What is a NACA Duct?
What Is It?
The words "NACA Duct" usually go by so fast that they sound like "Nackaduck". NACA ducts are those funny-looking clear inlets on the car windows. You'll also find carbon-fiber NACA ducts bringing air into the front of the car for brake cooling.
NACA (the National Advisory Committee for Aeronautics) was the predecessor of NASA (the National Aeronautics and Space Administration). If you think drag is a bad thing for a race car, talk to an aeronautical engineer. Every bit of drag on an airplane or rocket increases the amount of fuel needed to stay in flight and limits the maximum speed achievable. Launching spacecraft is expensive, which makes minimizing drag critical. The NACA duct brings air into a vehicle with minimal increase in drag. If you like old scientific reports, NASA has the original paper on the origin of the design on their website.
How Does It Work?
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. The best case for getting the maximum speed is to have a thin boundary layer with laminar flow. The thicker and more turbulent the boundary layer, the more the air will keep the car from going forward.
Ideally, you'd make the car a solid piece so that there are no holes or edges on which the air can catch; however, you have to get air into the car for cooling purposes (both the driver and the brakes). Making a hole is going to increase the drag significantly and that is where the NACA duct comes in.
NACA ducts have sloped inlets that gradually broaden. In the picture, the duct would be oriented so that the narrow side faces in the direction the vehicle is moving. The round tube on the left side of the picture would be inside the car and attached to a hose that would direct the air coming into the car to the driver's helmet, for example.
The air molecules closest to the car move at the same speed as the car. 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. The NACA duct is flush with the car’s surface and doesn’t significantly disrupt the streamlines of the car.