Concerning governor masking of carb ice.
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Concerning governor masking of carb ice.
Whilst explaining the phenomenon of carb ice i say:
1. Low pressure and vaporization of fuel lower temp in the
venturi
2. If the right conditions exist (high humidity, close temp/dew
point spread) moisture in the air may freeze
3. When the throttle valve is closed (low MAP) it may stick to
the ice and not open any wider when power is applied
NOW if i got that right
I cannot seem to understand how the governor can mask the signs of carb ice by rolling on the throttle more...
To maintain constant RPM wouldn't the throttle valve have to open more?
Thanks to anyone with more knowledge on the subject than myself!
1. Low pressure and vaporization of fuel lower temp in the
venturi
2. If the right conditions exist (high humidity, close temp/dew
point spread) moisture in the air may freeze
3. When the throttle valve is closed (low MAP) it may stick to
the ice and not open any wider when power is applied
NOW if i got that right
I cannot seem to understand how the governor can mask the signs of carb ice by rolling on the throttle more...
To maintain constant RPM wouldn't the throttle valve have to open more?
Thanks to anyone with more knowledge on the subject than myself!
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If you are cruising along and you start to get carb-icing, the governer will open the throttle more to keep the RRPM in the green, it will continue to do this until the throttle is wide open (all this happens without you noticing), then...the RRPM will start to drop as the icing gets worse.
If you didnt have the governer you would notice carb icing immediately as the RRPM would drop.
If you didnt have the governer you would notice carb icing immediately as the RRPM would drop.
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YOU SAID
"""3. When the throttle valve is closed (low MAP) it may stick to
the ice and not open any wider when power is applied"""
* Remember this is only one of the possible results of carb icing.
The other is that the venturi walls get ice on them making the 'pipe' smaller. This is just like closing the throttle as the flow of mixture is now restricted.
Then as Blithe said, the governor will open the throttle to maintain RPM.
cl12pv2s
"""3. When the throttle valve is closed (low MAP) it may stick to
the ice and not open any wider when power is applied"""
* Remember this is only one of the possible results of carb icing.
The other is that the venturi walls get ice on them making the 'pipe' smaller. This is just like closing the throttle as the flow of mixture is now restricted.
Then as Blithe said, the governor will open the throttle to maintain RPM.
cl12pv2s
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AFAIK, the throttle valve sticking to the ice is not the problem. It's that as ice forms, there is actually less space. Now, you can get carb icing at any power setting. But it's more likely at low power settings, because when you close the throttle, with ice being there, it closes that gap altogether, so the engine stops. And at that point of course...... 
If you have no governor, as the ice forms, the flow of mixture is restricted and the engine RPM drops. An unexplained drop in engine RPM in any aircraft with a carb is usually a sign of carb icing starting....and you can then do something about it before it's too late. But with a governor fitted, as the engine RPM starts to drop, the governor winds on the throttle. So you don't get any warning and don't know about it.
Now I think about it, even if the sticking to the ice bit is correct - and I suppose it could be - it makes no difference. The point is that the governor, by increasing engine RPM, masks the EARLY WARNING SIGNS of carb icing.
If you have no governor, as the ice forms, the flow of mixture is restricted and the engine RPM drops. An unexplained drop in engine RPM in any aircraft with a carb is usually a sign of carb icing starting....and you can then do something about it before it's too late. But with a governor fitted, as the engine RPM starts to drop, the governor winds on the throttle. So you don't get any warning and don't know about it.
Now I think about it, even if the sticking to the ice bit is correct - and I suppose it could be - it makes no difference. The point is that the governor, by increasing engine RPM, masks the EARLY WARNING SIGNS of carb icing.
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My 2c:
Everybody is right, the governor masks the powerloss to begin with.
To sum it up:
Looking at the carb, if the surcumstances are right (or wrong...) the carb-ice usually forms in two different places, the venturi where the fuel is introduced and vapourized, and at the throttle-valve.
The ice forming at the venturi is quite easy to handle since that's where the carb-temp is measured. Keep it out of the yellow and no ice will form.
