Two element aerofoils are provided, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge. The secondary element is deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle. The secondary element is configured to operate in airbrake mode when deflected by a respective the flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag. In at least some examples, the axial overlap is numerically greater than −0.5% of the aerofoil chord, at least for the design airbrake deflection angle. Also disclosed are methods for operating air vehicles, and methods for designing two-element aerofoils.