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.
Testing Your Aero Efficiency
This wing is set at a high...
This wing is set at a high angle of attack. Note the panels are offset in height from the right to left sides. It is the intent of the lower right side panel to allow the upper portion of the wing to receive air as the car slides somewhat sideways in the turns. What is forgotten is that in order for the wing to work, the air must flow efficiently over the top and bottom of the wing. The panel obstructs the flow under the wing where the lift is created. Furthermore, obstructions under the wing such as the flow off of the front wing, rollbars in close proximity to the wing, and the braces all decrease the usefulness of the wing. Its real function is often as a flat panel device rather than a wing. And as a high drag, high CG component that causes a high amount of load transfer.
Clean the car and the wings very well. Tape 2- to 3-inch pieces of yarn about every 10 inches all over the top and bottom of both wings and take video or still photos of the yarn at different speeds and different slide angles. If the yarn is waving all over the place and is not against the surface of the wing, it is stalled. When it is stalled, it's making less downforce and much more drag.
Get the wing angle down until the yarn is stable and against the wing surface. It should not be wiggling more than 1/4-inch. Watch what happens to the yarn as you start to slide the car versus going straight.
The endplates on Sprint Cars are primarily advertising space. They add very little to the efficiency of the wing. You may not be able to even measure the additional efficiency. The vertical endplate stiffeners should be blended as best as possible and should all be on the left side of the endplate to give the best drag reduction. The endplates act more as rudders in the turns and reduce downforce.
Notice that the right panel...
Notice that the right panel on this car extends 6-8 inches above the top of the wing and is set at a high angle of attack. The close proximity of the front wing to the rear wing causes turbulence in the flow of air over the big wing and reduces its efficiency.
In the turns, the aerodynamics of a Sprint Car make a radical change. The exception to this are with those who drive like Danny Lasoski usually does. If a car tracks through the turn or has a very small drift angle, the aerodynamics stay pretty close to the same as they are on the straights. If you throw the car into a slide, something very different happens.
As the car goes into a slide, the outside endplate blankets a section of the wing at an angle, reducing its downforce. There is more reduction in downforce at the rear of the wing than the front. As the slide angle increases, the downforce reduces more and the side plate acts more like a rudder.
On the left side of the wing, the endplate increases pressure on itself, acting more like a rudder and it increases pressure on the wing surface some, adding more downforce. As the slide angle in the turn increases, the more the downforce decreases, and the rudder force increases. The car is going through a transition from cornering with traction, to cornering with rudder force and reduced traction. The rudder force, or leaning on the side plates are holding you in the turn, but you are losing traction.
The more you lose traction, the harder it will be to get a good drive off the corner. If you slide a car in smoothly, like Dave Blaney does, the transition will be easier to manage. If you throw the car into the slide, the transition will be radical and upset the car. If you can anticipate the violent upset, you will be OK. The smoother the upset, the faster you should be. The less the slide angle, the more traction you will have.
Load Transfer Due To Drag
This front wing in a Micro-Sprint...
This front wing in a Micro-Sprint Car is set at a very high angle of attack. The rounded nose of the car won't direct the air the car drives through upward, but instead outward. So, the relative angle of attack is around 45 degrees, an amount that is sure to stall. There may be some flat panel effect, but compared to what a working wing would provide in the form of downforce, this design is not much help. Its close proximity to the hood reduces its effect even if the angle of attack is correct.
Bob Bolles and I have talked in length about something else going on here. The wing is stalled, creating a huge amount of drag. This drag is centered far above the car. There is a leverage arm from the ground to the center of the drag force. This drag causes load transfer to the back and is shifting its force causing what appears to be downforce on the rear wheels.
It is not airfoil induced, but drag induced from load transfer. It takes a tremendous amount of wasted horse power to make this work. This force is also trying to lift the front wheels off the ground. This force goes up with the square of your speed, meaning if you double your speed, you have four times the drag and a similar increase in load transfer.
The stalled wing also has other affects. It creates a large force trying to stop the car all the way around the track. When you lift the throttle the car has a tremendous stopping force. This stopping force may make up for the lack of braking power needed to enter the turn. The problem is it eats up your horsepower everywhere else. It could be using up more than 30 percent of your power. The drag also makes your speed more constant all the way around the track, constantly slower.
If you clean up the angles of the wings, which will take some testing, you will get more downforce and less drag. You will get a much higher straightaway speed if you do this. My question is; if you make the aerodynamics more correct, will you have the stopping power to make the turn? You may not. On the other hand, if it puts you in front on the straight, the other racers have to find a way around you.
