Study On Aeromechanical Stability Of A Helicopter With Inter-blade Dampers

As one of the basic problem of helicopter dynamics, the aeromechanical stability of the coupled rotor-body system has been paid a lot of attention. Among others, one method to solve this problem is to install lead-lag dampers. The conventional configuration for installing dampers is to put them between blades and hub. In 1990s, Eurocopter began to use the inter-blade dampers in EC155 which means to put every damper between two adjacent blades. The flight test of EC155 tells us that inter-blade dampers can work very well in stabilizing the system; another advantage of this configuration is it can make the hub system simpler and reduce the drag of air to the hub which is very important for helicopters with 4 or more than 4 blades. To study the influence of this damper configuration to aeromechanical stability, some work has been done in this thesis.First, dynamic equation of the rotor-hub system was modeled with the generalized Hamilton principle, which can be used to analyze the stability problem in different conditions like on-ground, hovering and flight. When fastening the fuselage, the model can also be used to analyze the stability problem of isolated rotor system. The equation was solved with both the time finite element method and the fourth order Runge-Kutta method.In chapter 3, the non-linear equation was linearized with the Jacobi Matrix, and then the linearized equation was transformed into fixed frame to analyze the stability problem with eigenvalue method (on-ground or hovering) or floquet theory (forward flight). Meanwhile, the equation was perturbed by sweep frequency excitation from steady state to get transit decay of lag response which was then transformed into fixed frame with a numerical Fourier Coordination Transformation (FCT). The fixed frame response along with the body response was analyzed via an FFT to determine modal frequencies. Damping was then determined via exciting the equation at the critical frequency to get the transit decay which was then analyzed with the Moving-block method.In chapter 4, the nonlinear dynamic equation of rotor-body system with inter-blade dampers (Taking all possible attachment offsets into consideration, spanwise offset, vertical offset and in-plane offset) was modeled. In order to analyze this model, a modified multi-blade coordinate transformation was proposed.At last, the aeromechanical stability problem was analyzed with the two...