Developing Thermoplastic Composites for Use in Commercial Aircraft
Thermoplastic composite materials (TPC) are gaining momentum for use in commercial airplanes and other aerospace applications, including electric vertical takeoff and landing (eVTOL) aircraft. TPC were once considered too expensive for applications other than small components. Now, material and processing advances are propelling TPC into the aerospace industry spotlight.
Why Thermoplastics? Why Now?
TPC are replacing thermoset composite materials (TS) and metal materials in a growing number of aerospace applications because TPC components and parts are lighter, more durable and more efficient to process, depending on the application. TPC properties also open design and manufacturing options not possible or easily achievable with TS or metals.
From an environmental sustainability standpoint, the lighter weight of TPC parts and structures helps airplane original equipment manufacturers (OEM) reduce aircraft fuel consumption and emissions. The weight reductions are dramatic – well over 2,000 pounds in some cases. In addition, TPC can be readily recycled and reused, whereas TS recycling, such as with pyrolysis, is a complex and energy-intensive process.
TS parts typically must be cured in an autoclave. TPC can be cured in-situ, during the assembly process. New automated fiber placement (AFP) and automated tape laying (ATL) equipment includes laser heads, which cure the TPC instantly as the fiber or tape is laid down. TPC are also compatible with compression molding, continuous compression molding, braiding and 3D printing manufacturing methods. In many applications, after the TPC undergoes these processes, there is little to no post-processing required other than some final trimming.
TPC opens the door for aircraft components and structures to be made with fewer overall pieces and steps. For example, a component that traditionally was made by fastening two or three parts together might now be a single, welded TPC component that, in turn, can now be welded onto the aircraft. This means there is less material waste, improving the buy-to-fly ratio.
TPC is still considerably more expensive than TS and some metals, but the economic model to use them makes sense now due to advances in TPC processing equipment. With this automated equipment, TPC component manufacturing is much faster and more efficient than TS production processes. The ability to weld TPC is important to their growing adoption. When OEMs can weld TPC parts instead of joining pieces with fasteners, adhesives, and brackets, they save weight and manufacturing steps. In addition, weldability lends itself to modular assembly processes.
For example, different parts and subsystems of an airplane can be made by off-site suppliers and then welded and wired into the plane body. This approach is commonly used in the automotive industry, known for its modular manufacturing efficiencies. By comparison, most commercial aircraft have a very long manufacturing cycle today.
All in all, OEMs can experience weight savings of 20 percent or more with greater TPC use. The finished product can be 30 percent to 40 percent less costly when the total cost of manufacturing is considered.
In particular, the prospect of out-of-autoclave composite processing is appealing to OEMs because autoclave cycles take time, energy and capital resources. Bottlenecks can build while hours pass, waiting for parts to finish their autoclave cure. In addition, TS prepregs require refrigeration, and even when kept cold, have a much shorter shelf life than TPC, which can be stored at room temperature. TS prepreg's need for refrigeration adds logistical challenges for OEMs and their supply chains to manage.
New applications for TPC in commercial aircraft include spars, stringers, the nacelle and empennage, among others. The European Union’s Clean Sky initiative has made great TPC strides with its Multifunctional Fuselage Demonstrator (MFFD) project. In an article about MFFD, Clean Sky said, “Pivotal to the project’s success is the extent to which composite thermoplastics can be demonstrated to be appropriate for unifying the functionality of systems, cabin and fuselage.”
TS-intensive aircraft are built from the inside out, which limits the OEM’s flexibility for how to outfit the plane interior over time. But a TPC-intensive aircraft is built from the outside in, giving OEMs greater control and choices over how the inside of the aircraft is built out and finished. “With a more modular form of design, it will be possible to adapt and modify cabin interiors if airlines wish to change the cabin elements,” the Clean Sky program said.
In addition to modular design options, TPC offer flexibility for creating contoured designs and complex shapes. TPC are more bendable than metals, making them ideal for circular or tubular structures with a large radius. For example, Daher uses TPC for a large air intake bulkhead for the Rolls Royce Ultra Fan engine. Several meters in circumference, the bulkhead is designed to be assembled in four sections. In a blog post about this project, Daher referred to how TPC help solve the aerospace industry’s “twin dilemmas” — environment and competitiveness.
TPC are likely to be a prime material choice for new blank sheet aircraft programs. For these programs, OEMs have a clean slate in terms of their chosen material-and-processing paths. It’s an opportunity to set up new manufacturing lines designed specifically for efficient TPC modular assembly.
TPC also are growing in popularity for eVTOL aircraft. The economic model for these aircraft requires that they be made much faster and less expensively than larger commercial airplanes. If they are limited by autoclave and freezer capacity, eVTOL OEMs are less likely to successfully scale up production. Instead, they need materials and manufacturing processes to finish aircraft parts in seconds vs. hours.
TPC are especially desirable for use in eVTOL aircraft propeller blades. TPC material imbues the blades with toughness and damage resistance so that they can withstand great stress. eVTOL aircraft have many propeller blades, so blade durability is important to the overall value proposition OEMs offer to their customers. The tougher the blade, the longer its lifespan.
Some OEMs are considering TPC for spacecraft launch vehicles, but more testing is required to examine how stable TPC remain amid extreme temperature fluctuations and how durable they are to withstand potential hits from meteor debris.
Specs, Qualification and Quality
Change does not happen overnight in the aerospace industry. Safety standards demand that materials and processes be rigorously tested and validated. Many TS materials, with their long history in aerospace applications, have been qualified by numerous OEMs and certified by the National Center for Advanced Material Performance (NCAMP). TPC are still the new kid on the block when it comes to certification. As of this writing, only one TPC had been certified by NCAMP. Others have been qualified by individual OEMs for specific applications.
TPC supply is relatively immature compared with TS supply. As more TPC are developed and NCAMP certified, TPC adoption promises to take off. With NCAMP certification, TPC materials have an open-source validation for use. TPC component suppliers will be free to work with all sorts of OEMs, beyond the largest players.
It’s also important to ensure material formatting specifications are tailored to TPC’s special nuances and performance characteristics. For example, experienced TPC formatters know precisely what tolerances should be included in a slit tape specification, down to the exact piece of machinery the tape will run on. A formatter also can collaborate with TPC materials suppliers upstream regarding what roll lengths and widths are most suitable for an application. Whether the material needs to be chopped for a compression molding process or slit into super-thin tape for an additive manufacturing process, the formatter can tailor the TPC to the desired end use.
With new TPC materials, processing equipment and welding advances, aircraft OEMs have a wealth of new opportunities for design and manufacturing innovation. The next generation of aircraft promises to be much lighter, greener and efficient to make, thanks in large part to the evolution of TPC.
This article was written by Ashley Graeber, director of sales and new business development, and Jim Powers, global thermoplastics market development manager, Aerospace division, Web Industries Inc. (Marlborough, MA). For more information, visit here .