Originally Posted by hin
So to start with, as altitude increases(let say pressure altitude),the air density decreases, and so as the drag(parasite drag), so the aircraft will have its maximum aerodynamic performance at its service attitude....
- that is where you went wrong. A/C generally fly at an IAS. At the same IAS, drag is the same (it actually increases with altitude, but that 'is not important right now') regardless of altitude.