Abstract:
A static discharge device includes a body and an electrical path carried in the body for discharging static electricity. Carried on the body is visual indicia indicating a present operational status of the electrical path to discharge static electricity effectively. The visual indicia is observable to an airplane operator or technician to quickly determine the operational or non-operational status of the static discharge device.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally airplane equipment, and more particularly to equipment for discharging static electricity from an airplane. 
     BACKGROUND OF THE INVENTION 
     Aeronautical flight is a complicated orchestra of engineering requiring considerations of lift, drag, aerodynamics, electrical engineering, electronics, hydraulic controls, material choices, dynamic control, weather, environmental control, and a variety of other issues. Safety, however, is a paramount goal of all flight solutions. Innumerable mechanism on board private and commercial airplanes are in place for safety reasons primary or solely. 
     As an airplane travels through the air, static electricity can build on the airplane. This static electricity is created by friction between the airplane body, or “skin,” and the rapid movement of air particles across that skin, much in the same way that static electricity is created by dragging one&#39;s shoes across a carpet. The electrical static normally produced during a flight is increased when the airplane flies through rain, snow, sleet, ice, or dust. Such precipitation is known as “p-static.” P-static is dangerous because it reaches levels of charge high enough to affect internal electronic controls and radio operations in the airplane. Pilots can hear the effects of p-static as high-pitched whining or popping over their radio systems. When p-static reaches high enough levels, it discharges from the airplane, and in doing so, can damage the airplanes electronic and radio components. In extreme cases, p-static can destroy electronic or radio components. 
     Static wicks have been used for over half a century to dissipate static electricity, and p-static in particular, consistently and rapidly, to prevent the buildup of dangerous levels of static electricity on the airplane. Static wicks are electrical components typically installed on the trailing edges of the plane, such as on the wings, ailerons, and tails. Static electricity on the plane moves across the plane body to the static wick where it can be let off, or discharged, into the air safely. 
     Static wicks are effective at discharging static electricity when operating properly. However, like most equipment, static wicks often require replacing. Typically, there will be a dozen or so static wicks distributed across the airplane. If one or several of the static wicks is or becomes inoperable during flight, static electricity can build do unsafe levels. It is difficult, however, to determine whether a static wick is operating properly. Typically, testing the operational status of a static wick requires a technician to remove the static wick from the airplane and take it to a bench for testing with a power supply and multimeter. This process can take hours. If several static wicks have to be tested, the testing process can be extremely lengthy, resulting in increased operational costs. An improved way to determine the operational status of a static wick is needed. 
     SUMMARY OF THE INVENTION 
     A static discharge device includes visual indicia for quickly indicating for observation by a technician the present operational status of the static discharge device. The static discharge device includes a body and an electrical path carried by the body for discharging static electricity. When the static discharge device is mounted to an airplane, and the electrical path is in an operational status, the electrical path discharges static electricity from the airplane into the air effectively to prevent damage to radio, electric, and electronic components of the airplane. When the electrical path is in a non-operational status, the electrical path does not discharge static electricity from the airplane into the air effectively, and damage to the radio, electric, and electronic components of the airplane is possible. The visual indicia of the static discharge device, however, indicates the present operational status of the electrical path to inform a technician whether the static discharge device needs to be replaced: when the electrical path is in the operational status, a first color is displayed by the visual indicia. Conversely, when the electrical path is in the non-operational status, a second color is displayed which is different from the first color and easily discernible. The technician thus can quickly and easily determine whether the static discharge device should be replaced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings: 
         FIG. 1  is a bottom perspective view of an aeronautical static discharge device installed on a trailing edge of a wing of an airplane; 
         FIG. 2  is a top perspective view of the aeronautical static discharge device of  FIG. 1  in isolation; 
         FIGS. 3A and 3B  are enlarged views of windows on an underside of the aeronautical static discharge device of  FIG. 1 , indicating the aeronautical static discharge device operating in an operational status and a non-operational status, respectively; and 
         FIG. 4  is a section view of the aeronautical static discharge device of  FIG. 1  taken along the line  4 - 4  in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference now is made to the drawings, in which the same reference characters are used throughout the different figures to designate the same elements.  FIG. 1  illustrates an aeronautical static discharge device  10 , hereinafter referred to as a “static wick  10 ,” installed on a trailing edge of an airplane wing  11 , such as an aileron. The static wick  10  is useful for discharging static electricity accumulated across the airplane into the ambient air to prevent radio frequency interference and damage to electronic equipment aboard the airplane, and for quickly determining a present operational status of the static wick to discharge static electricity from the airplane. The static wick  10  is elongate and extends from a proximal end  12  secured to the wing  11  to an opposed distal end  13  behind the wing  11 . The static wick  11  draws and conveys electricity from the wing  11  through the proximal end  12  toward the distal end  13  for discharge. 
