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Aviation History
1987
1987 - 2274.PDF
AEROTECH '87 Carbonfibre bolts tested Composite fasteners are being tested by aircraft manu facturers, reports Graham Warwick from Aerotech '87 at Birmingham. Lighter and cheaper than conventional steel and titanium bolts, the new fasteners have other advan tages. Carbonfibre bolts can be used in composite struc tures to reduce vulnerability to lightning strikes, while radar-transparent glassfibre bolts can be used in structures intended to reduce aircraft radar signature. Samples of the new composite fasteners are being supplied to aircraft companies for tests, reveals C. J. Fox & Sons, UK distributor for Cherry Textron and Tiodize products. Tiodize has developed a self-lubricating carbonfibre composite called Tribo/Comp TDF which can be hot- moulded into various shapes for a variety of applications, including channels, gears, bearings, and fasteners. Cherry is also working on composite fasteners, includ ing carbonfibre rivets with upsettable tips. British Aerospace is evalu- ting composite fasteners because it is concerned about possible lightning strike damage to the carbonfibre wing of the European Fighter Aircraft. In a paper presented at the Aerotech conference, experts from the UK Atomic Energy Authority's Culham Labora tory highlighted the risks of sparking between carbonfibre wing skins and the metal bolts securing them. Where the wing is also a fuel tank this brings the risk of explosion. The likelihood of a lightning arc attaching to a metal bolt on the surface of a carbonfibre wing was described as "particularly serious". On average an aircraft is struck by lightning once a year, the paper revealed. Despite advances in the co- bonding and co-curing of composite components, as detailed in a British Aero space paper describing the EAP fighter demonstrator's carbonfibre wing, fasteners will still be required. Although the EAP lower wing skin was co-bonded to the substruc ture, the upper skin was bolted in place. Bolts could also be required as backup, or "chicken", fasteners on bonded components, suppliers suggest. Although carbonfibre bolts are only one-third as strong as titanium examples, requiring more to be used, they are so much lighter and cheaper that there will still be an advan tage. Direct- drive EFA studied Advances in high-power elec tric motors make it likely that the European Fighter Aircraft (EFA) will have direct-drive fly-by-wire flight controls. Fairey Hydraulics has teamed with Moog and Rexrath in Germany, Micro- technica in Italy, and Inisel in Spain to bid for the EFA primary flight control actu ators, and is expected to offer a direct-drive solution. Fairey developed the Tornado taileron actuator, which is electrically signalled, but with hydraulic ampli fication of the input signal. There has been a rapid improvement in the power-to- mass ratio of electric motors in recent years, however, allowing their use in servo valves. In a direct-drive fly-by-wire system signals are sent from the digital flight control computer in the form of a power pulse, the duration of which determines the degree of control surface movement. In a quadruplex flight control system such as EFA's, each actuator would have four samarium-cobalt electric motors driving the servo valve. The advantages of direct drive include a significant reduction in complexity and weight, and an increase in reliability. In addition, the electric motor can be over driven by an enormous margin to clear a stuck valve, so increasing safety. In a typical direct-drive system the normal operating forces are around 2N, corre sponding to a current level of hundreds of milliamps. The maximum force needed to free a stuck valve is around 700N, corresponding to a current of a few amps—but only for very brief periods. Fairey has been working on direct-drive for several years, while consortium partner Moog, through its US parent, is supplying direct-drive actu ators for Sweden's Gripen. Three-in-one probe developed An air-data probe sensing pitot pressure, static pressure, and local angle of attack has been developed by Badin Crouzet. The delta-shaped vane, gimballed to align with the local airflow, has a pitot probe at its tip and static ports on either side which are also used to provide accurate angle of attack. Air-bag landing gear proposed Woodville Polymer, now part of Dowty and keen to empha sise its system design expertise, has revealed that it is studying a novel rough-field air-bag landing system for the C-130 Hercules. Designed to allow landings on unprepared strips too rough for the C-130's landing gear, the system comprises a series of elastomeric bags attached to the underside of the fuselage. These bags are inflated by ram air and kept topped up by an onboard compressor. When not in use they are vacuumed on to the fuselage to reduce drag. On a more conventional note, Woodville says that its polymer airframe seals will help Airbus achieve the effi ciencies it claims for the variable-camber wing on the new A330/A340 airliners. Woodville is also promoting its radar signature manage ment design services using structural and non-structural composite and elastomeric materials which absorb and reflect radar energy in a controlled fashion. Aerotech '89 planned Organised by the fledgling aerospace industries division of the 80,000-strong Institu tion of Mechanical Engineers, Aerotech '87 attracted 2,500 visitors to the conference and associated exhibition at Birmingham's National Exhi bition Centre. Almost 100 papers were presented in 24 half-day seminars—the primary reason for the event. The associated exhibition attracted some 114 exhibitors. Prompted by the success of the earlier Autotech event also organised by the IMechE, Aerotech was established as a forum both for information exchange, and to promote the Institution's two-year-old aerospace industries division, say officials. The Institution is already planning Aerotech '89. 16 FLIGHT INTERNATIONAL, 7 November 1987
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