 Zero-lift drag coefficient

The zero-lift drag coefficient $C_{D0}$ is the drag coefficient at $C_L=0$ for a stated configuration and flow condition. It is not necessarily the drag at zero angle of attack: a cambered airfoil can have a nonzero zero-lift angle of attack. In a simple parabolic drag polar, $C_D=C_{D0}+k{C}_L^2$, the lift-dependent term vanishes there.^[ref]

The zero-lift drag coefficient also need not be the smallest drag coefficient on a general polar. An original shifted parabolic drag polar,has minimum $0.018$ at $C_L=0.20$, while $C_{D0}=0.020$. Confusing the intercept with the minimum loses the shift caused by the particular aerodynamic configuration.

A measurement should locate the state where the measured lift crosses zero, then evaluate or interpolate drag at that state using compatible references. Zero readings can be affected by balance tare and systematic error. Measurements at several nearby angles of attack distinguish a crossing from one apparently small reading.

For a complete aircraft, the coefficient can include skin-friction, pressure and component-interaction effects under the selected decomposition. A finite-wing term proportional to $C_L^2$ can vanish at zero lift without making those other sources of drag vanish. Holding $C_{D0}$ constant while changing speed is itself an approximation if the Reynolds number, Mach number or flow state changes.