Instead of being adjustable...
Instead of being adjustable for angle, most Sprint Car wings are adjustable for fore and aft location. This adjuster moves the wing back and to a slightly higher angle. The idea is to move the center of force rearward as the track changes toward slick conditions. It might be of more use to cause a change of angle adjustment. High angles could be used for dry and slick conditions to promote high drag and cause more load transfer to the rear for more bite. Lower angles that are more efficient for downforce could be employed on tighter tracks for more overall traction. This is something that has not been considered before, but it is something you might think about.
I raced non-winged Sprint Cars and entered some WoO events without a wing. The car was radically different, and had four-wheel independent suspension. I built a hood that gave quite a bit of downforce with minimal drag. It wasn't as fast as a winged car, but it wasn't as different as you may think. I am not saying a Sprint Car shouldn't have wings. I am trying to show you that what the wings are doing is probably not what you think they are doing. My opinion is the racing is better without them.
As with most aeronautical problems, with both race cars and airplanes, there is a whole lot more going on than initially meets the eye. The more you evaluate the problem, the more difficult the problem gets. In this case, a flat sheet of metal or a piece of plywood would work about the same as the wings on a Sprint Car. So, why go to the trouble of having a bottom surface of the wing other than for structural reasons?
Wind Tunnel Errors and Why
Wind tunnels will not accurately tell us how our wings are working. That is because the air in a wind tunnel is charged with energy from the simple fact that it is in motion and the walls and the ceiling are too close to the car. In most wind tunnels, the air is moving in excess of 60 mph. This means that the air possesses energy and air in motion will desire to stay in motion greatly affecting its physical movement as it flows around the car and wings.
In contrast to the Sprint...
In contrast to the Sprint Car wing, the wings on most other types of race cars including this Grand Am Daytona Prototype car, are set high off the body and at a relatively low angle, around 10 to 14 degrees. It is known that an angle of attack greater than 16 degrees on these cars will produce a stall and high drag numbers. Compare that to a Sprint Car wing that is set at 25 to 35 degrees of angle.
In contrast, when we drive a Sprint Car into still air, that air has little or no energy. It is able to be pushed aside more easily and can move more quickly to fill the vacuum created behind objects such as the wings and the car itself.
It is this vast difference in the dynamics of still air verses moving air that causes errors in the evaluations in a wind tunnel. A more accurate way of evaluating your wings would be to do on-track testing utilizing the yarn tuft method I previously talked about and observe using a video recording.
Think about how your wing works and how you drive your Sprint Car. Top teams may have an edge here and have already re-evaluated their wing angles and driving attitudes. Watch the top teams as they race. If they crank angle out of the wings, they might just be looking for more, not less, downforce for tacky track conditions.
Higher wing angles might provide the needed bite on dry and slick tracks from more drag induced load transfer to the rear and that provides more acceleration that may be needed over top speed. With all of this new knowledge, you might need to re-evaluate your strategies concerning wing angles and placement. I hope so.
The sideboards on a Sprint...
The sideboards on a Sprint Car wing can obstruct the oncoming air when the car is traveling at an angle to the direction of travel. This sideways slide reduces the efficiency of the wing and also moves the center of force forward. Attempts are made to reduce this effect by lowering the right side panel, but it still sticks up 4-6 inches or more. That is enough to disturb the airflow and reduce what little downforce exists. This chart shows the area that is affected by the sideboard. Remember that both sides of the wing are affected by the board, so lowering the right side board only helps with the "flat panel' downforce, not wing-induced downforce because the underside is important also and is highly obstructed by the side panel.
This car is running at approximately...
This car is running at approximately a 20 degree angle, so the obstruction chart applies to this attitude. Granted, this is not an extreme angle of slide for a Sprint Car, but enough to matter. Imagine a car sliding at a 45 degree angle and what that would do to the wing efficiency. The more straight ahead you can run, the more efficient the wing will be.
The most useful aero testing...
The most useful aero testing is done on-track. Because we drive through still air as opposed to moving air flowing over our cars, wind tunnels might not tell us all there is to know about aero efficiency. Moving air has energy whereas still air does not. That makes a huge difference in how the air reacts to the surfaces on our race cars. Here, we see pre-prepared lines of yarn strips attached to a car for evaluation. On our Sprint Car, we could attach these onto the wing and body panels and take video using a GoPro camera or similar video source to see how the air is flowing over those surfaces. We could learn a lot about how everything works aero wise.