Wing Flaps & High-Lift Devices

Why a wing needs two personalities

The cruise condition and the landing condition are not just different speeds — they are different problems that demand different wings. A long-range airliner cruises at about Mach 0.78, which at typical cruise altitude works out to a true airspeed near 250 m/s. The lift equation L = ½ρV²S·C_L tells you that at that speed, even a modest lift coefficient produces all the lift you need. A cruise wing therefore wants to be small (low S), thin (low transonic drag), and to fly at a low C_L (around 0.5) with a high lift-to-drag ratio. The wing chord is short, the camber is gentle, and there is no surplus mechanism on the leading or trailing edge.

Landing approach is the opposite. The aircraft must fly slowly enough to stop on a 2-3 km runway after touchdown, which for a fully loaded 200-tonne airliner means an approach speed of roughly 70 m/s — about a quarter of the cruise speed, so about a sixteenth of the dynamic pressure ½ρV². To generate the same lift weight, the product S·C_L must therefore increase by a factor of 16. You cannot reasonably grow S in flight, so the entire factor has to come from C_L. A clean transport wing maxes out at C_L,max ≈ 1.5 — not nearly enough. Deploying flaps and slats raises C_L,max to 2.5–3.5, which closes the gap.

The alternative is a permanently large, draggy wing — and that is precisely what early airliners had. A DC-3 with its big, low-aspect-ratio wing landed at 60 mph but cruised at only 190 mph, with a fuel burn we would now find horrifying. The modern transport wing is small for cruise; high-lift devices borrow temporary area and camber for the few minutes of takeoff and landing when low-speed lift is actually needed.

Trailing-edge devices: the flaps

Flaps live on the rear part of the wing and add camber, area, or both. The simplest version is a plain flap: the trailing edge of the wing hinges downward. This adds camber but no area. A split flap is similar but only the lower-surface skin deflects, leaving the upper surface unchanged — this avoids creating a sharp adverse-pressure-gradient bump on the upper surface but is otherwise crude, and is now rare on transports (you can still find it on the Douglas DC-3 and on some piston singles).

The single-slotted flap is the workhorse of modern commercial aviation. As the flap deflects, a gap (slot) opens between the wing main element and the flap. High-energy air from the lower surface accelerates through the slot and re-attaches the boundary layer on the upper surface of the flap, allowing the flap to deflect more steeply without separating. The 787, A350, and A320neo all use single-slotted flaps.

The Fowler flap, patented by Harlan Fowler in 1924 and first flown on the Northrop Gamma, adds another trick: as the flap deflects, it also translates rearward on a track. The chord of the wing literally grows. Recall L = ½ρV²S·C_L: a larger S means more lift even at the same C_L. The 727, 737, 747, 757, and 767 all used multi-slotted Fowler flap systems with two or three slots, with the 747-200 sporting a triple-slotted design that pushed C_L,max past 3.0.

Smaller add-ons round out the trailing edge. The gurney flap is a small (1–2% of chord) right-angled tab fastened to the high-pressure side of the trailing edge, invented by Dan Gurney in the 1970s for Indianapolis race cars and quickly adopted on helicopter blades, sailplane wings, and some commercial-airliner horizontal stabilisers. It creates a tiny rear-stagnation-point shift that increases circulation, adding 5–25% to C_L,max at negligible weight cost. Vortex generators — small fences on the upper surface, just ahead of the flap hinge — energise the boundary layer and delay separation.

Leading-edge devices: slats, slots, and Kruegers

Trailing-edge flaps only get you so far. As C_L climbs, the suction peak on the upper surface near the leading edge grows. Eventually the adverse pressure gradient downstream of that peak is severe enough to separate the boundary layer, and the wing stalls — usually at α ≈ 14–17° for a clean airfoil. To go higher, you need to do something at the leading edge.

The slat is the leading-edge analogue of a slotted Fowler flap: a small auxiliary aerofoil that translates forward and downward, opening a slot. Air accelerated through that slot re-energises the boundary layer at the very leading edge of the main wing element, postponing separation to angles of attack 5–10° higher than the clean wing would allow. Frederick Handley Page patented the basic leading-edge slot in 1919 — it is one of the oldest high-lift devices, and almost every commercial airliner since the 707 has used some form of leading-edge slat. The A320 family uses slats; the 737NG and MAX use leading-edge slats outboard and Krueger flaps inboard.