     Referring now to  FIGS. 1 and 2 , the static wick  10  includes a base  14 , a shroud  15  extending from the base  14 , and a window  20  formed in the base  14  exposing a thermochromic applique which indicates a present operational status of the static wick  10  so that an airplane operator can quickly and easily observe and detect whether the static wick is in an operational status or a non-operational status. Further, and briefly,  FIG. 3A  illustrates how the window  20  displays the operational condition and  FIG. 3B  illustrates how the window  20  displays the non-operational condition. 
     Turning to  FIG. 4  now, a section view taken along the line  4 - 4  in  FIG. 2  illustrates internal components of the static wick  10  applied to the wing  11 . The base  14  is a generally elongate structure having a topside  21 , an opposed underside  22 , opposed sides  23  and  24  (shown more clearly in  FIGS. 1 and 2 ), and two internal chambers  25  and  26  (shown in  FIG. 4 ) separated by an internal wall  30 , where the internal chamber  25  is and will be referred to as a leading internal chamber  25 , and the internal chamber  26  is and will be referred to as a trailing internal chamber  26 . Both the topside  21  and the underside  22  at the base  14  are convex, such that the base  14  has a generally oval-shaped cross section. Along the base  14 , the edges between the topside  21  and the sides  24  and  25 , and between the underside  22  and the sides  24  and  25 , are radiused and smooth. 
     The base  14  has an axial slot  31  extending along the underside  22  which is opposed from the topside  21 . The leading internal chamber  25  is bound by the topside  21 , the sides  23  and  24 , and the slot  31  which is received and held against the wing  11 . Briefly, it is noted that the term “axial”, as used herein, should be understood to mean generally aligned along or with the static wick  10 , that the term “leading” should be understood to mean disposed generally toward the proximal end  12  or closer to the proximal end  12  than some reference part, and that, finally, the term “trailing” should be understood to mean disposed generally toward the distal end  13  or closer to the distal end  13  than some reference part. The base  14  is constructed of a material or combination of materials having the material characteristics of rigidity, strength, durability, and good electrical conductivity, such as a manganese-aluminum alloy. 
     A set screw  32  fixes and secures the static wick  10  in position on the wing  11 . The set screw  32  is a short threaded screw extending obliquely through a threaded mount  34  formed in the topside  21  of the base  14  toward and into the wing  11 . A socket  33  formed in the wing is sized to receive the set screw  32 , such that when the set screw  32  is threadably engaged through the mount and into the socket  33 , the set screw  32  is firmly engaged with the socket  33 , and forms a strong and rigid fastener between the static wick  10  and the wing  11 , that, together with the application of the slot  31  onto the wing  11 , firmly secures the static wick  10  on the wing  11 . Secured in this manner, the static wick  10  projects directly backwardly from the wing  11 , and is generally aligned horizontally with the wing  11 . 
     Referring to  FIG. 4 , the internal wall  30  partitions the leading internal chamber  25  from the trailing internal chamber  26  and isolates them from spatial communication with each other. The internal wall  30  extends completely from the side  24  to the side  25  and completely from the topside  21  to the underside  22 . The base  14  includes an open distal end  35  opposed from the internal wall  30 , and the trailing internal chamber  26  is bound by the topside  21 , the underside  22 , the internal wall  30 , and the open distal end  35 . A cutout  40  is formed through the underside  22  of the base  14  proximate to and trailing the internal wall  30 . As  FIG. 1  shows slightly more clearly, the cutout  40  is elongate, extends axially from the internal wall  30  toward the distal end  13  of the static wick  10 , and is flanked by the sides  23  and  24 , extending as it does substantially fully between the sides  24  and  25 . 
