Understanding Airflow – Knowing a Few Principles of How Air Flows past a Car Can Help You Make Improvements That Really Work

Since we can’t see airflow, there is usually a lot of mystery surrounding how it acts on a car. Unfortunately, airflow does a lot of things that are not obvious unless you have studied the subject extensively. Even then, it takes reliable research tools to really determine what is going on because everything depends on everything else. Exploiting positive airflow interactions and mitigating negative airflow interactions are the most effective ways to improve aerodynamic performance.

To get a feel for how big aerodynamic forces are, stick your hand out of an open window while you are riding on the highway. Aerodynamic forces change in proportion to airspeed squared, so twice as much speed produces four times as much force. Air flows around nearly everything on the car, so a big percentage of the car affects its aerodynamic performance. It also means that practically everything has a chance to mess up the airflow to the rear wing.

Some regions of the car are far more responsive to aero modifications than others. Determining where those areas are is a great first step in aero development. You might get a head start on that by examining modifications that have been made to similar cars that usually run at the front of the pack.

THE BALANCED APPROACH

The percentage of aerodynamic downforce on each axle will modify the cornering balance resulting from the mechanical setup more as the car goes faster, so it becomes extremely important in high speed corners. If you make an aerodynamic downforce improvement to the rear of the car, you will have to come up with a change to the front of the car that provides about half as much downforce increase to keep the ratio of front to rear downforce the same. Unbalanced downforce probably will not make you faster. It will definitely make the car harder to use effectively.

Drivers cannot sense front and rear downforce independently. Rather, we sense aerodynamic balance and total downforce. This situation is like the difference between a shower with a temperature and flow rate knob compared to one with hot- and cold-water knobs. Water temperature is like aero balance and water flow rate is like downforce. Temperature matters more than flow rate, and aero balance matters more than downforce.

In a competitive road course setup, the percentage of aerodynamic downforce on the front axle is between 40 percent and 45 percent. It is preferable for this percentage to remain constant, but pitch and heave changes usually affect the front wing, air dam, or underbody flow enough to change it significantly. If your car has a flat bottom with a really effective diffuser, the downforce center can shift all over the car in the course of a lap, including laterally.

STALL

If you have taken more than a few flying lessons, you are familiar with the drastic loss of lift caused by wing stall. If not, ask a friend who is a pilot to take you up and demonstrate a stall for you. Be prepared for a gut wrenching drop, and eat a banana or two before you go. Bananas taste about the same on the way out as they do on the way in.

Student airplane pilots are taught that a wing stalls at the same angle of attack every time, but that is not quite correct. A wing stalls when the aerodynamic boundary layer on the surface of the wing cannot remain attached to the surface in the presence of a local flow field that is decelerating too rapidly. As the local airspeed slows down, its energy level drops below the level required to remain attached to the surface. Boundary layer separation can be caused by either excessive angle of attack at a constant airspeed or by inadequate airspeed at a constant angle of attack. Because race car wings operate at a nearly constant angle of attack, they are stalled until the airspeed increases enough to generate attached flow under each wing and flap.

The science of aerodynamics is mostly about decelerating airflow in a controlled fashion. Speeding up airflow is easy. Any object in the flow field will accelerate the airflow around it. Keeping it attached to the surface through the region of deceleration is the hard part. If you choose to put clear tape on a wing for rock chip protection, it is best to wrap the whole wing all the way to the trailing edge. Airflow will separate at an aft-facing step easily, especially if the step is located in a region of rapid airflow deceleration.

There is a simple test you can do during a test day to find out what the attach speed is for your wing. Yarn tufts taped to the under side of each wing and a well placed video camera will show you the speed that the wing attaches. Just accelerate very slowly from a stop until all of the tufts lay flat against the surface. Of course it helps a lot if the video camera also has an airspeed data overlay. The attach event might be very crisp, with attachment happening over the whole surface at the same time. It might be gradual, with the trailing edge at the center of the wing attaching last. The wing is not attached until the flow is attached over the whole surface. Most wings have a little bit of stall hysteresis, so the wing will attach at a faster speed while accelerating than it stalls while decelerating. If the stall speed of your wing is faster than the slowest corner on the track, you have a problem. Small vortex generators or a lower angle of attack are the usual solutions to that.

INDUCED DRAG

Aerodynamic drag caused by production of lift or downforce is called induced drag. It is one of the major sources of aerodynamic drag on a downforce-producing race car, so minimizing that drag is a high-payoff modification. However, because downforce is a highly effective way to make the car faster, a compromise is required. The strongest correlations between lap time and car performance parameters are the power-to-weight ratio and the downforce-to-drag ratio. Determining the optimum downforce level, and therefore drag level, for each track requires a significant effort.

Induced drag is inversely proportional to wing span squared, so the ideal wing span is the maximum that you can get away with. Wing end plates increase the effective span, so bigger end plates actually reduce drag, particularly if they are joined to the body. If you can mount the wing by the end plates and get rid of the mount pylons, do it. The local airspeed under a race car wing is significantly faster than free-stream, so the drag caused by wing mounts is exaggerated by the increased local airspeed. The airflow separation caused by the pylons also costs you a fair amount of downforce. A few cars have hook-shaped wing pylons that wrap around forward of the wing and mount to the upper surface of the main plane in order to improve airflow under the wing. If that is an option for you, take it.

PRESSURE DRAG

The other major aero drag contributor is pressure drag. That is a result of airflow around an object that is not attached to the surface all the way around. The whole back end of a full-bodied car produces pressure drag. The second generation Honda Civic and first generation Insight both have well executed Kamm backs, which produce the minimum amount of base pressure drag for a length-limited body.

Pressure drag also comes from gaps and steps between body panels, windshields, cockpit and window openings, Gurney flaps, cooling flow paths, etc. Pressure drag is also the result of vortices that don’t generate lift or downforce, like the A-pillar vortices on cars with sizeable windshields. Air will belch out of any tiny gap on the sides, bottom, and top of the car and the cooling inlet ducts because the air pressure around the car is lower than the pressure inside. That leakage can create a lot of drag, so sealing all of the gaps can produce a surprising performance improvement from only a modest effort. However, never tape any of the water drain holes. You might forget about it, and bad things will happen the next time it rains.

ENGINE BREATHING

Engine intake air temperature matters more than intake air pressure, so locating the intake higher on the car is generally the right direction because air is hotter near the ground.

Exhaust exit location can make a very big difference to the aero performance of the car, either good or bad. The exhaust plume out the end of the pipe has a lot of momentum and it expands at about a 14° included angle, forming the aerodynamic effect of an expanding cone extending out from the exhaust exit and bending downstream some distance away from the exit. If the exhaust is aimed sideways out the side of the car, the exhaust plume adds a fair amount of unnecessary drag, but if it is located well, it can reduce pressure under the car.

Both the large-scale features and the small details of your car determine how much downforce, drag, and front-to-rear balance the airflow produces on it. Because wings are specifically designed to produce aerodynamic forces, they are particularly sensitive to small changes. The oil dot airflow analysis technique described in the October 2012 issue of Speed News is a great way to start the process of investigating and developing the aerodynamic performance of your car. My book, “Think Fast – The Racer’s Why-To Guide to Winning” has a whole chapter on the aerodynamics of race cars. Arming yourself with that information will make aerodynamic behavior less mysterious and more scientific. That is the first step toward using aerodynamics as a path to the podium.