Airfoil Comparisons

Updated 1/29/2023 to add NASCAR COT wing.

Airfoil Tools is an amazing website, and is the primary way I research wings. When I look at wings, the most important factor to me is the coefficient of lift (Cl), and I’m only interested in airfoils that have a Cl 1.5 or higher.

I also look at the efficiency of the wing, which is the coefficient of lift divided by the coefficient of drag (Cl/Cd). Wing efficiency is less important than downforce, because the only thing that matters is the efficiency of the vehicle, and that is typically achieved by using a wing with the greatest downforce. Even if wing efficiency isn’t that important, I wouldn’t use a really draggy wing, so I look for a Cl/Cd of 100 or higher.

So, lift should be at least 1.5, and efficiency should be at least 100. Those are nice round numbers, and easy to remember. I look at these two values at a Reynolds number (Re) of 500,000, because that represents a normal-sized wing at a realistic car speed. For a 9″ wing, 500k Re is 73 mph. For low speeds (or small wings) I’ll 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 the turbulent nature of a hardtop, nor what’s happening in a race, behind other cars, with cross winds and other factors.

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.

On a Miata, if you mount the wing at roof height, the ends of the wing are at 0 degrees, but the angle changes from -5 to -7 degrees along the roofline. If mount the wing closer to the trunk, that angle increases to nearly 15 degrees. You can read about that and see a video Visualizing Airflow.

Given that air hits a wing at different angles of attack across the wing, at some point along its length, a wing may be stalling. When wings stall, they lose downforce and gain drag. Some wings do this gracefully, with a very gradual falloff in downforce, and others stall dramatically, with downforce crashing and drag spiking way up.

An obvious way around this is to use a wing with a 3D profile, so that air hits the wing at the same angle across the entire wing. On a Miata, this is surprisingly easy, anmore on that in a future article.

Airfoil Comparisons

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 ordered these by Cl, which is 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!

Clark Y

The Clark Y airfoil (airfoil tools, wikipedia) is distinguished by a flat bottom, or when used upside down on a car, a flat top. Most wings are cambered on both sides, but the flat surface 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.

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 stalls gradually. This means it would perform well behind a highly cambered roofline, such as Miata, where the angle of attack changes a lot over the span of the wing.

NACA 6412

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.

Cambered Plates

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, this wing has a lot of drag and so the efficiency is quite miserable, 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.

LNV109A – NASCAR COT Wing

NASCAR flirted briefly with car wings, but after 93 races went back to spoilers.

The airfoil they chose was the Douglas/Liebeck LNV109A high lift airfoil. I don’t know what decisions went into that choice, but the numbers don’t excite me. I mean, it would be cool to have one for historical reasons, but I’m not actively searching eBay for one.

If you look at the Cl vs Alpha chart (which you can think of as how much downforce the wing makes at different angles of attack), you can see the wing has a max Cl of 1.7. Downforce peaks at 12 degrees and then falls off drastically. Compare this to the Clark Y above to see what I mean.

Next, take a look at the efficiency of the wing. Most wings have a bell-shaped curve with maximum efficiency in the middle of the range (say 5 degrees). What’s interesting here is that max efficiency occurs at 10 degrees, with a Cl/Cd is 117. Maybe that’s why NASCAR chose this airfoil shape? If you want a wing that creates close racing at high speed, this would be a good choice. But if your agenda is low lap times, I think there are better choices.

FX 72-MS-150A

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 Wortmann FX 72-MS-150A.

Some of these wings are sold as “universal” and so they are flat on the bottom with two mounting rails underneath. I modify these by adding wood to the bottom and rounding it.

By the numbers, this is a decent 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 6 degrees max. Behind a Miata hardtop, this zero degrees (flat across the top of the wing).

GOE 464

This is a very thin airfoil, almost potato chip in profile. The only reason I find this aIrfoil interesting is because APR makes a carbon fiber wing using a similar profile. APR’s GTC-300 wing has more camber, but the GOE 464 is close enough to look at some numbers.

The GOE 464 has a max Cl of 1.85, which is a lot of downforce, and the efficiency at 500k almost reaches 100. It’s an interesting wing, but the numbers aren’t blowing me away. Also, it’s so thin that it would be difficult for me to build and make rigid enough, and I feel there are better choices.

Better airfoils

The previous airfoils were all interesting in one way or another, and some can be bought or made pretty cheaply. But all of the following airfoils have superior numbers, and I feel would be better choices for a car wing. Choosing between them is difficult, as there’s always a trade off between lift, drag, and stall. Each airfoil has a niche where it outperforms the others, and I’d be happy with any of them.

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 that’s been proven many times over.

If you look at the data, peak downforce is around 10 degrees, which falls in line with CFD from 9 Lives. Max efficiency (Cl/Cd) occurs at around 3 degrees, which is where I’d set the wing on a Miata. This would put the angle of attack at 8-10 degrees over the roof, and 3 degrees at the wing ends. Perfect.

GOE 652

The first thing you notice about this wing is the thickness of the leading edge. It carries that thickness over much of the wing, and the 17% chord thickness is unusually phat. The purpose of that (I think) is to keep air attached at steep angles of attack.

The 652 has a very gradual stall, and should tolerate being set at too steep of an angle. For this reason, I believe this wing would be a good for a car with a highly cambered roofline, or where you have to mount the wing closer to the trunk. In both cases, there are large changes in apparent wind angle across the wing, and this wing won’t care as much as other wings.

This isn’t a particularly efficient wing, and even though it exceeds my threshold value of 100, it does that at a very low angle of attack. However, the high lift of Cl 2.0 puts 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. I’m definitely making one of these someday.

Eppler 420

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 article I looked at every 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’m not sure what’s happening around zero degrees, but I wouldn’t set it there anyway.

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.

Summary Data

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 somehow important for marketing, even if that doesn’t really matter.

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°	24 hrs of Lemons
NASCAR COT	1.75	113.7 @ 10°	Wing of yesterday
MIC FX 72	1.8	121.1 at α=6.75°	MicWing
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°	Warts and all
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.

My wings

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”.