{"references": ["Seiler, P., Pant, A. and Hedrick, K. \"Analysis of bird formations\",\nProceedings of 41st IEEE Conference on Decision and Control, Las\nVegas, Nevada, USA, December 2002.", "Poore, S. O., Sanchez-Heiman, A. and Goslow, G. E. Jr., \"Aircraft\nupstroke and evolution of flapping flight\", Nature, Vol 387, pp 799-802,\nJune 1997.", "Steven Hoa, Hany Nassefa, Nick Pornsinsirirakb, Yu-Chong Taib, Chih-\nMing Hoa, \"Unsteady aerodynamics and flow control for flapping wing\nflyers\", Progress in Aerospace Sciences, 39, 635-681, 2003.", "Bangash, Z. A., Sanchez, R. P. and Ahmed, A., \"Aerodynamics of\nFormation Flight, Journal of Aircraft\", Vol 43, No. 4, pp 907-912, July-\nAugust 2006.", "Vachon M., Ray R., Walsh K., Ennix, K., \"F/A-18 Aircraft Performance\nBenefits Measured During the Autonomous Formation Flight Project\",\nAIAA 2002-4491, Aug 2002.", "Bowles R. G. A. and Smith F. T., \"Lifting multi-blade flows with\ninteraction\", J. Fluid Mech., vol. 415, pp. 203-226, 2000.", "D. Fanjoy, and D. J. Dorney, \"A study of tandem-airfoil interaction in\ndifferent flight regimes,\" AIAA 97-0515, 1997.", "L. Zannetti, F. Gallizio, and G. M. Ottino, \"Vortex motion in doubly\nconnected domains,\" J. Fluid Mech., vol. 612, pp. 143 - 152, 2008.", "Z. Husain, M. J. Abdullah, and T. C. Yap, \"Two-dimensional analysis of\ntandem/staggered airfoils using computational fluid dynamics,\"\nInternational Journal of Mechanical Engineering Education 33/3.\n[10] Katz, J. and Plotkin, A., Low-speed Aerodynamics, Cambridge University\nPress, 2001.\n[11] A. J. Chorin, and P. S. Bernard, \"Discretization of a vortex sheet, with an\nexample of roll-up,\" J. of Computational Physics., vol. 13, No. 3, 1973.\n[12] A. Fage and F. C. Johansen, \"On the flow of air behind an inclined flat\nplate of infinite span\" Proc. Roy. Soc. A 116, 170, 1927.\n[13] T. Nakagawa, \"On Unsteady Airfoil-Vortex Interaction\", ACTA\nMECHANICA 75, 1-13, 1988\n[14] H. Wagner, \"Uber die Entstehung des Dynamischen Autriebes von\nTragflugeln\", Z.F.A.M.M., Vol. 5, No. 1, pp 17-35, 1925."]}

A potential flow model is used to study the unsteady flow past two airfoils in configuration, each of which is suddenly set into motion. The airfoil bound vortices are modeled using lumped vortex elements and the wake behind the airfoil is modeled by discrete vortices. This consists of solving a steady state flow problem at each time-step where unsteadiness is incorporated through the "zero normal flow on a solid surface" boundary condition at every time instant. Additionally, along with the "zero normal flow on a solid surface" boundary condition Kelvin-s condition is used to compute the strength of the latest wake vortex shed from the trailing edge of the airfoil. Location of the wake vortices is updated at each time-step to get the wake shape at each time instant. Results are presented to show the effect of airfoil-airfoil interaction and airfoil-wake interaction on the aerodynamic characteristics of each airfoil.