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@gbj_tester
blah blah Reynolds Number blah blah fineness ratio blah blah.
Basically, a circular cross section will get flow separation at a very low speed. The higher the speed, the longer the aerofoil has to get to avoid this. You always trade induced drag (shit that happens in the wake vortex, plus pressure build up on the nose) for skin friction (shearing between the boundary layer and the free stream), so each aerofoil is optimised for a specific speed. For bicycle frame tubes, the ideal fineness is somewhere around 6:1, i.e. the tube depth parallel to the flow should be around 6 times its width. The UCI limit on depth (orthogonal to the tube axis) of 3 times the width doesn't let you achieve this even after taking the angle of the tube axis relative to the free stream into account.