A NACA duct is a type of low-drag air intake vent used on a wide array of vehicles from space craft to airplanes, boats and automobiles. The intake vents were originally designed for jet engine use by the precursor agency to NASA, the U.S. National Advisory Committee for Aeronautics. The NACA duct also has shown to be a very effective air intake for cooling engine components as well as braking systems on race vehicles. The design and construction of the NACA duct is very efficient at bringing air into the vent without disturbing the flow of air over and around the vent. Ironically, the NACA duct does not work very well in its intended role as an air intake for jet engines, and the ram effect of a straight-on intake is much more effective at packing air into a jet engine.
Drag is the force of the air that is trying to prevent a vehicle from moving forward. Holes in the side of a vehicle, such as open windows and vents, add to the drag placed onto the vehicle. In a rocket or aircraft, drag equals fuel waste and a decrease in airborne time. In a racing vehicle, drag equals less top-end speed. Unfortunately, air must be taken in through the outside of all of these vehicles in order to cool the engines, brakes, drivers and crew.
By placing a NACA duct or several ducts on a flat surface, air can be taken from the area that is closest to the vehicle's service area without creating more drag. This lowest or closest level of air is the slowest-moving around the vehicle, so it may require more than one NACA duct to draw in a sufficient flow of cooling air. Even though efficient at reducing drag on a vehicle, most racing teams will cover a NACA duct with tape in order to gain valuable speed when timing a racing car for entry into a race.
A large hose fastened to the end of the NACA duct delivers fresh air from the outside of a vehicle and directs it to where it is most needed. The typical location for a NACA duct on a race car is the side window area, the hood, trunk lid and roof. On airplanes, the ducts can be found along the sides of the fuselage as well as the cowling area of the engines. Boats and water craft will typically use the ducts on the deck around the engine area to bring cooling air into the engine compartment.