Composite material-based UAV structural design and manufacturing technology is one of the key technologies influencing UAV development. What kind of fuselage materials and structural manufacturing processes are more suitable for the needs of UAVs? Today, Kzron will discuss and exchange ideas with you on topics such as UAV structure, manufacturing processes, and commonly used UAV materials.
Commonly Used UAV Materials
Commonly used materials for UAVs include airframe materials (including structural and non-structural materials), engine materials, and coatings. The most important are airframe structural materials and engine materials. Structural materials should have high specific strength and specific stiffness to reduce the structural weight of the UAV and improve flight performance. They should also have good machinability to facilitate the manufacture of required components.
The speed of a UAV determines the load on the fuselage, which in turn determines the choice of airframe materials. Low-speed UAVs mainly use wood, plastic, glass fiber, or carbon fiber composite honeycomb sandwich structures. High-speed UAVs often use aluminum alloy as the main material, with appropriate selection of titanium alloy or carbon fiber composite materials and other materials compatible with advanced manufacturing processes. This aims to increase load-bearing capacity and reduce structural weight.
1. Composite Materials Composite materials are primarily made by bonding matrix materials and reinforcing materials using specific processes. The most commonly used composite materials in drones are carbon fiber composites and glass fiber composites.
Composite materials possess advantages such as lightweight, high specific strength, high specific modulus, strong fatigue resistance, and strong seismic resistance. Furthermore, depending on the processing technology, they exhibit anisotropy, inherently allowing for design flexibility. The design can be optimized according to the aircraft’s strength and stiffness requirements without altering the structural weight. The corrosion resistance of composite materials meets the special requirements of long storage life in harsh environments for drones, reducing life-cycle costs for use and maintenance.
However, composite materials have poor electrical conductivity, making them susceptible to lightning strikes during thunderstorms, which can damage the airframe and electronic equipment. Additionally, their repairability is poor, and the mechanical properties are significantly reduced after repair.
2. Carbon Fiber and Glass Composites Carbon fiber and glass composites are mainly used in drones for two purposes: integral molding and localized reinforcement.
Currently, with the development of drone manufacturing technology, the former is more widely used. Manufacturing drone components such as fuselages, wings, and tail fins using a monolithic molding process offers advantages such as lightweight design and mass production capabilities. However, it suffers from poor repairability. Fiber materials used for localized reinforcement primarily include unidirectional carbon fiber sheets, bidirectional carbon fiber sheets, carbon rods, and carbon nanotubes, mainly used for localized reinforcement components in micro and lightweight drones.
3. Materials for Micro Drones
Micro drones typically weigh only tens to hundreds of grams and travel at speeds of tens of kilometers per hour. The load on their airframes is relatively low, so the strength requirements for airframe materials are lower than for traditional drones. Due to power unit limitations, micro drones have very high requirements for their own mass, necessitating materials with the lowest possible density.
Currently, micro drone airframes primarily utilize lightweight materials. Commonly used non-metallic materials include wood (including balsa wood, plywood, veneer, beech, birch, etc.), composite materials (including Kevlar, carbon fiber composites, and glass fiber composites), nylon, textiles, and plastics.
4. Materials for Lightweight Drones Lightweight drones have low speeds and their fuselages and wings bear relatively low loads; therefore, the selection of materials places greater emphasis on lightweight materials.
The typical material selection for lightweight UAVs involves using alloy steel and aluminum alloys for major load-bearing components such as the fuselage and landing gear (skids), lightweight materials like aerospace-grade laminates for secondary load-bearing components such as the fuselage frame, and glass honeycomb sandwich materials or glass fiber-carbon fiber hybrid structures for fuselage panels, skin, and fairings.
Lightweight UAVs primarily use glass fiber reinforced resin-based composite materials to reduce costs while maintaining strength.
5. Materials for Small and Large UAVs
As flight speeds increase and airframe loads grow, UAV materials are transitioning towards metal structures. The use of aluminum alloys in high-speed UAVs has significantly increased, with some using titanium alloys. Currently, small and large UAVs primarily use aluminum alloys for the skeleton, with other parts using a large amount of carbon fiber reinforced resin-based composite materials.
With the development of composite material technology, the use of composite materials in large UAVs is increasing, gradually replacing aluminum alloys and becoming the main material for high-altitude UAVs. The highest proportion of composite materials in modern manned aircraft is 50%, and in current advanced UAVs, the proportion of composite materials far exceeds 50%. For example, the Global Hawk, apart from its main fuselage structure, is entirely made of composite materials, accounting for 65% of its fuselage. In fact, many well-known drones use composite materials exceeding 90%.
There are also differences in the types of composite materials used in large and light drones. Light drones primarily use glass fiber and aramid fiber reinforced composites, while large drones mostly use higher-performance, more expensive carbon fiber reinforced resin-based composites, with some glass fiber and aramid fiber mixed in to reduce costs or adjust performance.
Structural Manufacturing Process
In the design and manufacturing of carbon fiber composite drones, the required structural elasticity and stiffness distribution are often achieved by adjusting the layup angle and the number of material layers. Carbon fiber composites are currently one of the most widely used reinforcing materials in drones. The manufacturing of drones using carbon fiber composites generally involves the following three molding processes:
First, autoclave molding. Autoclave molding is one of the high-performance molding processes for composite materials. For drones with high speed requirements, composite material components and major load-bearing components are often manufactured using this process. Autoclave-molded carbon fiber composite components have better internal quality and more uniform resin content, resulting in superior mechanical properties.
