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My two projects right now are learning to fly and 3D modeling a replacement blade for antique fan, and a question occurred to me:

Why are airplane propeller blades not shaped like household fan blades?

They're both designed to push the same fluid with some degree of efficiency, but household fan blades look a lot more like ship screws/boat propeller blades. Is it a weight issue? A visibility issue? A drag issue, and if so, how?

EDIT - This is not a duplicate of this question because the proposed answers a question about angle of attack and pitch, which are aviation jargon words that may mean nothing to novices. Pitch and angle of attack also have little to do with the front-view shape of a household fan or prop blade, which is likely the most obvious difference between the two types of blades in the eyes of someone without an aeronautical engineering degree.

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  • $\begingroup$ Possible duplicate of How does the Angle of attack vary from the root to the tip of a propeller for a fixed pitch prop? $\endgroup$ – Ryan Mortensen Aug 1 '19 at 21:42
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    $\begingroup$ @RyanMortensen That seems like a completely different question to me. That question is asking about the definition of the term "angle of attack" and the mathematics around it; this question is asking about the motivation behind a design decision. $\endgroup$ – Terran Swett Aug 1 '19 at 23:31
  • $\begingroup$ I'm not going to entertain a debate, this isn't the place. Ask on meta. Based on feedback I've seen from far more experienced users in the past, the main determining factor seems to be based on the resultant answers of a question, not the actual question itself. $\endgroup$ – Ryan Mortensen Aug 2 '19 at 1:35
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    $\begingroup$ @RyanMortensen, I agree with Tanner. The question you proposed does not answer my question, and even if it did, is not phrased in a way a relative newcomer would ever have asked it. I worded my question as I did because it is exactly the question I wanted to ask. $\endgroup$ – schadjo Aug 2 '19 at 12:38
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    $\begingroup$ You might want to post this on engineering as well, because the answers here don't get at the underlying issues in any coherent way. There are certainly differences in the fluid dynamics. These are characterized by differences in Reynolds number, thrust coefficient, and power coefficient between the two. And fans are often ducted to some degree. But the real difference is the cost of achieving acceptable performance. $\endgroup$ – Phil Sweet Aug 2 '19 at 21:10
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1) Airspeed, 2) Forward motion, 3) Size constraints. Just to begin with.

Household fan blades are extremely slow, so they need more chord to push a meaningful amount of air. Aircraft propellers approach the speed of sound at their tips, and low drag is critical. All things equal, more span and less chord is more efficient. Reducing the airspeed for props has diminishing returns, because aircraft themselves move forward through the air, so a very large propeller adds drag to the aircraft however slow it is.

In household fans, size constraints are more critical than efficiency. A high-span, low-chord (narrow) blade would be more efficient in a household fan as well - indeed, you can find household fans with such blades. But they have to either spin faster for the same amount of airflow, which adds noise, or be larger in diameter. Ceiling fans, which can be larger, have longer narrower blades than desk or floor-standing fans.

Ship propellers move in an extremely dense and viscous medium, which changes things even more. The drag of moving through water is extremely high and proportional to V³ power-wise. The thrust force they produce is at most proportional to V², and can be lost to cavitation at high velocity. So their velocity has to be kept as low as possible. The drag cost of adding more chord is also relatively small in water.

On large ships, propellers are already as large as they can be made without sticking out of the water at low draft (on merchant vessels) or reducing the number of shafts that can fit (on combatants). This allows them to spin slower and lose less power to drag.

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    $\begingroup$ Fan blades spinning as fast as propellers would also be a safety hazard, and also probably have extra trouble with vibrations. It's just a bad idea all around :D $\endgroup$ – Luaan Aug 1 '19 at 7:10
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    $\begingroup$ The key reason for this is that efficiency loss mainly occurs at the tip of a blade (where high pressure air can leak around to the low pressure side). Fat blades have more tip than narrow blades, so they're less efficient. $\endgroup$ – Robin Bennett Aug 1 '19 at 11:24
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    $\begingroup$ Submarines, which have no "sticking out of the water" problem, can have incredibly large props. $\endgroup$ – Carl Witthoft Aug 1 '19 at 12:41
  • $\begingroup$ @CarlWitthoft submarines have to work to a degree on the surface. And a screw that sticks out of the water is one someone can take a picture of... if the boat comes out of the water in drydock the screw gets shrouded. It might be more accurate to say that submarines have very cramped hulls. $\endgroup$ – Chris Stratton Aug 1 '19 at 14:57
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    $\begingroup$ @Luaan Some household fan blades actually do spin at similar RPMs to light aircraft propellers. They just tend to be rather small fans and usually have a shroud around them to prevent any danger. And they're also not nearly as long, so the blade tips aren't moving nearly as fast for the same RPMs. Also, not nearly as much mass per blade. Computer cooling fans are a common example of this. It's not unusual for them to be in the 1,500 - 2,000 RPM range, which is around the same as the PA-28s I've flown. $\endgroup$ – reirab Aug 1 '19 at 16:48
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From our local warehouse

