A Comprehensive Analysis of Ducted Fan UAVs: Structure, Principles, Applications, and Development

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A ducted fan unmanned aerial vehicle (UAV) is a type of unmanned aircraft that utilizes a ducted fan system as its core lift or thrust device. Its most distinctive feature is that the fan (or propeller) generating power is enclosed within a rigid annular duct (air duct). Through the synergistic effect of the duct and the fan, flight is achieved. This design stands in sharp contrast to the exposed propeller configuration of traditional multi-rotor UAVs, offering unique advantages in terms of safety, low noise operation, and spatial adaptability. It represents a crucial branch within the UAV technology field, tailored to meet specific scenario requirements.

1. Core Structure: Composition and Functions of the Ducted Fan System

The performance of a ducted fan UAV is directly determined by the combined design of the “duct + fan”. The compatibility between these two components and the technical parameters of each part collectively influence the UAV’s lift efficiency, flight stability, and operational safety. Its core structure can be broken down into three main parts:

(1) Duct (Annular Air Duct)

The duct is a key component that differentiates ducted fan UAVs from traditional ones. Typically fabricated from lightweight yet high-strength materials such as carbon fiber and ABS plastic, it has an overall annular or cylindrical shape. Its primary functions are manifested in three aspects:

• Optimizing airflow direction: It restricts the airflow generated by the rotating fan, minimizing “lateral diffusion loss” of the airflow. This encourages a greater proportion of the airflow to move along the axial direction of the duct, thereby significantly enhancing the UAV’s lift efficiency.

• Providing safety protection: It isolates the internal fan from the external environment, preventing the fan from colliding with tree branches, buildings, or personnel during flight. Additionally, it can reduce the noise produced by the rotating fan to a certain extent.

• Enhancing flight stability: During flight, the duct can counteract part of the crosswind interference through the air pressure difference. This makes it particularly suitable for operations in complex spaces such as low-altitude and narrow areas, improving the UAV’s anti-interference capability.

(2) Fan (Power Core)

Positioned at the center of the duct, the fan serves as the power source of the UAV. It usually adopts a multi-blade design, with the number of blades typically ranging from 4 to 8. Driven by a motor, it rotates at high speed to generate airflow. Compared with the propellers of traditional multi-rotor UAVs, the fan inside the duct has two notable characteristics: a smaller diameter and a higher rotational speed. Moreover, the shape design of the fan blades focuses on “adapting to the inner diameter of the duct”. This minimizes the frictional loss of airflow as it flows along the inner wall of the duct, further enhancing the power conversion efficiency.

(3) Drive and Control Components

The drive and control components are essential for ensuring the normal flight and attitude adjustment of the UAV. They mainly include a motor, an electronic speed controller (ESC), a flight control system, and some models are additionally equipped with deflectors. Among them, the motor is mostly a brushless motor, which boasts a high power density and can provide continuous and stable power for the rotation of the fan. The ESC is responsible for adjusting the motor speed to achieve precise control over the power output of the fan. The flight control system adjusts the flight attitude of the UAV, such as hovering, turning, ascending, and descending, by controlling the speed difference between different ducted fans. The deflectors are usually installed at the bottom or side of the duct. They assist the UAV in completing movements like turning by changing the direction of the airflow, thereby enhancing operational flexibility.

2. Working Principle: The Synergistic Mechanism of Airflow Restriction and Lift Enhancement

When the propellers of traditional multi-rotor UAVs rotate, the airflow diffuses in four directions: “up, down, left, and right”. This “wake loss” results in the waste of a portion of the power. In contrast, ducted fan UAVs, through their unique structural design, establish a synergistic mechanism of “airflow restriction – lift enhancement”. The specific process is as follows:

• Air intake and pressurization: When the fan rotates under the drive of the motor, it draws in air from the top of the duct. Once the air enters the interior of the duct, it forms high-pressure airflow under the action of the fan.

• Directional discharge of airflow: The annular structure of the duct exerts a physical constraint on the airflow, forcing the high-pressure airflow to be discharged only downward along the axial direction of the duct. This prevents the lateral diffusion of the airflow, concentrates the energy of the airflow, and reduces power waste.

• Assistance of the Coanda effect: As the airflow flows inside the duct, the “Coanda effect” occurs between the inner wall of the duct and the airflow. This effect refers to the tendency of the airflow to adhere to and flow along a curved surface. It further guides the direction of the airflow, enabling the airflow to be discharged more stably along the axial direction.

