Trailing-Edge Flaps - ATPL 081 — Principles of Flight

A wing shaped for cruise is the wrong wing for landing. In cruise the aircraft is fast, the required lift coefficient is small, and the designer wants a slim, lightly cambered section with as little drag as possible. On approach the same aircraft must fly slowly, and from the lift equation, flying slowly at the same weight demands a large $C_L$ . One fixed shape cannot serve both ends of the speed range well. The solution is to change the wing's shape in flight: high-lift devices, surfaces that fold out of the wing when slow flight demands them and disappear back into it for cruise. This lesson covers the devices at the back of the wing: the trailing-edge flaps; the next lesson covers the leading edge.

Camber on demand

A trailing-edge flap is the rear part of the wing section, mounted on hinges or tracks so that the pilot can deflect it downward. Deflecting the trailing edge down increases the wing's effective camber (the camber line acquires a much deeper curve) and some flap types also enlarge the wing area. From the coefficients lesson: more camber means more lift at every angle of attack, so $C_L$ rises at any given α and the maximum, $C_{L\,max}$ , rises with it. The price is also familiar: the deflected surface disturbs the flow and carries extra load, so $C_D$ rises too, and rises proportionally faster than the lift does.

Why is that trade worth making? Because of what it does to the slowest speed the wing can fly. The stalling speed comes from the lift formula, filled in at maximum lift:

A larger $C_{L\,max}$ under the square root means a lower stalling speed, and every operational speed built on top of the stall (rotation speed, approach speed, touchdown speed) comes down with it. The extra drag is not even always unwelcome: on approach it helps the aircraft descend more steeply and shed speed. Only where drag genuinely hurts, in the take-off climb, are flaps used sparingly: a small setting for some extra lift, not the full deflection.

The four flap families

Trailing-edge flaps come in four classic types, and the exam expects you to recognise each from a drawing. In order of increasing effectiveness:

Plain flap — the simplest: the whole rear portion of the section hinges down as one piece. Camber increases, lift rises, and the deflected surface adds a moderate amount of drag. Found on light aircraft, where simplicity beats the last bit of performance.
Split flap — only a plate on the lower surface hinges down; the upper surface keeps its original shape. The lift gain is slightly better than the plain flap's, but the wedge of separated, turbulent air trapped between the plate and the fixed upper shell produces much more drag. That drag made the split flap acceptable on older types, where it helped steepen the approach; modern designs avoid it, because the drag price is out of proportion to the lift gained.
Slotted flap — the flap is a separate small aerofoil, and when it deflects, a gap (a slot) opens between it and the wing. Air from the high-pressure region below the wing accelerates through the slot and flows over the flap's upper surface, keeping the flow attached where a plain flap's would have separated. Result: a larger lift gain with less drag. (What a slot does to a boundary layer is the next lesson's opening subject.)
Fowler flap — the flap first slides aft on tracks, and only then deflects down, usually with one or more slots. The aft travel enlarges the wing area (lift is bought with more square metres as well as more camber) which makes the Fowler the most effective type of all and the standard on transport aircraft. Its long aft travel also produces the strongest pitching-moment change, as the next sections show.

What flap does to the lift curve

Deflecting flap redraws the lift curve, and the exam asks for the comparison in four directions. All four are visible in the figure below. At any given angle of attack the flapped wing produces a higher $C_L$ : the whole curve is displaced upward, parallel to the clean one, because the added camber works at every angle. Read the same displacement horizontally and it says: any given $C_L$ is reached at a lower angle of attack: the flapped wing flies the approach with a flatter nose attitude. The peak of the curve, $C_{L\,max}$ , is higher with flap. But the peak arrives earlier: the critical angle of attack is reduced. The deep aft camber steepens the pressure recovery the boundary layer must climb on the rear of the section, so separation (the stall) sets in at a smaller angle of attack than on the clean wing.

The polar and the glide

On the $C_L$ – $C_D$ polar the story is one-sided: at every lift coefficient the flapped wing pays more drag, so the whole polar shifts to the right. The point that suffers most is the one the polar exists to show: the best lift-to-drag ratio. The tangent line from the origin to the flapped polar is shallower than to the clean polar: the maximum $C_L/C_D$ with flap is lower than the clean wing's. The flapped wing lifts more, but it flies less efficiently.

That ratio has a direct geometric meaning from the induced-drag lesson: in a glide, the best achievable glide angle is set by the maximum $C_L/C_D$ . Lowering flap therefore makes the glide steeper: the aircraft loses more height per metre flown. On approach that is a feature: a steeper, slower, drag-stabilised path down to the threshold. In a real engine-out glide it is the reason flap stays up until the field is certain: every stage of flap converts reach into descent rate.

