Patent Description:
Aerodynamic, also known as "wetted", exterior surfaces of aircraft can experience significant manufacturing tolerance variations during component fit-up. If gaps at faying edges (i.e. fastening joints) are fixed and/or locked in place with contoured fillers or shims, resulting variations can create turbulent air flows which may create erosion of aft joint surfaces. When the components are formed of composite materials, the erosion may actually produce delamination. As a result, special care must be taken in the manufacture of faying edges of wetted aircraft components at risk for erosion damage.

Such risks may be somewhat greater for certain thin-walled curvilinear aircraft components, including engine nacelle lipskins, leading edges of wings, and wetted areas aft of fuselage joints. To avoid critical damage, such thin-walled structures including the noted nacelle lipskins, for example, have been fitted with shims for assuring desired positioning of the thin-walled structures on underlying support structures such as bulkheads.

<CIT>, in accordance with its abstract, states a nacelle inlet provided for a turbo-fan engine for an aircraft. The nacelle inlet includes a lip skin having first and second pad-ups, an inner barrel including a forward flange and a bulkhead. The bulkhead, the lip skin and the inner barrel are attached together at mating surfaces. The bulkhead and the lip skin are attached together at mating surfaces. Each mating surface defines a surface geometry which approximates a developable surface having parallel ruling lines. The developable surfaces are formed by machining.

<CIT> in accordance with its abstract, states, in an air intake structure of an aircraft engine pod, the link or connection between the air intake lip, the front reinforcing frame and the acoustic panel. This link or connection is implemented in such a way that the lip can be dismantled without breaking the link between the frame and the panel. Moreover, the internal, rear part of the lip normally covers the front part of the panel, as well as the members ensuring the link between the latter and the frame.

Methods of applying the shims, generally referred to as "shimming", have traditionally required relatively tedious efforts to achieve desired levels of precision. Generally, custom contoured shims have been employed, even though applications of the latter have tended to be expensive and time-consuming.

Thus, there has been a continuing need to develop less expensive and more time-saving shimming methods and systems.

In accordance with one aspect of the present disclosure, a method is disclosed of assembling first and second curvilinear components to a support structure having an axis , the first component having a faying edge axially spaced and radially aligned with a faying edge of the second component; the method comprising:preassembling the first component to the support structure;conducting a preassembly measurement of radial gaps between the support structure and the faying edge of the second component to calculate an average value of radial gaps extending circumferentially between the second component and the support structure, including an assessment of whether the calculated average value of all radial gaps falls within structurally predetermined acceptable limits;selecting a specific shim thickness, from among prefabricated sets of shims, closest to that of one set that corresponds to the calculated average radial gap value;applying a plurality of shims of the specific thickness selected to the support structure in a position prospectively determined to be under the edge of the second component, each shim being spaced circumferentially about the support structure; and permanently securing the second component to the support structure, in a manner such that the faying edges of the first and second components are fixed in radial alignment with each other.

In accordance with another aspect of the present disclosure, a non-claimed assembly is disclosed. This assembly comprises first and second curvilinear aircraft components secured to a bulkhead, the bulkhead having an axis, and each component having a faying edge axially spaced from and radially aligned with the other; wherein the components are assembled so that a plurality of shims each of a specific thickness are in a position prospectively determined under the edge of the second component, each shim being spaced circumferentially about the bulkhead, wherein the shim thickness corresponds to a calculated average of radial gaps between the bulkhead and the faying edge of the second component, said average calculated gap value being assessed to assure that if falls within structurally predetermined acceptable limits; and the second of the components is permanently secured to the bulkhead with the plurality of shims incorporated in a manner such that that the faying edges of the first and second components are fixed in radial alignment with each other.

