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Source: NASA

The **drag coefficient** is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag. This equation is simply a rearrangement of the drag equation where we solve for the drag coefficient in terms of the other variables. The drag coefficient **Cd** is equal to the drag **D** divided by the quantity: density **r** times half the velocity **V** squared times the reference area **A**.

Cd = D / (A * .5 * r * V^2)

The quantity one half the density times the velocity squared is called the dynamic pressure **q**. So

Cd = D / (q * A)

The drag coefficient then expresses the ratio of the drag force to the force produced by the dynamic pressure times the area.

This equation gives us a way to determine a value for the drag coefficient. In a controlled environment (wind tunnel) we can set the velocity, density, and area and measure the drag produced. Through division we arrive at a value for the drag coefficient. As pointed out on the drag equation slide, the choice of reference area (wing area, frontal area, surface area, …) will affect the actual numerical value of the drag coefficient that is calculated. When reporting drag coefficient values, it is important to specify the reference area that is used to determine the coefficient. We can predict the drag that will be produced under a different set of velocity, density (altitude), and area conditions using the drag equation.

In this tutorial, you will learn how to use the Reference Values window for a 2D simulation- With Reference Values (Lenght, Depth, Area, etc.), you can find the Drag Coefficient. In this case, the experimental Drag coefficient (Cd) is 1.55.

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