Induced drag is mathematically proportional to which expression?

• A. Cl/(π AR e)
• B. AR e / Cl^2
• C. Cl^2/(π AR e)
• D. 1/(Cl π AR e)

Answer: (C)

Induced drag arises from wingtip vortices produced by lift, so its magnitude depends on how much lift is generated and how efficiently that lift is distributed along the span. The standard relation is CD_i = Cl^2 / (π AR e). Since D_i = q S CD_i, the drag itself scales with Cl^2 / (π AR e). This means induced drag increases with the square of the lift coefficient and decreases as the wingspan grows (higher AR) or as the efficiency factor e improves toward 1. The expression Cl^2/(π AR e) captures this dependence, making it the correct form. The other forms would imply the opposite relationships between Cl, AR, and e.