The design of the wings on...
The design of the wings on a Sprint Car leave much to be desired. Their function could be much different than we think it is. We will study how wing aero works and how it applies to a Sprint Car.
In an article I wrote in the Aug. '09 issue about race car aerodynamics, I basically said that wings on a Sprint Car are a joke. I say that because what you may think the wings are doing, and what they are actually doing, is probably not the same. The way the wings are mounted on the car, limits their aerodynamic efficiency to about the same as a piece of plywood. I can't explain everything in one article, but I will squeeze in as much as I can.
World of Outlaws rules limit the shape of the top and bottom of the airfoil to the point that they only act as an airfoil when at shallow angles of attack. The rules make it a very poor airfoil at best. By shallow angle of attack, I mean probably in the range of 10 to 14 degrees. Any damage to the airfoil or when dirt buildup occurs limits their efficiency further. Once the efficient angle of attack is exceeded, the drag numbers increase dramatically.
Drag Related to Angle of Attack
The greater the angle of attack, the higher the drag goes. It takes lots of power to overcome this drag. That power could instead be used for acceleration and top speed. The true angle of a wing relates to an imaginary line from the center of the leading edge through the center of the trailing edge, not including any wicker bill. The center of the downforce is far forward with this wing design, but if a wicker bill is added, the center of force moves rearward some.
The chord line of any wing...
The chord line of any wing is a line drawn between the middle of the leading edge and the rear tip of the wing, not including any wicker bills or other additions. The angle of attack is the angle of the chord relative to the direction of the oncoming air, which is influenced by the body panels. If the air is redirected to follow the front hood for example, then the angle of attack would be the angle formed between the chord line of the front wing and the direction of flow of the air. For most designs of Sprint Car hoods, this flow direction is almost parallel to the ground.
On an airplane the wing changes the angle that the air meets the horizontal tail. This is called downwash. Small changes of angle of attack of the horizontal tail also change the angle of downwash that the wing creates. The combination must be tuned for best performance. The same in reverse happens with race cars. The body panels also change the downwash.
A Sprint Car has radically different aerodynamics on the straight, compared to the turns. The affect varies depending on your driving style. Changing your driving style during a race will change the aerodynamics even further. This is due to the sideways attitude of the car relative to the direction of travel and occurs in varying degrees while in the turns. This is also one of the issues of angle of attack.
I will talk about the straight first. Remember always that the flow over the front of the car affects the back of the car and the flow over the back of the car affects the flow over the front. Any change to one, will change the other. The front wing makes some downforce and lots of drag. The force it makes is primarily flat plate, not aerodynamic. This is because the airflow efficiency under the wing is very poor. Depending on the angle at which you set the front wing, you can choke the flow between the hood and the wing. If this angle is set incorrectly, (as in too shallow relative to the hood) you increase drag and decrease downforce.
It is an important aspect...
It is an important aspect of wings that drag never goes away as the angle of attack increases contrary to the property of lift, or downforce in the case of the Sprint Car wing. We see in the chart that as the angle of attack is increased, the lift increases to a point. Once the angle becomes critical, lift falls off dramatically and drag continues to increase. High wing angles many times cause very little lift and a lot of drag. This contributes to high amounts of load transfer from front to rear and is often mistaken for added downforce. Load transfer to the rear tires does increase traction in the rear to promote bite off the corners under acceleration. This is useful for dry and slick conditions, but don't mistake downforce for load transfer.
If there is any obstruction behind the wing within about 15 feet, the obstruction will dramatically decrease the effect of the front wing. The air cleaner box does an excellent job of screwing up any possible efficiency the front wing may make. The air cleaner box could be significantly improved as I did back in 1980.
If the wing came out of the sides of the hood, you would reduce the drag and have the same downforce. The front wing changes the flow to the rear wing in an upward direction. So the rear wing does not see undisturbed air, but turbulent air in an upward direction. The airbox also disturbs the air to the rear wing. The actual angle of attack of the rear wing is something less than its angle to the ground because of the turbulence.
The angle of attack also changes as the rear of the car squats and the front rises under acceleration. The squat angle reduces downforce. The angle of both wings will cause a huge amount of drag. This drag results in very turbulent air behind the car and a lot of suction. If you are directly behind another car, the decrease in downforce from this turbulence will be severe, and the suction of the front car will pull you forward. This draft will pull you forward while at the same time reduce traction.
The rear wing is set at such a radical angle, that it is stalled. When an airfoil stalls, the downforce reduces dramatically and the drag increases dramatically. It still makes downforce, but it makes significantly less than it would if you reduced the angle of attack. You could test all this at a minimal cost.