Abstract:
A fairing for streamlining the junction between the horizontal stabilizer and the fuselage or the fin of an aircraft, comprising an upper fairing shell, a lower fairing shell, a leading edge extension, and several fittings to join the upper and lower fairing shells to the horizontal stabilizer. The upper and lower fairing shells are solid laminates of composite material that are designed with a shape, composition, and thickness such that they can be elastically deformed when they are fitted to the horizontal stabilizer, providing a contact force permitting them to lay permanently in contact with the fuselage or fin. A process for manufacturing the fairing shells.

Description:
FIELD OF THE INVENTION 
   The present invention refers to fairings that streamline the junction between the horizontal stabilizer (HS) and the fuselage or the fin of an aircraft. 
   BACKGROUND 
   Typical aircraft designs incorporate fairings streamlining the junction between the horizontal stabilizer and the fuselage or the fin. 
   The definition of the fairings may be divided conceptually in four parts:
         two parts, called fairings shells, that smooth the intersection between the horizontal stabilizer upper and lower surfaces and the fuselage or vertical stabilizer surface.   a third part, called leading edge extension (LEX), that smoothes the intersection between the leading edge surface of the horizontal stabilizer and the fuselage or vertical stabilizer surface.   the fourth part is comprised by several fittings that join the fairings to the horizontal stabilizer main structure.       

   The preferred materials for the parts mentioned above are:
         Upper and lower fairings: composite materials (solid laminate from tape and/or fabric prepregs).   LEX: metallic materials (formed sheet and machined aluminium alloy)   Fairing fittings: metallic materials (machined aluminium alloy)       

   One of the main requirements that govern the design of these fairings is to prevent the scooping effect of the negative air pressures minimising the gap with the contact surface especially in the frontal edge to the aerodynamical stream. This scooping may cause a blow out of the fairing due to internal over pressure. 
   Known fairings face this issue incorporating either a guiding profile in the frontal edge or a set of complex spring-loaded mechanisms that push the fairing shells against the contact surface. 
   This invention is intended to provide HS to fuselage or fin new fairings with the following advantages:
         Adaptability to more complex contact surfaces (i.e. with variable curvature).   Higher simplicity and lower cost (i.e. less number of parts).   Easier and lower cost installation and removal.   Improved aerodynamic smoothness.   Weight saving.   Easier and lower cost maintainability.       

   SUMMARY OF THE INVENTION 
   In a first aspect, the present invention provides a fairing for streamlining the junction between the horizontal stabilizer and the fuselage or the fin of an aircraft, comprising an upper fairing shell, a lower fairing shell, a leading edge extension, and several fittings to join the upper and lower fairing shells to the horizontal stabilizer in which the upper and lower fairing shells are solid laminates of composite material that are designed with a shape, compositions and thickness such that they can be elastically deformed when they are fitted to the horizontal stabilizer, providing a contact force permitting them to lay permanently in contact with the fuselage or fin. 
   In a second aspect, the present invention provides a process for manufacturing the above-mentioned upper and lower fairing shells that comprises the following steps: 
   a) Define the shape, laminate composition and thickness of the fairing shells complying with aerosmoothness requirements according to theoretical criteria; 
   b) Build a first finite element model of the fairing shells as installed in their fittings but before being deformed; 
   c) Test the deformation of the fairing shell models under predetermined values of aerodynamic suction; 
   d) Modify the definition of the shape, laminate composition and thickness of the fairing shell models according to the results of step c); 
   e) Build a second finite element model of the fairing shells as just installed in their fittings and deformed consequently; 
   f) Test the deformation of the fairing shell models and its separation from the fuselage under predetermined values of aerodynamic suction and predetermined enforced displacements of its edges; 
   g) Modify the definition of the shape, laminate composition and thickness of the fairing shell models according to the results of step f); 
   h) Manufacture a sample of the fairing shells using a modifiable tool able to adjust slightly its shape; 
   i) Test the separation of the fairing shell samples from the fuselage under predetermined loads; 
   j) Modify the definition of the shape of the fairing shell according to the results of step i). 
   The fairing according to this invention is designed with a deformation on assembly concept that enables the fairing to lay permanently in contact with the fuselage or fin structures without requiring any guidance in the frontal edge or other more complex mechanisms, even when subjected to the in-flight aerodynamic suction loads. 
   This fairing is conceived mainly to improve the design of movable fairings (i.e. those for trimable horizontal tails) but it may be applied to fixed fairings too. For movable fairings, the induced assembly load may vary along the operating range. 
   The design of fairings in accordance with the present invention is based on theoretical analyses supported by tests and practical developments that finally allow their manufacture and certification for commercial aircrafts. 
   The features, objects and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1 , which includes  FIGS. 1   a - d  shows the location of a fairing in an aircraft. 
       FIG. 2 , which includes  FIGS. 2   a - b , shows the complete assembly of the fairing according to the present invention in external and internal views. 
       FIG. 3   a  shows the upper and lower fairing shells, and  FIG. 3   b  shows a section A-A of the upper fairing shell. 
       FIG. 4 , which includes  FIGS. 4   a - c , shows the assembly of the leading edge extension. 
       FIG. 5  shows a typical fairing fitting. 
       FIGS. 6 ,  7  and  8  shows a flowchart of the manufacturing process of a fairing according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Fairings between horizontal stabilizers and fuselages or vertical stabilizers of aircrafts are mainly required to:
         Streamline the connection between both structural components providing the required aerodynamic smoothness. For trimable horizontal stabilizers, the fairing provides also aerodynamic continuity to the fuselage or fin surfaces.   Minimize the gap under load with the contact surface on the fuselage or the vertical stabilizer especially in the front edge to the aerodynamical stream.   Withstand and transfer aerodynamical loads to the main box of the horizontal stabilizer.   Be removable to gain access to the horizontal stabilizer central structure.       

