Flapping-Wing UAVs: Innovation and Exploration in Biomimetic Flight Technology

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1. Definition of Flapping-Wing UAVs

Flapping-wing UAVs, with the English name Flapping-wing UAV and abbreviation FWUAV, are biomimetic UAVs that take the flight modes of organisms such as birds and insects as imitating objects. Different from the flight modes of traditional fixed-wing UAVs, which generate lift through the relative motion between wings and airflow, and multi-rotor UAVs, which generate reaction force by blowing air downward with propellers, the core feature of flapping-wing UAVs is to generate lift and thrust through the periodic “flapping” motion of wings, thereby achieving aerial flight. This type of UAV integrates technologies from multiple disciplines, including biology, aerospace engineering, and materials science. It is committed to breaking through the limitations of traditional UAVs in terms of flexibility, concealment, and energy efficiency, and is an important branch of the development of UAV technology towards the “biological biomimicry” direction.

2. Core Principle: Biomimetic Flapping Mechanism

The flight principle of flapping-wing UAVs completely replicates the “flapping-wing motion” of organisms. It mainly realizes the coordination of lift and thrust through two key stages of wing flapping, so as to complete various flight movements.

Key Stages of Wing Flapping

• Downstroke Stage: During this stage, the wings move downward rapidly. By accurately adjusting the angle of the wings, i.e., the angle of attack, the airflow generates upward lift and forward thrust on the wings. This stage is the main power source for the flight of the UAV.

• Upstroke Stage: In this stage, the wings move upward slowly and, at the same time, fold and twist like birds retracting their wings to reduce air resistance and avoid offsetting the power generated in the downstroke stage.

• Coordination Control of Lift and Thrust: Through the precise control of flapping frequency (usually several to hundreds of times per second), flapping amplitude, and wing torsion angle, the dynamic balance of lift and thrust is achieved. Furthermore, the flapping-wing UAV can complete a series of complex flight movements such as hovering, forward flight, turning, climbing, and descending.

3. Core Characteristics: Comparison with Traditional UAVs

The design of flapping-wing UAVs originates from “biomimicry”, so their characteristics are based on the advantages of biological flight. However, limited by the current technical level, there are still certain limitations. Compared with traditional UAVs (fixed-wing and multi-rotor UAVs), each has its own advantages and disadvantages.

Comparison Dimension	Flapping-Wing UAVs	Traditional UAVs (Fixed-Wing/Multi-Rotor)
Flexibility	Extremely high, capable of realizing movements such as hovering, side flight, and sharp turns, and can well adapt to narrow spaces such as jungles and indoors	Fixed-wing UAVs cannot hover; multi-rotor UAVs have relatively strong flexibility but a larger turning radius
Concealment	Low noise during wing flapping, similar to the fluttering of insects. Moreover, their appearance can be designed into biomimetic styles, such as “mechanical birds”, making them difficult to be detected	Multi-rotor UAVs produce loud propeller noise; the flight trajectory of fixed-wing UAVs is easy to be identified
Energy Efficiency Ratio	High energy efficiency ratio. The flapping motion is more in line with the aerodynamic principle, and the endurance time is relatively longer under the same weight condition	Multi-rotor UAVs have high energy consumption and short endurance; fixed-wing UAVs have high energy efficiency but rely on runways.
Payload Capacity	Weak payload capacity. At present, the payload of mainstream models is mostly within 100 grams, which is mainly limited by the flapping power	Fixed-wing UAVs and large-load multi-rotor UAVs have strong payload capacity, which can carry items ranging from several kilograms to several tons
Wind Resistance Capacity	Weak wind resistance capacity. Small-sized models are easily affected by air currents due to their small wing area and light weight	Medium and large-sized fixed-wing UAVs and multi-rotor UAVs have a strong wind resistance capacity
Take-off and Landing Requirements	Extremely low take-off and landing requirements, capable of vertical take-off and landing and hand-launched take-off, without the need for runways or special take-off and landing platforms	Fixed-wing UAVs require runways or ejection devices to take off; multi-rotor UAVs need flat ground for take-off and landing.

4. Key Technologies: Core Elements Determining Performance

The R&D difficulty of flapping-wing UAVs is much higher than that of traditional UAVs, and their performance is mainly determined by the following four key technologies:

Flapping Mechanism Design

The main function of the flapping mechanism is to convert the motor power into the “biomimetic flapping” of the wings, and it is the most critical component of the flapping-wing UAV. At present, there are two mainstream design schemes:

• Rigid Flapping: Fixed wings are adopted, and the up-and-down flapping of the wings is realized through structures such as connecting rods and crankshafts. This design has a relatively simple structure and is suitable for small-sized flapping-wing UAVs.

