Flap types & effects
Flap: movable wing surface at or near trailing edge, between wing root and ailerons (TP 1102 ch.1).
- Both flaps extend and retract together.
- Change effective lift via camber and, on some designs, area.
Two families:
| Family | Motion | Effect |
|---|---|---|
| Plain | Part of main aerofoil (upper and lower surfaces) hinges down | No change in wing chord |
| Articulated (e.g. Zap, Fowler) | Moves aft as it extends | Increases effective chord; up to a given point in travel, much more lift for minimum drag |
Two more trailing-edge types (PHAK), ranked by lift produced:
| Type | Design | Effect |
|---|---|---|
| Split | Hinges down from lower surface only; upper surface undisturbed | Slightly more lift than plain; more drag (turbulent air pattern) |
| Slotted | Slot ducts high-energy air from lower surface to flap upper surface | Delays separation; much higher max coefficient of lift (CL-MAX) than plain or split; most common type today |
- Fowler = slotted variant: adds wing area to slot effect.
Slat (not a flap), leading-edge device (PHAK ch.6):
- Movable leading-edge airfoil; closed in normal flight.
- Extends at high AOA: air from below flows over upper surface, delaying separation.
- Leading edge; flaps are at or near trailing edge.
Extending flaps gives (some combination; TP 1102 ch.1):
- Lower stalling speed.
- Steeper approach without airspeed increase.
- Better forward visibility on approach (lower nose).
- Shorter take-off run.
Extending flaps also (TP 1102 Approach and Landing):
- Changes airflow over and under the wing and around the tailplane.
- Shifts trim requirements: retrim to hold desired descent rate and airspeed.
Trap: retracting flaps in flight needs extreme care, especially near the ground: sudden loss of lift and change in balance.
References
- TP 1102 Ex.2 Components (Flaps)
- TP 1102 ch.1 Basic Principles of Flight (Flaps)
- PHAK ch.6 Secondary Flight Controls (Flaps, Leading Edge Devices)
- TP 1102 Approach and Landing (Flaps and Trim)
- TP 13014 sample exam item 43 (flap extension speed)