 2009. 
     TECHNICAL FIELD 
     The present invention relates to a method for enhancing the aerodynamic efficiency of the vertical tail of an aircraft, as well as a vertical tail implementing such a method, and an aircraft comprising such a vertical tail. 
     The method is adapted for any type of aircraft and in particular for airplanes having their engines carried by airfoils. 
     BACKGROUND 
     It is known that the vertical tail of as aircraft is able to generate, as a result of the surface thereof, a side lift and an aerodynamic drag, and that it should impart to said aircraft the stability, the maneuverability and the balancing for every flight condition. 
     The surface of the vertical tail is sized so as to be able to compensate for the yaw effect created as a result of the breakdown of one of the engines of the aircraft, allowing to impart to the aircraft a sufficiently low minimum control speed should an engine breakdown, for meeting the take-off and landing performance of the aircraft. The dimensions of such a vertical tail are calculated at a low sideslip rate of the aircraft, for a control surface having, with respect to the stabilizer of the vertical tail, a maximum angular position. 
     Furthermore, it is known:
         from U.S. Pat. No. 2,356,139, a method for enhancing the aerodynamic efficiency of a vertical tail of an aircraft, comprising a vertical stabilizer and a control surface able to rotate with respect to such vertical stabilizer. To this end, said stabilizer is enlarged with an additional back stabilizer, the length of said additional stabilizer being higher than the height at which it meets the existing stabilizer. The aim of this additional stabilizer is to enhance a coefficient indicating the yaw torque produced by the whole vertical tail as a function of the angle of yaw of the aircraft;   from EP-0,887,256, a vertical tail of an aircraft comprising, more particularly, a vertical stabilizer, a control surface comprising two sections, and a spoiler; and   from document WO-00/07875, a vertical tail of an aircraft, being provided with a plate at the upper end thereof.       

     The aim of the invention is to increase the side lift of the vertical tail or to maintain the same side lift while decreasing the surface of the vertical tail. To this end, it provides a method allowing to enhance the local aerodynamic efficiency of the vertical tail throughout the height of the vertical tail, that is to bring the local value of the side lift coefficient closer to of the maximum acceptable value of the side lift coefficient beyond which there is a detachment of the air layer on the surface of the vertical tail. 
     SUMMARY OF THE INVENTION 
     To this end, according to the invention, the method for enhancing the aerodynamic efficiency of the vertical tail of an aircraft comprising a vertical stabilizer and a control surface able to rotate with respect to said stabilizer around a hinge axis, is remarkable in that the ratio between the control surface local chord and the vertical stabilizer local chord is varied along the height of the vertical tail in order to adapt the local value of the side lift coefficient applied to the vertical tail to a maximum acceptable value, the control surface local chord being the distance between the trailing edge of the control surface and the hinge axis and the vertical stabilizer local chord being the distance between the hinge axis and the leading edge of the stabilizer. 
     Advantageously, the maximum acceptable value of the side lift coefficient is the value beyond which, for a given angle between the control surface and the stabilizer, a detachment of the aerodynamic flow is observed on the surface of the vertical tail. 
     Furthermore, the variable ratio is determined between the control surface local chord and the vertical stabilizer local chord as a function of the local value of the side lift coefficient calculated with an angle of the control surface, with respect to the stabilizer, for which a detachment is observed of the aerodynamic flow on the surface of the vertical tail. 
     The ratio between the control surface local chord and the vertical stabilizer local chord is increased compared to a control surface local chord/vertical stabilizer local chord constant ratio, in the vicinity of the root of the vertical tail, and the constant ratio is decreased between the control surface local chord and the vertical stabilizer local chord compared to the ratio between the control surface local chord and the vertical stabilizer local chord, in the vicinity of the tip of the vertical tail. 
     In a first embodiment, the method allows to improve a generally trapezoid-shaped vertical tail, having a rectilinear trailing edge of the control surface and a hinge axis and for which the ratio between the control surface local chord and the vertical stabilizer local chord is constant, the improvement consisting in rotating the hinge axis in a given position, by an angle oriented so as to increase the ratio between the control surface local chord and the vertical stabilizer local chord in the vicinity of the root and to decrease the ratio between the control surface local chord and the vertical stabilizer local chord in the vicinity of the tip. 
     In a second embodiment, the vertical tail is achieved so that the ratio between the control surface local chord and the vertical stabilizer local chord allows a local value of the side lift coefficient, either constant or lower than or equal to the maximum acceptable value, along the height of the vertical tail, between the root and a given height of the vertical stabilizer, and a decreasing local value of the side lift coefficient between the given height of the vertical stabilizer and the tip. 
