Aerodynamic Coefficients

The Formulas

$ L = C_L \cdot \frac{1}{2} \rho V^2 \cdot S $ $ D = C_D \cdot \frac{1}{2} \rho V^2 \cdot S $

• $L$ / $D$: Lift / Drag Force (Newtons).
• $C_L$ / $C_D$: Coefficient of Lift / Drag (Unitless). Determined by shape and Angle of Attack.
• $\frac{1}{2} \rho V^2$: Dynamic Pressure ($q$).
• $S$: Wing Surface Area ($m^2$).

Coefficient of Lift ($C_L$) vs Angle of Attack ($\alpha$)

• Linear Region: As $\alpha$ increases, $C_L$ increases linearly.
• $C_{L_{max}}$: The maximum lift coefficient achievable.
• Stall: Beyond $C_{L_{max}}$ (Critical Angle of Attack, approx 16°), the airflow separates, and $C_L$ drops dramatically while Drag increases.
• Zero Lift Angle: For a symmetrical aerofoil, $C_L=0$ at $\alpha=0$. For a cambered aerofoil, $C_L=0$ at a slightly negative $\alpha$ (e.g., -4°).

Lift/Drag Ratio ($L/D$)

• Efficiency of the aerofoil.
• Max $L/D$: Corresponds to the most efficient angle of attack (approx 4°). Used for maximum range (in props) or maximum glide.
• At $L/D_{max}$, Drag is continuously minimum (Induced Drag = Parasite Drag).

Factors Affecting $C_L$

1. Angle of Attack: Primary factor.
2. Camber: More camber = higher $C_{L_{max}}$ (e.g., Flaps).
3. Aspect Ratio: Higher AR increases slope of lift curve.
4. Surface Condition: Ice/dirt reduces $C_{L_{max}}$ and Critical Angle.