TECHNICAL ARTICLE Effect of Skin Flexibility on Aerodynamic Performance of Flexible Skin Flapping Wings for Micro Air Vehicles H. Yusoff 1 , M.Z. Abdullah 1 , M. Abdul Mujeebu 2 , and K.A. Ahmad 3 1 School of Mechanical and Aerospace Engineering, Science University of Malaysia (Universiti Sains Malaysia), Engineering Campus, Nibong Tebal, Penang, Malaysia 2 Department of Aerospace Engineering, Putra University, Malaysia (Universiti Putra Malaysia), Serdang, Selangor, Malaysia 3 Department of Mechanical Engineering, Anjuman Institute of Technology and Management, Bhatkal, Karnataka, India Keywords Flapping Wing, Skin Flexibility, Lift, Drag, Angle of Attack, Micro Air Vehicle Correspondence M.Z. Abdullah, School of Mechanical and Aerospace Engineering, Science University of Malaysia (Universiti Sains Malaysia), Engineering Campus, Nibong Tebal, Penang Malaysia Email: ham_mid2003@hotmail.com Received: November 7, 2011; accepted: October 31, 2012 doi:10.1111/ext.12004 Abstract As part of the ongoing research on micro air vehicles, the present work focuses on the effect of membrane flexibility on the aerodynamic performance of flexible latex flapping wings. Wings with membrane thicknesses 0.37, 0.28, and 0.13 mm are chosen, which are named as least flexible (A), flexible (B), and most flexible (C), respectively. The experiments are performed in an air chamber of size 1.5 m × 1.5 m × 1.5 m, facilitated with wind velocities up to 15 m/s. The time-averaged lift and drag as functions of flapping frequency, forward flight velocity, the angles of attack (AoA), and advance ratio (J ). The novel electronic control system developed previously is used to monitor and measure the flapping frequency. It is found that the effect of flexibility on the aerodynamic performance mainly depends on the range of flight speed; at 7200 ≤ Re ≤ 18,000, the lift and drag increase with increase of flexibility, and at 18,000 ≤ Re ≤ 25,200, the lift decreases and drag increases with increase of flexibility. Hence latex compliant (Wing C) wings are advantageous in the low Re range, while the least flexible wing (Wing A) is preferable for higher range. Introduction Micro air vehicle (MAV) is a promising option for situations where a larger vehicle is impractical or impossible to be flown either autonomously or by a remote pilot. MAVs, by definition, are a class of unmanned aircraft with a maximum size limited to 15 cm, capable of operating speeds of 15 m/s or less. In other words, MAVs are characterized by low Reynolds numbers (Re) and low aspect ratio (LAR). 1 They are used in military and defense tasks such as over-the-hill battlefield supervision, bomb damage assessment, chemical weapon detection, etc. They also have applications in environmental study, agriculture, wildlife and traffic-monitoring. Fixed, flapping, and rotary wing MAVs are all viable candidates for these missions. Excellent reviews on this topic were provided by Shyy et al. 2,3 The rapid advance in MAV design has driven the researches on flight of insects, birds, and bats. LAR wings composed of thin and very flexible membranes are unique to flying and gliding mammals, such as bats, flying squirrels, and sugar gliders and these animals exhibit extraordinary flight capabilities with respect to maneuvering and agility that are not observed in other species of comparable size. 1 A few experimental works which are pertaining to the current study are summarized here. The founding researches on bat flight were summarized by Norberg 1 who studied the kinematics, aerodynamics, and energetics of the long-eared bat Plecotus auritus in slow horizontal flight. Similar study was reported by Aldridge, 4 on the greater horseshoe bat, Rhinolophus ferrumequinum, in horizontal flight at various speeds. Galvao et al. 5 reported experiments on the aerodynamics of compliant membrane wing models of LAR, performed at low Re, ranging from 30,000 to 100,000. Lift and drag coefficients over a range of angles of attack (AoA) from −5 to 60 ◦ and the Experimental Techniques 39 (2015) 11 – 20 © 2012, Society for Experimental Mechanics 11