A wing and tail are geometrically similar, while tail area is one-third of the wing area and distance between two aerodynamic centres is equal to wing semi-span (b/2). In addition, following data is applicable: ∈α = 0.3, CL = 1.0, CLα 0.08/ deg., c = 2.5 , b = 30m , CMac = 0, ηt = 1. The symbols have their usual aerodynamic interpretation. The maximum distance that the centre of gravity can be behind aerodynamic centre without destabilizing the wing-tail combination is