This　research　paper　addresses　the　formation　control　problem　in　a　swarm　of　quadrotor　unmanned　aerial　vehicles　(UAVs)　over　directed　interaction　topologies,　with　the　primary　objective　of　achieving　persistent　motion　based　on　bearing　measurements.　A　novel　bearing-only　formation　controller　is　proposed,　eliminating　the　need　for　relative　position　or　linear　velocity　measurements　between　quadrotors.　Specifically,　a　distributed　hierarchical　formation　control　scheme　is　adopted　for　the　quadrotors.　This　scheme　consists　of　an　outer-loop　translation　controller　designed　for　target　formations　using　only　bearing　vector　measurements,　as　well　as　an　inner-loop　attitude　controller　for　precise　attitude　adjustments.　To　overcome　the　absence　of　bearing　rate　measurement　in　the　translational　controller,　a　filter　is　introduced　to　estimate　it,　effectively　compensating　for　the　lack　of　linear　velocity　measurement.　Additionally,　a　differentiator　is　employed　to　estimate　the　high-order　control　input　of　the　translational　dynamics,　enabling　convenient　derivation　of　the　desired　angular　velocity　and　its　derivatives　required　for　the　torque　controller.　The　stability　analysis　of　the　underactuated　systems　of　multi-UAVs　under　directed　graphs　is　conducted　using　Lyapunov　stability　theory.　Simulation　results　demonstrate　the　convergence　properties　of　the　proposed　protocols,　while　real-world　tasks　validate　its　practicality.　©　2016　IEEE.