As a first year aerospace engineer student I had a problem today. The question was : Does the altitude effect the endurance of an aircraft (provided that the aircraft engines can generate enough trust at this higher altitude). Maximum endurance is minimum fuel flow [N/s] F = Ct T in a steady, horizontal, symmetric flight F = Ct D D = Cd (1/2) rho V² S D: Drag T: Thrust Cd: Drag coefficient Cl: lift coefficient Rho: air denisty V: velocity S: wing area So higher altitude = lower density = lower drag = lower fuel flow = higher endurance. But also true is that F = Ct (Cd/Cl) W and none of these parameters are effected by the altitude. Which one is correct and why? And why is the other one wrong? Kind regards, Alex Edit: D = Cd (1/2) rho V² S but V is sqrt (W*2*1/(S*rho*Cl)) so the rhos cancel eachother out. Is there a delete button somewhere?