Wings vs. Spoilers (part 2)
Don't just wing it. Know the theory behind the use of airfoils in motorsport.
In the previous part of this series comparing wings and spoilers, I made the assertion that both have value, and can even coexist on a performance car if correctly positioned. But why would you choose a wing over a spoiler, and in which situations might you want/need both? To get to an answer, I’ll start by first showing how a wing affects the aerodynamics at the back of the car. Although most add a wing to produce downforce, the influence that an airfoil can have on the size and shape of the drag envelope at the back of the car is perhaps the easiest to see in CFD imaging.
Here a two development images I captured from a recent baseline simulation and subsequent addition of a 72”, 14” inch chord wing on a BMW M2C. I’ll show both streamlines and an LIC (Line interval Convolution) pressure plot image, as they illustrate different aspects of the flow. LIC imaging is basically a way of combining local air pressure with a visual reference for flow direction. In both cases, the color coding for pressure represents: low = blue, yellow = ambient, red = high.
In these two images several interesting things can be seen. First, the general direction of flow off the rear of the vehicle has a downward trajectory and is slightly below ambient pressure as it leaves the rear deck lid. This indicates and area of lift and recirculation behind the vehicle. When one looks at the LIC image, it becomes even more evident what is happening, with the shape of the drag envelope clearly visible in the vector traces: a strong area of recirculation appears behind the bumper cover. The location of this primary region of recirculation is not uncommon, as the drag envelope attaches to the first part of the vehicle it encounters while recirculating (in this case the bumper)
By comparison, the imaging with the wing fitted tells a very different story about the flow, as we’ll see in the images below.
The streamline image at the top show how much more laminar the flow becomes when exiting from an airfoil. The low pressure beneath the wing also tends to pull the flow exiting the roof down and helps to keep it more attached to the rear window. In the LIC image, the reduction in size of the drag envelope and smaller area of recirculation can also be clearly seen. In addition, because the recirculation occurs in an area higher and closer to the rear of the vehicle, it allows the low pressure zone beneath the wing to relate more directly to the low pressure zone exiting the diffuser area. As far forward as the muffler, the increased air velocity there is adding a significant amount of rear underbody downforce as well.
So what can we take from all of this? Well, wings make their downforce differently and (to a degree, more efficiently) by increasing attached flow, having better interaction with rear underbody flow, and reducing recirculation (form) drag at the rear. Another interesting aspect is that the flow exiting the rear deck lid is flowing straight or slightly up (due to the rotation of the wingtip vortices) which adds a small amount of downforce as well vs. the downward flow vector seen without a wing. A view from the rear of the winged M3C shows this directional flow in action, along with the tendency for the flow exiting the roof to stay beneath the wing.
Conversely, the flow off of the rear of the stock vehicle (below) shows a much less vigorous flow attachment on the rear window, along with a less focused and more turbulent flow exiting the rear deck lid.
The above images also begin to explain why it is beneficial to run a wing lower and further back (if possible) for larger gains. Since the drag envelope behind a car is already lower than ambient pressure, it is an ideal environment to amplify the suction side of a wing. Also, the lower the wing can be run, the closer it is to the flow exiting the diffuser. If these two flows can be combined, they will to meet in the middle, further squeezing down the size of the drag envelope (and making both aerodynamic devices work better.)
This is one reason that you see such extreme wing placement on high end sports cars, DTM racers and those run in the prototype sports car classes in road racing (there are other examples as well, including the configurations run in Indycar and F1.)
So as you can see, wings offer some unique opportunities to improve the efficiency and reduce the drag of an overall aero package. This leaves only one question unresolved: how can they effectively be used in conjunction with a spoiler to improve the overall aero benefits? The easy answer is that SOME cars will not benefit from both and are better suited to choosing just one or the other!
In many race series, the rules only allow for the use of a spoiler (or a wing) and when they do, they often limit the height, location and/or surface area of the device. Obviously, in those cases the decision becomes easier, with most choosing to maximize the allotted specification set forth in the rules. Where the choice becomes more nuanced is in time-trial, prototype and even high-performance street cars, where the design parameters are more open. In these cases, it comes down to a relatively simple ratio: lift to drag.
Simply put, spoilers make CAN make significant downforce, but to match the same downforce made by a wing, the drag penalty will be far greater. Wings are not WITHOUT drag, but they make their downforce via the pressure differential across their surfaces (vs. using deflection to simply change air direction like a spoiler) Ironically, this difference come into play when they are used in combination.
The limiting factor of a wing’s ability to make downforce is the angle at which the air on the bottom (suction) side of the wing will detach from the airfoil’s surface (stall) and lose the ability to pull down on it. Since a spoiler is a great device for changing the direction of flow, they can sometimes be used to augment attached flow on the underside of a wing, allowing for a higher angle of attack to be run before stall occurs. There are many examples, but perhaps the most familiar to many track enthusiasts is the design used on the venerable Porsche 911 family for their many GT2 and GT3 performance variants.
As you can see from the above photos, the spoiler and lower curvature of the wing are concentric, allowing the spoiler to change the direction of the flow coming down the rear window of the car and match it to the flow on the underside of the wing. Additionally, on many newer track oriented sports car models, (such as the GT3/RS, et al) a swan neck mount is also used for the wing to allow even better flow attachment to the wing’s underside, improving performance even further. When seen in a “streakline” image (a particle-tracing technique used for airflow testing) it’s easy to see how the stalled flow is vectored in a more favorable direction for good flow attachment, while still allowing for a more aggressive AOA.
So there you have it, an explanation of the why for wings, along with some practical examples of the how. Spoilers and wings can be used individually, and together. Who knew? (Now you do ;) Perhaps spoilers have a bad name, since in some cases, spoiling your wing may actually a good thing!