In the age of smart cities, Unmanned Aerial Vehicles (UAVs) are crucial components in surveillance, traffic monitoring, and disaster relief. However, reliable and efficient communication between UAV and infrastructure is problematic since UAVs have onboard resources constraint and urban environment is dynamic constantly. In this paper, we propose a Lightweight Blockchain-Enabled Authentication Protocol (LBEAP) tailored for secure UAV communications in smart city environment. The proposed scheme combines a decentralized blockchain ledger with efficient lightweight cryptographic primitives to provide mutual authentication, data integrity and resist impersonation/replay attacks. In contrast to previous work that depends on hardware-intensive characteristics like PUFs or has a high consensus overhead, LBEAP is hardware-agnostic and computationally lightweight, which makes it appropriate for real-time swarm operations for memory-limited UAVs. Performance evaluations based on simulation and synthetic traffic data show that LBEAP can effectively reduce communication latency and energy consumption over time while maintaining a high level of security assurance. The protocol is highly scalable and can seamlessly integrate with IoT-centric smart city infrastructure, while providing a future-compatible solution for secure and independent UAV coordination.