The need for automation of unmanned aerial vehicles (UAVs) rises with the increase of inexperienced operators. Extensive research has gone into automating and understanding multirotor UAVs and fixed wing UAVs. As vertical take off and landing (VTOL) UAVs are a relatively new category, there is ample opportunity for research in automation and understanding. This report focuses on automating the landing of a specific VTOL UAV: the
Marlyn, created by ATMOS UAV.

Marlyn is a tailsitter, meaning that the centre of mass during the hover phase is relatively far away from the landing rods. This can lead to tip overs during landing. The centre of mass alone will cause a tip over moment at a pitch angle of θ ≤ −32.5 degrees. Experiments have been performed to find the relation between the incoming wind speed and the pitching moment generated by the wind. Adding the found pitching moment, the tip over limit angle decreases to θ ≤ −47.5 degrees. This corresponds to a wind speed of 8.4 m/s.

A surprising find was that not the wind nor the centre of mass was the biggest culprit for tip overs. The biggest factor was the reduction in controllability. Marlyn’s bottom tail motor is in line with the landing rod that first touches the ground. This gives Marlyn no way to counter a forward tip over once it has been set in motion.

The firmware of Marlyn (PX4) comes with a complete simulation environment based on Gazebo. This simulation runs a simple aerodynamic model. The parameters for this model are based on estimations gained from XFLR. To determine and measure the difference between the simulation and reality, a sensitivity study is done. This study shows the influence of the parameters gained from XFLR on the lift and drag forces. After the sensitivity study, the values from XFLR are compared to the experimental data used to find the relation between the incoming wind speed and the pitching moment generated by the wind. The absolute values deviated significantly, but the relative differences between the values was more or less constant.

To counter the tip overs, three strategies are proposed. The first strategy revolves around detecting the moment the bottom landing rod touches the ground. By killing the motors, the wind will always prevent the forward tip over by blowing Marlyn upright. Using the inertial measurement unit inside the autopilot to determine the impact showed to be non feasible given the current design. The impact forces were not significant enough on all surfaces to reliably be used as a trigger to kill the motors. Mechanical switches have been shown in previous research to not work reliably in all conditions, mainly failing in muddy or sandy environments. Range finders and vision based distance sensors showed the most promising, if it can be guaranteed that they always point straight down.

The second strategy is based on generating a moment using the touch down impact to fling Marlyn upright. To generate this moment, a horizontal impact is generated by allowing Marlyn to drift with the wind in a controlled manner. The path generated for this could be done using artificial potential fields. These can be used to safely find the optimal path to the ground while staying within safety bounds. To validate the feasibility of this strategy, a simple inverted pendulum simulation is used. The Gazebo based simulation could not be used as is, since the control loops that try to stabilise Marlyn would try to counter the moment enerated by the impact. The simple simulation has shown that there is merit in this strategy, but a lot of further research is required to validate that this also works in the real world.

The last strategy proposed tries to reduce the pitch angle during landing. This can be achieved via two methods that are discussed. The first rotates Marlyn so that the wings are not tangent but parallel to the wind. This method was discarded due to the actuators not being able to keep Marlyn in that unstable state for prolonged duration. The second method uses Marlyn’s ability to rotate her wing motors independently. By rotating both into the wind, these motors can generate a force to counter the drag caused by the wind. This in turn leads to a smaller pitch angle required to remain stationary. The Gazebo based simulation is used to validate this method’s feasibility. The wind speed at which the previously found tip over pitch angle was crossed was increased from 8.4 m/s to over 9 m/s. Due to instabilities in the simulation, wind speeds larger than 9 m/s could not be tested.

All strategies discussed have promising options. The most promising strategy to counter tip overs during the landing of the VTOL tailsitter UAV Marlyn is: Rotating the wing motors into the incoming wind to reduce the required pitch angle to remain stationary. Further research is required to validate this strategy in real world scenarios