  This paper presents the design and control of a novel quadcopter capable of
changing shape mid-flight, allowing for operation in four configurations with
the capability of sustained hover in three. The normally rigid connections
between the arms of the quadcopter and the central body are replaced by
free-rotating hinges that allow the arms to fold downward; no additional
actuators beyond the four motors that drive the propellers are used.
Configuration transitions are accomplished by either reducing or reversing the
thrust forces produced by specific propellers during flight. Constraints placed
on the control inputs of the vehicle prevent the arms from folding or unfolding
unexpectedly, allowing for the use of existing quadcopter controllers and
trajectory generation algorithms. 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. Furthermore, the ability to change configurations is
shown to enable the vehicle to traverse small passages, perch on hanging wires,
and perform simple grasping tasks.
