This Daytona Prototype is evaluated in the wind tunnel. The data gathered can give indicat
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, such as at Daytona, the drafting plays a significant role in
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.
In this comparison, the 20 car is situated lower and more level than the 83 car. The lower
The Influence of Aero Beyond The Car
When you were racing in still air, each pound of air would have 1 pound of inertia keeping it there. It's therefore easy to deflect that air, and it compresses easily. A car drives through that still air at a high speed and disturbs the air in several directions. Then, the rear of the car sucks the air back toward where it was to begin with. The perfect shape for a race car would move the air out of the way with the least amount of force, and then allow the air to flow back gently, undisturbed. This can't be done realistically, but it's still your goal.
Remember when Richard Petty started the Daytona 500 a few years ago and his hat flew off? He was about 20 or so feet from the race cars, yet the cars were dramatically upsetting the airflow enough to blow his hat off. If the walls of a wind tunnel are too close, they will negatively affect your wind tunnel data. The closer they are to the car, the more inaccurate the data is. If you put a vehicle in a wind tunnel, you restrain that airflow disturbance and actually artificially reattach the airflow back onto the vehicle.
Most teams will test on the racetrack. With sophisticated data systems, we can gain aero d
The data will show a higher downforce and higher drag, because you're compressing the air between the vehicle and the walls of the wind tunnel. This can also lower drag in some areas because you artificially reattached the flow onto the vehicle. One doesn't cancel out the other. You must manipulate all of the numbers from the wind tunnel data to have it make sense. Because you are putting a lot of inertia and energy into the air in a wind tunnel, the air doesn't behave the same way as it does in the real world. If it did, we wouldn't need test pilots.
When you see a smoke wand used in a wind tunnel, you see smoke roughly following the contour of the vehicle. The farther the smoke trail is off the surface of the vehicle when the wand is placed near the surface, the more airflow separation exists. This indicates more drag and less downforce. You want the smoke to lay on the vehicle from front to rear. The farther the smoke is off the car, the greater the flow separation and the greater the problem.
This car is fitted with tubing that runs from small holes drilled in the body to pressure
Changes to the rear flow of air will change the front flow characteristics and changes to the front flow will change the rear flow. Both drag and downforce are affected in this way. Any device you install across the airflow at the front of the car will separate the flow and significantly reduce good aerodynamic effects on the rear, as well as add drag. Never disturb the airflow at the front of the car.
Different Types of Airflow
Turbulent flow is where airflow boils and rolls in many different directions as it flows over the surface. This may be over the entire surface, or part of it. The less turbulent flow you have, the less drag you'll have. Most current race cars have turbulent airflow around them. The worst turbulence I ever saw was flight testing a certain low-wing aircraft. I saw some airflow actually going in the opposite direction of the aircraft, that means going forward!
Attached flow will significantly reduce drag and increase downforce or lift. If you get it attached over the entire surface, the drag reduction will really shock you. Usually, attached flow occurs over less than 10 to 25 percent of the front surface area. The more you get, the better you are. Attached flow is typically misinterpreted as laminar flow. The two are different.
If anyone talks to you about achieving laminar flow on a race car, he is misinformed. The only exception is with a very few Formula car wings. Those cars only achieve laminar flow over a very small part of the surface. Laminar flow is a very small, as in a few thousandths of an inch, layer of airflow that acts like ball bearings, further reducing drag.
By attaching yarn strips, we can visually see how the flow of air is progressing at the su
When you look up a laminar airfoil from, say, NASA to use it on a race car, odds are about 99 percent you won't actually achieve the laminar flow. You really don't want a laminar flow airfoil on a race car, because slight imperfections, dirt, rain, humidity can dramatically and suddenly reduce its performance. The surface must be clean, smooth and exceptionally accurate. If you put a pinstripe on some laminar airfoils, they will not make any lift or downforce. Laminar flow doesn't exist on any race cars in the U.S.
As a vehicle pushes through air, it creates an area of high pressure in front of it. This is called a bow wave. The bow wave on a race car will be somewhere between 10 to 20 feet in front of the car. This means that the car is affecting and changing the airflow that far in front of the car. The steeper the angle of the front of the car to the ground, the farther out in front of the car the bow wave will be. The farther out in front it is, the more drag the car will have.
