Amateur Aerodynamics

Driving back and forth to a city about an hour and a half from here for rehearsals and a concert recently, I wondered if I could measurably improve the stability of my Prius. I was transporting and playing this, a 2008 Zuckerman Flemish double harpsichord. You can buy kits to put an instrument like this together yourself. As it usually is here in the Midwest, it was windy all three days I had to make that drive and I found myself paying attention to how much I had to turn the steering wheel to keep the car going straight down the freeway. I wondered, is it possible to give my car better directional stability? And could I measure anything that would tell me, objectively, if a change I make has an effect?   Defining “Stability”   Stability, in aerodynamics, is a complicated subject. It depends not just on the aerodynamic characteristics of a car but also how these interact with the car’s suspension, tires, weight distribution, and handling tendencies. It also depends on how the driver

So far, I’ve lowered the aerodynamic drag of my 1991 Toyota Hilux by removing its mirrors, blocking part of the cooling air opening , and fitting a large front air dam . Together, these reduce drag by more than 10%--which means I’m about halfway to my goal of cutting the drag of this truck by 20% or more .   Moving on from the front surfaces, I thought I would trial some modifications to the airflow around the wheels next.   Keep It Simple   Right off the bat, I have a conundrum. The front wheel housings have a metal inner fender and a narrow plastic cover which seals the gap between inner and outer fenders. But in front of the tires, there are effectively no wheel housings; it’s open to the bumper, stock valance, and now air dam. According to various textbooks , wheel drag is typically reduced when wheels are enclosed in housings, with drag decreasing the smaller the ratio of the wheel housing volume to wheel volume. This may or may not hold true on my truck; I have no way of kn

  This summer, I tested a large spoiler at different angles, with and without fins , by measuring pressures on the rear window and base of a Prius.   Recently, I tested two variations on this. First, a Gurney flap is a short vertical spoiler placed at the trailing edge of bodywork or an existing wing or spoiler. I made a flap out of cardboard, about an inch high: Then, after reading about possible beneficial effects from a spoiler with a wavy edge rather than straight, I made another cardboard strip with a sinusoidal shape: Would there be any difference between the two?   Lastly, I took the 20 ° spacers I had fabricated for my previous test and taped the spoiler board to them, but this time with a slot or opening at the bottom. How would this compare to a spoiler with no slot? Results   As one might expect, the Gurney flaps did not increase pressure on the window as much as the large spoiler. And there was no difference in window or base pressures between the straight-edge and si

Several weeks ago, I wrote about an unusual base pressure reading while testing various spoiler angles on my car , a 2013 Toyota Prius. Specifically, the pressure at the center of the trunk, at 80 kph, was 0—no relative difference from atmospheric! This is unusual because there should be negative differential pressure on the rear surface of a car (its base ), where a turbulent wake forms behind it. What was going on?   Winds were calm that day, a rarity here in the American Midwest, which may have caused the odd pressure reading. I also had no wheel covers on the car since I had removed the wheels to have new tires mounted a few days before. This last observation led me to conduct some tests to see if the wheel design could be influencing base pressure.   This isn’t an outlandish idea, even if it appears so at first. Just this year Dr. Thomas Wolf, former head of aerodynamics at Porsche, published a paper detailing testing of wheel designs on the Porsche Cayenne which found that the p