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MG MGA - Heater Unit

Summer arrived here in Kirkham, England yesterday so I decided to refurbish the the heater box (just a paint job, no electrics tampered with). Job now completed and I need to reinstall it before the cold weather returns (summer here usually lasts 2 days). Two things now puzzle me:

1. I have a good grasp of aerodynamics from my flying days and I have an opinion on which way the fan should rotate when viewed from the engine compartment. Which is correct, clockwise or counter clockwise?

2. Not withstanding question 1, my fan rotates in the same direction, regardless of which way round I connect the leads from the fan motor. For a DC car I find this quite strange. Annoyingly, the fan is rotating in the opposite direction to that which I believe it should. However, I still get a good flow of air into the cockpit, so all in all I am puzzled and I am off to have a glass of wine in the last of the summer sunshine (24 C).

Barney, where are you?


Steve
Steve Gyles

The blower motor has wound feild coils rather then permanent magnets. Reversing the leads reverses the polarity of both the field coils and the armature. The result is the motor runs the same direction. This is the same reason that changing the car from positve ground to negative ground does not require changing the starter motor. To reverse the motor you would need to open it up and reverse the field coil connections.
J Heisenfeldt

The cage fan should rotate with the outer sharp edge of the blades leading. Air is thrown out by centrifugal force and will travel forward faster than the tip speed of the fan, therefore blowing more air in that direction.
Barney Gaylord

Thanks Barney. That's the way mine is working, so the unit is back in and the car is in for its annual test today.

However (yes, there is always a however with me!), I cannot get my mind round the logic of the way the drum fan runs. If the system was a standard propellor blade system akin to the main radiator and cooling blades it would be all quite easy to understand. What I do not understand on the drum fan is why the blade edges inside the drum are not facing the incoming air (see my schematic).

In my diagram air goes into the blue drum from the black trunking. At rest, this air has to be sucked in by the fan. When moving, the static (stationary) air is added to by the dynamic air being rammed down the black trunking. My diagram shows the green aerofoil blades , as fitted to the clockwise rotating fan. They are aerofoil sections with the leading edge facing forward on the outside of the drum (as you describe) - please excuse my home-made autoshape I created in Powerpoint). My understanding of airflows would suggest that air would be drawn in from outside the drum, not vice versa that actually happens. The more logical set-up to me would be for the drum to have the blade rotated anti clockwise by about 90 degrees (ish) as depicted in blue. The blades inside the drum would then face into the incoming air and make the whole system more efficient.

Puzzled

Steve


Steve Gyles

WOT was THAT all about?? Puzzled? Who needs a heater anyway? Barry
BM Gannon

Thought it might be a bit too technical for you Barry! In the 'heater off' position it can also blow ambient temperature air around your feet. I suppose you could replace the heater matrix with a small fridge matrix from your camper van and then you could keep your beers cool in the passenger footwell.

Steve
Steve Gyles

I'm not sure that it answers all your questions Steve,

http://en.wikipedia.org/wiki/Centrifugal_fan#Fan_blades

the root of the blade is usually radial and the tip curves forward or back or is straight depending on the application.
Dan Smithers

Dan

Thanks for the link. Makes sense and gives me comfort that I was not totally talking out of my ****. I spent a long time looking at the blade profile last night and I think that Smiths have optimised their single speed fan structure as a combination of benefits between forwards and rearwards facing blades) figs 3a and 3b in the link).

Steve
Steve Gyles

Of course being Wikipedia, it could be wrong - but probably isn't.

I have a hazy memory of being able to work these things out twenty years ago...

The real question is "if you put the fan on backwards will it suck instead of blow?"



Dan Smithers

Dan

Same this end. 18 years since I last flew profressionally and was required to have an understanding of all these things. However, the fan cannot be fitted the way round. It is an open ended drum.

Some people often say that about the fan at the front of the engine. However, if you look at it, the blade is exactly the same pitch to the airflow whichever way round you fit it. It is just the efficiency is impaired.

Steve
Steve Gyles

Steve,

I know, but wanted to put in my gripe about the usual debate about putting fans in backwards.

Actually, now I come to think about it, a centrifugal fan works with the rotor spinning in either direction - its efficiency will be different partly because it changes forward pinting blades into backward ones, but mostly because the outlet will be at the wrong angle.

dan
Dan Smithers

This thread was discussed between 07/05/2008 and 08/05/2008

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