This Daytona Prototype is...
This Daytona Prototype is evaluated in the wind tunnel. The data gathered can give indications of aero efficiency, but not exactly in the way it happens on the racetrack. We'll explore why the two don't necessarily correlate. -Bob Bolles
For a better understanding of aerodynamic testing, we asked an expert to provide us with some basic, but valuable information on the subject. The author is directly involved in using his knowledge of aerodynamics in the design and manufacturing of airplanes. His methodology must work or the plane will not fly. If we get it wrong, our race car is less efficient. If he gets it wrong, the plane crashes. We trust his approach to this because we know he has designed airplanes that fly very well.
Aerodynamics start to have a more noticeable affect on a vehicle at around 50 mph. If you're traveling slower than 50 mph, the weight of the aerodynamic devices are probably more of a penalty than any perceived gain in performance. Downforce and drag values go up roughly with the square of the increase in speed and the power required to overcome the drag forces goes up at a slightly steeper rate.
3-D computer modeling of aero effects is somewhat as good as the programmer, but not totally believable. Scale models tested in smaller wind tunnels give less accurate data. Efficiency is affected by poor fit, surface roughness, waviness and other disturbances. Most aero engineers ignore efficiency and are real proud of the coefficient of lift and drag. This line of thinking will steer you in the wrong direction when dealing with real world aerodynamics.
As airflow separates from the surface of the vehicle, drag will go up at an increased rate beyond its normal drag curve, and lift will go down beyond its normal curve. The drivetrain shapes, tire friction, tire pressures, tire heat, racetrack surface irregularity, toe angle, front camber angle, rear alignment, rear toe, and rear camber all affect the aerodynamic efficiency for a race car and with that the horsepower needed to overcome drag. It's easier to get a five horsepower gain in drag reduction than it is to squeeze 5 more horsepower out of the engine.
When cars run at high speeds,...
When cars run at high speeds, such as at Daytona, the drafting plays a significant role in how the car performs. The influence of the car pushing relatively still air out of the way extends far beyond the sides and top of the car. The "hole" left behind the car is felt as far back as four to five car lengths or more. -Bob Bolles
For these reasons, drag and horsepower calculations for cars are not comparable to conventional equations that are intended for the design of aircraft. If the data we get out of a test facility has to be manipulated, then it can be considered inaccurate data. If you manipulate the airflow, then the data is also inaccurate.
The wind tunnel is designed to move air into and around a stationary vehicle. Keep in mind that we don't race in 120-185 mph winds, we actually race at 120-185 mph through relatively still air. It's a different set of dynamics between the two conditions. The only thing we can hope for, as a NASA engineer once alluded to, is to find tendencies, not exact data.
The Energy Level of Moving Air
Air moving through a wind tunnel has a significant amount of energy whereas still air on a racetrack or on the road has virtually none. One pound of air displaces about 13.07 cubic feet of volume at sea level. If one pound of air is traveling 75 mph in a wind tunnel, it would have 110 pounds of inertia. There is approximately 20 pounds of air contained in the volume of the race car. That equates to 2,200 pounds of total inertia.
Each molecule of air has a lot of force trying to keep it going in the flow direction. It will take a lot of force to change its direction and once you do change its direction, it will carry a lot of force trying to keep it going in the new direction. Compress that high-energy air between the car and the walls of the wind tunnel and you introduce more variables for which you can account.