In accordance with yet another aspect of the present disclosure, a non-claimed assembly is disclosed, formed as a product-by-process, includes first and second curvilinear aircraft components secured to a bulkhead, the components each defining a faying edge axially spaced from and radially aligned with the other. The first component is preassembled to the bulkhead; the second component is then separately assembled to the bulkhead after conducting a preassembly measurement of gaps between the faying edge of the second component and the bulkhead. For this purpose, a predictive analytics model is applied to calculate average value of all radial gaps, and a specific shim thickness closest to one that corresponds to the calculated average gap value is selected from among prefabricated sets of shims. A plurality of shims from one of the sets is circumferentially applied to the bulkhead, with each shim from the one set having an identical thickness corresponding to the average gap value. The second component is then permanently secured to the bulkhead in a manner such that the faying edges of the first and second components are fixed in radial alignment with each other.

The features, functions, and advantages disclosed herein can be achieved independently in various embodiments or may be combined in yet other embodiments, the details of which may be better appreciated with reference to the following description and drawings.

It should be understood that referenced drawings are not necessarily to scale, and that disclosed embodiments are illustrated only schematically. Aspects of the disclosed embodiments may be combined with or substituted by one another, and within various systems and environments that are neither shown nor described herein. As such, it should be understood that the following detailed description is merely exemplary, and not intended to be limiting in either application or use.

The following detailed description includes methods as well as a product-by-process aspect for carrying out the present disclosure. Actual scope of the disclosure is as defined in the appended claims.

Referring initially to <FIG>, an aircraft turbofan jet engine <NUM> of the type utilized on a commercial jet airliner (not shown) includes a nacelle lipskin <NUM> mounted directly to a jet engine bulkhead <NUM> (hidden in <FIG>, but shown in <FIG>). The bulkhead <NUM> defines an axis "a-a" (<FIG>), and the lipskin <NUM> along with an inner barrel <NUM>, situated adjacent to the lipskin <NUM>, are both curvilinear components of a type that may be axially and radially secured to the bulkhead <NUM>, utilizing methods disclosed herein. The bulkhead <NUM>, as a critical support structure, may be formed of a metal, such as titanium or aluminum. The lipskin <NUM> will also normally be formed of a relatively thin-walled metal of aluminum alloy, so as to be able to withstand high temperature engine bleed air typically ducted through a leading edge <NUM> of the lipskin for deicing purposes during flight in known icing conditions. The inner barrel <NUM>, however, in the examples provided herein, is formed of a relatively light weight, but relatively rigid, composite material, and is secured to the bulkhead <NUM> in a preassembly process not described herein, although well known to those skilled in the art.

It is desirable that the lipskin <NUM> and the inner barrel <NUM> be axially spaced a slight distance apart, principally due to their differences in coefficients of expansion. As a result, an edge <NUM> of the lipskin <NUM> is ideally axially spaced from an edge <NUM> of the inner barrel <NUM> by a distance ranging from <NUM> to <NUM> thousandths of an inch (i.e. <NUM> to <NUM>), to accommodate axial expansion of the metal edge <NUM> of the lipskin <NUM> relative to the composite edge <NUM> of the inner barrel <NUM>. An arrow A indicates the relative movement of airflow over a so-called wetted faying joint <NUM>, which includes the described axially spaced edges <NUM> and <NUM>. In the manufacture of aerodynamic, or "wetted" joints, special care must be taken to avoid so-called aerodynamic steps, which can create erosion of areas aft of joints, such as the faying joint <NUM>, particularly when affected downstream structures are formed of composites. Such erosion may actually create significant damage, including delamination of the composite material.

