I am an automotive aerodynamicist and this is what I do.
Modern automotive aerodynamics really began in the 1920s when Chrysler built a small scale wind tunnel and hired Orville Wright to help conduct a comprehensive study of something like 50 test articles. Oh sure, there were earlier efforts to improve the aerodynamic performance of production cars, but the Chrysler effort was among the first scientific pushes by an automaker to reduce the amount of horsepower lost to the air. The resulting car, the Chrysler Airflow, was a sales flop. Despite being a colossal disappointment, it signaled the birth of my craft and I’m grateful for it.
Today, we use many tools to aerodynamically improve cars. The wind tunnel is still very much in use, though now it comes in many flavors (full-scale and sub-scale, static ground and moving ground, and some other stuff). We also use several types of Computational Fluid Dynamics software (CFD) in an attempt to predict the way air will flow around and through a car. It’s worth mentioning here that thermal engineers also use a ton of thermally coupled CFD to design, locate, and size things like heat shields and radiators. That is a whole different topic that, while related to general aerodynamics, is worth it’s own write-up.
As with any type of engineering, there are compromises to be
made when it comes to aerodynamics. If
nothing else mattered, it would be easy to make cars with super low drag. We would optimize to one shape, and then I’d
be out of a job because physics never change. In the real world, aero performance has to compete for attention with
thermal performance, vehicle weight, crash safety, pedestrian safety, and, most
importantly, the design studio.
To keep the car from melting down or catching on fire or just plain overheating, it’s important that cooling air flows through the car. Most cars get this cooling air from the grill, or under-car openings (look under a C5 Corvette for an example). The only problem with this is that air going through the car is going to have a net momentum loss, which translates to drag and lift. Both are bad for aero.
How about safety? How does this impact aero? Look at a Mercedes C Class front end from the mid 90s, and compare it with the nose on a brand new one. Notice how tall and blunt the new one is? That’s government safety requirements driving the shape of cars. You can see it on Corvettes, Mustangs, and Accords. There are similar safety regulations regarding tire coverage (when seen from dead-ahead or directly above). Every additional mandated shape change constrains us a little more and makes our job just a little more challenging. To be sure, I’m not saying safety is ruining aero, it’s just pushing us to get a little more creative than we otherwise would have been. I like being creative.
In general, the biggest battle that an aerodynamicist will fight is going to be with the design studio. The people that make cars look so good often don’t believe aerodynamic features are attractive. This means that in order to keep the car looking like something people want to buy, I need to find creative aero tricks that aren’t visible, or the designers need to find creative ways to hide my features. You’ll see examples of the former in places like little air dams in front of the tires, and the latter in little “kicks” built into the tail lights. Both are mostly out of sight but can help aero performance way more than you’d expect.
We often hear car companies bragging about the coefficient of drag (Cd) of their cars. Cd is a non-dimensional way of describing how slippery that car is, but it only tells part of the story. In reality, when calculating the drag on a car, you must also consider the frontal area (Af), or cross sectional area. If two cars both have a Cd of .32, but one has an Af of 2 square meters and the other is 3 square meters, the bigger car will incur a bigger aero penalty. This seems obvious, but when everyone is trying to blind us with Cd numbers, it’s easy to forget that equally critical Af term. So the next time you hear some auto engineer brag that they dropped the Cd on their new car by 10%, ask them if the Af changed. There’s a good chance it went up, potentially negating that Cd improvement. One important note here: bigger Af generally get you a roomier interior, so it’s not like they’re adding size just to be jerks. I just like to pick on Af because it makes my job harder!
Drag isn’t the only force we care about. Lift is also important. I will point out that 90% of the cars that we mortals own do not produce downforce. They almost all produce lift, and it’s important to keep in mind that that’s not necessarily a bad thing. In general, a car with low drag will have more lift than a car with higher drag. Reducing lift (or adding downforce) will almost always add drag because of how the physics shake out. So the deal here is that we spend a fair amount of time getting the balance between front and rear lift correct without adding unnecessary drag. If the lift values are out of whack, they can make a car act funny during things like emergency lane changes (see: Moose Test).
Tips and Tricks
So how do you get the aero performance to meet your targets? Well, in a perfect world the aerodynamicist can work with the studio designers, packagers, safety engineers, and a host of other people to get the basic shape the way we want it. For example, try keeping the front end kind of round while keeping the trailing edges nice and sharp is a good place to start. The current Lincoln MKZ and the Prius are good examples of drag-optimized cars.
What about when you want the car to look rad while still behaving well? If you look closely, you may notice some little tricks that can make a big difference. Look at the tail lights on a BMW 5 Series and you will see some small ridges that help kick the air off the car at the optimal point to reduce drag. In fact, you’ll see these ridges in a lot of tail lights if look closely.
Another trick is little air dams in front of the tires. These help keep air from getting into the wheel wells and impacting the spinning tires, which is generally bad for drag. In fact, you want the air to stay out from under a car as much as possible. That’s why you will see deep valences under the bumpers of most new full-size pickups. If the air does manage to sneak under, you want to get it out as quickly and efficiently as possible, so you may try to install flat shields that cover all the bumpy bits under a car. These smooth shields will generally also make a car quieter, which is good. Any time you can make something serve two purposes, you’re winning. The downside to underbody shields is that they can trap heat where you don’t want it, and they can add weight. This all has to be balanced to make sure your aero gain isn’t completely offset by a weight penalty, therefore netting zero MPG gain while costing money that could better be spent elsewhere.
Another trick that is becoming more common on SUVs is a feature that blends a roof-top spoiler to the sides of the car. This serves a similar purpose as those tail light kicks in that is kicks the air off the car, preventing it from “sticking” to a rounded rear shape. If air “sticks” to the rear of a car, it’s making drag. You want to do whatever you can to kick it off and never bother you again!
After an aerodynamicist spends all that time working with all the other people who influence the shape of a car, after all the CFD work is done and all the predictions have been made, you build a car and put it in a wind tunnel to see how you did. Sometimes you will start with a clay mock-up, sometimes you just build a running, driving prototype. While testing in the tunnel, you almost always learn that things aren’t exactly what you think they’d be. Sometimes drag is low, sometimes it’s high. Sometimes your lift numbers are all wrong. If things are good, you can sign the dotted line and certify that your design works as promised and the car can go to production. More often, you find out in the test that you need to make small changes to things like underbody shields or tail light ridges. Iteration with CFD will be the name of the game until you get the aero performance you targeted.
I have had three jobs since I graduated college with my aeronautical engineering degree. I worked as an aerodynamicist at a company that built business jets, then I worked as a wind tunnel test director at NASA, mostly doing rotorcraft research, and now I’m an aerodynamicist at a major auto maker. What I’ve learned from all this is that jets are easy, cars would be easy if they didn’t have to look good or be safe in a crash, and helicopters operate on miracles. They’re seriously the most aerodynamically and structurally challenging machines humans make.
Aaaaaand I’m spent.