The Importance of

AERODYNAMICSThe following presentation will provide

students with information about the RESEARCH of how the performance of vehicles is affected by air as they move

through at higher speeds. This is a branch of science dealing with objects as they move

through a fluid called Dynamics.

What is AERODYNAMICS?

• Aerodynamics is the science that does research to discover the effects on an object as it moves through air.

• Scientists have been studying how objects move through air for thousands of years. Sails on boats, arrows, bullets, and artillery shells, baseballs, golf balls and footballs have been tested and are still being researched to improve their accuracy, distance and flight all for the improvement of performance.

Automotive AerodynamicsFor this module, we will focus on the principles

of air that affect a vehicle’s abilities to reach its highest performance when accelerated to very high speeds. While a student can design a VW Beetle or Honda Accord or even a BMW 325i, the goal for this research project is to design for performance not looks or styling. Those goals are called AESTHETICS.

Most cars are not expected to go faster than 85 miles per hour. The next time you look at a car instrument panel look and see how fast the speedometer will measure.

• You will see the term, Dragster, many times while discussing vehicle aerodynamics. Be aware that most cars since the fuel shortages of the early 1970’s forced all passenger car makers to pay attention to how they could improve a models fuel mileage and range. Most specialty cars come from the modifications done on regular cars. Dragsters, hotrods, muscle cars, dune buggies, and now foreign or sport-compact “tuner” cars are all born from the cars made by Ford, GM, Dodge, Toyota, Honda, and Mitsubishi

Only a few true sport cars purposely built for maximizing performance over everything else are actually produced and ALL are very expensive. Most of the following sports cars sacrifice many features you see on even the cheapest economy cars today. No stereo or speakers, no electric windows or no roll down windows at all, forget about cup holders and even air conditioners are optional.

Where does everyone sit? Most have no more than 2 non-reclining racing seats. No sound insulation-who needs to talk when you can listen to the engine rev and tires chew the road.

Everything is sacrificed to reduce weight or save weight for the massive brakes, wheels and large exhausts. Compare these cars to the vehicles you rode in to school or to the store. Where would you put your book bag, sports equipment or grocery bags?

True Sports Cars

• Acura NSX

• Aston Martin

V12

• Chevrolet Corvette

• Dodge VIPER

• Lamborghini

• Mazda RX-7 3rd body style

Exotics

FerrariFord GT-40

True Sports Cars

• Toyota Supra

• Porsche 911

• McClaren 3 seats!

• Also and just as important was the difficulty in the ability to make the shape of the car out of steel. It was possible to make straight flat sides with corners or simple curves but it was way too expensive to make complex curves for most car buyers. Today, lightweight aluminum, high strength steel alloys, plastics, or plastic composites like fiberglass and carbon fiber give a vehicle the same strength at a greatly reduced weight. The same vehicle depending on the materials it was made out of would differ in weight in as much as 1000 lbs!

Big Cars= Big Drag

• Additionally, the pollution from big older engines made governments force car makers to use smaller cleaner running engines to reduce smog and other forms of air pollution. In order to keep the performance up designers began looking for ways the keep cars fast but not use up all their fuel to keep passenger cars going down the highway.

Reduce the Weight of the Car

• The first method designers tried was to make the cars weigh less so the engine making less power would not have to use all of the power to keep the car moving or accelerating too slowly. The problem designers ran into was in order to make it lighter this usually made the car smaller because they were still using the same material, mild steel, to make most of the body. Mild steel is relatively cheap and engineers and designers know how to build with mild steel- used a lot.

The Limits of Weight Reduction

• Although new materials continue to be develop by scientists and engineers, the need to improve performance led carmakers to start rethinking and asking questions they never bothered to ask before about how the shapes of their designs could if at all changed to somehow improve the PERFORMANCE of their cars. It is not that aerodynamics was discovered in the 1970’s but that the science of technology was finally considered as having benefit to cars… HMMM!

The Early Years

• The first attempts were to copy examples of other products that went fast and could be controlled at speed. Aircraft and jets, which had been designed using aerodynamics since their beginnings, all had common shapes and features: rounded edges, graceful and gentle curves, and low swept features and minimal parts stuck on the outside surfaces of the bodies.

Hi-Tech Wind Tunnel

• Designers began using these types of features and began to ask aerodynamicists how to test to see if the designed would work. Now car makers began to hire scientists instead of designers because, “FORM FOLLOWS FUNCTION” Depending on how much money a car maker/designer had they could measure or compare the efficiency of one design with another how much resistance, DRAG, was indicated by blowing air over a model in a machine called a wind tunnel. They would weigh how much force the car would push back or down on a scale and record the amount in pounds or kilograms.

