Sealing products and legal requirements - Parker Hannifin ...

More documents

Recommendations

Info

How to turn poor conductors into good ones EMI, grounding and lightning protection in composite-based aircraft Author: Christophe Loret, Conductive Compounds Product Manager, Chomerics Division Europe Composite materials have been enthusiastically adopted throughout the aircraft industry as they provide many advantages for designers. Their lack of electrical conductivity, however, is a notable disadvantage. The Chomerics Division of the Parker Seal Group Europe offers a range of highperformance conductive compounds such as coatings, sealants and greases that play an important supporting role in ensuring the safety and reliability of modern aircraft. Aircraft Design Steps Forward With advances in materials, structural design, and production processes, aircraft designers have been able to replace an increasing number of metal components with equivalents made from composites. Today, airliners such as the Boeing 787 and Airbus A350 consist of about 50% composites. Major structures include the fuselage and wing fairing, as well as large sections of the wings, fin and horizontal stabilisers. The Airbus A400, now being developed, is believed to be the first airliner to feature a 100% composite wing. One Step Back With the advantages of lower weight, however, have come a number of technical challenges since the 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 12 SealingReport · November 2011 systems such as navigation, fly-bywire and engine management systems. Outside the aircraft, storms are a major source of potentially disruptive electrical interference and physical damage through lighting 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. To cope with a direct lightning strike, a conventional metal aircraft is designed to conduct the energy, Exposed ground plane (wire mesh) Elelectrical component 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. 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 Mounting bracket Figure 1: Direct bonding of electrical modules to exposed metal mesh Composite structure
Nose gear wheel well copper-aluminium mesh, or an expanded foil, can be embedded in composite structures. Embedded conductors, however, cannot meet 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. In areas where electrical modules are bonded or grounded by direct connection to the airframe, a maximum impedance of 0.003Ω is specified but connections to the mesh can fail to meet this requirement due to environmental stress factors 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 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 Engine pylons and nacelles Figure 2: Conductive gaskets and sealants promote electrical continuity between metal-laminated composite structures. Main wheel wells lightning-strike sites. This coating may be in the form of an epoxy paint such as CHO-SHIELD 596, a twocomponent 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 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. Electrically conductive grease is typically applied at grounding connections, to support reliable electrical connectivity within the vibration and temperature ranges specified. Careful attention is paid to viscosity and surface-wetting properties, when formulating greases for aerospace applications. CHO-LUBE E117, which has resistivity better than 40mΩ/cm is an example of an aerospace-grade grease formulated to support electrical interconnections, improve metal to metal contact and provide long-term oxidation protection for exposed mesh or electrical terminals. Area forward of front spar Empennage All wing body fairing areas Area aft of rear spar In locations where electrical continuity and environmental protection is needed, a sealant such as CHO-BOND 1075 or CHO-BOND 1038 may be applied. Airframe features typically treated include screw holes, fasteners, antenna connection points and exposed conductors on external areas such as the wings, fuselage or tail. Where conductive gaskets are used to promote electrical continuity between several composite components, conductive sealant can be applied to provide additional continuity enhancement in areas where high vibration occurs. This may be around wheel wells, engine mounts, wings and the tail section, as depicted in figure 3. In addition to these techniques helping to optimise EMI performance throughout the airframe, the aircraft’s cabling is typically also provided with additional direct shielding against the effects of lighting strike and electrostatic discharge. Conductive zippered sleeving such as ZIP-EX, or a lightweight plastic conductive heat shrinkable tube such as CHO-SHRINK, can be used in place of standard metal braid to save weight. SealingReport · November 2011 13
Page 1 and 2: SealingReport Page 3 One groove - u
Page 3 and 4: One groove - unlimited potential Mo
Page 5 and 6: Profile Standard material E9 FKM V3
Page 7 and 8: Parker-Prädifa ranks among Baden-W
Page 9 and 10: On 28 September another “Health D
Page 11: Jochen Nigge, General Sales Manager
Page 15 and 16: aromatic hydrocarbons (PAH) have at

Sealing products and legal requirements - Parker Hannifin ...

Create successful ePaper yourself

Delete template?

Save as template?