Referring now to <FIG>, three representative cross-sections of the circumferentially extending faying joint <NUM> reveal an aerodynamic steps <NUM>' as a positive step in <FIG> depicts a virtual absence of any aerodynamic step, while <FIG> depicts a negative aerodynamic step <NUM>". Aerodynamic steps are created by radial offsets of the axially spaced faying edges <NUM> and <NUM>, as measured at outside diameters <NUM> and <NUM> of the nacelle lipskin <NUM> and inner barrel <NUM>, respectively. A step is identified as positive if the airstream (shown as arrow A) passes over the outside diameter <NUM> of the nacelle lipskin <NUM>, but directly impinges on the edge <NUM> of the inner barrel <NUM>. In fact, the edge <NUM> includes a chamfer <NUM> to minimize effects of faying joint erosion earlier described. Conversely, a step is identified as negative if the airstream passes over the outside diameter <NUM> of the nacelle lipskin <NUM> and flows radially above the edge <NUM> of the inner barrel <NUM>, as shown in <FIG>. Those skilled in the art will appreciate that neither a positive or negative aerodynamic step is acceptable if outside of certain structural limits.

The amount of aerodynamic step <NUM>' or <NUM>" is controlled by managing the circumferentially extending radial gaps 26a, 26b, and 26c, each gap delimited by an outer diameter <NUM> of the bulkhead <NUM> and an inner diameter <NUM> of the nacelle lipskin <NUM>, as shown respectively in <FIG>. It is noteworthy that the various gaps 26a, 26b, and 26c should not be confused with, nor do they directly correlate with, the positive or negative aerodynamic steps <NUM>' and <NUM>". For example, although the indicated gap 26a of <FIG> is virtually nonexistent, it is associated with the described positive step <NUM>'. On the other hand, the lack of aerodynamic step in <FIG> correlates with a discernable gap 26b; while the negative aerodynamic step <NUM>" is associated with an even larger gap 26c.

Referring now also to <FIG>, the methods of this disclosure offer effective control of depicted gaps 26a, 26b, and 26c, as well as of all others that may exist between the circumference of the nacelle lipskin <NUM> and the bulkhead <NUM>, in a manner that is more cost-effective and considerably less time-consuming than previous methods. Moreover, the methods disclosed herein, which can be generally referred to as "average thickness shimming", are particularly advantageous in situations wherein one axially spaced component is rigid, i.e. the inner barrel <NUM>, while the other component, i.e. the thin-walled lipskin <NUM>, is relatively compliant and/or flexible.

Referring now also to <FIG>, a cross-section of the nacelle lipskin <NUM> and adjacent inner barrel <NUM>, taken along lines <NUM>-<NUM> of <FIG>, reveals the circumferential faying joint <NUM>, as situated on an air inlet diameter <NUM> of the nacelle lipskin <NUM>. The nacelle lipskin inlet is treated herein as just one representative embodiment by which to describe the methods of this disclosure. Similar faying joints, such as that situated on the exterior or outside cowling <NUM> of the jet engine <NUM>, could be described similarly.

Referring now specifically to <FIG>, a shim <NUM>, which will be described in further detail below, is shown as applied between the bulkhead <NUM> and the nacelle lipskin <NUM>, for the purpose of minimizing/reducing positive or negative aerodynamic steps <NUM>' and <NUM>" that may otherwise exist circumferentially about the faying joint <NUM>. Also shown in <FIG> are fasteners <NUM> used to secure the bulkhead <NUM> and inner barrel <NUM> together, as well as fasteners <NUM>' used to secure the bulkhead <NUM> and nacelle lipskin <NUM> together with the shim <NUM> sandwiched between the latter elements and functioning as a spacer. Though mounted flush with the outer diameter <NUM> of the nacelle lipskin <NUM>, as well as with the outer diameter <NUM> of the inner barrel <NUM>, the fasteners <NUM>, <NUM>' may physically extend into a space defined by an inner diameter <NUM> of the bulkhead <NUM>, as shown.

Referring to <FIG>, a subassembly tool <NUM> enables the nacelle lipskin <NUM> to be installed vertically onto a bulkhead <NUM>, in a position adjacent to a composite inner barrel <NUM>, the latter having already been preassembled to the bulkhead <NUM>. In <FIG>, the axis a-a is shown as common to the nacelle lipskin <NUM>, the bulkhead <NUM>, and the composite inner barrel <NUM>. Since lipskin and inner barrel components are both fixed to the bulkhead, each of the bulkhead, inner barrel, and lipskin components will share the common axis a-a in their final assembled form.