Tuft Testing• Some car makers used a low

tech low cost method of testing by attaching five to seven inch long pieces of string in rows to the car to see where turbulence, Disturbed AIR, were created at different parts of the car while driving down a test track. Both of these methods are still being used today together to show the amount of turbulence or drag the shape feels as it moves through air at speeds as low as 35 miles per hour (MPH). Your car will be exceeding 150mph and cover 66 feet in as little as 1.5 seconds!

The Rookie Designer

• What are some things you can know about car design which will put you ahead of your classmates design?

• Recognize the purpose and goal of this module and remember it as you design you car: Performance! Go as fast as you can within the specifications (rules)

• Read and pay attention to the following aerodynamic testing results others have done before you.

• Notice how the shapes with flat rears have higher drag numbers than curves or tapers.

Results of Shapes

• Drag Coefficient means how much pull or drag each shape feels as it was moving through the air.

• Remember these are blocks.

• Which shapes cause higher drag? Curves or corners?

Frontal Area

Frontal Area should be as small as possible. It is the face or front surfaces of your car: front tires, bumper, grill, hood, windshield the air would run into as your car moves through the air. Reduce or avoid any flat vertical surfaces on the front. Wheels and tires are the worst! Hide them in the car!

Rake

• Side view showing Rake. The tilting or leaning back of a surface to deflect the impact of an object hitting a surface.

• Try to keep lines leaning back 30 degrees or less from horizontal. 7-10 degrees is best. Students should use a protractor to make sure the design has angles 30 degrees or less.

Rake Notice how much the windshield and front nose lean back

Taper

• A taper is the gentle reduction or shrinking of the size of a shape from the center to the end. This can be from the front to the center or center to rear. Scientists discovered that by tapering the shape of an objects sides it reduced the amount of drag by allowing the airstreams to gently come back together as they leave the objects rear.

• In vehicle design this is called boat-tailing

Tapered front and rear ends

Cropping the Tail

• Cropping is the process of removing material from the bottom rear of the object to allow an upward taper for airstreams to follow as they leave the vertical sides and under side of an object. Scientists and engineers discovered this design also acted like a upside down wing causing the rear of the object to hold to the ground MUCH better. The taper cannot be more than 20 degrees.

Cropped Tail Examples

Examples of cropped tails

No crop causes red swirl arrows-drag

Choosing Wheels and Tires

• Big wide wheels may look cool but they are bad in two ways:

• The bigger the wheel/tire the heavier it is and more power it will take to make it turn.

• The co2 cartridge pushes the car so you do not have to worry about traction.

• Friction Drag. The wider wheel touches more of the road grabbing and rolling against more road just like dragging you hand down a wall while running. The heat you feel is caused by the friction of the two surfaces. It is stealing your power! Naughty Friction!

More Evil Wheel Drag

• Wheels cause their own special drag that can be minimized.

• The wheel edges are corners all the way around the wheel on both sides of each wheel. If you measured the circumference of each wheel, multiplied each by 2 for both sides and then multiplied by 4 for all the wheels corners then draw a line to that length you would see how long a corner you would have on your car because of the sneaky wheels!

Wheel Problems

• Wheels and tires also spin through the air cutting and ripping through the air like an axe upwards, downwards, rearwards and forwards all at the time. If you were running a race would you like someone to hit down on your head and kick the bottom of your feet? Karate Chop!

• Research testing shows exposed wheels make up 45% of the total drag on a car.

• Hide the wheels in a hole in the car called a WHEEL WELL. The well allows the wheel (cylinder) to sit inside protected from the air.

Wheel Well Design

• In order to make wheel wells and the wheel/tire work the need to close to each other but never touch. Your car does not have a moving suspension or axle so the wheel well radius should be 1/8th in. or 4mm larger than the wheel.

• The outside edge of the wheel should be even with the outside of the well or air will rush into the well and cause a mess of turbulent air. (See illustration)

Porsche 993

Mazda RX-7 3rd body style

Dark area behind wheel is disturbed area causing serious DRAG

Shaded bulge around wheel as viewed from the top shows turbulent air causing drag

More drag as air finds tries to find a path around the spinning wheel

Notice the arrow inside the rectangular wheel is pointing in the opposite direction of the air going around the wheel. The spin of the tire is forcing air to be thrown forward smashing into the air going over the wheel!

Wheel spin throws air forward

Conclusion

• Reduce the size of the body reducing weight

• Shape of body should use curves or raked angles

• Frontal area should be a minimal as possible

• Taper the front and rear vertical corners

• Keep wheels out of flow of moving air