However, autoclave molding technology also has certain drawbacks. This process requires sophisticated equipment, resulting in high initial investment and processing costs, making it relatively uneconomical. From a cost perspective, low-temperature, low-pressure molding technology is often chosen as an alternative when budgets are limited. Furthermore, during the autoclave molding process of carbon fiber composites, the flow of resin, heat transfer, chemical cross-linking, and void formation interact and influence each other, increasing the difficulty of process control. Mistakes can lead to processing defects such as insufficient resin and high porosity.
Secondly, there is vacuum bag molding technology. Compared to autoclave molding, vacuum bag molding is simpler and requires less initial investment, with moderate operational difficulty. However, this molding method uses lower pressure and is only suitable for composite material components with less stringent quality requirements. It is often used to manufacture honeycomb sandwich structures and laminate structures with a thickness not exceeding 1.5mm. Due to its significant cost advantage, this process is widely used in the manufacturing of low-speed drones, as vacuum bag molding technology can meet the production requirements of most parts in the manufacture of small, low-speed drones.
In practice, vacuum bag forming requires prepreg application and wet application processes. Wet application is susceptible to human error, easily leading to uneven adhesive application, especially noticeable in sandwich structure forming. Furthermore, improper application direction can cause fiber bending and alteration, threatening the stability of the carbon fiber composite parts.
Thirdly, compression molding is suitable for manufacturing foam-core composite components. It combines the advantages of autoclave forming and vacuum bag forming, offering high production efficiency, high molding pressure, moderate equipment investment and cost, and good economic benefits. This molding process is widely used for parts of UAVs, such as control surfaces, that utilize foam sandwich structures. In the manufacturing of carbon fiber UAV wing panels, compression molding can also ensure the appearance quality and airfoil accuracy of the UAV wings, improving the overall manufacturing quality. However, pressure control is the most critical step in this process.
The application of carbon fiber composite materials is an important trend in UAV manufacturing development and an effective way to meet the requirements of UAV range, flight time, flight altitude, manufacturing cost, and stealth. The structural design and manufacturing process optimization of carbon fiber composite UAVs are currently key to the application of carbon fiber UAVs. The former requires the efforts of UAV 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 used in UAVs will effectively improve my country’s UAV manufacturing level.
UAV Structural Characteristics
Compared to manned aircraft, UAVs are smaller, eliminating the need for a cockpit and saving on personnel training costs. Only a small number of pilots are needed to operate the UAVs on the ground, eliminating personnel casualties. With increasingly mature intelligent technology, as long as the procedures are reasonable, there is virtually no risk of loss due to operational errors. Analysis shows that UAV structures have the following characteristics:
1. Small size and light weight. Comparing the Predator UAV and the U-2 high-altitude reconnaissance aircraft, the Predator has a wingspan of 14.85 meters, a total weight of 1420 kg, 295 kg of fuel, a range of 3704 km, and an endurance of 24 hours; while the U-2 has a wingspan of 24.38 meters, a total weight of 13154 kg, 4350 kg of fuel, a range of 4700 km, and an endurance of 8 hours. 1. The Predator drone is three times more efficient than the U-2. It eliminates the need for a life support system, reducing weight by 30-40%.
2. Low design load and low structural strength. Autonomous flight with standardized maneuvers results in relatively low flight overload; no risk of personnel injury, resulting in a low structural safety factor. Under comparable equipment standards, the design load is 40% less than manned aircraft. No personnel access channels are needed, minimizing openings. The structure transmits force directly, exhibiting good integrity, with minimal stress concentration issues in critical structures, and lower requirements for airframe fatigue life.
3. Utilizes largely integral molding processes, requiring less sophisticated processing equipment. New materials, advanced processes, and composite materials are widely used in drones. Typically, reinforcing fiber prepreg tape impregnated with matrix resin is laid layer by layer onto a pre-processed mold according to the required part shape, and then cured using a hot-pressing process. This results in complex, large structural components assembled from multiple parts mechanically connected by fasteners, such as reinforced panels. Integral molding can be achieved using pre-curing, secondary curing, or secondary bonding processes. Recently, textile preforms such as sewing and weaving, or resin transfer molding processes, have emerged to reduce machining and assembly workload. In short, this significantly reduces weight and manufacturing costs.
4. Convenient use and maintenance, low life-cycle costs. Drones have three low-cost characteristics: low material costs, low processing costs, and low use and maintenance costs. Overall, cost is roughly proportional to aircraft weight. Manned aircraft are heavy and expensive, with a pilot-to-aircraft maintenance ratio of 1.3:1; drones are lightweight and inexpensive, with a pilot-to-aircraft ratio of one to several sorties, and no maintenance-required aircrew, eliminating expensive training costs. The unit price of a drone is 70% lower than that of a manned aircraft. Furthermore, drones have maintenance-free storage. Compared to the life-cycle costs of missiles, for example, a medium-sized unmanned combat aerial vehicle capable of launching missiles and performing 15 missions over its lifespan, using maintenance-free storage, is similar to a single-use Tomahawk missile with the same combat effectiveness (unit price $1.4 million), making the cost-effectiveness much higher.
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