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Defrost fan or avionics cooling fan... or both?!

georgemohr

Well Known Member
Hi all,

The two fans that Vans specifies in the top of the glare shield.. are they intended to be defog for the canopy, or are they for cooling the avionics bay? And if they are only for defrosting, is there any "always on" cooling for avionics that folks have implemented?


Thanks!
George
 
fans

I built a -10. The fans help keep the avionics area cool and yes, they will help to remove fog on the windshield...I think defrost might be a bit strong.

You may want to put a speed control, or at least a switch on the fans. I have found that on high, the air is directed back into my face...
 
My -7 has four small computer cooling fans (1 3/4") across the span of the glare shield.

Originally I put them in to provide some defog and cooling air to my boxes underneath.

Now I find that one of the biggest benefits is that they distribute the hot air from the heater air inlet near my feet all around the cabin. Air rises up behind the dash and gets blown all around. Triple bonus.

The system has a speed control, but it's hidden below the dash (it is accessible in flight if needed) and controlled by a single on/off switch.

Good stuff
 
I was in 214VA this week and actually needed to defrost the canopy. The fans were pretty ineffective. Using a towel to wipe off the condensation had immediate results.

I’m glad I experienced it though ao I can think about how to improve the design.
 
Fan pics ...

Greg, here are the promised pics of the fans on my -7.
 

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I had fog in my cabin today too. That is a first for me. I opened the tip up canopy to taxi position and it cleared pretty fast. I may have to install a couple fans like in Bill's pictures. It is a bit freaky to think what could happen if it fogged up in flight.
 
several yrs ago around Cleveland on a rainy Fall day I had a fogged over windshield on the ground and kept wiping it with a towel during taxi. after takeoff it all cleared, quickly.
 
I have fogging regularly on (moderately) cold days 5-25F with sun, simple breathing in the cockpit causes it and with me alone. It will happen in a few minutes as I never installed a de-fog fan. I have to tip-up the canopy with the engine running to take off.

The 4 small fans look attractive for this purpose.
 
Bill,. I find that any condensation on the interior of the canopy is gone seconds after launch, even in the rain. Just the taxi and runup needs de-fog. I also feel they would help cool the radios as they get real warm on a summer day. Would 4 fans help move out the air under the panel enough to help? I am a little worried about drilling up from below with a big hole saw, though.
Been a real soaker around here lately. The -7 is nice and dry in the hangar but found a dead mouse inside today. Probably almost drowned outside and crawled inside and expired.
 
I have fogging regularly on (moderately) cold days 5-25F with sun, simple breathing in the cockpit causes it and with me alone. It will happen in a few minutes as I never installed a de-fog fan. I have to tip-up the canopy with the engine running to take off.

The 4 small fans look attractive for this purpose.

I have experienced this in the 6 many times. The 10 has a big fan on the dash and have never experienced it on that plane at least to any meaningful degree. I believe the fan is pushing around my moisture laden, exhaled air and not allowing much of it to reach the windshield and condense.
 
Portable Defoggers?

Has anybody tried one of those portable defoggers you plug into cigarette lighter type power? I get fog on the inside of my windshield sometimes in the morning and during taxi for takeoff at my airport you face the rising sun. It makes it very hard to see.
 
Thanks for the info. I sorta dislike that there's no "always on" vent for the avionics bay. Will have to think on this more.
 
A couple of comments:

This mod would be more difficult after the canopy "glass" is installed, but with a little ingenuity it should be possible. All my wiring is accessible. Just a matter of protecting the clear stuff while you're working. Blue tape and paper (maybe a thin sheet of something hard) are your friend. If you don't like the idea of drilling with a big hole saw (nor do I), use a Greenlee punch (perfect holes and no burrs to speak of). They require only a small pilot hole to get started.

And regarding the Defog/Defrost effectiveness. My earlier point was that it does a good job of moving heated air around in flight. Personally, I've never seen any condensation on the glass, but the plane is only a few months old, and this is not an all weather plane. They do move a lot of air, and my system is turned down to about half speed (at max it sounds like you're getting ready to levitate away!). So if you needed more, turn 'em up. Pretty much run mine full time.

In the end, opening the canopy might be enough to clear things up, and it's probably prudent not to go flying before your canopy has warmed a little to help prevent any unexpected IFR. I do think the fans help with more uniform warming. This system is all most useful after your heater is transfering some BTU's and sending them to your feet.

Hope this helps ...

Cheers
 
I used to get moderate fogging on my 7 in winter with no fans.
It would sometimes require a cloth to remove if it didn't clear up by takeoff time.
Heres my solution for the RV10. Shamelessly stolen from a local builder here.
It took a few iterations on the duct but it worked out ok on the 3rd go.

An axial fan with a 3D printed duct to take it to a slim auto style diffuser close to the windscreen base.
I didn't like the idea of big holes there. I also but a doubler around the slot for the diffuser. The slot is about 0.5" x 5"

IMG_3479.jpg

IMG_3478.jpg
 
A couple of comments:

This mod would be more difficult after the canopy "glass" is installed, but with a little ingenuity it should be possible. All my wiring is accessible. Just a matter of protecting the clear stuff while you're working. Blue tape and paper (maybe a thin sheet of something hard) are your friend. If you don't like the idea of drilling with a big hole saw (nor do I), use a Greenlee punch (perfect holes and no burrs to speak of). They require only a small pilot hole to get started.

Might be able to protect it with this. https://www.aircraftspruce.com/catalog/cspages/discoat4220plexiglass1.php
 
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