In order to simplify the description of the problems and to be convenient for mathematical manipulation, a series of hypotheses about the dynamic glide problem of unpowered aircraft were made. Under these hypotheses, a dynamics model of dynamic soaring, which is a 3-dimentional ordinary differential equations in the velocity space with a single input, was built. The largest growing area of mechanical energy in this velocity space, which is called energy-gained spindle, was obtained theoretically and the maximum rate of energy increasing was also deduced. The conclusion that the larger wind gradient, the smaller drag and the smaller area-mass ratio are more in favor of gaining energy for the engines aircraft gives a deeper understanding of the dynamic soaring energy and it can help the practice. 