This paper addresses simulation results about the altitude control of a quad-rotor unmanned flying vehicle (UAV) in the presence of uncertainty like wind gust. A sliding mode control was proposed, the gain of control can be decreased and, as a result, the chattering amplitude is reduced. The objective is to introduce an adaptation in the control law in order to decrease the gain to the minimal value preserving the sliding mode control and keeping his property of a finite-time convergence.