On many race cars, there are splitters attached to the front bodywork that act to split th
If the bow wave is strong enough, it will detach the flow completely over the rest of the car, at the point where the hood blends into a more horizontal surface. As it detaches higher, the airflow is actually causing suction or lift, counteracting the downforce you thought you were creating. The two probably can't cancel each other completely, but it can significantly reduce downforce and increase drag.
Separation of flow at the front of the car will also reduce the downforce created at the rear. This is because the air of normal pressure can't get back down to the level of the rear spoiler. Now, up to a certain point, increasing the angle of the spoiler will reattach some of the flow in front of it. If a car is sliding at some angle to its actual direction of travel, as in a dirt car, the sides of the car will detach some or all of the downforce flow. This is due to the abrupt change in direction of the oncoming air up over the side of the vehicle and it can't reattach to the top of the car. You'll also reduce some downforce because high pressure air on the side of the car will get under it, causing lift, unless you can seal the side to the ground.
The Sprint Car wing could be one of the most misused aero components in racing today. The
Drag and Moment Arm
Surfaces of the car that create downforce also have a moment arm. That's the distance from the wheel to the force. With a spoiler far back on the car, you could change downforce on the rear and it will affect the downforce on the front in the opposite direction. The height of the spoiler or airfoil affects the length of the moment arm. A Sprint Car wing has the most radical moment arm of any aerodynamic surface used in racing.
The bottom of the car is also important. Air does flow under there and it's usually under some suction force, so it has much less affect, but things can be done here to gain speed. Lining everything up will give you an improvement over certain designs, but yet more can be had. Look at how flows are detached under the car. You have to know what to look for, and not everyone sees it. There are ways of obtaining more downforce and less drag under the car. Look also at what the underside is doing under roll, squat, and dive. Look at how these conditions disturb the airflow and downforce. The more suction you produce, the more air wants to go under the car.
Homemade Wind Tunnel
You can make your own wind tunnel to get real data. It takes some good cameras, some small diameter yarn, and tape. Ideally, paint you car white, and do a good job with minimal overspray or "orange peel." A bad paintjob will give negative effects on airflow. Tape a 2- to 3-inch length of black yarn every 6 inches or so all over the car, including the sides--just eyeball it. Make sure the tape is put down smoothly so it doesn't disturb the airflow over the yarn.
The rear wing angle is critical. This isn't a good example of a functioning wing. It does
Drive the car at different speeds and take video or high speed still shots as the car goes by, or from inside the car. Don't put the camera where it will disturb the flow. Maintain a clean surface free of dirt. You can video from other cars along side, both close to and farther away. Look at how the yarn changes with another car in certain positions relative to your car. Different cars will affect your airflow a little differently, but generally in the same way.
Ideally, every yarn will lay flat on the surface and point straight back to the rear of the car. High pressure areas will do one or all of the following: lift the yarn, flutter the yarn violently, or point it off to one side. If you see these characteristics, you may be making more drag and less downforce than optimal.
Low pressure may do one or all of the following: lift the yarn in a more limp action, cause more of a limp flutter, and/or keep the yarn moving all over the place. These could be areas of lift. These indicators show more drag than optimal. How much affect do you get from getting it right? From what I see on a Dirt Late Model, you may be able to cut the drag in half. I accomplished attached flow on a 180hp aircraft and nearly doubled the cruise speed, with everything else being the same.
NACA ducts (short for National Advisory Committee for Aeronautics) offer the advantage of
Stall Angles of Spoilers and Wings
These are critical devices that need very small angular changes of 1 or 2 degrees at a time to see the results. Most airfoils stall at around 22 degrees. They still create lift or downforce beyond that angle, but it will make less than if you reduced the angle. As you approach stall or go past it, drag will go up considerably almost immediately. The gurney flap, wicker bill, spoiler strip, or whatever you want to call it can add downforce, but will also add drag.
Never position any of these devices angled forward beyond vertical. As the device points forward of vertical, you increase the stall effects negatively, and shorten the surface the force is generated through, and add a little lift. The steeper angles at which any of these devices are set will also move the downforce rearward, until stall. At stall, the downforce will move forward some.
