IRL: Open wheel car aerodynamics

Flying on the ground: Indy Car aerodynamics.

NAZARETH, Pa., April 18, 2002 -- Nazareth Speedway and Indianapolis Motor Speedway are the yin and yang of ovals on the Indy Racing League circuit. Nazareth, the one-mile rollercoaster that will host this weekend's inaugural Firestone Indy 225, is an archetypal short track. The 2.5-mile Indianapolis Motor Speedway, the hallowed home of next month's Indianapolis 500, is the grandest of superspeedways. These distinctive ovals demand radically different aerodynamic packages.

The art and science of aerodynamics is paramount in IRL competition. With a sculpted carbon-fiber body bristling with wings and aerodynamic devices, an Indy car is essentially an airplane that flies inverted at 230 mph, just inches above the ground.

"An IRL race car is shaped and tuned to make maximum use of ground effect," said Kevin Bayless, a chassis and aerodynamics consultant for GM Racing who works closely with Chevrolet teams. "Ground effect is the interaction between the moving vehicle and the stationary track surface. The underside of the sidepods forms a venturi between the car and the track. The air beneath the sidepod accelerates as it moves through the venturi, creating a low pressure area. The difference in pressure between the upper and lower surfaces of the sidepod produces downforce, or negative lift."

Downforce is the Holy Grail of auto racing because it dramatically increases traction by forcing the tires against the pavement. Increasing the vertical force on the tires with aerodynamic loads increases the amount of lateral force they can produce. More grip means faster cornering speeds and quicker lap times. An Indy car produces so much downforce at speed that it could literally drive upside down on the ceiling of a tunnel.

"The downside of downforce is that it almost always comes with the penalty of drag," Bayless explained. "Drag is the fore-and-aft force that pushes back against the car and slows it down."

Drag is the resistance of a body moving through the air. For example, the push you feel when you put your hand outside of a moving car is drag. The term "coefficient of drag," or Cd, is a reflection of the aerodynamic efficiency of a body's shape. Using the same example, when you turn your palm toward the wind, the coefficient of drag is high; when you turn your hand sideways, the drag is reduced.

The proverbial barn door has a Cd around 1.1; a streamlined teardrop has a Cd in the range of 0.1. While the production Corvette coupe that paces IRL events is an aerodynamic standout with a 0.29 Cd, an Indy car's drag-producing wings and underbody can produce a Cd as high as 0.9 when adjusted for maximum downforce.

"The amount of downforce an IRL race car produces can be adjusted to suit various tracks," Bayless continued. "At a high-speed track like the Indianapolis Motor Speedway, the cars are trimmed out to produce 1,700 to 2,000 pounds of downforce with minimum drag. On the other had, at a one-mile track like Nazareth, the cars are set up to produce up to 4,000 pounds of downforce. On a short oval, the increase in corner speeds produced by the extra downforce more than compensates for the speed penalty produced by additional drag on the straights."

IRL teams must work within the tightly defined limits of the rulebook when fine-tuning their race cars' aerodynamics. At most venues, IRL officials specify minimum rear wing angles and require wicker bills (perpendicular flaps at the trailing edge of the wings) to control speeds and to promote the wheel-to-wheel racing that has become a trademark of the series.

An Indy car's open wheels also produce significant aero forces, according to Bayless.

"A rotating cylinder like a tire and wheel combination creates both drag and lift in a moving air stream," he noted. "The spinning tire has very high velocity at the top and piles up air at the bottom, two conditions which produce lift. There also is tremendous turbulence behind the tire that creates drag.

"The turbulence produced by the open wheels disrupts the airflow around the rest of the car, affecting the aerodynamic performance of the sidepods and wings," he added. "The ramps in front of the rear wheels are designed to deflect the air to minimize these lift and drag effects. Of course, the ramps are also regulated by the IRL rules."

The complexity of the air movement around and through an Indy car makes wind tunnel testing a valuable tool. GM Racing engineers have tested a variety of race cars in the GM Aerodynamics Laboratory, including IRL cars.

The winds of change blow at hurricane force in the GM Aero Lab. Driven by a 4,000-horsepower electric motor and pushed by a 43-foot propeller, the wind is on call 24 hours a day to provide data on aerodynamic forces. The first full-scale automotive wind tunnel constructed in North America, the GM Aero Lab resembles an enclosed oval track, with air circulating through a closed loop at wind speeds up to 130 mph.

"The interactions between the car and the wheels, the radiator, the exhaust, the wings and all of the rest make it virtually impossible to predict the result of an aerodynamic modification," Bayless noted. "The wind tunnel allows you to evaluate accurately the effects of a change."

Just as F-16 fighter planes have replaced the biplanes of bygone days, the winged warriors of the IRL have made flying on the ground a science.

-gm-