Centre of pressure and pitch

The flap adds its new lift at the back of the section: the deflected surface and the deepened aft camber load up the rear of the chord. The wing's total lift therefore acts further aft: the centre of pressure moves aft when flap extends. Lift acting further behind the wing's reference point means a stronger nose-down pitching moment from the wing itself: strongest for the Fowler flap, which physically stretches the loaded chord aft. What the pilot finally feels also depends on how the flap's altered downwash strikes the tailplane, which can soften or even reverse the wing's contribution on some types, that interplay belongs to the stability module. For this lesson, hold on to the wing's own story: flap out, centre of pressure aft, the wing pitches nose-down, and the pilot counters it with elevator or trim.

Flap asymmetry

The two wings' flaps are one system, and the reason is what happens if they disagree. Suppose the left flap runs to the selected setting and the right flap jams clean: the left wing now flies at a visibly higher $C_L$ than the right at every angle of attack. The lift difference is a large, continuous rolling moment (the aircraft rolls toward the flapless wing) and the flap drag difference adds a steady yaw. Unlike a gust, this moment does not pass: the ailerons must hold it for as long as the asymmetry exists, and aileron authority is at its weakest exactly where flaps are used: at low speed. A large enough asymmetry, deflected far enough, can exceed full aileron and make the aircraft uncontrollable. This is why flap systems continuously compare the two sides and freeze both flaps the moment their positions diverge: a symmetric wing at the wrong setting is a manageable problem, an asymmetric wing may not be.

Take-off and landing with flap

Gather what the mechanism buys on the runway:

Lower speeds, shorter distances — flap raises the maximum lift coefficient, the stalling speed falls, and rotation, approach and touchdown speeds fall with it. Distance follows speed squared: the ground roll is governed by the kinetic energy at lift-off or touchdown, so cutting the speed by a fifth cuts the energy by roughly a third: both the take-off run and the landing roll shorten substantially.
Lower nose attitudes — the flapped wing reaches any needed lift coefficient at a smaller angle of attack, so the whole approach is flown with a flatter pitch attitude. The advantages are practical: the pilot sees the runway over the nose, the tail sits well clear of the surface in the flare, and the aircraft touches down close to its ground attitude instead of balancing on its main wheels at high pitch.
Different settings for different jobs — take-off uses a small flap setting: a useful lift gain at a modest drag price, keeping the climb gradient healthy. Landing uses a large setting: maximum lift for the slowest touchdown, with the extra drag welcomed as a speed brake on the way down.

Flap-setting errors

Because performance figures are computed for a specific flap setting, using the wrong setting (or moving flaps at the wrong moment) invalidates numbers the crew is depending on:

Wrong take-off setting — the speeds and distances in the performance calculation belong to the planned setting. With less flap than planned, the wing needs more speed to lift off: the computed rotation speed arrives before the wing is ready, and the take-off run lengthens beyond the computed distance. With more flap than planned, the extra drag slows the acceleration and, worse, cuts the climb gradient the obstacle calculation assumed.
Premature retraction — retracting flap just after lift-off, or early in a go-around, removes lift in one step: the stalling speed jumps upward toward the aircraft's actual speed, and the margin to stall buffet (the gap between flying speed and stalling speed) can shrink to nearly nothing while the aircraft is low and slow. The aircraft sinks as the lift drops. Flap is retracted on schedule, one stage at a time, as speed builds past each stage's safe minimum.
Late retraction — climbing with flap still extended means climbing with the flap's full drag penalty: the gradient and rate stay visibly worse until the wing is clean.
Premature or late extension on approach — extending early wastes fuel and time dragging the aircraft down the approach, and risks the flap-extension speed limits; extending late (or landing with less flap than intended) means a faster approach on a shallower path, flown at a more nose-up attitude, and a landing distance well beyond the computed figure. An approach that cannot be stabilised at the planned setting and speed is flown again.

What the exam asks

This lesson is 'describe the devices, then read the two graphs'. The overview, following the learning objectives:

Identify and describe the four trailing-edge flap types from diagrams: plain, split, slotted, Fowler

Describe how the increased effective camber raises $C_L$ and $C_D$ , and why that trade is beneficial

Describe the effect on the centre of pressure (aft), pitching moment (nose-down) and stalling speed (reduced)

Compare the lift curves: higher $C_L$ at any α, higher $C_{L\,max}$ , lower critical angle, a given $C_L$ at a lower α

Compare the polars: the maximum $C_L/C_D$ with flap is lower than the clean wing's

Explain the influence of flap deflection on the glide angle: steeper

Describe flap asymmetry and its effect on controllability

Explain the advantages of lower nose attitudes and why take-off and landing speeds and distances are reduced

Explain the effects of flap-setting errors on distances, speeds, climb and descent performance, and stall buffet margins