## 1 Introduction  
Rapeseed is one of the most important oil crops in the world. Aside from its oil crop function, it is also planted as important production sources of honey, forage, and manure and as tourist attractions [1]. Its multi-function application attributes determine that it is planted in different regions of China from plains to hills and from river valleys to shore fields. This situation also leads to a diversity of rapeseed planting methods including direct seeding and transplanting by manual labor, direct seedingand transplanting by ground machines, and direct seeding by aerial vehicles. Overall, the current mechanization level of rapeseed seeding in China is still quite low, far below grain crops such as wheat, rice, and corn. To exploit the multi-function application potential of rapeseed, increase the growers' income, and strengthen their enthusiasm to keep on planting it, there is an urgent need to raise the mechanization level of rapeseed planting in China to save production costs. Near-ground aerial seeding is a seeding method worth studying. Since the field surface is not touched while the unmanned aerial vehicle (UAV) works at high speeds, it can work in fields of different terrain conditions [2; 3; 4]. Aerial seeding would be a beneficial method supplementary to ground planting, providing a solution for rapeseed planting in regions where ground machines cannot enter or can only enter with very low economic efficiency. Furthermore, the fact that rape seeds are small and can germinate and grow easily after being spread to the soil surface increases the potential of applying aerial seeding. European and American countries have a long history of aerial seeding, especially for cover crops grown in forests and praaries, and the operation is usually done by manned helicopters or fixed-wing aircraft platforms [4; 5]. These kinds of equipment are large and expensive, and the seed landing spots are not accurately controlled. These features make them unsuitable for use in small-size fields, which are very common in southern China. In recent years, with the continuous advancement of technologies such as positioning and navigation, battery, and flight control, the application of small multi-rotor UAVs is developing rapidly, and they have been applied successfully in agricultural operations including remote sensing, mapping, and plant protection [6; 7; 8]. Some tech companies in China have started adding aerial seeding capabilities into their agricultural UAVs to make them more versatile and acceptable. Enterprises such as Zhuhai Yuren, Shenzhen D.J. [9], and Guangzhou X.A.G. [10] have added spreading devices or systems for granular materials to their commercial UAVs to expand their aircrafts' functions rather than plant protection only. The spreading system of DJ and some other enterprises adopted a centrifugal-disc seeder that was first proposed by [PERSON] et al. [11] from South China Agricultural University. Then, the XAG JetSeedTM spreading system and the system reported in [PERSON] et al. [12] utilized an external grooved wheel seed to quantify seed supply and a ducted fan to accelerate the seeds' movement. [PERSON] et al. [13] from Huazhong Agricultural University proposed a special centrifugal seeding system based on an XAG P20 UAV. The system could realize seeding rape seeds in lines with the help of a specially designed seed guiding tube. However, the tube was required to be manually hitched to the UAV by a person when the UAV was hovering, which posed an operational risk. Unlike the above aerial seeding systems, the aerial seeding system in this study was based on a miniature air-assisted centralized seed meter adopted from a ground rapeseed planter [14]. However, the seed meter was redesigned to fit a six-rotor UAV's structure and dimensions, especially on the seed feeding component to overcome problems of serious air backflow and difficult seed feeding after miniaturization.