Attitude modeling of the DelftaCopter

Abstract

Previous years have seen a rise in the use of Unmanned Aerial Vehicles (UAVs). Reaching a large endurance and range while being able to perform Vertical Take-Off and Landing (VTOL) landings allows a broad range of applications. For this purpose the DelftaCopter (DC) was developed, a tilt-body tailsitter UAV. It hovers using a single helicopter rotor for lift and transitions to forward flight by pitching its body down by 90°. In this forward flight state, wings generate the lift, while the helicopter rotor now provides thrust. The single rotor is more efficient than using multiple smaller rotors and helicopter swashplate is used for attitude and speed control. The heavy single helicopter rotor introduces significant gyroscopic moments, as is the case for all helicopters. In contrast with normal helicopters, the DC has a heavy fuselage putting the attitude dynamics between a helicopter and aircraft. In previous research, a controller based on a model incorporating the rotor as a rotating cylinder was implemented. This controller was unable to counteract the gyroscopic pitch-roll coupling, leading to the question of this thesis: how should the DC be modeled to allow control design.
In this thesis, the previous model is called the Cylinder Dynamics (CD) model, and is compared with another model from literature. The latter model, in this thesis called the Tip-Path Plane (TPP) model, includes the flapping dynamics through the tip-path plane dynamics and is also a linear state-space model. In flight tests, chirps were used to cover a broad frequency range. Fitting both the CD and TPP models on this flight test data, it is shown that the CD model lacks accuracy in the high-frequency area, while the TPP is able to accurately model these dynamics. This shows that the flapping dynamics are important to the attitude dynamics of the DC. An Linear Quadratic Regulator (LQR) controller was implemented based on the fitted TPP model, and shows adequate tracking performance, further validating the applicability of the model to the DC. For forward flight, extensions to the hover models are proposed. The extension including the elevator and aerodynamic damping is shown to simulate key dynamics of the DC in forward flight with reasonable accuracy. The parameters and eigenfrequencies of this model are not significantly different from the hover model. Therefore it can be concluded that the gyroscopic effect plays an important role in forward flight attitude dynamics. Another extension which estimates of angle of attack and sideslip using high-pass filtered rotational rates, yields better accuracy, but significantly changes the model parameters also present in the hover model. More research with angle of attack and sideslip vanes could validate this modeling approach. It was also found that for a new version of the DC with a smaller, more quickly rotating rotor, the modeling done before resulted in much worse fits. It was shown that the CD and TPP model response is much more comparable for this version. Control performance also suffers due to this lower accuracy model fit. Further research is required to understand why this is the case.