The supply of electrical power requires a generator, which must be driven from a prime mover, by some form of mechanical drivetrain. Such an electro-mechanical system will have natural resonant modes in both the electrical and mechanical subsystems. These subsystems are interfaced through the electrical generator for the useful transfer of power, but may also couple unwanted disturbances between the electrical and mechanical domains resulting in interaction. The interaction can lead to lifetime reduction in the mechanical components and instability in the electrical network, resulting in poor reliability for the wider system. Predicting the occurrence of interaction, through simulation, is challenging, requiring multi domain models, operating with different time scales. This paper analyses an aircraft auxiliary power offtake to produce a reduced-order mechanical drivetrain model, allowing the modal frequencies to be predicted and cross-domain interactions to be modelled. A purpose-built electro-mechanical test platform is used to validate the model and demonstrate how electrical disturbances are passed through to the generator to the mechanical system. Future research will use the test bed to demonstrate strategies for avoiding or suppressing unwanted interactions.