= Zero-lift drag coefficient
{title2=$C_{D0}$}
The \b[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>, $C_D=C_{D0}+kC_L^2$, the lift-dependent term vanishes there.<Induced Drag Coefficient by NASA Glenn Research Center>[Total drag decomposition, $C_D=C_{D0}+C_{Di}$]{ref}
The <zero-lift drag coefficient> also need not be the smallest drag coefficient on a general polar. An original <shifted parabolic drag polar>,
$$
C_D=0.018+0.050(C_L-0.20)^2,
$$
has minimum $0.018$ at $C_L=0.20$, while \b[$C_{D0}=0.020$]. 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.
Back to article page
