How Aerodynamics Work

Before we look at how aerodynamics is applied to automobiles, here's a little physics refresher course so that you can understand the basic idea.

As an object moves through the atmosphere, it displaces the air that surrounds it. The object is also subjected to gravity and drag. Drag is generated when a solid object moves through a fluid medium such as water or air. Drag increases with velocity -- the faster the object travels, the more drag it experiences.

We measure an object's motion using the factors described in Newton's laws. These include mass, velocity, weight, external force, and acceleration.

Drag has a direct effect on acceleration. The acceleration (a) of an object is its weight (W) minus drag (D) divided by its mass (m). Remember, weight is an object's mass times the force of gravity acting on it. Your weight would change on the moon because of lesser gravity, but your mass stays the same. To put it more simply:

a = (W - D) / m

(source: NASA)

As an object accelerates, its velocity and drag increase, eventually to the point where drag becomes equal to weight -- in which case no further acceleration can occur. Let's say our object in this equation is a car. This means that as the car travels faster and faster, more and more air pushes against it, limiting how much more it can accelerate and restricting it to a certain speed.

How does all of this apply to car design? Well, it's useful for figuring out an important number -- drag coefficient. This is one of the primary factors that determine how easily an object moves through the air. The drag coefficient (Cd) is equal to the drag (D), divided by the quantity of the density (r), times half the velocity (V) squared times the area (A). To make that more readable:

Cd = D / (A * .5 * r * V^2)

[source: NASA]

So realistically, how much drag coefficient does a car designer aim for if they're crafting a car with aerodynamic intent? Find out on the next page.

­ ­We've just learned that the coefficient of drag (Cd) is a figure that measures the force of air resistance on an object, such as a car. Now, imagine the force of air pushing against the car as it moves down the road. At 70 miles per hour (112.7 kilometers per hour), there's four times more force working against the car than at 35 miles per hour (56.3 kilometers per hour) [source: Elliott-Sink].

The aerodynamic abilities of a car are measured using the vehicle's coefficient of drag. Essentially, the lower the Cd, the more aerodynamic a car is, and the easier it can move through the wall of air pushing against it.

Let's look at a few Cd numbers. Remember the boxy old Volvo cars of the 1970s and '80s? An old Volvo 960 sedan achieves a Cd of .36. The newer Volvos are much more sleek and curvy, and an S80 sedan achieves a Cd of .28 [source: Elliott-Sink]. This proves something that you may have been able to guess already -- smoother, more streamlined shapes are more aerodynamic than boxy ones. Why is that exactly?

Let's look at the most aerodynamic thing in nature -- a teardrop. The teardrop is smooth and round on all sides and tapers off at the top. Air flows around it smoothly as it falls to the ground. It's the same with cars -- smooth, rounded surfaces allow the air to flow in a stream over the vehicle, reducing the "push" of air against the body.

Today, most cars achieve a Cd of about .30. SUVs, which tend to be more boxy than cars because they're larger, accommodate more people, and often need bigger grilles to help cool the engine down, have a Cd of anywhere from .30 to .40 or more. Pickup trucks -- a purposefully boxy design -- typically get around .40 [source: Siuru].

Many have questioned the "unique" looks of the Toyota Prius hybrid, but it has an extremely aerodynamic shape for a good reason. Among other efficient characteristics, its Cd of .26 helps it achieve very high mileage. In fact, reducing the Cd of a car by just 0.01 can result in a 0.2 miles per gallon (.09 kilometers per liter) increase in fuel economy [source: Siuru].

On the next page, we'll examine the history of aerodynamic design.

While scientists have more or less been aware of what it takes to create aerodynamic shapes for a long time, it took a while for those principles to be applied to automobile design.

There was nothing aerodynamic about the earliest cars. Take a look at Ford's seminal Model T -- it looks more like a horse carriage minus the horses -- a very boxy design, indeed. Many of these early cars didn't need to worry about aerodynamics because they were relatively slow. However, some racing cars of the early 1900s incorporated tapering and aerodynamic features to one degree or another.

In 1921, German inventor Edmund Rumpler created the Rumpler-Tropfenauto, which translates into "tear-drop car." Based on the most aerodynamic shape in nature, the teardrop, it had a Cd of just .27, but its unique looks never caught on with the public. Only about 100 were made [source: Price].

On the American side, one of the biggest leaps ahead in aerodynamic design came in the 1930s with the Chrysler Airflow. Inspired by birds in flight, the Airflow was one of the first cars designed with aerodynamics in mind. Though it used some unique construction techniques and had a nearly 50-50-weight distribution (equal weight distribution between the front and rear axles for improved handling), a Great Depression -weary public never fell in love with its unconventional looks, and the car was considered a flop. Still, its streamlined design was far ahead of its time.

