Prof. Inderjit Chopra Alfred Gessow Prof. of Rotorcraft Engineering Director, Alfred Gessow Rotorcraft Center Department of Aerospace Engineering, University of Maryland College Park MD 20742 [email protected] Tel: 301-405-192; Dr. Inderjit Chopra is the Alfred Gessow Professor in Aerospace Engineering and Director of Rotorcraft Center at the University of Maryland. He received his M.E. (Aero) from Indian Institute of Science, Bangalore in 1968 and his Sc.D. (Aero & Astro) from MIT in 1977. He worked at the National Aerospace Labs, Bangalore form 1966 to 1974. His research there included wind tunnel testing of scaled aeroelastic models of airplanes and launch vehicles. In 1977, he joined NASA Ames/Stanford University Joint Institute of Aeronautics & Acoustics, where he worked on the aeroelastic analysis and testing of advanced helicopter rotor systems. He has been working on fundamental problems associated with aeromechanics of helicopters, smart structures and micro air vehicles. His graduate advising resulted in 38 Ph.D. and 70 M.S. degrees. An author of over 165 archival papers and 270 conference papers, Dr. Chopra has been an associate editor of Journal of the American Helicopter Society (1987-91), Journal of Aircraft (1987-cont.) and Journal of Intelligent Materials and Systems (1997-cont.). Also, he has been a member of the editorial advisory board of various journals. He was the recipient of 1992 UM’s Distinguished Research Professorship, 1995 UM’s Presidential Award for Outstanding Service to the Schools, 2001 ASME Adaptive Structures and Material Systems Prize, 2002 AIAA Structures, Structural Dynamics and Materials Award, 2002 AHS Grover Bell Award, 2002 A. J. Clark School of Engineering Faculty Outstanding Research Award, and 2004 SPIE Smart Structures & Materials Lifetime Achievement Award. He has been a member of the Army Science Board (19972002) and NASA Aeronautics and Space Engineering Board (2007-cont.). He is a Fellow of AIAA, American Helicopter Society, ASME, Aeronautical Society of India and National Institute of Aerospace. Rotor Based Micro Air Vehicles: Challenges and Opportunities 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 proved 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 (gusts 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-the-art 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, 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 the main rotor by installing active turning vanes in the downwash of the 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 to 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 exploit the unsteady aerodynamic effects using a flapping rotor.