Unmanned aerial vehicle (UAV) has been developed and used for many years, and there are some other terms used to describe this market, including unmanned aerial vehicles (Umanned aerial vehicles) that are commonly used to describe small systems and unmanned aerial vehicles (Unmanned aerial system, UAS).
The US Air Force introduced the term ORB to describe autonomous or semi-autonomous small and medium-sized manned and unmanned aerial systems. ORB can carry people, objects or sensors, and can be used for commercial and military applications. Here, we will focus on drones and the market's demand for materials and aircraft structure. Currently, almost all UAV structures are made of carbon fiber composite materials.
Lightweight
All aerospace systems need to be lightweight. The lighter the structure, the higher the operating efficiency, the wider the coverage, the greater the payload, and the longer the stay at high altitude. Because drones are unmanned, they require sensors, cameras, and electronic equipment. Reducing the weight of the structure means that it can carry more sensors, more payload and stay in the air longer.
Small drones rely mainly on battery power, and the battery is very heavy, so this further needs to reduce the weight of the rest of the structure. Today, almost all UAV structures are made of carbon fiber composite materials. In sharp contrast, most of today's commercial aircraft structures are made of metals such as aluminum and titanium in addition to carbon fiber composite materials. The latest commercial aircraft systems use about 50% carbon fiber composite materials, and applications may increase further in the future.
Composite materials provide some advantages for light aircraft. Carbon fiber itself is light in weight and has a density of less than 2g/cm3. For comparison, the density of water is 1g/cm3, aluminum is 2.7g/cm3, and titanium is 4.5g/cm3. For carbon fiber composite materials, carbon fibers are usually embedded in a matrix material of epoxy resin or thermoplastic material, and the density of these materials is usually between 1 and 1.4 g/cm3. The carbon fiber composite material is usually composed of 35% to 45% carbon fiber, so the total density of the composite material is between 1.3 and 1.6g/cm3.
Another important reason is that carbon fiber has a high stiffness-to-weight ratio, also known as specific stiffness or specific modulus. Stiffness is a measure of the extent to which a material stretches when a load is applied. At a given load, the harder material will stretch less than the lower material. The higher the specific stiffness, the better the material applied for a critical stiffness structure.
High specific stiffness materials are widely used in the aerospace field. Generally speaking, the specific stiffness of titanium is 25, the specific stiffness of aluminum is 26, and the specific stiffness of carbon fiber composites is 113. For aerospace applications, stiffness is very important. For aerodynamics, it is desirable for the structure to remain relatively rigid to maintain its aerodynamic shape. In addition, stiffness is very important for rotating blades (such as rotors, propellers, or engine fan blades) and structures that withstand boost cycles.
Strength to weight ratio (specific strength)
Similar to the stiffness-to-weight ratio, aerospace structural design also requires materials with a high strength-to-weight ratio, which is also known as specific strength. Strength is the amount of load the structure can bear before it breaks or fails. The higher the specific strength, the better the performance of the material under a given structural load.
As mentioned earlier, carbon fiber has high stiffness, but it can break with little elongation. For metals, due to plastic deformation, metals tend to stretch and deform significantly before breaking. Metal structures sag easily when subjected to high loads, and may require a lot of force to tear and break the metal. Carbon fiber composites will not permanently deform, but will break with very little elongation.
Because the carbon fiber composite material is very hard, it needs to bear a large load before breaking. For aerospace, high specific rigidity and high specific strength materials are the first choice, and the combination of the two is the key driving force for material selection. The rigidity of its shape under load is better than that of permanently deformed materials, they should be able to withstand high flight loads without breaking. The specific strength of aluminum is 115, the specific strength of titanium is 76, and the specific strength of carbon fiber composite is 785.
Therefore, carbon fiber composites have a high specific stiffness and strength, which makes it the material of choice for aerospace applications. Figure 1 below shows the excellent specific strength and specific stiffness of carbon fiber.
Easy to design and assemble
Another important aspect of lightweight construction is design and assembly. One aspect of the design that can increase weight is the use of fasteners. Drilling holes used in the structure for fastener connection parts will increase the weight, and these holes will weaken the structure.
Connection points will also become stress concentration points, so they can withstand higher local loads than surrounding structures, so casting, forming, and molding techniques are needed to reduce the need for fasteners. Additive manufacturing techniques that use carbon fiber reinforced materials can also produce components that traditionally require complex geometries.
Honeycomb material with excellent performance and lighter weight
Honeycomb core materials are commonly used in aerospace systems. Structural stiffness, especially bending stiffness, will increase with increasing thickness. Since the honeycomb core material is mainly composed of air, it has an inherently lightweight material structure, which is used to increase the thickness without adding much weight. A typical use of honeycomb core material is to sandwich it between two composite panels. This makes full use of the stiffness and strength of the composite material, while increasing the bending stiffness of the structure with minimal additional weight. The honeycomb core material is made of plastic and paper-based systems or aluminum. Honeycomb core materials can also increase the impact life of the structure and help suppress noise from the engine and propulsion system.
Reliability
The application of carbon fiber composite materials in the aerospace field has a history of more than 50 years. They have also been used as the main structure of military and commercial aircraft and rotorcraft for more than 30 years. Carbon fiber composite materials have been approved for use by the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA), and have undergone important material and structural tests and developed design guidelines. Safe and reliable operation is the key to preventing casualties in civil aviation, but it is also very important for unmanned systems. If the UAV fails in military applications, the loss of information may cause significant losses.
Other excellent features
Composite materials can improve the electromagnetic properties of UAV systems. Because these systems are unmanned, they need to communicate efficiently and reliably with ground stations via wireless or satellite communications. Composite materials can be tuned to absorb certain electromagnetic frequencies and pass other frequencies. The composite material is used to protect the radome of the transmitting and receiving antennas. The material and structure of the radome can be designed to allow effective communication while shielding signals from other sources. Honeycomb core materials are also commonly used in radomes. Carbon fiber composite materials are also a key component of stealth technology, because carbon fiber composite materials can make drones "hidden" from being discovered by the enemy.
Composite materials provide more performance and further improve their reliability. They will not corrode, so there is no need for corrosion inspection and repair. Composite materials have strong fatigue resistance, so unlike metals, they will not form cracks under repeated cyclic loading. Composite structures have been part of military aviation and aerospace operations for decades, and perform well under extremely harsh environments and thermal loads. The composite material structure can be repaired using mature repair methods. At the end of its service life, the material can be recycled and reused, and reused in other applications.