Currently, the design and manufacturing technology of unmanned aerial vehicle structures with composite materials as the core is one of the key technologies influencing the development of drones. What kind of fuselage materials and structural manufacturing processes are more suitable for the needs of drones? Today, we will start from topics such as drone structures, manufacturing processes, commonly used drone materials, etc., to communicate and discuss with everyone.
Common materials for drones
The common materials used in drones include body materials (including structural materials and non-structural materials), engine materials, and coatings. The most important ones are the body structural materials and engine materials. Structural materials should have high specific strength and specific stiffness to reduce the structural weight of drones, improve flight performance, and also have good processability for easy manufacturing of required components.
The speed of the drone determines the load on the fuselage and also determines the choice of materials for the aircraft. The structure of low-speed drones mainly uses wood, plastic, glass fiber, or carbon fiber composite honeycomb sandwich structures. High-speed drone structures are mostly made of aluminum alloy as the main material, with appropriate use of titanium alloy or carbon fiber composite materials and other materials that are compatible with advanced manufacturing processes to increase load capacity and reduce structural weight.
1.Composite materials
Composite materials are mainly made by bonding base materials and reinforcing materials in a specific process. The most commonly used composite materials on drones are carbon fiber composites and glass fiber composites.
Composite materials have the advantages of being lightweight, high in specific strength and modulus, strong in fatigue resistance, and strong in earthquake resistance. At the same time, according to different processing techniques, it has anisotropic characteristics. It is inherently designable and can be optimized for aircraft strength and stiffness requirements without changing the structural weight. The corrosion resistance of composite materials can meet the special requirements of long storage life under harsh conditions for unmanned aerial vehicles, reducing the lifecycle cost of use and maintenance.
However, the conductivity of composite materials is poor, making them susceptible to lightning strikes in thunderstorm weather, leading to damage to the aircraft and electronic equipment. In addition, their repairability is also poor, with a significant decrease in mechanical performance after repair.
Sheets, bidirectional carbon sheets, carbon rods, carbon tubes, etc., are mainly used in the reinforcement parts of micro and light unmanned aerial vehicles.
2.Carbon fiber and glass composite materials
Carbon fiber and glass composite materials are mainly used for two purposes on drones: integral molding and local reinforcement.
Currently, with the development of drone manufacturing technology, the former has a relatively wide range of applications. The use of integral molding to manufacture drone components such as fuselage, wings, and tail fins has advantages such as lightweight and mass production. However, its repairability is relatively poor. Fiber materials used for local reinforcement mainly include unidirectional carbon sheets, bidirectional carbon sheets, carbon rods, carbon tubes, etc., which are mainly used in micro and light unmanned aerial vehicle (UAV) local reinforcement parts.
3.Materials for Micro Drones
The weight of a typical micro drone is only tens to hundreds of grams, with a speed of several tens of kilometers per hour. The surface of the aircraft bears relatively small loads, so the strength requirements for the materials used in the body are lower than those for traditional drones. Due to power limitations, micro drones have very strict requirements on their own weight, requiring materials with minimal density as much as possible.
Currently, the structure of micro drones mostly uses lightweight materials. Common non-metallic materials include wood (including light wood, paulownia wood, laminates, oak wood, birch wood), composite materials (including Kevlar material, carbon fiber composite material, fiberglass composite material), nylon, textiles and plastics.
4.Lightweight drone materials
Lightweight drones have low speed, and the fuselage and wings bear relatively low loads, so the emphasis in material selection is on “lightweight”.
The typical material selection for light unmanned aerial vehicles includes alloy steel, aluminum alloy, and other metal materials for the main load-bearing parts such as the fuselage and wing landing gear (skis), while lightweight materials such as aviation laminates are used for secondary load-bearing parts like fuselage frames. The body panels, skin, fairings, etc., are made of glass honeycomb sandwich materials or glass fiber-carbon fiber hybrid structural materials.
The composite materials used for light drones are mainly glass fiber reinforced resin-based composites, which reduce costs while ensuring strength.
5.Materials for small and large drones
With the increase in flight speed, the aircraft load increases, and unmanned aerial vehicle materials also transition to metal structures. The amount of aluminum alloy used in high-speed UAVs has significantly increased, with some using titanium alloy. Currently, aluminum alloy is mainly used as the framework for small and large UAV materials, while carbon fiber reinforced resin-based composite materials are widely used in other parts.
With the development of composite material technology, the amount of composite materials used in large drones is increasing, gradually replacing aluminum alloys and becoming the main material for high-altitude drones. In modern manned aircraft, composites account for up to 50%, and in advanced unmanned aircraft currently, the amount of composite materials far exceeds 50%. For example, except for the main structure of the fuselage of Global Hawk, all other structures are made of composite materials, with composites accounting for 65% of the fuselage. Even many famous drones have composite material usage exceeding 90%.
In the category of composite materials, there are differences between large drones and light drones. The composite materials used in light drones are mainly glass fiber and aramid fiber reinforced composites, while most of the composite materials used in large drones are carbon fiber reinforced resin-based composites with better performance and higher prices. In order to reduce costs or adjust performance, some glass fibers and aramid fibers are mixed in.
Manufacturing process of structure
In the design and manufacturing of carbon fiber composite drones, it is often achieved by adjusting the laying angle and material layers to meet the structural elasticity and stiffness distribution requirements. Carbon fiber composite materials are one of the most commonly used reinforcement materials in drones. The production of drones using carbon fiber composite materials generally involves the following three molding processes:
One is the hot press tank forming process. Hot press tank forming is one of the high-performance forming processes for composite materials. For drones that require high speed, this process is often used to manufacture composite components and main load-bearing components. The internal quality of carbon fiber composite components formed by hot press tank molding is good, with a relatively uniform resin content, and the mechanical properties are also excellent.
