Car Wings Examined

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.

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 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!

Clark Y

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.

Clark Y flat top.

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.

Wingmen Aerodynamics, flat-top fiberglass wing.

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.

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.

NACA 6412 looks good.

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.

NACA 9512 looks better.

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.

The simplest of wings, two cambered plates connected together.

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.

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 FX 72-MS-150A.

Made in China wings.

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.

MIC wing modified.

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.

GOE 464

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.

Potato chip profile.

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.

GTC-300 is not unlike GOE 464

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.

Get thee to church. Church Hollinger, that is.

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.

Big Wang with Gurney flap slot cut off for experimentation with chord.

GOE 652

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.

She thicc.

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.

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.

Eppler 420 is a solid all-around choice.

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.

Porshe Cayman R wing looks like an Eppler 420 at 5 degrees.

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.

It’s nice, I like.

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.

Selig 1223 (red) and 1223 RTL (green).

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.

AeroDesign wing from Australia appears to be Selig-ish.

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.

Re 1M, Ncrit=5

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 arguably important for endurance racing and momentum cars.

WingMax ClMax Cl/CdNotes
Clark Y1.4590.2 at α=4°Clark why?
NACA 64121.6111.3 @ 6.25°Good reference
Cambered plate1.742.5 @ 5.75°Lemony
MIC FX 721.8121.1 at α=6.75°McWing
GOE 4641.8597 at α=7.5°Potato chip
CH101.95132.4 at α=3.25°Effin efficient
GOE 6522.05102.8 at α=2.25°She thicc
Eppler 4202.1106.1 @ 6.25°Dude, 420
Wortmann FX 742.25115.5 at α=6.75°Gimme
s12232.397.4 at α=5.75°DF Queen
s1223 RTL2.3585.4 at α=5.5°DF King
Values at Re 500k, Ncrit=5

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.

How the players stack up at 500k Re.

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.

Wings!

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”.

Miata Spoilers

If you’re serious about downforce, use a wing; it can generate more downforce, and is more efficient than a spoiler. It begs the question, why would anyone want a spoiler?

  • Spoilers are usually cheaper than wings.
  • Some racing rules don’t allow wings, but allow spoilers.
  • A small spoiler can reduce both drag and lift.
  • Wings are often gaudy on a street car, but spoilers almost always make a car look cool. Not only my opinion, but NASCAR fans as well.

I’m going to build an adjustable-height 70-degree spoiler so I can find out what’s ideal on a Miata. But before that it’s worth looking at the existing literature and products.

How a spoiler works

Cars are basically shaped like airfoils, and as air moves over them, it creates lift. The faster the car goes, the more lift and instability is generated. A spoiler, as the name implies, “spoils” the airflow coming over the top of the car, fooling the air into behaving as if the car has a different profile. This cancels some lift, and often reduces drag as well.

Image result for with and without spoiler airflow
How a spoiler works.

A spoiler also concentrates high pressure air on the rear deck lid. Pressure is akin to weight, and so this adds downforce to the rear of the car.

A spoiler also moves the center of pressure rearwards, and like a streamer on a kite, this promotes stability.

Spoiler height

How high should a spoiler be? Let’s take a look at what the pundits say. In Race Car Aerodynamics, Katz shows two different graphs for spoilers. The first is based on spoiler height alone, at a fixed angle of 20 degrees from vertical, or what I’d call 70 degrees.

I’ve put some pencil marks on the graph and drawn some conclusions.

  • A low spoiler about 1″ tall reduces drag the most. It also adds a bit of downforce. From a drag and downforce perspective, it’s a win-win!
  • A 3″ spoiler doesn’t add any drag, and doubles the downforce of the low spoiler. In other words, you get something for nothing!
  • A taller spoiler adds downforce and drag, but downforce increases more rapidly than drag. The gift that keeps on giving!

So no matter what height spoiler you chose, it has a benefit. Based on theory alone, we should all have low spoilers on our street cars, and taller spoilers on our race cars (rules permitting).

Spoiler angle

Katz includes another graph on spoiler angle, this time using a fixed-height spoiler. Confusingly, this time the angle is measured from horizontal, not vertical, and the 70-degree angle from the previous graph isn’t included.

Some observations of this data:

  • Drag increases fairly linearly with angle.
  • Lift-drag ratio seems best at a very shallow angle, but this may simply be the low overall height of the spoiler. Also note that L/D ratio is at best 3:1, whereas a wing can be 12:1 or more, which is why you use a wing if you’re serious about downforce. (If you look at pressure plots of wings, you’ll notice they have about 3x more suction under the wing than pressure on top. Spoilers only make downforce from the top side, and so that’s why they are a lot less efficient.)
  • Increasing spoiler angle to 60-degrees or more increases downforce, but at a diminishing return.

Spoiler height and angle combined

Next I’ll look at my other favorite reference, Competition Car Aerodynamics. McBeath cites CFD work done on NASCAR spoilers, in which they changed both the spoiler height and angle. Now we’re getting somewhere.

I’ll use the above results to compare spoilers of different lengths and angles that result in a similar total height above the deck. Which in turn allows me to figure out the most efficient spoiler angle.

