The area of aerodynamics poses problems for amateurs and professionals alike. You can’t see the air you are trying to work with, and that air doesn’t react linearly. As a full-time aero engineer–you’ve likely seen my work at places like Daytona, Sebring and Talladega–I have five items that get under my skin when discussing operation aerodynamics.
Bad Question: How Much Downforce Does It Make?
This question is probably the first one that I get asked about an aero device. The real answer is “it depends.” How the device is incorporated into the whole car and the speeds involved are needed to accurately answer the question.
A better question, which should make the questioner think through the entire process, is this: How does this device work with the rest of the car? Adding devices at the back of a body can and often will change what happens at the front. It is a system to be considered as a whole. As a side note, mostly we don’t use forces for comparing aero, we use coefficients that are almost speed-neutral.
Bad Advice: Do It Like the Pros
While imitation is a high compliment, most professional series employ rules and limitations that prevent the engineers from getting too much performance. It might be size, location, adjustability or an outright ban on a device, but the limit is there. Use the pro series configurations as a starting point, not a final destination.
Bad Info: Spoilers and Vortex Generators Reduce Drag
Every time I hear this, I gnash my teeth. While there are a very few instances where this works, it is much more likely that spoilers and vortex generators increase drag. Forcing the air around stuff–like a spoiler–takes energy and therefore creates drag. This is a guarantee locally, but it may reduce drag effects elsewhere, providing a net drag reduction on the car. Remember, the entire car operates as a system.
Bad Plan: Bigger and More Is Better
Even at the professional levels, this crops up time after time. Usually it occurs with things like the angles of splitters and dive planes as well as the angle of spoilers and wings.
Once wings and spoilers achieve their peak downforce, more angle reduces the downforce. It does not disappear; it is just reduced, usually with increased drag.
Splitters and dive planes suffer by typically being made of materials that can bend under load. If you make those items too big, they will deform at speed and start causing issues like ground contact, flow separation that reduces downforce, or even ultimate failure and departure from the car. If the car feels unstable at the limit, back off angles and/or stiffen the aero devices to get a more consistent feel that will also make the car easier to drive at the limits.
Bad Logic: Aero Does not Work at Autocross Speeds
The very nature of aerodynamics says that something is happening if there is any airspeed. At autocross speeds, the effects are likely small compared to the end of the Mulsanne Straight, but aero is doing something.
Since the forces are a function of square of speed, aero forces at 60 mph are four times greater than at 30 mph–your typical range of speeds between the cones.
With that idea in mind, a dedicated autocross car will have much bigger aero devices than a track car just to get useful downforce. Also, autocross can often (legally) make some big power, so the drag impact can be ignored for all but the lowest-powered cars. This is a case for bigger is better, since the aero forces also increase directly with size.
Good Aero Logic
Remember, treat the car as a system with all of the aero parts working together. Determine the operating range for the car and components, and test to make sure you have not exceeded the peak operating conditions for the devices. Finally, think beyond professional series cars: They are working to a specific set of rules that may not apply to you.
Steve Stafford is a professional aerodynamicist working for factory efforts in NASCAR, IndyCar and IMSA. He’s also a frequent question-answerer on our message board.
