How does altitude affect drag? Consider two aircraft. Aircraft A flying at sea level with weight (W) and Angle of attack (α). Aircraft B flying at altitude h in level flight at the same weight (W) and same angle of attack (α) (same configuration as Aircraft A). Will the drag experienced by both the aircraft be the same?
Approximately, yes.
Both lift and drag depend on the density of air, so in order to generate enough lift at the same angle of attack in thinner air, an aircraft has to fly faster. Thus the same number of air molecules hit the plane every second and the drag is approximately the same.
This is a bit of a simplification (especially the bit about drag being caused by air molecules hitting the plane) as the change in temperature causes a slight change in viscosity, which affects one type of drag (detailed Peter's linked question). However it's a good enough approximation for most pilots, who use 'Indicated Air Speed' as a air-density-independent guide to aerodynamic speed.