-If the aerofoil of finite span is placed in a fluid stream then, lift on the aerofoil implies that pressure over the lower surface is higher than that over the upper surface.

-This pressure difference causes the field to flow from the lower to the upper surface around the tips.

-Due to inward flow on the upper surface and the outward flow at the lower surface, vertices are formed known as tip vertices.

-Training tip vertices cause a downward velocity $(U_i)$ known as downward velocity.

-Induced drag is expressed as

$F_{D_i}=C_{o_i}\times \frac{1}{2}\rho . A. U^2$

where $C_{D_i}=\text{coefficient of induced drag}$

$\therefore$ Total drag:

$F_{D_T}=F_{D_p}+F_{D_i}$

Total drag coefficient:

$C_{D_T}=C_{D_P}+C_{D_i}$

where $C_{D_P}=\text{profile drag coefficient}$

-Assuming elliptical distribution of lift on aerofoil, pr and t1 suggested following expression

Coefficient of induced drag

$C_{D_i}=\frac{C_L^2}{\pi(L/C)}$

where $y_c=\text{Aspect ratio}$

and

$\alpha _i=\frac{F_{D_i}}{F_L}=\frac{C_L^2}{\pi\frac{L}{C}}$