The methods presented herein are predicated on having a preassembled metallic bulkhead <NUM> already containing an inner barrel <NUM> formed of a composite material. Such approach has been found to be advantageous, particularly where the bulkhead <NUM> can be preassembled as several unitary pieces with the inner barrel <NUM>.

Referring to <FIG>, a prior art array of contoured shims 44a through <NUM>, each containing varied exterior contours 46a through <NUM> in accordance with prior art gap control practices, are shown as applied to a bulkhead <NUM>. The variability of the shims 44a through <NUM>, each having its own unique thickness and individually shaped exterior contour (i.e. one of contours 46a through <NUM>), demonstrates an undesirable complexity associated with prior art practice. Traditional aerospace structures have been joined together utilizing assembly tooling, with each set-up utilizing clamp-up loads of no greater than <NUM> pounds per linear foot (i.e. <NUM>/m). In highly contoured components, such as lipskins, for example, the traditional process of alleviating tapered gaps at faying edges has involved uses of such custom shims, resulting in inefficient cycle times, burdensome labor, and expensive equipment used for fabrication. Although newer technologies have enabled uses of predictive shim geometry to minimize amount of labor, such processes have required expensive and skilled metrology, along with specialized uses of CNC equipment. As such, the traditional approach has been considered to be cost prohibitive for sustaining high production rates.

Referring to <FIG>, this disclosure provides a means to replace custom contoured shims 44a-h of the prior art (<FIG>) with a plurality of equal-thickness flat standard shim stock, such as the shims 50a through <NUM> of <FIG>. Thus, this disclosure provides methods by which simple prefabricated same-thickness shims 50a through <NUM> (similar to the single shim <NUM> of <FIG>) can be readily used in applications requiring attachments of thin-walled components, including wing skins, leading edge skins, fuselage, and nacelle lipskins, to bulkheads. The compliance of thin-walled metal structures allows airfoil surfaces to "drape" over attachment hard points, such as bulkheads. The use of flat shim stock instead of custom contoured shims eliminates shim fabrication techniques that include the CNC machine time and labor earlier noted. Moreover, the flat shims 50a-h can be stored at points-of-use, thus eliminating a need for working inventory buffers. The shim material can be metal or a composite such as fiberglass or the like.

In one aspect, assuming just by way of example that nacelle lipskin <NUM> of the turbofan jet engine <NUM> has a thickness of between <NUM> and <NUM> inch (i.e. between <NUM> and <NUM>) and an air inlet diameter <NUM> of approximately <NUM> inches (i.e. <NUM>), the shims 50a through <NUM> may be formed of a fiberglass, each shim 50a through <NUM> having a length of <NUM> feet (i.e. <NUM>) and a width of <NUM> inch (i.e. <NUM>), as depicted in <FIG>. Sets of variable-thickness shims, schematically reflected as Shims <NUM> through <NUM> in <FIG>, include shims of different thickness, each set being of one specific thickness, and provided as part of a prefabricated inventory stock from which shims having specific thickness may be selected. From among the selectable choices, a unit-to-unit shim thickness variation of between <NUM> and <NUM> inch (i.e. between <NUM> and <NUM>) is available. Incremental thickness variations between the sets of shims are approximately <NUM> to <NUM> inch (i.e. between <NUM> and <NUM>). As further reflected in <FIG>, the shim sets <NUM>-<NUM>, with each set having its own distinct thickness, are prefabricated specifically for "Joint A", which represents the specific faying joint <NUM>, situated between the nacelle lipskin <NUM> and the inner barrel <NUM>, depicted in <FIG>, <FIG>, <FIG>.

