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Defrost fans - Part Deux

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kearney

Joined: 20 Sep 2008
Posts: 563

Posted: Tue Apr 28, 2009 3:09 pm Post subject: Defrost fans - Part Deux

Hi



Having given some thought to the defrost fans, I am wondering if a slot

would be more effective. Cars have a long strip that goes across the length

of the windscreen that allows warm air to pass along all sections on the

windscreen.



Would this arrangement be better than having a couple of hot spots on the

dash? I am not sure how to do this but I am wondering about the basic idea

only. Then again, a couple of fans may move so much air that this might be

moot.



My Cherokee has a couple of 3" slots that allow warm air to hit the

windscreen. The "blast" of hot air has caused a slight distortion in the

windscreen - something that may be worth considering as well.



Cheers



Les

#40643 - some assembly required

rv(at)thelefflers.com
Guest

Posted: Tue Apr 28, 2009 3:42 pm Post subject: Defrost fans - Part Deux

That's a common problem with Cherokee's.  Most people will install them with

the slot point towards the window, but it is really supposed to be pointed

away from the window.  Of course, on the Cherokee the defroster was heated

air as oppose to what most folks are just using ambient air from under the

console which the temperatures are elevated do to the avionics.  



I've thought about using the Cherokee approach, but I suspect that it would

be way too hot to use heated air.  I'm not sure that the slots do anything

better than just the standard computer fan that most folks use.



--

kearney

Joined: 20 Sep 2008
Posts: 563

Posted: Tue Apr 28, 2009 4:23 pm Post subject: Defrost fans - Part Deux

Bob



I will have a Subie under the hood so I will have hot air available. At

least where I fly, a fan moving ambient air would only be moving cold air,

at least until the cabin heat kicked in. Even in my Cherokee it takes a

while for the defroster to have an effect. Those flying in colder climates

might find the "fan" option to be a disappointment.



I have flown in very cold wx where it was difficult for the defroster to

keep up. I am uncertain how a fan only option would have fared.



Cheers



Les



--

Strasnuts

Joined: 10 Feb 2009
Posts: 502
Location: Salt Lake City, UT

Posted: Tue Apr 28, 2009 5:03 pm Post subject: Re: Defrost fans - Part Deux

Even cold "DRY" air defrosts the window in the citation.  It actually uses 

the Air conditioner for the air to pull the moisture out of it (evap fan). 

It doesn't matter if it is cold or hot, just dry.  It is a must if you are 

flying from a dry to a moisture climate to see when you land.

_________________
40936
RV-10 SB N801VR Flying
780 Hours
SuperSTOL 60 hours

pitts_pilot(at)bellsouth.
Guest

Posted: Tue Apr 28, 2009 5:32 pm Post subject: Defrost fans - Part Deux

Bob Leffler wrote:

 	  Quote: 	
		   

 

 That's a common problem with Cherokee's.

And mist aircraft with heated air directed at the windshield.
	


   Most people will install them with

 	  Quote: 	
		   the slot point towards the window, but it is really supposed to be pointed

 away from the window.  Of course, on the Cherokee the defroster was heated

 air as oppose to what most folks are just using ambient air from under the

 console which the temperatures are elevated do to the avionics.  

Not much heat from the avionics anymore!
	


 	  Quote: 	
		   

 I've thought about using the Cherokee approach, but I suspect that it would

 be way too hot to use heated air.  I'm not sure that the slots do anything

 better than just the standard computer fan that most folks use.

I would agree ..... circulating ambient air with a computer fan is 
	


probably all you need.  The air over the glareshield area is pretty 

stagnant and the moving air should be enough to keep the windshield free 

from condensation.

Linn

[quote] 

 --

AirMike

Joined: 27 Feb 2007
Posts: 514
Location: Nevada

Posted: Tue Apr 28, 2009 6:18 pm Post subject: Defrost fans - Part Deux

You can't beat the cost and ease of installation of one or two inexpensive computer fans (Less than $12) Plus since they are standardized, you can easily replace them if damaged. Fry's electronics even has models with a rheostat for less than $10.

_________________
See you OSH '18
Q/B - sold.

kearney

Joined: 20 Sep 2008
Posts: 563

Posted: Tue Apr 28, 2009 8:08 pm Post subject: Defrost fans - Part Deux

Hmmm



Always one to beat a dead horse, I'll ask a different question.



When starting on a cold day (say -15c), when the windows are frosted up,

will moving ambient air (temp -15c) over the windscreen remove frost. 



When I start my car in similar circumstances, the defroster puts out warm

air ....



