From another perspective

From another perspective Aerospace structural composites: hightech -- right? Carbon and aramid prepregs, toughened epoxies and bismaleimides, autoclaves, etc. you say. Not necessarily. It is true that composites in planes like the B2, F-22, Airbuses, big Boeings and helicopters usually are of this type. It may come as a surprise, however, to hear that most of the world's all-composite aircraft flying today have b e e n p r o d u c e d a different, far simpler way: from flbreglass fabric, by w e t layup, low temperature epoxies and oven cure, using a vacuum bag over a single mould. The catch lies in the worlds 'most' and 'all' -- since a handful of enterprising companies in Germany alone have built considerably more than 12 000 light fully composite aircraft (gliders, sailplanes and single engine sport craft) over the last 30 years, without resorting to the costly prepregs and autoclaves normally associated with aerospace composites. The key lies partly in the manufacturing method; the simplicity - as well as tight quality control and careful attention to detail. The Germans call it 'LPC' (low pressure composites), a technique spearheaded in the late 1950s by their various akaflieg (academic flight) university groups, with the Phoenix sailplane. Today, virtually all championship sailplanes feature fully composite airframes. The Germans summarize the process in a half dozen points, readily recognized by many a builder of other reinforced plastic structures. The major points are: use of a single negative mould; glass fabric sheets w e t laminated with a low temperature epoxy, use of light, core; a vacuum bag, foil and suction p u m p (7090 kPa) provides ample pressure for cure; use of a twostage temperature curing process. Nor are just small or exotic planes made this way. Dornier Seaster is building the world's only twin-

tuboprop all composite amphibian by this method, in Malaysia. FFT and its forerunners have p r o d u c e d hundreds of small, p o w e r e d sport planes of various types, while Grob -- which makes both championship sailplanes, and trainers, is n o w certifying a five seater, p u s h e r p o w e r e d business aircraft. All are composite/ reinforced plastic, built with the LPC method. And there's more. Grob, is constructing some of the world's largest composite aircraft: the Stratos I, a turboprop high altitude research aircraft possessing over a 30 m wingspan, and its n e w twin-engined Stratos II, pressurized high altitude plane for scientific purposes, with nearly the span of Boeing 747.

"Most composite

aircraft have been p r o d u c e d in a simple way."

Reinforced Plastics September 1994

Another unusual and innovative (from the point of view of aerospace composites) German aircraft is the Ruschmeyer R90, a snappy 4"seat, propeller p o w e r e d sport and business plane. Already certified and in production for a few years now, it has taken a different approach to low cost composite structures. THe R90's innovation lies in its choice of materials, for the plane's designers have chosen not epoxy but a vinyl ester (BASF Palatal A430) for the matrix resin. Interestingly, they claim superior properties to epoxy, eg. in stress and strain stability, service temperature, and environmental aspects. Clearly, aerospace is not merely the domain of exotic or complex composite materials, methods and structures. As shown above, there is a 'low tech' side to the widespread use of reinforced plastics in certain sectors of the aerospace industry that often goes almost completely unnoticed. Gary F. Turner Turner Research & Consulting, Munich, Germany

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0034-3617/94/$7.00© 1994, Elsevier Science Ltd