Abstract <p>The paper proposes an approach for fast tuning the nonlinear attitude control system applied for a quadcopter. First, a mathematical model of quadcopter’s motion is estimated by a simple identification process using a low-cost test bench. After that, the controllers’ parameters are computed via optimization-based synthesis using simulation-in-loop framework. The proposed approach therefore does not require the development of a complex mathematical model built on the theories of aerodynamics, flight dynamics and DC motors, as well as geometry and mass/inertial properties of a quadcopter. There is also no need to apply any algorithm for nonlinear control system analysis. Due to its simplicity, the proposed approach can be used for quick synthesis of new control systems or adjusting the existing ones. The adequacy of the proposed approach is confirmed by bench studies of identification accuracy and control performance. Besides this, the paper also firstly describes the mathematical principle of the square-root controller implemented in well-known flight controller software like Ardupilot.</p>