<p>As all phenomenons in nature are nonlinearity, meaning linearity is an ideal case, so large deviation exists for the existing linear control in academy and practice. For classical nonlinear controller design, the commonly used way is firstly to linearize the original nonlinearity, and then secondly apply the existing linear control theory to design the called linear controller. This paper is to alleviate this deviation through using differential geometry nonlinear control strategy for one interesting practical plant, i.e. unmanned helicopter. Specifically, firstly, the detailed physical modeling process for unmanned helicopter is shown through our own mathematical analysis, so that a general nonlinear system is obtained. Secondly, based on this nonlinear unmanned helicopter system without linearizing process, nonlinear controller design is derived in detail by virtue of differential geometry within state equation and state-observation equation respectively. Furthermore, thirdly, the perfect tracking is achieved during the framework of differential geometry nonlinear control. Fourthly, before applying the obtained nonlinear control, information of state variable is needed, so adaptive state estimation based on Lyapunov function is proposed to implement. Finally, one practical example is applied to prove our proposed theoretical results. Generally, this paper combines nonlinear control, differential geometry, unmanned helicopter, adaptive control together in order to design more advanced and accurate control in practice.</p>