Or, How Wings Work
An airfoil is a fairly simple device that revolves around some not-so-simple math. This tutorial will try to avoid most of that math and explain the basics of how an airfoil creates lift.
Airfoils, more commonly known as "wings", are devices that create differences in pressure. These pressure differences are applied over areas, namely the upper and lower sides of the wing, to create forces. The forces on each side of the wing are different, and this difference is lift.
Before diving into the fluid behavior, we'll look at the anatomy of an airfoil.
The first edge of the airfoil to meet the oncoming air is called the leading edge and the last edge that the air touches at it moves past the airfoil is called the trailing edge. The thickness of the airfoil is pretty self-explanatory. The chord line is an imaginary line that connects the leading and trailing edges. The camber of a wing is basically an approximation of its curvature. The camber line is an imaginary line that passes through the heighth-wise midpoints of the airfoil. The maximum camber is the largest distance between the camber and chord lines. The face of the airfoil with the highest pressure is called the pressure side while the face with the lowest pressure is called the suction side. The span of the wing, or wing span, is the length of the airfoil measured perpendicular to the chord. The aspect ratio is equal to the span divided by the chord length.
The camber of the airfoil is what allows it to generate lift. Because the surfaces are curved, and not parallel, the length of the upper and lower sides of the airfoil are different. As air flows past the airfoil (or more correctly, as the airfoil pushes through the air) the air must flow faster over the longer side than the shorter side. The faster moving air has a lower pressure than the slower moving air (remember Bernoulli's Equation from the Fluids Tutorial?). This pressure pressure difference creates a force in the direction of the suction side of the airfoil. This force is lift. As the air moving past the wing increases in speed, the pressure differential increases and the wing produces more lift.
Angle of attack is another way for an airfoil to produce lift. As angle of attack rises, so does lift. However, there is a penalty, the drag of the airfoil is also increased with higher angles of attack. Further, as angle of attack rises, it becomes harder for the airflow over the wing to stay attached. At a certain point, the skin friction and airflow momentum are no longer high enough to keep the flow attached and the velocity of air past the pressure side falls rapidly. This decreases the pressure differential and, therefore, lift. The airfoil is said to have stalled.
There are ways to allow the wing to function at higher angles of attack without stalling. One way is to install vortex generators on the suction side. These devices jut out into the airflow and impart extra energy and momentum into it. This added momentum helps keep the airflow attached over the wing.
Another way to allow high angles of attack, and one that is commonly used in motorsports, is the use of a Gurney Flap. A Gurney Flap is a simple device that is fitted to the trailing edge of an airfoil. It sticks up at 90° to the airfoil and serves two purposes. The first being that it collects air, similar to a spoiler (see the Aerodynamics Tutorial), producing an area of high pressure. This increases the pressure differential to develop more downforce. The second is that it generates a pair of vortices, spinning in opposite rotations, that create an area of low pressure behind the flap. This low pressure area literally pulls air around the suction side of the airfoil and helps prevent flow separation.
Aspect ratio plays an important role in the functionality of a wing. Wings with high aspect ratios (long spans and short chords) like those found on planes, generally exhibit higher efficiencies than those with low aspect ratios, like those found on race cars. Higher aspect ratios generally produce higher lift and less drag at lower angles of attack. However, a low aspect ratio wing is usually less sensitive to stall.
Many airfoils on racecars have plates on the end of their airfoils. Appropriately named end plates, the devices prevent wing-tip vortices from forming. A wing-tip vortex arises out of the pressure differential above and below the wing. Air flows around the edges of the airfoil to balance the pressure on either side of it. This decreases the pressure differential across the wing drastically reducing lift. The effect is especially critical on airfoils with a low aspect ratio, because the vortices infiltrate a larger percentage of the chord, producing a larger percentage decrease in lift. Placing end plates on the airfoil prevents these vortices from forming and has the added benefit of making it harder for competitors to copy your wing profile.
Most wings on racecars are actually multiple airfoils placed closely together. These wings are called multi-element wings. While more airfoils will produce more drag, the added amount of downforce can be worth the penalty. Flaps and slats placed at the leading and trailing edges of the main airfoil simulate a wing with more camber and a higher angle of attack, producing more lift. Stacking airfoils, similar to a bi-plane, also allows more downforce to be produced.
Front of Indy Car showing multi-element wing with end plates.
Rear Indy Car airfoil showing multiple elements, end plates, and Gurney flap.
References and Further Reading
Competition Car Downforce by Simon McBeath. Haynes Publishing, 1998.
Theory of Wing Sections. Abbott and Von Doenhoff. Dover Publishing, 1959.
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