A Krueger flap is a leading-edge high-lift device that swings down and forward from the underside of the wing to extend the effective chord and increase camber at low speeds. The driving component is a hinged four-bar linkage powered by a hydraulic rotary actuator, which both unfolds the panel and locks it against airloads. The purpose is to delay stall during takeoff and landing, when the wing is operating at high angle of attack. On a Boeing 747-400, deployed Krueger flaps raise the maximum lift coefficient from roughly 1.4 clean to over 2.4, cutting approach speed by around 30 knots.
Krueger Flap Mechanism Interactive Calculator
Vary deployment angle, lift coefficients, and approach speeds to see lift gain, speed reduction, and an animated Krueger four-bar deployment diagram.
Equation Used
This calculator compares the clean and deployed Krueger flap condition using the article values. The lift increment is the change in maximum lift coefficient, while the speed cut is the difference between the clean and deployed approach speeds.
- Clean speed default is derived from the article statement: 140 kt approach plus about 30 kt reduction.
- CL values are maximum lift coefficients for clean and deployed configurations.
- Speed cut is a comparison of stated approach speeds, not a full aircraft performance calculation.
The Krueger Flap Mechanism in Action
The Krueger flap sits flush against the underside of the wing leading edge in cruise. When the pilot selects flaps down, a hydraulic rotary actuator drives a four-bar linkage that rotates the panel forward and downward through roughly 130°, wrapping it around the leading-edge radius so that the panel's lower surface becomes the new upper aerodynamic surface ahead of the wing. The result is a longer chord, more camber, and a sharper leading edge geometry tuned for high angle of attack — exactly what you need on approach at 140 knots when the clean wing would already be stalling.
Design-wise, the panel is shaped this way because a simple hinged plate cannot match the curvature the flow demands at the wing leading edge. The Boeing 747 and 777 inboard sections use a variable-camber Krueger with a folding bullnose — a secondary linkage rotates a small nose segment so the deployed leading edge actually has the right radius for attached flow. If the bullnose timing is off by more than about 2°, the flow separates from the upper surface near the joint and you lose 10 to 15% of the expected lift increment. That is why the rigging tolerance on the bullnose linkage is held to ±0.5° at full deployment.
What happens when tolerances drift? The two failure modes we see most are linkage wear at the main pivot, which lets the panel sit slightly low and creates a step in the upper surface, and ice contamination of the stowed cavity, which can prevent full retraction and trigger a flap asymmetry warning. On the 747, an asymmetry of more than 3° between left and right Krueger groups commands an automatic shutoff of the high-lift system.
Key Components
- Krueger Panel: The aerodynamic surface itself, typically a machined aluminium or composite skin 1.5 to 3 mm thick. In the stowed position it forms part of the lower wing surface; deployed, it becomes the new leading-edge upper surface. Surface waviness must stay under 0.3 mm per 100 mm to keep the boundary layer attached.
- Four-Bar Drive Linkage: The kinematic heart of the system. Two pivot arms and a coupler convert rotary actuator motion into the compound forward-and-down sweep. Pivot bushing clearance is typically held to 0.05 mm — any more and panel position repeatability drifts past the 0.5 mm tolerance the aero shape requires.
- Folding Bullnose: A small secondary panel at the leading edge of the Krueger that unfolds during the last 20° of deployment. Without it, the deployed leading edge would be too sharp and the flow would separate. The fold timing is set by a cam-and-roller mechanism rigged to ±0.5°.
- Hydraulic Rotary Actuator: Provides the deployment torque, typically 8,000 to 15,000 N·m at 3,000 psi system pressure on widebody aircraft. The actuator includes an internal hydraulic lock that holds the flap deployed against airloads up to 250 KCAS without bleeding back.
- Torque Tube and Drive Shaft: Distributes actuation across multiple Krueger panels along the inboard leading edge so they deploy synchronously. Angular twist under load must stay under 1° end-to-end, or the outermost panel will lag and create a spanwise step in the leading edge.
- Position Sensor and Asymmetry Monitor: RVDT or proximity sensors at each panel report position to the slat/flap electronic control unit. If left and right groups differ by more than 3°, the system commands shutoff to prevent rolling moment asymmetry.
Real-World Applications of the Krueger Flap Mechanism
Krueger flaps live on the inboard leading edge of large transport aircraft where the wing is too thick at the root to package a slotted slat. They share the leading edge with outboard slats on most Boeing widebodies. You'll find them on every long-haul jet that needs a clean cruise wing combined with strong low-speed performance, and increasingly on laminar-flow research wings where any leading-edge step would trip the boundary layer.
- Commercial Aviation: Boeing 747-400 and 747-8 use variable-camber Krueger flaps across the entire inboard leading edge — three panels per side, deploying with the folding bullnose.
