The following graph illustrates the effects that air speed and AOA have on downforce only. The higher the air speed and AOA are, the higher the resultant downforce is.

CFD DATA (GRAPHS)

The following graph illustrates the effects that air speed and AOA have on drag only. The higher the air speed and AOA are, the higher the resultant drag is.

The following graph illustrates the effect that air speed and AOA have on both downforce and drag. The higher the AOA and air speed are, the higher the resultant downforce and drag are.

CFD DATA (RENDERED IMAGES)

The following image illustrates the pressure distributions across the surfaces of the airfoil. The units are in Pascals (Pa), where 1 Pa = 1.45 x 10^-4 Pounds Per Square Inch (lb/in^2).

The following image illustrates both the pressure distribution and streamlines associated with the airfoil. Free-stream air is shown at 73.85 meters/second (m/s), which is approximately 165 mph if 1 m/s = 2.237 mph.

The following image illustrates both the pressure distributions and vector fields around the airfoil.

The following image illustrates both the pressure distributions and vector fields around the airfoil (different view angle).

CONNECTING THE DATA TO REAL-WORLD APPLICATION

What we want to do now is to try to apply a bit of the CFD data and analysis to the real-world application of using this airfoil on a vehicle. First, we will first need to change a few things around. The first thing to change is the environment, because the mounted airfoil (both the center and outer sections) will never truly see any free-stream air flow. Even though the outer sections of the airfoil may be positioned beyond the vehicle's roof and body, and even though the air flow may be "cleaner" or be more parallel to the ground plane, the air around the sides of the vehicle is still affected such that it can no longer be considered to be free-stream air. Secondly, since we can no longer define angle with respect to the relative motion of free-stream air flow, it is helpful to introduce and use an additional term called "pitch." Previously, we had defined the AOA as the difference in angle between the center cross-section of the reference plane (a.k.a. "center reference line") and the vector that represents the relative motion of the undisturbed free-stream air flow. We will define pitch as the difference in angle between the center cross-section of the reference plane (a.k.a. "reference line") and a non-sloped ground plane that is parallel to the vehicle's direction of travel. A ruler that is placed on top of the center section of the GTC-series airfoil would be the real-life equivalent of the reference line. Additionally, to simplify the angle references and avoid confusion with AOA numbers, we will use only absolute values (i.e. positive numbers), in conjunction with either upward pitch (front higher than the rear) or downward pitch (front lower than the rear).

When the airfoil is mounted on the sedan-bodied vehicle, the center section actually be at an AOA that is greater than 0 degrees. This is because the air flow to the center section will have a tendency to follow the rear slope of the roof/glass/trunk area downward (see image below). This downward air flow will vary in smoothness (whether it's more laminar or more turbulent, more attached or more detached) depending on the shape of the vehicle (notice the air flow differences in the two vehicles shown below).

(Image from 3rd party source)

(Image from 3rd party source)

Knowing exactly what angles the air flows downward behind the roof and around the vehicle body is information that would be "nice-to-know," but in practice, we don't really need to know all the details. What we need to know is that the vehicle body has enormous effect on the air flow that reaches the airfoil. We should also know that the center section of the airfoil sees an effective AOA value greater than 0 degrees when the airfoil pitch is set to 0 degrees, and that the outer sections of the airfoil see outer-section-only AOA values around 15 +/- some value based on how the body affects the air flow. We already know from the CFD data that the outer sections of the airfoil begin to stall when the center AOA is around 10 to 12 degrees, with even greater stall at 12-13 degrees. Therefore, we can determine that the downward pitch of the airfoil should never need to be set beyond 12 to 13 degrees. In vehicles with steeper rear roof/glass/trunk slope, the airfoil pitch may never need to be set beyond 10-12 degrees. Remember, the inner or outer sections of the airfoil will begin to stall at effective AOA values of 25 to 27 degrees, with even greater stall at 27 to 28 degrees.

To re-iterate what was mentioned above, and to emphasize a few points:

1. At a center section pitch of 0 degrees, the effective AOAs of both the center and outer sections is greater than 0 degrees. --> The airfoil is already creating downforce at 0 degrees pitch.

2. At a center section upward pitch of greater than 0 degrees, the effective AOAs of both the center and outer sections will still be greater than 0 degrees up to a certain point. --> The airfoil is still creating downforce at "positive" pitch values.

3. A roof that slopes down to the trunk area gradually will allow the air to flow more smoothly (i.e. stay attached longer, be more laminar, less turbulent, etc.) than a roof that slopes down abruptly to the trunk area. --> Be very aware of how significant is the effect that the vehicle body shape has on the air flow to the airfoil when trying to determine airfoil placement and airfoil pitch.

AIRFOIL SETUP

Setting the Airfoil Height: As a general guideline, vehicles with steeper-sloped rear roof/glass/trunk areas are better suited to use higher airfoil mounting heights (just below the roof line). Vehicles with gradually-sloped rear sections (i.e. fastbacks) can work well with lower airfoil mounting heights. There is no strict rule for this, since every vehicle application is different. The GTC-300 airfoil comes with pedestals of a recommended height for each intended vehicle application. Additional height can be achieved by using optional risers.

Setting the Angle: It is helpful to know how the vehicle handles prior to installation of the airfoil. For the initial testing with the airfoil installed, we recommend setting the airfoil pitch to 0 degrees using an angle indicator tool. Test the vehicle's front vs. rear cornering balance at speeds above 45-55mph (on a familiar track surface). If the vehicle tends to understeer (feel "tight") too much at medium-high speeds, then dial in a bit of upward pitch. If the vehicle tends to oversteer (feel "loose") at medium-high speeds, then dial in a bit of downward pitch. Try to make very small adjustments as needed between each road test, and try to focus on adjusting one thing at a time (i.e. don't change the airfoil pitch, tires pressures, and shock settings all at once).

Initial angle setup with the airfoil pitch set to 0 degrees, as shown on the digital angle indicator:

Same as above, but with the angle indicator on top of the outer section:

Maximum Angle: We don't recommend setting the downward pitch greater than 12 to 13 degrees, even on vehicles with the most gradually-sloped rear roof/glass/trunk areas. On vehicles with steeper angles at the rear, the downward pitch should not be set greater than 10-12 degrees.

Recommended maximum angle, as shown on the digital angle indicator:

Same as above, but with the angle indicator on top of the outer section:

CONCLUSION

With so many vehicle configurations, track and road conditions, and weather and environment conditions, it is nearly impossible to predict exactly how an aerodynamic component will change a vehicle's performance and handling characteristics. Well-funded race teams continuously spend significant amounts of time and money to perform computer simulations, wind tunnel tests, track tests, and other research and development activities. For most folks, all these activities may not be within convenient access. Nevertheless, we can at least let the CFD data and analyses contained herein serve as starting guidelines in our quest to improve vehicle aerodynamic performance.

The information contained herein is property of APR Performance, and may not be reproduced in whole or in part without prior written consent from APR Performance.