FIELD OF THE INVENTION
This invention relates generally to the manufacture of mobile platforms and, more particularly, the assembly of skin panels to the airframes of aircraft.
BACKGROUND OF THE INVENTION
Aircraft bodies are assembled by fabricating a frame and fastening panels to the frame. Typically, the frame is an aluminum or titanium structure with ribs, stringers, and the like to distribute the loads imposed by the aircraft's weight and aerodynamic forces that act on the aircraft. The assembly process continues with panels being fastened to the structure to form the skin of the aircraft. Because these skin panel fasteners carry much of the load on the aircraft as shear stress, the fastening of the skin panels to the aircraft frame is a factor in how efficiently the aircraft carries the loads.
Increasingly, composite skin panels are being used to lighten the aircraft and improve its load carrying capability. Assembling the composite panels to the aircraft requires that a liquid shim be applied to the cured panel to fill any gaps that might otherwise exist between the panel and the frame. These gaps arise because the composite panels generally will not match the shape of the frame exactly. Rather, some variation will exist between the frame and the panel that may be several thousands of an inch in magnitude. Thus, the liquid shim compensates for the variation. Once applied to the composite panel, the liquid shim begins curing and eventually forms a portion of the panel. Next, the panel is mounted to the airframe and fastener holes are drilled through the panel and frame for subsequent installation of a fastener.
If the material of the structure and the panel are the same, the hole may be sized with one constant diameter through the two sub-assemblies. However, metals and composites behave differently when subjected to loads. For instance, interference fits are frequently selected between metallic structures and fasteners to improve fatigue life in the tensilely loaded panels and frames. On the other hand composites generally require a clearance between the composite and the fastener to prevent the composite from delaminating while installing interference fit fasteners through the composite panel.
To provide the interference fit and the clearance, the panel must therefore be removed from the airframe after the fastener hole is drilled to the interference diameter. The hole in the composite panel is then reamed to a slightly larger size to create the clearance. Thereafter, the panel is placed on the structure once again. Then, the fastener is placed in the hole having the two diameters and tightened into place.
Unfortunately, the process of providing the clearance causes several disadvantageous results. First, removing and reinstalling the panel consumes time and resources that could be employed for other useful activities. Second, because the panel has been moved after the initial hole was formed, the reamer used to enlarge the hole in the composite panel may be positioned off of the longitudinal axis of the hole. Accordingly, the reamed enlargement may be off-center, or eccentric, with respect to the axis of the hole (through the panel). Moreover, perfect re-alignment between the panel and the structure may not be re-acquired either. Further with these previous assembly methods, any interlaminar metallic burrs generated during the enlargement process must be removed manually from the airframe structure before fastening the panel to the structure.
In the alternative, the reamer could be brought to the panel while it is still mounted on the airframe and the hole enlarged. In practice, this alternative has produced poor assemblies because the thickness of the composite panel varies from the theoretical thickness across the panel (both by design and due to variations inherent to fabrication of composite parts). Moreover, the liquid shim applied to the panel may vary in thickness because of the gaps between the airframe and the panel and because of variations in how the liquid shim is applied. Thus, the operator (or numerically controlled machine programmer) does not know at what depth to stop the reamer before it engages the airframe. If the reamer is advanced too far, it creates a clearance within the metal and weakens the joint. If the reamer does not advance far enough, it leaves an interference fit in the composite and weakens the joint.
Thus a need exists to improve the assembly of composite members to metallic structures.
SUMMARY OF THE INVENTION
It is in view of the above problems that the present invention was developed. The invention provides improved mobile platform skin panel assemblies and methods of assembling the same.
More particularly, the present invention provides an assembly that includes a skin panel made from a composite material and a structure made from a metal. The assembly defines a hole through the panel and the structure with a first diameter through the metal and a second diameter through the panel, which has a thickness profile known in advance. The transition between the two diameters occurs within a pre-selected tolerance from the surface of the metal that is adjacent to the panel and another pre-selected tolerance from the surface of the panel that abuts the structure. In other words, the clearance reliably extends into the metal only to the extent of the first tolerance, whereas the interference fit reliably extends into the composite only to the extent of the second tolerance. Thus, the present invention provides superior joints between composite skin panels and metallic structures. In addition, the present invention provides assemblies with a structure having a first material and a member having a second material and a fastener hole therethrough, the materials requiring the hole to have two different diameters.