- Commercial Aviation: Boeing 777 uses seven Krueger panels per side on the inboard wing, paired with outboard slotted slats.
- Military Transport: Lockheed C-5 Galaxy uses Krueger flaps on the inboard leading edge to keep the upper surface clean for the high-aspect-ratio wing's cruise efficiency.
- Business Aviation: Bombardier Global 7500 uses a simplified Krueger on the inboard leading edge ahead of the engine pylon to manage flow at high angle of attack.
- Laminar Flow Research: NASA and Boeing Hybrid Laminar Flow Control test wings use Krueger flaps specifically because they leave the upper surface uninterrupted in cruise — critical for sustaining laminar flow.
- Cargo Aviation: Boeing 747-400F and 747-8F freighters retain the full inboard Krueger system for short-field operations at high gross weights.
The Formula Behind the Krueger Flap Mechanism
The figure of merit for any leading-edge device is the increment in maximum lift coefficient, ΔCL,max, that it produces when deployed. For a Krueger flap this depends on chord extension ratio, deployment angle, and whether a folding bullnose is fitted. At the low end of the practical deployment range — around 60° rotation, no bullnose — you get a modest ΔCL,max of about 0.3, mostly from chord extension. At the design sweet spot of roughly 130° with a properly rigged bullnose, you reach 0.8 to 1.0. Push beyond 140° and the panel starts to obstruct the flow rather than redirect it, and the increment falls away.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ΔCL,max | Increment in maximum lift coefficient produced by the deployed Krueger | dimensionless | dimensionless |
| Kb | Bullnose effectiveness factor — 1.0 with folding bullnose, 0.6 to 0.7 for a simple hinged Krueger | dimensionless | dimensionless |
| c'/c | Ratio of extended chord to clean chord | dimensionless | dimensionless |
| δK | Krueger deployment angle measured from stowed | degrees | degrees |
| η3D | Three-dimensional correction factor accounting for finite span, sweep, and panel gap losses | dimensionless | dimensionless |
Worked Example: Krueger Flap Mechanism in a 747-class widebody inboard Krueger panel
Take a 747-class inboard Krueger panel with a chord extension ratio of 1.12, a folding bullnose, a 3D correction factor of 0.85 for a swept inboard panel, and you are evaluating ΔCL,max across the practical deployment range to size approach speed margins.
Given
- Kb = 1.0 dimensionless (folding bullnose fitted)
- c'/c = 1.12 dimensionless
- δK,nom = 130 degrees
- δK,low = 60 degrees
- δK,high = 145 degrees
- η3D = 0.85 dimensionless
Solution
Step 1 — at nominal full deployment of 130°, compute sin(δK):
Step 2 — multiply through to get the nominal lift increment:
That is roughly the 0.7 to 0.8 increment Boeing publishes for the 747 inboard Krueger group, and it is what lets a 400-tonne aircraft approach at 145 knots instead of stalling near 175.
Step 3 — at the low end of the deployment range, 60°, which corresponds to a partial-deploy or transit position:
Wait — that is higher than nominal? It is, mathematically, but in reality at 60° the bullnose has not yet unfolded, so Kb drops to about 0.65. Recomputing: ΔCL,max,low,real = 0.65 × 1.12 × 0.866 × 0.85 = 0.536. That is the actual penalty of partial deployment — you lose roughly a quarter of the lift benefit. Pilots feel this as sluggish climb response if they retract Krueger before flap-up speed.
Step 4 — at the high end, 145°, the panel is over-rotated:
The math says the geometric projection drops, and on top of that the panel begins to obstruct the streamtube into the wing upper surface. Real wind-tunnel data shows a steeper falloff than this clean trig predicts — closer to ΔCL,max = 0.45. That is why production rigging stops at 130 to 135°.
Result
Nominal ΔCL,max works out to 0. 73, which on a 747-class wing translates to roughly a 25 to 30 knot reduction in stall speed at landing weight. The sweet spot is narrow — at 60° partial deploy you only get 0.54, and at 145° over-rotation you drop back to 0.55, so the 120° to 135° band is where the system actually earns its keep. If your flight-test data shows a measured ΔCL,max 15% below predicted, the usual suspects are: (1) bullnose timing off by more than 2°, leaving a sharp leading-edge radius the flow cannot follow, (2) excessive gap between the trailing edge of the Krueger and the wing lower surface — anything over 3 mm bleeds pressure and kills the suction peak, or (3) torque-tube wind-up from worn splines causing the outboard panel to lag the inboard by 4 to 6°, creating a spanwise step that triggers premature stall at the joint.
