Patent Publication Number: US-2002006078-A1

Title: Sound-absorbing and reinforcing structure for engine nacelle acoustic panel

Description:
[0001] The present invention relates to a sound-absorbing and reinforcing structure for engine nacelle acoustic panel designed to contain the engines of jet airplanes.  
       [0002] Each nacelle of a jet engine comprises a front portion set in a position corresponding to the inlet cone, the said front portion being structured in such a way as to ensure the best possible reduction of the acoustic emission produced by the engine.  
       [0003] In effect, the engines of jet airplanes are the cause, during their operation, of numerous sources of noise, which generate sound waves in various directions. A first source is represented by the internal noise, which is generated by the combustion and expansion of hot gases that traverse the turbine. A second source is represented by the reaction noise that is generated in the area where a mixture of gas and air flows and where the turbulence is maximum on account of the substantial difference in speed and temperature between the flows of the moving mixtures.  
       [0004] Another source of noise is the one due to the intermediate-pressure compressor present inside the nacelle. Finally, a further source of noise results from the operation of the fan of the jet engine.  
       [0005] The various relevant legal requirements and regulations which impose increasing reduction of noise levels in the vicinity of airports have given rise to an extreme interest in the progressive reduction of the noise produced by airplanes, with the provision of special acoustic shielding.  
       [0006] One method is that of using, in an area corresponding to the internal portion of the nacelle, on the side of the inlet cone, a series of layers of covering made of absorbent, porous material or sound-absorbing coating materials in which a series of Helmholtz resonators are made.  
       [0007] Alternatively, acoustic panels are employed that are able to attenuate the noise thanks to the adoption of peculiar constructional technologies that use both metallic materials and composite materials.  
       [0008] In the design of the inlet cone of a nacelle for reaction engines, the installation of absorbent structures thus involves an extremely sophisticated technology which combines the characteristics of adequate attenuation of the various noises generated with the characteristics for a general reduction of extension of all the mechanical parts that do not contribute to attenuation of the noise (the so-called “non-acoustic” parts), with the characteristics of extreme lightness, and, at the same time, considerable sturdiness and reliability of all the aircraft structures. Generally, structures of a traditional type do not take into account all these requirements, which, among other things, are in some cases even in contradiction with one another.  
       [0009] A purpose of the present invention is thus to provide a sound-absorbing and reinforcement structure for engine nacelle acoustic panel which will overcome the problems referred to above, i.e., to provide a reinforcement structure that will be able to attenuate as much as possible the noise coming from an engine nacelle of a jet airplane, reducing to the minimum the extension of the parts which do not contribute to attenuation of the noise, and at the same time that will guarantee the structural continuity of the entire acoustic panel.  
       [0010] Another purpose of the present invention is to provide a sound-absorbing and reinforcing structure for engine nacelle acoustic panel that will enable removal of the individual panel, meeting the requirements of maintainability, maximizing the effective geometrical area of the panels, ensuring respect of the streamlining in the assembly stage, and preventing any interaction with external atmospheric agents.  
       [0011] Not the least important purpose of the invention is that of providing a sound-absorbing and reinforcing structure for engine nacelle acoustic panels that is easy and economical to build and does not involve the use of complex or particularly costly technologies by virtue of the advantages achieved.  
       [0012] These and other purposes are attained by a sound-absorbing and reinforcement structure for engine nacelle acoustic panel according to claim  1 .  
       [0013] Advantageously, the present invention makes it possible to simplify the construction and assembly of the insulating coating materials of the type described previously and to improve their behaviour in use from the standpoint of lightness, strength and reliability of the aircraft, the production and running costs and of the sound-absorbing properties of all those portions of the acoustic panels that contribute to attenuating noise during flight. 
     
    
    
     [0014] Further purposes and advantages of the present invention will emerge clearly from the ensuing description and the annexed drawings, which are provided purely by way of explanatory but non-limiting example, and in which:  
     [0015] - FIG. 1 is a partial view of a nacelle, designed to contain a jet engine comprising a sound-absorbing and reinforcing structure according to the present invention;  
     [0016] - FIG. 2 is a fragmentary perspective partially cut-away pictorial view of the front portion, corresponding to an inlet cone of the nacelle appearing in FIG. 1, according to the invention;  
     [0017] - FIG. 3A is a schematic perspective view showing the jointing of two engine nacelle acoustic panels, which are shaped according to the geometry of the inlet cone of the nacelle and are jointed together; and  
     [0018] - FIG. 3B is an enlarged and partial view of a sound-absorbing and reinforcing structure for acoustic panels of a nacelle, provided in an area corresponding to the longitudinal jointing elements of the panels according to the invention. 
    
