OT: Running a pedestal fan in reverse - failed - a rant of sorts.

Yeah, you would expect better quality having spent all that money on it.

Wouldn't those two things cancel?

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John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
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{>8-} "putting the fan on backwards" ?>:-] ...Jim Thompson

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John Larkin wrote on 12/26/2017 1:21 PM:

Turning the blads on backwards only makes them work less efficiently. Turning the motor backwards makes the air blow the other way, but also works the blades less efficiently. Doing both makes the air blow the other way, but also allows the blades to work with the proper leading edge and so more efficiently.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

It seems to me that they would. Reversing the motor makes it blow in the opposite direction. Reversing the blades makes it blow in the opposite direction.

You'd need to mirror-image the blades to make them blow the other way without reversing the motor, ISTM. Just turning them round wouldn't do it.

Cheers

Phil Hobbs

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I'm useless at this sort of thing, so I figured I'd get some practice working it out:

A working solution to having a fan blow the other way is to turn it around. The effect of this relative to an observer in a fixed position is that the blades are rotated and the direction of spin is reversed. So to have the fan blow the other way without rotating it requires that the blades be rotated and spun in the opposite direction.

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From Wikipedia:

Propeller-driven aircraft generate reverse thrust by changing the angle of their controllable-pitch propellers so that the propellers direct their thrust forward. This reverse thrust feature became available with the development of controllable-pitch propellers, which change the angle of the propeller blades to make efficient use of engine power over a wide range of conditions. Single-engine aircraft tend not to have reverse thrust. However, single-engine turboprop aircraft such as the PAC P-750 XSTOL,[6] Cessna 208 Caravan, and Pilatus PC-6 Porter do have this feature available.[citation needed]

I studied the situation a bit more. You are correct, Phil.

No.

Imagine you have a piece of right-hand threaded rod. Whichever end you look at it from, it's still right-hand threaded.

Sylvia.

Reversing the blade does *not* make the fan blow in the opposite direction. Look at a fan blade and you will see this immediately.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Turning the blade around just changes the relative position of what you would normally call the leading edge. If the fan still runs in the same direction it will still blow in the same direction but the once was leading edge is now the trailing edge. Sylvia has the process right. Trust an ex-aircraft model flyer on it.

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I guess it's like screw threads, same sense from either end. If it's a shaded-pole motor, the only thing to do is heat-gun the blades.

--

John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

One option whether it's a PSC motor or a shaded pole motor is to reverse the stator, and deal with any wire routing issues that arise. However, in the case of the motor I had, it's been designed to make reversing it impossible - perhaps so that the people used to assemble it can't get it wrong.

Mind you, I'd have thought connecting the multiple stator wires to their leads would be the biggest source of error.

Sylvia.

I decided I was unwilling to be defeated, and a fair amount of soldering, heatshrinking and hot gluing later, I got it to work.

Sylvia.

If you reversed the motor, you would also reverse the blades because although the blades would pass air either way ,they work more efficiently

Many turbine propeller aircraft can reduce their landing roll by placing the propellers in reverse pitch. The blades are still turning in the same direction, but the thrust vector is reversed. They are not very efficient in this mode, but are still able to slow the aircraft.

To visualize what would happen in a pedestal fan, imagine turning it around

180 degrees. The motor is still spinning the propeller in the same direction, but the thrust vector is reversed in relation to the floor.

In order to reverse the thrust vector, either the propeller pitch has to be reversed, or the motor has to be reversed.

John is right. Doing both would cancel.

Wrong,reversing motor would reverse air flow, reversing the propeller would be same air direction but would put the leading edge in the right position for efficiency.

One issue I haven't addressed is that the fan is held on by a left-hand threaded nut, because a right-hand threaded nut will tend to undo itself in this application. Since I've reverse the direction, I really need the nut to be right-hand threaded, but there's no way I can change the threadedness of the shaft.

May need to use some more hot glue - we'll see.

Sylvia.