Also found on top of Nascar stock cars and other race cars. You might have seen them pop-up when there's some car accident on the track. The purpose of these flaps is to keep the car from flying. While flying is at times something fun to do, it's not exactly what you want to do when inside a car. Flaps are considered to be the best safety device since the roll cage. A short explaination of how they work:

Race cars are very aerodynamic. They are made to cut through the air very well, to lower air resistance and increase speed. However, air that goes over the car must travel farther than air under the car and moves faster than the air under the car. This creates lower-pressure on top of the car than under it that creates lift, much like an airfoil on an airplane. To keep the car on the ground, race cars normally have "upside-down" wings that push the car into the ground. However, when the car is spinning, sideways, going backwards, or whatever else, these upside-down wings don't work so well. Thus the car flies up into the air.

What the flaps do is counteract the flow of air above the car. When the car starts going sideways, the flaps come up, disrupt the smooth flow of air over the car thus ruining the low pressure that can be found up there and ruining the resulting lift.

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