Scat tubing is widely used to route air to brakes and other components for cooling. It doe
Interference drag is drag caused by compressing air with a disturbance protruding out of a surface that air flows over or around. The more the surface is flat, the greater the interference drag. The cross-sectional shape of the protrusion can also dramatically affect drag. A round object, like a front axle on a Sprint Car, has five times its diameter in equivalent drag. You don't need to know just what that means as long as you understand that a round tube placed at 90 degrees to airflow has huge drag. If you shape the same tube like a wing strut on an airplane, you'll see huge gains in reduction of drag. Most Formula cars incorporate this design aspect.
Make the components really aero-dynamic and drag will go down at an amazing rate. I did extensive flight testing on a 100mph aircraft with wing struts. By only changing the shape of the wing strut I discovered the following through observing actual flight test data: A design with a round tube strut yielded a 100mph top speed. Using a standard aircraft strut yielded a 127mph top speed. Using an optimized aerodynamic shape for the strut of my own design yielded a 136mph top speed.
There are millions of different designs of airfoils out there and they all do something different. For optimum performance in a race car, we want high downforce with minimal drag--not too different from an airplane--but there are other differences in airfoils. They are: the stall angle, max lift over drag angle, pitching moment, and the percentage of airfoil to which the flow is attached. Pitching moment means where the net lift or downforce occurs relative to the front of the airfoil.
In this application, sections of hard, smooth tubing could have been used to route the air
Plates increase the downforce somewhat while increasing drag and also provide sponsorship surface. As a vehicle starts to slide sideways, endplates will disturb the flow on the outside of the airfoil or spoiler and increase downforce on the inside. You'll lose some forward bite due to downforce reduction in the slide. Sprint Car endplates are ridiculous as an aid to the wing. They act more like a rudder or a barn door.
They cause the inside lean, or reverse roll angle and they're not allowing the car to perform its best way. The wings on Sprint Cars are set at angles way past stall. The added drag rotates around the contact point of the rear tire and the ground, causing a moment that transfers load from the front wheels to the rear wheels trying to lift the front wheels.
The front wing is also stalled, but in addition to the stall, the close proximity of the hood interferes with the predominant surface doing the work. A piece of plywood would work just as well as the wings on a Sprint Car, partly because of their regulated airfoil parameters and partly due to the stall angle. Endplates help more on formula cars. A properly designed wing sees zero effect from an endplate.
NACA Ducts for bringing air into the driver's compartment
These need to be placed somewhere where the yarn tufts are attached and straight. If they aren't, you may be placing these in a suction area and will get little flow, if any at all.
These should have a short vertical height and be as wide as possible for minimal drag. Their actual shape will change overall drag, and depending on how they are attached, can make lots of drag.
Scat tubing used for brake cooling and other things has its benefits. The benefits are bendability, and flexibility. At lower velocities they work pretty well. At higher velocities though, the effective diameter is much smaller than the measured diameter, significantly reducing airflow. You should use minimal scat tubing to transport air. Make ducts that have smooth insides, with blended curves. Keep the air velocity low.
The more smoothly surfaces are blended, the better chance you'll have for drag reduction and better downforce. Don't think that steep surfaces make more downforce, because they don't all the time. Most cars today have gone way beyond the best angle, and more is not always better. On the other hand, significant drag reduction and downforce improvement will require testing and optimum chassis set ups. Gains may require a slightly different driving style.
On dirt cars, there may not be enough braking force and traction to slow enough for the corner when you get the aero right. In fact, this may be the unintentional reason that the aerodynamics are stalled. Right now, many types of race cars are wasting lots of horsepower overcoming high wasteful drag. A Sprint Car on a quarter-mile, relatively slow track, probably won't see a gain here, but on longer tracks and with all other cars, there'll be a gain if you pay attention to what you're doing.
Every type of race car can achieve gains in lap times from paying close attention to the above issues. These includes Formula 1, Cup, Late Models, Sprint Cars, IndyCars, Grand Am Daytona Prototypes, and Sports Cars. Don't assume that because another team has the funds and resources to test that it has it right. The other team may just have it more right than you do.