Inquiring minds need to know....



Les



--

pitts_pilot(at)bellsouth.
Guest

Posted: Tue Apr 28, 2009 8:48 pm Post subject: Defrost fans - Part Deux

Les Kearney wrote:

 	  Quote: 	
		   

 

 Hmmm

 

 Always one to beat a dead horse, I'll ask a different question.

 

 When starting on a cold day (say -15c), when the windows are frosted up,

 will moving ambient air (temp -15c) over the windscreen remove frost.

If it's dry.  Cold air is typically dry, so it will remove the frost 
	


from the inside ..... but it may take time.



 	  Quote: 	
		   

 When I start my car in similar circumstances, the defroster puts out warm

 air ....

I think that's true only after the engine warms up!  My new truck puts 
	


out cold air when the engi9ne is cold ..... and I think it turns on the 

air conditioner to deliver dryer air ...... but i don't know what it 

does when the engine warms up.

 	  Quote: 	
		   

 Inquiring minds need to know....

 

Oh yeah, go ahead and start a food fight!!! <GR>
	


Linn

 	  Quote: 	
		   Les

Kelly McMullen

Joined: 16 Apr 2008
Posts: 1188
Location: Sun Lakes AZ

Posted: Tue Apr 28, 2009 9:03 pm Post subject: Defrost fans - Part Deux

Folks south of the border rarely fly with cockpit cold enough for

frost, only condensation, typically. That is why you are seeing these

answers. I'd think you would want some engine heat, and with your

water cooled engine you could just use an automotive heater core with

some ductwork to get what you want.

On Tue, Apr 28, 2009 at 9:44 PM, linn <pitts_pilot(at)bellsouth.net> wrote:

 	  Quote: 	

 Les Kearney wrote:

>

> 

>

> Hmmm

>

> Always one to beat a dead horse, I'll ask a different question.

>

> When starting on a cold day (say -15c), when the windows are frosted up,

> will moving ambient air (temp -15c) over the windscreen remove frost.

 If it's dry.  Cold air is typically dry, so it will remove the frost from

 the inside ..... but it may take time.

>

> When I start my car in similar circumstances, the defroster puts out warm

> air ....

 I think that's true only after the engine warms up!  My new truck puts out

 cold air when the engi9ne is cold ..... and I think it turns on the air

 conditioner to deliver dryer air ...... but i don't know what it does when

 the engine warms up.

>

> Inquiring minds need to know....

>

 Oh yeah, go ahead and start a food fight!!! <GR>

 Linn

>

> Les

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor
KCHD

n801bh(at)netzero.com
Guest

Posted: Wed Apr 29, 2009 12:52 am Post subject: Defrost fans - Part Deux

I did just that on my  V-8 powered Zenith 801. The pic of it is about 20 minutes into the video. 

http://video.google.com/videoplay?docid=-7272451917550730841&hl=en 

do not archive

Ben Haas

N801BH

www.haaspowerair.com



--------

rv(at)thelefflers.com
Guest

Posted: Wed Apr 29, 2009 3:18 am Post subject: Defrost fans - Part Deux

My comments are based upon feedback from several folks that are already

flying in that they only crack the heat vents in even really cold weather

because the heat output is more than ample to heat the cabin.  My concern

would be that it would be too hot to re-direct to the windshield.   With a

radiator, you may have better regulation of the temperature.



Granted in Ohio I haven't had to deal with heavy frost.  



Ambient air isn't just from avionics.  The front heat vent on the tunnel

should be providing heat that travels up through the panel, in which the

computer fans aid in circulating that warm air upwards towards the

windshield.  The question then to ask is at what temperature does this

approach become ineffective?   



I don't have any answers or solutions, just more questions.....



bob



--

Tim Olson

Joined: 25 Jan 2007
Posts: 2882

Posted: Wed Apr 29, 2009 3:50 am Post subject: Defrost fans - Part Deux

Absolutely right.  Plenty of heat available under the panel year  

round.  Front heat is enough to burn your foot if you put it on full.  

All that air goes under the panel.  My EIS was easily at 110F when it  

was 10F outside. Not only do you need a vent for defrost (fan) but you  

need it to keep avionics cool.  It's plenty warm to defrost.  I would  

NOT duct hot air to the vent or it could easily get too hot on the  

10.  And, I don't live in a hot area.  If the front seat people have  

adequate heat, so will the defrost.



