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I have a fan on a stand, to circulate air in my home. The three fab blades are somewhat "D"-shaped, with one edge being rounded and one being straight-ish or even curving inward.

Now, from my sailing, as well as my understanding of wing and propeller (edit: and low-noise computer fan) design, I would expect the outward-curving edge to be the leading one, but it's not. In the photo*, the fan spins clockwise to push air towards me. enter image description here *excuse potato quality, the fan was spinning so this is with reduced shutter time and high ISO.

In contrast, this low noise pc fan also spins clockwise, yet its blades do curve "backwards" in relation to their direction of spin (as I'd expect).

Wouldn't my fan be less noisy if it spun the other way, at least theoretically? Obviously, including inverting the blade tilt, in order to keep airflow direction.

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I believe the main reason for the different blade shapes is ducted vs non-ducted fan design and tip vorticity. PC fans are essentially ducted fans with a fan-blade shape that produces tip vortices. These vortices hit the edge of the duct, which dissipates (or weakens) the vortex. OTOH, the portable fan in your picture has no duct, so the shape of the fan blade (forward swept) is designed to direct the span-wise flow towards the inside of the fan rather than the edge. This allows the blade to produce more airflow, since less energy is used in producing a tip vortex.

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Most people like to have the air blown towards them - a direct flow of air feels like it helps.

The noise is mostly (probably) due to the motor noise - it is unlikely that the blade tips are going supersonic...

Edit based on comment - the shape of the blades will have been considered in terms of solidity (efficiency of moving the air) and also, but less so, in terms of flicker ie the gap between the blades as they rotate.

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  • $\begingroup$ Yes yes of course you are right. But I was merely considering the shape of the blades versus their direction of spin. $\endgroup$ – KlaymenDK Jul 28 '18 at 14:51
  • $\begingroup$ @KlaymenDK They are designed to be quiet over moving air. Where the blades on a propeller plane are designed to move air over being quiet ++1 $\endgroup$ – user4139 Jul 28 '18 at 15:49
  • $\begingroup$ @muse there is also the point that most airplane props are pitch controlled... $\endgroup$ – Solar Mike Jul 28 '18 at 16:31
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    $\begingroup$ I believe the main reason for the different blade shapes is ducted vs non-ducted fan design and tip vorticity. PC fans are essentially ducted fans with a fan-blade shape that produces tip vortices. These vortices hit the edge of the duct, which dissipates (or weakens) the vortex. OTOH, the portable fan in your picture has no duct, so the shape of the fan is designed to direct the span-wise flow towards the inside of the fan rather than the edge. This allows the blade to produce more airflow, since less energy is used in producing a tip vortex. $\endgroup$ – BillDOe Jul 28 '18 at 20:09
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    $\begingroup$ That should probably be an answer rather than a comment, @BillDOe $\endgroup$ – Jonathan R Swift Jul 28 '18 at 23:56

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