Aerodynamics / Aerodynamic Cost / Aero parts

It wouldn't be surprised if the aerofoil section used on bicycle frames is one of the NACA 4-digit aerofoils, so NACA 00XX range, probably a NACA 0012, where the '12' is the percentage of max. thickness to chord length.

UCI bikes are regulated to a thickness of just over 30% of chord length measured perpendicular to the tube's long axis. Measuring chord the aeroplane way parallel to the direction of travel, that's about 25% for the head and seat tubes and about 15% for the downtube. Because the downtube has a lot of sweep in aeroplane terms, it creates what would be spanwise flow if it were a wing, so thickness:chord measured in the direction of the flow field could be under 10% at some points if the basic tube shape was as thin as is permitted.