You are confusing weight, mass and downforce. Weight is the product of mass and gravity. You cannot increase it aerodynamically but you can add aerodynamic downforce to it. The mass of the a/c is not affected by increasing the downforce and so inertia is not affected. By increasing the downforce you are increasing the surface area in contact with the runway. The statement that friction does not increase with increased contact area is very simplistic and only applies if no other factor changes. It occurs because any increase in area must also be accompanied by a decrease in contact pressure. In the case of adding aerodynamic downforce to weight then the tyres deform more adding extra contact area but this is acheived at the same contact pressure. This increases the total friction available without a change in inertia.
Last edited by tightcircuit; 14th Oct 2012 at 09:29.