# A Study of Wind Shear Influences on the Aerodynamic Performances of a UAV Airfoil
###### Abstract  
In order to study the flight strategies of birds in shear wind and realize the long-range flight of an unmanned aerial vehicle (UAV), the aerodynamic performances of a UAV's airfoil under different shear wind are studied using numerical methods. The results are calculated using the RANS method in shear wind with a linear and logarithmic distribution of velocity. The results show that the slope of the lift curve, the maximum lift coefficient and the stall angle decrease when the velocity gradient is positive or increasing, conversely, they increase at a negative gradient compared to gradient-free wind fields. A positive gradient of wind significantly increased the maximum lift-drag ratio and the effective angle of attack. Compared with linear distributed shear wind with the same velocity at both the lowest and highest points of the flow field, logarithmic distribution decreases the slope of the lift curve, the maximum lift coefficient and the maximum lift-drag ratio and the effective angle of attack. Therefore, a shear wind with a positive gradient is beneficial to the increase of lift-drag ratio and more conducive to cruise flight, and a negative gradient is beneficial for increased lift and more conducive to take-off and landing of UAVs. The velocity distributions influence the aerodynamic performances of the airfoil which is related to the speed and gradient distribution of shear wind. s +
Footnote †: journal: Medical  
In nature, many birds have the ability to use wind field energy to achieve long-distance flight. For example, frigate birds and albatrosses use shear wind and updrafts [11] to achieve long-distance gliding flight with very low energy consumption by harnessing the energy of the wind field. The king of flight-albatross can traverse strong wind gradients near the sea surface [12] and climbs and glides down at high and low altitudes in shear wind fields. During the flight, they obtain frontal energy and offset drag consumption to achieve long-range flight [13]. Nowadays, the flight range of UAVs is constrained by power supply, and flight time and range are greatly limited. Studies have shown that there are two ways to extend UAV flight range. First, the endurance performance of a UAV can be improved by optimizing the aerodynamic layout and drag reduction design. However, the efficiency is limited. In the second, a UAV can use wind energy through flight attitude adjustment with the help of wind field flow. In this manner, its own energy consumption can be significantly reduced to achieve long-range flight. Therefore, according to the flight mode of albatrosses in shear wind, people have carried out studies on UAV trajectory planning [14], flight strategies [15] and dynamic glide monitoring [16] in shear wind. Flight trajectory optimization [17; 18; 19; 20] and flight attitude adjustment [21] in shear wind have been examined to achieve low-energy flight. However, the flight performance of a man-made glider controlled by a dynamic model is much less [22; 23] than that of the albatross. Imitating the flight attitude and trajectory of birds cannot achieve low-energy flight to achieve long-range flight, and analysis of aerodynamic performance under a flow field environment should be carefully performed to achieve excellent performances. Therefore, according to the safety and efficiency requirements of UAV flight, utilizing the shear wind energy during flying just like a bird can be a good bionic inspiration, which is considered to be favorable for UAV design. The shear wind effect on the aerodynamic performance of a UAV's airfoil is studied by numerical simulation. A series of shear winds are generated, and the impact of shear wind velocity distribution on aircraft flight performance is investigated. Lift enhancement, drag reduction and range increase are the expected result for UAV flight. How to perform flight control from the perspective of aerodynamics in shear wind is studied in the current research.