The Krueger flap is different. Instead of a separate aerofoil that translates forward, the Krueger hinges from the lower surface of the leading edge, swings down and forward, and forms a chin that increases the leading-edge camber. There is no slot. The Krueger preserves the clean upper-surface contour of the leading edge in cruise — important if you want laminar flow over the front portion of the wing — and is mechanically simpler than a tracked slat. Boeing has long favoured Kruegers, especially inboard of engine pylons where slat tracks would conflict with the high-bypass nacelle.

A fixed leading-edge slot is the simplest of all: a permanent gap near the leading edge. It works at all conditions, no actuators, but it adds drag in cruise — so it is found mainly on STOL aircraft like the Fieseler Storch and the Helio Courier, where short-field performance trumps cruise efficiency.

Worked example: how much runway do flaps save?

Take a 70-tonne narrow-body airliner with a wing area S = 122 m². At sea-level standard density ρ = 1.225 kg/m³, the lift balance L = W gives

V_stall = √( 2 W / (ρ S C_L,max) )

Plugging in W = 70,000 × 9.81 = 6.87 × 10⁵ N:

Clean wing,    C_L,max = 1.5   →  V_stall = 76 m/s = 148 kt
Flaps 25°,     C_L,max = 2.4   →  V_stall = 60 m/s = 117 kt
Full flaps + slats, C_L,max = 3.2 →  V_stall = 52 m/s = 101 kt

Landing approach speed is conventionally V_ref = 1.23 × V_stall. With clean wing the approach is at 182 kt; with full flaps and slats it drops to 124 kt — a 32 percent reduction. Stopping distance scales as the square of touchdown speed (kinetic energy is ½ m V²), so the runway required shrinks by roughly a factor of two. That is the difference between landing at a major hub and landing at a 1,500-m regional strip.

Real-world configurations

Aircraft	Trailing edge	Leading edge	Clean → flapped C_L,max
Cessna 172	Plain flap	None	1.5 → 2.1
DC-3	Split flap	None	1.4 → 2.0
Boeing 707	Double-slotted Fowler	Krueger inboard, slat outboard	1.5 → 2.8
Boeing 727	Triple-slotted Fowler	Krueger + slat	1.5 → 3.0
Boeing 747-200	Triple-slotted Fowler	Variable-camber Krueger + slat	1.5 → 3.2
Boeing 737NG/MAX	Double-slotted Fowler	Krueger inboard, slat outboard	1.5 → 2.9
Airbus A320	Single-slotted Fowler	Slat full span	1.5 → 2.7
Boeing 787	Single-slotted Fowler + droop-nose flaperon	Slat	1.5 → 2.6
Airbus A350	Single-slotted Fowler	Droop nose + slat	1.5 → 2.6
Fieseler Storch	Slotted flap	Fixed slot	1.4 → 2.5
Buccaneer (BLC)	Blown flap	Blown leading edge	1.5 → 3.9

The trend across two generations is striking: the 747-200 needed a triple-slotted Fowler to hit C_L,max ≈ 3.2; the 787 reaches C_L,max ≈ 2.6 with a single-slotted Fowler and lands almost as slowly because computational design has refined the cruise wing's clean C_L,max upward and the slat's contribution.

Boundary-layer control and powered lift

For aircraft that need more than even slatted Fowler flaps can deliver — STOL transports, naval aircraft with very short carrier decks, or research vehicles — engineers turn to active boundary-layer control. The simplest form is the blown flap: a slot just ahead of the flap hinge ejects high-pressure compressor bleed air over the deflected flap. The energetic jet keeps the boundary layer attached at flap deflections of 60–80°, generating extraordinary C_L,max values. The Blackburn Buccaneer naval bomber used BLC over its wings and tailplane to operate from short carrier decks; the F-104 Starfighter used BLC because its tiny wing would otherwise have given an unflyable approach speed.

