This article was originally posted by Mark Nokaoi to the VFR List and he has graciously allowed me to post it here as a reference for others. His response to the meaning of NACA answers a question that seems to be asked often about the VFR ducts.
NACA stands for "National Advisory Committee for Aeronautics" and is one of the predecessors of NASA. In the early days of aviation, NACA would mathematically define airfoils - e.g., NACA 071 - and publish them in large reference texts, from which aircraft manufacturers would select for their specific applications. As you probably know, an airfoil is a compromise of lift versus drag, and this relationship changes with the airflow velocity, density, etc, so no one airfoil is optimal for all conditions. As to the NACA duct, I'm trying to say this without sounding professional, but here it is in case you're interested.The purpose of the duct is to increase the flowrate of air through it while disturbing the aerodynamic boundary layer around the fairing as little as possible. When you increase the cross-sectional flow area of the duct, you decrease the static pressure and make the duct into a sort of vacuum cleaner, but without the drag effects of a plain scoop. The reason why the duct is very narrow, then suddenly widens in a graceful arc is to increase the cross-sectional area, but not so quickly that the airflow separates and causes turbulence (and drag). If there are any questions, see me after class.
- Mark
