What Is a Drag Coefficient?

In the field of fluid and aerodynamics, drag coefficient refers to the numerical figure that represents an object’s resistance — or drag — when it is moving against a fluid medium, which is most usually water or air. It can also factor in the surface area on which an object is standing, such as cement, grass, or water. The term is most often applied when making machines such as cars, airplanes, and ships.

Aerodynamists use the following formula to compute for an object’s drag coefficient: 2Fdd/pv2A. In this formula, “Fd” refers to the object’s drag force, or the energy that moves opposite the object’s direction. The “p” is the mass density of the medium, while “v” refers to the velocity or speed of the object. “A,” on the other hand, pertains to the reference area of the object.

The basic principle behind the drag coefficient’s formula is that the density of the fluid medium is proportional to the force it is giving against the object and to the squared speed of the object in relation to the fluid. This principle can be more obvious when the formula is inverted: Fd = (pv2 cdA/2)A. This also means that the drag coefficient can vary largely on how fast the air of the water passes through the object. The speed, in turn, can change with the shape of the object.

The general rule of thumb is that the wider the area that the fluid medium has to go through, the higher the drag coefficient. With a square and a cone, the wide area of the square allows more air to push against it, as opposed to the cone, wherein the air can rush off more quickly away from its pointed shape. In this way, a square-shaped object experiences more drag and has a tendency to travel slower, compared to a cone-shaped object.

This principle is often used in the designing of automobiles, especially for sports cars that rely heavily on speed. One can observe that racecars are smaller and have a smooth, inclined front. This is to let the air pass easier through the car without any obstructions, thus producing a lower drag coefficient, more speed, and more efficient use of the fuel. Sports cars also have a tendency to sit lower on the ground compared to normal cars, so the air that comes between the tires and the ground is reduced. In this way, the car has a better grip of the ground and can ride faster.