A variable lift, variable drag airfoil assembly including a leading edge and a trailing edge connected together in spaced apart relation and an elastic skin connected to and extending between the leading and trailing edges to define an airfoil profile. A resilient spring member positioned within said airfoil profile exerts biased outward pressure against a central portion of the lifting surface of the elastic skin to automatically create a varying thickness of said airfoil profile responsive to increased air speed over the airfoil profile wherein lift and drag of the airfoil assembly are proportionately reduced as airspeed increases. In a preferred embodiment, the airfoil assembly is inverted in the form of a rear spoiler of a land vehicle oriented transversely across a rear portion of the vehicle to produce an increasing downward road gripping force on the rear wheels of the vehicle with increasing vehicle speed while reducing the drag produced by the airfoil assembly.