When to Use a Krueger Flap Mechanism and When Not To
Krueger flaps compete with slotted slats and droop-nose devices for the leading-edge job. The choice depends on wing thickness at the root, cruise drag targets, and whether the wing is laminar-flow. Here is how the three stack up on the dimensions designers actually argue about.
| Property | Krueger Flap | Slotted Slat | Droop Nose |
|---|---|---|---|
| Typical ΔCL,max | 0.7 to 1.0 with bullnose | 0.9 to 1.2 | 0.3 to 0.5 |
| Cruise upper surface | Completely clean — no joint | Has a spanwise joint and seal | Clean — single piece |
| Mechanism complexity | High — four-bar plus folding bullnose | Medium — track and carriage | Low — single hinge |
| Best wing thickness range | t/c above 12% (inboard widebody) | t/c 8% to 14% (outboard) | t/c above 10% |
| Hydraulic actuation force | High — 8,000 to 15,000 N·m | Medium — track friction dominates | Low |
| Maintenance interval | C-check (~20 months) | C-check with track inspection | Light — fewer wear points |
| Compatibility with HLFC | Excellent — clean upper surface | Poor — joint trips boundary layer | Good |
| Typical aircraft fit | Boeing 747, 777 inboard | Airbus A320, 737 outboard | Concorde, some research wings |
Frequently Asked Questions About Krueger Flap Mechanism
Most simulation lift models apply a single ΔCL,max increment that scales linearly with deployment angle. The real Krueger does not work that way — the bullnose only unfolds in the last 20° of travel, and until it does, the leading-edge radius is wrong and the flow separates over the panel. So you get chord extension benefit early, but the camber and attached-flow benefit only switches on near full deploy.
If you are tuning a sim, model Kb as a step function: 0.6 below 110° deploy and 1.0 above 120°, with a ramp between. That matches Boeing flight-test data far better than a linear blend.
If your inboard wing thickness ratio is above about 12% and you want a clean cruise upper surface for drag, Krueger wins. The slotted slat needs track room inside the leading edge that just is not there on a thick root, and the cruise joint costs you 1 to 2 drag counts even on a conventional wing.
If your inboard t/c is below 12% or you need maximum ΔCL,max at any cost — short-field STOL work, for example — slotted slat wins by 0.2 to 0.3 in lift increment. The 737 and A320 went slat all the way for this reason. The 747 went Krueger inboard and slat outboard because the root is genuinely too thick to package a slat track.
In practice, no — and this trips up a lot of clean-sheet derivatives. The Krueger needs a hinge load path on the front spar lower cap that an existing wing usually was not designed for, plus a hydraulic torque tube running along the leading edge, plus cavity volume in the lower skin for the stowed panel. You are looking at a new front spar lower cap, new ribs at every actuator station, and a new lower leading-edge skin.
The Boeing 747-8 program kept the Krueger architecture from the -400 specifically to avoid this rework — they tweaked the bullnose schedule rather than touching the structure.
On a model, the most common cause is bullnose rigging difference of 1 to 2° between sides. That is invisible to the eye but it changes the leading-edge radius enough to shift the local CL,max by 5 to 10%. Pull the model and check bullnose deployed angle with a digital protractor referenced to the panel datum, not to the wing — referencing to the wing carries any panel-position error into the bullnose measurement.
Second common cause is gap-and-overlap variation at the panel-to-wing joint. A 1 mm gap difference between left and right is enough to produce measurable rolling moment in a half-model test.
Two loads dominate. First, aerodynamic hinge moment at the deployed condition — for a typical 1.5 m chord panel at 250 KCAS deployed, you are looking at 4,000 to 7,000 N·m per metre of span. Second, the inertia and friction torque to drive the panel through its sweep in the required time — usually 10 to 15 seconds full travel.
Size the actuator for 1.5× the maximum aero hinge moment plus a 2,000 N·m margin for ice and seal break-out. On widebody designs that lands you in the 8,000 to 15,000 N·m range at 3,000 psi. Undersizing here is a classic mistake — the panel deploys fine in static testing and then stalls partway through deployment when you hit the airloads at VFE.
Laminar flow on the upper surface needs the surface to be aerodynamically smooth and joint-free for the first 30 to 50% of chord. A slotted slat puts a spanwise joint right in that critical region — the joint trips the boundary layer to turbulent within a few millimetres, killing the laminar drag benefit you spent years engineering for.
A Krueger lives entirely on the lower surface in cruise. The upper surface is one continuous skin from leading edge to mid-chord. That is why every serious HLFC research wing — the Boeing/NASA 757 HLFC test, the European ALTTA program — has used Krueger flaps for the leading-edge high-lift function.
References & Further Reading
- Wikipedia contributors. Krueger flap. Wikipedia
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