Lightning damage resistant aircraft skin fasteners

A fastener providing protection against damage to an aircraft resulting from lightning strikes includes an elongated shaft having a head disposed at an upper end thereof, one, the other, or both of the shaft and the head containing a circumferential groove disposed proximate an intersection of the shaft and the head, a lower surface of the head including a plurality of grooves extending radially outward from the circumferential groove and terminating at an outer periphery of the lower surface. A tubular sleeve is disposed concentrically about the shaft and has a plurality of through-slots disposed therein. Each through-slot has a lower end disposed above a lower end of the sleeve, and an upper end disposed in fluid communication with the circumferential groove, such that superheated gases and particles generated in interfaces between the fastener and the adjacent skin due to lightning strikes are vented harmlessly to the atmosphere through the channels and grooves.

BACKGROUND

Technical Field

This disclosure relates to fasteners in general, and in particular, to aircraft skin fasteners that provide resistance to electromagnetic effect (EME) related internal damage to aircraft resulting from lightning strikes.

Related Art

In an ideal case, the EMEs of a lightning strike on an aircraft are limited to the theoretically continuous, electroconductive exterior skin thereof, in the manner of a so-called “Faraday Cage,” such that the interior of the aircraft should be relatively unaffected by the strike. However, as a practical matter, the skin, made of numerous individual panels, is actually discontinuous, and further, is pierced by a relatively large number of typically electroconductive fasteners, such as lock bolts, rivets or the like, used to attach the skin panels to associated structures, such as ribs, spars, stringers and the like. As a result, the ideal model does not hold true, and it therefore becomes desirable to develop designs and methods for protecting interior spaces of aircraft from EME-related damage resulting from lightning strikes.

FIG. 1is lower, side perspective view illustrating a conventional “lock bolt” fastener10, such as a HI-LOK fastener, of a known type typically used to fasten a skin panel12of an aircraft to an associated structure14, such as a wing rib, spar, stringer, longeron, frame or the like, and some sequential steps involved in the attachment of the skin12to the associated structure14using the fastener10. As illustrated inFIG. 1, the fastener10can comprise an elongated “pin” or shaft16, a head18disposed concentrically at an upper end of the shaft16, and lock nut or “collar”20having an internal thread that engages a complementary external thread disposed on a lower end portion22of the shaft16. The shaft16and head18typically comprise an electroconductive metal, e.g., steel, aluminum, titanium or an alloy of the foregoing.

In a typical assembly procedure, the skin12and the associated structure14are temporarily clamped tightly together, and a common hole is bored through the two parts. In the particular example embodiment ofFIG. 1, the head18of the fastener10is frustoconical in shape, i.e., intended to be countersunk below the upper surface of the skin12, such that the upper surface of the head18is disposed generally flush with the upper surface of the skin12, for streamlining purposes, and accordingly, the formation of the hole can include, or be followed by, the formation of a correspondingly shaped frustoconical counterbore in the skin12at the upper end of the hole. As illustrated in detail A ofFIG. 1, the elongated shaft16of the fastener10is inserted into the hole until the head18of the fastener10is seated within the corresponding counterbore, and the nut or collar20is then started onto the lower end22of the shaft16.

As illustrated in detail B ofFIG. 1, a “wrenching element”24disposed on the outer end of the collar20is then engaged and rotated by an installation tool (not illustrated) to advance the collar20upward along the shaft16and toward a lower surface of the associated structure16, as indicated by the arrow26, while the shaft16is prevented from rotating by the engagement of a socket27in the lower end of the shaft16with the installation tool. The wrenching element24is integrally coupled to the collar20through a stress-raising feature28, such as a notch or groove. As illustrated in detail C ofFIG. 1, continued rotation of the wrenching element24and collar20relative to the fixed shaft16with the installation tool eventually drives the upper end of the collar20into engagement with the lower surface of the structure14with a preselected amount of clamping force, at which point, the collar20becomes swaged into a locked position on the shaft16and against the associated structure14, while the wrenching feature24is sheared away from the collar20at the stress-raising feature28and subsequently discarded. Thus, in an installed state, a portion of the shaft16disposed below the lower surface of the head18and above the upper end of the nut or collar20is loaded in a preselected amount of tension, while the skin12and the associated structure14are pressed together by the fastener10with a corresponding amount of compressive force.

