 In still air the polar curve shows that flying at the minimum sink speed enables the pilot to stay airborne for as long as possible and to climb as quickly as possible, but at this speed the glider will not travel as far as if it flew at the speed for the best glide.When in sinking air, the polar curve shows that best speed to fly depends on the rate that the air is descending.Using Paul MacCready's theory, the optimal speed to fly for best cross-country speed may often be considerably in excess of the speed for the best glide angle to get out of the sinking air as quickly as possible.