In another preferred embodiment, the present invention provides a method of assembling composite panels to metallic structures. The method includes leaving the panel on the structure while a hole, which extends through the panel and structure, is enlarged down to between a pre-selected tolerance from the surface of the panel and another pre-selected tolerance in the structure. Thus, the present invention provides a superior method of assembling composite panels to metallic structures.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:
FIG. 1 illustrates a wing assembly in accordance with the principals of the present invention;
FIG. 2 illustrates a mechanical joint of the wing of FIG. 1 in cross section taken along the line 2-2;
FIG. 3 illustrates test results of mechanical joints constructed in accordance with the principles of the present invention;
FIG. 4 illustrates an instrument for verifying the mechanical joint of FIG. 2;
FIG. 5 illustrates another instrument for verifying the mechanical joint of FIG. 2; and
FIG. 6 illustrates a method in accordance with a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the accompanying drawings in which like reference numbers indicate like elements, FIG. 1 illustrates an assembly including a composite panel fastened to a metallic structure in accordance with the principals of the present invention. More particularly, FIG. 1 shows a wing 10 for use on a mobile platform (e.g. an aircraft) that has a structure 12 (including, for example ribs and spars) and a plurality of skin panels 14. The panels 14 define the outer mold line of the wing 10 and are fastened to the structure 12 with fasteners 16 (e.g. rivets) as shown. Also, a liquid shim 18 is applied to the inner surface of the panels 14. When the panel is pressed against the structure 12, the liquid shim 18 flows to fill gaps between the structure 12 and the panel 14. In a few isolated areas, the panel 14 will fit directly against the structure 12 and the liquid shim 18 will essentially flow completely out of these areas. In other areas, gaps, on the order of several thousandths of an inch, will exist where the liquid shim 18 will gather. A sealant 20 is also applied to the outer surface of the structure 12 to prevent moisture and other environmental contaminants from reaching the joint (i.e. the region where the panels 14 are adjacent the structure 12 with some quantity of liquid shim 18 and sealant 20 there between) following formation of the joint. Generally, the term “outer” refers to the panel 14 side of the joint, whereas “inner” refers to the structure 12 side of the joint. Further, “depth” refers to the distance from the outer surface of the panel 14 measured generally perpendicular thereto.
FIG. 2 illustrates the joint between the structure 12 and the panel 14 of FIG. 1 in cross section. In FIG. 2, the fastener 16 is not shown, though it fits in a hole 22. Generally within the structure 12, the hole 22 has a first diameter 24, whereas generally within the panel 14, the hole 22 has a second diameter 26. A transition region 28 connects the portion of the hole 22 having the first diameter 24 and the portion having the second diameter 26. While a transition 28 is shown that makes an acute angle with the axis 30 of the hole 22, step and curvilinear transitions 28 are also within the scope of the present invention.
The hole 22 includes an overall depth 32 that includes a first depth 34 defined by the first diameter 24 portion, a second depth 36 defined by the second diameter 26 portion, and a third depth 38 defined by the transition 28. Since the transition 28 may be a step, the depth 38 will henceforth be treated as being generally negligible. Similarly, the assembly 10 (e.g. the wing of FIG. 1) includes an overall thickness 40 associated with the locale of the hole 22. The overall thickness 40 includes a thickness 42 of the panel (which is measured in advance of the hole drilling and reaming method described herein), a thickness 44 of the liquid shim, a thickness 46 of the sealant, and a thickness 48 of the structure. The overall thickness 40 of the panel, the liquid shim 44, and (to an extent) the sealant 46 vary with each hole 22 location and, in general, between panels 12. Thus, the overall thickness 40 in the locale of each of the holes 22 is not known until measured at each hole location. For the same reason, the overall depth 32 of the holes are not known until the overall thickness 40 is measured. Various methods exist for measuring the overall thickness 40 (and therefore the overall depth 32) including sonar and laser-based measurements.
