What’s a NACA Duct?

Once upon a time, during a period in which our country — and the rest of the world — was in turmoil, the President of the United States wanted to create a governmental body that would be charged with coordinating critical scientific work being done by government with similar work carried out in industry and at universities.  Congress repeatedly refused to authorize the legislation.  The body wasn’t approved until three years after it was originally proposed – and then, it passed only because its creation was tacked onto the Naval Appropriation Bill and no one really noticed.

Sounds like our current Congress, doesn’t it?


This actually happened during World War I and the governmental body that was finally approved was the the National Advisory Committee for Aeronautics.  Its task was coordinating war-related projects between the government, industry and universities.  President Taft started the process in 1912, but it wasn’t until 1915 that President Wilson finally signed the legislation directing  that NACA (which was pronounced with each letter spoken, not as a single word)  would “supervised and direct the scientific study of the problems of flight with a view to their practical solution”.   One of the first appointees was Orville Wright.  (Yes, the logo — shown at left — is a little lame, but the government had a couple sort of important things going on at the time.)

NACA addressed their mission and much more – they were responsible for new, high-speed wind tunnels, engine test facilities, and lots of fundamental discoveries that facilitated commercial and military planes, including breaking the ‘sound barrier’.  Although the organization no longer exists (having morphed into the National Aeronautics and Space Administration in 1951), many of its discoveries and breakthroughs are still used today.

NACA Ducts

The words “NACA Duct” usually go by so fast that they sound like “Nackaduck”, but a NACA duct is really just a specialized type of duct.  Ducts are hoses or other devices used to move air from one place to another.  brings air into (in our case) a racecar in a very special way.   Clear NACA ducts can be seen in the rear windows of NASCAR cars – there are usually two or three in the window.  Carbon-fiber NACA ducts sit in the front of the car.

NACA Ducts

Aerodynamic forces — like drag — increase like the speed squared.  If you go twice as fast, you get four times as much drag.   If you think a race car going 200 mph experiences a lot of drag, think about an airplane at the speed of sound — which is 768 mph.  If a NASCAR race car could go the speed of sound, it would experience 15 times more drag than when it was going 200 mph.

The reason we don’t like drag, of course, is because drag always acts in the direction opposite the direction the car is moving.  No matter what direction you’re moving, drag is always trying to slow you down.  If you don’t care about being able to go faster, you still want to decrease drag because you need to use energy (gasoline) to overcome drag.  (I’m going to have an interesting article coming up on some nanotechnology being used to decrease drag in airplanes — that might be an option for NASCAR teams for Daytona and Talladega.)

Air must be brought into the car  to cool the brakes and to cool the driver. If you just pop out the rear window, you get a lot of air into the car — but you can’t direct where it’s going and you’ve just created a whole lot of drag.  The NACA duct was designed to bring air into a vehicle with minimal increase in drag.  (NASA has the original paper on the origin of the design on their website.)

How It Works


Some current road cars have air scoops sticking up out of the hood of the car to bring air into the engine – the problem with something poking out of the car is that it increases drag.  NACA ducts are submerged ducts – they are installed flat.  The picture at left is a closeup of a carbon fiber NACA duct.  The air comes into the narrow end and a hose is attached to the round part so that you can direct the air to the brakes or the driver’s airbox.

Drag is literally friction between air molecules and the surface of the car.  The air molecules closest to the car move at about the same speed as the car, but the air molecules further from the car’s surface move faster.  The boundary layer is the thin layer of air nearest the car’s surface.  The boundary layer can be thick or thin and the flow can be laminar (meaning in straight, smooth lines) or turbulent.  NASCAR aerodynamicists want  a thin boundary layer with laminar flow.  The thicker and more turbulent the boundary layer, the more drag.  The length and shape of the NACA duct creates counter-rotating vortices that deflect the boundary layer away from the intake, but draw in the faster-moving air above it, thus allowing you to get air into the car without creating a lot of extra drag.

During qualifying, you may see teams taping over the NACA ducts – in a sport where hundredths of a second count, eliminating even the small amount of drag the NACA ducts create can make a difference in where you start the race.  Like the engine opening, you can run a couple of laps without brake cooling or driver cooling, but you can’t run an entire race.

