Solving Ceiling Fan Woes | Home Improvement

[rsk]

There are three ceiling fans in my flat - one in the living room and one in each of the two bedrooms.
Two of them make wonderful breezes. The other one, in the main bedroom, seems to cut through the air without disturbing it at all. I've looked closely at them and they all seem to be the same design, blades the same size, no notable difference in angling or anything else I can see. It's not my imagination or perception, I tried holding a thin string in each room and in two of them it flutters wildly, in the main bedroom it hardly moves.

What should I be looking for? A standing fan? Or is there an easy fix you can suggest?

 

[Drakkith]

Is the distance from the fan blades to the ceiling the same for all three?

 

[rsk]

As far as I can tell, yes - they seem to be identical fittings

 

[Borg]

Try shifting your furniture around to see if the current arrangement is causing dead zones?

 

[Rive]

Maybe that specific one spinning reverse?

 

[rsk]

I have been thinking aboutthat (dead zones), about room size and so on and wondering what the physics of that looks like, trying to picture how to change or disprupt that. The living room is much bigger, but the two bedrooms are pretty similar in size and layout so I don't think it's that because in the other room there's no problem.

They are all spinning the same way, all seems to have blades tilted the same way and I can see no sign of a reverse switch anywhere - but then I'm in a place where it's unlikely to be cold, ever.

I've tried varying the speed to see if there's an optimum but no difference. I am completely mystified.

 

[rsk]

Now I'm really puzzled. I found this, which mentions both getting stuck in some kind of vortex loop and the need to keep dust off blades.
So I was going to go in there with something to wave underneath it to disrupt the vortex, but first decided to give the blades a wipe. I can't see the top of them, but can reach up. They didn't seem particularly thick with dust but, lo and behold! there now seems to be a good breeze coming from it.

Can dust really make such a difference? Or maybe, in tugging it to reach and wipe the blades I've re-aligned something somewhere?

 

[Borg]

Hard to say. Could have been this guy.

 

[DaveC426913]

Can dust really make such a difference?

Apparently yes.
Small fans and large, dusting the blades seems to dramatically improve output.

A clean fan slices through the air, creating lift, which, in a ceiling fan's case, results in axial flow (parallel to the axis).

Contrarily, I'd assume a dusty fan increases the stagnant layer so much that the fan stops slicing through the air and just starts acting like a blunt baseball bat. This won't produce much lift (axial flow), but will start the air moving concentrically (tangential to the fan tips) - a whirlpool with no down flow.

At least, that's my guess.

 

[hutchphd]

It doesn't take much sleet to really screw up an airplane wing. Maybe its the same ?

 

[DaveC426913]

It doesn't take much sleet to really screw up an airplane wing. Maybe its the same ?

True, although the reason I didn't get into that is because an airplane wing's lift is largely reliant on its airfoil shape - whereas most fan blades (esp. ceiling fans) are flat, i.e. rectangular in cross-section.

 

[BillTre]

Try introducing some kind of smoke source (like a stick of incense or a cigarette into the room.
This should allow you to see what the air flows are doing with and without the fan.

 

[rsk]

Well, I really am still amazed that a thin layer of dust could make so much difference. I slept under it last night, the first time I haven't needed aircon (and I really don't like sleeping with the air con on).
This weekend I'll give the other two a quick clean as well, just in case there's any gain there.

 

[phinds]

I watch a TV show called Air Disasters sometimes. It's like a high-tech detective story every week. One of the things I've seen is that a very small amount of roughly placed ice can pretty much destroy the lift of an airplane wing. One commentator said that if the wing were covered with low-grit sandpaper it would likely crash on takeoff.

 

[256bits]

I watch a TV show called Air Disasters sometimes. It's like a high-tech detective story every week. One of the things I've seen is that a very small amount of roughly placed ice can pretty much destroy the lift of an airplane wing. Once commentator said that if the wing were covered with low-grit sandpaper it would likely crash on takeoff.

Rime ice, or frost on a wing, - extra drag and reduced left, around 30%.
Reduce payload, increase takeoff speed and runway length.
Stall speed is increased.
De-ice the wing.

The crash part may be a bit extreme to the point of fearmongering for the traveling public who have booked a flight with winter conditions.
Pilots deal with this every day, and compensate for it.

In inclement weather, planes are usually grounded until conditions improve.

 

[256bits]

Well, I really am still amazed that a thin layer of dust could make so much difference. I slept under it last night, the first time I haven't needed aircon (and I really don't like sleeping with the air con on).
This weekend I'll give the other two a quick clean as well, just in case there's any gain there.

Surprised as you are.
Unless you had an inch layer of dust.

 

[rsk]

I'll be a bit more scared of flying now...

 

[DaveC426913]

Again though, most fan blades (except higher-end) are not airfoils - their cross-section is of constant thickness (some may have a curve, but many are simply rectangles).

 

[256bits]

I'll be a bit more scared of flying now...

No need.
Pilots are a well trained bunch of people.

 

[256bits]

No need.
Pilots are a well trained bunch of people.

Rather I should say commercial pilots who fly and hone their skills every day.

 

[Mark44]

Surprised as you are.
Unless you had an inch layer of dust.

Same here. It's difficult for me to imagine that a very thin layer of dust would make much of a difference.

 

[DaveC426913]

Same here. It's difficult for me to imagine that a very thin layer of dust would make much of a difference.

I think y'all are underestimating both how much dust accumulates, and how much it alters air flow.

 

[Mark44]

I think y'all are underestimating both how much dust accumulates, and how much it alters air flow.

To repeat what @256bits said, unless the dust is an inch thick, I don't see how it's going to make a big difference.

 

[rsk]

To repeat what @256bits said, unless the dust is an inch thick, I don't see how it's going to make a big difference.

I thought the same. They were dusty, no doubt, but it didn't seem like a thick layer. I thought 'well that won't make much difference' as I got down off the bed and went to switch the fan back on and wondering what to try next. I was astounded when I felt the breeze. I cannot believe that so little dust could make such a big difference, but unless it's fan fairies at work, there isn't another explanation.