     Referring back to  FIG. 4 , the cutout  40  holds the window  20 . The window is a transparent plug  41  formed of a material or combination of materials having rigid, dense, transparent material characteristics, such as acrylic, bicarbonate, polycarbonate, and the like. The plug  41  has a flat upper surface  42 , a convex lower surface  43 , and an integral, monolithic body  44  therebetween which has a 90% clarity. The convex lower surface  43  is flush with and contiguous to the underside  22  of the base  14 , so that the plug  41  neither protrudes past the underside  22  nor is receded within the base  14 . 
     A thermochromic applique  45  is applied to the upper surface  42  of the plug  41 . As the term is used herein, “thermochromic” is meant to include a substances or combination of substances which displays a first color in response to being at a first temperature and then displays at least one other color in response to being at least another temperature different from the first temperature. In other words, thermochromic indicates that a color change occurs when the thermochromic material changes temperature. The thermochromic applique  45  is a cured state of a chemical compound which is applied to the upper surface  42  of the plug  41  in liquid form. The thermochromic applique  45  is, by volume, composed of fifty to fifty-five percent ethylcyclohexane, fifteen percent butoxyethyl acetate, seven percent bees&#39; wax, one percent bisphenol, less than one-tenth percent formaldehyde, and the remainder as base acetone. The chemical compound, while in liquid form, is mixed to ensure a homogeneous consistency throughout. It is then applied, in liquid form, such as with a dropper or a brush, to the upper surface  42  of the plug  41  and cured, resulting in the dry, cured, finished thermochromic applique  45  having a thickness of 1.2 millimeters or less. 
     With reference still to  FIG. 4 , the shroud  15  is installed in the open distal end  35  of the base  14 . The shroud  15  is a generally elongate sleeve structure having a topside  50 , an opposed underside  51 , opposed sides  52  and  53 , and a single, long internal chamber  54  bound by a thin but rigid and strong sidewall  55 . Like the base  14 , along the shroud  15 , both the topside  21  and the underside  22  at the base  14  are convex, such that the shroud  15  has a generally oval-shaped cross section. Along the shroud  15 , the edges between the topside  21  and the sides  24  and  25 , and between the underside  22  and the sides  24  and  25 , are radiused and smooth. The shroud  15  has an open proximal end  60  and an opposed distal end  61 ; the open proximal end  60  is fit into and received by the open distal end  35  of the base  14 . At the engagement between the open distal end  35  and the open proximal end  60 , the base  14  and the shroud  15  have a similar cross-sectional profile, with the open proximal end  60  of the shroud  15  being just smaller in dimension than the open distal end  35  of the base. This allows the open proximal end  60  to be snug fit into the open distal end  35  of the shroud, providing a secure engagement which is further secured by the application of an adhesive between the open proximal end  60  and the open distal end  35  during assembly of the static wick  10 . 
     A discharge point  62  is formed at the distal end  61  of the shroud  15 . The discharge point  62  includes upper and lower let points  63  and  64 , each flanked axially by rigid guards  65 . The guards  65  are upward and downward protrusions of the sidewall  55  of the shroud  15  which protect the let points  63  and  64  from damage and protect operators from being injured by the let points  63  and  64 . The let points  63  and  64  are sharp metal points extending out of the shroud  14 . The let points  63  and  64  are the terminal discharge location for the static electricity which is discharged by the static wick  10 , as will be explained. The let points  63  and  64  are leads to an electrical ground, acting to discharge electricity into the air. The sharpness of the tips of the let points  63  and  64  improve the ability of the static wick to discharge static electricity quickly. 
     The shroud  15  is constructed of a material or combination of materials having the material characteristics of rigidity, strength, durability, and poor electrical conductivity, such as a silicon phenolic polymer or other plastic. The shroud  15  thus operates to contain electricity within the internal chamber  54  as it transmits through the static wick  10 . 
