Amateur Aerodynamics

If you read anything online about aerodynamics, you will come across something called a drag coefficient . Often, articles on blogs, magazine websites, or Youtube videos will explain this drag coefficient as a measure for comparing the aerodynamic efficiency of two cars or trucks. But what is a “drag coefficient” exactly?   Coefficients in Engineering   To answer that, we need to step back and look at the concept of coefficients more generally.   Coefficients are nondimensional numbers—that is, they have no units and don’t represent a measure of something physical like speed or force. For example, engineers working with compressible gases use several coefficients to determine properties of these gases, including reduced pressure ( p R ), reduced temperature ( T R ), and reduced specific volume ( v’ R ), where Coefficients of performance are used to calculate the efficiency of refrigeration or heat pump cycles, or the thermal efficiency of power cycles (like the home...

Bardeen Quad, UIUC, with the Mechanical Engineering Laboratory and Engineering Hall visible in the distance. When I came back to school and met with an advisor last summer before registering for my fall semester classes, I was gobsmacked to find that the university required me to take RHET 105, a 100-level freshman composition course. A perfect storm of stupidity happened to flail together: the Transfer Credit Office did not accept the freshman writing course I took in my first undergraduate degree program to satisfy this requirement (for whatever reason); SAT and ACT scores could not be submitted after admission to satisfy this requirement (my scores are more than twenty years old, but easily exceed the minimum for composition credit); none of my masters or doctoral coursework apparently satisfied this requirement, nor the fact that I have the master’s degree and have taught at 3 universities including this one . So, here I am—a victim of mindless university bureaucracy, stuck in a co...

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.   If you’ve ever taken a college physics class, you might remember something called the “Bernoulli equation.” This equation describes the relationship between internal energy, potential energy, and kinetic energy in a fluid—not just a fluid in motion, but any (incompressible) fluid (e.g. the “hydrostatic equation” is the Bernoulli equation simplified for a static fluid). By definition, the specific volume, v , of an incompressible fluid must remain constant, allowing us to divide out volume, V , and convert the energy terms into pressures. Bec...

Rear spoilers and wings still work even if a car has front-wheel drive. The claim: Spoilers are only for RWD cars since they make downforce at the rear. This myth has been around for a long time. I couldn't find any  examples of this misconception that didn't pile on additional myths. See if you can spot them all. This one can only be described as "confidently wrong." The reality: This myth is just one representation of a larger misconception. Online commenters seem to be fond of making pronouncements such as this, declaring with absolute certainty that some device or other will or will not work in a certain way. Often, their comments are accompanied by simple explanations. “That rear spoiler isn’t doing anything for your car because it’s FWD.” Or, “The angle of your wing is wrong; you should have a -5 degree angle from horizontal to make the most downforce.” Or, “Your spoiler is too high because it needs to ‘reach up’ only to the Template and it will create a locked ...

This summer road trip season may be my last for a few years, since I will be enrolled in school again full-time starting Monday and plan to take summer classes next year. Knowing this, I decided to go on my longest trip ever: starting from my home in Illinois, I drove to Phoenix and stayed with family, then over to Pasadena, up the Central Valley to San Francisco, through the Redwood Parks to Tacoma (my hometown), then across Washington and Idaho to Bozeman where I have more family, and finally to the Black Hills of South Dakota and then home—more than 6,000 miles in all. Top: Sunset at Colonel Allensworth State Park, California. Above: The Coastal Redwood is absolutely incredible; these trees in northern California, the Giant Sequoia to the south, and the Dawn Redwood in China are the last remnants of the Cretaceous forest that once covered most of the globe, according to the placards at Humboldt Redwoods State Park. Since I’m paying tuition now and need to be frugal, road tripping ef...

Setting out to modify your car’s aerodynamics can be daunting. Where should you start? What should you do first? What changes will give you the most bang for your buck?   Unfortunately, you’ll find a variety of answers to these on the internet—many of which will consider themselves the definitive, correct response, and most of which aren’t based on actual data or first-hand experience. Ignore them.   Identify Goals   The first thing you should do is identify your goals . If you don’t have something in mind you want to accomplish, you can’t plan out a way to get there. And the more specific you can be about where you want to end up, the better ability you will have to make effective modifications that accomplish your goals. Take some time first to identify these. Then, make a testing plan: what changes you want to test, how you will test them, and what you will do if the tests are successful or not. This plan can change as you go along (my plans change every f...