On Apr 29, 2009, at 6:19 AM, "Bob Leffler" <rv(at)thelefflers.com> wrote:



[quote] 



 My comments are based upon feedback from several folks that are  

 already

 flying in that they only crack the heat vents in even really cold  

 weather

 because the heat output is more than ample to heat the cabin.  My  

 concern

 would be that it would be too hot to re-direct to the windshield.    

 With a

 radiator, you may have better regulation of the temperature.



 Granted in Ohio I haven't had to deal with heavy frost.



 Ambient air isn't just from avionics.  The front heat vent on the  

 tunnel

 should be providing heat that travels up through the panel, in which  

 the

 computer fans aid in circulating that warm air upwards towards the

 windshield.  The question then to ask is at what temperature does this

 approach become ineffective?



 I don't have any answers or solutions, just more questions.....



 bob



 --

pitts_pilot(at)bellsouth.
Guest

Posted: Wed Apr 29, 2009 4:27 am Post subject: Defrost fans - Part Deux

Kelly McMullen wrote:

 	  Quote: 	
		   

 

 Folks south of the border rarely fly with cockpit cold enough for

 frost, only condensation, typically.

That's true ...... but some of us down south have experience up nawth.
	


  That is why you are seeing these

 	  Quote: 	
		   answers. I'd think you would want some engine heat, and with your

 water cooled engine you could just use an automotive heater core with

 some ductwork to get what you want.

Even down south we have a few cold days ..... usually on weekends when 
	


we're flying for food, and we use the defrosters to keep the3 windshield 

clear of that hot moist air that just sat down. <GR>



The point I was making was that the place on the windshield where the 

heated air hits will distort over time.  My Grumman, which has not spent 

a whole lot of time out of Florida, has that distortion.  The problem is 

that you're trying to heat the rest of the airplane with the hottest air 

you can get .... and that same heat is bouncing off the windshield. 

Maybe having a mixer with cooler air would work if you go with the 

heater core method.  Melting frost on the outside of the windshield is 

problematic ..... and a spray bottle with some de-ice fluid would be 

better.  I don't know, never tried it.  Just a thought.

Linn



 	  Quote: 	
		   

 On Tue, Apr 28, 2009 at 9:44 PM, linn <pitts_pilot(at)bellsouth.net> wrote:

> 

>

> Les Kearney wrote:

>> 

>>

>> Hmmm

>>

>> Always one to beat a dead horse, I'll ask a different question.

>>

>> When starting on a cold day (say -15c), when the windows are frosted up,

>> will moving ambient air (temp -15c) over the windscreen remove frost.

> If it's dry.  Cold air is typically dry, so it will remove the frost from

> the inside ..... but it may take time.

>

>> When I start my car in similar circumstances, the defroster puts out warm

>> air ....

> I think that's true only after the engine warms up!  My new truck puts out

> cold air when the engi9ne is cold ..... and I think it turns on the air

> conditioner to deliver dryer air ...... but i don't know what it does when

> the engine warms up.

>> Inquiring minds need to know....

>>

> Oh yeah, go ahead and start a food fight!!! <GR>

> Linn

>> Les

>

>

 

 

 

 

 

 

 ------------------------------------------------------------------------

 

 

 Checked by AVG - www.avg.com

kearney

Joined: 20 Sep 2008
Posts: 563

Posted: Wed Apr 29, 2009 4:50 am Post subject: Defrost fans - Part Deux

Hi Kelly



I do have a heat exchanger, driven off the water system, that provides heat.

I just couldn't see how moving cold air on the windscreen would remove the

kind of frost (not condensation) that I see up here north of the 49th. 



When I mentioned this discussion to Joan, my wife, she just laughed and

suggested that people need to come up here and see what real frost is like.



Cheers



Les



PS: How is your -10 coming along?



--

msausen

Joined: 25 Oct 2007
Posts: 559
Location: Appleton, WI USA

Posted: Wed Apr 29, 2009 4:49 pm Post subject: Defrost fans - Part Deux

I think the more important point is that if you have frost on the windshield, you probably have it elsewhere on the aircraft which legally (here in the US at least) has to be removed before flight so why wouldn't you remove it from the windscreen at the same time?



  You are basically correct though, cold air won't remove the frost from outside the window but any air warming the windscreen above 32F will eventually clear an area on it.  As previously stated, cold, dry air will remove moisture from the inside the windscreen.  I'm also willing to bet that adequate heat from the avionics, and off the heat muff, in a standard RV can be generated as fast as what a liquid cooled engine can generate in cold climates from a cold start ( the time you most likely would need to get rid of frost).  Possibly even quicker given the time it takes to warm up the engine, coolant, and produce noticeable output from the heater core.  My $0.02



Michael



--

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