Radio frequency energy harvesting (RF-EH) offers an unorthodox solution to the painstaking energy constraint drawback in wireless sensor networks (WSNs). In this paper, a data-gathering scenario from a UAV assisted WSN that consists of multiple sensor nodes (SNs) and power beacons (PBs) is considered. All the SNs are remotely powered by the PBs via wireless power transfer (WPT). In the first phase, a cluster-head (CH) is selected from the powered-up SNs and each SN periodically sends its observations to the CH. In the second phase, UAV powers the CH and collects the aggregated sensor observations from the CH. Under the assumption of Rayleigh and Rician fading channels, the outage probability at the UAV is derived and identified the time ratio of the proposed time-block structure that minimizes the outage probability. Furthermore, an effective algorithm is proposed to generate a solution for the process of CH selection. Finally, the achievable outage probability, throughput and the service range of the UAV are demonstrated for a given random setup of WSN through the theoretical and simulation results obtained.