Drag Polar

The drag polar is the whole drag of an aircraft split into a part that does not depend on lift and a part that is nothing but the price of lift. Written as $C_D = C_{D0} + C_{Di}$ it looks like an accounting convenience. It is closer to a law: with the induced term expanded, $C_D = C_{D0} + C_L^2/(\pi \cdot AR \cdot e)$, it becomes a parabola in $C_L$, and that parabola is the aircraft's performance in a single curve.

Read as forces against airspeed, the two halves move in opposite directions. Parasite drag is roughly constant in coefficient and therefore grows with the square of the speed. Induced drag falls with the square of the speed. Their sum is U-shaped, with a minimum where the two are exactly EQUAL — a tidy result that falls straight out of differentiating the sum, and one worth remembering because it identifies the best-glide condition without any arithmetic at all.

That minimum-drag speed is the same as the maximum-$L/D$ speed and it is the best-glide speed on the placard. Fly slower than it and total drag goes UP, which is the back side of the drag curve and the least intuitive fact in low-speed flying. Fly faster and drag rises too, but the extra speed is usually worth having, which is why the recommended glide speed for range in wind is a little above best-glide into a headwind and a little below it downwind.

The model has limits and it is honest to state them. The parabola is a good fit through the normal cruise and climb range and a poor one near the stall, where the polar bends away as separation begins. It also breaks down above the drag-divergence Mach number, where a wholly new wave-drag term appears that has nothing to do with lift. And $C_{D0}$ is not really constant even in the working range: on a real aircraft it creeps upward as $C_L$ rises and the boundary layer thickens, which is exactly the effect that fitted Oswald efficiency factors absorb.

Where the numbers land: a clean sailplane runs $C_{D0}$ near 0.008 to 0.012, a light aircraft with fixed gear and struts around 0.025 to 0.04, and an airliner in cruise configuration around 0.016 to 0.020. Gear and flaps roughly double the parasite drag of a light aircraft, which is the practical reason a go-around is initiated by cleaning up the aeroplane and not merely by adding power.