Some household fans are shaped like airplane propellers, the ones that need to move a lot of air at the highest speed. For a given motor, they have the highest efficiency, but the tip effect makes them noisy. They are best suited for industrial applications.

For inside the family home there are other considerations:

  • Silence. Best if we don’t hear the fan run at all.
  • Low air speed. We only want to feel the cooling effect of moving air, not have our hair blown out of shape.
  • Purchase cost. This is the one we directly see. The usage costs are hidden in the monthly electricity bill.

So for that purpose, a slow moving tip with many broad blades is best.

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  • $\begingroup$ Let's try helicopter vs ceiling fan: helicopter - symmetrical airfoil. Ceiling fan - flat. Note long thin fan blades will help reduce interblade turbulence issues for a high speed industrial fan, but optimal airmoving shape that produces the LOWEST "lift" and LEAST DRAG would likely be a thin plate. $\endgroup$ – Robert DiGiovanni Jul 31 '19 at 22:12
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    $\begingroup$ @RobertDiGiovanni ceiling fans tend towards flat blades because of their lower production cost and because they are often intended to operate in reverse with some efficiency, not because an airfoil would be bad. I have owned several standing fans with airfoil shaped blades, as well. $\endgroup$ – AEhere supports Monica Jul 31 '19 at 22:24
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    $\begingroup$ "usage costs are hidden in the monthly electricity bill." Maybe about 0.5-2$ USD per month? $\endgroup$ – chux - Reinstate Monica Aug 1 '19 at 7:06
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    $\begingroup$ @chux $2 per month adds up to the purchase price of a cheap desktop fan in a year. Of course you can pay 10 times as much for a fan that does exactly the same thing, if you want to :) $\endgroup$ – alephzero Aug 1 '19 at 11:23
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I think it should be obvious that a fan blade is basically a wing travelling in a circle. So the question is what makes a wing a good wing?

In theory and in practice (wind tunnel measurements or fuel consumption measurements of real planes) a long skinny wing is significantly more efficient than a short wide wing. The most efficient wing for a given airfoil is actually one with infinite wingspan. But since there are only a finite number of electrons in the universe it is not possible to build this perfect wing. Instead engineers try to make wings/propellers/fan blades as long and as skinny as practical given the limitations of materials.

You will even see this effect in household fan blades: in situations where span is not limited, such as ceiling fans, the fan blades tend to be long and skinny instead of short and wide like ship propellers. So why short and wide? Space restriction.

When the fan must fit on a tabletop, you cannot have 1 meter long blades. But as soon as this restriction is lifted you will see designers shift to long skinny blades like what you can find on industrial floor standing or wall mounted fans.

Granted, there are floor standing fans with the exact same blades as tabletop fans but this has more to do with reusing parts and economies of scale than aerodynamics.

Now, you can in theory also have an efficient short skinny fan on a tabletop fan: just look at drone/RC airplane propellers - they're often way shorter than the fan blades of tabletop fans. But you will need to move the blades really fast to move a given amount of air. This is very noisy (have you ever seen a drone fly?). So the second consideration is noise. To reduce noise you move the blades slowly. A slow blade doesn't move much air so you increase the chord to make them wide. This results in a very inefficient fan but efficiency is not your main concern: you are designing a non-moving machine sitting on a table drawing power from a socket in the wall. Noise reduction and size matters more.

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    $\begingroup$ +1 As an example, I used to pass through the VLC airport in the past, and it has massive fans on the ceiling, with long and thin blades not unlike the wings on a glider, and with a similar airfoil too. The fact that building maintenance often had them rotating backwards can be explained via Hanlon's razor. EDIT: see this question aviation.stackexchange.com/questions/61833/… $\endgroup$ – AEhere supports Monica Aug 1 '19 at 12:56
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Learning to flying and modelling a fan blade, both spin in the air but the propeller tries to move the plane, and the fan tries to move the air.