Also, FIG. 2 shows two tolerances associated with the hole 22 including a first tolerance 50 and a second tolerance 52. The tolerance 50 begins at the inner surface of the panel 14, extends into the panel 14 therefrom, and defines the allowable depth to which the first diameter 24 portion of the hole 22 may extend into the panel 14. Similarly, the tolerance 52 begins at the outer surface of the structure 12, extends into the structure 12 therefrom, and defines the depth to which the second diameter 26 portion of the hole 22 may extend into the structure 12. Between the tolerances 50 and 52 a variable thickness 54 portion of the joint exists defined by the local variations in the liquid shim 18 and the sealant 20.
As mentioned previously, the thickness 42 of the panel 14 also varies. The variations in the thickness 42 of the panel(s) arise from the complexity of the composite panels 14 and the design and manufacturing requirements, or preferences, for a given panel. For instance, the number of plies (e.g. carbon epoxy plies) in the panel typically varies with location on the panel 14. Also, manual lay-up methods and autoclave cure cycles are likely to cause further deviations from the theoretical thickness of the panel 14. To account for these variations, the following data may be loaded into a database for a numerically controlled machine prior to performing the drilling and reaming operations described for the present embodiment: a hole identification number, a hole location on the panel, and the actual panel 14 thickness 42. The methods discussed herein may also be performed manually, although automated machining is preferred. Whether the machining is performed manually or automatically, the machining parameters (e.g. drill feeds and speeds) can be changed as the tool(s) progress through the various layers of the assembly based on the stack-up information and the theoretical and actual thicknesses associated with a given hole identification number. As a result, it is possible to machine at the optimum rate for each material in the assembly and, thereby, reduce the cycle time for each hole. In particular, the machining speed may increase as progress is made through the softer materials (e.g. composites and aluminum) as compared to the speed preferred for the harder materials (e.g. titanium).
In accordance with the principals of the present invention, the variable thickness 54 region is defined by materials (the liquid shim 18 and the sealant 20) for which neither an interference fit, nor a clearance, with the fastener 16 (see FIG. 1) is preferred. That is to say that the liquid shim 18 and the sealant 20 will allow for either an interference fit or a clearance with the fastener 16. Accordingly, the transition 28 may occur anywhere within the portion of the joint defined by the combination of the tolerances 50 and 52 and the variable thickness 54. It has been found, in experimental operation, that the second diameter 26 portion of the hole 22 can be enlarged to the second diameter 26 without exceeding either tolerance 50 and 52 by using the variable thickness 54 portion to advantage, as illustrated in FIG. 3.
More particularly, FIG. 3 shows the results of experimental holes produced in accordance with the present invention. In particular, FIG. 3A shows a realistic worst-case example of a joint between a structure 112 and a composite panel 114. In this realistic worst-case scenario, only a small gap (of up to about 0.005 inches containing liquid shim 118) exists between the structure 112 and the panel 114 because almost all of the liquid shim was forced from between the structure 112 and the panel 114. Thus, the variable thicknesses employed to advantage by the present invention are at a minimum. Because experience has shown that the application of the sealant 120 is generally uniform, the present discussion will assume that the sealant is typically about 0.0056 inches thick and generally uniform. It will be understood, though, that the sealant thickness can vary. Further, the structure 112 could vary in thickness, although the structure 112 usually has a uniform thickness. However, in operation, whether the structure 112 is of uniform thickness, or not, has no bearing on the quality of joints constructed in accordance with the principles of the present invention.