  • 160mm spoiler, 20 degree angle, 54.7mm total height
  • 80mm spoiler, 40 degree angle, 51.4mm total height
  • 60mm spoiler, 60 degree angle, 52mm total height

It’s a bit difficult to see in this graph, but a 60mm spoiler set at 60-degrees is slightly better than a 160mm spoiler set at 20 degrees, even though the longer spoiler is a little bit taller. In other words, a higher angle works better. But it’s only by a small amount.

Based on Katz and McBeath, here is my simplified conclusion: The total height of the spoiler is all that matters.

NASCAR spoilers

NASCAR used rear wings for a short period of time and then switched back to spoilers. Not because they could get better performance from a spoiler, but because the series is always looking for ways to make racing both closer and safer, and the wing did neither. In addition, the fans didn’t like the look of a wing. To be fair, the CoT wing was hideous, see for yourself.

Yuck.

So we can’t look to NASCAR for the most effective spoiler design, because we know their priorities lie in close racing rather than outright speed. But it’s worth noting a few things about NASCAR spoilers.

  • NASCAR probably knows more about spoiler design than any other race series, and they still don’t settle on one design. In fact, the regulations change almost yearly. Looking only at the height, in 2016 it was 3.5″, in 2017 2.375″, and in 2019 8″.
  • Some years the spoilers were adjustable for angle, some years they were fixed, and there have been different heights, widths, and shapes throughout the years.
  • NASCAR uses the spoiler to balance not only the overall aero package, but as a way to balance the performance between different cars, and at different tracks.
  • When NASCAR reverted from rear wings to spoilers, they set the spoiler angle at 70 degrees. In 2019 the fixed angle remains 70 degrees. Interesting.

Here’s an excellent article on A comparative look at NASCAR’s new spoiler, old spoiler, and wing.

Nscs-newspoiler2010hi_medium
Click image to enlarge.

NASCAR spoiler shapes

The 2019 spoiler is flat across the top, but different shapes have come and gone.

Image result for nascar spoiler shape
Curvy, almost bat-wing style.
Image result for nascar spoiler shape
Convex top edge.
Image result for nascar spoiler
Concave top edge.

The size and shape of Miata spoilers

So now that we’ve looked at spoiler theories and real-world examples from NASCAR, let’s get down to what matters: Miata spoilers.

  • Miatas have a roofline that is peaked in the middle, and you might imagine that the ideal spoiler shape has a matching convex arc to it. Although like all things aerodynamic, this could be totally false, and maybe the sides should be taller.
  • The rear edge of the trunk is curved and so a curved spoiler would look more natural, and could be an easier DIY project as well. Also, a curved spoiler would be more rigid than flat. However, some race series say that the spoiler must be flat, with no curvature. Booo!
  • There’s no reason to “spoil” the air coming along the sides of the car, and so a spoiler much wider than the rear canopy seems like a waste. Although the exposed spoiler ends are probably adding downforce. Albeit not very efficiently, and at probably a different angle than is ideal for spoiling the roofline shape.

Miata products

This IKON spoiler is an attractive design, with a convex top edge and curved profile. It would be neat to see something like this with a flat extension that’s adjustable for height.

The Rocket Bunny spoiler is flatter across the top, taller, and with a steeper angle. I’d guess it’s slightly more effective than the Icon, but it has a tacked-on look that doesn’t really appeal to me.

And then there’s this JSP spoiler that looks like a wing, but isn’t (air isn’t going to flow under it, hence not a wing). The shape follows the curvature of the sides and roof, and this may be an efficient design. But meh to the looks.

Of course all of these spoilers have a fixed height and angle, so there’s no way to adjust the aerodynamic balance. On the other hand, the Blackbird Fabworx spoiler is large and adjustable for angle. I’m also not a huge fan of the way this one looks, but the beauty lies in the function.

Spoiler done right.

DIY spoiler, testing height

I made my own spoiler, it’s about 3.5″ tall and has some curvature to it that follows the trunk shape. It’s made of plywood and fiberglass, and there are 6mm T-nuts so I can add an extension.

With the low spoiler (without any extension), I ran very consistent 1:22s at Pineview Run. And by consistent, I mean 1:22.03, 1:22.05, 1:22.07, and in my second run, 1:21.99, 1:21.99 and 1:21.93. This was a hot day, and if I compare the times to previous ones, the track was definitely slower than normal.

With a 3.5″ extension (total 7″ height), my lap times were less consistent, most of them around 1:21.5, but my fast lap was a 1:21.03, almost a full second faster. But that one was an outlier, and if I average the five fastest laps, the taller spoiler was about .55 seconds faster than the lower spoiler.

The following table is an average of four back-t0-back runs, two with the spoiler extension, and two without. I’ve averaged the top six fastest laps.

