What is the best aerodynamic propulsion configuration for a multi-rotor drone?

What is the best aerodynamic propulsion configuration for a multi-rotor drone?

In the design of multi-rotor drones, the choice of the aerodynamic propulsion configuration is crucial to guarantee an optimal performance. The number of rotors as well as the number of blades and their design have a major impact on the efficiency, lift, and drag of drones. Thanks in particular to advances in CFD simulation (Computational Fluid Dynamics), the designers have powerful tools to determine the ideal configuration. Let’s explore the key principles related to these parameters leading to optimize the propulsion of UAVs (Unmanned Aerial Vehicles).

Multiply rotors: Advantages and disadvantages

Increasing the number of rotors on a drone generally improves its stability and load capacity. For example, octocopter drones, like the DJI Agras T40, can carry heavy loads while maintaining remarkable manoeuvrability, making them a preferred choice for agriculture or industrial inspections. However, increasing the number of rotors has its limits. Each rotor generates additional drag and turbulent interactions with the flows of the other rotors. This can reduce the overall efficiency of the system. A hexacopter drone, like the Yuneec H520E, has a good balance between stability and power consumption, but adding additional rotors can lead to increased complexity in managing motors and controllers.

To reduce these disadvantages, co-axial configurations (superimposed rotors) are used, making it possible to double the lift without significantly increasing the bulk. However, this configuration requires rigorous optimization to minimize losses due to induced vortices.

Influence of the number of blades per rotor

The design of a rotor blade plays a fundamental role in aerodynamic performance. Two-blade propellers, like those commonly used in recreational drones, offer low drag but generate less lift. On the other hand, three-blade or four-blade propellers, often used on racing drones or industrial UAVs, increase lift and allow better response to control, at the cost of higher energy consumption.

For example, the use of three-blade propellers on FPV drones improves manoeuvrability by reducing vibration, while maintaining high speed during tight turns. In contrast, long-range transport drones, like the WingtraOne GEN II, favor two-blade propellers to maximize efficiency over long distances.

The choice of blade number must also take into account subsonic speed limits in the flow through the rotors. Additional blades can generate turbulence effects and complex interactions in the airflow, requiring prior optimization by MRF (Multiple Reference Frame) simulation, the rotor performance can then be synthesized into a BEM model (Blade Element Momentum).

Evaluation of rotor performance: tests and calculations

Flight and wind tunnel tests

Practical tests, such as those carried out in a wind tunnel or in live flight, remain essential to validate the performance of the rotors. These tests make it possible to observe aerodynamic behavior under controlled conditions, to measure lift, drag, and turbulence effects. For example, wind tunnel tests on hybrid-powered drones, such as the Thales UAS100, have shown that minor adjustments to blade profiles can reduce drag by 15%.

However, flight testing presents logistical and financial challenges, including the need for accurate but expensive measurement tools such as on-board force transducers to collect real-time data.

Numerical simulation: CFD, MRF, and BEM

Numerical simulations offer a cost-effective and rapid alternative to evaluate rotor performance. CFD makes it possible to analyze in detail the air flows around the blades and to visualize the generated vortices. These simulations are particularly useful for optimizing designs before manufacturing prototypes.

The MRF and BEM methods, in particular, focus on specific models for evaluating rotor performance and aerodynamic interactions. BEM is commonly used for its speed in initial designs, while MRF provides more accurate results but at a significantly higher cost. By combining these methods, designers can refine designs and maximize the aerodynamic efficiency of drones.

Conclusion

The aerodynamic propulsion configuration of multi-rotor drones depends on a delicate balance between the number of rotors, the number and design of blades, the overall characteristics of the drone and its missions. While multiplying rotors improves stability and lift, it also creates challenges in drag and turbulent interactions. Blade choices must adapt to the drone’s specific application, while modern tools like CFD and MRF and BEM simulations offer powerful solutions for optimizing designs. This allows designers to create more efficient UAVs that are adapted to the complex needs of users.

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