# Is 4 more than 5?

I have heard that 4 fan blades on a ceiling fan move more air than 5? Is this true? Can this be true? Would 3 move more air than 4? Would I get eggroll with 6?

It all depends on fan speed, total fan blade area, blade pitch, blade shape. Assuming the same shape and pitch of blades and same RPM, I’d have to say 5 would move more than 4.

Doesn’t it have anything to do with balance? an even number has an equal opposite, but an odd number does not.

In general, for a fixed blade width, more blades is better. It’s obvious that one skinny little fan blade isn’t going to move much air, but a lot of skinny blades will – until you reach a point where the blades are so close together they interfere with each other. For example, if the blades actually overlap, then the trailing blade will be trying to move air that is already being moved by the leading blade.

The hard part of your question is determining when and how blades interfere. But widely-spaced, slow-moving blades (i.e., like a ceiling fan) don’t seem to me to be likely to interfere with each other much. Therefore (and this is just a guess) I would say that a five-bladed fan will move more air than a four-bladed fan, contrary to what you heard.

Howevere, there are probably other factors more important than the number of blades for any particular fan design. Blade angle, rotation speed, etc., are all considerations. Presumably the designer considered all these when he decided how many blades to put on the fan OR ELSE he just decided that five blades looked better than four blades and went with his heart, rather than his brain.

So while I believe it is generally true that more blades = more air moving, I’m sure it wouldn’t be hard to find examples of five-bladed fans that move less air than some four-bladed fans.

It depends on several things: The size and pitch of the blades, the speed of the fan, proximity to the ceiling, other air flow in the room, etc.

If two fans were exactly the same except that one had five blades and one had four, the five blade fan should move more air.

But the five blade fan will need a more powerful motor to turn the extra blade (to move the extra air). So it’s possible that most five blade fans turn a little slower so that they can use a motor with the same approx horsepower as most four blades fans. If this is the case, they would probably wind up moving about the same amount of air as the four blade fan.

Remember, moving air is just work that the fan does. Two fans using the same amount of electricity, and which have a similar efficiency, will move about the same amount of air, whether it is a two blade fan spining very quickly, or a six blade fan turning much more slowly. In other words, equal energy used, plus equal efficiency, equals about the same amount of work done.

Note however, that the amount of air moved and the velocity that the air is moved are two different things. A four blade fan moving the same amount of air as a five blade fan will probably be turning faster (see above) and so the air will probably be moving faster. This may give the impression of more air, when it is actually just a hiigher velocity.

Ugly

I think what we’re dealing with here is the bland statement that the fewer blades a fan has, the more efficient it is. As AWB says, it depends on several things. I think fewer blades will be more efficient than more in the abstract, that is, if the motor you are dealing with is equally efficient at all speeds. With a fan, I don’t know if that’s true. I have heard, on the other hand, that a two-bladed propeller is more efficient than a three-bladed one, at least on ships.

My guess is simply that each blade creates a little turbulence, so having fewer blades creates less turbulence even for the same total production (i.e. mass of air moved at a given speed). Obviously, turblence is going to reduce efficiency; if each blade messes up the air mass (“messes up” being defined as moving air in any direction other than the right direction), then two-blades will have less of their own inefficency to deal with. But that is only a guess.

As said above, plus remember that this is just comparing same style fans. In general, propeller-type fans are inefficient, and only good for low pressure applications. Centrifugals are used if you want to actually move any significant quantity of air against any sort of back pressure.

Here is the best I could find:
http://www.samurai.com/lists/trawler-world-old/1999/01/0709.html
It’s just a bunch of folks chatting, like us, but they are really into boats so maybe it’s worth visiting. On the other hand, air and water aren’t the same thing so maybe not everything they have to say is going to apply to fans.

This is complex.

The same question comes up, but in reverse, for windmill devices. It turns out that, for mills running at high speed, you want a very few very thin blades. The problem is that apparently you have a lot of friction at the ends of the blades, and overcoming this wastes a lot of torque, actually slowing the blade down. It’s true that those water-raising windmills that you see atop long poles on farms have a lot of blades, but that’s because having a lot of blades makes it easier to start from a stopped condition, to take advantage of light breezes, and to overcome rust and friction on a system that is a pain in the neck to keep properly lubricated.

So those turbine blades you see on the turbine farms in California have three very narrow blades. If each blade is spinning rapidly this is actually a very good condition – the odds of a rapidly spinning thin blade coming in contact with a molecule are better than the chances of a lot of slower blades, anyway – and that’s what you’re after.

No, especially since one is a compressible fluid, and one is considered to be essentially incompressible.

It’s unlikely that your ceiling fan are engineered well enough that there is much difference between four or five blades. I have seen single blade propellors on competition model aircraft in control line speed events. There is of course a counterweight but the theory behind the single blade is that at extremely high speed, 25,000rpm or more, is in air undisturbed by another blade.