  We present a novel approach to cooperative aerial transportation through a
team of drones, using optimal control theory and a hierarchical control
strategy. We assume the drones are connected to the payload through rigid
attachments, essentially transforming the whole system into a larger flying
object with "thrust modules" at the attachment locations of the drones. We
investigate the optimal arrangement of the thrust modules around the payload,
so that the resulting system is robust to disturbances. We choose the
$\mathcal{H}_2$ norm as a measure of robustness, and propose an iterative
optimization routine to compute the optimal layout of the vehicles around the
object. We experimentally validate our approach using four drones and comparing
the disturbance rejection performances achieved by two different layouts (the
optimal one and a sub-optimal one), and observe that the results match our
predictions.