• Flexible Flapping: The wings are made of elastic materials, such as carbon fiber and silica gel. During the flapping process, the wings will be accompanied by torsion and bending movements. This design is more similar to the flight mode of birds, with high lift efficiency, but the control difficulty is relatively greater.

Material Technology

The requirements for the materials of the fuselage and wings are “lightweight, high strength, and high elasticity”. For example, carbon fiber composites are often used for the wings to reduce weight, and shape memory alloys are used to realize adaptive deformation; the fuselage is mostly made of engineering plastics, such as ABS and PC, to reduce the overall weight.

Flight Control Technology

In order to cope with the changes in air currents, it is necessary to adjust parameters such as flapping frequency and wing angle of attack in real time. Since the flapping motion forms an “unsteady flow field”, that is, the airflow state changes with time, the control algorithms of traditional UAVs are no longer applicable. Therefore, it is necessary to specially develop “biomimetic control algorithms”. Some flapping-wing UAVs are also equipped with micro gyroscopes, accelerometers, and other equipment to achieve stable flight attitudes.

Micro Power and Energy Sources

Limited by size, flapping-wing UAVs need to be equipped with micro power systems. At present, the commonly used ones include coreless motors (weighing only a few grams) and micro fuel cells (with an endurance capacity 3-5 times longer than that of lithium batteries). However, mainstream models still use lithium batteries. Nevertheless, lithium batteries have the problem of limited energy density, resulting in the endurance of flapping-wing UAVs mostly being within 30 minutes.

5. Application Scenarios: Fields Highlighting Biomimetic Advantages

At present, the technology of flapping-wing UAVs is in a stage of rapid development, and their applications are mainly concentrated in scenarios that are difficult to be covered by traditional UAVs, as follows:

Reconnaissance and Surveillance Field

The appearance of flapping-wing UAVs can be disguised as birds and insects, such as “mechanical pigeons” and “mechanical bees”. They are very suitable for use in tasks such as urban street battles, jungle reconnaissance, and border patrols. It is not easy to be detected by enemy radars or personnel, and can effectively obtain intelligence information.

Ecological and Scientific Research Field

In ecological research, it can observe the behaviors of wild animals at close range, such as bird nesting and herd migration. Due to its low noise and little interference with organisms, it avoids the scare of wild animals caused by traditional UAVs; in environmental monitoring, small-sized flapping-wing UAVs can pass through the crown of trees and be used for work such as forest fire monitoring.

Indoor/Narrow Space Operation Field

In industrial scenarios, it can be used for factory equipment inspection and can pass through narrow spaces such as pipelines and shelf gaps; in disaster rescue, it can enter the interior of collapsed buildings to search for survivors; in the field of resource exploration, it can adapt to low and complex underground environments such as mines and carry out exploration work.

Consumer and Educational Field

Small flapping-wing UAVs, such as palm-sized models, have entered the consumer market. In the educational field, they can be used as popular science teaching aids to show students the principle of biomimetic flight; in the consumer entertainment field, they can be used as toys, such as “mechanical butterflies” and “biomimetic birds”, to meet people’s entertainment needs.

Military Field

In addition to the above-mentioned reconnaissance tasks, micro flapping-wing UAVs can also be used for tasks such as biochemical warfare agent detection, target indication, communication relay, and weapon launch. They are especially suitable for use in urban combat and can fill the blind areas that cannot be covered by satellites and reconnaissance aircraft. The on-board cameras, infrared sensors, or radars can transmit target information back in real time, and soldiers can grasp the positions of enemies through the displays on their palms. If an electronic nose is installed, it can also track human targets according to smells.

Civil Special Fields

In peacetime, flapping-wing UAVs are powerful tools for detecting nuclear, biological, and chemical pollution, searching for disaster survivors, and monitoring criminal gangs. In addition, the possibility of using them for pollination in indoor vertical farms is also being explored. Compared with rotating quadcopters, they can avoid the tearing of crops by blades; at the same time, because they can remain stable even in strong winds, they can also be used to drive birds away from airports, which greatly saves the labor costs of pest control companies and airport operators.

6. Current Development Status and Future Trends

(1) Current Development Status

At present, the technology of flapping-wing UAVs is still in the “small and medium-sized, professional” stage. The wingspan of mainstream products is mostly between 10-50 cm, the payload capacity is 50-200 grams, and the endurance time is 15-40 minutes. Their applications are mainly concentrated in professional fields such as military reconnaissance and scientific research monitoring, while consumer products are mainly toys and teaching aids with relatively basic performance.

