Time for some venting (no puns intended in this case ;)
This is admittedly a minor beef, but one that’s a constant source of irritation, so I'd like to get it off my chest: I hate hearing these two terms used wrong or interchangeably! So… for the sake of clarity (and my own sanity) I’d like to state their definitions. I know what you’re saying: “this is pretty basic stuff, so why even discuss it?” My Answer? Defining them is a great excuse to discuss their relative strengths and weaknesses as aerodynamic devices! But I’m getting ahead of myself.
Here are those definitions:
a spoiler is a barrier forcing air to flow over it, with no separation with respect to the car.
a rear wing is an inverted airfoil, separated from the car’s surface to allow air to flow above and below the element.
Right. Got it. So what else is there to talk about? Quite a lot as it turns out.
For example, you may be surprised to learn that a spoiler can actually be used to reduce drag (in addition to creating the downforce they are typically known for) So how does this work? Well, a picture is worth a thousand words, but I’ll attempt to explain a little about drag first… and then I’ll show you some diagrams.
As air moves over the curved upper surfaces of most modern cars, it is pushed up and over the vehicle body, increasing the airspeed (and reducing the pressure) of the flow. But as our old friend Mr. Newton tells us, since air has mass, what speeds up must slow down. When this high speed flow moves down the tail of the vehicle, it must eventually drop off the back of the body and is forced to rejoin (and match the speed of) the ambient airflow around the car. How smoothly this occurs is to a large extent what determines how much form drag a vehicle will make. Some examples:
When a spoiler is added, two things happen.
First, the airspeed coming off the roof of the vehicle is slowed before it leaves the trunklid, meaning that the speed differential between the surface flow and the ambient air around the car is reduced. This is good, since we are on the road to bringing these two velocities (and by association, pressures) back into agreement. As mentioned before, the faster and more smoothly we can make this happen, the less turbulence (a.k.a. pressure drag) there will be.
To that point, this high-speed upper body airflow is also beginning to cause lift (due to surface drag, which leads to boundary layer separation) as it moves rapidly across the roof. The separation will only increase at the rear as the car body drops away from this highly energized (but increasingly turbulent) flow unless something is done to ease the transition.
Second, the corresponding pressure of the air increases in front of the spoiler, which means (as Bernoulli has told us) that its speed has decreased. This higher pressure air exerts negative lift (downforce!) on the trunklid, and given the angle of the sheet metal, this actually produces a small amount of positive thrust, offsetting drag indirectly. In addition, this zone of higher pressure on the rear deck also means that the low pressure envelope behind the car cannot grow by feeding on the additional low pressure coming down the rear window as it would otherwise. This reduces form drag directly, as the smaller the drag envelope, the lower the recirculation turbulence (suction) behind the car will be.
So… there is your 1000 words. But what does this look like in a diagram?
As this diagram shows, the amount of high speed, low pressure flow available to feed the drag envelope behind the car is reduced with the use of a spoiler. This reduces form drag directly, while also adding a small amount of forward thrust by exerting pressure on the “fastback” portion of the vehicle body. A rare win-win!
So, spoilers CAN reduce drag. Which is pretty cool… but how about the downforce aspect of a spoiler? It all lies in the details, and how large and disruptive that spoiler’s plane is to the airflow as it attempts to exit the vehicle’s rear surfaces.
Small spoilers that are more parallel to the airflow (also called laminar lips or separation edges) are best for drag reduction, since they offer a more gradual speed reduction/pressure increase on the rear decklid and more gentle airflow blending. Some examples of this spoiler type:
More aggressive spoiler types are focused on maximizing the pressure increase/area of the rear decklid that experiences a pressure rise. These go by different names such as wicker bills, kickers, whale tails, or sometimes even Gurney flaps (a misnomer in this case, but I digress) An example:
And there you have it. Proof that in some cases, with the spoils come the victors ;) I realize that I haven’t yet discussed the relative merits of wings, but that will have to wait until part two of this article. Until then, I leave you with a reassuring visual that proves both are valuable (and can even co-exist!) when used properly. And it’s always good to know the theory behind your practice. Whether you use a spoiler (or just intend to wing it!)