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Aviation History
1981
1981 - 0149.PDF
FLIGHT International, 17 January 1981 139 "hybridise" Kevlar with carbon (see table), although Lockheed does not see the need for this. On the 727, a JT8D cowling panel has been made of a Kevlar/carbon hybrid combining the high strength and stiffness of carbon with the 40 per cent lighter weight, greater im pact-resistance and lower cost of Kevlar. On the 737, carbon spoilers have been installed on 28 aircraft in ser vice with seven major airlines, and have so far achieved two million land ings without major defect. On the 767, net weight-saving by composite components will be more than 2,0281b. Says Boeing's Bob Ham mer : "Present composites can be com pared in development to aluminium in the 1930s. The areas of further re search and development are almost limitless. Future generations of com mercial aircraft will have significantly greater amounts of composite material and, without any doubt, composites in primary structures. This evolution can occur only through the teamwork of further research by the composite scientist and ingenuity of application by the staff engineer and production designer." Helicopters Du Pont estimates that a pound of weight saved on a helicop ter is typically worth perhaps $300, and can be as high as $800. One such application is to the "doghouse" area around the engine. More than half the exterior of the Sikorsky S-76 Spirit is made of Kevlar- reinforced composites. There are 138 internal and 25 external components, saving about 30 per cent in weight. Sikorsky says that the 7ft stabiliser is cheaper than any the company has built. It is said to have failed on test at more than double the design ulti mate load. James Ray, Sikorsky composite pro grammes manager, says of the S-76: "You will find Kevlar in the nose section, cockpit canopy, upperdeck fairings, horizontal stabilisers, doors, pylons, floors—wherever we want to achieve high strength, stiffness and impact integrity while shaving off critical pounds." All S-76 components are formed from Kevlar 49 prepregs coated with epoxy resin and in a tacky state ("B-stage"). Adds Boy: "The entire aircraft is smooth, with flush sweep ing contours which would be difficult and very expensive to render in sheet metal. The lower drag as well as light weight add up to a 20 per cent in crease in range as well as one addi tional passenger." The Boeing Chinook will have Kevlar/Nomex-paper honeycomb fuel tanks, its floor will be made of (and supported by) Kevlar, and the engine cowlings and forward airframe are made of this material. A prototype Kevlar rotorblade for the Hughes AAH-64 battlefield heli copter, lighter than glassfibre, is being flight-tested. Blades for the Casa 212's The TriStar, opposite page, was the first passenger aircraft to make extensive use of Kevlar, the largest section evident in this view being the wing-to-body fairings (over and under). The other K. components are aileron, rudder and elevator trailing-edge wedges; fixed wing trailing-edge and leading-edge panels; the "Frisbee" fairing underneath the centre engine intake; and pylon fairings. Total weight saving is equivalent to four passengers with baggage. Above Boeing and Rohr have developed a Kevlarjcarbon lower cowl panel for the 727's side JT8D engines Hartzell propellers have been FAA certificated for 700hr or three years, whichever comes first. A glider such as the Applebay Zuni II saves 701b compared with glass- fibre, and Kevlar is being evaluated for airliner escape slides, saving more than 11 per cent weight and having a much higher flame-resistance than nylon. Cargo containers developed for Pan Am offer a 10-20 per cent weight- saving, and have been flying for 18 months without reports of damage. de Havilland Kevlar The de Havilland Dash 7 uses more than 5001b of Kevlar prepreg, saving more than 25 per cent weight compared with fibre- glass. Almost the complete cabin interior of the Dash 7 is made of Kevlar, including the floors. Cabin interior components are laminated and sandwich constructions using Nomex honeycomb varying in thick ness from :4 to 2in. Externally Kevlar is used for the nose avionics compart ment, the leading edge of the fore flap, wheel stoneguards (in the main landing-gear well), and various aero dynamic fairings. Special care has been taken, as on all aircraft, to protect external com posite structures from lightning. In the case of the avionics nose com partment, aluminium wire mesh pro tects the outer surface; the mesh provides electromagnetic shielding for the avionics, and prevents damaging induced voltages. The nose avionics compartment is the largest load-carrying Kevlar/ Nomex component on the Dash 7, forming the forward fuselage struc ture between nose radome and pres sure bulkhead. This nose saves about 30 per cent weight when compared with a glass /balsa construction as used in the Twin Otter. Dash 7 components under evalua tion and development by D.H. include tailcone, nose landing-gear doors (Kevlar/carbon hybrid) and wing leading-edges. In the tailcone, 80 de tail components in metal have been replaced by one in composite, with a 30 per cent weight-saving. The 36-passenger de Havilland Dash 8, now in the design stage, will probably use 50 per cent more Kevlar than the 50-passenger Dash 7. In addi tion to Dash 7 applications, wing-to- fuselage fairings, dorsal fin, flap shroud, aft engine nacelles and land ing gear doors are being studied. Some of these components could be of hybrid construction using carbon. Development background The impact- tolerance of carbonfibre plastic laminates can be significantly improved by incorporating surface layers containing Kevlar fibres. Such laminates have been tested by the Royal Aircraft Establishment. Accord- The de Havilland Dash 7 has many Kevlar parts, including this honeycomb nose compartment • 1
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