[katieHaylor (OP)]

Steve asks:

Why do the propellers on planes have two blades and on ceiling or other fans they have 3,4 or more?
Are they more efficient with two blades, at higher speeds?

Can you help?

[Kryptid]

One of the biggest limiting factors on the thrust generated by a propeller is tip speed. For the sake of efficiency, you'd want to make the propeller blades as long as possible but not so long that their tips will travel faster than the speed of sound as they rotate. This puts a limit on both the rotational speed of the propeller and the total propeller diameter. You can increase the amount of thrust the propeller generates, and thus increase the aircraft's speed, by increasing the number of blades the propeller has. However, you don't get something for nothing: those extra blades create more drag and thus decrease fuel efficiency. There are some aircraft with tips that exceed the speed of sound on a regular basis, but those are usually reserved for the very fastest of propeller aircraft (such as the Tupolev Tu-95 Bear).

EDIT: Oops, I misunderstood the question. I thought you were asking why different aircraft have different numbers of propeller blades. My best guess as to why ceiling fans have more blades is so they can maximize the amount of air that they move to provide the best cooling effect.

[evan_au]

In general, the greater the number of blades in a propeller, the greater the amount of thrust you can deliver (or power you can extract).

The thrust/power gains decrease with increasing numbers of blades. However, the weight and cost keep increasing in proportion to the number of blades.

For airplanes and wind turbines which need to adjust the blade angle during normal operation, the mechanical complexity increases enormously with the number of blades.

For balance reasons, you would not have less than 2 blades (or 1 blade + a counterweight). So wind turbines usually settle on 3 blades.

The windmills used to pump water in rural areas have many blades, almost filling the whole circle.
See: https://pixabay.com/en/windmill-western-south-dakota-sky-821344/

[Colin2B]

As Kryptid points out aircraft can have more than 2 blades, Spitfire had 3, many large passenger planes have 4, I’ve seen 6 and in a museum I’ve seen 1 - counterbalanced.

[RD]

 As you increase the number of blades on a plane propeller, there will be a point where efficiency begins to decrease because the blades are cutting through the turbulent wake from the other blades, rather than cutting through still-air.


https://en.wikipedia.org/wiki/Disk_loading

[Bored chemist]

I wonder if big, slow, flattish blades- like you often see on a fan, are quieter.

[Kryptid]

I wonder if big, slow, flattish blades- like you often see on a fan, are quieter.

I'm inclined to say yes, as exhaust velocity (or in this case, the velocity of the air behind the propeller) is strongly correlated to the volume of noise created by an aircraft engine. Moving a large volume of air slowly versus moving a small volume of air quickly can generate the same amount of thrust, but the slower moving air makes less noise.

[evan_au]

I count 24 blades on an Airbus 380 engine. This is effectively a giant fan that provides most of the thrust of this jet engine.
See: http://www.airliners.net/photo/Airbus/Airbus-A380-841/1571442/L

I wonder if big, slow, flattish blades- like you often see on a fan, are quieter.

Certainly, slower → quieter. Turbulence is noisier than laminar flow, and turbulence is more likely with faster speeds.
 
Size has to do with it, too, but I haven't quite got my head around Reynolds numbers.

But I think that flat blades have more to do with ease of manufacture than with quietness. I expect that:
- a curved blade of the same area would be quieter than a flat blade, as it gently accelerates the air from its initial speed to its final speed
- a blade that is twistedalong its length would be quieter, as the linear velocity is higher towards the tip, and so the blade needs a different angle of attack at the tip compared to near the hub
- a blade that is thin on the leading and trailing edges (but thicker in the middle) would be quieter, as it would slice through the air more gently
- some bumps on the leading or trailing edges (like humpback whale flippers) can apparently reduce turbulence when it is approaching stall speed 

I notice that my ceiling fan (4 flat wooden blades of fairly uniform thickness) is probably designed that way to minimise damage in case of collisions with tall people. Plus the flat blades stack easily for shipping.

Curved blades require some metal stamping or plastic moulding technique, and may be harder to stack for shipping.

[jeffreyH]

About 10 years ago I used to use pipe flow equations in hydrological software. I didn't need to bother about Reynolds numbers. That had already been included in the maths.

[kazbert]

I'm guessing that residential ceiling fans are designed more for aesthetics than aerodynamics.  They don't spin very fast.  Industrial ceiling fans have more engineering thought put into them.  A Costco near my home recently installed several enormous ceiling fans above the cash register lines.  The fans have five skinny blades and a diameter so large they look like re-purposed main rotors from Comanche helicopters.  Those Costco fans don't spin very fast but they move a lot of air.