Amateur Aerodynamics

Perhaps the oldest work in aerodynamics—the study of moving air—can be found in an unexpected place. Long before wind tunnels and airfoils, before the Wright brothers and powered flight, airships and streamlined automobiles, pipe organ voicers figured out how to manipulate airflow to create a wide variety of sounds. Detail of the Zliten mosaic (2nd century CE), depicting the playing of several instruments including a hydraulis, the predecessor of the modern pipe organ. (image credit: Wikimedia Commons) A Brief History   The roots of modern organs trace back to ancient Greece, where an engineer named Ctesibius invented a water-powered organ around 246 BCE called the hydraulis . This instrument disappeared from western Europe after the fall of Rome but evolved into the pipe organ in the Eastern Empire and was reintroduced around 800 CE. Since then, organs have underpinned Western music; the instrument shares its name, organum  ("instrument" or "tool"), with the first ...

In the first installment of this series, I wrote about the fallacy of using a “template” to guide aerodynamic modifications . This second part brings us to a related topic: testing. Testing is an Absolute Necessity, Not an Unnecessary Frivolity   The claim: Since we know what the “ideal” shape looks like and can predict airflow over a car, all we have to do is build something to that shape and it will be guaranteed to work. There’s no need to test anything.   Eliminating guesswork by...more guessing. Sounds legit. The reality: Because of the complexity of the flow around a car, “automobile aerodynamics is still dominated by empiricism,” as Hucho wrote in Aerodynamics of Road Vehicles . Simulations have replaced some of the development previously done in wind tunnels as computers have become more powerful and faster, but there is still no way (and likely never will be) to design a low-drag car by following simple principles of shaping such as the “template” discussed previou...

To test for changes in drag , you will most likely need some sort of throttle position sensor (TPS) display. This helps to ensure that your throttle is open at the same angle during constant-throttle testing or to note the change in throttle angle during constant-speed testing. Here’s how I wired a display in my 1991 Toyota pickup.   To Wire or Not to Wire   If you have an OBD2 car—most cars built in or after 1995 or 1996—you can read throttle position from a computer (or even an app on your phone) plugged into the OBD port, usually located under the steering column. Here's the OBD port on my 2013 Prius. For those of us with older cars, it isn’t as simple. On the truck, I’ll have to find the wire that carries the TPS signal to the engine control unit (ECU), tap into it, and send that voltage to a separate display that I’ll mount on the dash.   Reading Wiring Diagrams   Before doing anything else, I found and purchased a factory wiring manual for my truck. This one ca...

A few weeks ago, I covered the benefits of tuft testing , how to do it and what it can show you about the airflow over your car. You may recall that tufts align themselves to the velocity vector wherever you attach them to a surface. Now we’ll look at pressure testing , which is the other side of that coin since the behaviors of pressure and velocity in a flow are related.   Background and Theory   To understand how aerodynamic pressures change, we first need to understand what a frame of reference is. As you push a car through air, the car moves relative to the air—moving car, stationary air. In a wind tunnel, rather than pushing the car through the air, the air is pushed past the car—moving air, stationary car. However, both of these describe the same situation: relative motion between car and air. Which one you imagine “moving” depends on your frame of reference.   It's easiest for most people to think of air moving past a car rather than the other way around. Usin...

An important part of any curriculum in the sciences, mathematics, or engineering is the concept of significant figures. Enroll in a degree program in any of these areas and the first thing you will learn in your first semester—I’m not exaggerating here, it’s literally the first topic covered in multiple introductory classes—is how to use significant figures. What are they? And why are they important? Let’s find out.   Estimation and Uncertainty   When we measure things, our results are subject to uncertainty. The degree of uncertainty is determined by the method and accuracy of the measurement, but there is always uncertainty . If I use a ruler marked off in millimeters (like the tape measure above) to measure something and it looks like it is 25 mm long, it might actually be 25.0001 mm or 24.999999 mm—but my ruler, which doesn’t have that degree of accuracy, won’t show that. So how do we arrive at a length for this object?   Well, we estimate the last number beyond the...

For a lot of people interested in modifying their car’s aerodynamics, drag reduction is the primary goal (perhaps more so now that gas prices have shot up). In a bit of good news, my license plate is relevant again. Drag is any force that resists the car’s motion along its longitudinal axis i.e. an imaginary line that runs from back to front along the centerline. Mechanical drag is, as its name suggests, produced by the inertia of and friction between the various mechanical components of the car and/or the road. Aerodynamic drag is the resistance generated by air as it flows over, under, and around your car. Because of this focus on aerodynamic drag reduction, measuring changes in drag becomes an important task—perhaps THE most important in any home modifier’s development program . Fortunately, you don’t need access to a wind tunnel or a supercomputer running CFD simulations to measure drag. There are some simple test methods you can use on the road to measure the drag changes whi...

Frequently, online articles or forum posts about aerodynamics are succinct, with simple explanations and guidelines for modifying your car. The problem with a lot of these is that they are wrong . Not just fuzzy on a few details here and there but outright, completely, 100% incorrect. They are not borne out by the vast body of literature on car aerodynamics or by direct, hands-on experimentation. The counter to these, unfortunately, cannot be as simple because, as we will see, the science of air flows is by its very nature complex and hard to understand. This series will argue against some of the most insidious myths, debunked with my own test results and citations from experts as supporting evidence (which I encourage you to read for yourself ). Aerodynamic "Templates" and "Ideal" Shapes Aren’t Applicable to Real Cars   The claim: Everything we need to know about car aerodynamics was discovered in the 1920s. Using a single ideal profile, we can create a universall...