Abstract:
Device to reduce the lateral force generated by an aerial refueling boom ( 11 ) of an aircraft characterized in that it comprises at least one plate ( 31 ), said plate ( 31 ) comprising two cantilevered wings ( 32, 33 ), said wings ( 32, 33 ) comprising perforations ( 34 ), so that the wake produced in the boom ( 11 ) has a lower dynamic pressure than that of the free stream.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a system to perform aerial refueling of aircraft, and in particular to a device to reduce the side force produced by an aircraft refueling boom. 
       BACKGROUND 
       [0002]    One of the procedures currently in use to perform aerial refueling of aircraft involves the use of a cylindrical beam (boom) with a telescopic tube through which fuel is passed from the tanker aircraft to the receiver aircraft. The boom is articulated with two degrees of rotational freedom at the attachment point to the tanker. A pair of independently moving fins (ruddevators) attached to the boom at a distance to the articulation, allow the boom operators to fly the boom, controlling its position in space, to guide the telescopic tube into the receptacle of the receiver. 
         [0003]    The boom cross section must be symmetric and have a low drag, so that for a given control force in the ruddevators a large operational flyable envelope can be achieved. This need for a low drag boom section is even more important when the boom is to be capable to refuel large aircraft, as this operation is performed in specific conditions that increase the drag. 
         [0004]    The low thickness—low drag airfoils typically used in aeronautics generate a large amount of lateral force (lift) when they are subjected to local sideslip (incident current out of the section plane of symmetry). For the typical applications of these airfoils, this is beneficial, as the requirement is to have the largest lift to drag ratio possible. Nevertheless, for the particular application of the boom at sideslip, this lateral force is detrimental, as it opposes the control force introduced by the ruddervators, thus reducing the operational envelope of the system. This problem is aggravated by the boom flexibility. The lateral force generated by the ruddevators to control de boom twist the boom in the sense to increase the local sideslip, with the corresponding increment of the opposing force generated by the boom and associated reduction of the flyable envelope. 
         [0005]    A possible way to solve this problem is to use circular or quasi-circular boom cross-sections, which generate much less lift than an airfoil, but they also produce a much larger drag than an airfoil, with the corresponding impact on the operational envelope. 
         [0006]    Another way to address this issue is by the use of thick airfoils (70% thickness ratio for instance) that when at sideslip (due to a turn as rigid solid of the boom around the articulation or due to its elastic deformation) generate negative lift which adds to the ruddevators control force and does not oppose it. This type of airfoils has several disadvantages:
       non-linearities in the lift-incidence relationship, which complicate the control laws design;   larger drag with the corresponding impact on the operational envelope;   high sensitivity to variations in section geometry and flight conditions (Mach number, Reynolds number, elevation angle, turbulence) which complicate control and may even induce the loss of the inverse lift effect;   they are prone to aeroelastic instability problems (divergence, flutter, galloping, etc.).       
 
         [0011]    The present invention comes to solve these drawbacks. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention relates to a passive device that provides a significant reduction of the lateral force generated by an aerial refueling boom at sideslip even when airfoil sections are used. 
         [0013]    For this reason, the device comprises a plate located at the leading edge of the boom airfoil that generates a low dynamic pressure wake around the boom. Alternatively a series of plates can be arranged at different positions along the boom chord. 
         [0014]    The plate of the device according to the invention comprises several perforations and has irregular edges to limit its negative effect on drag and to diminish at the same time the possibility of generating flow induced oscillations. 
         [0015]    The device according to the invention allows regulation of its effect by modifying its length, width, porosity and number of plates in series used. 
         [0016]    Other features and advantages of the present invention will be disclosed in the following detailed description of an illustrative embodiment of its object in relation to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  shows a schematic lateral view of the type of aerial refueling system to which the present invention is applied. 
           [0018]      FIGS. 2   a  and  2   b  show the definition of the angles that define the position in space of the boom. 
           [0019]      FIG. 2   c  shows an example of the elevation-azimuth/roll envelope of an aerial refueling boom. 
           [0020]      FIG. 3  shows a scheme of the way the device is mounted on the boom and a detailed zoom of the device. 
           [0021]      FIG. 4  shows the physical mechanism by which the device reduces the lateral force on the boom. 
           [0022]      FIGS. 5   a  and  5   b  present the physical reason for the use of the perforations on the device. 
           [0023]      FIGS. 6   a ,  6   b , and  6   c  show different types of irregular finishing of the device edges. 
           [0024]      FIG. 7  presents a scheme with the conditions to be taken into account in the selection of the device width. 
           [0025]      FIGS. 8   a  and  8   b  show the physical mechanism by which the same effect can be achieved using either a single, wide plate at the leading edge, or several plates of reduced width placed in series along the chord. 
           [0026]      FIGS. 9   a  and  9   b  show the physical mechanism that allows regulating the amount of lateral force eliminated as a function of the device width. 
           [0027]      FIG. 10  presents a scheme of the ruddevators used to control the flow of the device according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    One of the procedures currently in use to perform aerial refueling of aircraft involves the use of a cylindrical beam (boom)  11  with a telescopic extension  12 , through which fuel is transferred from the tanker aircraft  13  to the receiver aircraft  14 , as shown in  FIG. 1 . The boom  11  has an articulation  15  with two degrees of rotational freedom (elevation-roll or elevation-azimuth) at the attachment point to the tanker  13 . A pair of independently moving fins (ruddevators)  16  attached to the boom at the opposite side to the articulation  15 , allow the boom operators (not shown), by means of independent variations of the incidence of each fin in respect to the incident current, to fly the boom  11 , controlling its position in space, to guide the telescopic tube into the receptacle of the receiver. The flyable envelope  21  is defined by means of two angles, the elevation angle γ and the lateral angle (roll or azimuth) θ, as shown in  FIG. 2 . 
         [0029]    The boom  11  cross section (section perpendicular to its longitudinal axis) must be symmetric to have equal behavior to port and starboard side and introduce a low drag, so that for a maximum given control force in the ruddevators  16  the largest possible operational flyable envelope  21  can be achieved. Alternatively, a low drag reduces the amount of ruddervators  21  deflection needed to achieve a certain elevation angles, thus resulting in additional spare control power to reach higher lateral envelope angles  21 . The need for a low drag boom section is even more important when the boom is to be capable to refuel large aircraft, because this operation requires specific conditions:
       higher Mach number of flight   thicker booms  11  to increase the fuel flow rate   longer boom  11  to increase separation between the tanker  13  and the receiver aircraft  14 .       
 
