<p>As all natural phenomena are nonlinear systems, so the problem of controlling nonlinear systems to get better performance is a major interesting subject. Because unmanned helicopter is underactuated, highly nonlinear and coupled system, this paper considers nonlinear controller design for nonlinear unmanned helicopter through combining some related knowledge, for example, differential geometry, exact linearization, closed loop pole placement, perfect tracking, speed gradient algorithm, convergence and robustness, etc., thus resulting to our named nonlinear geometric control analysis. More specifically, firstly after modeling the motion of unmanned helicopter through force analysis and aerospace analysis, one nonlinear dynamical equation is obtained. Secondly, nonlinear geometric control strategy is proposed to design one nonlinear controller for unmanned helicopter through exact linearization, closed loop pole placement and differential geometry together without any linearization. Thirdly, consider the control mission of perfect tracking, speed gradient algorithm is applied to generate one satisfied nonlinear controller, and its convergence is also analyzed through classical Lyapunov function analysis. Fourthly, to extend speed gradient algorithm for more general corrupted case, its modified form is also given for bounded disturbance and the modified convergence is also proven in our own mathematical derivations, corresponding to robustness. Finally, to prove the efficiency of our theoretical results in unmanned helicopter flight control system, a practical platform is established to run some example simulations.</p>