Amateur Aerodynamics

Tuft testing is an experimental methodology most amateur aerodynamicists are familiar with, even if the vast majority of them have never actually tried it. This is too bad, since tuft testing can give us a lot of good information about the quality of the flow over a car.   We all know that this involves taping bits of yarn to your car, like what the actors playing Carroll Shelby and crew do to the Ford GT in a ( comically reductive ) scene in Ford v. Ferrari (2019). The tufts as depicted in that film are pretty much useless for trying to determine what the "engineers" were attempting to ascertain (magnitude and direction of front lift), but in real life they can be tremendously instructive if applied with purpose and interpreted correctly. So, what is the physical explanation behind tuft testing, why does it work, and what can it actually tell us? Stress   To explain tuft testing, we first need to define some terms and concepts, beginning with stress.   Divide force b...

In the US, many students' first introduction to aerodynamic principles happens in basic physics or natural sciences coursework with a lesson on the "Bernoulli effect." Unfortunately, for a lot of people this is also the end of their education in aerodynamics, leading to a lot of incorrect understanding, weird theories, and gross oversimplification—like the incorrect idea that a fully streamlined body with no flow separation will have no pressure drag ( it still does ).   You probably recall the gist of it: as the velocity of a stream of air increases, its static pressure is reduced. This qualitative relationship is always true of low speed flows with no energy addition, since (total) pressure is a measure of the (total) energy in a volume of air and, because energy cannot be created or destroyed spontaneously, when more of that energy is used in the kinetic energy of the flow there must be less internal energy (static pressure) available. This is one explanation of how ...