Control design for multi rotors UAV is an important challenge for engineers and scientists, due to the fact that the standard configurations are under-actuated, highly nonlinear, and unstable systems. In this paper, a wind force compensation strategy is proposed for a quadrotor. This strategy relies on a second order sliding mode controller based on the super twisting algorithm (STA) with an observer. Second order sliding mode technique ensures robustness to external disturbances and time varying, parametric and nonlinear uncertainties. Integration of an observer in the closed-loop system is needed for states reconstruction and the estimation of unknown external forces such as the wind effect. This estimation will allow a better monitoring of the system's status than passive robustness, providing the opportunity for recovery tactics such as an emergency landing when the external perturbations become too strong for the system. The effectiveness of the proposed strategy is compared to an adaptative gain controller through simulation and validated in real experiments on a quadrotor.