An Aeromechanical Stability Analysis For Tandem Helicopter

The intention of this paper is to investigate the aeromechanical stability of the tandem helicopter in hover.Firstly,the rotor/body coupling dynamic analysis model of the tandem helicopter is established.The blade dynamics model includes the flap and lag of the blade and linear damping and elastic restraining stiffness are added to the flapping and lagging hinges,taking into account the influence of aerodynamic forces and interference between front and rear rotors,The equation of the single blade is derived by Newton’s method.The fuselage is assumed to be an elastic beam in the body dynamics model,considering the first-order elastic mode of the fuselage,the finite element method is used to derive the equation of the fuselage.On this basis,small disturbance hypothesis and multi-blade coordinate transformation are applied to the blade and fuselage motion equations,and the rotor/body coupling disturbance motion equation is obtained.Finally,choosing a tandem helicopter as an example,the eigenvalue analysis method is used to analyze the air resonance stability,The influence of parameters such as height difference between front and rear rotors and fuselage stiffness on air resonance is obtained.The research in this paper shows that when the regressing lag mode of the rotor is coupled with the first-order elastic mode of the fuselage within the rated speed range,this will lead to the occurrence of dynamic instability of the tandem helicopter in hover.The greater height difference between front and rear rotor be,the worse the stability of the regressing lag mode and the fuselage’s first-order elastic mode coupling will be.When the damping coefficient of the lagging hinge and the stiffness of the fuselage are larger,the stability of the coupling between the regressing lag mode and the fuselage’s first-order elastic mode is better.