Through the above mechanism, when a ducted fan UAV is flying at a low speed, the lift generated per unit power is higher than that of an exposed propeller. The efficiency can typically be increased by 20% – 40%, which is the core reason for its advantages in low-speed and hovering scenarios.

3. Main Characteristics: A Comprehensive Analysis of Advantages and Limitations

The structural design of ducted fan UAVs determines their unique performance characteristics. They not only possess significant advantages in specific scenarios but also have certain limitations due to their design features. The specific manifestations are as follows:

(1) Core Advantages

• High safety: The duct fully encloses the fan, eliminating the exposure of rotating components. This effectively prevents the risk of scratching personnel or damaging external objects during flight, making it suitable for operations in human-centric environments such as indoor spaces and crowded areas.

• Low noise level: The duct structure can block the aerodynamic noise generated by the rotating fan. Practical tests have shown that its noise level is 10 – 15 decibels lower than that of multi-rotor UAVs of the same size. It is thus suitable for scenarios with high noise reduction requirements, such as urban patrols and film and television shooting.

• Strong spatial adaptability: With a compact overall structure, the duct diameter usually ranges from 20 to 60 centimeters. It can fly flexibly in narrow spaces, such as inside factories, between buildings, tunnels, or mines. Moreover, it is less likely to be caught by obstacles, reducing the risk of flight accidents.

• Good stability at low speeds: The restrictive effect of the duct on the airflow ensures that the UAV maintains a more stable flight attitude in low-speed and light wind conditions. It can achieve precise hovering, making it suitable for operations that require high precision, such as material delivery and equipment inspection.

(2) Main Limitations

• Low efficiency at high speeds: When the UAV flies at high speeds, the duct creates additional friction with the air, resulting in air resistance. This leads to a shortened endurance time, which is usually only 10 – 25 minutes, shorter than that of traditional multi-rotor UAVs of the same class.

• Weak load-carrying capacity: The duct structure increases the overall weight of the UAV body. At the same time, the low efficiency at high speeds limits the effective conversion of power output. As a result, its load-carrying capacity is usually less than 1 kilogram, and only a few large-sized ducted fan UAVs can have a load capacity of up to 5 kilograms.

• Poor wind resistance: Most ducted fan UAVs have small body sizes and low weights. Additionally, the duct is susceptible to airflow impact in strong wind conditions, leading to reduced stability. Generally, they can only fly safely in winds below level 3 (wind speed ≤ 5.4 meters per second).

• High manufacturing costs: The production of the duct requires high-precision forming processes to ensure the smooth flow of airflow inside. Meanwhile, the matching design between the fan and the duct is relatively complex. These factors contribute to a manufacturing cost that is 30% – 50% higher than that of traditional multi-rotor UAVs with the same performance.

4. Typical Application Scenarios: Scenario Adaptation Based on Core Advantages

Leveraging its core advantages of “safety, low noise, and compactness”, ducted fan UAVs are mainly applied to operational tasks in low-altitude, short-distance, and complex environments, covering multiple industrial fields. The specific scenarios are as follows:

(1) Indoor/Narrow Space Operations

• Equipment inspection and patrol: They can enter the interior of factories to inspect the status of equipment such as boilers and pipeline welds. They can also conduct maintenance work on elevator shafts and ceilings inside buildings, and penetrate into tunnels and mines for environmental exploration, without the concern of the propellers being damaged by obstacles.

• Material transportation: In hospital scenarios, they can deliver medicines and medical devices to isolation wards. In laboratory environments, they can safely transfer samples, avoiding direct contact between personnel and reducing the risk of cross-infection.

(2) Human-Centric Scenarios

• Security and patrol: In crowded events such as large-scale concerts and exhibitions, they can be used to monitor crowd density and promptly identify potential safety hazards. In areas such as urban communities, campuses, and scenic spots, they can carry out daily security patrols to improve security efficiency.

• Film and television shooting: They are suitable for indoor follow-up shooting and close-up shot capture. Their low-noise characteristic can avoid interfering with on-site audio recording, ensuring shooting quality and reducing the workload of post-production audio processing.

(3) Special Industry Requirements

• Fire rescue: They can enter the interior of fire sites (in environments with low visibility and narrow passages) to detect the location of the fire source and transmit real-time on-site images, providing support for rescue decision-making. Some large-sized models can also carry fire-fighting equipment to assist in fire-extinguishing operations.