In the process of technological development, some representative products and achievements have also emerged. For example, the “Yun Xiao” flapping-wing UAV developed by Northwestern Polytechnical University of China set a Guinness World Record with an endurance time of 154 minutes in 2022; the Xi’an Aisheng ASN211 micro flapping-wing aircraft made its first flight in 2010; the German company Festo launched biomimetic UAVs such as “Swift” and “SmartBird”; the French Ministry of National Defense launched the “Biomimetic Bird” project; China’s “Carrier Pigeon” UAV, as a military reconnaissance flapping-wing aircraft, has a realistic appearance and has undergone public tests; the Harvard RoboBee is a micro flapping-wing robot, weighing only 80 milligrams, which is mainly used to study the flight mechanism of insects. In addition, a team from South Australia, Singapore, China, and Taiwan jointly designed a flapping-wing aircraft weighing 26 grams and about 25 cm in length. It can complete movements such as hovering, diving, gliding, and braking like a swift, and can also quickly switch between various flight attitudes in the air. This design has been described in the journal Scientific Reports.

However, flapping-wing UAVs still face many challenges at present. In terms of the technical stage, they are in the transition stage from technical maturity to commercialization. Although some products have realized short-term flight and mission load integration, the endurance and load are still limited by the battery energy density and flapping mechanism efficiency, resulting in low endurance time and mission load capacity; in terms of flight control, because flapping-wing flight involves unsteady aerodynamics, the flight control algorithm still needs to be further optimized; in terms of cost and reliability, the manufacturing cost of high-precision structures is relatively high, and the reliability in complex environments needs to be verified. At the same time, birds have different reactions to flapping-wing UAVs that are similar to themselves in size and shape. Small birds are easily scared, while large flocks of birds and larger-sized birds sometimes attack the flapping-wing aircraft, which is also a problem currently faced. Moreover, at present, only 10% of biological flight can be replicated at most, and there is still a large gap compared with real biological flight. Birds and insects have multiple sets of muscles, which can realize complex movements such as rapid flight, wing folding, and body twisting. Their wings have high flexibility, can rotate the body in mid-air, and can flap the wings at different speeds and angles. For example, the maximum cruising speed of an ordinary swift can reach 31 meters per second, which is equivalent to 112 kilometers per hour or 90 miles per hour. These are all difficult for current flapping-wing UAVs to achieve.

(2) Future Trends

Technological Innovation Direction

• Material Innovation: Develop and apply lightweight and high-strength materials, such as carbon fiber and biomimetic films, to improve the durability of flapping-wing UAVs, further reduce the weight of the fuselage, and enhance the structural strength.

• Power and Energy Optimization: Develop hybrid power modes, combining fixed-wing and rotor modes to expand their application scenarios; at the same time, improve the energy utilization efficiency by adopting fuel cells, solar power supply, and other methods, increase the endurance time to several hours, and break through the current limitation of short endurance.

• Intelligent Upgrading: Combine artificial intelligence technology, use machine learning to optimize the flapping frequency and path planning, develop more advanced biomimetic control algorithms, realize functions such as autonomous obstacle avoidance of flapping-wing UAVs and autonomous learning of bird flight strategies (such as avoiding obstacles and gliding using air currents), and improve their intelligence level.

Application Expansion Direction

• Enlargement and Heavy-Load Development: Break through the load limitation, develop models that can carry small sensors (such as infrared cameras and gas detectors), expand their application scope in fields like scientific research and monitoring, and enhance their actual operational capacity.

• Swarm Collaboration: Imitate the flight mode of bird flocks to realize the collaborative operation of multiple flapping-wing UAVs, such as carrying out large-area reconnaissance and material delivery, thereby improving operation efficiency and task completion rates.

• Multi-Functional Integration: Integrate micro sensors, communication modules, and other equipment to expand their applications in civil scenarios, such as agricultural pollination and equipment inspection, and enable them to play a role in more fields.

Industry Standard Direction

With the continuous maturation of flapping-wing UAV technology, it is expected that a comprehensive industry standard will be gradually established to standardize various links, including R&D, production, and application, thereby promoting the industrialization process and facilitating the healthy and orderly development of the flapping-wing UAV industry.

7. Conclusion

As a cutting-edge technological product integrating bionics, aerodynamics, and intelligent control, flapping-wing UAVs have not yet realized large-scale commercial application at present. They still face challenges in terms of endurance, load, control algorithms, cost, and reliability, and there is a large gap compared with biological flight. However, relying on their unique concealment, high maneuverability, and adaptability in narrow spaces, they have shown important value and broad application prospects in fields such as military reconnaissance, scientific research monitoring, and disaster rescue. With the continuous progress of material technology, power and energy technology, and artificial intelligence technology, as well as the gradual improvement of industry standards, flapping-wing UAVs are expected to break through the existing technical bottlenecks in the future, achieve innovative breakthroughs in more professional fields, become an important force in the diversified development of UAVs, and may even reshape the way we handle aerial operations, data collection, and urban planning, opening a new chapter in aerial robot technology.