In the example shown by FIG. 3A, the joint includes a ⅜″ composite panel 114 and a ⅜″ aluminum structure 112. The hole 122 was drilled through the joint with a bit to produce the interference fit diameter 124. Tolerances 150 and 152, of 0.010 inches each, were selected for the joint based on good engineering practice to prevent delamination of the panel 114 and to create a satisfactory interference fit with the structure 112. Next, the hole was reamed (from the panel side of the joint with the panel 114 remaining on the structure 112) to enlarge the portion of the hole 122 in the panel 120 to the clearance diameter 126. The reamer was allowed to advance into the panel 120 a distance equal to the measured thickness of the panel 114 and liquid shim 118 and then withdrawn. The hole 122 was thereafter inspected to determine where the transition 128 actually occurred with respect to the inner surface of the panel 114 and the outer surface of the structure 112. The process was repeated for the remainder of the holes 122 desired for securely fastening the panel 114 to the structure 112.
The results showed that the average transition 128 occurred within the liquid shim 118 in accordance with the principals of the present invention. That is to say, the transition 128 lies within the variable thickness of the liquid shim 118 where the joint requires neither a clearance nor an interference fit. The process was also shown to possess a Cpk (process capability index) of 1.45 that is satisfactory for most applications, even here in the realistic worst-case example.
FIG. 3B shows a more typical joint. Here, the liquid shim 118 (gap) is about 0.10 inches thick. The resulting average transition 128 again fell within the variable thickness of the liquid shim 118, thereby creating a satisfactory joint. The Cpk of the process operating on a typical joint was found to be about 2.05 and more than adequate to ensure the quality of the joint. In all cases, the depth of the reaming operation possessed a standard deviation of about 0.0028 inches using otherwise conventional drilling and reaming techniques.
For purposes of demonstration, an abnormal situation, wherein little (or no) liquid shim (gap) exists in the joint at the locale of the hole 122 was also tested, as shown by FIG. 3C. The resulting transition 128 occurred at a depth 0.0012 inches into the composite panel 114, well within the 0.010 inch tolerance 150. With a standard deviation of 0.0028 and a Cpk of about 0.86 the vast majority of the transitions 128 were found to be within the tolerance 150 in the panel 112, or in the sealant 120. A few transitions 128 were found to lie just outside of the tolerance 150. Given the number of holes 122 employed per joint, the factors of safety utilized, and the availability of inspection tools for the holes 122, the worst-case scenario of FIG. 3C also produces a satisfactory joint. Of course, controlling the drilling and reaming subprocesses to a smaller standard deviation will eliminate the existence of even those few transitions 128 that lie outside of the tolerance 150. In summary of the current embodiment, the present invention provides suitable joints even in the worst-case scenario involving no liquid shim 118 at a particular hole 122 location in the joint.
Moreover, because the panel 114 and the structure 112 remain generally adjacent each after the application of the liquid shim 118 and the sealant 120, no manufacturing debris, or other contaminants, will be found in the joint. Thus, the overall joint is stronger than provided by previous methods of assembly. Moreover, the drilling and reaming may be performed by using the same conventional 6-degree of freedom robot that can remain stationary relative to the axis 130 of the hole 122 (except as it traverse the axis 130). Thus, eccentricity of the clearance diameter 126 portion of the hole 122 (with respect to the interference diameter 124 portion) is significantly reduced over that of previous processes. For the joints illustrated by FIG. 3, the vast majority of holes 122 had an eccentricity of less than about 0.0013 inches, thereby yielding improved fit between the fastener and the panel 114 and structure 112 assembly. The “one-up” method of the present embodiment is, thus, particularly well suited to applications wherein the panels 114 are sealed, or bonded, to the structure 112 and thereafter left in place for the remainder of the aircraft assembly. Of course, the term “one up” refers to one of the advantages of the present invention in that the panel 114 may be lifted “up” into place on the structure 112 (or otherwise moved into place) only once with no subsequent disassembly required.
In the alternative to the one-up method, the present invention may also be employed where no sealant, or bonding agent, is applied between the panel 14 and the structure. For instance, the panel 114 may be clamped to the structure 112, an initial one-diameter hole drilled, and the hole may then be enlarged through the panel 114. Thereafter, the panel 114 may be unclamped from the structure 112 and, if desired, removed for de-burring and other operations prior to subsequent assembly operations.
