Reduced obstruction self-sealing fasteners for multi-layer panels

Abstract: A fastener system (100,100') for securing a multi-layer panel (200) having a first surface adapted to be secured to a substructure (300), a second surface awash in a fluid flowing thereacross, and a plurality of spaced fastener-receiving holes arranged in a predetermined array in the second surface. Each fastener (100,100') includes a threaded shank (112,112') having a diameter smaller than any of the fastener-receiving holes, a load-bearing flange (124,124') located adjacent the shank having a diameter greater than the shank diameter (112,112') for pressing the first surface of the panel against the substructure (300), an unthreaded shank (130,130') extending away from the load-bearing flange (124,124') and having a diameter smaller than the threaded shank diameter, and a second flange (140,140') having a diameter greater than the diameter of the load-bearing flange (124,124'). The second flange (140,140') is located on the side of the unthreaded shank (130,130') opposite the load-bearing flange (124,124'), and functions to seat against a recessed region of the second surface of the panel. A surface (148,148') on the second flange (140,140') adjacent the unthreaded shank (130,130') engages with and seals against a correspondingly configured recess (248a,248b) in the panel second surface.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to reinforced laminar flow control (LFC) structures used in aerospace applications, and more particularly to a fastening system using threaded fasteners to secure truss core LFC panel structures to underlying aircraft substructures, such that the head of each fastener lies flush with, and seals the opening In the outer surface of the LFC panel structure through which the fastener has been inserted, while a load-bearing portion adjacent the fastener threads functions to secure the panel structure to the underlying substructure. 2. Background of the Invention Recently, designers of aerospace vehicles have developed skin or panel structures which facilitate conformance of moving air or other fluid to the surface over which it flows. Such structures, known as laminar flow control (LFC) structures, in recent years have taken the form of reinforced truss core panel members. These panel members must be secured to underlying airframe substructures via suitably chosen fastening systems. One of the most critical difficulties in securing these structures to the airframe is in maintaining the precision of fit of the secured parts. Often manufacturing tolerances of the panel structures as well as the fastening mechanisms present the greatest obstacles in achieving the desired precision. To overcome,this problem, fastening systems of various forms have been tried, from simple fasteners, such as rivets or bolts, to more complex devices, such as intricate latching mechanisms. Still, precision of fit remains a problem. Moreover, the key objective of today's aerospace industry is to achieve optimum laminar flow. Thus, flow contacting surfaces of fastened panels, as well as of the fasteners securing the panels must present surfaces which act to minimize turbulence of the moving fluid. To date, no known system achieves these objectives. Attention is drawn to EP-A-0 209 778 which discloses a fastener having a threaded shank together with a support ring and a bolt shaft having an unthreaded shank and an end portion onto which a screw head is mounted, on the side of said support ring opposite said threaded shank. Said screw head having a diameter greater than the diameter of said support ring. In accordance with the present invention a fastener system as set forth in claim 1 is provided. Preferred embodiments of the invention are disclosed in the dependent claims. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a novel fastening system of laminar flow control panel structures which will provide precise and secure attachment to underlying airframe substructures, and which will overcome all the deficiencies and drawbacks of currently known systems of like kind for attachment of such laminar flow control structures. Another object of the present invention is to provide a novel fastener system simultaneously securing the laminar flow control panel to the underlying airframe substructure and sealing the aperture in the panel into which the fastener is inserted. These and other objects are achieved by providing a fastener system according to claim 1. The laminar flow control panel, comprised of a reinforced multi-layer member typically having at least inner and outer panel skins (and more preferably being of the type comprising a truss core construction), is secured to underlying substructure, as for example a wing or body support in an airframe. In particular, the fastener system includes a plurality of fasteners arranged at appropriate fastening locations, each having a conventionally threaded shank for attachment to the underlying substructure, an extended shank member including a first flange portion and a second flange portion spaced apart by a distance equal to the distance between the inner and outer surfaces of the panel, and exhibiting characteristics of the type described in detail below. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side sectional view of a fastener element in accordance with the teachings of the present invention; and Figure 2 is a side sectional view of a variation of the fastener element of Figure 1 in accordance with the teachings of the present invention; and Figure 3 shows a section of a multi-layer (eg., a truss core) panel with the two fasteners illustrated in Figures 1 and 2 disposed in respective positions securing the panel to an airframe underlying substructure. DETAILED DESCRIPTION OF THE INVENTION Referring now to Figures 1 and 2, there are shown two fasteners 100 and 100', respectively, contemplated by the present invention. These fasteners are essentially identical in structure with the exception of the undersurface of the second flange portion 140, 140' (as will be explained below). The following description is directed to the fastener shown in Figure 1; however, the fastener shown in Figure 2 exhibits similar structure which can be identified by the use of the similar ("primed") reference numerals used in both Figures. In Figure 1, the fastener 100 includes a