Period characteristics of radiated noise at high frequency is investigated by a method based on short time Fast Fourier Transformation, and propellerspeed is estimated.
用短时傅立叶分析的方法研究了辐射噪声在高频段时频变化的周期性，估计了目标的螺旋桨转速。
2
The Sikorsky X2 has a pusher propeller mounted at the rear to obtain its high forward speed.
美国西科斯基公司的X2拥有一部安装在后方的推进螺旋桨，这使它获得高速的前飞速度。
3
Instead, slight speed variations on each propeller perform that function automatically.