Abstract: The dynamic characteristics of gearbox-arm component are analyzed to improve the flight stability of a heavy-duty oil-powered four-rotor aircraft. The Lanczos eigenvalue solver is used to solve the modal of gearbox-arm component. The vibration acceleration value at the center of the component is collected through experiment，and the length-to-diameter ratio of mandrel is optimized. Runge-Kutta method is used to solve the dynamic equation of the component，and the law of vibration and influencing factors of the component are summarized. The results show that the mandrel's length-to-diameter ratio greater than 20 will produce larger bending vibration. The length-to-diameter ratio (<14) of the mandrel can be optimized to change the natural frequency of its vibration，and avoid the excitation frequency and frequency multiplication. The vibration acceleration increases first and then decreases with the increase in rotating speed，and reaches the maximum when the excitation frequency is equal to the natural frequency of the system. The stiffness of the system is improved to achieve vibration reduction by optimizing the angle of the inclined support.

Key words: four-rotoraircraft, gearbox-armcomponent, dynamiccharacteristics, modalanalysis, vibrationacceleration, stiffness, length-to-diameterratio