Preforms for acute structural edges

[0001] 1. Field of the Invention

[0002] The present invention relates generally to preforms for use in construction of vehicles and relates specifically to preforms used for forming acute structural edges.

[0003] 2. Description of the Related Art

[0004] When constructing vehicles, it is often desirable or necessary to create edges formed using acute angles. This is especially true when constructing air vehicles, such as manned or unmanned aircraft and missiles, which may require a sharpened edge for smoothly penetrating the airflow or providing a smooth convergence of separate flows.

[0005] FIG. 1 shows a prior art aircraft 11 and illustrates the numerous locations of acute structural edges. The most common use of acute edges is in leading edges 13, 15 and trailing edges 17, 19 of wings 21, 23, respectively. Leading edges 13, 15 ensure that wings 21, 23 pass easily through a longitudinal airflow, whereas trailing edges 17, 19 allow the separated airflows on either side of wings 21, 23 to smoothly converge at the rear of wings 21, 23. Inlet lips 25 separate onrushing air into flows entering engine nacelles 27 and flows continuing on the outer surfaces of aircraft 11. Chines 29 are formed along longitudinal edges, providing desired aerodynamic effects and a blended shape for reducing radar cross-section.

[0006] FIGS. 2 through 4 illustrate prior art methods for forming acute structural edges. Though described in terms of directions relative to their orientation in the figures in the present application, it should be noted that the edges may be located in various orientations, including orientations opposite of those shown or in inclined or vertical orientations.

[0007] In FIG. 2, assembly 31 is formed by inserting a rigid edge member 33 between an upper planar element 35 and a lower planar element 37. Edge member 33 and elements 35, 37 may be formed from various materials, including metals and composites. When assembled, upper surface 39 of edge member 33 registers with the outer surface of element 35 and lower surface 41 registers with lower element 37, forming smooth, continuous outer surfaces for undisturbed airflow. A forward edge 43 has an acute included angle and is located on the forward portion of edge member 33. Members 45 extend from a rearward portion of edge member 33 for fastening edge member 33 to elements 35, 37. Members 45 are inwardly offset toward a plane (not shown) bisecting edge member 33 and parallel to forward edge 43, forming rearward-facing shoulders 47. The forward ends of elements 35, 37 abut shoulders 47. Typical methods of fastening edge member 33 to elements 35, 37 include adhering members 45 to the inner surfaces of elements 35, 37 and mechanically fastening members 45 to elements 35, 37.

[0008] FIG. 3 shows assembly 49, which is formed by assembling upper element 51 with lower element 53, elements 51, 53 being formed of rigid materials, such as metals or composites. Elements 51, 53 terminate in forward ends 55, 57, respectively, each forward end 55, 57 having an increased thickness and mating surface. When elements 51, 53 are assembled to each other, the mating surfaces are adjacent at seam 59 and forward ends 55, 57 are aligned to form forward edge 61. Elements 51, 53 are fastened together at forward ends 55, 57 by fastener 63, for example, a double-countersunk fastener, which extends through the thickness of the assembled forward ends 55, 57.

[0009] Like assembly 49 in FIG. 3, edge assembly 65 in FIG. 4 comprises upper element 67 and lower element 69. Upper element 67 has a forward end 71 having an increased thickness and a downward-facing mating surface. Lower element 69 has a forward end 73 that also has an increased thickness and an upward-facing mating surface. Elements 67, 69 are aligned and joined to each other, the mating surfaces being adjacent at seam 75 and forward ends 71, 73 aligning to form forward edge 77. Rather than being joined using fasteners, elements 67, 69 adhered to each other or bonded in a co-curing process.

[0010] While these prior art assemblies have satisfactorily been used to form acute edge structures, improvements are needed to reduce weight and cost and increase structural integrity of edge structures. Additionally, improvements are needed to improve the ability to repair damaged edge structures and to allow for tailored material and structural properties.

[0011] An apparatus and method are provided for constructing acute structural edges. A preform has a body having an acute-angle edge at one end and connecting areas extending from the opposing end for bonding the preform to converging planar elements of a wing or other similar structure. In several embodiments, the acute angle of the preform generally matches the angle relative to each other of converging ends of the planar elements, the preform forming the outer end of smooth, continuous outer surfaces after assembly. The connecting areas may be connected to interior or exterior surfaces of the elements, depending on the orientation of the preform and elements. In another embodiment, the preform is placed between the outer ends of the elements for aligning and supporting the elements at the desired location and angle relative to each other.

[0012] The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings.

[0013] FIG. 1 is a perspective view of a prior art aircraft illustrating the locations of edge structures.

[0014] FIG. 2 is a schematic, cross-sectional view of a prior art edge structure.

[0015] FIG. 3 is a schematic, cross-sectional view of a second type of prior art edge structure.

[0016] FIG. 4 is a schematic, cross-sectional view of a third type of prior art edge structure.

[0017] FIG. 5 is a schematic, cross-sectional view of an edge structure of the present invention.

[0018] FIG. 6 is a schematic, cross-sectional view of a second embodiment of an edge structure of the present invention.

