Wing Efficiency Doesn’t Matter

A wing that is set up properly doesn’t add a lot of drag. My real-world testing showed that my 9 Lives Racing 60″ wing set at 4.4 degrees AOA and roof height added .03 to the coefficient of drag (Cd) of my Miata, no matter if it was an open top, OEM hardtop, or fastback.

On a Miata with an OEM hardtop, adding a wing accounts for an increase of 6.25% drag, which is slightly more than the two mirrors combined (see Where Drag Comes from on a Miata). That’s nothing compared to the benefits of a wing, and yet wing drag and wing efficiency are a constant source of conversation and consternation. There are much better things to concentrate on, like the lift/drag ratio of the entire vehicle. Let’s get into it.

The effect of changing wing angle

If you want to go faster around a track, then downforce outweighs drag reduction. Every. Fucking. Time. And yet people make purchasing decisions based on which wing has less drag than another. Or they adjust wing angle for more efficiency rather than more downforce. Going after efficiency only makes you go slower, and I’ll prove it to you with a few simulations.

For the simulations I’ll use a Miata with round numbers: 2400 lbs, 120 hp, 1.2g, Cd .45 (plus wing), Cl 0 (plus wing). This represents the average Miata with bolt-ons running on 100 TW R-comps or Super 200 tires. The Cl and Cd values will change with each configuration, as I adjust wing angle. I’ll simulate a 9 Lives Racing wing, using their published CFD data for wing angles of 0, 5, and 10 degrees. Many wings operate in a similar window, and so this is mostly a generic wing choice. I’ll run the make-believe car around Mid Ohio, which has both fast and slow sections, and is about average for a race track. (I’ll also simulate the car at 75 hp, but more on that later.)

Here’s how it shakes out:

Wing angle 0, 5, and 10 degrees at 120hp, and then reduced to 75 hp.

In the table above, the 10-degree setting wins with a 1:40.75 lap (100.75 seconds). It’s only a little faster than the 5-degree setting, but both of them beat the zero-degree wing by a fair margin.

Does the most efficient setting ever work? In a straight line, yes. And even in this simulation, the zero degree setting has the highest top speed. On an oval track zero degrees would probably work well, but on a typical race track, maximum efficiency never wins.

Take a look at the last three columns where I reduced power to 75 hp. This is like a Miata running on three cylinders. In this detuned state, the 5-degree angle wins over 10 degrees, but just barely. Meaning, if your car has 75 hp, then you can babble on about optimizing your wing angle for less drag. Everyone else STFU.

Wing maximum efficiency is baloney

Given that the most efficient wing angle was the slowest, it begs the question: does wing efficiency matter? As a static number of “maximum efficiency”, no. At a certain coefficient of lift, a tiny bit.

You can research this yourself on Airfoil Tools, or read the article I wrote Car Wings Examined. The gist of that blog post was that there are different wings for different uses. Of the 1638 airfoils in the Airfoil Tools database, there is no single wing that is the most efficient at all speeds. I used the site’s search feature to find the most efficient wing, and each of the wings below is the “most efficient wing” at a different Reynolds numbers (which you can think of as different speeds, or different size wings, or both).

Which is the most efficient airfoil? All of them.

Consumers who believe that a wing that has a 17:1 lift/drag ratio is going to be better than a wing with a 12:1 ratio (or whatever) is a victim of marketing, bad assumptions, and lack of knowledge. The airfoil that has the highest maximum efficiency won’t make the most downforce when set at a high angle. Likewise, the airfoil with the most total downforce is probably not very efficient at low angles.

To sum it up, as it relates to car racing, an airfoil’s maximum efficiency is total bullshit.

I’ll tell you what’s to blame for this: advertising. Wing manufacturers like to compare dick sizes and somehow wing efficiency became their ruler. Touting their higher “17:1 lift/drag ratio” gives them a chubby. Every wing manufacturer seems to do this, and so I understand having to keep up with the Jones’s, but it’s still utter and complete nonsense as it relates to the only thing that matters – lap times.

It would be better if wing manufacturers stopped competing with useless information and told us the maximum coefficient of lift, what angle of attack that occurs at, and the lift/drag ratio at that point. That’s the data we need to make purchasing decisions.

Aerodynamic efficiency of the vehicle

I previously mentioned that a wing added .03 drag to the vehicle, for a total of .48 Cd. That might or might not seem significant to you, so let me put this another way: the wing is responsible for 1/16th of the drag of the entire vehicle. If you read my post on Where Does Drag Come From, you’ll see that reducing drag on any other part of the car is going to return larger gains. Optimizing for wing drag is a waste of time.

In the OptimumLap output (the spreadsheet image above), notice the Aero Efficiency field, the fourth row from the bottom. Aero Efficiency is a measurement of the total lift/drag ratio of the vehicle, and this increases with wing angle. Now wait a goddamn minute; increasing wing angle makes the wing less efficient, right? Correct. You have to think of aero as a system, and a wing is just part of that system. A wing doesn’t add a lot of drag to the entire vehicle no matter what you do, but can add a lot of downforce at a much higher rate.

A Miata with the wing set at the highest-downforce least-efficient wing angle (without stalling) gives the entire vehicle the most efficient lift/drag ratio. If you care about aerodynamic efficiency, care only about this.

