Viscous Fan - Remove?

[Guest Alan_builder]

the "pull" fan is still better as it was better to begin with.

 

Hi Richard, thanks for keeping it simple. Could you answer in the same way is the difference in pull and push fan's efficiency is theoretical or is it a phenomena?

 

Thanks

 

Daren

If you looked at a GP1 racing boats propeller it would look like an airplane's propeller long thin and for the boat sharp (just like a food blender), you have described the slow propeller for super-sized ships (non plaining craft or heavy displacement) and river Thames craft where fuel efficiency is the factor dictating design as they are limited to 7Kn (River Thames). The propeller blade pitch / shape / design is set to speed at which the propeller is passing through the media, then you modify the shape to get the thrust for the mass of thing you are moving through the media.

[fry61]

Will not doubt for a moment the words of wisdom above BUT are not all fans sucking air on one side & then pushing air on the other? Same fan -sucks air in-blows air out can't be doing one without the other --- can it??

 

 

 

Probably a bumble bee thing.

[Guest Alan_builder]

Will not doubt for a moment the words of wisdom above BUT are not all fans sucking air on one side & then pushing air on the other? Same fan -sucks air in-blows air out can't be doing one without the other --- can it??

 

They do but have you taken into account the air which falls over the end of the blade "shorting" out the blade reducing it's efficiency. Better described as on an airplane wing at the tip there is often an up stand to stop the afore said problem, it effectively makes the wing longer. Do you remember the Jumbo jet (747) when it first came out? No wing tip up stands, now 747-400 has wing tip up stands?

[agent_zed]

well on my 2b i have managed to get away with mounting the fan in front of the rad. without an cowling at all even the plastic surround it came with. haven't so much as had a hint of overheating although i am only running a 1.6 pinto so maybe would be worse in a more powerful engine.

 

i remember reading about how you can't make a propellor spin too fast as the outer tip would break the sound barrier whilst further down the blade it would not (shorter distance to travel) causing all sorts of problems with resonence etc. Also in water it creates a negative pressure and allows airbubbles to form reducing propulsion. Hopefully somewhere near what i read. actually a little interesting to read about if you like that sort of thing

[richardm6994]

Hi Alan, keeping it simple..........errrmmm.......well you've got to look solely at the blade and blade's surface to explain this (ignore air movement entering / exiting the fan).

 

 

Basically when a blade is "pushing" a fluid, it is also compressing it (positive blade pressure), however when a blade is "pulling" a fluid, it is streching it (negative blade pressure). These positive / negative pressures are immediate to the blades surface and leading edges and have nothing to do with the air velocities or pressures exiting / entering the fan.

 

The positive blade pressure creates air resistance and thus reduces "pushing" fan's efficiency. This positive pressure also destabilises the blade's "boundry" layer (a thin film of zero velocity are on the blade's surface), which is needed for the fan to work..........but now we're getting way to deep and I'm sure many people reading this have dropped to sleep!

[Guest The Modfather]

 

 

Daren

If you looked at a GP1 racing boats propeller it would look like an airplane's propeller long thin and for the boat sharp (just like a food blender), you have described the slow propeller for super-sized ships (non plaining craft or heavy displacement) and river Thames craft where fuel efficiency is the factor dictating design as they are limited to 7Kn (River Thames). The propeller blade pitch / shape / design is set to speed at which the propeller is passing through the media, then you modify the shape to get the thrust for the mass of thing you are moving through the media.

 

High speed water screws (as they're called) are thin blades to avoid the water passing over them to boil, which deteriorates the blade surfaces astonishingly quickly and causes blade fatigue. Cannot remember the tech term for this action, but that is why they are thin, to reduce surface area for high speed. Liners etc don't need the high speed so use a wider bladed screw and can use a lower revving engine, as the wider blade pushes more water. Thought that was obvious?

[Redspec]

Cannot remember the tech term for this action?

 

Cavitation???

[Guest The Modfather]

Yep, that's it. I was otherwise preoccupied when writing and couldn't think of it, lol

[steve in stockport]

I can hear the distant popping of an exhaust going on in the background =@

[Guest Alan_builder]

Hi Alan, keeping it simple..........errrmmm.......well .........but now we're getting way to deep and I'm sure many people reading this have dropped to sleep!

 

That makes sense to me and a good explanation, thanks a lot.

No not fallen asleep went out to lunch.

[fry61]

They do but have you taken into account the air which falls over the end of the blade "shorting" out the blade reducing it's efficiency. Better described as on an airplane wing at the tip there is often an up stand to stop the afore said problem, it effectively makes the wing longer. Do you remember the Jumbo jet (747) when it first came out? No wing tip up stands, now 747-400 has wing tip up stands?

 

That's the reason for the close fitting cowl Or cowl fixed to blades; still don't get the cigar Alen.

[Guest chris brown]

Zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

[Guest peter2b2002]

dump the old fan , don't know what is better push or pull, all I know is on my RR I took off the viscus fan and fitted 2 push fans from an 3ltr Audi , engine warms up a lot qiucker, but when the fans come on the rengine revs drop , so it would seem that the fitted viscus fan will take a few bhp , + my 2B use to have the fan infront of the rad all ducked in now it has a pull fan behind the rad ( thanks Jase) runs cooler now

peter2b