The limited energy and high mobility of unmanned aerial vehicles (UAVs) lead to drastic topology changes in UAV formation. The existing routing protocols necessitate a large number of messages for route discovery and maintenance

greatly increasing network delay and control overhead. A energy-efficient routing method based on the discrete time-aggregated graph (TAG) theory is proposed since UAV formation is a defined time-varying network. The network is characterized using the TAG

which utilizes the prior knowledge in UAV formation. An energy-efficient routing algorithm is designed based on TAG

considering the link delay

relative mobility

and residual energy of UAVs. The routing path is determined with global network information before requesting communication. Simulation results demonstrate that the routing method can improve the end-to-end delay

packet delivery ratio

routing control overhead

and residual energy. Consequently

introducing time-varying graphs to design routing algorithms is more effective for UAV formation.