Unmanned Aerial Vehicles (UAVs)
Unmanned aerial vehicles (UAVs) have been developed and used for many years. There are other terms to describe this market, including drones, generally used to describe small systems, and unmanned aerial system (UAS), often used more frequently by military organizations. The U.S. Air Force has recently introduced the term ORB to describe small-to-medium manned and unmanned aerial systems that are autonomous or semi-autonomous. ORBs can carry people, cargo, and/or sensors and can be used for commercial and military applications. We will focus here on UAVs and the materials and aerostructures needs of this market.
Lightweight
All aerospace systems need to be lightweight. The lighter the structure, the more efficient it operates, the longer range it can cover, the greater payload it can carry, and the longer it can stay aloft. Since UAVs fly unmanned, they require sensors, cameras, and electronics. Reducing the weight of the structure allows it to carry more sensors, more payload, and/or stay up in the air longer. Small UAV drones primarily rely on batteries for power, and batteries are heavy, so this creates further need to reduce the weight of the rest of the structure. Today, almost all UAV structures are made from carbon fiber composites. This is in contrast to piloted aviation, where a large percentage of the structure today is made from aluminum and titanium in addition to carbon fiber composites. Newer commercial passenger-carrying systems use around 50% carbon fiber composites, and this will likely further increase on future platforms.
Composites offer several advantages for lightweight aerial vehicles. Carbon fiber is inherently lightweight with a density around 2 grams per cubic centimeter (g/cm3). For reference, water is 1 g/cm3, aluminum is 2.7 g/cm3, and titanium is 4.5 g/cm3. For carbon fiber composites, the carbon fiber is embedded in a matrix material that is generally an epoxy or thermoplastic material, and these typically have a density between 1 to 1.4 g/cm3. Carbon composites are usually composed of 35% to 45% carbon fiber, so the overall density of the composite is in the range of 1.3 to 1.6 g/cm3.
Another important measure is stiffness-to-weight ratio, also referred to as specific stiffness or specific modulus. Stiffness is the measure of how much a material stretches when a load is applied. A stiffer material will stretch a smaller amount for a given load than a less stiff material. The higher the specific stiffness, the better the material for a given stiffness critical structural application. High specific stiffness materials are generally used in aerospace. For reference, titanium has a specific stiffness of 25, aluminum has a specific stiffness of 26, and carbon composites have a specific stiffness of 113 [Wikipedia – Specific Stiffness]. For aerospace applications, stiffness is very important. For aerodynamics, you want the structure to remain relatively rigid to keep its aerodynamic shape. Also, stiffness is very important for rotating blades such as rotors, propellers, or engine fan blades and for structures undergoing pressurization cycles.
Similar to stiffness-to-weight ratio, aerospace structure design also needs high strength-to-weight ratio materials. This is also referred to as specific strength. Strength is the amount of load a structure can take before it breaks or fails. The higher the specific strength, the better the material will be for a given structural load. As discussed, carbon fiber has high stiffness, but it can break with a small amount of elongation. Metals tend to stretch and deform significantly before breaking due to plastic deformation. We know that a metal structure tends to dent when subjected to a high load, and it can take a significant amount of force to tear and fracture the metal. Carbon fiber composites don’t permanently deform but will fracture after a small percentage of elongation. Because carbon composites are very stiff, it takes a lot of load before they will break. For aerospace, high specific stiffness and high specific strength materials are preferable, and the combination is the key driver for material selection. Stiff materials that keep their shape under load are better than materials that deform permanently, and they should withstand high flight loads and not break. For reference, aluminum has a specific strength of 115, titanium has a specific strength of 76, and carbon composites have a specific strength of 785 [Wikipedia – Specific Strength]. As you can see, carbon fiber composites have high specific stiffness and strength, which makes it the material of choice for aerospace applications. A good representation of this is shown in Figure 1, where various materials are mapped out for specific stiffness and specific strength. As shown, carbon fiber composites are in the upper right quadrant showing a good combination of specific strength and stiffness.
Figure 1 – Specific Stiffness vs. Specific Strength
(Courtesy www-materials.eng.cam.ac.uk)
Another aspect of lightweight structures is design and assembly. One aspect of a design that can add weight is the use of fasteners. Drilled holes in the structure for fasteners to connect components can add significant weight because these holes weaken the structure. Attachment points also become stress concentrations so they can see higher local loads than the surrounding structure, so more material structures through such technologies as infusion, forming, and molding processes that will reduce the need for fasteners are needed. Additive manufacturing, using carbon-fiber reinforced materials, can also allow for the production of complex geometry components that traditionally would require complex attachments.
