Airfoil Tools is a really neat site. It serves as a catalog of existing airfoil shapes, and allows you to compare them, simulate different speeds and angles of attack, and draw and plot your own wing shapes. Naturally most of these wings are designed for airplanes, but some have been used on cars (upside down) to create downforce. Cars don’t go nearly as fast as planes, and so the ideal shape of a car wing is different, and optimized for Reynolds numbers on the lower side of the spectrum.
The Reynolds (Re) number can be thought of as a resistance number. At 5 mph, you can put your hand out the window and move it back and forth with little resistance. At 60 mph, there’s more resistance; it’s almost like moving your hand through water. What’s important is that wings generally work better with more resistance. The shapes are more efficient at high speed (or underwater).
The airfoil tools calculator has values that can be used to simulate Re numbers (car speeds). For example, a wing with a 10″ chord that is traveling through the air at 67 mph, and at 68-degrees F has a Re number of around 500,000. Doubling the speed doubles the Reynolds number. When using the Airfoil Tools calculator, I set the values to 200,000, 500,000, and 1,000,000, which are realistic Reynolds numbers for most cars and wings.
In Competition Car Aerodynamics, , Simon McBeath frequently uses the NACA wings as examples, such as the NACA 6312. In NACA 4-digit wings, the first digit is the camber as a percentage, the second digit is the location of camber in relation to the chord length, the third and fourth digits are the percent thickness of the chord. So, a NACA 6312 wing has 6% camber, the max camber occurs at 30% of the chord length (from the front), and the thickness of the wing is 12% of the chord length.
McBeath discusses these form factors and how they allow a wing to generate more downforce.
- More camber creates more downforce, but too much and you get separation underneath at the trailing edge.
- Moving camber rearward generates more downforce, but further forward is more efficient for low-drag, low-angle applications.
- A thickness of around 12% is quoted as being best for flow separation, but a thicker wing doesn’t create much more drag at car speeds, and could be better when run at a high angle of attack. The author suggest around 16-18% thickness as being ideal. Multi-element wings can also benefit from more thickness.
First I want to give a shout-out to 9 Lives Racing who makes a very strong, economical, and proven wing. I don’t know if it originated from a NACA profile, and I’ve tried to find a NACA shape that’s the same, but I can’t find an exact match because it has more camber.
For fun, I’ll plot a wing that looks similar. Start at the NACA 4-digit airfoil generator and plug in some numbers. At first I’ll try 8% camber, 50% chord position, and 12% thickness and I get a wing that looks kind of like the 9LR wing (although 9LR has more camber).
Now I want to take McBeath’s advice and increase the thickness to 16% and I’ll add as much camber as the tool allows (9.5%).
To my eye it looks chubby, especially the leading edge. It’s a lot of effort to build a wing, and it would take a lot of trust to build one based on an online calculator and other people’s suggestions for what might work. A couple proven wing shapes I’m interested in building are the Eppler E423 high lift airfoil, and the Chuch Hollinger CH 10-48-13 high lift low Reynolds number airfoil. And then again there’s the smart move, which is to buy the 9 Lives Racing wing and move onto other projects.
Another thing I discovered using the Airfoil Tools page is that most wing shapes operate best in a fairly narrow range of angle, and that this angle of attack is dependent on speed. Let’s see what the Airfoil Tools page can show us on how wing angle affects downforce and drag.
First I’ll select a wing, and the NACA 6412 is as good as any for this demonstration. Next I’ll set the Reynolds number, and then the NCrit range between 5 and 7. Click Update Range and look at the resulting graphs.
First up is Cl vs Alpha. Cl is the coefficient of lift, which is downforce to you and me, and Alpha is the wing’s angle of attack. The more wing angle, the more downforce, up until about 9 degrees. After that flow separations occur and the wing creates less downforce.
Next take a look at Cd vs Alpha, which is drag vs wing angle. This wing has the least drag around zero degrees. There are three lines in these graphs that represent different speeds, but you can see that drag goes up quite a bit after 10 degrees.
The next one I’m interested in is Cl/Cd vs Alpha, which is how efficient the wing is at different angles. The wing is most efficient between 5-7 degrees, depending on speed. This is where the Reynolds number comes into play. These three lines indicate three different speeds, and you can see at 200k (the lowest line), the wing just isn’t very efficient. Going faster makes a larger Re number, and the wing works better.
So you might think to set this wing at about 6 degrees, so that the wing performs at its best lift/drag ratio over a range of speeds. However, this isn’t necessarily the best setting for your car.
- More downforce, at the expense of more drag, is almost always faster. You can read more about that in Downforce vs Drag.
- Having an over abundance of rear downforce will make the car understeer at high speed (safer), which means you can tune the car for oversteer at low speed (funner). In the end, the best setting is how you want to adjust the balance.
If you look at the open-air CFD data for the 9 Lives Racing wing, it also operates in a similar range for angle of attack, but with more camber, it’ll probably stall earlier than the 6412 wing. As you can see in the chart below, the 9LR wing makes a tiny bit more downforce at 10 degrees than 5 degrees, but with a lot more drag. It may be starting to stall at 10 degrees, and the downward wash of most rooflines makes the 10-degree setting a bad one.
Also included in their chart is data for Gurney flaps and dual-element wings, which is the subject of the next post.