Abstract:
An external part of an aircraft (such as a fairing) comprising a skin made with flexible materials attached to rigid supporting elements (such as longerons and frames) arranged in, at least, two directions. The skin comprises inner inflatable panels in all bays delimited by the rigid supporting elements and/or an external inflatable panel. The skin is joined to the supporting elements so that its external surface complies with aerodynamic requirements.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the European patent application No. 15382356.2 filed on Jul. 3, 2015, the entire disclosures of which are incorporated herein by way of reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the structure of aircraft external parts undergoing, mainly, pressure loads such as fairings and, particularly, a belly fairing. 
       BACKGROUND OF THE INVENTION 
       [0003]    The aircraft belly fairing stores equipment and systems located between the wing and the fuselage such as air conditioning equipment, fuel systems and the landing gear. The size and location of the landing gear bay is the main constraint for the ventral fairing design. 
         [0004]    The main objective of the ventral fairing aerodynamic design is to minimize the negative interferences between the wing, the fuselage and the ventral fairing, in order to achieve the least aerodynamic drag penalty possible and to improve the airflow adherence conditions around the wing surface. 
         [0005]    From a structural point of view, known belly fairings are formed by a rigid skin and an inner reinforcing structure comprising longerons and frames arranged in perpendicular directions. The skin is usually formed by several panels (particularly composite honeycomb panels) bolted or fastened to the longerons and frames. 
         [0006]    This invention is addressed to an improvement of known belly fairings and similar external parts of aircraft in weight and drag reduction as well as in assembly and maintainability using inflatable structures. 
         [0007]    The use of inflatable structures in aircraft is not new and some proposals for deployable structures and/or structures able to changes of shape are known, such as U.S. Pat. No. 5,681,010 that discloses a deployable, inflatable aerodynamic control structure for aerospace vehicles which has a desired non-circular or non-cylindrical cross-section, U.S. Pat. No. 7,185,851 B2 that discloses a wing that includes an array of inflatable chambers with generally circular cross-sections or US 2005/0151007 A1 that discloses an inflatable, rigidizable wing for a terrestrial or planetary flying vehicle that can be deployed from an initially packed condition to assume its functional shape by means of an inflation gas. 
         [0008]    However inflatable structures will be used in the present invention in a very different way to these proposals. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides an aircraft external part of an aircraft (such as a fairing) comprising a skin made with flexible materials attached to rigid supporting elements (such as longerons and frames) arranged in, at least, two directions. The skin comprises inner inflatable panels in all bays delimited by the rigid supporting elements and/or an external continuous inflatable panel. The skin is joined to the supporting elements so that its external surface complies with aerodynamic requirements. 
         [0010]    In an embodiment, the skin comprises a continuous external layer and inner inflatable panels made with a drop stitching technique to control that their shape fit the space of the bays and provide the required out of plane inertia, when they are inflated, for backing the continuous external layer. 
         [0011]    In another embodiment, the skin comprises a continuous external layer and inner inflatable panels as an integral solution. Integration can be performed by bonding inner inflatable panels to the external continuous layer or using an external continuous skin as the panel external layer. 
         [0012]    In another embodiment, the skin comprises an external inflatable panel and an internal backup formed by inner inflatable panels. 
         [0013]    In another embodiment, the skin comprises an external inflatable panel and an internal backup formed by a lattice structure. 
         [0014]    In another embodiment, the skin comprises an external inflatable panel and inner inflatable panels as an integral solution. Integration can be performed by bonding the external layer of inner inflatable panels to the internal layer of the external inflatable panel or using the internal layer of the external inflatable panel as the external layer of the inner inflatable panels. 
         [0015]    Other characteristics and advantages of the present invention will be clear from the following detailed description of embodiments illustrative of its object in relation to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1 a    is a schematic plan view of an embodiment of an aircraft external part according to this invention and Figure lb is a sectional view by the plane A-A of  FIG. 1   a.    
           [0017]      FIGS. 2 a  and 2 b    are schematic views of joining areas of the aircraft external part of  FIGS. 1 a   - 1   b.    
           [0018]      FIG. 3  is a schematic sectional view of an inflatable panel used in embodiments of this invention. 
