## 1 Introduction  
Over a twenty-year period, 40% of all weather-related aircraft accidents that involved U.S. airlines were related to rainfall, which is more than the share of accidents attributed to wind shear and turbulence [1], and the majority occurred during takeoff and landing. It has historically been difficult to explore the impact of rain-induced penalties on the flight performance, and to develop countermeasures; so far, there is no certain way to deal with the threat of extremely heavy rainfall to flight safety. For commercial aircraft, the safest procedure has been simple avoidance: not taking off, or flying at a distance, until the torrent passes. Unlike heavy and powerful commercial and military aircraft, the aquatic UAV [2] has a small size, light mass, and low rotational inertia [3], and so it is more sensitive to the impact of precipitation and the drag force of adhering water; the adhesion of the liquid film has a more significant adverse effect on its flight in air and its exit from water [4, 5]. Very few physical tests of aircraft in rain have been reported, and most of them are relatively outdated [6, 7, 8]. At present, the methods for evaluating the impact of precipitation on aircraft are mainly the real-world evaluation of the aircraft [9], model tests [10], and numerical simulations using computational fluid dynamics [11]. Testing with the actual aircraft is the closest to the actual situation in the natural environment, but factors such as high costs, environmental-variable interference, and safety risks limit this research method. A cost-saving yet accurate research method is to select a rationalized model, use it in physical precipitation experiments, and combine the results with numerical simulations. If the rapid and efficient desorption of the adherent water from the surfaces of aquatic UAVs [2; 12] can be achieved, then the negative flight effects from precipitation and adherent water can be reduced to low levels. Aquatic UAVs can enter the water, and they also require excellent dehydration capabilities; the solution for precipitation may be the same one that enables flight promptly after submersion, but there are few reports on how to actively reduce the water adhesion on the model after extended solid-liquid interaction. The aquatic UAV [13; 14] is subject to rapid density changes and complex solid-liquid-gas phase transients that can impair the flight balance when transitioning from water to air. The current micro aerial vehicles (MAVs) are limited in their application because they can only fly. Once the MAVs can be both underwater and in the air [15], they potentially have applications in search and rescue, expeditions, and even military reconnaissance. However, because an MAV has no high-power propulsion system, it must be designed to minimize drag under water and to be efficient upon returning to flight. The inability to rapidly and efficiently desorb the adherent water from surfaces is one of the main bottlenecks that is currently limiting the development of aquatic UAVs. Unique biomimetic surfaces [16] can substantially reduce the adhesion of water. The water droplets on lotus leaves after rain are almost spherical, and they easily roll off, even at a very low inclination; this phenomenon is known as the \"Lotus effect\" [17]. To characterize the \"lotus effect\", the static contact angle (\(\theta\)) and roll-off angle (\(\theta\)s) should be considered simultaneously [18], and the superhydrophobic state is achieved when \(\theta>150^{\circ}\) and \(\theta\)s \(<10^{\circ}\)[19]. The feathers of some small aquatic birds, such as kingfishers [20; 21; 22] and pigeons [23], also exhibit similar superhydrophobic, which is not only due to the hydrophobicity of the feathers (\(\theta\) as high as \(160^{\circ}\)), but also to the anisotropy [24; 25]. There are essential differences in the mechanisms of the liquid repellency exhibited by superhydrophobic feathers and the \"lotus effect\".