A New Way to Utilize Air Forces
In race car aero design, one often overlooked component of aero down-force is what is called Flat Plate Aero. On this Outlaw Late Model, the front nose is angled down at about a 45 degree angle to create Flat Plate down-force. We’ll explain how that works.
This is not your average, run of the wind tunnel aero story. Nope, this one you’ve most certainly never heard before. It is sure to shake up a few individuals in the industry who engage in the promotion of wind tunnels and extol the benefits thereof. This is for you, the racer, not the professors or proponents of the more conventional aero technology. It will make sense and you’ll see my point unless you have an agenda that unfairly contradicts this point of view.
First off, these are not my thoughts or conclusions, far from it, although I have come to completely agree with them. This comes from persons deep within the aero design and engineering industry who rather than teach the properties of aero, live the properties of aero and there is a marked difference between the two.
When you design an airplane that you will ultimately have to fly, you must make sure you understand the subject of aerodynamic engineering. The classroom or cozy control room in a wind tunnel presents little risk and there is nothing to prove the results wrong, until you “fly” the airplane or race car.
A senior NASA scientist once told a friend of mine that wind tunnels give you tendencies and probabilities’ only, and that is why we have test pilots. Not one airplane ever designed and “flown” in a wind tunnel ever flew in test flights the exact same way. It’s the same with race cars. Otherwise, every multimillion dollar F1 team would have the perfect aero properties in their cars design, and we know for a fact they don’t.
Why Wind Tunnels Have Errors – The whole premise of a wind tunnel is to move air at a given speed into and over an object to measure lift, down-force, drag, etc. In a wind tunnel, the air is accelerated to some predetermined speed and the stationary object is bombarded by the oncoming, energized air that has a lot of momentum. This is not what happens in nature with airplanes and race cars. They travel through relatively still air that has no energy or momentum.
Many aero engineers will tell you, “Well, it’s the same thing.” No it is not. Air moving at 60-100mph has a lot of energy and does not want to deviate from its path. When this energized air hits an object, the way it moves around that object is much different than if that object were moving through still air at the same speed. It has to.
Most veteran aero engineers, if they are honest, will tell you that the primary problem with wind tunnel data is converting it to reality. There exists very expensive Computational Fluid Dynamics software programs that attempt to duplicate the wind tunnel data and/or make it more real world. The conversion is very complicated and obviously not perfectly accurate. And again, that’s why we have race tracks and test pilots.
The other thing that causes errors in the data we get from a wind tunnel test is the close proximity of the walls. Most wind tunnels are too small because it is cost prohibitive to build it big enough so that the walls don’t influence the results. Air is disturbed well beyond the distance to the walls in most wind tunnels when using full scale models. The presence of the walls restricts the movement of the surrounding air and causes a compression which alters the results.
The only “wind” tunnel that comes close to reality is the one Chip Ganassi is said to own called Laurel Hill in Pennsylvania. It is actually a tunnel through a mountain. There a test vehicle is run for a mile through the tunnel, through still air, and the pressure distribution that produces drag and down-force is measured. At least the test object is traveling through the air and not the other way around, like an airplane or race car really does. While it still has walls that are too close, the results are much closer to reality.
The ideal aero testing for race cars would be done on a smooth flat surface outdoors at speeds that replicate the speeds the car would experience on a race track. Then the load and pressure sensors would record the aero influence on the car, much like test flights do with airplanes.
An airplane flies due to lift, but one seldom talked about component of lift is Flat Plate aero affect. It is the lift provided by the flatter underside of the wing moving through air at a slight angle that counts for about 25% of the lifting component. The low pressure on the upper side, which can be explained by understanding Bernoulli’s principle, reacts with the higher atmospheric pressure on the underside to provide the other 75% or so of lift. We can utilize the FP aero affect alone on our race cars where Bernoulli’s does not exist.
That Being Said – Now that we have that out of the way, let’s get into how aero really works. The age old depiction of an airplane wing, and one that I have used, is not perfectly correct. Yes, the air traveling over the airplane wing travels farther and faster and thus has less pressure than the air traveling under the wing. This is the Bernoulli principle and it provides lift. But that’s not the only component of wing lift.