    
     [0019] With reference to the above figures, the number  20  designates a nacelle, designed to contain a jet engine for a jet aircraft, which is generally mounted on a support underneath the wing  31  of an aircraft.  
     [0020] The nacelle  20  includes a thrust-reversing device, comprising a sleeve  21  which translates backwards and forwards, so as to cover and uncover, alternately, cascaded structures  22 , designed for reversing thrust.  
     [0021] In an aircraft, the thrust-reversing device is mounted on the nacelle  20  and is positioned at the front, with respect to the location of the jet engine, which is inside the nacelle  20 , whilst the cascaded structures  22  are spaced out on the circumference inside the nacelle  20 .  
     [0022] Whilst the aircraft is in flight, the translating sleeve  21  is in a position where the cascaded structures  22  are enclosed. In order to set the aircraft in a condition for landing, the sleeve  21  is displaced from the closing position to a retracted position by means of actuator devices  28 . This arrangement enables the exhaust gases to flow onto the cascaded structures  22  so as to slow the aircraft down during descent for landing.  
     [0023] The translating sleeve  21  is generally made up of one or more metal outer semi-cylindrical panels  23  and one or more inner semi-cylindrical acoustic panels  24  built, according to a sandwich structure, of both a metal material and a composite material, which are connected to the panels  23  in an area corresponding to the portion  26  of the nacelle  20 .  
     [0024] The outer metal layers  23  and the inner acoustic panels  24  are joined together in a rear area and diverge with respect to one another towards the portion  26 , so as to identify a chamber designed to contain the cascaded structures  22  and the associated supports.  
     [0025] Traditionally, in order to form an acoustic panel  24  a sandwich structure is used.  
     [0026] In particular, the acoustic panel  24  has an inner layer  32 , which is perforated and is made of a composite material, an outer layer  34 , which is not perforated and is also made of composite material, and an intermediate layer  36 , which is made of a honeycomb structure and is pressed between the two aforementioned layers  32 ,  34 .  
     [0027] The acoustic panels  24  are then treated so as to constitute a single ring-shaped curvilinear structure such as the one illustrated in FIG. 3A, which is then jointed longitudinally in two or more points.  
     [0028] In particular, FIG. 3A shows two acoustic panels  24 , jointed at the reference points  40  so as to constitute a curvilinear structure.  
     [0029] The longitudinal jointing elements  40  may present various constructional solutions; however, the aim of the one represented in enlarged detail in FIG. 3B is to extend the acoustic region to the majority of the element  40 , guaranteeing structural continuity, enabling the individual panel  24  to be removed, i.e., meeting the requirements of maintainability, ensuring respect of the requirements of aerodynamic contouring in the assembly stage and preventing any interaction with external atmospheric agents.  
     [0030] In detail, the constructional solution outlined above envisages proceeding in such a way that, regardless of the number of panels  24  used, the portions  34 A,  34 B of each panel  24  are connected to a metal butt strap  35  by means of a series of rivets or fasteners  38 , ensuring the continuity of fan flow along the layers  341 ,  342  (backskin) of the respective portions  34 A,  34 B, that are without acoustic treatment.  
     [0031] The portion  36 A, consisting of a honeycomb structure, is acoustically treated in order to contribute to the attenuation of noise and is held in a position adjacent to the butt strap  35 , as represented in FIG. 3B, thanks to the action of the said connection members  38 , so as to contribute to the strength of the jointing elements  40  and of the entire structure, which, as a result, presents a considerable compactness and is not particularly affected by mechanical stresses.  
     [0032] In addition, the layers  321 ,  322  of the respective portions  34 A,  34 B undergo acoustic treatment, and likewise the layer  361  of the portion  36 A, in order to improve the mechanical characteristics further.  
     [0033] Finally, it is to be emphasized that a structure made like the one represented in FIG. 3B, which is envisaged at the longitudinal joints between acoustic panels  24 , makes it possible to obtain a better insulation from noise and improved mechanical properties. Furthermore, in this way each panel  24  can be removed and replaced individually and easily.  
     [0034] In particular, the presence is important of the metal layer  35 , which is used as butt strap of the structure, and of the connecting members  38 , which make it possible to withhold the butt strap  35  in position with respect to the portions  34 A,  34 B of the panel  24 , and which enable the honeycomb portion  36 A to be kept in place.  
     [0035] In addition, in the proximity of the jointing elements  40 , as is clearly visible from FIG. 3B, it is envisaged to position a honeycomb portion  36 A, which overlaps at least partially the layers  32 ,  34 ,  36  of the panel  24 , and has a width approximately equal to that of the jointing element  40  and a longitudinal dimension slightly smaller than the thickness of the panel  24 . Finally, the portion  36 A, in a preferred, but non-limiting, example of embodiment, has an approximately trapezoidal shape.  
     [0036] From the foregoing description the characteristics and advantages of the sound-absorbing and reinforcing structure for acoustic panels of nacelles, which forms the subject of the present invention emerge clearly.  
     [0037] It is evident that numerous variations may be made to the said sound-absorbing and reinforcing structure for acoustic panels of nacelles, which forms the subject of the present invention, without thereby departing from the principles of novelty inherent in the inventive idea, as likewise it is clear that, in the practical embodiment of the invention, the materials, shapes, and dimensions of the details illustrated may be any whatsoever according to the particular requirements, and the said details may be replaced by others that are technically equivalent.