The Benefits of Aerodynamics
Airflow has always played a part in vehicle performance but never more so than today
The art of aerodynamics has come a long way in recent years. Decades ago, developers would just stick wings on rear ends and see what happened. Today the process is far more complex. aerodynamic engineering involves countless hours of wind-tunnel testing and advanced computer simulations. But if aerodynamics is now so advanced, why are there still cars like the Honda Civic Type R, covered in gaudy adornments, when others such as Ferrari's 488 GTB don't feature anything nearly as obvious?
Getting Down with the Force
To understand how aero works, look to the skies. The same principles that keep aircraft aloft are used to stick cars to the ground. The curved profile of an airplane's wing deflects airflow, forcing some air over the top and some under the bottom. Because the top surface curves more, the air has to travel farther and therefore faster than the air going underneath. According to Bernoulli's principle — invoked in Swiss physicist and mathematician Daniel Bernoulli's 1738 book, "Hydrodynamica" — air traveling at a fast speed has a lower pressure than slower-moving air. This pressure difference means the wing is pushed upward, producing lift and keeping an airplane in the sky.
Turn the wing upside down, and the same principles push a car harder into the asphalt, giving more grip. The most common example of this is downforce generated by the humble fixed wing. Using the same simulation tools that top manufacturers use to design their cars, we can illustrate how a wing makes downforce.
In this two-dimensional computational fluid-dynamics simulation, the blue area underneath the wing shows Bernoulli's principle at play. The faster-moving air creates a low-pressure zone, forcing the wing down. However, before you bolt a wing to your car, take a minute to consider the red area on the wing's leading edge. Unfortunately, all of this downforce comes with aerodynamic drag.
Manufacturers often quote their cars' drag performance using a metric called the drag coefficient, or Cd for short. The lower this number, the more easily the car slips through the air. It's important because there is a relationship between speed and drag: The faster you go, the harder the air pushes on your car, squared. This is one of the reasons why the new Bugatti Chiron has 300 hp more than its predecessor, the Veyron Grand Sport Vitesse, but can only manage 6 mph more in top speed.
Manufacturers use countless aerodynamic control surfaces to create downforce, and we would need to write an aerodynamic bible in order to cover them all, so for now we'll stick to the most well-known: the fixed wing.
The fixed wing's first automotive use stretches back to the 1920s with the Fritz von Opel RAK 2. This speed-record machine used 24 rockets to reach 147.8 mph. Learning directly from the folks in the aeronautical industry, Opel added two upturned airfoils to the sides of the RAK 2 to keep the vehicle from lifting off the ground.
It took until the late 1960s to see the first appearance of the more conventionally mounted, fixed rear wing courtesy of Colin Chapman and his successful Formula 1 chassis, the Lotus 49. Midway through the 1968 F1 championship, Chapman bolted an expansive rear wing directly to the rear suspension, above the engine and driver to reach clean air — in other words to move it out of the aerodynamic wake produced by cars running ahead of it on the track. This initially gave Lotus a vastly superior competitive edge, but it was soon banned due to a collection of serious crashes.
Convention dictates any part that generates downforce will have some drag associated with it. So how to achieve the holy grail of aero, to have downforce in the corners and no extra drag in a straight line? This is where active aero comes into play.
Active aero is any aerodynamic control surface that provides the best of both worlds. For straight-line speed, a wing can either retreat into the bodywork — minimizing drag — or extend to give downforce in the corners. A great example of this is the McLaren P1's active rear wing. When minimal drag is ideal and at low speeds, the wing sits flush with the rear bodywork; at higher speeds, it is lifted via two hydraulic struts, generating downforce. In Race mode, like with the new Ford GT, the wing reaches its highest position and most aggressive angle of attack, forcing the P1 into the ground for extreme lateral grip. The wing also folds forward dramatically under heavy braking to provide an airbrake effect.
Probably the first instance we can find of this type of tech is on the 1966 Chaparral 2E. This race car used a large rear wing that could be augmented to offer more or less downforce when needed. Nissan took this type of tech one step further two years later with its grand prix-winning R381 chassis. Nissan split the rear wing left to right to offer different levels of downforce for the inside or outside wheel.
Spoiling the Flow
Not every device attached to a car's rear is a true wing, though. Spoilers are more common than wings on modern road cars, but they are quite different. The most important surface of a wing is the underside, where the greatest pressure difference is realized. Spoilers don't have an underside; they simply deflect airflow from its path and are used mainly as pop-up devices on road cars. This helps with stability at higher speeds by separating the airflow at the trunk lip. A good example of this is the three-piece split spoiler seen on the 2017 Porsche Panamera Turbo.
When we consider that a wing's underside is the most important surface for producing downforce, we stumble across an issue with conventional wings: Most are connected to the car via two struts located underneath. The pillars can actually take away up to a third of the wing's performance since the struts disturb airflow over the underside. This is why we see top-mounted or "swan neck" wings on some race cars and even hypercars such as the Koenigsegg One:1. These mounting positions make the most of the air flowing over the wing's surface.
State of the Art
Modern aero is so advanced that we can get a whole bunch of downforce without having to resort to large appendages that cause lots of drag. Take the Ferrari 488 GTB: It creates 50 percent more downforce than its predecessor, the 458 Italia, with no brash add-ons. It achieves this with clever aero tricks such as using Ferrari's blown spoiler.
In the 488's case, air flowing over the roofline is guided down past the engine cover and into an opening in the rear bodywork. This air is then passed over an internal spoiler, which can have a much larger angle than if it were on the surface. Not only this, but the blown spoiler also takes advantage of something called the Venturi effect. This occurs when air squeezed into a smaller area is accelerated. Combining the blown nature of the spoiler and acceleration due to the Venturi effect, the 488 GTB's rear spoiler outperforms a conventional fixed wing.
Future Aerodynamic Tech
So we've seen the evolution of fixed control surfaces, from permanently mounted rear wings to modern active aero. Where do we go from here? One answer seemingly straight out of science fiction could be the way forward. Introducing plasma flow control.
Activate the plasma flow controllers when extra grip is desired, shut them off when it isn't.
Plasma flow control is still in an early research and development stage but shows potential for use in ultra-performance cars. Electronic devices placed inside the bodywork — as to not affect airflow — can manipulate the surrounding air with no moving parts. High-voltage alternating current is passed across two electrodes, which creates low-temperature plasma. This plasma can ionize air molecules passing over a surface, speeding up the airflow.
This type of tech could increase a car's downforce dramatically without creating as much drag and aero disruption as modern active wings do. Activate the plasma flow controllers when extra grip is desired, shut them off when it isn't. Even with them switched on, their flushness with the car means they create less drag than conventional or active wings and spoilers as they also have no moving parts and no frontal area, the latter being aero efficiency's nemesis. Although only mentioned briefly by McLaren in relation to its 2016 MP4-X F1 concept car, this technology shows a lot of promise and has caught the attention of the likes of NASA. But don't get too excited. It presently needs very high voltages and wouldn't be efficient in a performance car — at least, not yet.