Flight Guidance, Navigation, and Control systems are the crucial core of any flying vehicle, particularly in UAVs. GNC algorithms should be tested and verified using a validated nonlinear 6-DOF model to be evaluated before actual flight. As a result, increasing UAV modeling accuracy and an open-loop simulation model that describes the UAV
behaviors for various inputs are highly needed. In this work, a nonlinear model of UAV is proposed. The inertia mass model is validated utilizing experimental measurements using the pendulum method; subsequently, the behavior of the actuators in conjunction with the UAV's deflection component is validated using system identification. Additionally, a test
setup has been devised to measure the propulsion model characteristics, and the experimental results are analyzed. The aerodynamic coefficients and derivatives at various angles of attack are estimated using semiempirical software (DATCOM). The dynamic model is based on motion equations to simulate the overall model. This work advances
by incorporating the sub-models mentioned above into a unified openloop flight simulation model for the UAV based on the improvements made to the previous sub-models. By adopting this integrated approach, we can thoroughly simulate the UAV's behavior under different flight conditions, which assesses the efficiency of our design processes, and
the results confirm the accuracy and dependability of each sub-model.  Additionally, we analyze the UAV's stability in an open-loop system, considering both lateral and longitudinal aspects, which evaluates the UAV's capacity to maintain balance and respond to control commands in various flying situations, which is crucial in the stability and safety of
the UAV in real-world operational settings, hence confirming the overall success of our integrated simulation design.