Krueger Flap: The Leading-Edge Panel That Unfolds From Under the Wing

What a Krueger Flap Is and Where You Find It

A Krueger flap is a leading-edge high-lift device that increases a wing's lift at low speed by hinging a panel out of the lower wing surface and forward, ahead of the fixed leading edge. When retracted it sits flush in the underside of the wing; when deployed it adds both chord (area) and camber to the leading-edge region, raising the maximum lift coefficient so the aircraft can take off and land at lower speeds.

Its defining trait is that it comes from below. A slat pulls the actual leading edge forward on tracks; a Krueger flap leaves the upper surface and the true leading edge alone. That distinction drives where it is used:

• Boeing 707 and 747 — early and large-scale production users; the 747-400 and 747-8 use variable-camber Krueger panels across the entire inboard leading edge, three panels per side.
• Boeing 727 and 777 — inboard Krueger flaps paired with outboard slats; on the 777 the inboard Krueger also seals the gap between the engine strut and the inboard slat.
• Lockheed C-5, Boeing YC-14, business jets — inboard Krueger systems where a clean, laminar-compatible upper surface matters.

How It Works: The Four-Bar Linkage and Folding Bullnose

Deployment is a compound rotation, not a simple translation. A four-bar linkage, driven by a hydraulic rotary actuator, swings the panel forward and downward through roughly 130° (practical range about 120°–135°). The surface that was the wing's lower skin literally flips over to become the new aerodynamic upper surface at the leading edge, producing a much more pronounced, blunt nose radius that keeps the flow attached at high angle of attack.

A flat panel alone would present a sharp, flow-separating edge, so a folding bullnose — a smaller secondary segment on its own linkage — unfolds during the final ~20° of travel. It rotates tangent to the main panel to restore the correct leading-edge radius for attached flow.

• The bullnose raises the leading-edge effectiveness factor (K_b) from roughly 0.6–0.7 to 1.0.
• Variable-camber Krueger (VCK) flaps go further: a flexible fiberglass panel is bent to an optimum low-speed shape, with an aluminum folding nose set tangent to it — developed at Boeing by James Cole and Richard Weiland in the mid-1960s.

Key Quantities and a Worked Example

The aerodynamic payoff scales with two things the Krueger flap adds: extra chord (area) ahead of the wing, and extra camber. The lift equation is L = ½·ρ·V²·S·CL, where ρ is air density (~1.225 kg/m³ at sea level), V is true airspeed, S is wing reference area, and CL the lift coefficient. Because stall happens at CLmax, the useful figure of merit is how much the device raises CLmax.

• Clean wing CLmax: ~1.4 (747-class airfoil).
• With inboard Krueger deployed (plus trailing-edge flaps): over 2.4.
• The nominal leading-edge increment alone works out to ΔCLmax ≈ 0.73.

Worked example. Stall speed goes as V_stall ∝ √(1/CLmax). Going from CLmax = 1.4 to 2.4 gives a speed ratio √(1.4/2.4) = √0.583 = 0.764 — a ~24% cut in stall speed. For a landing approach near 155 kt clean, that is roughly a 30-knot reduction, matching flight data. The rotary actuator holds this against airloads up to ~250 kt, delivering on the order of 8,000–15,000 N·m at 3,000 psi with an internal hydraulic lock.

Design, Rigging, and Operation in Practice

Krueger flaps are almost always on/off (two-position) devices tied to the flap lever, extending automatically as trailing-edge flaps are selected for takeoff or landing and retracting for cruise. They are structurally simple relative to slats — no long external tracks protruding into the airflow — which is part of their appeal on the inboard wing where the engine strut interrupts a continuous slat run.

Getting the geometry right is unforgiving. The folding bullnose typically needs ±0.5° rigging tolerance; deviations beyond about 2° can cost 10–15% of the device's lift because the leading-edge radius no longer matches the local flow. Practical design and operation rules:

• Symmetric deployment is critical — an asymmetric leading-edge condition produces a strong rolling moment, so systems interlock left/right panels.
• Nose-up pitching moment: deploying the Krueger shifts the aerodynamic center and adds a nose-up moment that the trim system and tail must absorb.
• Fits a small cruise leading-edge radius: because the device is stowed below, the cruise airfoil can keep a thin, low-drag leading edge optimized for high-speed cruise.

Krueger Flap vs. Slats and Droop Noses

The closest cousin is the leading-edge slat. A slat translates forward on curved tracks and opens a slot that ducts high-energy air over the wing's upper surface, delaying separation; it typically yields a slightly larger ΔCLmax (~0.9–1.2 slotted) than a plain Krueger. But the slat becomes the leading edge, and its tracks and slot create surface discontinuities.

• Krueger advantage — laminar flow. Because the Krueger stows in the lower skin and never disturbs the upper surface or true leading edge, it avoids the steps and gaps that trip a laminar boundary layer into early turbulent transition. This is why NASA/Boeing's 2015 ecoDemonstrator used a 6.7 m variable-camber Krueger to shield a natural-laminar-flow wing from insect contamination — with a projected fuel-burn benefit near 15%.
• Droop (hinged) leading edge rotates the nose down about a hinge with no slot; it is mechanically simplest but gives the smallest lift gain (~0.3–0.5) and is common on fighters and gliders.
• Fixed slot is a permanent duct — great for STOL bushplanes but a constant cruise-drag penalty.

Failure Modes, Trade-offs, and Significance

The Krueger's blunt deployed nose is aerodynamically effective but drag-heavy; it is strictly a low-speed device and must be stowed for cruise. Its trade-offs and failure modes are mostly mechanical:

• Asymmetric deployment (a jammed panel or failed actuator on one side) is the headline hazard, producing a roll and stall-margin split; certification and interlock logic exist specifically to guard against it.
• Bullnose mis-rig or fatigue in the many-bar linkage degrades the leading-edge radius and lift; the tight ±0.5° tolerance makes it maintenance-sensitive.
• Airload and thermal cycling on the thin fiberglass VCK panels can cause delamination or cracking over service life — an inspection item.
• Bird strike / debris: the exposed deployed panel and its linkage are vulnerable during the takeoff and landing phases when they are extended.

Its significance is outsized: by delivering large low-speed lift without compromising the cruise leading edge, the Krueger flap let Boeing build thin, efficient high-speed wings on the 707 through 777, and it is now central to making practical, contamination-tolerant laminar-flow wings — a leading lever for future fuel savings.

Leading-edge high-lift devices compared

Device	How it moves	Upper surface at cruise	Typical ΔCLmax	Where used
Krueger flap	Panel hinges forward/down from lower surface, ~130°	Untouched (smooth, laminar-friendly)	~0.7–0.9	747/767/777 inboard, 707, 727 inboard
Slat	Segment translates forward on tracks, opens a slot	Forms part of the moving leading edge	~0.9–1.2 (slotted)	A320, 737 outboard, 777 outboard
Droop/hinged leading edge	Nose rotates down about a hinge, no slot	Rotates down, no slot energizing	~0.3–0.5	Fighters, some sailplanes
Variable-camber Krueger (VCK)	Flexible panel bends to shape + folding bullnose	Untouched; bullnose sets correct radius	~0.8–1.0	747-400/-8, 777 inboard, laminar-wing research
Fixed slot	No motion; permanent duct	Permanently ducted	~0.2 (drag penalty)	STOL bushplanes (e.g. Fieseler Storch)