Innovative Composite Stabilizer Flies on Bell Helicopter MAPL

13 May 2005

A lightweight, low cost composite stabilizer manufactured by Fiber Innovations of Walpole, MA using Braiding and RTM processes recently flew on the Bell Helicopter MAPL (Modular Affordable Product Line) demonstrator aircraft.

Fiber Innovations, supplier of lightweight composite structures for aerospace and defence applications, have produced a one-piece, constant section horizontal stabilizer, designed to provide a cost saving compared with a conventional composite design by reducing parts count, subassemblies and associated surface preparation, bonding and inspection steps, according to the company.

The RTM composite stabilizer also features simplified attachment to the fuselage, further reducing assembly cost.

The 11 square foot horizontal stabilizer is a fixed control surface that passes through the fuselage and is attached by four bolts. The stabilizer also contains provisions for attachment of a leading edge slat and two vertical tip fins.

Bell Helicopter said that the 17 pound structure was manufactured using a combination of braiding, hand layup 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.

The stabilizer is a multi-cell box beam structure (pictured), 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.

Bell Helicopter

Internal bushing blocks are moulded into the stabilizer to carry attachment loads at the 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 need. 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.

Prior to flight test, extensive static and fatigue testing was performed to verify the structural integrity of the low cost RTM stabilizer design. Static testing on the stabilizer was performed at 200F in an environmental chamber to simulate humidity conditions. The stabilizer was successfully tested to 390% of Design Ultimate Load.

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