Automobile Aerodynamics and Equivalent Flat Plate Area

Today I'm all wound up in numbers. I read an article in Flying magazine today by Peter Garrison called "How Clean is Your Car?". The discussion was mainly geared towards the differences between aircraft drag and motor vehicle drag. I found his discussion about the hp required to overcome drag at certain speeds.

Try to stick with me on this one...Every vehicle has a certain amount of drag and we want to equate it to a number that we can apply to hp. These calculations negate the drivetrain friction, we are looking at the purely aerodynamic drag. First we need to take the flat plate area and the drag coefficient to get the equivalent flat plate area.

To get the flat plate area you can find the information online like I did at MadAboutKitCars.com . If you want to do it by hand take a head on view of a car and lay grid paper over it, fill in all the areas occupied by the frontal view of the car and that gives you the flat plate area in square feet.















The coefficient of drag (cD) for your vehicle is a characteristic amount of aerodynamic drag caused by fluid flow. This number is fairly difficult to determine on your own but the website I mentioned before ( MadAboutKitCars.com ) has cD for quite a few vehicles.

Now that you have these two numbers you can determine the equivalent flat plate area (EFP). This number is used as a way to equate vehicles of different sizes. One thing a lot of keyboard cowboys like to point out is how a 2004 Dodge Durango (cD=.39) and is more aerodynamic than a 1986 Porsche 911 (cD=.40). We have all seen someone park their ginormous SUV next to a 911 and noticed how huge it is by comparison. The Durango may have a flat plate area of 30 square feet whereas the 911 has a flat plate area of around 15 square feet. The SUV may be more "streamlined" but it certainly has to push a lot more air than the 911. So the flat plate area is taken into account to give an accurate representation of the total aerodynamic drag load for a given shape.














Multiply your flat plate area by the coefficient of drag and you can determine the aerodynamic drag load. The equivalent area for a Durango is around 11.7 square feet. Given that information and the fact that the air pressure on a flat plate at 60mph is 13psi the total drag load for the Durango is 152 pounds. For the 911 at the same speed the drag load is 81 pounds.

Here are a couple of other calculations I did.....From the calculations you can see that the horsepower required to overcome drag goes up exponentially with speed. The actual function is that the horsepower required increases as the square of speed.

Why do I find this interesting? The Porsche 911 in it's original form wasn't known to be an aerodynamically perfect automobile, it was however fairly efficient. Despite it's aerodynamic efficiency the early 911s can only manage in the 15mpg in the city and mid 20s on the highway. As a result of engine technology the Prius is rated at 48mpg city and 45mpg highway.

Is the Prius as fun to drive as the 911? Not unless you get your jollies playing the gas mileage game (some people do, really). Is the Prius more socially responsible? Yes on a daily basis the Prius is going to cause a lot less environmental damage. So have your Prius for daily driving if you want and your 911 for fun on the weekends.

If I was looking at the environmental responsibility aspect of this discussion I would ask the typical Prius driver how many cars they have purchased in the last 38 years. If you are buying a hybrid because it's fashionable then please don't claim to be helping the environment, you know you aren't going to drive it until the wheels fall off. What will happen is you'll drive it until the Prius isn't fashionable and buy something else. The environmental damage from producing a new car for people every two years is significant.

References:

MadAboutKitCars.com

Air load on a flat plate from NY Times Article Circa 1910