Our mechanical engineer talks about automotive aerodynamics in the first of a two-part article.
BY the time a spacecraft leaves the last layer of Earth’s atmosphere, it no longer needs to be streamlined. That is because, out in space, there is very little air to cause any resistance. In fact, there are so few air molecules that even the sun’s rays have nothing to reflect off . Which is why outer space appears to be perpetual night.
Down on Earth, however, air (which consists mainly of nitrogen and oxygen) has an average density of 1.2kg per cubic metre. This is enough to reflect light and, yes, enough to create buoyance for aeroplanes to fly. There’s impedance to motion, too – an effect we call aerodynamic resistance, or drag.
Air is technically classified as a fluid, because it can flow over, below or around a solid object. In extreme conditions, fast-moving air (or simply wind) can exert a tremendous force. That it can propel a sailboat or blow down trees and buildings alike is sufficient to illustrate the power of air/wind.
Wind resistance and the behaviour of air around a moving automobile are critical aspects of automotive design, because they affect the car’s fuel economy, stability and noise. Motor vehicles, after all, travel on Earth, so they have to move through air.
Any object moving through air is opposing forces exerted by air, with said forces getting stronger as the speed of motion goes up. In fact, the laws of physics have defined the drag (or resistance due to air) to be proportional to the square of the speed.
So, if the speed is doubled (x 2), drag increases by four times (2 x 2), and if speed is tripled (x 3), drag increases by nine times (3 x 3), and so on. The takeaway is, drag consumes energy by being a resistance to motion.
We’ll delve deeper into automotive aerodynamics in our next issue.