However, the hot press tank forming technology also has certain shortcomings. This process requires high equipment requirements, and both the initial investment and processing costs are relatively high, resulting in poor economic efficiency. From a cost perspective, when budget is limited, low-temperature and low-pressure forming technology is often chosen as an alternative. In addition, during the hot press tank forming process of carbon fiber composite materials, interactions such as resin flow, heat transfer, chemical cross-linking, and void formation will occur which increases the difficulty of process control. Once there is a mistake in control, processing defects such as inadequate resin content or high porosity may occur.
The second is the vacuum bag molding process. Compared with the hot press tank process, the vacuum bag molding process is relatively simple, and does not require high initial investment in the early stage, with a moderate level of operational difficulty. However, this molding method has lower pressure and is only suitable for composite components with low quality standards requirements, mostly used in manufacturing honeycomb sandwich structures and laminated plate structures not exceeding 1.5mm. Due to its cost advantage, this processing method is widely used in the manufacturing of low-speed drones because vacuum bag molding technology can meet most of the production requirements for parts in small-scale low-speed drone manufacturing.
In practical operation, because the forming of vacuum bags requires processes such as pre-impregnated material laying and wet lay-up, and wet operations are influenced by human factors, it is easy to cause uneven coating of adhesive solution. This situation is particularly evident in the formation of sandwich structures. In addition, unreasonable brushing direction can also easily lead to bending and alteration of fiber orientation, threatening the stability of carbon fiber composite manufacturing components.
The third is compression molding process. Compression molding process is more suitable for the manufacturing of foam core composite components, which combines the advantages of hot press tank forming technology and vacuum bag molding process. It has high production efficiency and large forming pressure, moderate equipment investment and cost, and good economy. The parts such as drone control surfaces that use foam sandwich structure mostly adopt this molding process. In the manufacturing of carbon fiber drone wing panels, using compression molding process can also ensure the appearance quality and wing shape accuracy of drone wings, improving the overall manufacturing quality of drones. However, pressure control is the most critical step in this process.
The application of carbon fiber composite materials is an important trend in the development of drone manufacturing, and it is also an effective way to meet the requirements of drone range, flight time, altitude, manufacturing cost, and stealth. The structural design and manufacturing process optimization of carbon fiber composite material drones are currently key to the application of carbon fiber drones. The former requires efforts from drone designers, while the latter requires continuous experimentation and innovation in the research and application of carbon fiber composite materials. The increasing proportion of carbon fiber applications in drones will effectively improve China’s level of drone manufacturing.
Characteristics of drone structure
Compared to manned aircraft, drones are smaller in size, do not require cockpit space, and can save on personnel training costs. Only a small number of pilots are needed to operate the drone from the ground, eliminating personnel losses. With the advancement of intelligent technology, as long as the program is reasonable, there is basically no risk of operational errors leading to losses. According to analysis, drones have the following characteristics:
1.Small in size and light in weight. Compared with the Predator drone and U2 high-altitude reconnaissance aircraft, the Predator has a wingspan of 14.85 meters, a total weight of 1420 kilograms, fuel capacity of 295 kilograms, a range of 3704 kilometers, and an endurance time of 24 hours; while the U2 has a wingspan of 24.38 meters, a total weight of 13154 kilograms, fuel capacity of 4350 kilograms, a range of 4700 kilometers, and an endurance time of 8 hours. The Predator is equivalent to three times the efficiency of the U2. Drones do not require life support systems and can reduce weight by thirty to forty percent.
2.The design load is small, and the structural strength is low. Automatic flight, standardized movements, relatively low flight overload; no risk of personnel casualties, low structural safety factor. Under the same grade of equipment, the design load is 40% less than that of manned aircraft. No need for personnel to enter or exit passages, fewer openings. Direct transmission of structural forces, good overall integrity, not serious issues with concentrated stress on key structures, and low requirements for aircraft fatigue life.
3.Use more integrated molding processes, with low processing equipment requirements. New materials and advanced composite materials are widely used in drones. Generally, reinforced fiber pre-impregnated tapes with matrix resin are used. According to design requirements, they are layered on molds that have been pre-processed into the desired shapes and then solidified into an integral shape through a hot pressing process. Complex large structural components formed by multiple small components assembled together using fasteners. For example, ribbed wall panels can be integrally molded using processes such as initial curing, secondary curing, or secondary bonding. Recently, there have been textile preforms or resin transfer molding processes such as sewing and weaving to reduce mechanical machining and assembly workload. In short, significant weight reduction and manufacturing cost reduction are achieved.
4.Easy to maintain, low lifecycle cost. Drones have three low-cost characteristics: low material costs, processing costs, and maintenance costs. Overall, the cost is basically proportional to the weight of the aircraft. Manned aircraft are heavy with high costs; in terms of maintenance and operation, there is a ratio of 1.3 pilots to 1 aircraft. Drones are light with low costs; in terms of maintenance and operation, there are multiple flights for each pilot compared to one aircraft, without the need for maintenance personnel or expensive training fees. The unit price of drones is seventy percent less than manned aircrafts’. Additionally, drones do not require maintenance storage like missiles do; when comparing the lifecycle cost between drones and missiles that can be launched interchangeably by medium-sized unmanned attack aircraft executing 15 missions using non-maintenance storage versus similar combat effectiveness one-time-use Tomahawk missiles (priced at $1.4 million each), the cost-effectiveness is much higher for drones.
