What would you do?

[trampbike]

Ok, perfect now you've narrowed down the problem.
Do you now agree at least that warming the air right in front of the leading edge above 0 degrees does not mean that you can be sure to not be picking up ice (dry or wet, ice could still build up)?

Now, for the specific case of dry wing staying dry while wet wing accumulating ice or keeping a constant amount of ice:
The dry wing will reach the temperature equilibrium much faster than the wet wing. After some time (let's say when flying in non-icing condition), the whole wing should reach some sort of an equilibrium (it doesn't mean it's at a constant temperature though). That is all there is to it. If your dry wing is above 0 degrees, you would need an awfull lot of supercooled water droplets for it to be picking up ice (it is possible for it to happen, but it is less likely). On the other hand, while your wet wing stays at 0 degrees, every single supercooled water droplets that hit it will stick. Now obviously a lot of it will melt too, but at different rate. Pretty sure that so far you agree with everything I wrote.

Now here comes the problem: If we are in the specific situation where ice would not build up on a dry wing but would on a wet wing, it means that the energy exchanges are not exactly the same! How could that be? (am I right understanding that this is where you have yet to find a mechanism to explain this?)

Think about how the air is able to transfer energy to the ice: hint: http://en.wikipedia.org/wiki/Square-cube_law
and how the wing can transfer energy to the ice (hint: what is the overall temperature of your wet wing compared to the dry wing?)

[photofly]

I've written and deleted more answers to this than I should have spent time on.

You need to consider the concept of thermodynamic equilibrium between the surface of the wing/ice, the super-cooled liquid droplets, and the heated air. If an equilibrium can be established, then there can only be one, and it can't depend on initial conditions.

If you think an equilibrium fails to be established, you need to say why, in clear terms.

I see no reason why the initial state of the wing (wet or dry) should make any difference to the establishment of an equilibrium.

PS short posts, without links to external webpages (bad) and books (worse) are appreciated.

[trampbike]

If an equilibrium can be established, then there can only be one, and it can't depend on initial conditions.

If you think an equilibrium fails to be established, you need to say why, in clear terms.

I thought you'd figure it out, sorry. Square-cube law, when you get more volume of ice on your wings, the rate of melting is going to be lower than when you have a very small volume on it (like when you start off with a dry wing and the new ice gets melted very rapidly as is touches the wing).

I've spent way too much time on this. If you don't think my hints were helpful, then maybe you should just sit down and work it out from the start.
Please if you ever get some ice on your wings, don't try to shed it off by going fast, unless you're able to fly very very fast.

[Colonel Sanders]

Trampbike: have you ever picked up a load of ice?

[Pop n Fresh]

So when I do finally troll you I get a straight answer? Unfair.

[photofly]

To answer my own question: here's a reason why a dry wing might not pick up ice when a wet wing does: the chemical or microscopic surface of a dry wing might repel the initial layer of water droplets before they freeze (think "Rain-Ex"), but once overwhelmed, ice can continue to build in a scenario where the first layer wouldn't form. I've no idea if that happens, but it's physically plausible.

What's not plausible is that ice can continue to form in a scenario where the incoming air/moisture is heated above freezing. Not even if the wing is already covered in ice. While water can maintain its liquid state below zero degrees (and there are good energy-related reasons why equilibrium isn't established) I don't know of any process where ice can continue to exist in thermal contact with something that's above zero degrees.

I secretly suspect a Nobel prize awaits someone who can demonstrate water ice stable above the freezing point of water.

[trampbike]

What's not plausible is that ice can continue to form in a scenario where the incoming air/moisture is heated above freezing.

We are spinning in a tight circle here... I'll ask again: why do you assume that the water droplets will also have time to be warmed above 0 degrees.
The air/moisture might heat up above 0 like you calculated earlier, but that doesn't mean the water droplets will. They might still hit the wing at a below freezing temperature. If they accumulate faster than they melt, your total volume of ice increase. Of course at a high enough speed, your rate of melting will certainly be greater than the rate of catch, and after a point you'll end up with a dry wing that'll not catch any more ice.