     Still referring to  FIG. 4 , like the base  14 , the shroud  15  is hollow, having the internal chamber  54  which extends completely through the shroud  15  and is coupled in open spatial communication with the trailing internal chamber  26 . An electrical path  70  is formed in the static wick  10  and extends throughout the static wick  10  from the proximal end of the static wick  10  proximate at the wing  11  to the upper and lower let points  63  and  64  at the distal end  13  of the static wick  10 . The electrical path  70  is identified generally in  FIG. 4  with an arrow marked with the reference character  70 ; the electrical path  70  includes the upper and lower let points  63  and  64  and other structure as well which communicates electricity to the let points  63  and  64 . The electrical path  70  includes the internal wall  30 , which is constructed of the same material or combination of materials as the base  14  and thus has good electrical conductivity. The internal wall  30  is integrally formed to the base  14  which is in contact with the wing  11 , at the proximal end  12 , at the slot  31 , and at the engagement between the set screw  32  and the socket  33 , so that the proximal end  12  and slot  31  of the base  14  are in good electrical communication with the wing  11 , and thus, the internal wall  30  is also in good electrical communication with the wing  11 . 
     The electrical path  70  includes other structure. From the internal wall  30  to the distal end  13  of the static wick, the trailing internal chamber  26  of the base  14  and the internal chamber  54  of the shroud  15  cooperate to form a single internal chamber through which a carbon tube  71  carrying a fiberglass filament  75  extends. The carbon tube  71  extends from a leading end  72 , proximate to the plug  41 , to a trailing end  73 , proximate to the discharge point  64 . The leading end  72  is disposed just above the plug  41 , at a generally intermediate location axially along the plug  41 , and an air gap  74  separates the plug  41  and the thermochromic applique  45  applied thereto, from the leading end  72  of the carbon tube  71 . A consumable spiral-wound wire  80  is tightly wound into a spiral or helix around the carbon tube  71  proximate to the leading end  72  and extends, in a spiral wind, from the leading end  72  to the internal wall  30 . The spiral-wound wire  80  is in a press-fit engagement against the internal wall  30  such that the spiral-wound wire  80  is biased into the internal wall  30  and flush against the internal wall  80  in direct contact. Further, the spiral-wound wire  80  is secured on the carbon tube  71  with a high-temperature thermally-resistant ceramic paste. The spiral-wound wire  80  is a resistor coupling the carbon tube  71  and fiberglass filament  75  in good electrical communication with the internal wall  30 . 
     At the distal end  13  of the static wick  10  and within the shroud  15 , the fiberglass filament  75  exits the trailing end  73  of the carbon tube  75  and is connected to each of the upper and lower let points  63  and  64 . The let points extend through the sidewall  55  of the shroud  14  and terminate just below the ends of the guards  65 . 
     In operation, the static wick  10  is highly effective at discharging static electricity from the airplane and at informing an operator that the static wick  10  is no longer functioning properly. Several static wings  10  will be installed on an airplane, generally distributed evenly across the trailing edges of the wings, though perhaps in other locations on an airplane. Referring to  FIG. 1 , when positioned properly on a wing  11 , the static wing  10  presents the underside  22  downward toward the ground. An operator walking underneath performing a pre-flight check may look up at the underside  22  and see the window  20 . The plug  41  in the window  20 , having a transparent material characteristic, reveals the color of the applique  45  applied to the upper surface  42  of the plug  41 . Referring now to  FIG. 3A , the applique  45  presents a first color, such as red.  FIG. 3A  indicates the applique  45 , visible through the transparent plug  41  in the window  20 , has a first color. Red is a preferred first color because in aeronautics, the color red often signifies that an instrument, or a device for which an instrument is displaying information, is operating normally. However, one having ordinary skill in the art will readily appreciate that any desired color may be used for the first color. 
     As the plane flies and moves through the air, static electricity will build on the airplane. This static electricity is created by friction between the airplane body, or “skin,” and the rapid movement of air particles across that skin. If the plane files through rain, snow, sleet, ice, or dust, increased static electricity, or p-static, will be formed. P-static has the potential to affect internal electronic controls and radio operations in the airplane, or even damage or destroy electronic or radio components. When the airplane is properly built so that the trailing ends of the wings are coupled in electrical communication with the rest of the airplane, the static electricity conveys through and across the airplane to the trailing ends. Static electricity tends to collect in concentration points. The static wick  10  provides such a concentration point because of its size and shape on the wing  11 . 
