What is Aerodynamic Drag?
Aerodynamic drag is the force opposing the motion of an object through a fluid (gas or liquid). It arises from the difference in pressure between the front and rear of the object (pressure drag) and from friction between the fluid and the object's surface (skin friction drag). Drag increases with the square of velocity, meaning doubling your speed quadruples the drag force.
The drag equation combines four factors: fluid density, velocity squared, drag coefficient, and reference area. The drag coefficient is a dimensionless number that characterizes the aerodynamic efficiency of a shape, with lower values indicating less drag.
Drag Equation
Drag Coefficients
| Shape / Object | C₅ | Notes |
|---|---|---|
| Sphere | 0.47 | Smooth surface, moderate Re |
| Flat Plate (normal) | 1.28 | Perpendicular to flow |
| Streamlined body | 0.04 | Teardrop / airfoil shape |
| Modern sedan | 0.25 - 0.35 | Typical production cars |
| SUV / Pickup | 0.35 - 0.45 | Less aerodynamic shape |
| Cyclist (upright) | 0.9 - 1.1 | Frontal area ~0.5 m² |
| Skydiver | 1.0 - 1.4 | Position dependent |
Frequently Asked Questions
Why does drag increase with the square of velocity?
Doubling velocity increases both the mass of air encountered per second (proportional to v) and the kinetic energy of that air (proportional to v). The combined effect gives v^2. This is why aerodynamic drag becomes the dominant resistance force at highway speeds, far exceeding rolling resistance.
What is the power needed to overcome drag?
Power equals drag force times velocity (P = Fd * v). Since drag is proportional to v^2, power to overcome drag is proportional to v^3. Doubling speed requires 8 times the power to overcome aerodynamic drag, which is why fuel economy drops dramatically at high speeds.
How does altitude affect drag?
At higher altitudes, air density decreases (approximately 1.225 kg/m^3 at sea level vs 1.007 kg/m^3 at 2000m). Lower density means less drag, which is why aircraft cruise at high altitudes for fuel efficiency and why cars set speed records at places like Bonneville Salt Flats (1300m elevation).