But don’t misunderstand this I’m saying and set your wing to 10 degrees! You have to take into account the downdraft angle of air induced by the roofline shape, which changes depending on the height of the wing, and changes across the length of the wing as well. You can read about that in my post on Visualizing Airflow, but at roof height on a Miata, the downdraft angle is an average of about 5 degrees (5 degrees in the center, 7 degrees at the wing stands, and zero at the ends). When I say the wing is most efficient at 10 degrees, that’s free stream air. On a Miata, the best you can do with the average car wing is 5 degrees. Unless you can get the wing higher, where there’s less change in the angle of the air coming over the roof, or you’re using a wing that can operate at a higher angle of attack.

The fact is that different airfoils work better at different angles of attack. Most of the wings I’ve reviewed in Car Wings Examined have a range of about zero to 10 degrees before stalling, but some work up to 15 degrees. I typically cite 9LR wings for examples because a) awesome, b) tested it, c) have not found anything better. I’m making some wings myself, but I don’t think there’s going to be a marked improvement in the shape. I’m just going after more chord and I like DIY projects.

When drag matters

I oversimplified to exaggerate a point, and there are some outside cases where wing drag matters. Certainly anywhere top speed is more important than cornering speed, such as land-speed records. An oval track is another area where wing drag reduction could be beneficial over downforce. But I haven’t seen Miatas setting land-speed records or racing oval tracks, and for normal racetracks where Miatas race, there are very few situations where wing drag matters. The only time it does is with low powered vehicles (under 75 hp) and the following corner cases.

Endurance racing

I’ve run a lot of endurance racing simulations in OptimumLap, and downforce wins over drag 99% of the time. This is true even when fuel consumption forces you to take an extra pit stop over the course of a day. However, there are some cases where you can optimize your fuel consumption and lap times using a lower-drag lower-downforce configuration, and ultimately complete more laps.

That one in a hundred times is always the result of one fewer fuel stop. If you’re racing in AER, with 90 minute stints and 3 minute pit stops, then this will never work. If you’re racing in 24 Hours of Lemons (where stint time is unregulated), and can do a two-driver change the first day, it can work. For Lucky Dog and Champcar where there are 2-hour stints and 5-minute pit windows, you have to be right on the cusp of a 1:55 stint time, and then wing drag matters. Although a single full-course yellow incident during a stint would have the same effect.

Watkins Glen

At Watkins Glen, in a Miata with less than 100 hp, drag matters. I’ll spare you the data, but I ran the simulations and the 5-degree and 10-degree wing angle came out to exactly the same lap time for a Miata with 100 hp. Both of these beat the most efficient wing setting by half a second. When I increased the power above 100 hp, the high-downforce setting won every time. Ergo, if you have less than 100 hp, then drag matters, at this track and similar tracks. At most any other track, wing drag doesn’t matter, go for maximum downforce.

Speaking of Watkins Glen, I’ve never been to an endurance race there when the track wasn’t under full-course yellow for less than 45 minutes. Someone always hits a wall (not pointing fingers, one time it was me), and this brings out the pace car and many laps under caution going 40 mph. This usually happens three or four times in a 8- to 9-hour day. In these situations you save a lot of gas, and optimizing your wing for more efficiency would just slow you down when the track goes green.

Bad wings and bad setup

I tested a cheap 53″ double wing at Watkins Glen, and it had almost as much downforce as my 60″ 9 Lives wing, but a shit ton more drag. A shit ton in technical terms is 300% more drag than the single wing, and this changed the total vehicle Cd from .48 to .55. Even worse, the rear-biased drag lifted the front of the car, and so the front lost .2 Cl downforce! In this case, wing drag absolutely matters.

It’s worth noting that OptimumLap simulations predicted the car would go faster with the double wing than without it, and so even a crappy, draggy, $70 wing is better than no wing at all.

A similar situation would occur if you set a single element wing with too much angle, and put the wing into a stall condition. This is often the result of setting the wing angle without accounting for the downwash angle of the roof.

Image from F1 Dictionary

Aero balance

Going after maximum wing downforce may produce a slower car. If reducing wing angle makes the car go faster, then either the wing was set too high (stalling) or the car understeered so much that it ruined the balance of traction between front and rear tires.

An understeering car can be fixed in a number of ways, the easiest is adding chassis rake (raising the rear of the car, or lowering the front). According to Supermiata alignment settings, Miatas are sensitive to chassis rake and this can be an easy way to adjust balance. Another way is to reduce the front roll couple by increasing rear spring rate or sway bar thickness, or decreasing those in the front.

Aerodynamic balance is really a topic unto itself, and I’ll get into that in the future. But if you don’t care about lap times, and you’re just having fun in HPDE, play with wing angle and aero balance. Less angle means less rear grip, and many people enjoy a bit of oversteer, even if the car has slower lap times and is less stable at high speed.

Finally, if you’ve set your wing up correctly, you can ignore wing drag and efficiency as variables of any consequence. The only thing that matters is the aerodynamic efficiency of the entire vehicle. The benefits of downforce far outweigh the penalties of drag, and as a whole, the vehicle will have the highest aerodynamic efficiency with the wing set at the highest, least efficient angle. The next time I hear someone talk about reducing wing drag, or how efficient their wing is, I’m going to punch them in the dick point them to this article.

One thought on “Wing Efficiency Doesn’t Matter”

  1. However.. one could increase the wing efficiency and reduce the drag of their 9LR Big Wáng by going with their dual element setup compared to cranking it down to 10° with a 3/4″ gurney flap.. but who in their right mind would do such of a thing.. I’ll see myself out and eventually correct the error of my ways..

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s