Data dissemination is playing a crucial role in improving the connectivity and performance in hybrid vehicular ad-hoc networks (VANETs) by exploiting both vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication links, which can achieve effective data sharing and distribution between road-side units (RSUs) and vehicles. Recently, unmanned aerial vehicles (UAVs), acting as flying base stations (BSs) or relays with caching capability, have been widely investigated as an effective and enhanced communication support from the sky to provide the ground users improved quality of services (QoS) in a variety of circumstances. In this paper, by fully exploiting the advantages of UAVs introduced in VANETs, we design an advanced UAV-aided cooperative data dissemination scheduling strategy to improve the data dissemination performance in VANETs. Considering the mobility of the involved UAVs, we further propose a three-dimensional (3D) spatial dynamic programming (SDP) algorithm for the trajectory scheduling of UAVs to optimize the network transmission utility. Simulations results verify that, compared with other data dissemination strategies, our proposed UAV-aided cooperative data dissemination strategy can efficiently achieve a better system performance in terms of downloading progress and transmission delay.