Manufacture - machining (E.D.M.) Composite materials and sandwich casings

COMPOSITE MATERIALS AND SANDWICH CASINGS 

Fig. 22-14 Some composite material applications.

43. High power to weight ratio and low component costs are very important considerations in the design of any aircraft gas turbine engine, but when the function of such an engine is to support a vertical take-off aircraft during transition, or as an auxiliary power unit, then the power to weight ratio becomes extremely critical.

44. In such engines, the advantage of composite materials allows the designer to produce structures in which directional strengths can be varied by directional lay-up of fibres according to the applied loads.
45. Composite materials have and will continue to replace casings which, in previous engines, would have been produced in steels or titanium. By-pass duct assemblies comprising of three casings are currently being produced up to 4ft-7in. in diameter and 2ft-0in in length using pre-cured composite materials for the casing fabric. Flanges and mounting bosses are added during the manufacturing process, which are then drilled for both location and machined for peripheral feature attachment on C.N.C. machining centres, which at one component load, completely machine all required features. Examples of composite material applications are illustrated in fig. 22-14.
46. Conventional cast and fabricated casings and cowlings are also being replaced by casings of sandwich construction which provide strength allied with lightness and also act as a noise suppression medium. Sandwich construction casings comprise a honeycomb structure of aluminium or stainless steel interposed between layers of dissimilar material. The materials employed depend upon the environment in which they are used.