first shank portion 110, a first flange portion 120, a second shank portion 130, a second flange portion 140, a third shank portion 150 and a third flange portion 160. The first shank portion 110 is preferably provided with conventional bolt threads 112 having an angle of inclination relative to the longitudinal axis of the fastener shank portions suitable for the retention needs of the bolt system. The first flange portion 120 has a lower planar surface 122, a circular periphery 124 which is concentric with the first shank portion 110 and which has a diameter greater than the diameter of the first shank portion, and an upper surface 126. The second shank portion 130 has a diameter which is less than the diameter of the first shank portion 110 and a length which approximates the distance between the inner and outer panel layers. The lower region of the second shank portion merges with the upper surface 126 of the first flange portion in a contoured manner. The second flange portion 140 has a lower section 142 of circular configuration having a diameter no greater than the diameter of the first flange portion 120. A flange 144, having an upper face 146 and a lower face 148, is situated atop the lower section 142 and exhibits a circular periphery concentric with the first shank portion and the first flange portion. The diameter of the flange 144 is greater than the first shank portion 122. The upper region of the second shank portion 132 merges with the lower section 142 of the second flange portion 140, and the lower region of the second shank portion merges with the upper surface 126 of the first flange portion to define a concave exterior surface on the second shank portion 130. Situated atop the second flange portion 140 is a third shank portion 150 and a third flange or nut member 160. The lower end of the shank portion 150 is designed so that, after a torque in excess of a predetermined amount is applied to the flange member 160, the shank portion lower end 152 separates from (twists off) the upper face 146 of the flange 144 of the second flange portion. Nut member 160 is shown as including faces 162, 162 adapted for engagement by conventional torque applying tools. The fastener of Figures 1 and 2 are substantially identical, with the exception of the lower faces 148 and 148' of the second flange portions 140 and 140', respectively. In the fastener of Figure 1, the lower face 148 is substantially perpendicular to the longitudinal axis of the first, second and third shanks. In the fastener of Figure 2, the lower face 148' is disposed at an oblique angle relative to the longitudinal axis of the first, second and third shanks. The function of the lower faces 148 and 148' will be described in connection with the structure illustrated in Figure 3. Referring now to Figure 3, there is shown a laminar flow control multi-layer panel member 200 fastened to an underlying substructure 300 with fasteners 100 and 100' provided in predetermined opening locations in the panel member 200. The internal layers of the panel member 200 illustrated in Figure 3 are of the type known as truss core panel layers, which when the panel is fabricated, exhibit characteristics of corrugated members. Such truss core panels are well known in the art, and no claim to novelty for such panels, per se, is made in this application. However, it is to be noted that when using truss core panels with the fasteners of the present invention, the fasteners are preferably to be located substantially centrally of the channels 220 formed between corrugation sidewalls 222, 224. When used in this manner, it can be seen that the necked shank region 130, 130' of the fastener presents minimum obstruction to the cross-sectional area of the channels 220, and thus in applications where fluid flows within the channels, interference with the flow is minimized. It is also to be noted that when the fasteners are properly secured in their respective apertures, the top surface of the fastener "cap" portion 140 lies substantially flush with the top surface 230 of the panel. In this manner, the fastener cap portion maintain integrity of a fluid-tight seal with the panel top surface to contain the hydraulic fluid system within the panel. Moreover, the "cap" portion top surface and the panel outer skin present a substantially unbroken, smooth surface without any discontinuities to any flowing fluid environment within which the outer skin outer surface is awash, so that laminar flow control at the panel outer skin can be achieved. As shown, the second flange portions 140, 140' of the fasteners 100 and 100', respectively bear against correspondingly configured aperture seats in the outer skin 210 of the panel 200. Thus, the fastener 100' shown on the left side of Figure 3 has its surface 148' bearing against a correspondingly configured surface 248a in the outer skin 210 of the panel. Similarly, the fastener 100 shown on the right side of Figure 3 will have its surface 148 positioned adjacent a correspondingly configured seat 248b in a sealing manner. It is contemplated that an appropriate sealing compound or material (as discussed below) will be applied to the underside of the fastener at surface 148 or 148' so that a fluid-tight leak-proof seal can be obtained. The first, load-bearing flange portions 124, 124' of the fasteners 100 or 100' are brought to bear against the inner skin of the panel via the threaded shank. The surfaces 122, 122' press against the panel inner skin and transfer the securing load to the load-bearing flange portions. Neck portions 130, 130' interconnect the load-bearing flange 124, 124' with the "cap" portion 140, 140' of the fasteners. Thus it is apparent that there has been provided, in accordance with the present invention, a novel fastening system securing a multi-layer panel to underlying substructure which is simple in construction and function, requires relatively small cost to manufacture, and yet fully satisfies the objectives, aims, and advantages set forth above. The multi-layer panel contemplated in the present invention can take the form of a truss core panel having inner and outer skin members. The skin members can be solid as when the panel's principal function is structural, or the skin members can be perforated to permit passage of fluid between the ambient and the interior of the panel, as when the panels are used