## 1 Introduction  
In recent years, Martian aerial vehicles have become a hot research topic due to the need for the performance of an aerial survey on Mars [1]. Compared with ground-based Martian rovers, aerial vehicles have unique advantages of denying terrain, fast flying, and large area coverage, making them suitable for tasks that ground rovers cannot accomplish [2]. Various forms of Martian aerial vehicles, such as the micro air vehicle [3, 4], flying wing airplane [5], and rotor crafts [6, 7], have been designed to achieve various tasks. For example, the National Aeronautics and Space Administration (NASA) Ames Research Center designed a Martian helicopter to study highlands on Mars where the Martian rovers cannot reach [8]. [PERSON] et al. conducted a parametric design of a Mars helicopter for pit crater exploration [9]. The Jet Propulsion Laboratory even designed and fabricated a Mars helicopter to test the feasibility of an actual helicopter flying on Mars [10], called the Ingenuity Helicopter, and it has been successfully flied on the red planet [11, 12]. However, designing a Martian aerial vehicle is very challenging since the atmospheric environment of Mars is quite different to Earth's, such as having very low air density,pressure, and temperature [13]. The Martian air, mostly consisting of carbon dioxide, has a density and pressure as low as about 0.017 kg/m\({}^{3}\) and 600 Pa, respectively. The unique atmospheric conditions require Martian aerial vehicles to fly in a compressible flow with a very low Reynolds number, which is not normally seen in Earth aerial vehicle applications [14]. Therefore, an understanding of the aerodynamic performance of airfoils, especially for rotorcraft and propeller-based aircraft, becomes critical. Compared with a conventionally high Reynolds number flow, flow at a low Reynolds number shows unique behavior including separation, transition, and reattachment on the wing surface, which strongly affects the aerodynamic performance of airfoils [15,16]. There are experimental [17,18] and numerical studies [19,20] available for helicopters [21] and fixed-wing aircraft [22,23] used on Mars, and most of them have focused on the aerodynamic performance of airfoils in low-Reynolds-number flows. [PERSON] et al. experimentally investigated five thin airfoils at \(Re=1.1\times 10^{4}\) and with a Mach number of 0.2 in a Mars wind tunnel [15] using the pressure-sensitive paint technique. [PERSON] et al. in Tokyo University carried out a numerical study on the aerodynamic optimization of airfoils for flying-wing type Martian aircraft [5]. [PERSON] et al. conducted a simulation work on the accuracy of low and high-fidelity numerical approaches in predicting the aerodynamic forces at low Reynolds numbers and moderate subsonic Mach numbers [24]. The results prove that 2D Navier-Stokes computations could be used for airfoil shape optimization in the context of low Reynolds number with reasonable accuracy. [PERSON] et al. carried out a direct numerical simulation on a triangular airfoil to understand its nonlinear lift at a low-Reynolds-number compressible flow by comparing the results with experimental data from the Mars wind tunnel at Tohoku University [25]. Their results revealed that the nonlinear aerodynamic behavior of the triangular airfoil is brought by the lift enhancement attributed to the large leading-edge vortex generated. However, most of the study focused on the characteristics of 2D airfoils, while the rotor of a Mars helicopter works in a condition with a significant three-dimensional effect; moreover, the flow characteristics are much more complex than a two-dimensional airfoil section. Therefore, further research is needed. Several universities in China are now undergoing experimental research on this topic. For example, [PERSON] and [PERSON] et al. from the Harbin Institute of Technology carried out several experimental investigations on the hover performance of a single rotor system for a Mars helicopter in a Martian atmosphere simulator [21].