  We introduce UFO, a modular aerial robotic platform for transforming a rigid
object into a multirotor robot. To achieve this, we develop flight modules, in
the form of a control module and propelling modules, that can be affixed to an
object. The object, or payload, serves as the airframe of the vehicle. The
modular design produces a highly versatile platform as it is reconfigurable by
the addition or removal of flight modules, adjustment of the modules'
arrangement, or change of payloads. To facilitate the flight control, we
propose an IMU-based estimation strategy for rapid computation of the robot's
configuration. When combined with the adaptive geometric controller for further
refinement of uncertain parameters, stable flights are accomplished with
minimal manual intervention or tuning required by a user. To this end, we
demonstrate hovering and trajectory tracking flights through various robot's
configurations with different dummy payloads, weighing ~200-800 grams, using
four to eight propelling modules. The results reveal that stable flights are
attainable thanks to the proposed IMU-based estimation method. The flight
performance is markedly improved over time through the adaptive scheme, with
the position errors of a few centimeters after the parameter convergence.