   With reference to  FIGS. 1-5 , the components of the fairing  11  of this invention are detailed described below, as well as their assembly: 
   Upper and Lower Fairing Shells  15 ,  17   
   These consist of two CFRP (Carbon Fiber Reinforced Plastic) solid laminate shells  15 ,  17  are screwed to dedicated fittings  25  attached to the horizontal stabilizer  5  main structure. Both fairing shells  15 ,  17  are then removable to allow access to the horizontal stabilizer (HS)  5  central structure. 
   The manufactured shape of these shells  15 ,  17  differs from the shape once installed on the aircraft so that the induced elastic deformation provides a permanent contact force (on this way the fairing acts as a spring) and so prevent the separation from the contact surface  7  even when loaded by the aerodynamic pressures in flight. The development process of these parts may be arduous and constitutes an important challenge of the present invention. 
   For designs of fairing shells with moderate curvature even in two directions, an optimised mixture of unidirectional tape and plain fabric prepregs should be enough to meet both strength and stiffness requirements. Fairing shells with high curvature may require some slots, carefully defined, to permit functional and/or structural deformation compatible with the required strength and stiffness. 
   For movable fairings, materials at the contact edge are to be chosen to ensure good sliding, wearing and sealing behaviour in combination with the fuselage or fin skin materials. Teflon materials are a good choice for this application. 
   Leading Edge Extension (LEX)  19   
   This consists of a metallic assembly comprising a fixed plus a movable part. The latter is hinged on the former and pushed on to the fuselage or fin  7  by a spring mechanism. This mechanism ensures the permanent contact with the fuselage or fin surfaces  7  when aerodynamic suction is applied. 
   Both the fixed and the movable parts are basically composed of:
         An external skin made of aluminium alloy formed sheet, chemically milled if required.   An aluminium alloy machined rib. Ribs in both parts are hinged one to each other in a scissors type assembly.   A mechanism composed of two links plus a spring ensures the permanent contact between the LEX and the fuselage or the fin surface  7  even when loaded.       

