The effects of horizontal tail geometry and position on longitudinal flapping-wing micro aerial vehicle dynamics were studied using wind tunnel and free-flight experiments. Linearised models were used to analyse the effect on the dynamic properties of the ornithopter. Results show higher steady-state velocity and increased pitch damping for increased tail surface area and aspect ratio. The maximum span width of the tail surface is also found to play an important role in determining dynamic behaviour, in particular when the distance between the tail surface and the flapping wings is large. Steady-state conditions can be predicted accurately using linear functions of tail geometry for this ornithopter. Predicting dynamic behaviour is more complicated and requires further study. However, the observed trends in some of the model parameters suggest that future models explicitly including the tail geometry may be used to design flapping-wing robots with desirable dynamic properties.