In another known type of lock-bolt fastener that does not involve rotation of the collar20relative to the shaft16, such as a HUCK fastener (not illustrated), the shaft16can comprise a breakaway extension, or “pin-tail,” disposed on a lower end of the shaft16that is gripped by an installation tool and pulled axially outward relative to the collar20, such that the collar20is forced axially over a series of thread-like corrugations on the circumfery of the lower end of the shaft16and into a compressive engagement with the lower surface of the structure16. As above, when a pre-selected amount of compressive force on the skin12and associated structure14has been reached, the collar20becomes swaged onto the shaft16and against the associated structure14in a locked position, and the pin-tail on the shaft16is sheared away and discarded.

Another type of conventional aircraft skin fastener10commonly used in lighter aircraft applications is a rivet, which, like the lock-bolts described above, comprises an elongated shaft16and a head18, but which omits a nut or collar20in favor of a radial expansion of the lower end portion22of the shaft16as a technique for compressing the skin12and the associated structure14together, which expansion can be effected through a variety of well-known mechanisms.

While the first embodiment of skin fastener ofFIGS. 3A, 3B and 4can provide good protection against lightning-induced EME-related damage to interior spaces within aircraft having conventional metal, e.g., aluminum alloy, skins, its use, like that of the conventional fastener ofFIGS. 1 and 2, may be contraindicated in some aircraft with composite skins, i.e., skins made of laminated sheets of carbon fibers embedded in a relatively soft, dielectric polymer-resin matrix, because the elongated, relatively hard shafts of the fasteners can have a tendency to gall or seize when inserted into the corresponding bores formed in the skins to receive them, resulting in a deformation of the bore and an abrading-away of the relatively softer material of the composite skin into the interface between the fastener and the skin.

However, as illustrated inFIG. 5, it is known that this problem with composite-skinned aircraft can be successfully addressed by the provision of a complementary, thin-walled, metal sleeve244disposed concentrically around the shaft216and below the head218of each fastener200. The sleeve244, which can include an upper end portion246that is complementary in shape to the head218, can be inserted into the corresponding bore in the aircraft skin before the body of the associated fastener200is inserted therein. The thin wall of the sleeve244enables it to deform radially inward when being inserted into the corresponding bore, rather than galling the adjacent skin. The body of the fastener200, i.e., the shaft216and the head218, can then be inserted into the relatively hard sleeve344without galling.

Many skin fasteners like those described above and currently used in commercial airplanes are not designed for problems caused by lightning strikes because they do not allow for the safe escape to the atmosphere of superhot EME-related gases and particles that can be generated in the interfaces between respective ones of the fasteners and the aircraft skin surrounding them. This can be critically important in locations near or within the wing tanks of aircraft, where a lightning strike could result in a particularly undesirable outcome, and this is true whether the aircraft skin comprises a metal, e.g., an aluminum alloy, or a composite material, such as a carbon-fiber/resin layup. Thus, while there are designs for accommodating lighting strikes in airplanes, alternative or additional systems are needed.

Prior art efforts have attempted to address the problem, for example, by insulating the upper surfaces of the fasteners with a layer of a dielectric material, by making all or a portion of the fasteners themselves of a dielectric material, or by encapsulating the lower end portions of the fasteners in a dielectric material. While these efforts in some cases have met with some measure of success, none provides for the venting to the atmosphere of pressurized, superhot EME-related gases and particles resulting from a lightning strike, while simultaneously blocking the flow of those gases and particles to interior spaces of the aircraft.

Accordingly, a long-felt but as yet unsatisfied need exists in the industry for aircraft skin fasteners that are strong, reliable and cost effective as conventional fasteners, yet which also provide robust protection against EME damage to interior spaces of an aircraft, including metal- and composite-skinned aircraft, resulting from lightning strikes.

SUMMARY

In accordance with the present disclosure, embodiments of aircraft skin fasteners are provided that are strong, reliable and cost effective, and which also provide robust protection against EME-related internal damage to aircraft, including metal- or composite-skinned aircraft, resulting from lightning strikes.

