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This paper describes a fundamental experimental study, which involved systematic performance and flowfield measurements (PIV) to understand and optimize the hover performance of a MAV-scale helicopter rotor operating at Reynolds numbers lower than 30,000. The rotor parameters that were varied include blade airfoil profile, blade chord, number of blades, blade twist, planform taper and winglets at blade tip. Blade airfoil section had a significant impact on the hover efficiency and among the large number of airfoil sections tested, the ones with the lower thickness to chord ratios and moderate camber (4.5% to 6.5%) produced the highest rotor hover figure of merit. Increasing the solidity of the rotor by increasing the number blades (with constant blade chord) had minimal effect on efficiency; whereas, increasing the solidity by increasing blade chord for a 2-bladed rotor, significantly improved hover efficiency. Moderate blade twist (-10t o -20˚) and large planform taper (larger than 0.5) marginally improved rotor efficiency. Rotor blades with small winglets (height ≈ 6% of rotor radius) at the tip also improved hover performance. While using winglets, the flowfield measurements showed a diffused tip vortex, which could reduce the induced aerodynamic losses. Spanwise lift distribution obtained using sectional bound circulation computed from the measured flowfield correlated well with the load cell measurements. The optimal rotor designed based on the understanding gained from the present study produced a figure of merit of 0.67, which is the highest value of FM ever reported in the literature for micro-rotors operating at these low Reynolds numbers. INTRODUCTIONMicro Air Vehicles (MAVs) are small-scale aerial platforms, which are envisioned to have a wide range of both military and civilian applications. Being small and compact systems, MAVs offer several advantages such as portability, rapid deployment, real-time data acquisition capability, low radar cross section, low noise signatures and low production cost. The micro air vehicle concept was first proposed by DARPA back in 1997 [1] and according to DARPA's original definition, the size of these vehicles has to be within 6 inches (0.154 m) with a gross weight of 100 grams (including 20 grams payload) and a flight endurance of 60 minutes. However, even after substantial progress in the last two decades, the fact that none of the current MAVs are even close to achieving this endurance goal (60 mins), is a true testament to the difficulty of this problem. The key reasons for this are the inefficiencies associated with the low Reynolds number aerodynamic regime at which these vehicles operate and challenges in smallscale power generation and storage. Note that, one of the least understood aspect of small-scale flight is its aerodynamic performance, which is the focus of the current study.From an aerodynamics perspective, a key challenge for a MAV designer is the low lift-to-drag ratio of even the most optimized airfoil geometries at low Reynolds numbers. Several fi...

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This paper describes the experimental design studies conducted to maximize the hover endurance of a micro air vehiclescale quadrotor helicopter (gross weight < 50 g) by careful design and selection of the individual subsystems while understanding their interdependencies. The different subsystems of the quadrotor that were examined include the rotors, motors, electronic speed controllers, gear transmission systems, battery, and airframe structure. An optimized micro air vehicle-scale rotor with a diameter of 110 mm and a figure of merit of 0.67 served as the basis for the vehicle design. Brushed and brushless motor systems were paired with the optimized rotor and were examined for maximum system level efficiency and capability to produce the required thrust. Propulsion systems using brushed motors with speed controllers and the optimum gear ratio showed up to 35% improvement in efficiency over brushless outrunner motors with speed controllers. Lithium-ion polymer batteries were compared and evaluated on voltage drop-off characteristics and achievable endurance versus weight. Quadrotor airframe weight has been minimized to 7.4% of the gross weight, representing a 30-50% decrease compared existing similarly sized quadrotors. The highest endurance was obtained using brushed motors with a 5.33:1 gear ratio and a 650 mAh battery. Presently, the longest continuous hover endurance achieved with this design is 31 min.

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