Vertiflite May/June 2021

Nearly 30 years after the Apache stabilator, Bell test flew induction-welded thermoplastic ruddervators and access panels on the V-280 Valor advanced tiltrotor. Arnt Offringa, director of the GKN Aerospace Global Technology Center, in Hoogeveen, The Netherlands, explained, “With thermoplastic composites, the ruddervators are welded together, drastically reducing the amount of bolts and therefore assembly cost. On a total part level, typically a cost reduction of 20% is feasible.” The thermoplastic ruddervators also cut weight about 10% compared to thermoset parts. Induction welding permits a large number of internal ribs strengthening a thinner skin. Excess production material from the Valor ruddervators was also reprocessed to mold two access panels. Thermoplastic composite promises better structural and mechanical properties for crashworthy air taxi structures that resist “ramp rash” — the inevitable ground contacts during busy operations. “It’s a tougher material,” observed Peryea. “Today, if you’re using thermosets, you typically use bolted joints, whereas with thermoplastics, the entire joint is welded together. You end up with a stronger bond.” He concluded, “Certainly, it’s a lighter, stronger structure,” adding, “Thermoplastics have better repair characteristics that thermosets. You can do field repairs easier than conventional thermosets. Those repair processes are being developed right now. It will be cheaper to repair aircraft damaged in operation. The materials are more durable. Thermoplastics are pretty much immune to fuel or hydraulic fluids.” Unlike thermoset prepreg — reinforcing fabric that has been pre- impregnated with a resin system — with limited working times, thermoplastic composites need no refrigeration. Peryea conceded, “There’s notmuch difference inmaterial properties froman aircraft performance perspective. Fatigue lives will be very similar. The weights will be higher and the costs will be higher with thermosets. Thermoplastics are lighter and cheaper, but it’s more the maturity of developing thermosets for rotor blade applications that would take additional time and maturity for certification requirements. It’s a little bit more of a challenge. Fortunately, there are automated means to lay-up rotor blades in practice with existing OEMs.” Try and Certify For all their apparent advantages, use of thermoplastics versus thermosets for AAM depends largely on process maturity. Toray’s Howell said, “There are going to be some weight advantages. For example, with thermosets you have to do adhesive bonding. You also have to do riveting because they don’t trust the bond. For a thermoplastic part, you can do welding. You don’t have that same concern you have with adhesive bonding. These parts are like one laminate together. You can eliminate rivets. Welding is a great way to eliminate weight.” Qualifying thermoplastic materials and certifying processes are essential to overall air taxi certification. According to Peryea, “There is certainly certification data out there for existing parts that are being built today, but it’s the induction welding process; that process needs to have certification data for a manufacturing process. That is currently in work today at Triumph Aerospace Structures. They’re developing the design data necessary.” AAM generally implies smaller aircraft to operate from city vertiports, and thermoplastic composites have so far lent themselves better to small parts than airliner-size wings and fuselages. “Historically, it’s been stamp-formable shapes that have been looked at mainly in smaller sizes,” observed Howell. “Now,

Automated tape laying (ATL) provides one means of reducing touch labor, shortening manufacturing time and cutting composite part costs. (Toray Advanced Composites)

where the industry is headed for these large commercial aircraft,” said Howell. “They’re looking at entire fuselages and entire wings made by automated fiber placement. They’re looking at in-situ fabrication where you lay up the part over stringers that are already in place. When you put heated material over that you get consolidated structure in-situ.” GKN today gives the Leonardo AW169 light helicopter a thermoplastic horizontal tail 15% lighter than a thermoset alternative, but the rotorcraft industry has been slow to adopt big thermoplastic parts. In 1990, McDonnell Douglas built a full-scale horizontal stabilator for the AH-64 Apache out of graphite-reinforced PEEK. The static-tested stabilator weighed 18% less than the original metal assembly, and adhesive bonds cut the number of mechanical fasteners from 3,300 in the metal stabilator to just 148 in the thermoplastic substitute. Nonetheless, the thermoplastic stabilator was never flown. Without tailored processes, thermoplastic composites could not deliver production cost savings versus thermosets. Jonathan Sourkes from TXV Aero Composites told the VFS eVTOL symposium about advances in thermoplastic composite manufacturing including automated tape and fiber placement, compression molding and stamp forming, resin transfer molding and thermoplastic welding. TXV Aero Composites in Bristol, Rhode Island, offers continuous fiber and injection molding materials with PEEK and polyaryletherketone (PAEK) resins for cost-effective hybrid overmolding. With efficient processes, TXV analyses show thermoplastic composites cut manufacturing costs 30–50% versus thermosets. The Bell V-280 Valor has flown with thermoplastic composite ruddervators that eliminate fasteners to cut cost and weight vs. thermosetting composite parts. (Bell)

May / June 2021

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