Abstract:
A propeller blade assembly including a propeller blade having a leading edge, and a trailing edge extending between a tip and a hub, and an electrically conductive band extending longitudinally on either or both of the leading edge and trailing edge. The electrically conductive band secured to the leading edge of the propeller serves as an entry point on the aircraft and conductive channel into the airframe thereby avoiding the potentially damaging effect of the lightening on the carbon fiber composite propeller.

Description:
FIELD OF THE INVENTION 
     The technology disclosed herein relates to an apparatus and system for a guard band for protecting aircraft composite propellers against damage from lightning strikes. 
     BACKGROUND 
     This present invention generally relates to a propeller blade constructed from a fiber-reinforced material, and more particularly to an arrangement for protecting the structural integrity of a plastic propeller blade against damage from lightning strikes. Propeller blades are among aircraft components most frequently struck by lightning. Carbon fiber-reinforced plastic materials are especially sensitive to lightning damage due to their partial electrical conductivity because the fiber is conductive while the plastic matrix is insulating. Carbon fiber blades are also more conductive than Kevlar blades and are therefore harder to protect. Typically, this protection is provided by a surface film of lightning strike protective material (e.g., cooper or aluminum) which is laid on the outer surface of the composite blade. This type of construction; however, adds complexity and weight to the propeller design. Moreover, with conductive materials, such as with Carbon fiber, the desired blade protection is difficult to achieve due to the possibility of “puncture” of the protective surface into the carbon material. This puncture allows current flow into the carbon material which will result in damage to the blade. This is the primary reason for the high dielectric glass that covers the carbon fiber material in this type of protection scheme and if it is not strong enough, then the voltage created by the spark gap will break down the dielectric and arc to the carbon fiber, causing damage to the blade. This protection scheme is also very susceptible to manufacturing issues and therefore the reliability of the design is very suspect. 
     Therefore, it is an object of the present invention to provide a propeller blade constructed from a composite material with lightning protection that does not require lightning protection material incorporated into the material lay-up itself. 
     For the foregoing reasons, there is a need for a propeller system capable of being fabricated into a configuration that will provide high performance aerodynamic characteristics with lightweight, high structural integrity and lightening protection in a single package. 
     SUMMARY 
     The effects of lightning on aircraft skins, both metallic and composite include: 1) melting or burning at lightning attachment points; 2) resistive temperature rise; 3) magnetic force effects; 4) acoustic shock effects; 5) arcing and sparking at bonds, hinges and joints; and 6) ignition of vapors within fuel tanks. 
     Not all materials will suffer these effects equally. Aluminum skins will suffer from melting from long duration dwell times at lightning attachment points. While they will be subject, like composites, to acoustic shock damage, their greater ductility and malleability will likely enable them to survive. Composites will suffer the most rapid rise in temperature and acoustic shock waves. Carbon composites are conductors, albeit resistive conductors and they are therefore subject to the same influences as metal structures, although in different degree. They are, for example, subject to magnetic forces, as well as arcing and sparking at bonds and resistive heating. Non-conductive composites, such as fiberglass and aramid fiber reinforced plastics will be subject to dielectric breakdown, surface flashover and puncture. 
     Aircraft structures include the outer skins of the aircraft, together with internal framework, such as spars, ribs, frames, and bulkheads. Lightning currents must flow between lightning entry and exit points on an aircraft and tend to spread out as they flow between attachment points, using the entire airframe as a conductor. Any conductive material, metal or conductive composite with which most of these structures are fabricated becomes part of the conductive path for lightning currents. 
     In metal structures, the current density at any single point in the airframes is sometimes sufficient to cause physical damage between lightning entry and exit points. Only if there is a poor electrical bond (contact) between structural elements in the current flow is there likely to be physical damage. On the other hand, where the currents converge to the immediate vicinity of an entry or exit point, there may be sufficient concentration of magnetic force and resistive heating to cause damage. 
     As previously noted, composites reinforced with carbon or boron fibers have some electrical conductivity, because of this, their behavior with respect to lightning differs not only from nonconductive materials, but from that of aluminum (which is much more conductive). In carbon and other conductive composites, resistive heating has an entirely different effect. As temperatures rise, the resin bonding the carbon fibers begin to break down, typically as a result of burning or pyrolysis. If the gases which the burning resins give off are trapped in a substrate, explosive release may occur with attendant damage to the structure. 
     The principal risk to a rotating propeller struck by lightning is structural failure, and particularly of a component undergoing considerable centrifugal forces. If the punctured skin is comprised of unidirectional cloth plies, the ply laminates may allow damage to propagate further, at least on the surface ply. Many factors influence damage. Unlike most aluminum alloys, which are ductile and will deform, but not break, carbon fiber composite materials are stiff and may shatter. Consequently, the hazards associated with a lightning strike upon a rotating composite propeller are significant and not to be readily dismissed. 
     There are several trends in small aircraft operations which may cause greater exposure of aircraft everywhere to lightning strikes in the future, including: 1) longer range capabilities of small airplanes; 2) increases in the number of small aircraft and rotorcraft equipped for instrument flight rules (IFR) flight; and 3) increasing the use of radar and direct route navigation aids in general aviation aircraft, permitting IFR flight under adverse weather conditions. It is these factors that warrant continued diligence in the design and operation of aircraft with respect to the hazards lightning may present. 
     Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawings in which like numerals represent like components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of a propeller blade with an embodiment of an electrically conductive leading edge guard shown offset from the blade; 
         FIG. 2  is a perspective view an embodiment of a propeller blade with an electrically conductive band secured to the leading edge of the blade; and 
         FIG. 3  is a cross section of an embodiment of the propeller blade taken along line  3 - 3  in  FIG. 2  and depicting the electrically conductive band secured to the blade. 
     
