September 16, 2019
On November 2, 1947, the H-4 Hercules took flight. This 200-ton aircraft, designed by Howard Hughes and popularly known as the Spruce Goose,
featured a pioneering innovation.
World War II demands had metal in short supply at the time of the Spruce Goose’s construction. Being an inventive engineer, Hughes constructed the wing spars from a composite of thin layers of birch wood and plastic resin, marking one of the earlier uses of composite materials in aircraft design.
Since the H-4 Hercules, composite materials have evolved significantly. Their lightweight properties and superior strength present ideal solutions to design problems across industries, leading to increased applications in manufacturing.
This is especially true in aerospace manufacturing, where the use of composites has doubled every five years since 1987. With technological advancement and increased demand, however, have come a host of difficulties in sourcing and management.
To understand these challenges, it is important to understand the nature of composites and how they relate to aircraft design and production.
First, a basic definition.
A composite is two or more different materials combined to create a superior product. The combination creates improvements in the new product (stronger, lighter, more flexible) that the individual materials cannot achieve separately.
This technology dates back to the ancient Egyptians, who combined mud and straw to create stronger bricks, more resistant to shrinkage.
Modern composites, or fiber reinforced polymers (FRPs), are constructed by embedding continuous straight fibers (e.g. carbon, glass, aramid) in a polymer matrix (e.g. phenolic, polyester, epoxy). The materials are laminated layer-by-layer to build up a final structure, which can be used for a multitude of industrial applications.
In aircraft design and manufacturing, FRPs have been revolutionary.
Perhaps the most significant benefit of composite applications in aircraft manufacturing relates to weight. Carbon fiber-epoxy mixes are the most common FRP in aerospace, being significantly lighter than metal-based materials while still retaining strength.
A lighter aircraft has a better lift-to-weight ratio, meaning improved fuel-efficiency. With increasing fuel prices and environmental lobbying pressures, this is of critical importance. Weight reduction also allows for the possibility of longer flights.
In addition to creating lighter, more fuel-efficient aircraft, the use of composites helps reduce operating costs for manufacturers. FRPs are easier to manipulate into complex and custom shapes than metal. This customizability means Original Equipment Manufacturers (OEMs) can:
Reduce the number of parts making up a component
Reduce the need for fasteners and joints (which often create weak points in a component as rivets cause stress concentrations that lead to cracks)
Reduce assembly time as a result of fewer parts
Composites hold an advantage over metal-based materials from an engineering perspective as well. Conventional metal materials are usually isotropic, meaning they have the same strength and stiffness in all directions. Fibrous composites, however, are anisotropic. Their strength and stiffness depend on the direction of the load in respect to the orientation of the fibers. This is sometimes overcome by stacking multiple layers with the fibers running different directions.
More ingeniously, the anisotropy of composites can be exploited to help reduce material usage. Layers of laminate are stacked in a specific sequence designed to withstand the precise loads it will be subjected to. This means a more efficiently crafted aircraft and reduced material usage.
Further benefits of FRP usage in aircraft manufacturing include:
High damage tolerance: increased survivability for passengers in case of an accident
Fatigue and corrosion problems practically eliminated: maintenance costs significantly reduced
Thermal stability: fiber composites don’t expand or contract excessively with fast and extreme temperature changes (100⁰F on ground vs -67⁰F at altitude)
These benefits make clear why aerospace OEMs have increased demand for FRP materials. The Boeing 787 Dreamliner, a commercial aircraft with an airframe comprised of 50% composites, affirms that increased application is a continuing trend.
But while the Boeing 787 symbolizes the future of composites application in aerospace, it is also representative of the difficulties facing the sourcing and distribution of these materials.
For further discussion of these issues, read Part 2 of this article series: Supply Chain Woes.