Honeycomb core materials are commonly used in aerospace systems. Structural stiffness, especially in bending, is enhanced by thickness. Honeycomb core materials, inherently lightweight material structures because they are composed mostly of air, are used to increase thickness without adding much weight. A typical use of honeycomb core material is to sandwich it between two composite face sheets. This takes advantage of the stiffness and strength of the composite while increasing the structural bending stiffness with minimal added weight. Honeycomb core materials are made from plastic and paper-based systems or aluminum. The honeycomb core material can also improve impact survivability of the structure and can help to dampen sound coming from engines and propulsion systems.
Reliable
Carbon fiber composites have been used in Aerospace applications for over 50 years. They have been used as primary structures on military and commercial aircraft and rotorcraft for well over 30 years. Given this history, the behavior, response, and lifetime of carbon composite structures are well known and understood. They are approved for use by the Federal Aviation Administration (FAA) and European Union Aviation Safety Agency (EASA). Significant material and structural testing have been carried out on carbon composites, and the design guidelines are well established. Reliable operation is critical for passenger aviation to prevent loss of life, but it is also very important for unmanned systems. If the UAV were to fail in a military application, the loss of information could lead to loss of life. Failure of UAVs for commercial applications could also lead to loss of life if it came down over a populated area.
Composites can contribute to the electromagnetic properties of the UAV system. Since these systems are unmanned, they require efficient and reliable communication with ground stations through wireless or satellite communications. Composites can be tuned to absorb certain electromagnetic frequencies and pass other frequencies. Composites are used for radomes that protect the transmit and receive antennas, and the radome materials and structure can be engineered to allow for efficient communication while blocking out signals that are coming from other sources. Honeycomb core materials are also commonly used in radomes. Carbon composites are a key component in stealth technologies, as they offer the ability to “hide” the UAV from enemy detection.
Composite materials offer even more properties that enhance their reliability. They do not corrode, which eliminates the need for corrosion inspection and mitigation. Composites are very fatigue resistant, so unlike metals, they won’t form cracks over time of repeated cyclic loading. Composite structures have been a part of military aviation and space operations for decades, and they perform very well under extremely harsh environmental and thermal loading. Composite structures can be repaired with well-established patching methods, and at the end of service life, the materials can be recycled and reused in other applications.
Affordable
Carbon composite can deliver more strength per unit weight than most metals. Upfront costs per unit weight are higher than most metals, but the total lifecycle cost of carbon composites is lower. Lifecycle costs factor in the cost of building the UAV, the maintenance, inspection and refurbishment costs, operational costs, and the end-of-life retirement costs. From a weight and strength perspective, composites are competitive with metals for high-rate production of small and large parts on the aircraft, and this includes attritable aircraft that may only have a single use.
There are many existing and emerging manufacturing technologies that can significantly reduce the product development and manufacturing costs of building the UAV. These include tooling, automated manufacturing, direct processing, and additive manufacturing.
A typical aerospace composite material construction is the prepreg. The prepreg is a unidirectional sheet of carbon fiber that is pre-impregnated with a resin or epoxy. These prepregs are pliable and flexible at room temperature and become more pliable and flexible as you increase the temperature. At elevated temperature the epoxy cures and hardens to become rigid to form the final shape of the structure. Because of the flexibility of the prepreg below the curing temperature, these plies can be stacked and formed on top of each other and then the laminate cured.
Thermoplastic matrix materials are also available, and these can also be formed into prepregs. The difference is thermoplastic materials do not cure but they crystallize on cooling from an elevated temperature, making them rigid. If you reheat the thermoplastic composite, you can re-form or repair the material, whereas with epoxy-based systems, once they are cured, they remain rigid. It is also important to note that epoxy-based prepregs need to be kept in a freezer until they are used, as curing can occur slowly at room temperature. Epoxy prepregs have a shelf life in the freezer beyond which they cannot be used. Thermoplastic materials do not have this limitation, as they can be stored at room temperature until their use. Thermoplastics are generally more expensive than epoxies, but there are additional handling and storage costs with epoxy-based composites. Depending on the use case and manufacturing method, thermoplastic composites may have higher or lower total production costs compared to epoxy-based systems.
For making composite laminated structures, one needs a tool surface on which to lay down the individual prepreg plies, and tooling can be expensive to machine. Steel tooling can also cause issues because the steel expands at a much higher rate than the composite, so upon heating to the cure temperature, this thermal expansion mismatch can lead to warping problems with the composite. As a result, the composites industry has developed composite tooling materials that are easy to machine, have a thermal expansion similar to the composite, and can hold up well under repeated heating and cooling cycles. Composite tooling has a much higher return on investment compared to machined steel tooling, so this is more attractive to smaller lot production runs.