As the 1950s and '60s came about, some of the biggest advancements in automobile aerodynamics came from racing. Originally, engineers experimented with different designs, knowing that streamlined shapes could help their cars go faster and handle better at high speeds. That eventually evolved into a very precise science of crafting the most aerodynamic race car possible. Front and rear spoilers, shovel-shaped noses, and aero kits became more and more common to keep air flowing over the top of the car and to create necessary downforce on the front and rear wheels [source: Formula 1 Network].

On the consumer side, companies like Lotus, Citroën and Porsche developed some very streamlined designs, but these were mostly applied to high-performance sports cars and not everyday vehicles for the common driver. That began to change in the 1980s with the Audi 100, a passenger sedan with a then-unheard-of Cd of .30. Today, nearly all cars are designed with aerodynamics in mind in some way [source: Edgar].

What helped that change to occur? The answer: The wind tunnel. On the next page we'll explore how the wind tunnel has become vital to automotive design.

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To measure the aerodynamic effectiveness of a car in real time, engineers have borrowed a tool from the aircraft industry -- the wind tunnel.

In essence, a wind tunnel is a massive tube with fans that produce airflow over an object inside. This can be a car, an airplane, or anything else that engineers need to measure for air resistance. From a room behind the tunnel, engineers study the way the air interacts with the object, the way the air currents flow over the various surfaces.

The car or plane inside never moves, but the fans create wind at different speeds to simulate real-world conditions. Sometimes a real car won't even be used -- designers often rely on exact scale models of their vehicles to measure wind resistance. As wind moves over the car in the tunnel, computers are used to calculate the drag coefficient (Cd).

Wind tunnels are really nothing new. They've been around since the late 1800s to measure airflow over many early aircraft attempts. Even the Wright Brothers had one. After World War II, racecar engineers seeking an edge over the competition began to use them to gauge the effectiveness of their cars' aerodynamic equipment. That technology later made its way to passenger cars and trucks.

However, in recent years, the big, multi-million-dollar wind tunnels are being used less and less. Computer simulations are starting to replace wind tunnels as the best way to measure the aerodynamics of a car or aircraft. In many cases, wind tunnels are mostly just called upon to make sure the computer simulations are accurate [source: Day].

Many think that adding a spoiler on the back of a car is a great way to make it more aerodynamic. In the next section, we'll examine different types of aerodynamic add-ons to vehicles, and examine their roles in performance and providing better fuel mileage.

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There's more to aerodynamics than just drag -- there are other factors called lift and downforce, too. Lift is the force that opposes the weight of an object and raises it into the air and keeps it there. Downforce is the opposite of lift -- the force that presses an object in the direction of the ground [source: NASA].

You may think that the drag coefficient on a Formula One racecar would be very low -- a super-aerodynamic car is faster, right? Not in this case. A typical F1 car has a Cd of about .70.

Why is this type of racecar able to drive at speeds of more than 200 miles an hour (321.9 kilometers per hour), yet not as aerodynamic as you might have guessed? That's because Formula One cars are built to generate as much downforce as possible. At the speeds they're traveling, and with their extremely light weight, these cars actually begin to experience lift at some speeds -- physics forces them to take off like an airplane. Obviously, cars aren't intended to fly through the air, and if a car goes airborne it could mean a devastating crash. For this reason, downforce must be maximized to keep the car on the ground at high speeds, and this means a high Cd is required.

Formula One cars achieve this by using wings or spoilers mounted onto the front and rear of the vehicle. These wings channel the flow into currents of air that press the car to the ground -- better known as downforce. This maximizes cornering speed, but it has to be carefully balanced with lift to also allow the car the appropriate amount of straight-line speed [source: Smith].

Lots of production cars include aerodynamic add-ons to generate downforce. While the Nissan GT-R supercar has been somewhat criticized in the automotive press for its looks, the entire body is designed to channel air over the car and back through the oval-shaped rear spoiler, generating plenty of downforce. Ferrari's 599 GTB Fiorano has flying buttress B-pillars designed to channel air to the rear as well -- these help to reduce drag [source: Classic Driver].

But you see plenty of spoilers and wings on everyday cars, like Honda and Toyota sedans. Do those really add an aerodynamic benefit to a car? In some cases, it can add a little high-speed stability. For example, the original Audi TT didn't have a spoiler on its rear decklid, but Audi added one after its rounded body was found to create too much lift and may have been a factor in a few wrecks [source: Edgar].

In most cases, however, bolting a big spoiler on the back of an ordinary car isn't going to aid in performance, speed, or handling a whole lot -- if at all. In some cases, it could even create more understeer, or reluctance to corner. However, if you think that giant spoiler looks great on the trunk of your Honda Civic, don't let anyone tell you otherwise.

For more information about automotive aerodynamics and other related topics, breeze on over to the next page and follow the links.