Supercooled water droplets will continue to accumulate for a longer period on a wet wing than on a dry wing, because in the first case your leading edge temperature can't be above 0 degrees (while there is ice) because of the absorption of heat to melt a given amount of water. As they accumulate, the volume of ice gets larger, and the larger it gets, the harder it is for the air to warm it up (square-cube). A dry wing getting hit by the same amount of droplets might be able to melt them right away, simply because 1)The surface/volume ratio of the ice is much greater and therefore air will warm it up faster and 2)The whole wing will be warmer, and by conduction will heat up whatever hits the leading edge faster. Keep in mind that the wet wing will not see its temperature increase as long as there is ice. The ice can't be warmed from below by conduction in this case.

If an equilibrium can be established, then there can only be one, and it can't depend on initial conditions.

Very true, when the processes are the same. In this case, the initial conditions change a little bit the processes.

I've pretty much reached my english writing skills limit during this discussion (it is not my first language), so I'm not sure I would be able to be more concise and clear.

Trampbike: have you ever picked up a load of ice?

No, but I'm in luck because I'm sure you have some sweet pictures of you flying the L39 or another awesome aircraft that will prove very well how previous personal experiences affect physical phenomenons.

[photofly]

Very true, when the processes are the same. In this case, the initial conditions change a little bit the processes.

Actually not true. The process entirely irrelevant. The point about thermodynamics is that you don't have care about the energy exchange mechanism, as long as there is one. As long as there's a path, some path, any path to equilibrium - that's where you end up. Therein lies the value of the study of thermodynamics.

To the case in hand:

Let's split the incoming air and supercooled liquid droplets into two distinct phases. (In other words let's take to extreme your point that the SLD doesn't have time to warm in the heated air before it impacts the wing.) Here's why it doesn't matter.

In extremis, either the warm air heats the wing first and then the SLD freezes on the wing, or at the other extreme, SLD freezes on the wing and then the warm air heats it. In either case, all three systems (SLD, wing, and warm air) are in good thermal contact, and reach an equilibrium temperature. The same equilibrium temperature either way around. That's the zeroth law of thermodynamics.

But the SLD and the warm air have an infinite thermal mass. There's an infinte amount of both flowing over the wing, and only a finite mass of wing. The wing can get hotter or colder as it gains or loses thermal energy, but the incoming flows of air and SLD are continually renewed at the same two temperatures.

So when the three systems - wing, air, SLD come to an equilibrium temperature it can only be at the mean temperature of the SLD and the warm air, weighted by their initial temperatures and thermal masses. The initial temperature of the wing is irrelevant.

What is this "mean" temperature of the SLD and the warm air? Why, it's exactly the temperature we calculate that a parcel of air, containing SLD, would reach if adiabatically compressed and left for the SLD to warm and reach equilibrium with the air. That's the temperature the wing will reach.

If that temperature is above freezing, then no ice can accrete, and any existing ice must melt.

[trampbike]

How long will it take to reach said equilibrium? Do you think you'll be able to stay airborne long enough for the wet wing to dry? It all depends on the rate of catch. Do you also think that the rate of catch will be the same for a dry and an iced up wing? Do you also assume that the air is heated up all around the wing or just forward of the leading edge?

edit: I've mentionned this a couple of times before, but what do you think might happen to the ice you'll melt as it moves aft on the wing?

[trampbike]

Very true, when the processes are the same. In this case, the initial conditions change a little bit the processes.

Actually not true. The process entirely irrelevant. The point about thermodynamics is that you don't have care about the energy exchange mechanism, as long as there is one. As long as there's a path, some path, any path to equilibrium - that's where you end up. Therein lies the value of the study of thermodynamics.

My bad, I should have worded it differently: The actuall amount of energy that is exchanged in your 2 systems is not necessarily the same, since some factors will change your rate of catch.

[photofly]

How long will it take to reach said equilibrium?

Oh, I think it will take a couple of microseconds for the immediate surface of the ice on the wing to reach equilibrium with both the hot air and SLD, and start to warm up. Ice doesn't conduct heat very well so very little energy is going to be conducted back into the body of any ice.

If the wing is bare painted aluminium, although aluminium conducts heat well, it doesn't really matter. It's not going to ice up first and then melt.

[AuxBatOn]

From experience, I need to have a TAT of 5-10 degrees on a surface for it not to pick up ice.

