Level Unaccelerated Flight Performance
The thrust required for horizontal flight must compensate the different formas of drag, i.e.
- The parassitic drag, varying with $\bar{q} \sim V^2$.
- The induced drag, varying with $1/\bar{q} \sim 1/V^2$.
It can be proved analytically that, assuming a parabolic drag polar, the condition for flight at minimum $T_\mathrm{R}$ occurs when the the aircraft develops a drag coefficient whose value is given by the two equal halves: $C_{D0} = C_{D\mathrm{i}}$.
