Leading Edge Devices
Leading edge devices are moveable or fixed structures at the wing leading edge. They improve stall behaviour at high angle of attack and raise CL_max.
Working principle
At high α the flow separates on the upper surface → stall. LE devices delay separation by:
- Energising the boundary layer (via nozzle/slot effect).
- Reducing the effective angle of attack (slot geometry allows more favourable inflow).
Types
1. Fixed slot
- Permanent opening between a permanent leading-edge element and the main profile.
- On some Cessna 152 and Helio Courier aircraft.
- Effect: CL_max increase about +0.2 to +0.4; α_stall raised by about 5°.
- Disadvantage: constant drag increase also in cruise.
2. Moveable slat
- Extends at high α (or low speed, design-dependent).
- At low α retracted — minimal drag.
- At high α extended — CL_max increase.
- Examples: all modern airliners (A320, B737, B777, A380).
- Effect: CL_max +0.5 to +1.0.
3. Krueger flap
- Leading-edge flap swinging forward and down (instead of rearward like a slat).
- Effectively increases LE camber.
- Examples: B747 inner wing, some high-performance designs.
- Effect: CL_max +0.5; shifts α_stall.
4. Drooped leading edge
- Permanently drooped (no movement).
- Effect: higher CL_max at higher α — good for gliders and some STOL aircraft.
- Examples: some Pilatus PC-12 configurations, gliders.
Slot mechanism in detail
The slot between LE device and main profile lets air flow from underside to upper side. This energetic air re-energises the boundary layer on the upper surface → later separation → higher maximum α.
Effect on CL-α curve
With slot/slat extended:
- CL_max rises by 0.3 to 1.0 (type-dependent).
- α_stall rises by 5° to 8°.
- Lift curve shifts right and up.
Comparison flaps vs LE devices
| Aspect | Flaps (TE) | LE devices |
|---|---|---|
| Location | trailing edge | leading edge |
| CL_max increase | +0.5 to +1.0 | +0.3 to +1.0 |
| α_stall shift | usually lower | higher |
| Drag increase | high | low (slat) |
| Pitching moment | usually nose-down | usually neutral or slightly nose-down |
| In GA | very common | rare (fixed slots Cessna 152 var.) |
| In airliners | mandatory | mandatory |
Combined use
Modern airliners use both systems:
- Take-off: slats + flaps 5° (B737), slats + flaps 1+F (A320).
- Landing: full slats + full flaps.
- CL_max can rise from ~1.4 (clean) to 3.0+ (full configuration).
Vortex generators (alternative)
Vortex generators are small vanes on the profile upper surface that produce controlled vortices → energise the boundary layer → later separation. Similar effect to a slot but passive (no moving parts). Examples: some Cessna 172 modifications, many gliders.
Practical PPL relevance
- Cessna 172 classic: only flaps, no slats.
- Trainers with slats are rare.
- Awareness for observing airliners important.
Vne with slats?
Slat configurations have own Vfe / Vle limits in the POH. Some modern gliders: Vle for slats about 70-100 kt.