In a descent through ice, I'll set my speed to get at least 5 degrees Celcius on the inlet temperature in order to avoid picking up ice.

This is regardless of adiabatic whatever bullshit theory. In an airplane, theory seldom served you well. Practical and concrete knowledge is far more important.

[trampbike]

Yep, so it melts, but it continues to accumulate ice, it might never end up being dry. During this time, the rest of your wing does not heat up. How long can you last in such conditions? Would you be able to fly longer in these conditions with an initially dry wing?

You also seem to assume that all of the heat from the air is absorbed by the wing or the ice, and that it is absorbed equally on the dry wing and the wet one.

[photofly]

You're confusing heat and temperature.

It doesn't need to absorb all the "heat" from the air to reach the same temperature as the air.

[trampbike]

It's not going to ice up first and then melt.

Oh really why not?

[photofly]

It's not going to ice up first and then melt.

Oh really why not?

Because if the wing is warmed enough to melt ice, no ice can accrete in the first place. The end result is either a wing with no ice, or a wing with ice. It can't be first one then the other. Or are you proposing some kind of ongoing oscillation between two states?

[trampbike]

You're confusing heat and temperature.

Don't worry, I'm not.

AuxBatOn, have you ever been in a situation where you had ice, picked-up some speed and were able to shed the ice while still in icing conditions?
Also, on the Hornet, what are the recommendations to prevent icing: speed up before to keep it dry, or speed up once you see ice accumulates?

It's not going to ice up first and then melt.

Oh really why not?

Because if the wing is warmed enough to melt ice, no ice can accrete in the first place. The end result is either a wing with no ice, or a wing with ice. It can't be first one then the other. Or are you proposing some kind of oscillation between two states?

It will ice up on contact, even on an above 0 surface. Wether it will melt away or accrete (or freeze up further aft on the wing!) is a function of the rate of catch and the amount of heat the wing and the air can provide.

[AuxBatOn]

Again, from experience, I'll try to avoid ice accumulation because it's more difficult to get rid of ice than to avoid it. I'll turn the engine anti-ice on and accelerate to maintain > 5 degrees on the inlet anti-ice.

But I'n sure our theorist will refute that.

[photofly]

I could rebuke you, or refute your comment. I don't think I could rebuke your comment. And in no sense am I "yours".

[AuxBatOn]

Whatever semantics. My English is not perfect. You are nitpicking at shit to does not matter.

[photofly]

Well, I don't really want to do either. I don't have a quarrel with you; I'm not sure why you're trying to pick one with me.

[trampbike]

I actually like talking theory, but I really think that is this case, there are way too many variables (surface conduction, airflow disruption, phase changes, rate of catch, shape changes, heat dissipation, compressive heating, etc) for us to be able to "prove" in any rigorous matter our points.

My point was from the beggining that you might not be able to shed ice even if you could get the air in front of the leading edge above 0 degrees. It hasn't changed since. The discussion has been interesting, but honestly photofly, I'm not too sure what your intial point was and what it is now... You seemed to pick on some of the points that I made and ignore many others that I repeated over and over again, so I have no idea on which point do we now agree on that we didn't at the beggining of the discussion. Maybe it's a language issue here for me.

[photofly]

The discussion has been interesting, but honestly photofly, I'm not too sure what your intial point was and what it is now..

I've found it interesting to think about what happens when air containing SLD is heated ahead of a wing. You made some interesting points and I learned some stuff by thinking it through.

However anyone who thinks that a good ice removal strategy is to fly at 185 knots just because a CX2 calculator says the TAT is just above zero is a good candidate for removal from the gene pool.

[photofly]

I wonder, if it's sufficiently cold outside, if it's possible to increase your ice catch rate through heating the air?

[trampbike]

Water will generally not go from ice to supercooled water, it will either stay solid, sublimate, or become a liquid above freezing. So I would say that no, heating the air around the wing should not be a problem in this case. Again, it depends on how the water moves with the airflow around the wings, and without actual wind tunnel datas, I would not dare go there.

However if you show up with a warm wing and fly slowly into ice crystals or snow, now you might accumulate some as it melts partially on the wing and is allowed to stick to it.