[citation][nom]LMF5000[/nom]You're confusing up profile drag and lift-induced drag. Wikipedia will be able to explain them better than me, but the basic concept is that threre are two primary kinds of drag on a wing/propeller shaped object like a fan: Profile drag is the smaller of the two. It arises because you're pushing the blades through the air and overcoming the friction between the air and the blade's surface. You'd get profile drag even if the blades were flat and weren't pushing any air.Lift-induced drag is caused by the blades "pushing" the air. The greater the angle of attack of the blades, the more air they push, and the higher the lift-induced drag. So if the blades were flat, you'd get zero lift induced drag (but still get profile drag), but in an actual fan with the blades angled, you'd get both profile drag and lift-induced drag (which is the larger of the two).By adding dimples, you're not increasing air flow "because you are increasing the resistance", but rather the dimples decrease profile drag (because like a golf ball, the dimples make the air flow turbulent instead of laminar, so you reduce air resistance). The fact that resistance is reduced (not increased) was written in the last line of the first paragraph of the article. My guess is that lift-induced drag remains unchanged by the dimples. So there you go you'll get the same amount of airflow but need a less powerful motor to overcome the combined drag, leading to quieter running.[/citation]
Except that dimples do not always reduce profile drag.
The reason that the dimples work on a golf ball is that turbulent boundary layers are more resistant to flow separation. This means that the flow stays attached to the surface of the ball farther around the back of it, causing it to have a smaller region of stagnant air behind it. Less stagnant air = less drag.
With an airfoil or streamlined object, such as a fan blade, a turbulent boundary layer is not beneficial. Although it is still resistant to flow separation, an airfoil or streamlined shape is not prone to flow separation regardless, even with laminar flow. Because flow separation is not a problem, skin friction drag is the majority of the profile drag (rather than pressure drag, as with a sphere). With a turbulent boundary layer, which is what the dimples are meant to induce, the skin friction drag actually increases. There's a reason that airplanes aren't dimpled. In fact, on the Boeing 787 (now in testing), a tremendous amount of effort went towards decreasing the amount of turbulence on the front part of the wings (especially on the engines), maintaining laminar flow as long as possible to reduce drag.
(Oh, and induced drag is effectively unchanged)
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