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# Geometry shape selection of NACA airfoils for Mars rotorcraft

Acta Astronautica, (2019): 300-309

Abstract

Mars rotorcraft, as an aerial auxiliary platform can assist Mars rover to complete exploration missions, is of great significance in the field of planetary exploration. The complex terrain on Mars limits the working range of the Mars rover, whereas a rotorcraft can acquire high detection speed and efficiency, even fixed-point sample using...More

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Introduction

- The feasibility of a small scale aircraft for the exploration of Mars is of great concern.
- The lift and drag characteristics and mechanical power required of rotor system can be directly measured by a hover test stand which can vary the rotational speed and collective pitch angle of rotor in a large range.

Highlights

- The feasibility of a small scale aircraft for the exploration of Mars is of great concern
- Since the flight condition on Mars is a thin atmosphere with low density of air and low temperature, the rotor blade of the vehicles must meet the challenge of poor aerodynamic characteristics resulting from the low Reynolds numbers and high Mach numbers present in the spanwise direction
- Where Clα is the lift-curve slope, which is a function of α and is nonlinear in the low Reynolds number regime; α is the angle of attack; α0 is the zero-lift angle of attack, which is zero in the computational fluid dynamics simulation; θ is the blade pitch angle; λ is the rotor inflow ratio caused by the climb velocity and induced velocity, which is a function of α; and r is the nondimensional radial distance (r = y/R)
- The ultralow density of air and low temperature on Mars require the Mars rotorcraft to fly in the low Reynolds number and high Mach number regime, which is not present in conventional Earth aircrafts
- The rotor with collective pitch angles ranges between 24∘ and 40∘ at the rotational speed ranges between 4500 r/min and 6000 r/min can achieve the required thrust, as shown in Fig. 8(a)
- In the rotor blade design, more than 10% of the blade root is to be cut-out, which is slightly lower than that of the traditional earth rotorcraft [29,30]
- The Blade Element Momentum Theory-computational fluid dynamics model is proposed to evaluate the rotor performance during the hovering process, and the results show that the rotor can generate a thrust to balance a 250 g aircraft under the collective pitch angle of 20. 1∘ to obtain a figure of merit of 0.36

Results

- This work focuses on the hover performance test for a full-scale rotor to optimize the airfoil, select a suitable collective pitch angle, and the rotational speed.
- Where Clα is the lift-curve slope, which is a function of α and is nonlinear in the low Reynolds number regime; α is the angle of attack; α0 is the zero-lift angle of attack, which is zero in the CFD simulation; θ is the blade pitch angle; λ is the rotor inflow ratio caused by the climb velocity and induced velocity, which is a function of α; and r is the nondimensional radial distance (r = y/R).
- To obtain the aerodynamic characteristics of the airfoils, CFD method is adopted to calculate the airfoil coefficients and analyze effect of airfoil parameters on airfoil performance in the low Reynolds number regime.
- It means the proposed BEMT-CFD model, to a certain degree, can be used to predict the rotor hover performance with collective pitch angle over a range from 0 to 20∘ with error less than 10%.
- To accurately measure the rotor performance, the seesawed hover stand in Fig. 7 is designed to measure the thrust, required power and rotational speed.
- The rotor with collective pitch angles ranges between 24∘ and 40∘ at the rotational speed ranges between 4500 r/min and 6000 r/min can achieve the required thrust, as shown in Fig. 8(a).
- It means a higher CT/CP and CT3/2/CP of the rotor with collective pitch angle around 34∘ can be obtained by decreasing the rotational speed of the rotor, in a reasonable range of the required power.
- Fig. 12 shows the power loading as a function of the generated thrust of the rotor with different collective pitch angles.

Conclusion

- For the single rotor configuration of Mars rotorcraft, the suitable collective pitch angle is about 36∘, and the rotational speed is around 5000 r/min, leading to a FM of 0.58 and a PL of 0.47 N/W.
- The BEMT-CFD model is proposed to evaluate the rotor performance during the hovering process, and the results show that the rotor can generate a thrust to balance a 250 g aircraft under the collective pitch angle of 20.

- Table1: Optimization for the airfoil parameters
- Table2: Comprehensive comparison for high-performance airfoils
- Table3: Rotor performance predictions at different collective pitch angles θ

Funding

- We acknowledge the financial support for this work provided by the National Natural Science Foundation of China under Grant (61403106) and the partial support by the Program of Introducing Talents of Discipline to Universities under Grant (B07018)

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