Abstract:
In accordance with the present invention an aircraft floor panel is provided comprising a honeycomb core element having an upper core surface, a lower core surface, and a core thickness. An upper face sheet assembly is mounted to and seals the upper core surface and includes at least one upper material sheet impregnated with an upper epoxy resin. A lower damping face sheet assembly is mounted to and seals the lower core surface and includes at least one lower material sheet infused with a highly damped lower epoxy resin. The lower damping face sheet assembly dampens vibrational noise.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to an apparatus and method for the damping of aircraft interior floor panels, and more particularly to an apparatus and method for integrating such damping directly into composite aircraft floor panels.  
       BACKGROUND OF THE INVENTION  
       [0002]     Commercial aircraft must not only meet strength, weight and cost objectives they must also be designed to achieve adequate customer satisfaction. One arena of customer satisfaction arises regarding passenger comfort. The reduction of interior noise levels is considered critical to passenger comfort. A wide variety of noise sources come from many locations outside and inside the airframe. Aircraft design benefits are achieved by addressing these noise sources and minimizing their impact on passengers.  
         [0003]     One known noise source into the main cabin is through the aircraft floor panels. The floor panels are typically constructed using lightweight designs with high rigidity. These very light weight structural solutions are typically not efficient at reducing noise that transmits through them. Because the floor panels are stiff, they have fast traveling bending waves which tend to match well with acoustic waves which produce a low coincidence frequency. The floor panels, therefore, have a high radiation efficiency which is bad for interior noise.  
         [0004]     A current methodology for dealing with such resonant floor panels is through the use of add-on damping patches bonded to the underside of the floor for the purpose of increasing damping. This approach adds extra weight, fabrication costs, and installation costs to the floor panel design.  
         [0005]     What is needed is a design and method for increasing damping in floor panels that was more weight and cost effective than current add-on damping techniques. Additionally, it would be highly desirable to have a design and method for such floor panels that simplified the time and complexity of fabrication.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the present invention an aircraft floor panel is provided comprising a honeycomb core element having an upper core surface, a lower core surface, and a core thickness. An upper face sheet assembly is mounted to and seals the upper core surface and includes at least one upper material sheet impregnated with an upper epoxy resin. A lower damping face sheet assembly is mounted to and seals the lower core surface and includes at least one lower material sheet infused with a highly damped lower epoxy resin. The lower damping face sheet assembly dampens vibrational noise.  
         [0007]     Other objects and features of the present invention will become apparent when viewed in light of the detailed description and preferred embodiment when taken in conjunction with the attached drawings and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an illustration of an aircraft in accordance with the present invention.  
         [0009]      FIG. 2  is a detailed illustration of a portion of a floor panel for use in the aircraft illustrated in  FIG. 1 .  
         [0010]      FIG. 3  is a detailed illustration of an alternate embodiment of the portion of the floor panel in  FIG. 2 , the detail illustrating a hybrid thick face embodiment.  
         [0011]      FIG. 4  is a detailed illustration of an alternate embodiment of the portion of the floor panel in  FIG. 2 , the detail illustrating a thick core embodiment.  
         [0012]      FIG. 5  is a detailed illustration of an alternate embodiment of the portion of the floor panel in  FIG. 2 , the detail illustrating a mixed face sheet embodiment. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0013]     Referring now to  FIG. 1 , which is an illustration of an aircraft  10  in accordance with the present invention. The aircraft  10  includes a plurality of aircraft floor panels  12  positioned throughout its interior, which serve both as structural and functional platforms within the aircraft  10 . It is known that existing aircraft floor panels  12  do not adequately inhibit undesirable noise transmission within the aircraft interior. The present invention contemplates a novel approach to the reduction of such noise through the use of an aircraft floor panel design with fully integrated damping characteristics.  
         [0014]     Referring now to  FIG. 2 , which is a detailed illustration of a portion of an aircraft floor panel  12 . The illustrated embodiment is one of a basic damped panel  14 . The basic damped panel  14  is comprised of a core element  16 , preferably a honeycomb core. At least one embodiment contemplates the use of a nomex core for use as the core element  16 . Nomex is a phenolic reinforced aramid paper honeycomb core available from the Hexcel Corp., Stamford Conn., and MCGill Corp of El Monte Calif. Preferably, the core element  16  is constructed to have approximately 9 lbs./cubic feet density, 0.11 to 0.14 inch cell size, 475 psi minimum average L-shear property, and 260 psi minimum average W-Shear property. This specification establishes the requirements for heat resistant nonmetallic honeycomb core for aircraft structural and general purpose applications. The material is fabricated from an aramid paper and stabilized by phenolic resin. The honeycomb core element  16  is comprised of a core thickness  18  that is approximately 0.46 inches. The core element  16  further includes an upper core surface  20  and a lower core surface  22 . The terms upper and lower additionally refer to their orientation after installation within the aircraft  10 .  
