Drag is the price of moving through air. Aircraft engineers split it into two kinds.

1. Parasite drag. This comes from the shape and the skin: the pressure drag of the fuselage, tyres and struts pushing air aside, and the skin friction of air rubbing along the surface. It behaves like the lift: D_p = ½ρV²S × C_D0. It grows with the square of the speed.

2. Induced drag. This is the price of making lift. A wing with finite span pushes air down, and the air curls around the wingtips into swirling wingtip vortices. That wasted energy shows up as a backwards force: D_i = ½ρV²S × C_L² / (π e AR). Here AR is the aspect ratio (span² divided by wing area; long thin wings have a high AR) and e is an efficiency factor below 1.

Put together, the total drag coefficient is C_D = C_D0 + C_L² / (π e AR).