Bell Flies Composite Horizontal Stabilizer on MAPL

Walpole, MA, May 6, 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.The one-piece, constant 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.

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. The 17 pound structure was manufactured using a combination of braiding, hand layup of dry woven fabrics and resin transfer molding (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, comprised of four full span internal spars. The spars are produced by automated triaxial braiding, where bias ply AS4 graphite fibers in the shear webs are machine-placed (i.e. braided) over four mandrels at high speed simultaneous with placement of the uniaxial fibers 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 molded into the stabilizer to carry attachment loads at four central bolt locations. Similarly, a molded 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 fiber preform assembly is resin transfer molded in a closed cavity mold 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 centerline to facilitate extraction after molding. After trim, drill and inspection of the single molded 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 200° F in an environmental chamber to simulate humidity conditions. The stabilizer was successfully tested to 390% of Design Ultimate Load.

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