| dc.description.abstract | Motivated by the limited landing space on board a ship, this thesis investigates the landing
of the Skywalker X8 fixed wing unmanned aerial vehicle (UAV) in a net. Further motivated
by the way birds abruptly, and equally elegantly, reduce their velocity when landing on
a perch or the branch of a tree, a perched landing strategy utilizing the increased drag
experienced for large angles of attack is used to minimize the velocity at impact with the
net. This is accomplished by expressing landing as an optimal control problem, taking
advantage of a nonlinear model of the X8 that is valid for high angles of attack. At this
stage, the optimal control problem is only concerned with the three longitudinal degreesof-
freedom. It is solved in an open source optimization framework, using a nonlinear
interior point method. Further, a nonlinear model predictive controller (NMPC) that can
control the X8 throughout the landing is developed. At the heart of the optimal control
problem lies a linear model, blended with a flat plate model to increase its validity for
high angles of attack in lift, drag and pitch moment. The linear model is developed using
an easy-to-use computational fluid dynamics (CFD) modeling software. In addition a six
degrees-of-freedom software-in-the-loop (SITL) simulator is developed for future use in
testing of hardware-near implementations of the controller, and to allow for validation
of the model through pilot testing. Comparison of six different landing scenarios yield a
wide range of landing velocities, depending on the constraints of the scenario. Simulations
with the developed NMPC show that the same performance is achievable through control
of the X8 under minor environmental disturbances. From this it is concluded that the
perched landing strategy will lead to a considerable reduction in terminal absolute velocity,
compared to a low angle of attack approach. It is found to be advantageous to start from
a low altitude, landing into an elevated net. However, whether an equally large reduction
is possible in practice depends on the capabilities of the real-time implementation and
the validity of the model, particularly the propeller model, the pitching moment and drag
coefficients. Finally it depends on how the lateral degrees-of-freedom are affected by the
high angle of attack flight. | |
