四旋翼小型无人直升机的建模与仿真。对直升机建立模型，进行物理分析，列出微分方程并线性化，得到传递函数和状态空间方程。-Modeling and simulation of small unmanned four-rotor helicopter. Helicopter model, physical analysis, and differential equations listed linearization to obtain the transfer function and state space equation.