Hover Performance of a Small-Scale Helicopter Rotor for Flying on Mars

摘要

The present study is in response to increased interest towards assessing the feasibility of a small-scale autonomous helicopter (gross weight less than 1 kg) for Martian exploration. An autonomous rotorcraft may be ideally suited for such an application because of its unique advantages, which include the ability to take off/land vertically on harsh terrain, and greater speed, range, and field of view, when compared to a traditional surface rover. The atmospheric conditions on Mars present a unique set of design challenges. Even though the Martian gravity is only about 38% of Earth's gravity, the Martian average atmospheric density is about 70 times lower than Earth's atmospheric density. Therefore, the rotors would be operating at extremely low Reynolds numbers, even lower than 5000 for a small-scale helicopter. However, the Mach number will be significantly higher (M > 0.4) because of the higher tip speed required (due to lower density) and because of the fact that the speed of sound on Mars is only about 72% of the speed of sound on Earth. This low-Reynolds-number, high-Mach-number flow condition on the blade imposes severe constraints on the rotor design. The solution proposed in the present study involves scaling up the rotor size to produce the required thrust at acceptable Mach and Reynolds numbers. The hover performance of a full-scale rotor for a 200 g Martian coaxial helicopter was experimentally evaluated in an evacuation chamber, where the exact Martian air density was simulated. The maximum figure of merit obtained for the baseline rotor was less than 0.4 at an operating Reynolds number of 3300 and Mach number of 0.34. Increasing the Reynolds number at a constant Mach number by changing the air density increased the figure of merit of the same rotor to over 0.6 at a Reynolds number of 35,000. As the Reynolds numbers was decreased to ultralow values (Re < 5000), the blade collective pitch angle for maximum figure of merit increased even up to 30 deg. A key conclusion from this study is the feasibility of small-scale hovering flight on Mars with a realistic endurance (12-13 min).