In one example embodiment, a fastener comprises an elongated shaft and a head disposed at an upper end of the shaft. The shaft includes a plurality of grooves disposed around a circumfery thereof. Each groove extends upwardly to and radially outward along a lower surface of the head and terminates at an outer periphery of the lower surface. Each groove has a lower end disposed above a lower end portion of the shaft.

In another example embodiment, a fastener for attaching an aircraft skin to an associated structure comprises an elongated shaft having a head disposed at an upper end thereof, one, the other, or both of the shaft and the head containing a groove disposed proximate to an intersection of the shaft and the head, a lower surface of the head including a plurality of grooves extending radially outward from the groove and terminating at an outer periphery of the lower surface. A tubular sleeve having a plurality of through-slots disposed therein is disposed concentrically about the shaft. Each through-slot has a lower end disposed above a lower surface of the skin or the associated structure, and an upper end that is disposed in fluid communication with the groove of the shaft.

In yet another example embodiment, a method for protecting interior spaces of the aircraft from EME-related damage resulting from lightning strikes to the aircraft comprises attaching a skin of the aircraft to an associated structure by a plurality of elongated fasteners, and providing a plurality of elongated channels in interfaces between each fastener and the aircraft skin disposed adjacent thereto. Each channel has a closed lower end disposed above a lower surface of the aircraft skin or the associated structure and an open upper end disposed at an upper surface of the skin so as to vent superheated EME-related gases and particles harmlessly from the channel to the surrounding atmosphere.

A more complete understanding of the aircraft skin fasteners of the present invention, together with the novel methods for making and using them, as well as a realization of additional advantages thereof, will be afforded to those of skill in the art by a consideration of the following detailed description of one or more example embodiments thereof. Reference will be made to the various figures of the appended sheets of drawings, which are briefly described below, and within which like reference numerals are used to identify like ones of the elements illustrated therein.

DETAILED DESCRIPTION

In accordance with the present disclosure, various embodiments of aircraft skin fasteners are provided, together with methods for making and using them, that are strong, reliable and cost effective, and which also provide robust protection against EME-related interior damage to aircraft, including metal- or composite-skinned aircraft, resulting from lightning strikes.

It is generally accepted that both metal (typically aluminum alloy) and composite (typically, carbon- or graphite-fiber-and-epoxy-resin matrices) structural components currently being incorporated into the bodies of advanced aircraft will eventually be subjected to naturally occurring lightning discharges, or “strikes,” during flight. In a typical lightning strike incident, the lightning strikes, or “attaches,” at one extremity of an aircraft, and departs, or “detaches,” from another extremity, resulting in a very large, momentary flow of electrical current through the body of the aircraft between the two points.

The more severe, or “primary,” strikes tend to attach to and detach from the body of the aircraft at features that are located at or near protuberances located at the extremities of the body (e.g., the nose, tips and leading edges of wings, stabilizers, vertical fins and rudders, engine nacelles, and the trailing edges of rudders, elevators, and ailerons), and are characterized by a fast-rising, high-peak current (2×1O5amp) and a large energy transfer density (25×106amp2sec) having frequency components of from between about 1 kHz to about 1 MHz. These strikes can cause severe structural damage to aircraft structures and their contents if the energy of the strike is not efficiently conducted through the outer skin of the aircraft without damage.

Additionally, other “secondary” parts of the structure, located between the typical primary attachment and detachment points, can be subjected either to primary, or to lesser discharges, referred to as “swept-stroke” lightning strikes. The latter are characterized by a fast-rising current having the same frequency spectrum as above, but with 1×105amp peaks and energy transfer densities of 0.25×106amp2sec, and can also result in severe structural damage to unprotected structures.

The skin of an aircraft typically comprises a plurality of panels that abut one another at seams, and which are attached to associated structure, such as ribs, spars, longerons, frames, stringers and the like, by fasteners, such as lock bolts or rivets. The problem can be exacerbated in aircraft with composite skin panels because the energy of the strike is not conducted through the composite material efficiently, due to its relatively lower thermal and electrical conductivities. Therefore, to prevent or reduce damage to a composite aircraft resulting from either type of strike, it can be desirable to connect the attachment and detachment points of the strike with a continuous, highly electroconductive path that is capable of carrying a momentary, high-density electrical current without damage, such that the electrical current of the strike is substantially diverted through the conductive skin path, rather than through other portions of the aircraft that cannot tolerate such a current flow without damage. The problem, although still present, is substantially reduced in aircraft with metal, i.e., highly electroconductive, skin panels.

