Fixed-pitch quadrotors are popular research and hobby platforms largely due to their mechanical simplicity relative to other hovering aircraft. This simplicity, however, places fundamental limits on the achievable actuator bandwidth and the possible flight maneuvers. This project shows that many of these limitations can be overcome by utilizing variable-pitch propellers on a quadrotor. I performed a detailed analysis of the potential benefits of variable-pitch propellers over fixed-pitch propellers for a quadrotor. This analysis is supported with experimental testing to show that variable-pitch propellers, in addition to allowing for generation of reverse thrust, substantially increase the maximum rate of thrust change. I also developed a nonlinear, quaternion-based control algorithm for controlling the quadrotor with an accompanying trajectory generation method that finds polynomial minimum-time paths based on actuator saturation levels. I implemented the control law and trajectory generation algorithms on a custom variable-pitch quadrotor, utilizing my own autopilot. Flight tests highlight the benefits of a variable-pitch quadrotor over a standard fixed-pitch quadrotor for performing aggressive and aerobatic maneuvers.
The videos below show the prototype variable-pitch quadrotor that I built. The quadrotor is controlled using the developed trajectory generation algorithms.
It probably goes without saying, but the variable-pitch quadrotor didn't always work. One of the most notable fails was when our autopilot power distribution board short-circuited during a crash, causing the battery to ignite. You can see the burn marks on the autopilot and the floor.