TIJ1O - Aerodynamics

Lesson in Slideshow

Our pneumatic cars will all start as a block of wood. We will make a hollow chamber inside the car for placing a small Co2 canister. This canister is filled with Co2 under high pressure thus allowing you to propel your car forward. All cars will be propelled the same way. When designing, take time to decide which aerodynamic design is best suited for your car

Think about how you want your car to perform. Should I make a longer car or should I make it as small and lightweight as possible? Should I instead, focus on making my car as aerodynamic as possible? Remember the laws of physics that will affect your car when designing.

First we'll cover what will be powering our cars. Compressed air. This will create thrust, the power needed to move our cars.

THRUST

Thrust is what will force our cars forward. Thrust is basically the principle that for every action there is an equal and opposite reaction. As we charge our cars with air, the second we allow the air to escape out the back of our car, it will force the car forward. think of blowing up a balloon. As more and more air is blown into the balloon, more and more pressure is created. Then when the balloon is let go the air rushing out forces it to fly across the room. This creates thrust, the force that moves the balloon forward.

DRAG

We've touched on drag in the laws of physics, but now we need to look at it more in depth. What makes one car more aerodynamic than the next? It all comes down to reducing drag. Remember, drag is the resistance on an object as it moves. As speed increases, so does drag.

Look at the picture of the F1 car at the top of the page, then compare it to the picture of a transport truck.

Other than the size of the truck, what makes the F1 car more aerodynamic? Simply its low profile? Its spoiler? Many factors contribute to the F1 cars low drag. We'll examine each of them in depth.

First we must think of the basic shape. The transport looks like a smoothed rectangle. Not exactly the first thing you think of when you think aerodynamic. The F1 car however doesn't really look like a conventional shape. How do we know which shaped are aerodynamic? We can use this table from Wikipedia to help.

Drag Coefficients explained in the 1930's. Start the video at 2:20.

So what does this table tell us?

Look at the shapes, then look at the numbers beside them. The lower the number, the more aerodynamic the shape is. That means the lower the number the more easily it moves through the air. Here's a more practical example.

Look at these two examples. The truck and the car. The truck has a drag coefficient of about 0.50. The car (The Volkswagen XL1) has a drag coefficient of 0.15!

The image tells us that the most efficient shape is the streamlined body. It looks like a stretched- out circle. So why aren't more cars shaped this way? First think of practicality. The length and shape of the car would be very awkward to drive and park. There would be a great deal of wasted space. Cars must not only be aerodynamic but more importantly they must be practical.

Practicality is as important a factor when designing a car as aerodynamics. Other limitations are harder to think of, but they exist. For example, where the gas tank is accessed on a car. The tank must be accessible from the side to use gas pumps properly. Other things that affect car shape are laws, for example the term "street legal". In fact some laws exist as to how high a bumper can/must be off of the road.

There are more and more factors that affect the design of a modern car than simply aerodynamics. But that's a discussion for another class. Let's get back to aerodynamics.

We've covered the basic shapes that are more aerodynamic, lets talk about the eddies that were mentioned in the black and white film. We could see in the video, the swirls that were left behind some of the less aerodynamic shapes. These "eddies" created more drag. Think of sticking your hand out the car window. As you bring your hand in closer to the car, behind the mirror, what happens? The wind resistance disappears. In fact you can bring your hand further back from the mirror and see how big the eddy created by the mirror actually is.

Where else would there be eddies on a vehicle? Think of Mr. Foster's truck. Not the mirrors, but further back. The box of the truck! The wind rushing off the cab of the truck moves just like the water in the black and white video. It swirls in the box and creates more drag, increasing the wind resistance. How could the truck become more aerodynamic? By adding a cap on the box. Some box covers have a low profile and only cover the top, but a more aerodynamic choice would be to cover the entire box and carry on with the shape of the cab.

Back to our dragsters.

We've covered form, drag, and which shapes are more or less aerodynamic. It's up to you how you want you're dragster to be shaped. But we need to think of another factor that will affect our dragster. The same forces that affected our top will come into play. Contact with the floor. All cars will get four wheels and two axles. Its up to you where to put them on the car but we can cover some basics to ensure the wheels turn smoothly.

Make sure there are no bumps or defects on your tires. Even the slightest imperfection can slow your car down. When installing the axles, be sure they are installed perpendicular to the body of the car. This means ensure the axels are installed at a right angle to the car body.

We've covered all the major factors that are going to affect our dragsters, and how to deal with them. Ensure your car is aerodynamic by using all of the information on this page. Watch the video in the portfolio section on how to draw a dragster in sketchup. Good luck!