A system for protecting a composite-skinned aircraft from damage caused by lightning strikes is described in U.S. Pat. No. 7,554,785 to A. V. Hawley, incorporated herein by reference in its entirety, and includes the creation of a “Faraday cage” on the exterior surface of the aircraft body by the provision of a continuous, electroconductive grid that extends to the outermost lateral periphery of the body. The conductive grid provides preferential attachment points and conductive paths for lightning strikes on the surface of the aircraft, thereby shielding the interior of the grid from lightning damage. A similar result is obtained in a metal-skinned aircraft, provided that adjacent skin panels are electroconductively coupled to each other in an effective manner.

However, even if such protective measures are undertaken, the fasteners used to attach the aircraft's skin to associated structure, which are typically electroconductive, can comprise a weak link in the “Faraday cage” of the aircraft's skin, because of the penetrating discontinuities that they introduce into the skin of the cage. In particular, some lightning strikes can produce a high-energy electrical arc at the interface between a fastener and the surrounding aircraft skin, which can result in the production of superheated gases and particles within the interface that, if not effectively blocked or safely vented to the atmosphere, can penetrate downward along the interface and into interior spaces within the aircraft.

FIG. 2is partial cross-sectional view of the conventional aircraft skin fastener10ofFIG. 1, shown attaching an aircraft skin12to an associated structure14, and experiencing a lightning strike, as indicated by the lightning “bolt”30. As those of some skill will understand, although the shaft16and head18can be sized to fit as closely as practical within the corresponding bore and countersink in the skin12, one or more discontinuities or gaps can still exist within the interface32between the fastener and the surrounding skin12, even if only at a microscopic level.

As discussed above, the high voltage and current imposed on and through the fastener10and skin12by a lightning strike30produces high current density at contact points across these gaps which can result in the generation of superheated gases, e.g., superheated microscopic combustion particles of, e.g., paint, primer, lubricants, and skin material, such as metal, carbon fibers, resin, and the like, within the interface32, together with an attendant elevated pressure. As illustrated inFIG. 2, these high-pressure, superheated gases and particles34can then be forced upwardly and/or downwardly within the interface32and be exhausted therefrom at the upper and/or lower ends of the interface32.

Thus, as indicated by the arrow36inFIG. 2, the superheated gases and particles34can be vented harmlessly upward to the atmosphere above the skin12. However, as indicated by the arrow38inFIG. 2, the superheated gases and particles34can additionally (or alternatively) be vented from a lower end of the interface32downward and into an interior space of the aircraft disposed around the lower end portion22of the fastener10, e.g., within the interior of a fuel tank, with potentially undesirable consequences.

Prior art efforts aimed at preventing the latter occurrence have mainly been directed toward 1) preventing the occurrence of electrical arcs within the interface32, or 2) blocking the exit of the superheated gases and particles34from the lower end of the interface32, e.g., with a dielectric gasket, or by encapsulating the collar20and lower end portion22of the shaft16with a dielectric material.

However, the present disclosure contemplates another solution, namely, providing a path of relatively low resistance for the venting of the superhot EME-related gases and particles34upwardly to the atmosphere surrounding the upper surface of the skin12, while simultaneously impeding their downward flow into the interior spaces of the aircraft. This can be effected, for example, by providing a plurality of elongated channels within the interface32, each having a closed lower end disposed above a lower surface of the skin12or the associated structure14, and an open end disposed at the upper surface of the skin12so as to preferentially vent the EME-related gases and particles34from the channels and into the atmosphere.

FIG. 3Ais side elevation view of a first example embodiment of a lightning damage resistant aircraft skin fastener100capable of effecting the foregoing “preferential venting” technique, andFIG. 3Bis a cross-sectional view of the fastener100, as seen along the lines of the section3B-3B taken inFIG. 3A.FIG. 4is a partial cross-sectional view of the example skin fastener100, shown attaching an aircraft skin112to an associated structure114, and experiencing a lighting strike130. As can be seen inFIGS. 3A, 3B and 4, the example fastener100, like the conventional fastener10ofFIGS. 1 and 2, includes an elongated shaft116and a head118disposed at an upper end of the shaft116.

