  Traditional aerial vehicles are usually custom-designed for specific tasks.
Although they offer an efficient solution, they are not always able to adapt to
changes in the task specification, e.g., increasing the payload. This applies
to quadrotors, having a maximum payload and only four controllable degrees of
freedom, limiting their adaptability to the task's variations. We propose a
versatile modular robotic system that can increase its payload and degrees of
freedom by assembling heterogeneous modules; we call it H-ModQuad. It consists
of cuboid modules propelled by quadrotors with tilted propellers that can
generate forces in different directions. By connecting different types of
modules, an H-ModQuad can increase its controllable degrees of freedom from 4
to 5 and 6. We model the general structure and propose three controllers, one
for each number of controllable degrees of freedom. We extend the concept of
the actuation ellipsoid to find the best reference orientation that can
maximize the performance of the structure. Our approach is validated with
experiments using actual robots, showing the independence of the translation
and orientation of a structure.
