Lift augmentation and drag reduction devices
Overview
Lift augmentation and drag reduction devices tests high-lift systems and the hardware used to cut drag. Expect questions on flap types, slats, vortex generators, spoilers and fairings.
Core rules
- Trailing-edge flaps increase camber and, on Fowler designs, wing area and chord, raising C_Lmax and usually drag.
- Plain, split, slotted and Fowler flaps differ in how they manage the boundary layer. Slotted flaps re-energise the flow; Fowler flaps slide aft and down.
- Leading-edge slats and slots delay the stall by allowing air to reach the upper surface at a higher AoA; Krueger flaps are leading-edge devices.
- Vortex generators mix high-energy air into the boundary layer, delaying separation and improving aileron effectiveness.
- Spoilers destroy lift and add drag; they are used for descent, speed braking and roll control.
- Drag reduction: retractable gear, fairings, gap seals, winglets and laminar-flow surfaces.
Worked example
A Fowler flap extends chord by 15 % and raises C_Lmax from 1.4 to 2.2. At the same weight the stall speed changes by √(1.4/2.2) = 0.798, a 20 % reduction - but approach drag is much higher, so the approach is steeper.
Traps
- Flaps increase lift but also drag and pitching moment; full flap may reduce climb gradient.
- Spoilers reduce lift - they are not flaps.
- Slats and slots improve C_Lmax but add complexity and drag.