Laminates are often made with hand lay-up processes. The prepreg plies are cut and then laid down and stacked on a tool surface. Hand lay-up provides a lot of flexibility and can be quite useful during the prototyping stage but can be very inefficient for production operations. Over the last two decades, automated lay-up equipment has been developed and used extensively in aerospace production. Automated Tape Lay-up (ATL) and Automated Fiber Placement (AFP) machines have been developed by several equipment manufacturers. These are essentially robotic systems that lay down tape or fiber tows onto a tool surface. These machines are much faster than hand lay-up systems, require much less human labor, and are more repeatable. Computer software is available to translate a computer-aided design (CAD) file of the laminate configuration into machine programming to run the AFP/ATL machine much in the same way as computer-aided machining (CAM) software is used today for machined parts. Other automated systems are available for pultruded composite beams, filament winding, thermoforming, ply cutting, and fiber patch placement. All of these systems reduce the overall costs of composite structure manufacturing by significantly reducing labor costs and increasing manufacturing throughput in a reliable and repeatable manner.
Direct processing methods are another way of taking advantage of the material properties of composites while reducing the costs of producing parts. One of these methods is resin infusion onto a dry laminate or fabric. Here, instead of pre-impregnating a fiber sheet with resin, the fiber architecture is laid down in a tool and then the resin is infused into the structure. Fiber tapes and fabrics are available that have a small amount of plastic on them so they can tack down to adjacent layers to hold the structure together before resin injection. A mold is then closed onto the fiber preform and resin is either injected under pressure into the tool in a Resin Transfer Molding (RTM) process or pulled into the tool using Vacuum Assisted Resin Transfer Molding (VARTM) process. Both RTM and VARTM are done at elevated temperatures to help the resin flow into the preform. Once the resin has fully impregnated the pre-form, it is cured to harden the structure. This process can also be used with thermoplastic matrix materials where the thermoplastic material hardens as it cools down but can be reheated to reform if needed. Direct infusion processing methods can significantly reduce costs by eliminating some steps in the overall process. Dry fiber preforms can often be constructed less expensively than laying up a laminate of prepreg. Preforms can also incorporate braiding, fabrics, or stitching to create more three-dimensional fiber architectures in the structure. The infusion and curing process can occur in a single step, saving time and cost.
Chopped fibers can also be included in a thermoplastic material used for injection molding. Traditional injection molding plastics and equipment can be used that are suited for high-volume, low-cost production. Adding chopped fiber into the thermoplastic can provide added stiffness and strength to the injection-molded part, which can reduce the overall weight of the part to meet the structural requirements. Cost of injection mold tooling can be high, and the fibers can increase the wear on the tool, so this is applicable only for high-volume applications. For many of the drone applications that are high volume, injection molding of fiber-reinforced plastics can make sense. Overmolding can also be used here to provide a different material on the outside of the structure.
Additive manufacturing is another technology that can reduce the overall structure costs for a UAV system. Additive manufacturing relates to manufacturing processes that build up a structure layer by layer. One can claim composites as the original additive manufacturing process, where layers were used with hand lay-up to create the structure. A variety of additive manufacturing technologies and methods have been developed over the years, and companies are actively exploring and implementing this technology. AFP/ATL processes are clearly additive manufacturing methods for composite laminate structures. These processes continue to be refined using more sophisticated robotic systems to create complex three-dimensional geometries with continuous fiber. Additionally, chopped fiber is being incorporated into existing additive manufacturing methods to create composite structures. Fused deposition modeling (FDM) and selective laser sintering (SLS) both have the ability to incorporate chopped carbon fiber to create parts that are stiffer, stronger, and lighter weight than parts with just the plastic alone. These methods are particularly well suited for brackets, ducts, and handles, where the ability to create the complex shapes without tooling provides a significant cost advantage.
Conclusions
The UAV industry will continue to grow and provide value to commercial and military organizations as well as consumers. These vehicles can help keep us safe, deliver the goods we need and use in a more efficient manner, and protect those who defend our freedoms both at home and abroad. Lightweight, reliable, and affordable materials will help to enable this industry moving forward. Carbon composite materials, honeycomb core structures, additive manufacturing, and dielectric tuned materials are key parts of this industry today and will continue to enhance the functionality and efficiency of these vehicles in the future.
