# Aerodynamic Coefficients

> Understanding Lift and Drag formulas and their coefficients.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-01 — Subsonic Aerodynamics
- **Topic code:** 081-01-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/subsonic-aerodynamics/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.

---

Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.