Referring now to <FIG>, a first flow chart details several steps of a first method of utilizing flat shims 50a through <NUM> to achieve the above-described securement of the nacelle lipskin <NUM> to the bulkhead <NUM>. Step <NUM> involves first preassembling the inner barrel <NUM> to the bulkhead <NUM>. Step <NUM> involves a preassembly measurement of radial gaps <NUM> (such as those of 26a, 26b, and 26c of <FIG> and <FIG>) between the bulkhead <NUM> and faying edge <NUM> of the nacelle lipskin <NUM>. The latter is conducted by applying predictive analytics to calculate an average value of such radial gaps <NUM> existing circumferentially between the nacelle lipskin <NUM> and the bulkhead <NUM>. Any predictive analytics model applied would include assessment of whether the calculated average value of all radial gaps falls within any predetermined allowable localized offsets from nominal, so as to fall within structurally acceptable limits.

More specifically, any applied predictive analytics model may involve a preassembly measurement of a lipskin <NUM>, relative to a bulkhead <NUM> to which the lipskin is to be assembled, by obtaining point data of as-fabricated lipskin surfaces to establish a point cloud surface reflective of that lipskin, as will be appreciated by those skilled in the art. The as-fabricated point cloud surface can then be used to determine whether any relative local waviness (i.e. surface variation) within a faying edge <NUM> of the as-fabricated lipskin <NUM> will deflect beyond predetermined limits upon assembly. Such determination could be aided by a computer algorithm for making direct comparison of an actual as-fabricated surface to an averaged shim value (or "shimmed") surface position, thus aiding in prediction of any relative amounts of offset between the as-fabricated and shimmed positions. A structural analysis based on specific design features of the lipskin could be applied to establish predetermined maximum offset limits for any local area of the faying surface <NUM>, the latter having a surface waviness in the as-fabricated condition (the offset being the calculated difference between the faying surface and the shimmed surface position). The shimmed surface position would be determined by calculating how much the surface must be moved or shifted in order to be aligned with a corresponding faying surface <NUM> of the inner barrel <NUM>. The calculation of difference between an as-measured and an as-fabricated inner barrel <NUM>, and an as-measured and an as-fabricated lipskin <NUM>, in which each is independently measured, and wherein a point cloud surface for each has been determined on a preassembly basis, along with any structural analysis establishing hard deflection limits, would thus always be conducted in advance of actual assembly.

Continuing the description of the flow chart of <FIG>, in Step <NUM>, a specific shim thickness is selected, from among prefabricated sets <NUM>-<NUM> of shims (<FIG>) including shims 50a through <NUM>, that corresponds closest to the calculated average value of all radial gaps <NUM>, including the described representative gaps 26a, 26b, and 26c.

In Step <NUM>, the plurality of selected shims 50a through <NUM>, all sharing the same specific thickness, is circumferentially applied to the bulkhead <NUM> in a position prospectively determined to be under the edge <NUM> of the nacelle lipskin <NUM>.

Finally, in Step <NUM>, the nacelle lipskin <NUM> is permanently secured to the bulkhead <NUM>, incorporating the shims contained between the latter structures, and in a manner such that the edges <NUM> and <NUM> of the respective nacelle lipskin and bulkhead components are fixed in radial alignment with each other.

Referring now to <FIG>, a second flow chart details several steps of a second method of utilizing flat shims 50a through <NUM>, which also achieves the above-described securement of the nacelle lipskin <NUM> to the bulkhead <NUM>. Again, Step <NUM> involves first preassembling the inner barrel <NUM> to the bulkhead <NUM>. However, in this second method, no predictive analytics are employed for completing the preassembly measurement of radial gaps <NUM>. Instead, the radial gaps <NUM> (as represented by gaps 26a, 26b, and 26c of <FIG> and <FIG>) are physically measured, and an average value of all circumferential radial gaps <NUM> that exist between the nacelle lipskin <NUM> and the bulkhead <NUM> is calculated.

