MAV08 1st US-ASIA MAV conference and competition
"Rotor Based Micro Air Vehicles: Challenges and Opportunities” Inderjit Chopra Alfred Gessow Professor & Director Alfred Gessow Rotorcraft Center, Department of Aerospace Engineering University of Maryland, College Park, MD 20742 A Micro air vehicle (MAV) is envisaged to be a small-scale autonomous flying vehicle (with no dimension larger than 15 cm) intended for reconnaissance over land, in buildings and tunnels, and in other confined spaces. While some progress has been made in this field, no vehicle has been able to achieve long-loiter time (over 60 minutes) and hover at weights less than 100 grams with a payload of about 20 grams. Several factors contributed to this poor performance including lack of understanding of aerodynamic, structural, and propulsion physics at the micro-scale. Also, none of these configurations can carry avionics packages that would permit robust navigation in complex urban environments. In contrast, nature has evolved thousands of miniature flying machines (insects and small birds) that perform far more difficult missions. While details underlying the operational success of biological fliers remain an ongoing research endeavor, a general picture is emerging that indicates that the overwhelming superiority of biological fliers over existing MAVs stems from two fundamental factors: ability to generate lift more efficiently than existing technologies and ability to store and release energy efficiently. Two efficient hovering configurations can be used to generate sufficient thrust to sustain weight; rotary-wings and flapping-wings. The rotary-wing approach has proven successful in the high Reynolds number regime (>106), where inertial forces dominate flow characteristics. However, in the low Reynolds number regime that scales MAV flight physics, it is not clear which solution is more efficient. Hence, both hovering concepts are being examined at this time. To develop such vehicles, challenges include: low Reynolds number flow regime (~104), low altitude environment (gust and obstacles), size and weight constraints, compact power generation and storage, micro actuators, strong aeroelastic couplings, and stringent navigational and guidance requirements. Among hovering air platforms, rotor-based platforms appear more advanced at this time than flapping-wing-based vehicles. The objective of this presentation is to cover the state-of-art on design concepts and aeromechanics of rotor-based MAVs, and identify key barriers for future research. Covered configurations will include: single main rotor with turning vanes, coaxial rotor, shrouded rotor, flapping rotor, and cycloidal rotor systems.
Because of dominant viscous effects at low Reynolds numbers environment, hover figure of merit of current MAVs ranges from 0.4 to 0.65, a number far below the full-scale value of about 0.8. Cambered plates with maximum camber (6.75%) ahead of mid-chord, and a sharp leading edge exhibited the best hover performance. Detailed flow visualization of a single rotor using laser sheet showed evidence of highly non-ideal inflow wake distribution and a significant blocked flow in the center. At a given disk loading, there is a specific combination of rotor speed and collective pitch at which the power loading becomes optimum. It is possible to counteract the torque of main rotor by installing active turning vanes in the downwash of main rotor. Incorporation of duct around a rotor resulted in an increase of total thrust by 25% for a given electric power. A coaxial rotor configuration is compact, but can be less efficient in hover because of aerodynamic interference between rotors. Providing a shroud around a single rotor would not only improve hover performance of the system, but would also serve as a safety feature. However in forward flight, the shrouded rotor resulted in more drag and pitching moment compared to the free rotor. In a cycloidal rotor, it is envisaged to have a superior aerodynamic efficiency than a conventional rotor. Thee-bladed cycloidal rotor was found to be more efficient than a six-bladed rotor. To improve rotor performance of a single rotor especially at high thrust levels, it may be possible to
MAV08 1st US-ASIA MAV conference and competition
exploit the unsteady aerodynamic effects using a flapping rotor.
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