Anti Ice bleed air
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Anti Ice bleed air
Don't know if this question goes here.
I'm curious to know how much bleed air you need in order to provide heat anti-ice instead of typical boots de-ice on a TP or smaller jet. Does anyone know?
I understand there is a penalty to pay somewhere but is a heated system completely out of the question in turboprops?
I'm curious to know how much bleed air you need in order to provide heat anti-ice instead of typical boots de-ice on a TP or smaller jet. Does anyone know?
I understand there is a penalty to pay somewhere but is a heated system completely out of the question in turboprops?
the Convair had a hot wing , however I believe they kind of had a dedicated janitrol or equivalent. The leading edge has to be hot enough, to vaporize the icing, and not flow back. Realistically, this is a no issue.
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The reciprocating Convairs have a wing heated by augmentor duct air from the exhaust. This brings to mind an important distinction.
There are two types of heated wing systems. The system used by the Convair is known as a "running wet" system. It is not capable of evaporating the water impinging the wing. Rather, it simply prevents it from freezing. This usually, as in the Convair, leads to re-freezing aft of the protected surface. The result is typically a sawtooth pattern of thin ice ridges, more longitudinal than lateral.
The more common installation is known as a "fully evaporative" system. This is designed to evaporate the liquid water impinging the wing. The advantage, of course, is the wing will be long gone before the water re-condenses.
One caveat is that all evaporative systems necessarily transition through a running wet period while warming up to operating temperature. This requirement is what drives things like minimum N1 during descent; it wouldn't do to have the system cool into a running wet condition when the power is retarded. This is also one of the shortcomings of such designs as the MD80, in which the wing is cooling while the tail is being heated, and vice versa.
Both types of systems can be used as either anti-ice or de-ice systems. Boeing currently recommends a de-ice method in most designs, although once you select it, it functions as an anti-ice system if you just leave it on.
The required energy is driven by the wing surface area that is intended to be heated, which in itself is a function of how much chord will be protected. It also depends on the operating speeds in the design specification, and whether the system will run wet or evaporate. It can be pretty substantial. Most turboprops try to recover as much energy from the gas turbine as possible; anything that goes out the tailpipe is essentially wasted (yeah, they count it as thrust, but...). So bleed air is a precious commodity, and the energy balance between needs and availability drive manufacturers toward boots. I suppose you could hang a couple of Allisons out there or something like that, but then your specific fuel consumption goes through the roof and the customers don't really care for that.
Hope that is helpful
There are two types of heated wing systems. The system used by the Convair is known as a "running wet" system. It is not capable of evaporating the water impinging the wing. Rather, it simply prevents it from freezing. This usually, as in the Convair, leads to re-freezing aft of the protected surface. The result is typically a sawtooth pattern of thin ice ridges, more longitudinal than lateral.
The more common installation is known as a "fully evaporative" system. This is designed to evaporate the liquid water impinging the wing. The advantage, of course, is the wing will be long gone before the water re-condenses.
One caveat is that all evaporative systems necessarily transition through a running wet period while warming up to operating temperature. This requirement is what drives things like minimum N1 during descent; it wouldn't do to have the system cool into a running wet condition when the power is retarded. This is also one of the shortcomings of such designs as the MD80, in which the wing is cooling while the tail is being heated, and vice versa.
Both types of systems can be used as either anti-ice or de-ice systems. Boeing currently recommends a de-ice method in most designs, although once you select it, it functions as an anti-ice system if you just leave it on.
The required energy is driven by the wing surface area that is intended to be heated, which in itself is a function of how much chord will be protected. It also depends on the operating speeds in the design specification, and whether the system will run wet or evaporate. It can be pretty substantial. Most turboprops try to recover as much energy from the gas turbine as possible; anything that goes out the tailpipe is essentially wasted (yeah, they count it as thrust, but...). So bleed air is a precious commodity, and the energy balance between needs and availability drive manufacturers toward boots. I suppose you could hang a couple of Allisons out there or something like that, but then your specific fuel consumption goes through the roof and the customers don't really care for that.
Hope that is helpful
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Thanks for the input.
I fly a Caravan in the north so I know about not having excess power... So, basically it's an economical choice in the design phase and I guess you could have heated wings if you throw 1000hp on the nose and use the 325 excess for anti-ice. Not exactly what management would like I suppose.
The thing that bothers me is that we're expected to use good judgement and fly in most icing conditions without killing ourselves, yet, there is no interest in providing decent protection. The boots generally work but don't provide any extra margin. As for TKS, the system may work when there's fluid but once you run out your in barney.
I read something about Boeing installing electric heating on the slats instead of pumping warm air on the 787. Supposedly this would cut energy consumption for anti/de-ice by 40-50%.
Would that be a better option?
