25 May 2023
Moldable, weldable thermoplastic composites may make electric vertical takeoff and landing (eVTOL) aircraft affordable in mass production.
Making lightweight, crashworthy aerostructures affordable is essential for advanced air mobility (AAM), given the huge number of aircraft needed to unclog roads. AAM fleet projections vary greatly, but a presentation in January for the Vertical Flight Society’s Eighth Annual Electric VTOL Symposium by Toray Advanced Composites senior application engineer DeWayne Howell baselined 5,000 deliveries a year around 2040. Howell noted his conservative estimate is about five times more than the current helicopter industry output and comparable to specialty automobile production.
In high-volume manufacturing, thermoplastic composites can help churn out aerostructures faster and cheaper than commonly used thermosets. Howell acknowledged, “There’s a lot of work to be done to… have materials and production processes that will allow us to do that.” He concluded AAM mass production will require partnerships between original equipment manufacturers (OEMs) and material suppliers to build prototypes, refine processes, qualify materials and certify aircraft.
The Jaunt production plan adopts a mix of thermoplastic processing techniques for the fuselage, tailboom and wing. (Jaunt Air Mobility)
In December, Toray in Morganville, California, announced a long-term supply agreement with Joby Aviation for carbon fiber-reinforced composites to build eVTOL air taxis. The material maker offers composites impregnated with either thermosetting or thermoplastic resins. Thermosets cured in pressurized autoclaves undergo a chemical reaction to achieve full strength. Thermoplastics melted in molds re-solidify without chemical change. Joby refused to discuss material tradeoffs, but Howell at Toray summarized, “If you’re going to make a typical thermoset part, you have to give time for the resin to cross-link and make that chemical change to solidify. With thermoplastics, if they’re stamp-formable, you can produce parts in minutes rather than hours.”
In February, Jaunt Air Mobility — headquartered in Dallas, Texas — announced a Small Business Technology Transfer (STTR) contract from the US Air Force Research Laboratory to work on thermoplastic technologies and low-cost production techniques for eVTOL aircraft. The company is developing its four-passenger Journey Reduced-rotor Operating Speed Aircraft (ROSA) with mostly thermoplastic structure. Jaunt CEO Martin Peryea explained, “The entire airframe is going to be made out of thermoplastic materials. All the major primary structural components will be based on thermoplastics. The outside body, tailboom, rudder, stabilizer, all of that will be made of thermoplastic materials.” He added, “The rotor blades, because of the maturity of the manufacturing technology, those will use conventional thermosets.”
Jaunt plans first flight of its electric gyrodyne next year and aims at certification and full-rate production in 2026. The company ultimately plans to turn out 2,500 aircraft a year at each of two manufacturing centers. Thermoplastics are key to mass production. Peryea offered, “It’s the automation of hot press forming very large structural components and welding those together with very minimal touch labor — there’s a significant cost savings associated with that. You can produce at much higher rates and volumes.” Toray, Hexcel, and other material makers supply snap-cure thermosets that save manufacturing time, but thermosets deny manufacturing engineers the chance to melt and re-form complex parts several times in successive production steps.
Thermoplastic airframe substructure for the Journey will be compression-molded from chopped-fiber composites. Wing skins, wing spars and tail booms will all be layed up by automated fiber placement. Thermoplastics make it possible for large sections of the Journey to be hot press formed to the required shape and welded or fused together using induction heating without fasteners. Eliminating fasteners helps cut the weight of thermoplastic assemblies 2–10% compared with thermoset structures. Jaunt has yet to finalize its material selections. “We’re still optimizing a couple of different materials that lend themselves to this type of manufacturing process,” said Peryea.
Thermoplastic composites eliminate the costly refrigeration required with thermosets and lend themselves to faster, more automated processes. According to Jaunt chief engineer for aircraft certification Dr. Sanjay Gattani, “Typical thermoset composite cure cycles will not be able to support rate [production] at the affordability target.” The ROSA maker’s main objective during Phase I of the Air Force project is to model a thermoplastic composite wing structure and use advanced manufacturing techniques to achieve its low-cost commercial production targets. Jaunt worked with the Air Force to determine detailed mission and vehicle requirements subsequently used to develop a basic wingbox model and structural design criteria.
Toray has supplied thermoplastic prepreg for compression molded aerospace parts. (Toray Advanced Composites)
Georgia Tech is under contract to Jaunt to design the internal structure for the thermoplastic composite wing and establish design allowables — stress, strain and stiffness values — for the material system. Gattani explained, “Georgia Tech will utilize this information provided by Jaunt to develop an internal structural layout for a representative thermoplastic wingbox structure and establish a load envelope consisting of aerodynamic, propulsive and ground loads. This structure will be optimally sized using Georgia Tech’s computational tools and design allowables for the material system to obtain structural weight and performance.” The end deliverable to Jaunt will be a computer-aided design wingbox model, a finite-element model with material data and a report detailing performance comparisons and design methodologies.
Jaunt and Georgia Tech in turn are collaborating with Triumph Aerospace Structures to integrate the company’s patented induction welding process into high-rate production. Triumph modeling will identify key manufacturing cost drivers. The structures supplier in Spokane, Washington, already produces thermoplastic parts for Boeing and other OEMs. “They hot press-form about 700 parts for the industry.” Peryea offered. “These parts are used to tie primary structure together. They’re secondary structural components. They’re also used to hang a lot of the aircraft systems. What the industry is doing from a technology development aspect is to make large structural, primary structural, components using a hot press forming technology.” Triumph has already fabricated a Journey wingbox from thermoplastic composites.
We Have History
Thermosetting composites already pay off throughout the aerospace industry with weight savings and fatigue and corrosion resistance. Thermoplastics have less history, but Howell noted, “Contrary to popular belief, thermoplastic composites have been used on aircraft for over 30 years.” Airbus uses thermoplastic shear clips between frames and stringers with Toray fibers pre-impregnated with polyphenylene sulfide (PPS) or polyetheretherketone (PEEK) resins.
Automated tape laying (ATL) provides one means of reducing touch labor, shortening manufacturing time and cutting composite part costs. (Toray Advanced Composites)
GKN Aerospace induction-welds the fiber-reinforced PPS rudder and elevator of the Gulfstream G650 business jet. Eddy currents in the conductive carbon fiber heat laminate plies from the inside and fuse mating parts without fasteners or adhesives. “This is 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.
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.”
The Bell V-280 Valor has flown with thermoplastic composite ruddervators that eliminate fasteners to cut cost and weight vs. thermosetting composite parts. (Bell)
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 not much difference in material properties from an 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, with automated fiber placement thermoplastics for fuselage- or wing-size parts, you don’t have to apply pressure.” The materials engineer said, “That part of the industry is not mature yet… We’re not there yet, but it’s coming.”
Thermoplastic and thermoset composites have broad application in different parts of AAM vehicles. (Toray Advanced Composites)
Thermoplastics may make air taxis more crashworthy and damage resistant. Howell said, “Thermoplastics fail much differently than thermosets. It’s graceful failure. The fibers are still going to work, but the matrix has a lot more strain-to-failure. Instead of the pieces falling apart, the polymers still hold together. It’s not as catastrophic, but it’s still going to fail. It’s going to help because it’s higher strain to failure in the matrix material.”
Jaunt's Peryea noted that, “All the non-destructive inspection processes apply to thermoplastics.” He said, “We are really designing to commercial aviation design practices, for which the frequency of inspection is reduced compared to the design practices currently used in the rotorcraft industry.”