Quote:
Originally Posted by jamesqf
I think you mean cross-sectional area? It's something people tend to forget when they talk aerodynamics. Cd is a function of shape, so a low Cd is better, but drag is Cd * cross-section. So even though the semi may have a high Cd, the cross-section relative to total weight is much smaller, so drag per pound is less.
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No actually. Frontal cross sectional area has already been mentioned several times. Frontal cross sectional area is also proportionally smaller on a semi, but it is not the [I]only[I] factor. A semi with a trailer a mile long will have a different Cd than a "normal" semi even though both have the same frontal cross sectional area. The people who have to deal with Reynolds numbers and long pipes could tell you all about that subtle difference. Also, the high end competitive boat people pay a lot of attention to "wetted area" since their "aerodynamic" losses go up much faster with a lot of exposed surface area compared to moving objects in air.
Admittedly, it is not a major factor.
Finest regards,
troy