| Description |
The goal of this dissertation is to model and control the behavior of in-ground-effect (IGE) on multirotor unmanned aerial vehicles (UAVs). Ground effect, or rotor IGE, is a common phenomenon experienced by rotorcraft aerial vehicles when taking off, landing on, hovering around, or flying near surfaces or obstacles. Rotor IGE is caused by aerodynamic interaction between the rotor wake and the nearby obstacle. In particular, the deformed wake causes changes in the induced velocity, which leads to drastic changes in rotor thrust and torque that make flight control in confined spaces difficult and challenging. Many of the existing models for IGE are based on work on helicopters from the early 1940s and more so from the 1950s. Many of these models, unfortunately, are not directly applicable to smaller rotorcraft aerial vehicles due to the assumptions that were made. Also, many of these models suffer from singularities at certain heights, and many of them are computationally heavy. The contributions of this dissertation are computationally-light and accurate models of IGE and the development of feedback controllers that are effective at handling IGE. First, a quasi-steady IGE model for a single rotor that predicts a finite maximum IGE thrust ratio is developed. An empirical approach is used to establish the base exponential function in the quasi-steady model, followed by exploiting blade element theory (BET) and the semipositive induced velocity assumption to relate two IGE model coefficients to blade geometry. The changes in the rotor IGE for various multirotor configurations are studied, characterized, and modeled with respect to the number of rotors, rotor rotation direction, and minimum rotor tip-to-tip distance. The quasi-steady model also incorporates a newly-discovered fountain-vortex thrust loss effect. The quasi-steady model is experimentally validated for off-the-shelf and variable pitch propellers. Second, the quasi-steady model is extended to capture dynamic IGE by considering vehicle flight states and the partial ground effect where a portion of a rotor operates within the ground-effect region. More specifically, using blade element theory, rotor IGE thrust ratios in forward and axial flight are derived as a function of the advance ratio and climbing speed in the IGE regime. A rotation-based IGE test stand that simulates forward flight is created and used to characterize dynamic IGE and to validate the analytical results. The advance ratio, rotor disk angle of attack (AOA), and various multirotor configurations (transverse and tandem rotor) are investigated using the test stand. The partial ground effect is empirically characterized, and the behavior is incorporated into the dynamic IGE model. Finally, the developed IGE models are exploited for vehicle motion control to account for IGE on a quadrotor helicopter (quadcopter) aerial vehicle flying near the ground surface. Specifically, a feedback-based nonlinear disturbance observer controller and a feedforward IGE compensator are designed, simulated, and implemented. Simulation and experimental results validate the effectiveness of the IGE model and flight controller to compensate for IGE when flying close to the ground. |