For this purpose, after preassembling the inner barrel <NUM> to the bulkhead in accordance with Step <NUM>, the Step <NUM> provides that the nacelle lipskin <NUM> is physically clamped to the bulkhead <NUM>, with the faying edge <NUM> of the nacelle lipskin <NUM> in a predetermined position. All circumferential radial gaps <NUM> (e.g. gaps 26a, 26b, and 26c) extending between the nacelle lipskin <NUM> and the bulkhead <NUM> are physically measured.

In Step <NUM>, the average value of the radial gaps <NUM> is calculated, and any proposed average calculated value assessed to assure that it falls within structurally predetermined acceptable limits before its acceptance. Thus, even though the nacelle lipskin <NUM> is physically clamped to the bulkhead <NUM> in the assembly process of <FIG>, any as above-described local deflection limit with respect to as-fabricated waviness will still apply. Such deflection limit will be based on the structural analysis of the as-designed part to establish a specific "predetermined deflection limit", even though a "predictive analytics" process including the point cloud surface modeling generation as described in connection with the assembly process of <FIG> is not employed. As such, any predetermined deflection limit established by preassembly structural analysis applies to the assembly process of <FIG>.

Continuing the description of the flow chart of <FIG>, a shim of a specific thickness is next selected from among prefabricated sets of shims <NUM>-<NUM> (<FIG>) including the plurality of shims 50a through <NUM>. The specific shim thickness is selected from one of the sets that corresponds closest to the calculated average value of all radial gaps <NUM>. The average calculated value of all circumferential gaps <NUM> includes all gaps between lipskin and bulkhead, including the representative gaps 26a, 26b, and 26c earlier described.

In Step <NUM>, the nacelle lipskin <NUM> is unclamped from the bulkhead <NUM>, and a plurality of selected shims, all sharing the same specific thickness (e.g. shims 50a through <NUM>) is next circumferentially applied to the bulkhead <NUM> in a position prospectively determined to be under the edge <NUM> of the nacelle lipskin <NUM>.

Finally, Step <NUM> provides that the nacelle lipskin <NUM> is permanently secured to the bulkhead <NUM>, incorporating the selected shims between the latter structures in a manner such that the edges <NUM> and <NUM> of the respective nacelle lipskin and bulkhead components are fixed in radial alignment with each other.

Referring now to <FIG>, a fuselage <NUM> is depicted in an exploded view to reveal fuselage sections 60a through 60d. This disclosure may be also applicable to securing butt joint edges <NUM> and <NUM> of respective fuselage sections 60b and 60c, for example as applied to interior frame bulkhead structures (not show), as will be appreciated by those skilled in the art.

Claim 1:
A method of assembling first and second curvilinear components (<NUM>, <NUM>) to a support structure (<NUM>) having an axis (a-a), the first component (<NUM>) having a faying edge (<NUM>) axially spaced and radially aligned with a faying edge (<NUM>) of the second component (<NUM>); the method comprising:
preassembling the first component (<NUM>) to the support structure (<NUM>);
conducting a preassembly measurement of radial gaps (26a-c) between the support structure (<NUM>) and the faying edge (<NUM>) of the second component (<NUM>) to calculate an average value of radial gaps (26a-c) extending circumferentially between the second component (<NUM>) and the support structure (<NUM>), including an assessment of whether the calculated average value of all radial gaps falls within structurally predetermined acceptable limits;
selecting a specific shim thickness (50a-h), from among prefabricated sets of shims (<NUM>), closest to that of one set that corresponds to the calculated average radial gap value;
applying a plurality of shims (<NUM>) of the specific thickness (50a-h) selected to the support structure (<NUM>) in a position prospectively determined to be under the edge (<NUM>) of the second component (<NUM>), each shim (<NUM>) being spaced circumferentially about the support structure (<NUM>); and
permanently securing the second component (<NUM>) to the support structure (<NUM>), in a manner such that the faying edges (<NUM>, <NUM>) of the first and second components (<NUM>, <NUM>) are fixed in radial alignment with each other.