Every wing is a compromise. The aerofoil that cruises efficiently at high Mach is hopeless at approach speed. High-lift devices are how you break that compromise on command.
Flaps and slats are among the highest-yield topics in EASA ATPL Principles of Flight, and among the most misremembered. Get the lift curve straight and the exam questions answer themselves.
The problem: lift falls with the square of speed
Lift follows L = ½ ρ V² S CL. In level flight lift equals weight, so as you slow down, the only way to keep that equation balanced is to raise the lift coefficient, CL.
A wing raises CL by increasing angle of attack, until it reaches the critical angle and stalls. That ceiling, CLmax, sets your stall speed: VS is proportional to one over the square root of CLmax. Raise CLmax and the stall speed drops.
That is the entire job of a high-lift device: buy more CLmax so the aircraft can fly slower, approach slower, and land shorter.
Trailing-edge flaps: more camber, more lift, lower stall angle
Extend the flaps and you increase the wing's camber. More camber means more lift at any given angle of attack, so the whole lift curve shifts up and to the left. CLmax rises, which is the point of the exercise. But there is a catch that trips up half the candidates: the stalling angle of attack decreases.
The cambered section is already working harder at a given geometric angle, so the airflow separates at a lower angle than on the clean wing. You get more lift, but you reach the stall at a smaller nose-up attitude.
Flaps also add drag, and they generate a nose-down pitching moment as the centre of pressure moves aft. That is why extending flap changes your trim.
The four flap types you must know
- Plain flap: the trailing edge hinges down. Simple; increases camber only.
- Split flap: the lower surface deflects down. Similar lift to a plain flap but much more drag.
- Slotted flap: a gap lets high-energy air from below re-energise the flow over the flap, delaying separation. The most common airliner flap.
- Fowler flap: moves aft on tracks before deflecting down, so it increases wing area as well as camber, for the biggest CLmax gain of the four.
Only the Fowler meaningfully increases area. That distinction is a favourite EASA question.
Leading-edge slats: delaying the stall to a higher angle
A slat is a small aerofoil that extends from the leading edge, opening a slot. That slot ducts high-energy air over the upper surface and re-energises the boundary layer, delaying the point at which airflow separates.
The result is the mirror image of a flap in one key respect: a slat increases the stalling angle of attack. The lift curve is extended to the right, reaching a higher CLmax at a higher angle. Slats add little extra drag, which is why they earn their keep on both take-off and landing.
Do not confuse a movable slat with a fixed slot, or with a Krueger flap, which is a leading-edge flap that adds camber rather than opening a slot.
Put them together: the approach configuration
Flaps raise CLmax but lower the stall angle; slats raise CLmax and raise the stall angle. Deploy both and you get the highest CLmax of all with a usable stalling angle, and therefore the lowest possible stall speed. That is exactly why a landing configuration uses full flap and slats together.
It also explains the sequence you feel on approach: slats and a little flap first to bring the speed back, then progressively more flap as you configure to land, each stage lowering the speed at which the wing will stall.
The traps EASA sets on this topic
Three catch students at almost every sitting:
- Flaps increase the stalling angle. They do not. Trailing-edge flaps lower it; leading-edge slats raise it.
- Flaps and slats do the same job. Both raise CLmax, but they act at opposite ends of the wing and move the stalling angle in opposite directions.
- All flaps add wing area. Only the Fowler flap does. Plain, split and slotted flaps add camber, not area.
Anchor every answer to the lift curve above and you will not be caught.
EASA ATPL flaps and slats: FAQ
Do flaps increase or decrease the stalling angle of attack?
Trailing-edge flaps decrease the stalling angle of attack while increasing CLmax. Leading-edge devices such as slats increase the stalling angle. That opposite behaviour is a classic EASA exam distinction.
What is the difference between a slat and a Fowler flap?
A slat is a leading-edge device that opens a slot to delay separation and raise the stalling angle. A Fowler flap is a trailing-edge device that slides aft to increase wing area and camber, giving the largest CLmax increase of the common flap types.
Why do high-lift devices reduce stall speed?
Stall speed is inversely proportional to the square root of CLmax. By raising CLmax, flaps and slats let the wing support the aircraft's weight at a lower speed, so the stall speed falls.
Do slats add a lot of drag?
No. Slats re-energise the boundary layer and add relatively little drag, which is why they are used at both take-off and landing, whereas large flap settings add significant drag.
Want to drill this until the lift curve is automatic? Practise EASA ATPL Principles of Flight questions with worked explanations at atpltraining.io.
Photo by Sergey Koznov on Unsplash
