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.

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.