How does this factor into design? Propellers stand above and apart from fans in that they generate lift not only by deflecting air (bottom lift), but also from their motion through the air (top lift). This is best explained by viewing the lift curve vs AOA of an airfoil from 0 to 45 degrees. Lift will increase up to stall, then decrease, then increase again up to 45 degrees. This means you have to move LESS air to get the same lift with an airfoil up to stall.

The fan really is opposite, unless you want it to pull it self around the room. The fan blade is designed to move air, period. Sensible design would make it compact, and thin flat blades with a wider chord would be fine, as one just looking for a cooling breeze from their product. Keeping the original style of fan on the "antique" may make it more valuable.

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    $\begingroup$ You have a good point, but it doesn't need to involve the dubious concepts of 'top' and 'bottom' lift. Simply put, propellers are optimised to create lift (thrust) while disturbing the air as little as possible. This favours a large diameter in the first place, to move more air with the lowest possible speed. A fan is almost the opposite: it needs a certain diameter and move the air at a decent speed. (A cooling fan, that is; requirements may be different for a ventilation fan). $\endgroup$ – Zeus Aug 1 '19 at 1:24
  • $\begingroup$ "A cooling fan, that is; requirements may be different for a ventilation fan" which may be different from a jet fan! Look at the shape of those airfoils! $\endgroup$ – Robert DiGiovanni Aug 1 '19 at 6:10
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    $\begingroup$ @Zeus Tanner Swett published an airfoil Clift vs AOA graph that shows the "top lift" contribution at (lower drag) lower AOA as a lift maxima, which drops to a minima at around 30 degrees AOA, before rising to a maxima (with loads of drag) at 45 degrees. This is the Rosetta Stone for many applications. However, your point that the moving wing "disturbs as little air as possible" is an excellent way of describing it (hence less drag for a given lift!). Thanks. $\endgroup$ – Robert DiGiovanni Aug 1 '19 at 6:19
  • $\begingroup$ @RobertDiGiovanni Friendly nitpick: consider using the singular form of minimum and maximum when appropriate. $\endgroup$ – AEhere supports Monica Aug 1 '19 at 7:37
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Price is the main reason. Ceiling fans just stir the air around and flat paddles are the cheapest way to go.

More expensive fan blades can have some aerofoil shape and even winglets at the tips but this is mainly for show since these blades don't have twist.

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  • $\begingroup$ But wouldn't a more wing-shaped, narrower blade use less material and be less expensive? $\endgroup$ – schadjo Aug 1 '19 at 15:08
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Most of the other answers are already correct. Just one more aspect: Blade chord length.

With something that spins as fast as an airplane propeller, the blades need to be as light as possible, to minimize centrifugal loads. So it's better to give them a short profile chord length and high area loading (how much force they produce per surface area of blade), which means small chord length and turning the flow comparatively sharply, which requires a large pressure difference between the pressure and suction side of the fan blades. For a desk fan, any old plastic will be strong enough, so those blades can work with a very low surface loading (long profile chord length) and turn the flow slowly, with a small pressure difference, which is quieter.

The same applies for ship propellers: Since water is so dense, the structural loading on those blades is pretty high. If you tried to get the same thrust from a propeller with short chords (at equal radius), the blades would have trouble carrying the bending loads, and the sharper turning would also lead to cavitation [1] quicker.

[1]: cavitation happens when the pressure on the suction side of an underwater propeller falls so low that water evaporates. This effectively separates the flow, which reduces suction, so the freshly-evaporated water condenses again, water rushes back to where the steam bubble was... That can break a propeller very quickly.

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Some rare designs do look more like a fan blade. The Antonov AN-70 is a good example of this, sporting huge fan blades on its D-27 propfan. The scimitar props do make for a rather efficient design, but they are extremely noisy which limits their usability around many commercial airports.

enter image description here

Image credit : By Tangopaso - Own work, CC BY-SA 3.0

Some additional discussion can be found in this question :

What are the advantages of more than 4 propeller blades?

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  • $\begingroup$ Those props are wild. $\endgroup$ – schadjo Aug 13 '19 at 12:11

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