Amateur Aerodynamics

Mirrors are an easy drag problem to spot: they stick out from the car body, disrupting flow. This is easy enough to guess, but how and how much do they adversely affect things? To answer that we need to do some testing.   The Problem with Mirrors   The problem with mirrors is twofold: first, in order for them to function they need a large, flat surface on the back side, which contributes to high drag for the mirror itself. Second, regardless of the drag of the protuberance such as a mirror, when it is brought close to the body of a car the total drag is more than the sum of the two. This phenomenon is called “interference drag.”   You can get an idea for why this is by looking at a tuft test of a window surface behind a mirror. Here’s the flow over my 1991 Toyota truck’s side window with the factory mirror—which is not large by truck standards—in place:   Enjoy the soybean fields. If you follow this blog, you're going to see a lot more of them.   Notice how the tufts are disrupte

Updated August 28, 2023 I’m subtitling this post “don’t believe all the data you read.”   There’s a lot of stock put in fuel economy tests to verify changes in drag. Someone makes an aerodynamic change to their car and runs an “A-B” test , one run (sometimes more) in each of opposing directions measuring fuel economy , to “prove” that drag has been reduced . These results are then posted online uncritically; I’ve done it myself in the past.   There are a couple of things wrong with these tests.   The Normal Distribution   Statistics help us evaluate the truth of real-world claims. One of the foundations of this branch of mathematics is the “normal distribution”: the discovery that natural systems follow the same pattern of variability, with a certain percentage of measured results falling within a certain deviation of the simple average. For example, over the years I’ve owned my Prius, I have kept track of the fuel economy displayed on the factory gauge and the calculated MPG from

Updated August 14, 2023 Several months ago I had the opportunity to check out my local university’s engineering library for the first time. Automotive engineering materials are on the fourth floor, near aeronautics. I was dismayed to find that there was only a single book there on the subject of automotive aerodynamics, Aerodynamics of Road Vehicles (4 th ed.). This book: This is a good book to have (that’s my copy above, which I bought several years ago), but it is 25 years out of date. This was the last version edited by Wolf-Heinrich Hucho, published in 1998; the SAE released a comprehensively updated 5 th edition in 2016 edited by Thomas Schuetz of BMW. Curiously, my own library on automotive aerodynamics dwarfs that of this well-known engineering school. Here’s (some of) what you can find on my bookshelves. Fluid-Dynamic Drag (Sighard F. Hoerner) Published by the Author, 1965 This tome, although of limited direct applicability to modern vehicle aerodynamics, is still a f

The spoiler temporarily taped in place. Interior/exterior painter's tape like this is perhaps the most useful tool in the home modifier's toolbox. When I finally decided to stop guessing and start testing, one of the first things I did was investigate the changes a small spoiler could introduce to the flow over my Prius. Like you, I’ve noticed that tons of cars now have tiny spoilers. You can usually find them on the back of sedans, whether integrated like this, on the Hyundai Sonata: Or as an add-on part, attached with permanent tape to the trunk lid as on the Toyota Corolla: These must do something, I thought, otherwise they wouldn’t be popping up everywhere. But they’re so small; surely they can’t make that much difference? Boy, was I wrong. Materials To find out, I ordered a 21mm tall lip spoiler strip from a well-known online retailer, as well as a manometer (pressure-measuring device) from the same place and a bunch of pressure-sensing disks from Scanivalve that are de

Updated September 13, 2023 Tuft testing is an incredibly powerful technique for visualizing airflow. Tuft testing is one of the easiest, cheapest, and most enlightening methods for measuring airflow over your car. Tuft testing is so effective because the behavior of the tufts tells you something about both the characteristics of the boundary layer and the velocity at any spot on your car. Background and Theory To see why tufts are such a great way to test, we first have to look at the paths that air can take over a car and the various ways we have available to visualize them. Aerodynamicists typically use three different illustrations of airflow: pathlines, streaklines, and streamlines (there are more, but we won't get into those here). If we divide the air into tiny “parcels” of constant volume (the technical term is Lagrangian parcels ), the path that a single parcel follows as it moves around and over the car forms a pathline , a record of its movement over a certain elapsed

Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo

Fortuitously, it was around this time that a new user appeared on the Ecomodder forum. He was a journalist and author, and I had seen articles of his posted on the popular Autospeed website, which it turned out he ran. He was writing a new book on techniques of aerodynamic modification for hobbyists , he posted, and was looking for cars to feature in the book. Specifically, he wanted examples of modifications that had been tested . I emailed some pictures of my car and got the response I should have expected: sorry, but these modifications haven’t been tested and can’t be in the book. That changed everything. I had done some rudimentary fuel economy testing at one point—recording average mileage over a mile-long section of road with and without solid wheel covers—but beyond that, nothing. I don’t think I had even realized before this that I could test for changes in drag and lift. I had enrolled in an engineering degree program but was only in my first year and still pretty much c