The ice forming at the valve is only forming when the throttle is beeing closed, since by doing so, the air has to go through a smaller area and now there's a second venturi with no temp measurement. The more you close it, the greater the temp drop will be. This is the reason that the R22 and H300CB says full carb heat below 18" MAP no matter of carb-temp. My guess beeing that below 18" the pressure and subsequent temp drop is greater at the valve then at the venturi. Long before the throttle valve sticks to the ice, the engine will have been choked, and your nose WILL initially be pointing very much to the left
/2beers
Everybody is right, the governor masks the powerloss to begin with.
To sum it up:
Looking at the carb, if the surcumstances are right (or wrong...) the carb-ice usually forms in two different places, the venturi where the fuel is introduced and vapourized, and at the throttle-valve.
The ice forming at the venturi is quite easy to handle since that's where the carb-temp is measured. Keep it out of the yellow and no ice will form.
The ice forming at the valve is only forming when the throttle is beeing closed, since by doing so, the air has to go through a smaller area and now there's a second venturi with no temp measurement. The more you close it, the greater the temp drop will be. This is the reason that the R22 and H300CB says full carb heat below 18" MAP no matter of carb-temp. My guess beeing that below 18" the pressure and subsequent temp drop is greater at the valve then at the venturi. Long before the throttle valve sticks to the ice, the engine will have been choked, and your nose WILL initially be pointing very much to the left
/2beers
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Has any one elce ever seen petrol frozen . i use to have a citron that whould stop when the wether was realy cold , then i had to get out in the cold , lift the hood and pull the air filter pipe off . then you could look down the carb and see the ice that had choked it . with a fag lighter it could be lit and whould burn off very quick , hand ovet top to put it out , pipe back on and away to go untill it did it again 10 mijles down the road .
I think that this happens at about -40 . With the air speed in the carb ( 4 miles an hour to 1' drop ) and the drop in presure whould explain this i think or am i wrong . I have all ways asumed that it was the fuel mainly not the moisture .
I think that this happens at about -40 . With the air speed in the carb ( 4 miles an hour to 1' drop ) and the drop in presure whould explain this i think or am i wrong . I have all ways asumed that it was the fuel mainly not the moisture .
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In the case of no governor, the pilot would need to adjust the throttle often.
How could the pilot sense a creeping carb ice condition? Or understand that the throttle is opening up over time?
In a fixed wing the throttle knob creeping closer to the panel is obvious. I do not see how a helo pilot can notice carb ice, from the throttle position, with or without a governor.
Just one R22 flight for me , so I don't know the answer.
How could the pilot sense a creeping carb ice condition? Or understand that the throttle is opening up over time?
In a fixed wing the throttle knob creeping closer to the panel is obvious. I do not see how a helo pilot can notice carb ice, from the throttle position, with or without a governor.
Just one R22 flight for me , so I don't know the answer.
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Slowrotor,
Your question is in fact your answer.
In an aircraft that requires manual application of the throttle control, the pilot flying along quite happily straight and level will notice by the fact that he / she must raise the collective or roll on throttle in order to maintain the straight and level flight. If he / she raises the collective, then the rpm will drop requiring more throttle. If he / she just rolls on throttle, then eventually the pilot will notice this trend (may be run out of throttle) and decide this must be carb icing.
It is by noticing the continual need for more throttle in that the pilot can deduce the carb icing.
With the R22, the govenor does this. In order to let the govenor do its stuff, most R22 pilots hold the collective very lightly. Some (me included) hold the collective below the throttle control. Nevertheless, R22 pilots don't have an accurate way of guaging how open the throttle is.
cl12pv2s
Your question is in fact your answer.
In an aircraft that requires manual application of the throttle control, the pilot flying along quite happily straight and level will notice by the fact that he / she must raise the collective or roll on throttle in order to maintain the straight and level flight. If he / she raises the collective, then the rpm will drop requiring more throttle. If he / she just rolls on throttle, then eventually the pilot will notice this trend (may be run out of throttle) and decide this must be carb icing.
It is by noticing the continual need for more throttle in that the pilot can deduce the carb icing.