   The complete LEX assembly is removable from the HS  5  box since is screwed to the skins and inboard fixed leading edge rib. 
   Materials at the contact surfaces are again chosen to ensure a good sliding, wearing and sealing behaviour. 
   For fixed fairings, a conventional simple Leading edge extension is applicable. 
   The design of the LEX may have to comply with the applicable airworthiness requirements regarding bird strike resistance. 
   Fairing Fittings 
   The upper and lower CFRP fairings shells  15 ,  17  are bolted to a series of aluminium alloy machined fittings  25  (channel type) fastened to the HS  5  main structure. Floating anchor nuts are provided to allow the fairings installation. 
   These fittings  25  are required to maintain the fairing shells  15 ,  17  on the installed position transferring both assembly and aerodynamical loads to the main structure of the horizontal stabilizer  5 . 
   Complete Fairing Assembly 
   The final assembly of the fairing is accomplished by joining afore mentioned parts. 
   Fairing fittings  25  are attached thru fasteners (either fixed or removable) to the upper and lower HS  5  main box skin covers. A precision tooling will be required to locate these fittings  25  if fairings are to be interchangeable. Some designs permit the adjustment on assembly of these fittings. 
   Upper and lower fairing shells  15 ,  17  are then bolted to the fixed fittings  25  using, for example, bolts  26  shown in  FIG. 5 . Hole sizes and fits at these joints should be carefully defined to reduce manufacturing costs and ease assembly process. If fairings are to be interchangeable, a precision tool is required to drill holes in both fairing shells  15 ,  17  and fittings. 
   Another special tool may be required to install the fairing shells  15 ,  17  on the fittings  25  as both will certainly not match and some amount of force on assembly will be required. Special care will be put in defining the tightening torque of the bolts as tests have revealed a major influence on the fairing behaviour. 
   Finally, the leading edge extension  19  is screwed to the horizontal stabilizer  5  main structure. A special tool is required to limit the aperture of the movable part while assembling. 
   Development Process of Upper and Lower Fairing Shells 
   The complete design and engineering process of the upper and lower fairing shells shown in  FIGS. 6-8  comprises the following steps 
   Step  41 : Define the shape, laminate composition and thickness of the fairing shells complying with aerosmoothness requirements according to theoretical criteria. 
   Step  43 : Build a detailed finite element model of the fairing (“installed but not enforced shape”) and attachment fittings. Use non-linear (large displacements) elements and locate contact simulation elements between fairing and fittings. 
   Step  45 : Test the deformation of the fairing shell models under predetermined values of aerodynamic suction.
         Sub-step  47 : Obtain the deformed shape of the fairing once loaded with the applicable negative (suction) aerodynamic pressure distribution. Apply a factor on the load according to experience to be on the conservative side.   Sub-step  49 : Fix the fairing and fitting thicknesses so that deformation and stress levels maintain within reasonable values according to experience.       

   Step  51 : Modify the definition of the shape, laminate composition and thickness of the fairing shell models according to the results of step  45 . The laminates are designed to minimize the spring-back of the manufactured fairings (expected spring-back may be analysed by using the above mentioned detailed F.E. model). 
   Step  61 : Build a second finite element model of the fairing shells as just installed in their fittings and deformed consequently. 
   A symmetry of the deformed shape resulting from sub-step  49  is performed. 
   A finite element mesh is projected into this symmetrical shape and a second detailed finite element model representing the fairing “installed just on fittings” and without any other deformation on assembly is built. Contact elements at the free edge of the fairing to be able to control the eventual separation from the contact surface when loaded are provided. 
   Step  63 : Test the deformation of the fairing shell models and its separation from the fuselage under predetermined values of aerodynamic suction and predetermined enforced displacements of its edges. 
   Sub-step  65 : Apply enforced displacements at the edge of the fairing (so that the fairing is forced to lay on the installed position).
         Sub-step  67 : Control if deformed shape matches, within tolerances, with theoretical shape.   Sub-step  69 : Apply both aerodynamical acting suction plus enforced displacements at the edge representing the “completely installed” position.   Sub-step  71 : Control the eventual separation from the contact surface.   Sub-step  73 : Check the strength of the fairing.       

   Step  75 : Modify the definition of the shape, laminate composition and thickness of the fairing shell models according to the results of sub-steps  67 ,  71  and  73 . 
   Step  77 : Manufacture a sample of the fairing shells using a modifiable tool able to adjust slightly its shape.
         Sub-step  79 . Define the shape of the fairing shells to be manufactured taking advantage of the automatic transfer capability of surfaces from CAE to CAD tools.   Sub-step  81 : Manufacture a sample of fairing shells. Use a modifiable tool to be able to adjust slightly the shape if required during the development process.       

   Step  83 : Test the separation of the fairing shell samples from the fuselage under predetermined loads.
         Sub-step  85 : Verify the actual shape of the manufactured fairings and report deviations with respect to the theoretical.   Sub-step  87 : Test if the manufactured fairings comply with the maximum allowed gaps at the contact edge when installed and under prescribed loads. Use a test rig representative of the joint of the fairings to the horizontal stabilizer and the contact surface of either the fuselage or the fin. Account for adverse tolerance stack up.   Sub-step  89 : Control requirements.       

   Step  91 : Modify the definition of the shape of the fairing shell according to the results of sub-step  89  until obtaining positive test results  93 . 
   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.