In the particular example fastener100illustrated inFIG. 4, the tightening mechanism disposed at the lower end122of the shaft116comprises a threaded collar120, and the head118is frustoconical in shape for countersinking below the upper surface of the skin112, as discussed above. However, it should be understood that the functioning of the novel venting features described below are independent of both the type of tightening mechanism and the shape or installation position of the head118of the fastener. Thus, for example, in some embodiments, the head118of the fastener100could comprise, e.g., a conventional “pan-head” that is disposed below or on top of the upper surface of the skin114when installed.

As can be seen in, e.g.,FIGS. 3A and 3B, the elongated shaft116of the fastener100includes a plurality of longitudinal grooves140disposed around a circumfery thereof. Each of the grooves140extends upwardly to, and radially outward along, a lower surface142of the head118, and terminates at an outer periphery of the lower surface142of the head118, and each groove140has a lower end that is disposed above the lower end portion122of the shaft116, or even higher, e.g., above a lower surface of the aircraft skin112, or anywhere in between. In this regard, it should be understood that, as discussed above, the current flowing through the body of an aircraft as the result of a lightning strike130is confined substantially to the skin112of the aircraft, and accordingly, substantially all of the arcing that takes place in the periphery132between the body of the fastener100and the skin112, as well as the superheated products134resulting therefrom, occur within the thickness of the skin112.

Thus, in some embodiments, the lower ends of the grooves140can be disposed slightly above the lower surface of the skin112. However, as illustrated inFIG. 4, in some embodiments, it may be desirable, e.g., for manufacturability reasons, to make the grooves140longer, such that the lower ends of the grooves140are disposed at a distance D that is slightly above the lower surface of the associated structure114. In either case, as discussed above, the lower ends of the grooves140function to block the flow of the superheated gases and particles134downward to the lower end of the periphery132, while the upper ends of the grooves140function to provide openings through which the superheated gases and particles134can vent harmlessly to the atmosphere.

The number and the cross-sectional shape and area of the grooves140can vary widely, depending on the particular application at hand, and can be formed in the body of the fastener100using a variety of techniques, including, for example, extruding, upset die-forming, casting, machining and roll-forming. All other things being equal, the greater the volume being vented to the atmosphere, the better the venting.

While the “sleeved fastener” approach ofFIG. 5can eliminate the galling problem in composite-skinned aircraft, it is subject to the same vulnerability to EME-related lightning strike damage as the conventional fastener10ofFIGS. 1 and 2. In particular, the addition of the sleeve244creates a second interface between the body of the fastener200and the adjacent skin, i.e., one between the outer surface of the fastener body and the inner surface of the sleeve244, and one between the outer surface of the sleeve244and the surrounding skin of the aircraft, within which, in response a lightning strike230, lighting-induced arcing and the concomitant generation of pressurized, superhot gases and particles234can occur. And, as illustrated inFIG. 5and described above in connection with the conventional fastener10ofFIG. 2, these superhot arcing products234can, under the appropriate conditions, exit the two interfaces at their respective upper and lower ends, i.e., harmlessly upwards or undesirably downwards, as respectively indicated by the arrows236and238.

Accordingly, as discussed above in connection with the first fastener embodiment100, it is desirable to provide a preferential venting of the superhot EME-related gases and particles234upwardly to the atmosphere surrounding the upper or outer surface of the skin, while simultaneously impeding their downward flow into the interior spaces of the aircraft.

A second example embodiment of a sleeved fastener300for attaching a composite aircraft skin to an associated structure while providing resistance to lighting-induced EME-related damage in accordance with the present disclosure is illustrated inFIGS. 6A-6C, wherein the tensioning feature, e.g., a locking nut or threaded collar of the fastener300, has been omitted for purposes of illustration. As can be seen in these figures, the fastener300comprises an elongated shaft316having a circumferential groove348disposed proximate to an intersection of the shaft348and a lower surface350of a head318located at an upper end of the shaft316. In the particular example embodiment illustrated, the head316is frustoconical for countersinking purposes, but as discussed above, the shape of the head316, whether it is countersunk or not, and the technique used at the lower end to compress the skin and associated structure together with the fastener300are unrelated to the venting function of the EME-related damage resistant features of the fastener300.

