Airfoil Tools is an amazing website, and is the primary way I research wings. When I look at wings, the most important factors to me are the maximum downforce (Cl), and the efficiency (Cl/Cd). For downforce, I ignore anything under Cl 1.5, and for efficiency, I’m only interested in Cl/Cd of 100 or higher. These are nice round numbers, and easy to remember.
I look at these two values at 500k Reynolds, because that represents a normal-sized wing at a realistic car speed. For a 9″ wing, 500k Re is 73 mph. If I have a single bone to pick with Katz’s Race Car Aerodynamics, it’s that he commonly cites Reynolds numbers of 2 million, which would be a 9″ wing traveling at 280 mph!
For low speeds (or small wings) I might look at Re 200k, and for huge wings or really fast speeds, I’ll look at 1 million. But there’s really no reason to look outside of the 200k-1M Reynolds numbers and 500k is a happy medium.
I also set the turbulence value to Ncrit=5 because Airfoil Tools doesn’t allow me to set it any lower (in some cases there’s data for lower Ncrit numbers, but it’s rare.) The default setting in Airfoil Tools is Ncrit=9, and that replicates what the wing would experience in a wind tunnel, and doesn’t represent what’s happening in a race, behind other cars, with cross winds and other factors that create turbulence.
The next important factor I look at is how the wing deals with stalling at high angles of attack. I look at this for two reasons. 1) Most cars will go faster with the wing set to maximum downforce rather than maximum efficiency, and 2) car roofs are often cambered and this means air hits the wing at different angles across its length. The fact is, at some point along its length, a wing may be stalling. When some wings stall, they do so gradually, and that’s good. Other wings stall dramatically, and that’s bad – lift takes a nosedive and drag spikes way, way up.
Now that you know my criteria for looking at wings, in the rest of this post I examine different wing profiles and give my thoughts on them. I’ve put these in order by how much downforce they make. At the end of this post I’ve included a table with summary values and some parting thoughts.
With all of that front matter and grey matter out of the way, let’s check out some wings!
The Clark Y airfoil (wikipedia) has a flat top, which can make it easier to manufacture, and for an airplane, it’s good for training because it has gentle stall characteristics. But as a car wing, the flat top means more drag and less downforce than a wing with a cambered topside. As such, the Clark Y doesn’t quite meet my downforce threshold of 1.5 Cl at 500k, and the efficiency (Cl/Cd) is below 100 as well. Personally, I wouldn’t use this wing profile.
Wingmen Aerodynamics makes a wing with this profile, or something very similar. I’ve seen this wing at a couple different races, and I was immediately impressed with the build quality. From the construction pictures on Facebook, it doesn’t appear that they need to make the top flat – it’s not like they are using a flat piece of foam (or whatever) to simplify construction. So for the amount of work that goes into making such a wing, I would have chosen a different profile.
If you have one of these wings, I’m not trying to make you feel bad; I’m sure your car goes a lot faster with it than without it! Also, the flat top has two advantages: it’s easy to set the wing angle, and it doesn’t fall off a cliff when it stalls. This means it would perform well behind a highly cambered roofline, such as Miata, where the wind angle changes over the length of the wing.
MacBeath often cites the NACA profiles as examples in his books, and for good reason, they are easy to understand. The first number in 6412 means percent of camber relative to chord (6%), the second number is where the camber occurs (4 means 40% of the chord), and the third number is thickness (12%). NACA 6412 meets my criteria for a good car wing with a Cl over 1.5 and a Cl/Cd over 100.
I like NACA profiles for another reason: they allow me to change the variables and see what happens to lift, drag, and efficiency at different Reynolds numbers and angles of attack. For example, I’ve read that maximum lift on single-element wings occurs at 12% thickness, and after experimenting with different NACA profiles that are identical in other respects, I know this to be true.
You can also use the NACA 4-digit generator to create your own wing profiles. For example, this is a NACA 9512. I’ve maxed the camber allowed in the tool (9.5%), set the camber further rearward (50%) to increase lift, and used the max lift thickness of 12%. I’m certain this would be a good car wing.
Probably the easiest way to make a wing is to cut a metal pipe lengthwise into strips, and then lay two of the curved pieces on top of each other. Put a semi-circular nose on it and weld the three pieces together. This is a cheap and easy way to make blades for small wind turbines, I don’t see why you couldn’t do the same for a car wing. It’s so DIY I want to make one for 24 Hours of Lemons.
You can make these wings in different thicknesses, and at 12%, it has a high Cl of 1.7. However, the efficiency is less than half of my threshold value of 100. Still, for a low-speed wing where drag is inconsequential (autocross), this would totally work. And for 24 Hours of Lemons, it’s better than a snowboard, skateboard, angled plywood, etc.
I have three different made in China wings, one came as a double wing, the other two are single wings. They are fun for experimenting with, cheap, and disposable. Whoever designed them chose a similar profile for all of them, which is akin to the FX 72-MS-150A.
Some of these wings are sold as “universal” and so they are flat on the bottom because they have two mounting rails underneath. I modify these by adding wood to the bottom and rounding it.
By the numbers, this is a good airfoil for a car, it makes a lot of lift (1.8) and is very efficient (121). The only drawback is when this wing stalls, it falls out of the sky. This isn’t a wing that you want to set for maximum downforce. Make sure you take into account the downwash angle on the roof and then set the wing to around 5-6 degrees max. Behind a Miata hardtop, this is about 1 degree negative.
This is a very thin wing, almost potato chip in profile. The only reason I find this airfoil interesting is because the APR GTC-300 carbon fiber wing uses a similar profile. APR’s wing has more camber, and I didn’t find an exact match on Airfoil tools, but the GOE 464 is close.