     In this second embodiment, the method can improve a generally trapezoid-shaped vertical tail and having a rectilinear trailing edge of the control surface and a hinge axis for which the ratio between the control surface local chord and the vertical stabilizer local chord is constant, the improvement consisting in rotating the hinge axis in a given position, by an angle oriented so as to increase the ratio between the control surface local chord and the vertical stabilizer local chord in the vicinity of the root and to (Decrease the ratio between the control surface local chord and the vertical stabilizer local chord in the vicinity of the tip, and giving to the trailing edge of the control surface a concave curved shape. 
     Preferably, the control surface is pivoted around the hinge axis by an angle ranging from 0 to 45° with respect to the vertical stabilizer for determining the varying ratio between the control surface local chord and the vertical stabilizer local chord. 
     Obviously, it results from the foregoing that the present invention also relates to a vertical tail implementing the previously described method, and an aircraft comprising such a vertical tail. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The FIGS. of the appended drawings will better explain how this invention can be implemented. In these FIGS., like reference numerals relate to like components. 
         FIG. 1  schematically shows the tail of the fuselage of an aircraft with a vertical tail of the state of the art. 
         FIG. 2  is an enlarged section view, along the line II-II of  FIG. 1 , of the vertical tail. 
         FIG. 3  shows a plot for the evolution of the ratio A 1  between the local value of the side lift coefficient Ky and the constant value of the side lift coefficient Cy calculated for a minimum control speed, as a function of the height, relatively to the span, of the vertical tail of  FIG. 1 . 
         FIG. 4  illustrates a first embodiment of the vertical tail according to this invention. 
         FIG. 5  shows a plot for the evolution of the ratio A 1  as a function of the height, relatively to the span, of the vertical tail of  FIG. 4 . 
         FIG. 6  illustrates a second embodiment of the vertical tail according to this invention. 
         FIG. 7  shows a plot for the evolution of the ratio A 1  as a function of the height, relatively to the span, of the vertical tail of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     The tail  1  of the fuselage of the aircraft, shown on  FIGS. 1 ,  4  and  6 , carries a substantially trapezoid-shaped vertical tail  2  and a horizontal tail  3 . 
     The vertical tail  2  is bounded by a trailing edge  4 , a leading edge  5 , an upper end or tip  6  and a lower end or root  7 . The vertical tail  2  comprises a vertical stabilizer  8  and a control surface  9  hinged to the vertical stabilizer  8  via a hinge axis  10  slightly ahead of the back end edges  11  of the vertical stabilizer  8 . The span of the vertical tail  2  between the tip  6  and the root  7  is referred to as E. 
     At a height h of the vertical tail  2  corresponding to the distance from the line II-II to the root  7 , there are considered the control surface local chord I being the distance between the trailing edge  4  and the hinge axis  10 , and the vertical stabilizer local chord L being the distance between the hinge axis  10  and the leading edge  5 , as illustrated on  FIG. 2 . 
     In the state of the art described on  FIGS. 1 to 3 , the I/L ratio is constant whatever the height h. 
     On the plot of  FIG. 3 , three curves C 1 , C 2  and C 3  are shown, in a bell shape each corresponding to the evolution of the previously defined ratio A 1  as a function of the height h relatively to the span E, for three different angles of the control surface  9  with respect to the vertical stabilizer  8 . A horizontal right line C 4  corresponds to the maximum ratio A 1  beyond which a detachment  12  of the air flow on the vertical tail  2  is observed. 
     As can be seen, for each curve C 1 , C 2  and C 3 , the ratio A 1  increases up to a limit height, and subsequently decreases down to becoming nil at the span E. The limit height is located closer to the tip  6  than to the root  7 , as the air flow is submitted the most to aerodynamic disturbances on this part of the vertical tail  2 . 
     The angle of the control surface  9  with respect to the Vertical stabilizer  8  is 10°, 20°, 45°, respectively for the curves C 1 , C 2  and C 3 . The ratio A 1  of the three curves C 1 , C 2  and C 3  is proportional to the angle of the control surface  9 . Therefore, whatever the height h relatively to the span E, the curve C 3  is above the curve C 2  being in turn above the curve C 1 . 
     The position of the two curves C 1  and C 2  under the right line C 4  for any height h comprised within the range [ 0 , E], illustrates that, for angles of 10° and 20°, the local side lift of the vertical tail  2  is lower than the maximum acceptable side lift without any detachment of the air layer, whatever the height h. 
     The curve C 3  is under the right line C 4  for a height h being comprised in the ranges [0, h 1 ] and [h 2 , E], and is above the right line C 4  for a height h being comprised in the range [h 1 , h 2 ] 
     In other words, for the angle of 45°, the local side lift of the vertical tail  2  is:
         lower than the maximum side lift for a height h comprised in the ranges [0, h 1 ] and [h 2 , E];   higher than the maximum side lift for a height h comprised within the range [h 1 , h 2 ].       

     The deviations between the local side lift of the vertical tail  2  and the maximum side lift indicate that the I/L ratio being constant in the state of the art, whatever the height h, is not adapted. 