for laminar flow control applications. In accordance with the invention, fastener-receiving apertures are provided through the thickness of the panel according to predetermined design requirements, and in advance of assembly of the panel to the underlying substructure. The fasteners include upper and lower flanges which provide separate and distinct functions. The lower flange of the fastener provides a load-bearing surface for holding one layer of the multi-layer panel against the underlying substructure. The upper flange includes a surface that coacts with an outer layer of the panel to seal and maintain the interior of the multi-layer panel at a predetermined positive or negative pressure. The upper and lower flanges are connected by a distinct separate necked region which is configured to minimize fluid flow obstruction as well as to maintain the second flange or "cap" portion in a sealed and seated position. The upper fastener flange provides no load-bearing function. Its sole purpose is to seal the aperture through which it has been inserted, while eliminating distortion of the outer skin of the multi-layer panel member. This is particularly important since the layers of the panel member, and the panel member itself, may be extremely thin, for example, in the order of .064 to .101 cm (.025" to .040"). The upper flange also assists in preventing ingress of fluid through the fastener-receiving aperture in the panel member's outer skin into the interior of the panel member when the fastener upper flange is brought to bear against the outer skin. Contributing to preventing leakage of fluid into or out of the panel member interior is a sealing material which is disposed below the undersurface of the upper flange. Such a sealing material can be polysulfide, silicone or polyurethane, or any other equivalent materials. As has been illlustrated in Figure 3, the interior sheets of the multi-layer panel 200 delimit channels or flow paths within which fluids (eg., cooling fluids or pressurized fluids) can be passed. With the inner and outer skins of the panel member 200, a hydraulic fluid transport system is formed which must be maintained at a predetermined level of positive or negative pressure. The fasteners of the present invention maintain the integrity of this system by preventing leakage at the fastener-receiving apertures, while securing the panel member to its underlying substructure. While the invention has been described relative to specific embodiments thereof, it also embraces any alternatives, modifications, and variations that will be apparent to those skilled in the art in light of the foregoing description, and which fall within the scope of the appended claims.

Claims: A fastener system including a multi-layer laminar flow control (LFC) panel (200) adapted to be secured to an underlying structure (300), and a plurality of fasteners (100, 100'), the LFC panel having one surface (230) awash in a fluid flow and a plurality, of spaced fastener-receiving holes arranged in a predetermined array, one fastener being disposed in each of the fastener-receiving holes, each fastener including a threaded shank (112, 112') for attaching the fastener to the underlying structure, the threaded shank having a diameter smaller than any of the panel-receiving holes, a load-bearing flange (122, 122') unitary with and located adjacent the shank, the load-bearing flange having a diameter greater than the shank diameter and seating on a first layer of the panel to urge the panel first layer into secure engagement with the underlying support structure, an unthreaded shank (130, 130') unitary with the load bearing flange and extending away from the load-bearing flange on the side opposite the threaded shank, and a non load-bearing second flange (144, 144'), having a diameter greater than said load bearing flange, on the side of the unthreaded shank opposite the load-bearing flange, the second flange snugly engaging in a recess in a second layer which comprises the one surface of the LFC panel awash in the fluid flow, so that the second flange presses against a recessed region (248a, 248b) in the panel, the second flange having a lower surface (148, 148') which is drawn into sealing engagement against the recessed region in the LFC panel, and an upper surface which is drawn flush with the one surface of the LFC panel to provide a substantially umbroken smooth surface, when the threaded shank is drawn tightly into threaded holes in the underlying support structure. The system of claim I, wherein the lower surface of the second flange (148') is disposed at an angle to the unthreaded shank and the upper surface (100') is circular in plan. The system of claim 1, wherein the second flange lower surface (148) comprises a planar region disposed at substantially ninety degrees to the unthreaded shank. The system of claim 1, wherein the LFC panel includes an upper layer (210), a lower layer and a truss core (224) disposed between the upper and lower layers, the truss core defining channels through which fluid can flow, and the unthreaded shank has a diameter smaller than the diameter of the threaded shank so that when each fastener has been secured in its respective fastener-receiving hole, fluid flowing in the channels passes therethrough without being obstructed by the unthreaded shanks. The fastener system of claim 1, wherein the lower surface of the second flange is coated with a sealing material. The fastener system of claim 1, wherein the second flange snugly engages the panel one surface without transferring loads thereto so that deformation of the one surface of the LFC panel is prevented. The fastener system of claim 1, wherein a fastener-driving means (160, 160') is provided on the upper surface of the fastener second flange, the fastener-driving means extending away from the second flange on the side thereof opposite the unthreaded shank, the fastener driving means being rotatable relative to the fastener second flange when the fastener has been secured to the underlying structure so that the fastener driving means separates from the second flange leaving a substantially planar surface.

Inventors: DECOUX STEVEN P, DECOUX, STEVEN P.
Applicants: ROCKWELL INTERNATIONAL CORP, ROCKWELL INTERNATIONAL CORPORATION
Application year: 1993
Main classification: F16B5
IPC codes: E04C2, F16B33, F16B31, B64C21, F16B35, F16B5