         [0033]    All these elements produce an increase in the drag of the boom  11 . 
         [0034]    The low thickness—low drag airfoils typically used in aeronautics generate a large amount of lateral force (lift) when at incidence (flow not contained in the section symmetry plane). For the typical applications of these airfoils, this is beneficial, as the requirement is to have the largest lift to drag ratio possible. Nevertheless, for the particular application of the boom  11  this lateral force is detrimental, as it opposes the control force introduced by the ruddervators  16 , thus reducing the lateral operation (roll or azimuth) of the envelope  21 . This problem is aggravated the more flexible the boom  11  is (the flexibility being a consequence by the need to have a light structure). To ease visibility of the receiver receptacle  17  between the boom  11  and the receiver aircraft  14  by the boom operator, ruddevators  16  are typically placed on the upper part of the boom  11 . The point of application of their forces is, in consequence, above the boom  11 , so that the generation of a lateral control force in the ruddevators  16  generates a twist of the boom  11  in the sense to increase its local sideslip. This results in an increment of the opposing force generated by the boom  11 , with the consequent reduction of the control capacity of the ruddevators  16  and the reduction of the flyable envelope  21 . 
         [0035]    The present invention attempts to solve the drawbacks of the solutions used to solve this problem until now. 
         [0036]    The device according to the invention comprises a plate  31 . This plate  31  substitutes the leading edge of the boom section  11 , or alternatively is mounted on top of said leading edge of the boom section  11 . As shown in  FIG. 3 , the plate  31  comprises two cantilevered wings  32  and  33 , which comprise perforations  34 , with a circular shape or with any other shape. 
         [0037]    As depicted in  FIG. 4 , the plate  31  produces two effects:
       avoids the acceleration of the flow on the suction side  41  of the leading edge of the boom section  11 , responsible for the generation of the suction peak  45  typical of airfoils; and   generates a low dynamic pressure wake  44 , which surrounds the boom  11 , reducing the lift due to the pressure differences between the suction side  42  and the pressure side  43 , located at the central and rear parts of the boom  11  chord.       
 
         [0040]    The perforations  34  of the lateral wings  32  and  33  of the plate  31  must have a number and size enough to produce a porosity that avoids excessive blocking of the flow passing through them, so as to produce the following two beneficial features:
       reduce the aerodynamic drag introduced by the plate  31 ; and   avoid massive flow separations  51  that might introduce large oscillating forces on the boom section  11 , as shown in  FIGS. 5   a  and  5   b.          
 
         [0043]    With the aim to avoid the possibility of large high-energy coherent vortices, the edges of the plate  31  have circular  61 , square  62 , triangular  63  or similar irregular shapes to help “breaking” this vortices, as respectively shown in  FIGS. 6   a ,  6   b , and  6   c.    
         [0044]    As the extension of the low dynamic pressure wake  44  introduced by the plate  31  increases with the plate  31  width, this width is selected so that this low energy wake  44  completely covers the boom  11  section for all the elevation angles γ of interest, as shown in  FIG. 7 . Should the resulting width be excessive and limited by any other design consideration, it would be possible to achieve the same effect using two  81 , 82  or more plates of smaller width placed in series along the chord, so that each one of them actuates on a zone of the boom  11  section as shown in  FIGS. 8   a  and  8   b.    
         [0045]    The effect of the plate  31  on the boom lateral force can be modulated in several ways:
       varying the length of the boom  11  covered by the device;   varying the plate  31  width so that the wake  44  affects a larger or smaller portion of the boom  11  chord, as previously described and depicted in  FIGS. 9   a  and  9   b;      varying the plate  31  porosity  34  to introduce a larger or smaller dynamic pressure loss in the flow that passes through it.       
 
         [0049]    As the plate  31  has relatively sharp edges, its wake has low sensitivity to changes in the flight conditions (Mach number, Reynolds number, boom elevation angle, turbulence) which assures the aerodynamic behavior of the boom  11  with the device of the present invention to be consistent and easy to control. 
         [0050]    According to another embodiment of the invention, the ruddevators  16  used to control de booms, and which preferably have a dihedral angle of 45° with respect to the boom  11  of the device according to the invention, can have arranged on them some fences  101  that perform the function of flow control as shown in  FIG. 10 . These control fences  101  are preferably arranged perpendicular to the ruddevators  16 , and have a shape that is similar to the airfoil section of said ruddevators  16 . The control fences  101  are placed close to the root of the ruddevators  16 , from the leading edge to the trailing edge. The control fences  101  are formed by thin plates, from 2 to 6 mm thick. The function of these control fences  101  is to stabilize the device of the aerial refueling in the wing envelope in extreme loading conditions, or maximum sweep, which is the unstabilizer aerodynamic flow, avoiding said sweep or airflow movement to the external part of the ruddevators  16 , to shield the ruddevators  16  from the possible negative influences of the wake generated by the plate  31 . 
         [0051]    Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.