Amateur Aerodynamics

One of the highlights of my last semester as an aerospace engineering student was a new class, Sustainable Aviation. Previously, this course was offered as a Special Topics class outside of the regular curriculum but it’s now part of the course catalog of electives in AE—and I think that’s great. Despite the current political climate in the US, sustainability is as much of a problem as ever and will continue to be, as humanity will have to grapple with issues of climate-changing pollution and resource usage into the indefinite future. And the longer we wait to address it, the more of a problem it will become; to put things in perspective, global mean surface temperature (GMST) averaged over the last three years has already exceeded the 1.5°C limit agreed to in the Paris accords.   Even though the semester is over and I’m no longer a student, sustainability is still on my mind. And it turns out, I’ve found the perfect example of one of the major challenges of sustainability right ...

  We’ve all read or heard about the fact that colder air increases aerodynamic drag due to its increased density, reducing your BEV range or ICEV fuel economy in winter. What most people don’t know is   how much   aerodynamic drag increases with dropping temperature, or how much benefit there is on a hot day. Is it significant? Let’s figure it out. Yes, we have a reindeer farm in central Illinois.  Merry freakin' Christmas. Standard Atmosphere Model   To do that, we first need an appropriate model of atmospheric parameters such as temperature, pressure, and density, and how they change with altitude.   In the Aerospace Engineering program I'm about to complete, we use the 1962 International Standard Atmosphere (ISA). This model was derived from decades of atmospheric measurements, from which a set of equations were developed that adequately reproduce variations in temperature, pressure, and density as altitude changes. Up to 36,089 ft, a  thermocline ...

Most people are idiots when it comes to aerodynamics.   I know; I'm an idiot too. I used to think I "knew" about aerodynamics because I had read a lot of things by posters on the Ecomodder forum, looked at a few cars at auto shows, and used terms like "Kammback" and "pressure recovery"—but without actually knowing the first thing about the science of aerodynamics. Did any of these modifications reduce drag? Probably, based on the car's long-term fuel economy. How much? I have no way of knowing since it was all done by guess. In the last couple of years, I've gone back to school to get a degree in aerospace engineering. I've read fluid mechanics and aerodynamics textbooks and journal articles. I've taken coursework in compressible and incompressible flows, flight mechanics, CFD, and applied aerodynamics. And most importantly, I've spent the last several years figuring out how to test things myself on my own cars, building interesting...

The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...