The Shape That Makes Flight Possible
The airfoil — the cross-sectional shape of a wing — is where lift is born. Refining airfoils and assembling them into a three-dimensional wing is one of the central tasks of aeronautical engineering, balancing lift, drag, structural weight, and the speed range over which the aircraft must perform.
Airfoil Geometry and Nomenclature
Every airfoil is described by a standard vocabulary:
- Chord line: the straight line from the leading edge to the trailing edge.
- Chord (c): the length of that line.
- Mean camber line: the line halfway between the upper and lower surfaces.
- Camber: the maximum distance between the camber line and the chord line — the airfoil's curvature.
- Thickness: the maximum distance between the upper and lower surfaces.
- Leading edge radius: the bluntness of the nose, affecting stall behavior.
A symmetric airfoil has no camber (upper and lower surfaces mirror each other) and produces zero lift at zero angle of attack — useful for tail surfaces and aerobatic aircraft. A cambered airfoil produces lift even at zero angle of attack.
The NACA Airfoil Series
The National Advisory Committee for Aeronautics (NACA) created systematic airfoil families still used and referenced today.
| Series | Example | Encoding |
|---|---|---|
| Four-digit | NACA 2412 | 2% camber, at 40% chord, 12% thick |
| Five-digit | NACA 23012 | Design lift & camber position, 12% thick |
| Six-series | NACA 64-212 | Laminar-flow design, low drag bucket |
The four-digit code is the easiest to read: first digit = maximum camber (% chord), second = location of max camber (tenths of chord), last two = thickness (% chord). The six-series airfoils were designed to maintain laminar flow over a larger portion of the chord, reducing drag — important for efficient cruise.
From Airfoil to Wing: Planform Parameters
A wing is more than an extruded airfoil. Its three-dimensional shape — the planform — strongly affects performance.
Aspect Ratio
Aspect ratio (AR) compares span to chord:
AR = b² / S
where b is span and S is area. A high aspect ratio (long, slender wing) produces weaker wingtip vortices and therefore lower induced drag, which is why gliders and surveillance aircraft have very long wings. A low aspect ratio (short, stubby wing) is structurally lighter, rolls faster, and tolerates supersonic flight, favoring fighters.
Taper
Taper ratio is the ratio of tip chord to root chord. Tapering the wing (narrower at the tip) approximates the ideal elliptical lift distribution that minimizes induced drag, while reducing structural weight and root bending moment compared with a rectangular wing.
Sweep
Wing sweep angles the wing rearward (or, rarely, forward). Sweep reduces the airspeed component perpendicular to the leading edge, delaying the formation of shock waves and the associated drag divergence at transonic speeds. This is why nearly all jet airliners and fast military aircraft use swept wings. The penalty is reduced low-speed lift and a tendency toward tip stall, mitigated by twist and high-lift devices.
High-Lift Devices
A wing optimized for efficient high-speed cruise produces too little lift at the low speeds needed for takeoff and landing. High-lift devices temporarily reshape the wing to raise its maximum lift coefficient.
- Trailing-edge flaps (plain, split, slotted, Fowler) increase camber and, for Fowler flaps, area — sharply increasing lift (and drag) for approach and landing.
- Leading-edge slats and slots open a gap that re-energizes the boundary layer, delaying separation to a higher angle of attack and raising maximum lift.
- Krueger flaps extend from the lower leading edge to increase camber on takeoff.
Deployed together, these devices can roughly double the wing's maximum lift coefficient, lowering stall speed and allowing safe slow flight near the ground.
Wingtip Vortices and Winglets
Because the wing carries higher pressure below than above, air leaks around each tip, rolling up into a powerful wingtip vortex. These vortices are the physical origin of induced drag and also create the hazardous wake turbulence that forces separation between landing aircraft. Designers fight them with:
- Winglets — vertical or canted tip extensions that recover some of the spilling energy and effectively increase aspect ratio, cutting fuel burn by a few percent.
- Raked wingtips and tapered, washed-out tips that spread and weaken the vortex.
Designing the Whole Wing
Wing design is a balancing act. The airfoil sets the basic lift and drag character; aspect ratio and taper control induced drag and structural weight; sweep extends the usable speed range; high-lift devices restore low-speed performance; and winglets trim the induced-drag penalty. Every choice trades against the others, and the final wing is a compromise tuned to the aircraft's mission — efficient cruise for an airliner, agility for a fighter, or endurance for a glider.