The objective of this paper is to address stability of Unmanned Aerial Vehicles (UAVs). The paper first derives the equations of motion for a generic UAV that account for both rigid-body and elastic degrees of freedom in coupled form, as well as nonlinear structural and unsteady aerodynamic effects. The equations are used to compute trim states for steady level flight at desired altitudes and speeds. Aircraft stability is addressed by linearizing the equations about the desired steady level flight, and solving the corresponding eigenvalue problem. The paper shows that other aircraft models widely used for addressing stability can be obtained from the “full model” as special cases. The five aircraft models considered in the paper are 1) full model, 2) linear model (the full model with a linear structural model), 2) nonlinear restrained aircraft model (full model with no rigid body degrees of freedom), 3) linear restrained aircraft model (the full model with no rigid body degrees of freedom, and with a linear structural model), 4) rigid aircraft model (the full model with no elastic degrees of freedom). The comparisons of these models are illustrated in a numerical example involving a high-altitude-long-endurance unmanned aerial vehicle.
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