ABSTRACT
Analysis of helicopter stability and control is a very complicated dynamic problem involving the coupling of rotor motion with the fuselage degrees of freedom. In general, this problem can be simplified by assuming that blade flap dynamics is much faster than fuselage dynamics, and therefore, the steady-state flap response to fuselage perturbation motion is treated in a quasi-static (or also known as “low-frequency approximation”) manner. In addition, the lead-lag and the torsion motions of the rotor blade can be neglected. The reason for this approximation can be attributed to the fact that the steady-state hub loads due to the flap motion of the blade are large compared to the hub loads due to the lag or the torsional motion. It is important to note that, for ground and air resonance problems, lag motion plays an important role, whereas in helicopter stability and control problems, lag mode is generally neglected.
