A device such as an aerofoil which in use is subject to fluid flow, includes an outer surface part the geometry of which is variable to affect the fluid flow, the device including a support structure which supports the outer surface part, the support structure being internal of the device and including a plurality of support members of composite material the geometry of the support structure being changeable by an actuating apparatus, between a first stable geometry and a second stable geometry to effect variation in the geometry of the outer surface part, the support members providing structure stiffness to the outer surface part.