As illustrated in, e.g.,FIG. 6B, the lower surface350of the head318includes a plurality of grooves352extending radially outward from the circumferential groove348and terminating at an outer periphery of the lower surface350of the head318, i.e., at the intersection of the lower surface350of the head318and the upper surface of the head318. A relatively thin-walled tubular sleeve344is disposed concentrically about the shaft316and has a plurality of helically extending through-slots354disposed therein. Each of the through-slots354has a lower end356and an upper end358in fluid communication with the circumferential groove348in the shaft316when the body of the fastener is fully seated within the sleeve344. As discussed above, the lower ends356of the through-slots354can be located either slightly above a lower surface of the aircraft skin, or in an alternative embodiment, at a distance D slightly above a lower end of the sleeve344, as illustrated inFIG. 6A. Similar to the sleeve244of the conventional sleeved fastener200ofFIG. 5, the sleeve344can include a funnel-shaped upper end portion346that is complementary in shape to the head318.

As may be noted in the figures, the through-slots354in the sleeve344extend helically upward in the sleeve344from their lower ends356at a helical angle of θ, rather than generally vertically, as in the case of the longitudinal grooves140of the first embodiment100above. As those of some skill will understand, a composite aircraft skin typically comprises a “layup” of a plurality of layers, each of which includes a planar arrangement of electroconductive fibers, e.g., carbon fibers, that are intentionally disposed at different angular orientations relative to those of the other layers, for example, at 0 degrees, 45 degrees, 90 degrees, 135 degrees, and so on, for reasons of strength. Thus, the fibers, through which the electrical current resulting from a lightning strike flow, intersect the bore of the fastener300at a plurality of discrete angles. By arranging the through-slots354helically, the effective width of the slots that are disposed in opposition to respective ones of the layers of fibers is increased, relative to through-slots354that are disposed longitudinally, i.e., orthogonally to the fiber layers.

The circumferential groove348functions in the manner of an exhaust “collector,” “header,” or “manifold,” in that it serves to receive or collect the superhot EME-related gases and particles334from each of the through-slots354in the sleeve344, and then conveys them into the plurality of grooves352in the lower surface350of the head316, such that they vent from the grooves352and into the atmosphere at the upper surface of the aircraft skin. Thus, and depending on its particular shape, the groove348can be disposed almost entirely within the circumfery of the shaft316of the fastener300, almost entirely within the lower surface350of the head318of the fastener300, or at a position intermediate those two positions, provided only that the continuity of the venting paths from the through-slots354to the circumferential groove348, and thence, from the circumferential groove348to the radial grooves352, is maintained.

As illustrated inFIG. 6C, the operation of the fastener300during a lightning strike330is similar to that of the first embodiment of fastener100above, except for the function of the slotted-walled sleeve344. Thus, the superhot EME-related gases and particles334generated in the interfaces between the body of the fastener300and the sleeve344, and between the sleeve344and the skin surrounding them expands preferentially into the through-slots354, then upwardly and into the circumferential groove348disposed proximate to the intersection of the shaft348and the lower surface350of the head318, and thence, through the radial grooves352in the lower surface350of the head318and into the atmosphere above the upper surface of the aircraft skin.

As in the first example fastener100described above, the number, width and helical angle θ of the through-slots354can vary widely, depending on the particular application at hand, and the sleeve344can be manufactured using a variety of known fabrication techniques. As above, the greater the volume defined by the through-slots354being vented to the atmosphere, the greater is the venting protection afforded.

Other configurations are also contemplated. For example, although grooves140and352are generally illustrated as longitudinal in shape (e.g., straight), and through-slots354are generally illustrated as helical in shape (e.g., curved), other configurations may be used. For example, any of grooves140, grooves352, through-slots354, and/or related features may be implemented as longitudinal and/or helical in shape as appropriate in various embodiments.

As those of some skill in this art will by now appreciate, and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of making and using the lightning damage resistant aircraft skin fasteners of the present disclosure without departing from the spirit and scope thereof. In light of this, the scope of the present invention should not be limited to those of the particular embodiments illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.