The disclosure is directed to parallel hybrid drive assemblies for lightweight unmanned aerial vehicles (UAVs). Specifically, the disclosure is directed to hybrid drive assemblies and control systems for UAVs, utilizing continuous belt tension modulation to couple and decouple an electric motor and an internal combustion engine. In some embodiments, this is achieved through the use of a tensioner module that is configured to couple and decouple the electric motor and the internal combustion engine by continuously and selectably modulating belt tension on drive elements of each of the electric motor and the internal combustion engine.