         [0015]     The lower core surface  22  is sealed by way of a lower damping face sheet  24  affixed thereto. The lower damping face sheet  24  is preferably constructed of a lower material sheet  26  impregnated with a highly damped lower epoxy resin  28 . Although a variety of highly damped lower epoxy resins are contemplated, in one embodiment the highly damped lower epoxy resin  28  is a low temperature cure epoxy resin. In another, it is specifically Duralco Superflex epoxy resin 4538N available from Cotronics Corp., Brooklyn, N.Y. Although Duralco epoxy resin is preferred, the present invention contemplates the use of a variety of two part low temperature cure (cure room temperature 70° F. to 180° F.) and low glass transition (−40° F. to 40° F.) temperature epoxy resins. The lower damping face sheet  24  has a lower sheet width  30  of approximately 0.02 inches comprising a 6 mil fiberglass scrim  32  and a 12 mil Super Flex epoxy cloth sheet  34  (3K-70-PW cloth). The combination of the highly damped lower epoxy resin  28  in use with the superflex cloth sheet  34  allows the lower damping face sheet  24  to induce improved damping characteristics into the floor panel  12  without a negative impact on manufacturing.  
         [0016]     The floor panel  12  further includes an upper face sheet  36  comprised of an upper material sheet  38  impregnated with an upper epoxy resin  40 . In one embodiment the upper face sheet  36  is also utilized for damping and therefore is an upper damping face sheet  36  and the upper epoxy resin  40  is also a highly damped upper epoxy resin. The upper face sheet  36  is utilized to seal the upper core surface  20 . The upper damping face sheet  36  may be formed in a construction identical to the lower damping face sheet  24 . Testing has indicated that the dual damped surfaces provide a flex load of 85 pounds and a deflection at 100 lbs of 1.8 inches. Alternately, the upper face sheet  36  may be formed with materials to generate high rigidity in comparison to the low rigidity of the lower damping face sheet  24 . This provides a unique combination of rigidity above and damping from below. Since the upper face sheet  36  will largely experience compression and the lower damping face sheet  24  will largely experience tension during loading, the combination of upper rigid and lower flexible allow for an improved damping response without sacrificing overall floor panel  12  rigidity.  
         [0017]     An additional embodiment is illustrated in  FIG. 3  which may be referred to as a hybrid thick face  42  embodiment. In this embodiment, the core element  16  utilized has a core thickness  18  of 0.46 inches of non-metallic honeycomb with a density range of 3.7 to 4.3 lbs. per cubic feet. In addition the lower damping face sheet  24  and an upper damping face sheet  36  utilize an intermediate modulus (22-25 MSI) toughened carbon pre-preg tape and that is a style 3k-70-PW carbon fiber cloth. This specification establishes requirements for 350 F cure toughened-epoxy pre-impregnated carbon fiber unidirectional tape and fabric products. The 3k-70-PW specification is a 3000 filaments/yarn, 12.5±0.5 warp count per inch, 12.5±0.5 fill count per inch, plain weave, worked. The lower damping face sheet  24  and an upper damping face sheet  36  have a lower sheet width  30  and upper sheet width  44  of approximately 0.034 inches each. This is preferably accomplished through the use multiple layers  45  of tape and low strength cloth combinations. The hybrid thick face sheet design  42  was tested and found to provide a flex load of 340 lbs and a deflection at 100 lbs of only 0.278 inches.  
         [0018]     A third embodiment is illustrated in  FIG. 4  which may be referred to as the thick core  46  embodiment. In this embodiment the core thickness  18  was increased to approximately 1.25 inches. The upper and lower sheet thicknesses  30 ,  44  were kept at 0.017 inches apiece (preferably comprised of 2-ply 3k-70-PW cloth at 0.085 inches per ply). By increasing the core thickness  18  while retaining original sheet widths  30 ,  44  the flex load was found to be 269 lbs and the deflection at 100 lbs was found to be 0.2 inches. It should be understood that all test data in this patent is for illustrative purposes only in regards to the improvement of damping and flex characteristics.  
         [0019]     Finally a fourth embodiment is illustrated in  FIG. 5 , which may be referred to as the mixed face sheet  48  embodiment. In the mixed face sheet  48  embodiment, the upper face sheet  36  is comprised of a high strength face sheet while the lower face sheet  24  is a low strength damping face sheet. The high strength face for example can be comprised of carbon/graphite tape laminate such as sold by Hexcel Corporation under the trade name AS 4 /Magnamite. In this embodiment the core thickness  18  may remain the same, but the upper face sheet  36  is preferably comprised of grade 190 tape with plies of 0/90 at 0.0075 inches combined to arrive at a upper sheet width  44  of 0.017 inches. The lower damping face sheet  24  is comprised of 0.024 inches of carbon/epoxy cloth such as the 3k-70-PW cloth (preferably comprised of 3 plies of 0.0085 cloth) impregnated with a two part epoxy resin with superior flexibility at low temperatures. This combination of upper face rigidity with lower face flex provides a unique combination of benefit in combating vibrations. It should be understood that in this embodiment, the lower damping face sheet  24  may still be designed to be high in tensile strength to prevent failure while still being low strength and flexible in the designed range required for damping. This proposed embodiment was found to provide a flex load of 310 pounds while having a deflection at 100 lbs of only 0.55 inches.  
         [0020]     While the invention has been described in connection with one or more embodiments, it is to be understood that the specific mechanisms and techniques which have been described are merely illustrative of the principles of the invention, numerous modifications may be made to the methods and apparatus described without departing from the spirit and scope of the invention as defined by the appended claims.