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A lightweight, low-cost composite stabilizer manufactured using braiding and RTM processes recently flew on the Bell MAPL (Modular Affordable Product Line) Helico p ter demonstra tor aircraft. The one-piece, co n stant section horizontal stabilizer was designed to provide significant cost reduction compared with a conventional composite design by reducing parts count, subassemblies and associated surface preparation, bonding and inspection steps. The RTM composite stabilizer also features simplified attachment to the fuselage, further reducing assembly cost.
(Published on January-February 2006 – JEC Magazine #22)
The one-square-metre horizontal stabilizer is a fixed control surface that passes through the fuselage and is attached by four bolts.
It also contains provisions for attachment of a leading edge slat and two vertical tip fins. The eight-kilo structure was manufactured by Fiber Innovations (Walpole, Massachusetts) using a combination of braiding, hand lay-up of dry woven fabrics and resin transfer moulding (RTM). The graphite/BMI part reduces the weight of the existing composite stabilizer and provides a significant cost reduction through reduced parts count and elimination of secondary bonding.
Braided box beams
The stabilizer is a multi-cell box beam structure, comprised of four full-span internal spars. The spars are produced by automated triaxial braiding, where bias-ply AS4 graphite fibres in the shear webs are machine-placed (i.e. braided) over four mandrels at high speed simultaneous with placement of the uniaxial fibres in the spar caps.
The braided mandrels form four box beams, and feature skin thickness taper to match the spanwise load distribution.
Internal bushing blocks are moulded into the stabilizer to carry attachment loads at four central bolt locations. Similarly, a moulded channel is provided at the leading edge to accommodate a nut plate strip used for slat attachment. The braided mandrels are overwrapped with woven biaxial and unidirectional fabric to form the stabilizer skin, including local buildups as needed. Flanges for the attachment of the tip planes are integral with the skin.
The entire fibre preform assembly is resin transfer moulded in a closed cavity mould using Cytec 5250-4 one-part bismaleimide (BMI) resin. The high temperature BMI resin enables the stabilizer to endure engine exhaust temperatures present in portions of the rotor downwash. The mandrels are split at the stabilizer centreline to facilitate extraction after moulding. After trim, drill and inspection of the single moulded unit, the tip planes and leading edge slat are mechanically fastened and the unit is ready for airframe installation.