[0019] FIG. 7 is a schematic, cross-sectional view of a third embodiment of an edge structure of the present invention.

[0020] FIG. 8 is a schematic, cross-sectional view of a fourth embodiment of an edge structure of the present invention.

[0021] FIG. 9 is a schematic, cross-sectional view of a fifth embodiment of an edge structure of the present invention.

[0022] FIG. 10 is a schematic, cross-sectional view of the preform of FIG. 8 illustrating fibers within the preform.

[0023] FIG. 11 is a schematic, cross-sectional view of the preform of FIG. 8 illustrating fibers and a core within the preform.

[0024] FIGS. 5 through 9 show several edge assemblies using embodiments of the preform of the present invention. The preforms are preferably woven from selected fibers in a three-dimensional weave pattern, with fibers running in lateral, longitudinal, and vertical directions. The three-dimensional weave increases the strength of the preform and allows for tailoring of characteristics of the preform through selection of fiber materials and their position within the preforms. Preforms may alternatively be formed by other methods, including braiding, knitting, and stitching, that can be used to assemble fibers in an interlocking, three-dimensional pattern with control of fiber type and orientation. After completion of the edge assembly process, the fibers of the preforms are located within a rigid matrix, such as cured resin, providing the preforms with rigidity. As in the previous sections, the various embodiments are described in terms of directions relative to their orientation in the figures, though the edges may be located in various orientations, including orientations opposite of those shown or in inclined or vertical orientations.

[0025] Edge assembly 79 of FIG. 5 comprises preform 81 and upper and lower planar elements 83, 85 of a structure, such as an airfoil. Elements 83, 85 are formed from a rigid material, for example, cured composites or metal. Preform 81 is formed to have a body 87 and upper and lower extensions 89, 91 extending from a rear portion, or base, of body 87. The base may be arcuate, as shown, or may be planar. Extensions 89, 91 have inner surfaces 93, 95 and outer bonding surfaces 97, 99, respectively, inner surfaces 93, 95 terminating in tapered ends. Though not required in the most embodiments of the present invention, the use of tapered ends is preferred, as the tapered ends spread mechanical loads across a larger surface and provides better resistance to peel loads than a preform in which the longitudinal fiber layers all terminate at the same length. In some cases, however, truncated ends may be preferred, such as for repairs.

[0026] Body 87 has an upper surface 101 and a lower surface 103, or flanks, and bonding surfaces 97, 99 are generally parallel to surfaces 101, 103. Surfaces 101, 103 converge to an edge 105, or crest, at the forward end of preform 81. Extensions are inwardly offset toward a horizontal plane (not shown) that bisects body 87, forming shoulders 106 on a rear portion of body 87.

[0027] Preform 81 may be infused with resin prior to assembly for co-bonding to rigid planar elements 83, 85 (e.g., metal or cured composites) or for co-curing with uncured, composite elements 83, 85. Preform 81 may alternatively be cured prior to bonding to rigid elements 83, 85 or may be infused with resin after assembly, for example, by a resin-transfer molding process (RTM). Preform 81 may also be attached by mechanical fasteners.

[0028] In an assembly procedure of an uncured, infused, preform 81 and rigid elements 83, 85, preform 81 is positioned near the outer ends of planar elements 83, 85, and surfaces 101, 103 of preform 81 are generally aligned with the outer surfaces of elements 83, 85. Surfaces 97, 99 are placed adjacent the inner surfaces of elements 83, 85, and the outer ends of elements 83, 85 are placed against shoulders 106. Preform 81 is then cured to form a rigid edge structure, bonding extensions 89, 91 to the inner surfaces of elements 83, 85. After assembly, the outer surfaces of elements 83, 85 and surfaces 101, 103 of preform 81 form smooth, continuous surfaces.

[0029] FIG. 6 shows edge assembly 107, comprising preform 109 and upper and lower planar elements 111, 113. Preform is formed to have body 115 shaped like that in the embodiment of FIG. 5, but differs from the previous embodiment in that only one extension 117 extends rearward from body 115. Body has outer surfaces 119, 121, or flanks, that converge to an edge 123, or crest. Extension 117 has upper and lower bonding surfaces 125, 127 that are generally parallel to surfaces 119, 121 of preform 109. Surfaces 125, 127 are inwardly offset, creating shoulders 129.

[0030] Assembly of preform 109 and elements 111, 113 is accomplished in a manner similar to that described for the previous embodiment. Preform 109 may be infused with resin and cured prior to assembly by co-bonding or mechanical fastening. Alternatively, preform 109 may be assembled uncured, or it may be infused after assembly. Bonding surfaces 125, 127 are bonded to the inner surfaces of elements 111, 113, and body 115 is cured to form a rigid edge with surfaces 119, 121 parallel to the outer surface of a corresponding element 111, 113.

