Defrost ideas

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?

Umm...

If you were getting "freezing moisture" on the windshield, where else might you be getting "freezing moisture"?...

I assume you wouldn't intentionally fly into freezing moisture, that being said, I mounted a fan on the upper side of the firewall heater plenum, mounted a 2" aluminum flange to it and routed it to a defrost duct on pilots glareshield. I have not used it to remove ice from the outside of the windscreen and don't know if it will, but works nicely to defrost the inside windscreen.

rocketman1988 said:

If you were getting "freezing moisture" on the windshield, where else might you be getting "freezing moisture"?...

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This was just a local flight in the pattern, surprised to see a buildup so quick when there wasn?t real visible moisture but obviously was there, was a dreary day. Learned something again. Wasn?t up long enough to see anything on airframe. I can see though a need for better defrosting.

rocketman1988 said:

If you were getting "freezing moisture" on the windshield, where else might you be getting "freezing moisture"?...

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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.

Been there done that...VMC, not in the clouds.

My neighbor's Baron, although not equipped for known icing, has an alcohol prop and windshield. It's nothing more than an AN fitting and a piece of aluminum tube in front of the windshield. I'm sure one could rig up a heated windshield washer nozzle from a car and strap in a small tank/pump to be used in the winter.

Unless you could throw a serious amount of heat at the windshield, hence why hotplates are a thing, I doubt any amount of cabin heat would be enough to keep ice from sticking.

Yes...

Yes, a defroster is not a bad idea, however, if you are seeing it on the windshield, it is most likely happening at other areas on the airframe, typically on small radius curves and anything that protrudes into the flow.

It is truly AMAZING how fast the "freezing moisture" can accumulate, if the right conditions are present.

Some years ago a family friend flew a 182 on a cross country and encountered ICE, (there, I said it). Thankfully, he landed at his destination...with almost two inches on the airframe and a ball on the spinner the size of a softball. You may have read about it in the AOPA magazine.

Also, some years ago, my family was snowmobiling in the Northwoods of WI when we came upon a guy flagging us down in the middle of a trail. It turns out the King Air that he and his family were riding in had crashed in a clearcut area. We carried him to the nearest stop on the trail, only to find the Sheriff's department getting ready to walk a grid through the woods to find the downed airplane. Weather conditions, at the time? FREEZING FOG. That King Air was on about a 15 mile ILS final and didn't make it...the crash broke the airplane in three pieces. Amazingly, everyone, including the dog, onboard survived; the worst injury was the pilot's broken leg.

Point is, "freezing moisture" can and will accumulate, sometimes after than expected. Our RVs are wonderful airplanes but they are still no match for Mother Nature.

Be careful out there!

The empennage surfaces my be considered small radius curves protruding into the airflow.

Yep...

Yes, they are...

Be careful of unintended consequences

To clear ice from on the outside of the windshield by blasting heat on the inside would lead me to reflect on my days of stress equations for heat transfer across material. Here the cold outside surface will be in tension while the hot inside surface will be in compression. While quality steel can accommodate some of these conditions, I would not want to test the plexi we use in our airplanes.

I had clear ice once on the glass and leading edge. About a quarter of an inch accumulated on the windshield and leading edges in less than 30 seconds. Departing the icing conditions cleared it up in a few minutes. This experience lead me to study compressive heating (OAT reading is up to 5 degrees or so above actual OAT) and super cooled liquid in cloud tops.

Here is an OAT correction graph:http://aviationandaccessories.tpub.com/TM-1-1510-225-10/css/TM-1-1510-225-10_640.htm

Carl

rvdave said:

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?

Click to expand...

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.

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.

Paddy said:

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.

Click to expand...

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

Hot plates

echozulu said:

it could possibly be equivalent* to hot plates that cessna's and piper's use on their windshields.

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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.

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

Paddy said:

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.

Click to expand...

echozulu said:

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.

Click to expand...

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?

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

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...

rvdave said:

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?

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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!

Paddy said:

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.

....

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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.

I'm VFR-Day only, but I've often thought that having some kind of "windshield washer spray" could save my bacon in the event of oil on the windshield, as well as ice. It seems like an automotive windshield washer cold be easily adapted for that purpose.

rvdave said:

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?

Click to expand...

Was this typical condensation on the inside of the windshield or on the outside? Icing comes from liquid moisture hitting the plane and then freezing. This requires "visible moisture,"such as in clouds, fog or rain. The air always includes moisture in a gaseous state (at least down to a temp below 0) , however, this gaseous moisture mixed with air will not cause icing. It is only an icing problem when the air is fully saturated with moisture and the remainder stays in liquid form and creates clouds or fog. What makes a cloud visible is the suspended water or ice particles in a liquid or solid state.

I am not the most experienced guy, however, I have flown MANY hours at freezing temps. I have never seen icing form without visible moisture and this is backed up by all of the educational material in the IFR world. The only way that moisture converts from a gas to a liquid is through condensation. As long as your airframe is the same temp as the air, it can't cause condensation.

Larry