Common Misconceptions in Aerodynamics: Part 9
Aerodynamics Matter Regardless How Fast a Car Goes
The claim: Drag only matters if you go above a certain speed. 45 mph/50 mph/55 mph/60 mph—take your pick, as the critical speed depends entirely on the person reporting this myth.
The reality: People who repeat this claim are telling only part of the story—as is true of many, if not all, of these misconceptions. To see why, we need to go back (again) to some fundamentals of fluids in motion.
Assuming no energy is added or taken away from the system (and assuming an incompressible fluid, etc.), this relationship should hold true.
One-half of the product of density and the square of velocity is something we call “dynamic pressure,” and it is, as its form suggests, simply the kinetic energy of an incompressible flow with volume divided out (recall that the kinetic energy of a solid is equal to one half of the product of its mass and the square of its velocity. Dividing mass m by volume V gives density ρ; the inverse of this, V/m, is specific volume, v). In ground vehicle aerodynamics, gravity can (usually) be ignored since its effect on airflows is negligible when considering the flow around a car. This leaves a modified form of Bernoulli’s equation,
As you might guess, this is where the negative relationship between pressure and velocity is described: as one goes up, the other must go down to satisfy the equation. Rewriting the equation makes this clearer:
(Fun fact: you can also derive this relationship between velocity and pressure from an energy model of an incompressible fluid moving through a converging duct [nozzle] or diverging duct [diffuser], as well as a more complicated method involving vector math and linear algebra).
|Drag doesn’t suddenly matter at 60 kph—it isn’t an on/off switch. (Also, this should read "its relationship to speed is not directly proportional"—since it is proportional to speed squared!).|
...and bicycles. The effects of aerodynamic drag--especially how it changes with body position and headwinds--can be easily felt on a bicycle, and measured. For example, my best 40 km time trial on this bike:
...was 62 minutes. However, on this bike:
...that improved to 56 minutes, more than 4 kph faster).
But there is no discrete point at which aero drag “matters” and below which it doesn’t, only a speed above which aero drag predominates and below which it doesn’t but may still be substantial. And keep in mind, that critical speed is different (and usually unknown without sophisticated measuring equipment) for every car model.