• High-risk environment operations: In high-risk areas such as nuclear power plants and chemical plants, which are exposed to nuclear radiation or chemical pollution, they can replace personnel to complete tasks such as equipment inspection and environmental monitoring. This prevents personnel from being exposed to dangerous environments and ensures their safety.

(4) Military and Civilian Expansion Scenarios

• Military field: They can be used for reconnaissance and strike missions. Some models, combined with stealth design and precision strike technology, have become derivative models of cruise missiles. Additionally, their strong wind resistance (Note: This is a relative advantage in specific military scenarios; in general, their wind resistance is poor as mentioned earlier) makes them suitable for use as shipborne UAVs to meet the take-off and landing requirements of ships.

• Logistics and distribution: In the “last-mile” distribution in cities, their safety advantage can prevent threats to pedestrians. For example, Amazon has tested ducted fan UAVs for cargo delivery.

Consumer products: Some racing UAVs and shooting UAVs adopt a ducted design to enhance flight stability and improve the control experience and shooting effects.

5. Key Differences from Traditional Multi-Rotor UAVs

To gain a clearer understanding of the positioning of ducted fan UAVs, a comparison can be made with traditional multi-rotor UAVs (such as the DJI Phantom series) from multiple core dimensions. The specific differences are as follows:

Comparison Dimension	Ducted Fan UAV	Traditional Multi-Rotor UAV
Power Device	Fan enclosed in a duct	Exposed propeller
Safety	High (no exposed rotating components)	Low (propellers are prone to injuring people or damaging objects)
Noise Level	Low (10 – 15 decibels lower than multi-rotor UAVs)	Medium (obvious aerodynamic noise)
Spatial Adaptability	Strong (compact, suitable for narrow spaces)	Weak (propellers are prone to getting caught on obstacles)
Endurance Time	Short (10 – 25 minutes)	Long (25 – 40 minutes)
Wind Resistance	Weak (≤ level 3 wind)	Strong (some models can resist level 6 wind)
Applicable Scenarios	Indoor, human-centric, narrow environments	Outdoor, open spaces, long-distance operations

6. Future Development Trends: Technological Breakthroughs and Scenario Expansion

With the continuous advancement of related fields such as material technology and flight control algorithms, the limitations of ducted fan UAVs are gradually being overcome, and their application scenarios are constantly expanding. The main future development trends are reflected in the following aspects:

(1) Integration with eVTOL and Urban Air Mobility (UAM)

Ducted fan technology has become one of the important technical routes in the fields of electric vertical take-off and landing (eVTOL) aircraft and urban air mobility (UAM). Some enterprises, such as Joby Aviation, have developed eVTOL models with a tilting ducted design. In the future, these models are expected to be applied to scenarios such as short-distance urban manned flight and air taxis, providing support for the construction of an urban three-dimensional transportation network.

(2) Application of Hybrid Power Systems

To enhance endurance and load-carrying capacity, hybrid power systems will be gradually applied to ducted fan UAVs. By combining turbofan engine technology with electric drive technology, it is possible to ensure power output while reducing electrical energy consumption, extending the endurance time, and meeting the requirements of longer-distance operations such as long-distance logistics distribution and large-scale patrols.

(3) Optimization of Materials and Processes

With the research, development, and application of lighter and higher-strength materials (such as new carbon fiber composites), the weight of the ducted fan UAV body will be further reduced, which is conducive to improving the load-carrying capacity and endurance. Meanwhile, the improved precision of the duct forming process can reduce airflow friction loss and further optimize the lift efficiency.

(4) Upgrading of Flight Control Algorithms

To address the issue of poor wind resistance, the optimization of flight control algorithms, such as the integration of more precise wind field sensing and attitude adjustment modules, can enhance the stability of the UAV in low to moderate wind speed environments. Furthermore, the upgrading of intelligent obstacle avoidance algorithms will also improve its autonomous flight capability in complex environments and reduce the operational difficulty.

Conclusion

Ducted fan UAVs belong to the category of “scenario-specific” UAVs. Their design sacrifices certain aspects of endurance, load-carrying capacity, and wind resistance in exchange for high safety, low noise, and strong spatial adaptability. Their core value lies in addressing the pain point that traditional multi-rotor UAVs cannot operate safely in “indoor, human-centric, and narrow environments”. Currently, they have demonstrated significant application value in fields such as fire rescue, equipment inspection, and logistics distribution. With continuous technological breakthroughs, their applications in areas such as refined urban management, industrial inspection, and manned flight will be further expanded, making them an indispensable and important direction in the evolution of UAV technology.