Ground resonance is a rotorcraft instability that involves an interaction between the rotor and its support, consisting of the fuselage and the landing apparatus. In cases where the rotor lacks symmetry, the equations of motion are periodic. This paper compares the classic time-periodic Floquet approach to investigate stability with the truncated linear time-invariant harmonic decomposition. Then, various cases are presented dealing with variations in some basic rotor characteristics, such as the number of blades. These findings show the capacity of the decomposition to represent time-periodic modes and present a criterion for the optimal application of this decomposition, which is extremely advantageous because it leads to a linear model for further analyses.