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F1R 11-29-2018 01:03 PM

Quote:

Originally Posted by rvdave (Post 1305788)
Yesterday while flying in northern Michigan marginal vfr conditions below the clouds and freezing temps I was picking up freezing moisture on the windscreen. Although I have defrost fans on the glare shield it wasn’t enough heat to take away or disperse buildup so landed right away with no factor.
Has anyone ducted air from floor heat to optionally divert warm air to defrost fans? I’m thinking of adding somehow a coupling and scat to floor heat outlet and a way to switch between the two. Any ideas?

If on the inside , from a cold windscreen and sweaty clothing or breathing and humidity inside the cabin, I have found a small blast stream of cold outside air keeps the inside surface very clear. from experience, this even works well down to and at -20C . The cold air simply sublimates away any moisture frozen on the inside with the added advantage of no thermal shock from hot on the inside but cold on the outside.

If however you were getting freezing moisture on the outside.... best to stay out of icing conditions or get out ASAP. RV's and most other light piston singles are just not FIKI aircraft.- Unless you spend a bundle on electric hot edges, and or
an alcohol prop and windshield system like Rocket Bob mentioned, and even then unless you hire a tanker with a water spray system to fly above and in front at several hundred K$ for all the test and paperwork you still won't be approved for flight into known icing. And don't forget your fuel tank vents or you will be landing with a collapsed fuel tank/ leading edge- just ask Bruce Bohannon what happened when he iced over the fuel tank vents on one of his time to climb or altitude record attempts.

Canopy icing is the least of the icing problems.

Lots of days it is best to stay home or get a motel.

Paddy 11-29-2018 01:05 PM

Heated Windshield
 
To expand on Carl's point, the amount of heat required to melt ice in a 150-200mph sub-freezing airstream is not trivial. In other words, it's not coming from any size or temperature of defrost fan inside the cockpit. Just look at the windshield protection on business jets or even light singles with FIKI certification.

Now, if you're looking for a defogger to keep the INSIDE of the windshield free of condensation on a cold day, I've found a couple of 80mm computer fans drawing warm air up from underneath the glareshield to perform quite well.

echozulu 11-29-2018 02:02 PM

Quote:

Originally Posted by Paddy (Post 1305827)
To expand on Carl's point, the amount of heat required to melt ice in a 150-200mph sub-freezing airstream is not trivial. In other words, it's not coming from any size or temperature of defrost fan inside the cockpit. Just look at the windshield protection on business jets or even light singles with FIKI certification.

Now, if you're looking for a defogger to keep the INSIDE of the windshield free of condensation on a cold day, I've found a couple of 80mm computer fans drawing warm air up from underneath the glareshield to perform quite well.

While you can't use it as de-ice, it could possibly be equivalent* to hot plates that cessna's and piper's use on their windshields. So it would inhibit ice from forming further, potentially obstructing your view.

TBH, I haven't even done back of the napkin calculations or have any sort of FIKI experience so this is just a rough guess. I'm pretty far from this point in my build, but I figure even though I don't intend to ever encounter icing conditions, if it has any sort of benefit at all it may be worth it.

*Depending on how hot the air is coming out

Paddy 11-29-2018 03:13 PM

Hot plates
 
Quote:

Originally Posted by echozulu (Post 1305838)
it could possibly be equivalent* to hot plates that cessna's and piper's use on their windshields.

That's exactly my point. Those hot plates are small glass rectangles placed on the OUTSIDE of the windshield and draw in excess of 20 Amps when in operation! That's a lotta heat and way more than would be possible to get to the exterior surface of a plexi windshield by blowing any amount of hot air on the inside surface while in flight.

dtw_rv6 11-29-2018 05:39 PM

The heat gun in my shop draws way less than 20A. No way am I going near my canopy with it!


Quote:

Originally Posted by Paddy (Post 1305848)
That's exactly my point. Those hot plates are small glass rectangles placed on the OUTSIDE of the windshield and draw in excess of 20 Amps when in operation! That's a lotta heat and way more than would be possible to get to the exterior surface of a plexi windshield by blowing any amount of hot air on the inside surface while in flight.


rvdave 11-29-2018 11:28 PM

Quote:

Originally Posted by echozulu (Post 1305800)
While that is true, I think with how icing accumulates it'd be pretty helpful to have a defrost duct for the windshield. If you run into icing conditions, at least being able to see out the window might save you, and it's cheaper and easier to implement than putting in prop heat and deicing fluid.

That?s what I?m wondering, an automobile defrost keeps the windshield from icing up to a point but I also realize there is a reason hot plates are used. Is it the high speed of the cold hitting the windscreen that overcomes the ability of hot or warm air to prevent ice buildup?

dtw_rv6 11-30-2018 06:26 AM

q = m x C x DT

q = amount of heat energy gained or lost by substance
m = mass of sample
C = heat capacity (J oC-1 g-1 or J K-1 g-1)
Tf = final temperature
Ti = initial temperature

You have to get Tf above 32 in order to thaw the windscreen. The colder it is outside Ti, the more amperage you need.

The problem here is that you have a boatload (essentially endless supply) of freezing air and water hitting the windscreen so you are essentially trying to boil a gallon of water with matchsticks.

Oh, and to top it off, all of that heat has to go through the plexi. I’d want to see some data on what that material does long term when exposed to a steep thermal gradient.

The alcohol spray mentioned earlier would be far more practical IMHO. Yet again, you really need to research and understand what it is going to do to plexiglass.

(With apologies to any real Mechanical Engineers)
Don

rocketman1988 11-30-2018 06:49 AM

and...
 
...and your car windshield is GLASS, not plexiglass.

At one time I owned a C-421. It actually had an option for a heated windshield. That windshield was over $20k...and that was 20 years ago...

The hot plate could probably be adapted for the RV, however, the best solution is NOT to be in icing conditions in an RV...

Paddy 11-30-2018 08:06 AM

Quote:

Originally Posted by rvdave (Post 1305923)
an automobile defrost keeps the windshield from icing up to a point - break - Is it the high speed of the cold hitting the windscreen that overcomes the ability of hot or warm air to prevent ice buildup?

Good discussion! An automobile windshield isn't designed to prevent ice buildup while driving. The defrost function is primarily designed to bring the glass temperature up to the ambient temp inside the vehicle, so condensation doesn't form on the inside surface. Most modern vehicles generally have enough excess heat to melt accumulated snow just enough to be swept off by the wipers after an overnight accumulation, but that's about it.

Cars don't encounter icing like airplanes, because they don't generally drive in visible moisture (clouds) in below freezing temperatures, or operate where supercooled droplets land on them and freeze on contact. The other factor, as you point out, is the speed of the airstream. The faster you go, the faster the air sucks away the available heat. To replicate what an airplane has to handle, imagine driving a car through freezing fog or an ice storm at well over 100mph!

az_gila 11-30-2018 10:00 AM

Quote:

Originally Posted by Paddy (Post 1305961)
Good discussion! An automobile windshield isn't designed to prevent ice buildup while driving. The defrost function is primarily designed to bring the glass temperature up to the ambient temp inside the vehicle, so condensation doesn't form on the inside surface. Most modern vehicles generally have enough excess heat to melt accumulated snow just enough to be swept off by the wipers after an overnight accumulation, but that's about it.

....

I believe the modern car systems also cool the defrost air before heating it up to decrease the humidity level of the air blown onto the inside of the windshield.

In non-icing but high humidity scenarios this would have more effect than simply blowing inside air through the glare shield.


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