ConfigurationAvg LapSimulatedHPLbsCgCdCl
Low Spoiler1:22.01:21.1111224001.00g.44-0.25
Tall Spoiler1:21.451:20.6311224001.00g.45+0.20

I added .01 to the Cd as a guess, but drag isn’t that consequential anyway. I came about the Cl figure by changing that value in OptimumLap until I got the .55 delta in lap time. It seems absurd to think a spoiler can make a .45 swing in Cl, but that’s what the simulation says. Interestingly, this is also the value cited for a 8″ tall spoiler in MacBeath’s Competition Car Downforce.

In Race Car Aerodynamics, Katz cites several examples of spoilers, but none that go as high as 7″. In his examples, the relationship between height and coefficient of lift is nearly linear, and from 0″ to 4″ there’s a change of about .4 in Cl. So if I extrapolate those values from a 3.5″ spoiler to 7″, I’d only expect to see a change of .4 Cl, which is again pretty close to the test result.

Whatever the case, a 7″ tall spoiler works on a Miata. Now I have to make a taller one and test that.

Miata vs RX-7 Aero

Not so different. Yet so different.

In 1993, the Mazda Miata had a coefficient of drag of .38, and the RX-7 had a Cd of .29. Same manufacturer, same year, both two-door sports cars, and yet the RX-7’s had 20% less drag.

There’s nothing magical about the RX-7 shape, and if you compare its Cd to new cars, it’s only so-so. In The Most Aerodynamic Cars You Can Buy Right Now there are many cars with Cds from .27 down to .22, and a unicorn at .189. (Follow along in this Aero Timeline and see how Mercedes has incrementally improved their aero from high .4s down to .24 complete with wind-tunnel smoke trails.)

But let’s stick with the same year and manufacturer, and see what would happen if you could magically put a RX-7 body on a Miata, and what that would do for performance and fuel economy.

Calculating top speed

To calculate top speed, I’ll use the RSR Bonneville Aero-Horsepower & Drag Loss Calculator. I’ll enter data for a 1993 Miata, with frontal area of 18 sq feet and a Cd of .38. Miatas of that vintage had about 116 crank hp, and if I multiply by .82 to simulate driveline losses, that’s about 95 hp at the rear wheels. (You can argue driveline losses, I’m using figures from Competition Car Aerodynamics.)

First I want to calculate top speed, so I’ll throw some numbers into the calculator until the Horsepower Needed field reads 95. Turns out that 116 mph is the top speed.

Now I’ll drop a RX-7 body on the Miata, and drop the Cd to .29. The top speed goes up by 10 mph to 126 mph. Wow!

However, top speed is rarely important, so I’ll plug in some more common values. I’ll use 60 mph to represent the exit of a corner, and 90 mph to represent a faster section of track. How much power is required to go that fast, and how much power remains?

CdHP to go 60 MPHHP RemainsHP to go 90 MPHHP Remains
.3816.578.546.848.2
.2913.881.237.857.2

At 60 mph, the low-drag RX-7 body has an additional 2.7 hp available over the standard bodywork. Meh. At 90 mph, there’s an additional 9 hp available from the sleek RX-7 body. Wow! Obviously, the faster you go, the more important drag becomes.

Simulating lap time

Drag is obviously important, but more important is lift (downforce). We need the numbers for both drag and lift in order to calculate a lap. I don’t have any published numbers for lift on a Miata, but the Hancha group did CFD testing and I’ll use their lift value of 0.27. In Race Car Aerodynamics (p. 19) Katz lists the RX7 at .24 lift, and AutoSpeeds article on Aero Testing even breaks that down into front lift vs rear. Let’s plug these values into OptimumLap and simulate lap times at my local track, Watkins Glen International.

MiataRX7 MiataDelta
Drag.38.29.09
Lift.27.24.03
Lap2:34.932:33.381.55

So a Miata with a RX-7 body would go over 1.5 seconds faster than a stock Miata. Some people would give their left nut for a second-and-a-half per lap. I’m betting that with windows open, which is how I’ve always raced, the RX-7 advantage would be even higher. This because the Miata hardtop is quite wide, and acts as a parachute, especially when in yaw.

Fuel economy and race strategy

In sprint racing, fuel economy is meaningless, but in endurance racing, it can be important. Especially if longer stints will allow you to do one fewer pit stop during the race, or if your car is right on the cusp of doing the maximum allowed stint. OptimumLap shows a 2.5% decrease in fuel economy using the RX7 body. That doesn’t seem like much, but it can be a big difference.

Let’s use my race Miata as an example. It burns about 7 gallons per hour, and with its 12.7-gallon gas tank, it can go about 1:50 before the tank runs out. This is not a problem in AER where stints are 90-minutes long. But in Champcar or Lucky Dog, stints are two hours long, and I end up doing an extra stop each day. In cases like this, 2.5% fuel economy can be a huge deal.

So not only is the RX7-bodied Miata going 1.5 seconds faster per lap, it’s doing that while burning 2.5% less fuel. If I calculate the total number of laps per stint, the driver in the stock Miata can do 42.6 laps per stint. The driver in the RX-7-bodied Miata can do 44.1 laps.

Imagine if Mazda made a RX-7-bodied Miata, without the design compromises of a convertible top. It would be sleeker, lighter, more rigid… and probably fall flat in sales. Ah well, it would have been great on track!