Most every plane cruises with a wing attitude where the underside of the wing is at a small angle (the front higher than the back) to the air it is moving through. So, it acts much like a water ski to assist in holding up the airplane. This is called Flat Plate (FP) aero.
Many aerodynamicists tend to discount this affect, but it can be 20-25%, or more, of the total lift component. I think it is more, but without real world testing, I’ll stay with the published data. This is where we get into an important area of race car aero design.
In a race car, we may have flat plate areas where we can develop down-force to increase the grip of the tires. That is called FP aero down-force, but what about other areas of the car that are flat? What about the sides? We’ll get into that in a minute.
Down-force From Flat Plate – The sloped front of an Outlaw Late Model, the nose and hood on a ‘70’s era super late model, the nose on a modern dirt late model and the wing on a sprint car all offer FP aero down-force possibilities.
One of the early proving grounds for short track racing was the annual Daytona Speedweeks races at New Smyrna Speedway. Back in the late 1970’s and early ‘80’s, teams from all over the country would show up with “stock” bodied super late model cars that adhered to the strict rules of their perspective sanctioning bodies only to discover that there were no body rules at NSS. The local teams already had their car bodies tricked up.
In the later 1970’s and early ‘80’s, at tracks that did not have strict body rules, such as New Smyrna Speedway, the smart teams utilized Flat Plate aero to provide lots of down-force on the front and rear (note the huge whale tail spoiler). The roof panel was even angled to provide a surface for FP aero down-force.
Those who remember noticed that after about the second or third night of racing in the nine night series, most of the cars had been transformed into sleek, wedge shaped oddities with huge fantail rear spoilers attached. This is what we now understand to be an effective use of FP down-force.
With each of the examples, we may, or may not, have a low pressure on the other side of the FP, but the FP still generates its own force. The angle is critical, just as it has proven to be with airplane design, but up to a certain angle of about 15-20 degrees, a lot of force can be developed.
As for the wing on a sprint car, most of the down-force from a sprint car wing is FP derived from air flowing over the top flat plate. It acts much more like an outlaw nose. The air going across the underside, while being curve shaped like a wing, is disturbed and does not provide the “lift” that we see on an airplane wing. And you can have too much angle in that wing. As the speed increases, the wing angle must be reduced in order to have maximum down-force.
Flat Side Lateral Force – The concept of FP aero can be turned 90 degrees to assist in helping the car to turn. If a race car has a relatively flat side on the side of the car to the outside of the turns, then if the car is run at an angle to the direction of travel, it can produce a force in a direction opposite to the centrifugal, or lateral, forces.
Cars where this can be a benefit are easy to pick out. Late Model dirt cars, dirt Modifieds, Outlaw late models are just a few. And, if we look at the large sideplates on a sprint car wing, we see where FP aero can be used if the car runs at just the right angle through the air.
For modern race cars, we can utilize the flat sides, like on this dirt Modified, to provide FP aero to help push the car to the inside of the turns using side-force. On this type of car, teams can also angle the roof mounting so that it pushed down for down-force.
In the case of the dirt Late Model, we know these teams rear-steer the car so that the rear is running outside the front tires. This puts the large flat side of the car at just the right angle to the air to produce FP forces that are opposite of the lateral forces. Not equal to, but opposite in some amount. The tires still have to do the rest of the work, but the car goes faster with this FP advantage.
It’s a similar situation with Sprint Cars. I have watched Outlaw sprint cars qualify at Volusia during Speedweeks at upwards of 135 MPH average. The tires cannot hold the car in the turns at that speed. There must be FP aero down-force and side-force helping out big time
Dirt Late Model cars have flat sides like the Modified, and utilize rear steer to cause the rear of the car to run outside the front end. This angle creates side-force to push the car towards the inside of the turns. The other side probably creates a suction, or low pressure to also pull the car towards the inside of the turns. The high angle and flatter surface of the hood also provides FP down-force to help the car turn.
Sprint Cars with wings are much faster than those without because of the FP down-force from the large flat upper surface of the wing. They also produce a lot of side-force from the flat panels on the wing pushing the car towards the inside of the turn. These two forces help these cars run much faster than those without wings.
This pavement Modified has a modern nose that is shaped to trap the air and keep it on the angled nose by utilizing raised ribs on each side. With no fenders on the front, low pressure down-force does not exist, so the teams have gotten creative and use any part of the car they can to capture FP down-force.