The GOE 464 has a max Cl of 1.85, which is a lot of lift, and the efficiency at 500k almost reaches 100. It’s an interesting wing, but would be difficult for me to build, and I feel there are better choices.
The previous airfoils were interesting in one way or another, but I personally wouldn’t put the effort into building a wing using those shapes. All of the following airfoils are superior, and would be better choices for a car wing. There’s always a trade off between lift, drag, and stall, and so each wing below has a niche where it outperforms the others.
Church Hollinger CH10
Any wing that makes around Cl 2.0 is in the category of ultra high lift. The most efficient of these is the Church Hollinger CH10. At 500k, this wing has a Cl/Cd of 132, which blows away the others.
The 9 Lives Racing wing is a close cousin in shape, although the Big Wang has more camber. Adding camber adds downforce, but this usually peaks at around 10% of chord, which is where the CH10 sits as is. I’m sure that Elan figured it all out, and my guess is that those changes add downforce, at the expense of some drag. In any case, this is a great wing for a car, and would be a solid choice for a low powered car or endurance racer.
The first thing you notice about this wing is the blunt and rounded nose. The purpose of that is to create a thicker boundary layer, which delays separation at steep angles of attack. It carries that thickness over much of the wing, and the 17% chord thickness is phat.
In some ways the GOE 652 airfoil is the opposite of the FX72, because the 652 has a very gradual stall. Meaning this wing is tolerant of being set at too steep of an angle. This would be a good wing on a car with a highly cambered roofline, or where you have to mount the wing closer to the trunk. In these cases, there are large changes in apparent wind angle across the wing, and this wing won’t care that much. For the same reasons, this isn’t a great candidate for a 3D wing, it just wouldn’t be necessary.
The high lift of Cl 2.0 and efficiency over 100 put this wing into elite company. I’d wager this would make a good upper element for a dual-element wing, not just the because of the shape, but because the added thickness would make it stiffer in a smaller chord.
The Eppler 420 isn’t as efficient as the CH10, but has slightly more downforce and a gradual stall. It’s a good all-purpose shape, and because it’s thicker, would be a strong contender for either element in a dual-element wing. As an all-purpose wing, it’s hard to choose between the CH10 and E420. The former is more efficient, the latter makes more downforce.
It’s also a pretty good wing for low Reynolds (low speed or small wing). The Porsche Cayman R has a tiny rear wing, and it’s probably not a coincidence that the profile looks a lot like the Eppler 420.
Wortmann FX 74-CL5-140
This airfoil wasn’t on my radar, but in writing this blog I looked at almost every single airfoil on the Airfoil Tools site. Glad I did, this one is a keeper! With a Cl that’s nearly the same as the Selig wings, and an efficiency closer to the CH10, this wing sits in rarefied air.
You don’t get something for nothing, and the tradeoff is a steep drop when it stalls. If you want to go after maximum lift with this airfoil, mount it high where the angle of wind doesn’t change much. I have more to say about this wing after reviewing the next wings. I’m tempted to build one, so stay tuned on that.
Selig 1223 and 1223 RTL
The Selig 1223 and the Selig 1223 RTL are Downforce Royalty. The RTL version is slightly thicker, which results in higher lift and drag. The RTL can be set to 15+ degrees and approaches a Cl of 2.5. That’s huge.
Both airfoils make a lot of downforce, but also a lot of drag, and their Cl/Cd efficiency is less than 100 at all angles. Ergo, I would use this airfoil for low speed or for a car with a lot of power. Those are also usecases for a dual element wing, which might be a better choice if your racing rules allow that.
Let’s compare the two Selig wings to the FX74. These graphs are from Re 1M because I plan to use these for a larger chord wing. In the comparisons you can see how the S1223 wings are clear winners in downforce, but the FX74 is far more efficient at Cl 2.25 and below. I’ve drawn a blue dashed line at 2.25 Cl, and you can see that the FX74 has a sweet spot where’s it’s making a lot of downforce without much drag.
Here are all the wings sorted by Cl. For each wing, I’ve also listed the max efficiency, and the angle where that occurs. Note that the fastest way around the track is often right around max downforce, not max efficiency. But efficiency is arguably important for endurance racing and momentum cars.
|Wing||Max Cl||Max Cl/Cd||Notes|
|Clark Y||1.45||90.2 at α=4°||Clark why?|
|NACA 6412||1.6||111.3 @ 6.25°||Good reference|
|Cambered plate||1.7||42.5 @ 5.75°||Lemony|
|MIC FX 72||1.8||121.1 at α=6.75°||McWing|
|GOE 464||1.85||97 at α=7.5°||Potato chip|
|CH10||1.95||132.4 at α=3.25°||Effin efficient|
|GOE 652||2.05||102.8 at α=2.25°||She thicc|
|Eppler 420||2.1||106.1 @ 6.25°||Dude, 420|
|Wortmann FX 74||2.25||115.5 at α=6.75°||Gimme|
|s1223||2.3||97.4 at α=5.75°||DF Queen|
|s1223 RTL||2.35||85.4 at α=5.5°||DF King|
This is how five of the airfoils stack up at 500k Re. I didn’t include the RTL because that’s in the previous charts. I’ve pointed out a few areas that differentiate one wing from another.
Here’s a collection of wings and cutoffs in my shop. Two are missing, the GLTC 250 sq-in wing that I sent to Justin Lee, and the upper element of that wing, which is lounging in a corner somewhere.
From the bottom and then left to right: Selig 1223 RTL 41” x 16” prototype; MIC 110cm x 11.6cm chord (I have three of these); APR GTC-200 59.5” x 8” (end cap shown, the wing is on a car); MIC 135cm x 14.2cm; 9 Lives Racing Big Wang cutoff; 9LR-based wing 48” x 11.4”.