Investigation on propulsion of flapping wing with modified pitch motion Tiauw Hiong Go and Wang Hao Division of Aerospace Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Abstract Purpose – With possible practical application in a micro aerial vehicle (MAV), propulsion characteristics of a flapping wing with modified pitch motion are investigated both theoretically and experimentally in this paper. Design/methodology/approach – Modified pitch motion is defined as a sinusoidal pitch motion with the pitch axis outside the wing chord line. Based on the momentum theory, an analytical model is developed to analyze the propulsion characteristics of the defined flapping wing. Following that, a water tunnel study of the effects of pitch axis distance, pitch frequency, and stream velocity on thrust generation is carried out. Thrust is directly measured using a 1-D load cell and the flow visualization is captured using a high speed video camera. Findings – It is found that shifting pitch axis outside wing chord line benefits the thrust generation significantly. Positive average thrust is produced at a relatively low frequency and increases almost quadratically with the motion frequency. The effect of stream velocity on the thrust time history is signified but has little effect on the average thrust magnitude. Practical implications – Compared to other types of flapping wing motions, the proposed flapping can be achieved with simple mechanism and thus has the edge in practicality for propelling MAV or other submarine systems. Originality/value – The paper provides useful aerodynamic characteristics of a type of flapping wing motion which possesses mechanical simplicity and relatively large thrust generation at low-flapping frequency. This flapping wing has the potential to provide propulsion for a MAV or other submarine systems. Keywords Aerodynamics, Dynamics, Motion Paper type Research paper Nomenclature A in ¼ area of input flow in X direction A out ¼ area of output flow in Y direction A wing ¼ area of wing platform C T ¼ instantaneous thrust coefficient  C T ¼ average thrust coefficient C d ¼ viscous drag coefficient F cv,x ¼ force on control volume in X direction F cv,y ¼ force on control volume in Y direction _ m in ¼ input mass flow rate in X direction _ m out ¼ output mass flow rate _ m wing ¼ input mass flow rate in Y direction U 1 ¼ stream velocity V pitch ¼ velocity due to pitch motion V pitch ¼ average velocity due to pitch motion V x ¼ flow velocity in X direction V out ¼ average output velocity b ¼ wing span f ¼ frequency k ¼ reduced frequency, pfC/U 1 C ¼ wing chord length L ¼ instantaneous lift R ¼ pitch axis distance to wing-leading edge (positive if away from trailing edge) R/C ¼ dimensionless pitch axis distance T ¼ instantaneous thrust T net ¼ net thrust T ¼ average thrust X, Y ¼ Cartesian coordinates a 0 ¼ pitch amplitude a m ¼ mean angle of attack in pitch motion a ¼ instantaneous pitch angle of attack r ¼ flow density Dm ¼ instantaneous mass inside control volume g ¼ ratio of flow going backwards to forwards I. Introduction Flapping wing propulsion is unique in terms of its capability of providing both lift and thrust for flight vehicles. Driven by flapping wings, birds and insects have demonstrated their superior flight performance, especially in terms of agility and mobility. Hence, the investigation on the use of flapping wing propulsion in micro aerial vehicle (MAV) is very attractive. Flapping wing consists of two main elements: wing attributes and flapping motion. Wing attributes may consist of different geometrical properties such as wing areas, aspect ratio and cross-sections, and material properties such as wing stiffness. The flapping motion element might consist of any motion as long as it is feasible to implement and capable of generating the desired lift and thrust. There have been a lot of efforts to investigate flapping wing propulsion from both the wing attributes and flapping motion aspects. Heathcote et al. (2008) have studied the wing flexibility effects and found that a wing with medium flexibility produced maximum thrust for pure plunge motion. The current issue and full text archive of this journal is available at www.emeraldinsight.com/1748-8842.htm Aircraft Engineering and Aerospace Technology: An International Journal 82/4 (2010) 217–224 q Emerald Group Publishing Limited [ISSN 1748-8842] [DOI 10.1108/00022661011082696] 217