The Airflow Field Characteristics of the Unmanned Agricultural Aerial System on Oilseed Rape (<i>Brassica napus</i>) Canopy for Supplementary Pollination

Pollination success is essential for hybrid oilseed rape (OSR, <i>Brassica napus</i>) seed production, but traditional pollination methods are not efficient. The unmanned agricultural aerial system (UAAS) has developed rapidly and has been widely used in China. When flying, the wind field generated by the rotors overcomes the UAAS gravity, and it blows and disturbs the crops below, which helps the pollen spread. In order to investigate the distribution law of the three-dimensional (direction <i>x</i>, <i>y</i>, <i>z</i>) airflow field, experiments involving three levels of flight speed (FS) at 4.0, 5.0, and 6.0 m/s, and three levels of flight height (FH) at 1.5, 2.0, and 2.5 m were conducted in the OSR field by using an electric four-rotor UAAS P20. The effects of FS and FH on airflow velocities (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>x</mi></msub></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>y</mi></msub></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>z</mi></msub></mrow></semantics></math></inline-formula>) were analyzed. High-speed dynamic camera (HSDC) technology was used to capture the swings of OSR plants under airflow field disturbance. OSR pollen samples were collected during the experiments. The results showed that the airflow field in the direction <i>x</i> was mainly concentrated on the center of the flight path (S3), and the maximum wind velocity of direction <i>x</i> was 8.01 m/s (T1, S3). The direction <i>x</i> airflow field width was distributed almost symmetrically, but the center position shifted easily, due to crosswind. The airflow field in the direction <i>y</i> was distributed on both sides of the center flight path, and the velocity was generally larger, with the maximum at 7.91 m/s (T1, S2). The airflow field in the direction <i>z</i> was distributed irregularly, and the velocity was small. The FH had highly significant impacts on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><msub><mi>v</mi><mi>x</mi></msub></mrow></semantics></math></inline-formula> (<i>p</i> < 0.01), and the interaction of FS and FH had significant impacts on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><msub><mi>v</mi><mi>x</mi></msub></mrow></semantics></math></inline-formula> (0.01 < <i>p</i> < 0.05), while the FS had no significant impact on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>x</mi></msub></mrow></semantics></math></inline-formula> (<i>p</i> = 0.70804 > 0.05). The FS, FH, and interaction of FS and FH all had highly significant impacts on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>y</mi></msub></mrow></semantics></math></inline-formula> (<i>p</i> < 0.01). The swings of the OSR plant captured by the HSDC proved that the UAAS airflow field could effectively blow the OSR plant. The swing amplitude changes showed a positive correlation with airflow velocities (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>v</mi><mi>x</mi></msub></mrow></semantics></math></inline-formula>) in general. Although the observed OSR plant swung forward and backward repeatedly, there was a law of first forward, and then backward, and forward again at the beginning of each swing. The pollen collected on the sampler verified that the UAAS airflow field could help with pollen spread. The research results provide technical support for UAAS application on supplementary pollination for hybrid OSR seed production.