           [0019]      FIGS. 4-8  are schematic sections views of five embodiments of an aircraft external part according to this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In a first embodiment (see  FIGS. 1 a -1 b , 2 a -2 b   ), an aircraft external part  10 , such as a belly fairing, comprises longerons  15  and frames  17  as rigid supporting elements) made of metallic or composite materials in a similar way to known belly fairings, and a skin made of a flexible continuous external layer  21  and inner inflatable panels  23  in the bays delimited by the longerons  15  and the frames  17 . 
         [0021]    The inner inflatable panels  23  are always inflated when the aircraft external part  10  is mounted in the aircraft. They could be uninflated when the aircraft external part  10  is removed from the aircraft for maintenance purposes. 
         [0022]    The continuous external layer  21  may be made, for instance, with a base fabric of aramide and a coating of CSM (Chlorosulfonated Polyethylene) or TPU (thermoplastic polyurethane). 
         [0023]    The inner inflatable panels  23  are made using a drop stitching technique, i.e., they are made (see  FIG. 3 ) with two plies  11 ,  13  of a similar material as the continuous external layer  21 , joined together by a plurality of substantial inextensible elements  12  to restrict the extent of separability of the plies  11 ,  13 . A general description of the technique can be found in U.S. Pat. No. 2,743,510. 
         [0024]    The drop stitching technique allows controlling the configuration of the inner inflatable panels  23  when they are inflated to comply with two main requirements:
       Having a controlled curved shape so that they can adapt to the shape of the aircraft external part  10 .   Providing out of plane inertia for backing the continuous external layer  21 .       
 
         [0027]    The inner inflatable panels  23  are joined to the longerons  15  and frames  17  by suitable joining means  27  (see  FIG. 2 a   ) including a rigid plate  28  providing a rigid-rigid connection to a fastening means  19 . 
         [0028]    The continuous external layer  21  is joined to the two outer longerons  15  by joining devices  25  that include means for applying a pretension T to the continuous external layer  21  to keep the required shape together with the backing provided by the inner inflatable panels  23 . The ends  22  of the continuous external layer  21  are configured with a rigid plate  29  disposed within a fold providing a rigid-rigid connection to the fastening means  24 . 
         [0029]    If the aircraft external part  10  has convex and concave areas (see  FIG. 4 ), the continuous external layer  21  shall be also joined to an intermediate longeron  15  between a concave and a convex area through suitable joining means  26  including means for applying a pretension T. In the non-convex area  16  (seen from the outside of the aircraft external part  10 ), the continuous external layer  21  shall be bonded to the contiguous inflatable panel  23  to be pretensioned by it. 
         [0030]    In a second embodiment (see  FIG. 5 ), the skin is a single piece comprising an external continuous layer  31  joined to the two outer longerons  15  by joining devices  35  and inner inflatable panels  33  integrated into the external continuous layer  31 . 
         [0031]    In a third embodiment (see  FIGS. 6-7 ), the skin comprises an external inflatable panel  41  joined to the two outer longerons  15  by joining devices  45  and an internal backup formed by inner inflatable panels  43  in the bays delimited by the longerons  15  and the frames  17  (see  FIG. 6 ), joined to them by suitable joining means (similar to those shown in  FIG. 2 a    for joining the inner inflatable panels  23  to the longerons  15 ) or a lattice structure  44  (see  FIG. 7 ) that can be formed, for instance, by a triangular meshing adapted to the curved shape of the aircraft external part  10 . The external inflatable panel  41  is pretensioned when it is inflated. The external inflatable panel  41  and the inner inflatable panels  43  are preferably made using a drop stitching technique. 
         [0032]    In a fourth embodiment (see  FIG. 8 ), the skin is a single piece comprising an external inflatable panel  51  joined to the two outer longerons  15  by joining devices  55  and inner inflatable panels  53  integrated into the external inflatable panel  51 . 
         [0033]    The main advantages of the fairings of the invention with respect to known fairings are:
       Weight and cost savings.   Better assembly and maintainability as they can be easily managed in uninflated state.   Drag reduction due to the replacement of several honeycomb panels by one continuous skin.       
 
         [0037]    Although the present invention has been described in connection with various embodiments, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention as defined by the appended claims. 
         [0038]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.