With respect to FIG. 4, a pair of instruments 200 and 300 are shown as used for inspecting holes 222 and 322 in accordance with the principles of the present invention. Generally, the instruments 200 and 300 are used to verify the end point of the clearance diameter portion of the hole. The instrument 200 includes a boroscope eyepiece at the proximal end (not shown) and a mirror 202 at the distal end. The distal end is shown inserted in the hole 222 and further includes a pair of diametrically opposed apertures 204, an index mark 206, and a flat distal end 208. The flat distal end 308 is adapted to engage the transition 228 as shown. Thus, when the flat distal end 208 engages the transition 228 the instrument 200 is held in a fixed position relative to the transition 228. Preferably the outside diameter (of at least a portion) of, the distal end is the same as the clearance diameter 226, thereby further facilitating holding the instrument 200 in fixed relationship to the transition 228. The mirror 202 is positioned so that it allows a user looking through the eyepiece to see out through the aperture 204 and inspect the side of the hole 222 in the vicinity of the transition 228. In particular, the index mark 206 is also viewable by the user and set (inside the distal end of the instrument 200) at a distance from the distal end equal to the tolerance 252 associated with the structure 212. Thus, as the user views the side of the hole 222, via mirror 202, the user can judge whether the index mark 206 is deeper than the outside surface of the structure 212. If so, the clearance diameter 126 portion penetrates too deeply into the structure 112 indicating an unacceptable hole 222. Such a situation is illustrated in FIG. 4B. In contrast, FIG. 4A shows a hole 222 wherein the index mark 206 is at the outer surface of the structure 222. Thus, FIG. 4A illustrates an acceptable hole 222. Accordingly, the instrument 200 is referred to as a “too deep” gage 200.
FIGS. 4C and 4D illustrate the corresponding “too shallow” gage 300 in a hole 322 that is acceptable (FIG. 4C) and in an unacceptable hole 322 (FIG. 3D). The differences between the “too deep” gage 200 and the “too shallow” gage 300 include the presence of a step 308 on the sides of the distal end and the location of the index mark 306. The step 308 is adapted to engage the transition 328 and generally corresponds in shape to the transition 328. Thus, the index mark 306 may be positioned at a distance equal to the tolerance 350 associated with the panel 314 from the step 308 as shown. A user may view the index mark 306 and the side of the hole 322 via the mirror 302 to judge whether any of the liquid shim 318, sealant 320, or structure 312 is visible between the index mark 306 and the distal end. If so, the hole 322 is deep enough (i.e. is within or deeper than the tolerance 350). If not, then the transition 328 is too shallow (i.e. the clearance diameter 326 portion of the hole 322 does not extend far enough through the panel 320 to produce a satisfactory joint).
While the exemplary fastener holes previously discussed were generally orientated perpendicularly to the mating surfaces of the structure 12 and panel 14 (see FIG. 2), the present invention is not thereby limited. Rather, in a preferred embodiment, a joint with a “ramped” pair of mating surfaces is provided. That is to say that the axis 30 (see FIG. 2) of the hole is orientated at an acute angle with respect to the mating surfaces. In the present embodiment, the depth 36 of the clearance diameter 26 portion of the hole 22 is measured along the axis 30 of the hole 22. Thus, the intersection of the axis 30 with the plane defining the end of the clearance diameter 26 portion lies within the acceptable depth range 56. Ramped holes 22 are inspected by, for example, rotating the “too deep/too shallow” gage 400 in the hole 22 until the views through the pair of diametrically opposed apertures 404 are the same. If both views are neither too deep, nor too shallow, the hole 22 is considered acceptable.
Lighting may also be provided internal to the instruments 200 and 300 to enable the user to view the visible differences between the structure 212, the panel 214, the liquid shim 218, and the sealant 220. The instruments 200 and 300 also enable the user to inspect the sides of the hole 222 to determine whether any chips or cuttings were caught between the outer pitch of the drill bit (and reamer) and the sides of the hole 322 by turning the instrument while traversing the axis of the hole 322. Thus, the composite panels 220 and 320 may be inspected for internal machining damage that would otherwise be hidden.