     Referring now to  FIG. 4 , static electricity passes from the wing  11  into the base  14  which is constructed of a material or combination of materials having good electrically conductive material characteristics. The electricity moves through those locations at which the base  14  is in contact with the wing  11 , including the proximal end  12 , the set screw  32 , the slot  31 , and the internal wall  30 . The electricity does not communicate through the base  14  further down the base  14  past the internal wall  39 , because the base terminates in the open distal end  35 , to which the non-conductive shroud  15  is coupled. Thus, electricity passes through the electrical path  70 : from the internal wall  30  to the spiral-wound wire  80 , which is in direct contact with the internal wall  30 . The spiral-wound wire  80  is tightly wound, and formed of a combination of elements, namely 65% nickel by weight, 15% chromium by weight, and 20% iron by weight. The spiral-wound wire  80  begins to heat as electricity passes through the spiral-wound wire  80 , performing like a resistor to reduce the flow of electrical current through the electrical path  70 , to lower the voltage level in the electrical path  70 , and to produce heat. The electricity passes through the spiral-wound wire  80  and into the carbon tube  71  and the fiberglass filament  75  carried within the carbon tube  71 . The electricity passes further down the electrical path  70  by traveling down the carbon tube  71  and fiberglass filament  75  toward the distal end  61 . At the let points  63  and  64 , the electricity is discharged into the air. The ambient air flowing over the wing  11  is an electrical ground, and the sharp tips of the let points  63  and  64  are electrically coupled to that electrical ground and act to discharge electricity from the electrical path  70  into the air. The ambient air flowing over the wing  11  and the static wick  10  present a low voltage, and the wing  11  presents a source of high voltage, such that there is a large difference in voltages between the wing  11  and the ambient air. The static wick  10  and the electrical path  70  form a circuit coupled to the voltage source and the voltage ground, with the spiral-wound wire  80  defining a resistor in that circuit. Thus, as electricity passes from the wing  11  to the ambient air  10 , the spiral-wound wire  80  begins to heat. Typically, the resistance of the spiral-wound wire  80  is between 118.57 and 121.53 Ohms, depending on the amount of p-static building on the airplane. With continued use, however, the spiral-wound wire  80  will fail, as will now be explained. 
     As a resistor, the spiral-wound wire  80  can endure a maximum continuous voltage load for a period of time before it fails. Further, the spiral-wound wire  80  can endure a maximum pulse voltage load for a very brief period of time before it fails; in other words, the spiral-wound wire  80  can endure a short-time overload. Failure of the spiral-wound wire  80  is defined as the inability of the electrical path  70  to discharge static electricity fast enough to prevent damage to the radio, electric, and electronic components of the airplane. This occurs when the resistance of the spiral-wound wire has reached 164.102 Ohms. At 164.102 Ohms, the spiral-wound wire provides too much resistance in the electrical path  70  to shed static electricity from the wing  11  to the ambient air, and damage to the radio, electric, and electronic components is possible. It is important to understand that failure is not the complete termination of the ability of the static wick  10  to discharge electricity, but is rather the inability of the electrical path  70  to discharge static electricity fast enough to prevent damage to the radio, electric, and electronic components of the airplane, which defines the non-operational status of the static wick  10 . 
     As described above, the spiral-wound wire  80  is spaced apart from the thermochromic applique  45  by an air gap  74 , such that the thermochromic applique  45  and the spiral-wound wire  80  are not in direct contact. The air gap  74  thus acts as a heat exchange buffer and allows heat to transfer from the spiral-wound wire  80  to the thermochromic applique  45  through convection, that is, by heating the air gap  74 , and the air contained with the trailing internal chamber  26 . 
     The thermochromic applique  45  is responsive to a heat change. The thermochromic applique  45  changes color from the first color to the second color in response to a heat change. This color change results from a change in the density of the chemical compound of the thermochromic applique  45 , which is in response to the change in temperature of the thermochromic applique  45 . When the thermochromic applique  45  reaches 40 degrees Celsius, the thermochromic applique begins to change color from the first color to the second color. The temperature change of the thermochromic applique  45  is due to heat radiating off of the spiral-wound wire  80  as its resistance changes due to degradation of the spiral-wound wire  80 . When the resistance of the spiral-wound wire  80  reaches 136.42 Ohms, the spiral-wound wire  80  produces heat such that the thermochromic applique  45 , spaced apart from the spiral-wound wire  80  by the air gap  74 , heats to 40 degrees Celsius, thus initiating the color change in the thermochromic applique  45 . 