[0031] In FIG. 7, edge assembly 131 is formed of planar elements 133, 135 and preform 137. Each element 133, 135 has a forward flange 138, 139 that is angled inward relative to the remainder of elements 133, 135. Body 141 of preform 137 has outer surfaces 143, 145, or flanks, that taper to a forward edge 147, or crest. Extensions 149, 151 extend from the rear portion of body 141 and are formed to have outer bonding surfaces 153, 155 and tapered inner surfaces 157, 159. Channels 161 are formed in the rear portion of body 141 for receiving flanges 138, 139.

[0032] Edge assembly 131 is assembled by inserting each flange 138, 139 into a channel 161 of preform 137 and placing bonding surfaces 153, 155 adjacent the inner surfaces of elements 133, 135. The inner surfaces of channels 161 are adjacent the outer surfaces of flanges 138, 139. As described above, preform 137 may be attached to elements 133, 135 by co-curing, co-bonding, secondary bonding, and/or mechanical fastening.

[0033] FIGS. 8 and 9 show edge assemblies 163 and 165 formed using preform 167. Referring to FIG. 8, planar elements 169, 171 are attached to preform 167. Preform 167 has a body 172 having outer surfaces 173, 175, or flanks, that converge to a forward edge 177, or crest. Extensions 178, 179 extend rearward from the rear portion of body 172, outer surfaces 181, 183 being parallel and continuous with surfaces 173, 175 of body 172. Inner surfaces 185, 187 of extensions 178, 179 are tapered toward a rear edge. Elements 169, 171 have forward flanges 189, 191, respectively, oriented to be slightly angled inward relative to the remainder of elements 169, 171, matching the angle of the taper on inner surfaces 185, 187.

[0034] To assemble edge assembly 163, planar elements 169, 171 are bonded or otherwise fastened to inner surfaces 185, 187. The outer surfaces of flanges 189, 191 are placed adjacent inner surfaces 185, 187, the angle of flanges 189, 191 following the taper of inner surfaces 185, 187. Extensions 178, 179 are positioned so that their rear, terminal ends lay at the intersection of flanges 189, 191 and the remainder of elements 169, 171, providing for a smooth, continuous transition from outer surfaces 173, 175 to the outer surfaces of elements 169, 171.

[0035] FIG. 9 shows edge assembly 165, comprising preform 167 and planar elements 193, 195. Rather than being attached to the inside of preform 167, elements 193, 195 are attached to outer surfaces 173, 175 of body 172 and outer surfaces 181, 183 of extensions 178, 179. Elements 193, 195 are positioned so that the forward portion of element 193 slightly overlaps the forward portion of element 195, forming forward edge 197. Preform 167 is located within a volume enclosed by elements 193, 195, and in the assembly shown, no portion of preform 167 forms an outer surface of edge assembly 165 or is exposed to airflow. Rather, preform 167 provides structural support and holds elements 193, 195 in the desired positions.

[0036] As mentioned above, preforms of the invention may be formed entirely of one of various types of fibers, or the preforms may be formed of multiple types of fibers for forming preforms having desired characteristics, such as increased mechanical stiffness, reduced electrical conductivity, reduced emissivity, and reduced reflectivity. Alternatively, the preforms may be formed of one or more types of fibers, with fibers in selected portions of the preform having coatings on the fibers or being otherwise altered to provide desired characteristics.

[0037] FIG. 10 is a schematic, cross-sectional view of preform 167 showing the use of two different types of warp fibers in the weave. Fibers 198 are indicated with an x, and fibers 199 are indicted by a -. Fibers 198, which are located in extensions 178, 179 and the rear and interior portion of body 172, are preferably higher strength fibers, for example, carbon fibers. Fibers 198 are positioned in locations where stresses are higher, such as bonding areas. Fibers 199, which may be glass fibers or similar types, are located in outer surfaces 173, 175, reducing electrical conductivity in outer portions of the preform. Fibers 199 allow for tailoring of electrical conductivity, emissions of internal electromagnetic energy, and radar wave reflectivity.

[0038] Referring to FIG. 11, a cross-sectional view of a preform 201 shows an insert 203, which may be formed of foam or honeycomb and is located in the interior of preform 201. A three-dimensional weave of fibers 198 and 199 is produced around insert 201, which provides a rigid structural member for support during weaving. Fibers 198, 199 and insert 203 may be coated or otherwise modified to produce desired characteristics, such as those discussed above. Preform 201 is shown as having a shape like that of preform 167, though an insert like insert 203 may be used to form any of the preforms within the scope of this invention.

[0039] Many advantages are realized from using the present invention. Edge structures can be easily formed from preforms that allow for tailoring of the characteristics of the preform by altering the types and locations of materials in the preform. These structures are lightweight, and can be tailored for desired strength, flexibility, emissivity, and reflectivity. The preforms provide for lower cost of assembly and increased structural integrity. Reparability is enhanced, since the preforms may be installed and removed as a unit, which also decreases the man-hours required to repair a damaged edge assembly.

[0040] While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, though shown as flat, the planar elements may be curved, and the preforms may be formed and cured to match the curvature of the elements. Also, preforms may be formed from a variety of composite materials, including metal- and ceramic-matrix composites or fiber-reinforced adhesives.