Although NACA ducts were originally designed for high-speed aircraft, they can’t produce large airflow or high-pressure airflow, which means that they really aren’t used anymore in the jet engine applications for which they were originally designed.  They are, however, ideal for cutting down the drag on racecars.

Is There Really a Second-Place Curse?

One of the commentators after the final race in Homestead mentioned that Jimmie Johnson should be happy he finished in third because it allows him to avoid the “dreaded second-place curse”.
Anytime someone says something like that, it makes me wonder whether there really is a curse, or whether that person had just been talking to Carl Edwards.  So I analyzed a little data and guess what… there really IS a second place curse.

I used data from the last twelve years — from racing-reference.info, bless them!  After trying a couple of different approaches to making the data easy to visualize, I ended up with something a little more complicated than I would have liked.

Bear with me – it’s not as yucky as it looks.  I have plotted on the horizontal axis the place in which a driver finished in the first year listed, which we’ll call “X”.  I then calculated the change in positions of the same driver the next year (X+1) and plotted that on the vertical scale.  So the first set of data has X = 2000 and X+1=2001.

  • A positive number on the vertical axis means that the driver finished better by that many places in the following year. For example, +5 means that the driver finished five places better the next year than they finished the year before.
  • A negative number on the vertical axis means they finished worse the next year. A -5 means they moved down five spots in the final standings.

I went through and removed any special cases — like Mark Martin running full time one year, but not the next, Busch brothers missing races (that’s a different kinds of curse), people retiring, etc.  The graph below summarizes the top 16 finishing places and the change in final standing over the last twelve years.

There’s an obvious statistical implication:  If you finish second, for example, you have only one place to move up and forty one places to move down.  You’re either going to win the championship next year, become second again, or move down.  The probability is that you’re going to finish worse than second.

To look at the data in a slightly different way, I plotted it the same way they plot the daily activity of the the stock market:  the symbol shows you the average.  One line extends up to the maximum increase in position and one line extends down to the largest drop in position.


The first-place curse

In fact, if we’re going to call dropping in the standings a “curse”, then there is clearly a first-place curse that affects everyone except Jimmie Johnson.  Mose drivers who win the championship one year inevitably finish worse the next year.  When I say ‘drop in points’, it’s not a huge drop:  nine places was the most anyone who finished first dropped.

The average first time finisher fell almost five positions.  That’s including four consecutive ‘0’s due to Jimmie Johnson.  If we exclude Jimmie just because what he did was really unprecedented (and unlikely to be duplicated), the average first-place finisher falls almost seven positions the next year – about the the same as the second-place driver.

The second-place curse

Second place shows a very similar story, only worse.  There is only one case in twelve years in which the second place finisher one year won the championship the next year.  That was Jimmie Johnson.  Whoops – Rick pointed out my mistake.  It was 2001 -2o02 and the driver was Tony Stewart!  On average (including Jimmie), the second place finisher finishes about seven positions lower the next year.

The three biggest drops in point standings (-15, -13, -11, -9 and -7) are due to Martin, Edwards, Biffle, Edwards and Hamlin.  There are no extenuating circumstances like crew chief changes, owner changes, etc. on which to blame the drops.  Four out of five of those drivers were all driving for Roush at the time… maybe there’s a Roush curse?

The bad news for Jimmie Johnson… and everyone else who made the chase

Here’s the bad news for Jimmie:  Yes, he avoided the second-place curse; however, no third-place driver has gone on to finish first or second the next year.  The best they’ve done was to match their third-place finish.

Yep, perhaps there’s a third-place curse as well, as third-place drivers finish an average of three places lower the following year.

In fact, you don’t find a finishing position in which there is an average probability of bettering your finish until 7th place.  On the graph above, you can see that the majority of finishes were improvements, although without one -11 change, it would be a much more positive number.  After that, it’s an oscillation between slightly better and slightly worse.

A caveat of this data analysis is that the Chase sort of messed things up going out past 10 because a driver in the Chase can’t finish lower than 10th, even if he misses races or otherwise would have fallen much lower without the Chase format.