Coupling is considered between fluid flow and a freely moving body shorter than the development length in an oncoming boundary layer or channel flow but longer than the flow thickness. The body lies within the core of the flow. The coupling occurs between the inviscid-dominated displacement and the viscous–inviscid pressure, the latter acting to move the body. This interaction can be unstable. It is found however that three factors serve to stabilise the interaction as each one alters the decisive balance of angular momentum. One is a 10 % shift forward in the position of the centre of mass. The second is a degree of flexibility in the body shape by means of its response to the induced pressure force. Third is a slight streamwise movement of the body which is sufficient to modify the viscous–inviscid pressure response and again produce stabilisation. The effects are largely independent of the lateral position of the body.