How Does An Aircraft Wing Work?

A wing generates lift by changing the direction and momentum of air that flows over the wing. Air that moves over the wing is displaced downward after the trailing edge. The downward displacement of airflow results in an opposite upward reaction force (i.e., lift). The lift is proportional to the direction and volume of the displaced air, and the rate at which the air is displaced. Lift is also a function of wing efficiency, which is derived from wing design.

Airflow Over a Wing

Direction of Displacement

The downward direction of airflow displacement is a result of the wing cross-sectional shape or airfoil (Fig. 1).

Fig. 1: Airflow over a wing cross-section (airfoil) in level flight.

Airflow initially moves upward over the leading edge of the wing, which results in negative lift. The airflow turns to follow the downward contour of the wing surface, and continues to move downward after the trailing edge of the wing. The negative lift is overcome by the positive lift that is generated from the downward displacement. The lower surface of the wing also contributes some lift by deflecting airflow downward. Lift is significantly improved at low altitudes (such as during takeoff and landing) when the displaced air mass strikes the ground.

Volume of Displacement

The upper surface area of a wing determines the volume of air that is displaced at a given forward speed and is therefore the primary factor for lift. For example, the wing area of a cargo aircraft is large for the heavy load that the aircraft must carry. Some aircraft have two wings to increase lift (Fig. 2).

Fig. 2: Antonov An-2 cargo aircraft (photo by Kevin Heap).

Rate of Displacement

Lift is also determined by the rate of displacement, which is a product of the curvature on the upper surface of the wing and the upward tilt or angle of attack (AOA) of the wing.

For a given forward speed at zero degrees AOA (or level flight), the rate of displacement is a function of the curvature on the upper surface of a wing. Air flows over a wing in layers, and the velocity of each layer is lower with the proximity to the wing surface (the layer that is closest to the wing surface has almost zero velocity). The differences in velocities between layers create pressure differences that cause the airflow to bend and follow the contour of the wing surface.

Airflow velocity increases with the amount that the flow must bend to follow a curved surface. At higher AOA, the airflow over the wing bends at a greater degree to follow the wing contour than at zero degrees AOA. The increase in airflow velocity over the wing means that airflow is displaced downward by the wing at a higher rate, with a corresponding increase in lift.

To maintain lift at low speeds (such as during aircraft landing), the AOA must be high to compensate for the reduced volume of airflow over the wing. However, at excessive AOA (more than 15 degrees), the airflow can no longer follow the contour of the upper surface. The airflow detaches from the wing in an uncontrolled manner and the wing stalls with a sudden loss of lift (Fig. 3). At low speeds, devices such as leading edge slats and trailing edge flaps are deployed during takeoff and landing (Fig. 4). The slats and flaps are extended beyond the edges of the wing to effectively add curvature and generate additional lift with a low AOA.

Fig. 3: Airflow over a wing cross-section (airfoil) in stall condition.

Fig. 4: Fieseler Storch aircraft. Leading edge slats and trailing edge flaps add
lift to reduce landing speed and takeoff distance (photo by Glenn Denny).

Wing Efficiency

Lift and Drag

The measure of wing efficiency is the proportion of lift to drag that is generated by the wing within its operational speed. Drag is the normal byproduct of lift, and is minimized by matching wing design with the operational requirements of the aircraft. For example, the airfoil shapes in Fig. 1 and Fig. 4 have a concave lower surface that improves lift at the expense of higher drag and lower top speed, while the airfoil shape in Fig. 5 achieves the opposite result.

Fig. 5: High speed airfoil.

Drag is also caused by the resistance of airflow to the outer surfaces of a moving aircraft. Rough or uneven surfaces, and projections such as struts, rivets, wire bracing and non-retractable landing gear (Fig. 4) are sources of drag, and are therefore absent in high speed aircraft such as fighter planes (Fig.6).

Fig. 6: F-35 fighter aircraft .

Aspect Ratio

Wing efficiency is significantly affected by the aspect ratio of the span (wing length) to the chord (the distance between the leading and trailing edges). A wide span and a narrow chord improves the distribution of air pressure across the wing surface, and minimizes vortices that normally form at the wingtip (Fig. 7) which create drag. Vortices occur when the pressure difference between the upper and lower surfaces of a wing is short-circuited at the end of a wing, and air moves across the wing, rather than from leading to trailing edge. The problem is less important at aspect ratios of 7:1 or greater, where the tip area is small as a percentage of the total wing area.

Fig. 7: NASA photo of wingtip vortex as revealed by colored smoke.

The aspect ratio of a wing is limited by the weight of the additional supporting structure and the reduction in aircraft maneuverability. Sailplane wings (Fig. 8) have aspect ratios of 10:1 or greater to develop maximum lift at low speeds. In contrast, the maneuverability, speed and strength requirements of a fighter aircraft (Fig. 6), result in wings with low aspect ratio.

Fig. 8: High aspect ratio wings enable a sailplane to glide over long distances (photo by Christian Boysen).

Angled winglets at the wingtips (Fig. 9) effectively increase wingspan and aspect ratio, and also block the short-circuiting effect. Winglets are common in large passenger aircraft as a means to improve fuel efficiency.

Fig. 9: Winglet.

Wing Taper

Wing efficiency can be improved by tapering both chord and thickness from the wing root (where the wing joins the fuselage of an aircraft) to the wingtip (Fig. 10). Efficient wings usually combine high aspect ratio with tapering thickness and chord.

Fig. 10: U2 reconnaissance aircraft. The wings provide lift at extreme altitudes.

Stall Characteristics

A wing will stall for a given load below a certain speed or at an excessive AOA. The stall characteristics of a wing are controlled by the thickness and shape of the airfoil. Thick wings produce a less abrupt loss of lift during a stall condition than thin wings. Subsonic-speed wings have a typical thickness of approximately 13% of the chord (Fig. 5) and very thick wings may have a ratio of 16%. Stall characteristics are also improved by a surface curvature that gradually changes airflow velocity over a large area of the leading edge (Fig. 5).