Drag Equation from the Drag Coefficient

Drag is written exactly like lift, with the same $q$ and the same $S$, and that shared reference area is doing more work than it looks. Because the two equations divide by the same thing, the ratio $L/D$ equals $C_L/C_D$ exactly, and the whole of aircraft performance can be argued in coefficients without ever converting back to newtons.

It also creates the most reliable way to be wrong about aeroplanes in public. A car's drag coefficient is divided by its FRONTAL area; an aircraft's is divided by its WING PLANFORM, which for a typical light aircraft is something like fifteen times the frontal area. So a clean sailplane at $C_D \approx 0.01$ and a modern saloon car at $C_d \approx 0.30$ are not thirty times apart in any physical sense — the two numbers are not measured against the same denominator, and comparing them directly is meaningless. When you want to compare bodies of different kinds, compare drag AREA, $C_D S$, which has units of area and is denominator-free.

Aircraft drag is conventionally split in two, and the split is the subject of the drag polar page. Parasite or zero-lift drag is what the aeroplane costs simply for existing in a moving airstream: skin friction over every wetted surface, form drag from the shape, interference drag where the wing meets the fuselage, and cooling drag through the engine baffles. Induced drag is the price of the lift, and it is a wholly different animal.

Two speed regimes break the constant-$C_D$ assumption and both catch people out. At low Reynolds number — a model aeroplane, a small drone, an insect — the coefficient rises sharply as viscosity takes over, so a scale model is proportionally draggier than the aircraft it represents. At high Mach number the coefficient rises even more sharply as shock waves form on the upper surface, and drag divergence is the reason airliners cruise at Mach 0.78 to 0.85 and not faster: a few hundredths of a Mach number past the divergence point costs more fuel than the time is worth.

Finally, remember what a drag force actually buys you when it is multiplied by speed. Power required is $D \times V$, and it is the power curve, not the drag curve, that decides how an aeroplane climbs. The two have their minima at different speeds, which is why best-glide and best-rate-of-climb are different numbers on the same placard.