Spoiler Down-force Vs. Drag – When racers have run tests with their cars in a wind tunnel, they experiment with spoiler angles. Sometimes the results can be miss-interpreted. Here is what I mean. A more vertical spoiler will produce much more drag than one angled at say, 55 degrees.
The wind tunnel operator must compare changes in loading on each axle with the total loading of the car. So let’s say a vertical spoiler added 50 pounds to the rear axle, but removed 45 pounds from the front axle. This is only 5 pounds of added down-force combined with a lot more drag, and represents more of a displacement of loads due to the leverage of drag. If the total loading on the car does not increase, you have no gain. You might as well take weight from the front of the car to the rear and remove the spoiler.
With the 55 degree spoiler angle, we might see a 40 pound increase in the rear loading and a 15 pound decrease in the front loading due to the same leverage affect from drag, but in this case the drag is much less. In this example, we see a total increase in the vehicle loading of 25 pounds from actual aero FP down-force.
There is still the low pressure verses high pressure down-force that utilizes the Bernoulli principle. This can be a very significant source of added grip for the front of the car. Modern asphalt late model bodies are shaped to take advantage of this effect.
At the rear of certain bodied cars, we can create low pressure (Bernoulli principle) down-force by routing air out of the rear cavity under the rear deck. This gets tricky with current strict body rules, but most tech officials are looking elsewhere for “cheating.”
Traditional Low Pressure Down-force – In a more traditional sense of looking at aero down-force, there is the phenomenon of high pressure opposite low pressure. The shape of the body on our race car can help produce low pressure inside the body panels to create even more down-force. What we need to do is design our bodies so that we can direct some of the air we are driving through around the car in order to vacuum air out from within the area inside the body.
If we can use the swift flow of air that is flowing past the sides of the car to help vacuum air out of the engine compartment under the hood, we can lower the pressure along the underside of the hood, similar to the airplane wing. A higher pressure on one side of an object will push that object in the direction of the lower pressure, or high towards low.
To accomplish this, teams use wider, angled front noses that will direct the displaced air around the sides of the car past the wheel wells in such a way that a low pressure area is created just outside the wheels. Air is pulled out of the engine compartment to fill this “void” and the pressure under the hood is reduced.
The average atmospheric pressure at sea level is 14.7 pounds per square inch on all sides of an object, even on our bodies. If we reduce the pressure under the hood to 14.5 PSI, a drop of only 0.2 PSI, over an area of just a square yard we would generate about 260 pounds of down-force (0.20 X 36² = 259.2 pounds).
Rear Aero – At the rear of the car, we can manipulate the shape of the spoiler, the rear window posts and the body just in front of the rear wheel wells. By routing the air that is flowing past the sides of the car out and to the sides of the rear wheel wells, a similar suction effect takes place to create a low pressure area under the rear deck. There are obvious limits as to body shape in this area, but a little reshaping can help.
Spoiler angles have been a source of debate for some time now. What steep angled spoilers do for the most part is create high drag numbers. This transfers weight from the front tires to the rear tires from the cantilever effect. They produce little in the way of down-force. An angle of 55-60 degrees can reduce drag and produce down-force that will add to the total load of the car and help produce more overall grip.
Race tracks that are longer and have more banking require less down-force and would benefit from reduced drag. We can rethink how the air flows past the roof (green house area) and onto the rear spoiler. If we reshape the post that connects the rear window with the side window openings, we can direct air away from the spoiler and greatly reduce aero drag.
Conclusion – The key goals with stock car aero design and development is to create a body shape and running attitude that will provide more down-force and side-force to enhance turn speeds, produce less drag, and promote a more balanced race car. We can now see where and how we can utilize Flat Plate aero to increase down, as well as side, force.
Everything we do has to be done within certain limits. If we work hard to develop 600 pounds of down-force on the front end and the rear is not able to keep up with that high amount of grip, then the car will be loose and nobody can drive a loose car fast. So there are limits to how far we can go.
Work towards a good balance of front to rear aero down-force to help produce more overall grip. Do not overdo your efforts to help the car aerodynamically at the expense of handling efficiency. Make sure the basic chassis setup is balanced, and then the combination of both aero down-force and handling will enhance your on-track performance.