  This paper presents the design and control of a novel quadrotor with a
variable geometry to physically interact with cluttered environments and fly
through narrow gaps and passageways. This compliant quadrotor with passive
morphing capabilities is designed using torsional springs at every arm hinge to
allow for rotation driven by external forces. We derive the dynamic model of
this variable geometry quadrotor (SQUEEZE), and develop an adaptive controller
for trajectory tracking. The corresponding Lyapunov stability proof of attitude
tracking is also presented. Further, an admittance controller is designed to
account for changes in yaw due to physical interactions with the environment.
Finally, the proposed design is validated in flight tests with two setups: a
small gap and a passageway. The experimental results demonstrate the unique
capability of the SQUEEZE in navigating through constrained narrow spaces.