FIG. 5 shows a combined “too deep/too shallow” instrument 400. The instrument 400 differs from the instruments 200 and 300 in that, the instrument 400 includes both of the index marks 406 and the step 408. In particular, the index mark 406′ is positioned a distance from the step 408 equal to the tolerance 452 associated with the structure 412. Thus, it indicates whether the hole 422 is too deep. The index mark 406′, on the other hand, is positioned a distance equal to the clearance 450 associated with the panel 414 from the distal end, thereby indicating whether the transition is too shallow. Accordingly, the one tool 400 may be used to simultaneously determine whether the hole 422 is deep enough and whether the hole 422 is shallow enough (i.e. within an acceptable range of depth).
In another preferred embodiment, the present invention provides a dial indicator for inspecting the depth of the transitions. The dial indicator includes a plunger operatively connected to a depth dial gage. Further, the end of the plunger is adapted to engage the transition of a hole, thereby enabling the inspection. To inspect a hole, the indicator is zeroed by fully depressing the plunger against a hard surface. Then the plunger is inserted into the hole and allowed (by a biasing member such as a spring) to extend to the depth at which it stops. Generally, the depth at which the plunger stops indicates the location of the transition. However, debris in the hole may cause the dial indicator to indicate a transition depth shallower than the true transition depth. Also, erosion (particularly of the liquid shim and sealant) caused by chips being caught between the drill bit, or reamer, during the machining of the hole, may allow the plunger to extend beyond the true transition, thereby indicating a transition depth larger than the true transition depth. Accordingly the boroscope based instruments 200, 300, and 400 are preferred over the dial indicator of the current embodiment.
With reference now to FIG. 6, a method in accordance with yet another preferred embodiment is illustrated. As shown, the method 500 includes laying up a plurality of composite panels and subsequently curing them in operation 502. Usually in parallel with operation 502, an airframe is fabricated (or assembled) as in operation 504. Liquid shim is then applied to the cured panels. Before the liquid shim cures, the panels are pressed against the airframe to cause the liquid shim to fill the gaps there between. See operation 506. The liquid shim is allowed to cure. Before final assembly of the panels to the airframe, operation 508 applies sealant to the airframe. Thereafter, the panels are mounted (with jigs or other support equipment) to the airframe as in operation 510. Holes having a diameter that will cause an interference fit with the fasteners are drilled through the panels and through the structure at pre-selected locations. See operation 512. While leaving the panels on the airframe (as in operation 514) the holes are then reamed to the larger clearance diameter to a depth that is pre-selected to cause an acceptable joint in operation 516. While many types of tools may be employed to enlarge the hole, either step or flat-bottom reamers are employed in preferred embodiments of the present invention. Depending on the tool selected, the set-up values (e.g. step reamer lengths—to the step and the total length) are measured in advance and included in the machining program to enable the machine to ream the hole to the specified depth (e.g. to the bottom of the flat reamer or to the step of the step reamer). Of course, the overall thickness of the panel(s) is known or measured before pre-selecting the depth of the enlargement. At reference 518, the holes are then inspected to determine whether the diameter transitions are within the acceptable ranges (neither too deep, nor, too shallow) with, for example, the instruments provided by the present invention. Fasteners are inserted into the holes and the panels are fastened to the structure as shown at operation 520. Thus, the superior joints discussed herein may be assembled by use of the present embodiment.
In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. In particular, satisfactory joints are provided with the panels remaining on the airframe once placed thereon. Thus, the present invention reduces the cost of assembling aircraft. For the same reason, the present invention provides joints having superior mechanical properties (e.g. strength, fit, noise or rattling because of poor “fit up”). Additionally, the present invention provides improved inspection tools over those previously available.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.
As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. For example, whereas the foregoing discussion involved composite members being assembled to metallic structures, the present invention is not limited thereby. Rather, any assembly with materials requiring having fastener holes with two different diameters is within the scope of the present invention (e.g. titanium and aluminum). Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.
Patent Application
20060010689
