Abstract— Comparative study on flight performance and longitudinal stability of biplanes with gap variation and monoplane Micro Air Vehicles (MAVs) are presented in this paper. The aerodynamic modeling is based on the experimental data collected at low Reynolds Number (~150,000) in a low-speed wind tunnel. A rigid flat plate with an aspect ratio of one and three different planform shapes (Zimmerman, inverse Zimmerman and Elliptical) are used to study the effect of gap between two wings on the flight performance characteristics. The trim states across a velocity spectrum of 5 and 15 m/s are evaluated using a nonlinear constrained optimization scheme based on hybrid Sequential Quadratic Programming (SQP) and quasi-Newton methods. The stability of these trim points is assessed numerically using Runge-Kutta methods. There is an evidence of emergence of Limit-Cycle Oscillations (LCO) at high angle of attack. The onset and amplitude of LCOs is earlier and larger for biplanes than monoplanes.