What Is a Multi-Element Airfoil?
How they work for aircraft takeoff/landing, motorsports downforce,
and how they are designed by engineers.
Quick definition
A multi-element airfoil is a lifting surface made from two or more cooperating profiles—typically a main element plus leading-edge slats and/or trailing-edge flaps. By carefully positioning the elements (gaps, overlaps, and deflections), designers dramatically increase lift (for aircraft) or downforce (for cars) at low to moderate speeds without making the wing excessively large.
Why multi-element airfoils work
- Circulation & camber boost: Slats and flaps increase effective camber, strengthening circulation and lift.
- Slot effect (boundary-layer control): The gap between elements jets high-energy air over the next element, delaying separation and letting the system reach higher lift coefficients before stalling.
- Fowler motion: Many flaps translate rearward and rotate, increasing wing area and camber simultaneously.
- Load sharing: Each element carries part of the pressure jump, reducing peak adverse gradients on any single surface.
In practice, a single-element airfoil might achieve a CL,max around ~1.4 (order of magnitude), while a well-designed multi-element system can exceed ~2.5–3.0+ depending on geometry, Reynolds number, and deflection schedule. (For cars, think of “negative lift” or downforce rather than positive lift.)
Where you see them in the real world
Jet airliners (takeoff & landing)
Airliners need huge lift at low speeds to operate from practical runways. On approach and takeoff, they deploy leading-edge slats and multi-segment trailing-edge flaps to raise CL,max, allowing lower approach speeds, shorter distances, and improved safety margins. In cruise, devices retract to reduce drag.
Business jets, turboprops, and STOL aircraft
Many business jets and turboprops use slats and flaps for field performance. Short-takeoff-and-landing (STOL) aircraft do the same, sometimes adding devices like fences, cuffs, Krueger flaps, or blown flaps to energize flow and improve controllability near stall.
Uncrewed aircraft & model aviation
UAVs benefit from high-lift systems for heavier payloads or shorter fields. Multi-element tails or deployable flaps are common on fixed-wing drones that must launch and recover in tight spaces.
Motorsports & performance cars
Racing wings often use two or more elements (plus Gurney flaps) to produce large downforce at modest speeds, improving grip in braking and cornering. Rules usually cap element count and geometry, so careful design of slot gap, overlap, and flap angle is crucial to hit the aero targets without stalling the wing.
Key design choices
- Architecture: How many elements? Slat + single flap, double-slotted flap, or more?
- Gap & overlap: Tiny changes (millimeters) in the slot can make or break high-lift performance.
- Deflection schedule: Angle and translation vs. speed/phase (takeoff vs. landing) or, for cars, vs. ride height/attitude.
- Reynolds/Mach effects: Section choice and flap geometry depend on size and speed regime.
- 3D integration: Wing twist, endplates/fences, tip effects, and flap track fairings all matter.
- Structures & mechanisms: Added complexity, weight, and maintenance vs. performance gains.
- Noise & certification: For aircraft, aero-acoustic considerations can drive geometry and schedules.
A practical workflow for designing multi-element airfoils
- Define the mission: Field length, stall margins, approach/takeoff speeds (aircraft), or target downforce/drag window (cars).
- Choose a baseline section: Start with a main element suited to the Reynolds number and thickness needs.
- Select devices: Slat type and size; flap type (plain, split, single-slotted, double-slotted, or Fowler); Gurney height.
- Set initial geometry: Gap/overlap and hinge lines; add mechanical constraints for real deployable hardware.
- Analyze 2D performance: Sweep angles of attack and device deflections to map CL, CD, Cm, and stall behavior.
- Scale to 3D wing/car installation: Include spanwise effects, endplates/fences, and local ground effect (cars).
- Optimize the schedule: Create “takeoff” and “landing” (or “low-speed” and “high-speed”) settings; validate against constraints.
- Iterate with CFD and tests: Refine details such as slot curvature, fairings, and sealing strategies.
Design faster with Hanley Innovations software
Hanley Innovations provides tools that streamline multi-element airfoil and wing design—from early concepts to practical, test-ready geometries:
- MultiElement Airfoils – Rapidly configure slats, flaps, gaps, and overlaps; evaluate high-lift performance across deflection schedules. Ideal for airliner high-lift studies, STOL concepts, UAVs, and motorsports wings.
- 3DFoil – Analyze full wings and tail combinations quickly, explore stability derivatives, and build trim maps that incorporate your high-lift settings.
- Stallion 3D – Move to full-3D CFD when you need richer flowfield details (pressures, forces/moments, and flow features) on real geometries, including multi-element systems and car wings.
Ready to accelerate your high-lift or downforce project?
Visit Hanley Innovations to explore MultiElement Airfoils, 3DFoil, and Stallion 3D.
FAQ
How many elements are “too many”?
Diminishing returns set in as mechanical complexity, drag, and sensitivity increase. Most practical systems use one slat and
one or two flap elements; motorsports rules often limit element count explicitly.
Do Gurney flaps count as an element?
They’re typically treated as a device on an element rather than a full element, but they can significantly boost
lift/downforce at the right Reynolds numbers.
What’s the most sensitive parameter?
The slot (gap and overlap) and the deflection schedule. Small tweaks here can change peak performance and stall character.
