Relationship between CL and Speed

081-01-07Pablo Asensio Martínez2026-04-011 min

The Balance

In steady, straight, and level flight: $ Lift = Weight $ $ C_L \cdot \frac{1}{2} \rho V^2 \cdot S = Weight $

The Relationship

Since Weight ($W$) and Wing Area ($S$) are constant (mostly): $ C_L \cdot V^2 \approx Constant $

  • High Speed ($V$): Requires Low $C_L$ (Small Angle of Attack).
  • Low Speed ($V$): Requires High $C_L$ (Large Angle of Attack).

Flying Slower

To maintain level flight while decelerating:

  1. Speed ($V$) decreases -> Dynamic pressure decreases -> Lift would decrease.
  2. Pilot increases Angle of Attack -> $C_L$ increases -> Lift is restored to equal Weight.
  3. Limit: Eventually $C_{L_{max}}$ is reached (Stall Speed).

Altitude Effect

$ EAS \approx Constant \cdot \sqrt{Weight} $ Typically cruise is flown at specific Mach or CAS.

  • As altitude increases ($\rho$ decreases), TAS must increase to maintain same Lift (if $C_L$ is constant).