  A novel quadcopter capable of changing shape mid-flight is presented,
allowing for operation in four configurations with the capability of sustained
hover in three. This is accomplished without requiring actuators beyond the
four motors typical of a quadcopter. Morphing is achieved through
freely-rotating hinges that allow the vehicle arms to fold downwards by either
reducing or reversing thrust forces. Constraints placed on the control inputs
of the vehicle prevent the arms from folding or unfolding unexpectedly. This
allows for the use of existing quadcopter controllers and trajectory generation
algorithms with only minimal added complexity. For our experimental vehicle at
hover, we find that these constraints result in a 36% reduction of the maximum
yaw torque the vehicle can produce, but do not result in a reduction of the
maximum thrust or roll and pitch torques. Experimental results show that, for a
typical maneuver, the added limits have a negligible effect on trajectory
tracking performance. Finally, the ability to change configurations is shown to
enable the vehicle to traverse small passages, perch on hanging wires, and
perform limited grasping tasks.