     The thermochromic applique  45  typically changes from the first color to the second color from one of the leading and trailing ends to the opposed other of the trailing and leading ends, respectively, but may change gradually as well. When the thermochromic applique  45  reaches 74 degrees, the thermochromic applique  45  has completely changed to the second color, and is “locked” to the second color, such that the change is irreversible and the thermochromic applique  45  will not change color back to the first color.  FIG. 3B  shows the thermochromic applique completely changed to the second color, which is preferably yellow. Yellow is a preferred second color because the shroud  15  is preferably yellow, and the colors would match. However, one having ordinary skill in the art will readily appreciate that any desired color may be used for the second color, as long as such second color is appreciably and observably different from the first color. 
     Prolonged exposure of the spiral-wound wire  80  to the maximum continuous voltage load will cause the spiral-wound wire  80  to fail such that its resistance exceeds 164.102 Ohms. The spiral-wound wire  80  will remain in the operational status to operate normally (herein defined as the electrical path  70  discharging electricity at a rate sufficient to prevent damage to the radio, electric, and electronic components of the airplane) so long as the maximum continuous voltage load is not exceeded. Typically during operational status, the spiral-wound wire  80  will have a resistance of between 118.57 and 121.53 Ohms. However, prolonged exposure to the maximum continuous voltage load will eventually cause degradation and oxidation of the spiral-wound wire  80  such that its resistance rises to 164.102 Ohms, at which point the heat produced by electrical current flowing through the spiral-wound wire  80  will transfer by convection through the air gap  74  to heat the thermochromic applique  45  to 74 degrees Celsius. At this point, the thermochromic applique  45  will completely change color from the first color to the second color, and the change will be irreversible. 
     Sudden exposure of the spiral-wound wire  80  to a maximum pulse voltage will also cause the spiral-wound wire  80  to fail such that its resistance exceeds 164.102 Ohms. The spiral-wound wire  80  will operate normally so long as the maximum pulse voltage load is not exceeded. However, exposure to the maximum pulse voltage load for more than a few seconds will cause sudden degradation and oxidation of the spiral-wound wire  80  such that its resistance suddenly to 164.102 Ohms, and the heat produced by electrical current flowing through the spiral-wound wire  80  will transfer by convection through the air gap  74  to heat the thermochromic applique  45  to 74 degrees Celsius. At this point, the thermochromic applique  45  will completely change color from the first color to the second color, and the change will be irreversible. 
     Thus, as the airplane flies through the air, through clouds, through weather, through rain, snow, and lightning, the voltage load on the static wick  10  will change. The electrical path  70  may function properly for a long time. However, prolonged exposure to weather and p-static will cause the electrical path  70  to fail as described above. Further, a catastrophic event, such as a lightning strike, may cause the electrical path  70  to suddenly fail. When the electrical path fails  70 , two things happen: the static wick  10  can no longer discharge electricity fast enough to prevent potential damage to the radio, electric, and electronic components of the airplane, and the thermochromic applique  45  will change from the first to the second color. When the airplane lands and a pre-flight check is performed, the operator will look up at the window  20  and see that the thermochromic applique  45  has changed from the first color to the second color. This indicates that the static wick  10  has failed and should be replaced. Replacement is easily accomplished by rotating and backing the set screw  23  out of the socket  33  in the wing  11  and removing the static wick  10 , then applying a new static wick  10  and securing it similarly as before. The newly-installed static wick  10  may now operate to discharge static electricity from the airplane as it flies, and the old static wick  10  can be discarded. The entire checking and removal process takes only a few minutes. 
     A preferred embodiment is fully and clearly described above so as to enable one having skill in the art to understand, make, and use the same. Those skilled in the art will recognize that modifications may be made to the described embodiment without departing from the spirit of the invention. To the extent that such modifications do not depart from the spirit of the invention, they are intended to be included within the scope thereof.