I fly a Caravan in the north so I know about not having excess power... So, basically it's an economical choice in the design phase and I guess you could have heated wings if you throw 1000hp on the nose and use the 325 excess for anti-ice. Not exactly what management would like I suppose.
The thing that bothers me is that we're expected to use good judgement and fly in most icing conditions without killing ourselves, yet, there is no interest in providing decent protection. The boots generally work but don't provide any extra margin. As for TKS, the system may work when there's fluid but once you run out your in barney.
I read something about Boeing installing electric heating on the slats instead of pumping warm air on the 787. Supposedly this would cut energy consumption for anti/de-ice by 40-50%.
Would that be a better option?
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I haven't looked at the 787 system, but Boeing is typically pretty good at identifying where they need ice protection and where they don't. Note that none of their tails are protected, and they have no history of any tail icing issues. The 767/757 family is only protected on the outer slats. So they can and do identify the minimum surface area needing ice protection, which allows them to reduce the energy requirement.
In doing so, they are taking advantage of scale. We know that the critical factor in ice is the k/C number, which is the ratio of ice shape height to chord length. A Boeing has a large chord; so the same k value, for example from precisely the same icing encounter, will result in a much lower k/C value for the Boeing than for the Cessna single (all other variables being constant, which is frankly impossible to achieve).
In my opinion, too little is still known about thin ice. We have also known for many years that thin, sandpaper ice is very dangerous stuff in the right circumstances.
The real problem with the acceptability of any ice protection system is that the icing threat, unlike the thunderstorm threat, does not produce repeatable results. Other variables in ice shape, such as horn location, angle, and height, runback, roughness, etc. all play a role in the aerodynamic degradation. One airplane may have no significant degradations, and a few minutes later the next guy falls out of the sky. In many cases, the wing performs just fine until the critical angle of attack is achieved. There is no way for a pilot to know what his modified lift curve looks like while in icing, so he takes away the perception that "it carries a lot of ice"...all the while unaware of how close the the cliff, or peak in the lift curve, he was operating. The vast body of successful experience reinforces the notion that we know what we are doing, and allows us to conclude that boots are acceptable. Every once in a while, a booted airplane turns turtle, and it is easy to direct the cause at the operating decisions made by the crew. In some events, that is valid; in others, it is not.
There have been many efforts at developing low energy systems for smaller aircraft, typically using electrical power. Some are very promising. But there is no manufacturing incentive, because there is no metric to use which will point to one or another system as superior. They are all either certificated, or not. And the gamble of building your brand new Super XYZ 100 around an ice protection system that has yet to be certificated or proven in operation is pretty daunting.
That said, to my knowledge, the only hot wing airplanes to suddenly stop flying in ice were being operated without the ice protection system selected on. This usually happens because a lot of crews operating heated wing aircraft are under the impression that a little ice is no problem. That may be true, until you run the angle of attack up the lift curve slope towards the top, whereupon you find that the curve breaks a whole lot sooner than you thought it would.
In doing so, they are taking advantage of scale. We know that the critical factor in ice is the k/C number, which is the ratio of ice shape height to chord length. A Boeing has a large chord; so the same k value, for example from precisely the same icing encounter, will result in a much lower k/C value for the Boeing than for the Cessna single (all other variables being constant, which is frankly impossible to achieve).
In my opinion, too little is still known about thin ice. We have also known for many years that thin, sandpaper ice is very dangerous stuff in the right circumstances.
The real problem with the acceptability of any ice protection system is that the icing threat, unlike the thunderstorm threat, does not produce repeatable results. Other variables in ice shape, such as horn location, angle, and height, runback, roughness, etc. all play a role in the aerodynamic degradation. One airplane may have no significant degradations, and a few minutes later the next guy falls out of the sky. In many cases, the wing performs just fine until the critical angle of attack is achieved. There is no way for a pilot to know what his modified lift curve looks like while in icing, so he takes away the perception that "it carries a lot of ice"...all the while unaware of how close the the cliff, or peak in the lift curve, he was operating. The vast body of successful experience reinforces the notion that we know what we are doing, and allows us to conclude that boots are acceptable. Every once in a while, a booted airplane turns turtle, and it is easy to direct the cause at the operating decisions made by the crew. In some events, that is valid; in others, it is not.
There have been many efforts at developing low energy systems for smaller aircraft, typically using electrical power. Some are very promising. But there is no manufacturing incentive, because there is no metric to use which will point to one or another system as superior. They are all either certificated, or not. And the gamble of building your brand new Super XYZ 100 around an ice protection system that has yet to be certificated or proven in operation is pretty daunting.
That said, to my knowledge, the only hot wing airplanes to suddenly stop flying in ice were being operated without the ice protection system selected on. This usually happens because a lot of crews operating heated wing aircraft are under the impression that a little ice is no problem. That may be true, until you run the angle of attack up the lift curve slope towards the top, whereupon you find that the curve breaks a whole lot sooner than you thought it would.