A more aggressive concept is upper-surface blowing (USB): the engine exhaust itself flows over the upper surface of the wing, attaching by Coandă effect and creating lift through entrainment. The Boeing YC-14 and McDonnell Douglas YC-15 used USB; the Antonov An-72/74 still does. Vectored thrust takes the idea further: a Harrier or F-35B redirects the engine plume itself downward, supplementing aerodynamic lift with direct thrust lift for vertical takeoff.

The opposite of blowing is sucking: laminar flow control uses perforated skin and suction pumps to draw the boundary layer away through tiny holes, keeping the flow laminar at much higher Reynolds numbers than would otherwise be possible. NASA's F-16XL test programme and a few experimental gliders have flown LFC, but the maintenance burden of keeping the holes clean has so far kept it out of production.

Trade-offs: weight, drag, complexity

Every flap slot is a separate aerofoil with its own tracks, rollers, linkages, fairings, and actuator. A triple-slotted Fowler system on a 747 wing weighs hundreds of kilograms per side and adds bulky external fairings that disturb the otherwise clean cruise wing. Each kg of structural weight costs about 0.05 kg of additional fuel per flight hour — and a 747 flies 30,000 hours over its life. Wing flap mechanism complexity also drives a substantial fraction of routine maintenance: actuators, tracks, position sensors, and seals all need periodic inspection and rework.

The trend since the early 1980s has therefore been toward simpler, lighter, more aerodynamically refined high-lift systems. The 757 used a double-slotted main flap; the 777 used a single-slotted main flap with a smaller second element; the 787 and A350 use a single-slotted Fowler with a sophisticated leading-edge droop nose. Computational fluid dynamics now lets engineers extract from a single slot the C_L,max that earlier designers needed three to achieve.

A brief history

• 1919 — Handley Page slat. Frederick Handley Page patents the leading-edge slot, originally as a fixed slot. Demonstrated dramatically when a Handley Page biplane flew at half its previous stall speed.
• 1924 — Fowler flap. Harlan Fowler patents the chord-extending track-mounted flap. Tested on the Northrop Gamma in 1932 and adopted on the Lockheed Constellation, DC-4, B-29, and almost every transport since.
• 1930s — Split flap. Orville Wright and J. M. H. Jacobs jointly develop the split flap; it becomes the standard on the DC-3 and B-17.
• 1940s — Krueger flap. Werner Krueger develops the leading-edge hinged Krueger flap at the AVA Göttingen wind tunnel; first widely used on the Boeing 707.
• 1957 — Buccaneer BLC. Blackburn Aircraft introduces production blown-flap BLC on a carrier-capable strike aircraft.
• 1970s — Gurney flap. Dan Gurney's race-car trailing-edge tab is adopted on aircraft, especially helicopter blades and horizontal tails.
• 2000s — Single-slotted return. The 787 and A350 demonstrate that with modern CFD, single-slotted flaps suffice — closing the curtain on the multi-slot era for new transports.

Common misconceptions

• "Flaps create lift directly." Flaps increase the wing's C_L at a given α; the lift is still generated by the same Newton/Bernoulli mechanism. Flaps are a knob, not a source.
• "More flap = more lift, always." Beyond a certain deflection angle, drag rises faster than lift, and the lift-to-drag ratio collapses. Full flaps are for landing, where you want the drag; takeoff uses a smaller setting (often 5–20°) to maximise lift without excessive drag.
• "Slats and flaps do the same thing." Slats extend the stall angle; flaps shift the entire C_L–α curve up. Both increase C_L,max but by different mechanisms — they are complementary, not redundant.
• "Triple-slotted is always better." Each slot adds weight, mechanism, and cruise drag. Modern airliners have moved away from triple-slotted designs because the trade-off is no longer favourable.
• "Flaps make the plane go slower." Flaps allow slower flight, but they also let the plane fly steeper descents (more drag) and shorter landings. A pilot uses flaps to expand the slow-flight envelope, not as a brake per se.
• "A gurney flap is just a tiny flap." It works by shifting the Kutta condition at the trailing edge, not by adding camber. The fluid-mechanical mechanism is different from a deflecting trailing-edge surface.