# CLMAX Augmentation (High Lift Devices)

> Using flaps and slats to increase maximum lift coefficient for landing and takeoff.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-01 — Subsonic Aerodynamics
- **Topic code:** 081-01-09
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/subsonic-aerodynamics/clmax-augmentation/

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## Purpose
To increase $C_{L_{max}}$ allows the aircraft to fly at lower speeds (Stall speed is reduced) safely for Take-off and Landing.
$$ V_{stall} = \sqrt{\frac{2W}{\rho S C_{L_{max}}}} $$

## Trailing Edge Flaps
Increase camber and (usually) surface area.
1.  **Plain Flap**: Simple looking hinge. Increases camber. Small $C_L$ increase.
2.  **Split Flap**: Lower surface hinges down. High Drag.
3.  **Slotted Flap**: Gap allows high energy air from bottom to re-energize boundary layer on top. Delays separation.
4.  **Fowler Flap**: Moves back and down. Increases **Surface Area** ($S$) and Camber. Very effective.

**Effects**:
*   Increases $C_{L_{max}}$ significantly (up to 50-90%).
*   Reduces Stalling Angle of Attack (e.g., from 16° to 12°).
*   Increases Drag (Desirable for landing).
*   Nose-down pitching moment.

## Leading Edge Devices
1.  **Krueger Flaps**: Hinged flap on leading edge.
2.  **Slats**: Moveable leading edge section. Creates a **Slot**.
3.  **Slots**: Fixed gap.

**Effects**:
*   Re-energizes boundary layer.
*   **Increases** Critical Angle of Attack (e.g., from 16° to 22°).
*   Increases $C_{L_{max}}$.

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