    
    
     DETAILED DESCRIPTION 
     While  FIG. 1  illustrates an embodiment of a composite aircraft propeller blade  10  and how the blade is supported for rotation by a propeller collar  12 . Each collar  12  has a snap ring receiving groove  15  and an inner cylindrical bore  16 . As those skilled in the art will recognize, blade  10  is the type having a collar  12  which inserts into a metallic hub of the propeller configured for retaining a set of variable pitch blades and mounting to an engine/crank shaft. The propeller blade  10  has a leading edge  18 , and a trailing edge  20  extending between a tip  26  and a shank  28 . Proximate the blade in  FIG. 1  is an electrically conductive band  24  that can extend longitudinally on either or both of the leading edge and trailing edge but in  FIG. 1  resides over the leading edge  18 . 
       FIG. 2  depicts the electrically conductive band  24  held in position on the blade  10 . The band  24  is preferably held in position using a special waterproof, weather resistant and insulating adhesive, such as Hysol® EA 9359.3 (made by Henkel). The conductive band  24  is preferably fabricated from a nickel alloy with a preferred nickel content of no less than 98.5% and a thickness of no greater than 0.033 inches and preferably extends in the range of from 0.80 to 1.25 inches on each side of the leading edge with the preferred span at 1.02 inches on each side of the leading edge as shown at “P” in  FIG. 3 . The band will be at its greatest thickness at the crown and taper to a lesser thickness at the flared edges. 
     As fabricated, and as best seen on  FIG. 1 , the conductive band  24  is flared at the shank (root)  28  of the blade. Importantly, the flared end “F” of the conductive band  24 , when installed, does not come into contact with a snap ring assembly (not shown) located in the snap ring receiving groove  15 . A gap  30  between the flared end of the band and the snap ring receiving groove  15  in the range of from 0.70 to 0.80 inches is preferred. The flared end of the conductive band preferably spans a greater width than the remainder of the band. The flaring of the band allows the electrical current a greater length of conductive material from which to arc to the snap ring assembly adjacent to collar  12 . 
     The expanded length of the flared shank end of the blade reduces the prospect of an excessively powerful charge exiting the conductive band at a discrete point and possibly vaporizing the exit point of the electrical arc on the conductive band. This discrete type of electrical arcing could lead to damage to the blade, the band and the hub if not properly controlled. 
       FIG. 3  is a cross sectional view of the blade  10  taken along line  3 - 3  in  FIG. 2 .  FIG. 3  reveals the interior structure of the blade and details the placement of the band  26  over the leading edge  18  of the blade. The band  26  is preferably symmetrically positioned over the leading edge with an equal portion of the band descending over each side. 
     The composite propeller is fabricated in accordance with standard industry practice well known by those skilled in the art. Multiple layers, or plies, are built up to produce the desired configuration of the propeller. The final layer of the blade  10  (not shown in the Figures) is a non-conductive glass that effectively insulates the blade from the conductive band  24 . The propeller system is compliant with the lightning strike provisions of the Federal Aviation Administration policy No. ANE-2001-35.31-R0 titled Policy for Bird Strike, Lightning, and Centrifugal Load Testing for Composite Propeller Blades and Spinners. 
     A lightning strike is essentially a high amplitude direct-current pulse with a well-defined waveform. A lightning flash initially attaches to, or enters, an aircraft at one spot and exits from another. Usually these entry and exit points are extremities of the aircraft such as the nose, wing and empennage tips, propellers and rotor blade, engine nacelles and other significant projections. 
     Aircraft propellers are frequent targets for lightning strikes thereby precipitating the need for the technology disclosed herein. The general location of propellers, front for traction or rear for pusher account for their high probability of lightning strike attachment. As the lightning attaches to the propeller blade at some point between the tip  26  and the shank  28  of the conductive band  24 . The electrical charge will pass to the metallic hub and then conduct through the gears and bearings supporting the propeller or rotor shaft. The electrical current then travels through the bearings, which are supported on insulating lubricant films. Finally, the electrical charge travels through the airframe and exits to ground, typically at some aft or lower location on the aircraft. Thus, the electrical charge follows a ground path that includes a path through the propeller hub. 
     While the preferred form of the present invention has been shown and described above, it should be apparent to those skilled in the art that the subject invention is not limited by the figures and that the scope of the invention includes modifications, variations and equivalents which fall within the scope of the attached claims. Moreover, it should be understood that the individual components of the invention include equivalent embodiments without departing from the spirit of this invention. 
     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims. Not all steps listed in the various figures need be carried out in the specific order described.