     The invention consists in varying the I/L ratio for bringing the ratio A 1  of the curves C 1 , C 2  and C 3 , from now on designated as C 1 , C′ 2  aid C′ 3 , closer to the maximum ratio A 1  of the right line C 4 . 
     The varying I/L ratio is determined as a function of the ratio A 1  calculated for an angle of the control surface  9 , with respect to the vertical stabilizer  8 , resulting in a detachment  12 , corresponding to the curve C′ 3  on  FIGS. 5 and 7 . For instance, such an angle is 45°. 
     A first embodiment of the vertical tail  2  according to the invention and the plot for the evolution of the ratio A 1  of the curve C′ 3  as a function of the height h, relatively to the span E, are respectively shown on  FIGS. 4 and 5 . 
     In this first embodiment, the trailing edge  4   a  of the control surface  9  corresponds to the rectilinear trailing edge  4  of the vertical tail  2  of  FIG. 1 . The hinge axis  10   a  has pivoted around a point V, with respect to the hinge axis  10  of  FIG. 1 , by an angle θ oriented in the anti-clockwise direction. 
     The varying I/L ratio is increased with respect to the constant I/L ratio, in the vicinity of the root  7 , whatever the height h comprised within the range [ 0 , hv], so as to allow to increase the ratio A 1  of the curve C′ 3 , comparatively to the ratio A 1  of the curve C 3 , in the vicinity of the root  7 . It is decreased with respect to the constant I/L ratio in the vicinity of the tip  6 , whatever the height h comprised within the range [hv, E], so as to allow to decrease the ratio A 1  of the curve C′  3 , with respect to the ratio A 1  of the curve C 3 , in the vicinity of the tip  6 . It is equal to the constant I/L ratio for hv, the ratio A 1  of the curve C′ 3  being equal to the ratio A 1  of the curve C 3 . 
     Therefore, the absolute value of the maximum difference C′ 3 ( h )−A 1  is lower than the absolute value of the maximum difference C 3 ( h )−A 1  for any height h of the vertical tail  2 . The local value of the side lift coefficient Ky of the curve C′ 3  is therefore brought closer to the value of the maximum side lift coefficient Ky of the right line C 4 , for any height h, the value of the side lift coefficient Cy being a constant value. 
     A second embodiment of the vertical tail  2  according to the invention and the plot for the evolution of the ratio A 1  of the curve C′ 3  as a function of the height h, relatively to the span E, are respectively shown on  FIGS. 6 and 7 . 
     In this second embodiment, the trailing edge  4   b  of the control surface  9  has a concave curved shape. It could have another concave shape that is not curved. The hinge axis  10   b  is pivoted around a point W, with respect to the hinge axis  10  of  FIG. 1 , by an angle θ′ oriented in the anti-clockwise direction. 
     The varying I/L ratio is determined so that the ratio A 1  of the curve C′ 3  is equal to the maximum ratio A 1  for any height h comprised within the range [ 0 , hi], and decreases within the range [hi, b]. 
     The values A 1  of the curves C′ 1  and C′ 2 , being calculated with the determined varying I/L ratio and for angles of 10° and 20°, are two constant values being lower than the maximum ratio A 1  for a height h comprised within the range [ 0 , hi], and decrease for a height h comprised within the range [hi, E]. 
     For each one of the three curves C 1 , C′ 2  and C′ 3 , the absolute value of the maximum difference C′ 3 (h)−A 1  integrated onto the height h describing the range [ 0 , E], is lower than the absolute value of the maximum difference C 3 (h)−A 1  integrated onto the height h describing the range [ 0 , E]. The local value of the side lift coefficient Ky of the curves C′ 1 , C′ 2  and C′ 3  is thus brought closer on average to the value of the maximum side lift coefficient Ky of the right line C 4 , the value of the side lift coefficient Cy being a constant value. 
     The second embodiment is advantageous in that it decreases the size of the vertical tail and thereby, the drag applied to it, comparatively to the first embodiment, while optimizing bringing the local value of the side lift coefficient closer to the maximum acceptable value. 
     Alternatively, it would be possible to modify the leading edge instead of the trailing edge, or to modify both of them, so as to bring closer or to match the ratio A 1  of the curve C′ 3  and the maximum ratio A 1  of the right line C 4 . 
     Thus, both embodiments allow the local value of the side lift coefficient to be brought closer to the maximum acceptable value of the side lift coefficient. Moreover, advantageously, they allow the bending stresses resulting from the angular positions of the control surface with respect to the vertical stabilizer to be lower than those resulting from the configuration of the vertical tail of the state of the art, as the side lift applied to the vertical tail is more distributed toward the root than toward the tip. It result therefrom that significant mass gains are achieved for implementing the vertical tail of the aircraft.