As a wing moves through the air, the airfoil is inclined to the flight direction at an angle. The angle between the chord line and the flight direction is called the angle of attack. Angle of attack has a large effect on the drag generated by an aircraft.
The magnitude of the drag generated by an object depends on the shape of the object and how it moves through the air. For airfoils, the drag is nearly constant at small angles (+/- 5 degrees). As the angle increases above 5 degrees, the drag quickly rises because of increased frontal area and increased boundary layer thickness. As an object moves through the air, air molecules stick to the surface. This creates a layer of air near the surface (called a boundary layer) which, in effect, changes the shape of the object. The flow turning reacts to the boundary layer just as it would to the physical surface of the object. To make things more confusing, the boundary layer may lift off or "separate" from the body and create an effective shape much different from the physical shape. This occurs at higher angles of attack; determining the drag when the flow is separated is very difficult. The separation of the boundary layer explains why aircraft wings will abruptly lose lift at high inclination to the flow. This condition is called a stall.
On the slide, the flow conditions for two airfoils are shown on the left. The shape of the two foils is the same; the lower foil is inclined at ten degrees to the incoming flow, while the upper foil is inclined at twenty degrees. On the upper foil, the boundary layer has separated and the wing is stalled. Predicting the stall point, the angle at which the wing stalls, is very difficult mathematically. Engineers usually rely on wind tunnel tests to determine the stall point, but this must be done very carefully, matching all the important physical parameters.
The plot at the right of the figure shows how the drag varies with angle of attack for a typical airfoil. At low angles, the drag is nearly constant. Notice on this plot that at zero angle, a small amount of drag is generated because of skin friction and the airfoil shape. At the right of the curve, the drag changes rather abruptly and the curve stops. In reality, you can set the airfoil at any angle you want. However, once the wing stalls, the flow becomes highly unsteady. And the value of the drag can change rapidly with time. Because it is so hard to measure such flow conditions, engineers usually leave the plot blank beyond wing stall.
Since the amount of drag generated at zero angle and the location of the stall point must usually be determined experimentally, aerodynamicists include the effects of inclination in the drag coefficient. As the angle increases, the lift coefficient also increases. This greatly affects the amount of induced drag produced by a wing.
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