Micro Air Vehicles (MAVs) have the potential to revolutionize our sensing and information gathering capabilities in environmental monitoring and homeland security areas. Due to the MAVs" small size, flight regime, and modes of operation, significant scientific advancement will be needed to create this revolutionary capability. Aerodynamics, structural dynamics, and flight dynamics of natural flyers intersects with some of the richest problems in MAVs, including massively unsteady

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... al separation, transition in boundary layers and shear layers, vortical flows and bluff body flows, unsteady flight environment, aeroelasticity, and nonlinear and adaptive control are just a few examples. A challenge is that the scaling of both fluid dynamics and structural dynamics between smaller natural flyer and practical flying hardware/lab experiment (larger dimension) is fundamentally difficult. In this paper, we offer an overview of the challenges and issues, along with sample results illustrating some of the efforts made from a computational modeling angle. 2 flows, unsteady flight environment, aeroelasticity, and nonlinear and adaptive control are just a few examples. The large flexibility of animal wings leads to complex fluidstructure interactions, while the kinematics of flapping and the often spectacular maneuvers performed by natural flyers result in highly coupled nonlinearities in fluid mechanics, aeroelasticity, flight dynamics, and control systems. The agility and flight performance of natural flyers is of particular interest to the aerospace community, from the viewpoints of both fundamental engineering science and the development of miniaturized flight vehicles. For all of the maturity of aerodynamics as an engineering discipline, our understanding of flight in natural flyers presently stands far from complete. There are several distinct features of natural flyers in their flight characteristics. For example, (i) for natural flyers, there is substantial anisotropy in the structural characteristics between the chordwise and spanwise directions, (ii) natural flyers employ shape control to accommodate spatial and temporal flow structures, (iii) natural flyers accommodate wind gust and accomplish station keeping with several established kinematics patterns, (iv) natural flyers utilize multiple unsteady aerodynamic mechanisms for lift and thrust enhancement, and (v) natural flyers combine sensing, control and wing maneuvering to maintain not only lift but also flight stability. In principle, one might like to first understand a biological system, then abstract certain properties and apply them to MAV design. A challenge is that the scaling of both fluid dynamics and structural dynamics between smaller natural flyer and practical flying hardware/lab experiment (larger dimension) is fundamentally difficult. Regardless, in order to develop a satisfactory flyer, one needs to meet the following objectives:  generate necessary lift, which scales with the vehicle/wing length scale as 3  (under geometric similitude); however, oftentimes, a flyer needs to increase or reduce lift to maneuver toward/avoid an object, resulting in substantially more complicated considerations;  minimize the power consumption. An optimal design based on a single design point, under a given steady freestream value, is insufficient; instead, we need to develop knowledge base guiding future design of MAVs across a range of wind gust and flight speed and time scales so that they can be optimal flyers defined by the entire flight envelop. In this paper, we will offer our perspective regarding the issues, progress, and challenges associated with unsteady low Reynolds number aerodynamics pertaining to MAV development. In particular, we will discuss plunging, pitching and flexible wings. In the following, we will first review the scaling parameters, and then discuss kinematics used by natural flyers. The wing flexibility and its implications will also be reviewed.