Research on the Influence of UAV Rotors on the Measurement Results of a 2D Ultrasonic Anemometer

Technical Article | Hongyu Ultrasonic Wind Measurement Technology

Introduction

UAV-based atmospheric pollutant source tracing technology has developed rapidly in recent years [1]. A growing body of research worldwide has combined unmanned aerial vehicles with atmospheric pollutant detection equipment to identify and trace pollution sources [2]. During UAV-based pollutant source tracing, a relatively straightforward way to anticipate the direction and speed of pollutant dispersion is to equip the UAV with a wind speed and direction sensor and measure changes in the near-ground atmospheric wind field.

Traditional methods for measuring low-altitude atmospheric wind fields mainly rely on meteorological towers or wind masts [3–5]. These structures are heavy, must be installed away from high-rise buildings and trees, and are primarily suited to measuring vertical wind profiles. Consequently, meteorological towers and wind masts have limitations when measuring horizontal wind fields close to the ground [6–7]. In 1992, Holland et al. first used a fixed-wing UAV equipped with a Pitot tube to measure high-altitude wind fields. However, because of the flight characteristics of fixed-wing UAVs, the method was limited mainly to horizontal wind measurements.

Compared with fixed-wing UAVs, multirotor UAVs can ascend and descend vertically and hover in place. When equipped with a wind sensor, they can not only measure horizontal wind fields but also support analysis of vertical wind-field variations. Jiang Ming et al. [9] used a multirotor UAV equipped with a 2D ultrasonic wind sensor to perform low-altitude wind measurements during a 40 m hover. The results showed good agreement between the UAV-mounted ultrasonic sensor and static measurements from a gradient tower at the same altitude, with an average wind-speed deviation of 0.46 m/s, demonstrating the feasibility of wind measurement using multirotor UAVs. A number of studies have also investigated algorithms for 2D ultrasonic wind measurement on multirotor UAVs.

During UAV flight or hovering, the turbulence generated by rotating rotors inevitably disturbs the airflow around the onboard anemometer. Therefore, both the choice of anemometer and its mounting position on the UAV can affect the measurement results. In this study, a compact ultrasonic wind speed and direction sensor independently developed by Hongyu based on the principle of acoustic resonance is mounted on a multirotor UAV to investigate near-ground wind fields. The objective is to determine how rotor-induced turbulence influences UAV-based wind speed measurements.

1. Analysis of the Principles of UAV-Based Wind Measurement

As illustrated in Figure 1, the turbulence generated by a multirotor UAV's rotors is directed predominantly downward, as indicated by the arrows. Because the rotors are located at a certain distance from the center of the UAV, a relatively quiescent airflow region exists around the central area. If a compact or miniature 2D wind sensor, whose dimensions are smaller than this quiescent region, is mounted above the UAV or below its belly within this region, the horizontal wind-speed measurement is expected to be largely unaffected by the rotor-generated downward turbulence.

Figure 1. Schematic diagram of UAV-mounted wind measurement

2. Experimental Program

2.1 Experimental Equipment

The experiment used two Hongyu HY-SA256 2D ultrasonic wind speed and direction sensors manufactured in-house, together with a six-rotor UAV.

2.1.1 HY-SA256 Wind Speed and Direction Sensor

As shown in Figure 2, the HY-SA256 is an ultrasonic anemometer based on an acoustic-resonance measurement principle. It is specifically designed for installation on small aircraft and unmanned platforms to monitor wind speed and direction in low-altitude regions.

[Figure / Product image: HY-SA256 UAV-specific weather station]

Figure 2. Miniature ultrasonic wind sensor

2.1.2 Six-Rotor UAV

As shown in Figure 3, the experimental platform was a V6 six-rotor UAV with 400 mm arm length and 400 mm rotor diameter. In other words, the minimum distance from each rotor to the UAV center was 200 mm. The included angle between adjacent arms after deployment was 60°.

[Figure / Product image: Six-rotor UAV]

Figure 3. Six-rotor UAV

2.2 Experimental Design

2.2.1 Experimental Setup 1 — F1030

As shown in Figure 4, one HY-SA256 ultrasonic wind sensor was installed above the UAV and another below the belly. The support rod for the upper sensor was 600 mm long. Both sensors were positioned along the UAV centerline. With the UAV stationary on the ground, the rotors were operated without taking off, and wind speed was measured. The data refresh interval was 1 second, and the total test duration was 120 seconds.

[Figure / Product image: Six-rotor UAV equipped with HY-S256]

Figure 4. UAV wind measurement test

2.2.2 Experimental Setup 2

Using the sensor arrangement in Setup 1, the lower sensor remained in its original position while the support rod of the upper sensor was changed to 300 mm and then 0 mm, with the sensor placed directly at the UAV center. Wind speed was measured under each configuration. The sampling interval was 1 second and the total test duration was 120 seconds.

