JEC COMPOSITES MAGAZINE - Issue #68 Special JEC Asia - October 2011 - 90

aeronautics
About Chomerics
Chomerics specialises in the development and application of electrically- and thermally-conductive materials in electronics, transportation and alternative energy systems. Chomerics provides EMI shielding and thermal management solutions for telecommunications, information technology, medical devices, automotive, military, commercial and consumer electronics industries.

a conventional metal aircraft is designed to conduct the energy, by means of electrical continuity, towards an exit point, usually at the front of the aircraft. The airframe itself is able to dissipate an appreciable quantity of the strike energy to prevent damage to electrical systems and internal structure equipment.
Fig. 1: Direct bonding of electrical modules to exposed metal mesh

One step back
With the advantages of lower weight, however, have come a number of technical challenges since composites themselves are not electrically conductive. Modern aircraft have numerous electrical systems capable of generating electro-magnetic interference (EMI) which can potentially disturb the operation of sensitive systems such as navigation, fly-by-wire and engine management systems. Among these are fluorescent lights, light switches, dimming circuits, AC-powered window heaters, motors and generators, data cables and power lines running through the aircraft, and transmitters such as radio and radar.

In areas where electrical modules are bonded or grounded by direct connection to the airframe, a maximum impedance of 0.003 Ω is specified but connections to the mesh can fail to meet this requirement throughout environmental stresses such as vibration and temperature variation. Also, at the sites where grounded or bonded modules are attached (Figure 1), the exposed metal mesh can be vulnerable to oxidation that further impairs electrical conductivity.

Applied performance enhancement
If extra metal were added to overcome these challenges, the additional weight would discount some of the advantages of the new materials. Hence there is a need for effective, lightweight technologies that will enable designers to optimise conductivity in specific areas of the airframe. This goal is met through a variety of conductive compounds, such as adhesives, sealants, coatings and greases. A silver-bearing conductive coating can be applied to complement the embedded metal mesh and improve the shielding performance of an enclosure enhancing protection at lightningstrike sites. This coating may be in the form of an epoxy paint such as CHOSHIELD® 596 two-component EMI shielding formula, or a polyurethane coating such as CHO-SHIELD 4994, which has high solvent rub and wear resistance and can be used externally; it is also compatible with primers and top-coat systems. In areas where high corrosion protection is needed, such as antenna covers, a copper-based urethane coating such as CHO-SHIELD 2002 is formulated to remain stable and inhibit corrosion when exposed to high temperature or humidity.

Recovering lost properties
Designing-in structures built with composites means designing out the interference protection inherent in their metallic predecessors. To restore lost shielding, bonding or grounding properties, woven or non-woven copper-aluminium mesh, or an expanded foil, can be embedded in composite structures. The embedded metal is specified to provide an optimal combination of electrical conductivity, weight, and corrosion resistance. Solid metal strips can be used in the radome area to handle very high concentrations of lightning energy. Embedded conductors, however, cannot solve all of the electrical challenges that come with increasing use of composites. Ensuring reliable electrical continuity between individual composite parts after the airframe is assembled, for example to promote conduction of lightning energy, can be challenging.

jec composites magazine / No68 October 2011
Outside the aircraft, storms are a major source of potentially disruptive electrical interference and physical damage through lightning strike impact. In addition, friction caused by phenomena such as vibration and airflow can cause potentially hazardous electrostatic charges to accumulate on the outside of the plane. A conventional, predominantly metal airframe allows designers to take advantage of the Faraday cage effect to protect equipment against interference originating inside or outside the aircraft. There are abundant opportunities to bond and ground items of equipment reliably by connecting directly to a convenient surface. To cope with a direct lightning strike,

About Parker Hannifin Corporation
With annual sales exceeding $10 billion, Parker Hannifin is a diversified manufacturer of motion and control technologies and systems, providing precision-engineered solutions for a wide variety of commercial, mobile, industrial and aerospace markets. The company employs approximately 62,000 people in 48 countries around the world.

JEC COMPOSITES MAGAZINE - Issue #68 Special JEC Asia - October 2011

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