In this letter, we study unmanned aerial vehicle (UAV) enabled Internet-of-Things (IoT) system, where a rotary-wing UAV is employed to collect data from a set of IoT devices. By introducing the propulsion energy model for rotary-wing UAV, we aim to minimize the maximum energy consumption among all devices, while satisfying the UAV energy budget. The problem is formulated with a non-convex programming by jointly optimizing the communication scheduling and transmit power allocation for devices as well as the UAV trajectory. An efficient solution is proposed by applying the alternating optimization and successive convex approximation techniques. Simulation results show that the proposed solution can achieve significant gains over benchmark schemes for data collection over IoT system.