[Figure / Product image: 300 mm support-rod configuration]

[Figure / Product image: 0 mm support-rod configuration]

2.2.3 Experimental Setup 3

Using the arrangement in Setup 1, the upper sensor remained fixed. The lower sensor was first placed on the ground directly below the UAV center, then moved to the ground beneath the rotor edge, and finally placed on the ground directly below the midpoint between two adjacent rotors. The UAV remained on the ground while the rotors operated, and wind speed was measured at each of the three locations. The data refresh interval was 1 second, with a total test duration of 120 seconds.

[Figure / Product image: Three-position comparison]

2.2.4 Experimental Setup 4

Using the sensor arrangement in Setup 1, the UAV was hovered at heights of 60 m and 100 m above the ground for wind speed measurements. Data were refreshed every 1 second, with a total test duration of 120 seconds.

[Figure / Product image: High-altitude UAV hovering measurement]

Figure 5. Wind measurement during UAV hovering

3. Analysis of Experimental Results

3.1 Results of Setup 1

[Figure / Product image: Setup 1 data results]

Figure 6. Results from Setup 1

As shown in Figure 6, the wind-speed data measured by the two sensors mounted above and below the UAV were nearly identical.

3.2 Results of Setup 2

[Figure / Product image: Setup 2 data results]

Figure 7. Results from Setup 2

Figures 7(a) and 7(b) show no significant difference between the wind speeds measured above and below the UAV. This indicates that, within the quiescent region around the UAV center, wind-speed measurements are not significantly affected by the length of the sensor support rod.

3.3 Results of Setup 3

[Figure / Product image: Setup 3 data results]

Figure 8. Results from Setup 3

As shown in Figure 8, among the four measurement conditions a/b/c/d in Setup 3, only the wind-speed measurement taken directly below the center of the UAV was unaffected by rotor rotation.

3.4 Results of Setup 4

[Figure / Product image: Setup 4 data results]

Figure 9. Results from Setup 4

As shown in Figure 9, there was no significant difference between the wind speeds measured by the two sensors while the UAV hovered at 60 m and 100 m.

4. Conclusion

A multirotor UAV has a relatively quiescent airflow region around its center. When a miniature wind sensor (with dimensions smaller than this quiescent region) is mounted within this area for airborne hover measurements, the measurement results are largely unaffected by the downward turbulence generated by the rotors. In addition, the measured wind speed is not significantly dependent on the length of the support rod.

References

[1] Liu, K. Design and Implementation of a UAV-Based Visualization System for Atmospheric Pollution Source Tracing [J]. Electromechanical Information, 2021(20): 38–40.

[2] Qu, Y.W., Wang, T.J., Yuan, C., et al. Research Progress on UAV-Based Detection and Source Tracing of Atmospheric Fine Particulate Matter and Ozone Pollution [J]. Environmental Science, 2023, 44(12): 6598–6609.

[3] Huang, B.C. Principles and Applications of Structural Wind-Resistance Analysis [M]. Shanghai: Tongji University Press, 2001: 47–49.

[4] Li, Z.N., Yu, M., Wu, H.H., et al. Correlation Study of Measured Wind Fields and Wind Pressures for a Low-Rise Model Building [J]. Journal of Hunan University (Natural Sciences), 2016, 43(5): 70–78.

[5] Huang, B., Li, Z.N., Zhao, Z.F., et al. Near-Ground Impurity-Free Wind and Wind-Driven Sand of Photovoltaic Power Stations in a Desert Area [J]. Journal of Wind Engineering and Industrial Aerodynamics.

[6] Li, Z.N., Wu, W.X., Wang, Z.F. Experimental Study of Near-Ground Wind Field Characteristics in the Suburbs of Beijing [J]. Journal of Building Structures, 2013, 34(9): 82–90.

[7] Hu, S.Y., Li, Q.S. Field Measurement of Wind Loads on Low-Rise Buildings (I): Characteristics of the Near-Ground Boundary-Layer Wind Field During a Landfalling Typhoon [J]. China Civil Engineering Journal, 2012, 45(2): 77–84.

[8] Holland, G.J., McGeer, T., Youngren, H. Autonomous Aerosondes for Economical Atmospheric Soundings Anywhere on the Globe [J]. Bulletin of the American Meteorological Society, 1998, 73.

[9] Jiang, M., Shi, J., Li, J.Q., et al. Experimental Study of Multirotor UAV-Mounted 2D Ultrasonic Wind Sensors [J]. Foreign Electronic Measurement Technology, 2021, 40(5): 88–94.

Product & Application Context

The HY-SA256 is positioned as a compact 2D ultrasonic wind speed and direction sensor for small aircraft, UAVs, and unmanned platforms. Its compact 50 mm × 50 mm form factor, 56 g weight, RS485 digital output, and wide operating-temperature range make it suitable for low-altitude atmospheric measurement, UAV-based environmental monitoring, atmospheric pollutant source tracing, mobile meteorological observation, and other applications where low weight and compact integration are critical.