With the R22, the govenor does this. In order to let the govenor do its stuff, most R22 pilots hold the collective very lightly. Some (me included) hold the collective below the throttle control. Nevertheless, R22 pilots don't have an accurate way of guaging how open the throttle is.
cl12pv2s
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Thanks for the responses!
i like the idea of the "2nd venturi" @ the butterfly creating a lower temp than at the primary venturi. It does explain the "full carb heat below 18" placard.
-keep it in the green-
J.
PS
if flying @ high elevation does anyone lean/enrich the mixture when flying a robbie?
i like the idea of the "2nd venturi" @ the butterfly creating a lower temp than at the primary venturi. It does explain the "full carb heat below 18" placard.
-keep it in the green-
J.
PS
if flying @ high elevation does anyone lean/enrich the mixture when flying a robbie?
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Concerning governor masking of carb ice.
bauldrik. I too had a vehicle that suffered from carb ice. As you describe, whip off the air filter after grinding to a halt, and there it is. A ball of iced petrol sitting on top of the main jet. forcibly remove it or just wait till it melts, and away you go, well at least for another two or three miles. Having suffered carb ice where it is just an inconvenience I can see how easy it would be to get caught out in a situation where its very very inconvenient. The vehicle in question a Fiat Topolino had a little flap on the air filter onto which was stamped. IN COLD WEATHER MOVE FLAP TO THIS POSITION. I now read the instructions before use. Bug
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bug
Ta , i know about the little flap but the car was a £50 form scrap yard so had the pipe from the manifould missing . I am engineer but have seen the error in this and am now training on the 22 .
All the best David
All the best David
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that wasn't frozen gas, it was ICE!
Gasoline doesn't freeze at any temperature found on the Earth's surface, or even inside a venturi. Too warm!
Those crystals in the carb throat were Hard Water (a.k.a. Ice) which had crept in mixed with the gasoline, then precipitated out in the carb venturi because of the usual stuff: expansion cooling AND evaporative cooling.
Thus not all cases of aviation carb ice are water-vapor-sublimating-onto-venturi; just 99% of them. The other 1% is water droplets arriving from the fuel tank and atomizing with the gasoline at the jet.
Another reason for not letting water cohabit in your fuel tank. Besides the fact that it's a lousy fuel.
Those crystals in the carb throat were Hard Water (a.k.a. Ice) which had crept in mixed with the gasoline, then precipitated out in the carb venturi because of the usual stuff: expansion cooling AND evaporative cooling.
Thus not all cases of aviation carb ice are water-vapor-sublimating-onto-venturi; just 99% of them. The other 1% is water droplets arriving from the fuel tank and atomizing with the gasoline at the jet.
Another reason for not letting water cohabit in your fuel tank. Besides the fact that it's a lousy fuel.
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Years ago I used to run a classic racing engine on avgas which suffered horribly from ice. Problem almost eliminated by coating venturi and throttle valve with spray on teflon (PTFE). A reasonably common dodge in classic racing 20 years ago.
Edit: Just for interest - Im not suggesting you do this on your Robbie!
Edit: Just for interest - Im not suggesting you do this on your Robbie!
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An interesting point for all you robbo pilots,
when you are pulling 23" manifold power the butterfly is not fully open as you would think!!
It opens to approx 52 degrees from the fully closed position, just imagine how much of a gap that leaves for the mixture to flow thro'........not a lot!!
when you are pulling 23" manifold power the butterfly is not fully open as you would think!!
It opens to approx 52 degrees from the fully closed position, just imagine how much of a gap that leaves for the mixture to flow thro'........not a lot!!
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The ice in the carb is mostly from the air that is pulled in with the fuel. When the fuel sprays into the air, the evaporation is responsible for a big temperature drop, as is the pressure drop when the air is pulled through a nearly closed throttle. The combination supercools the airborne moisture, which then sticks to the walls of the carb. The frozen mixture is ice and gasoline mixed, so it will burn (sort of).
The danger is from carb throat restriction (thanks Whirly!) and also that the newly formed carb throat is ragged and makes for turbulent flow of the air, thus bad for mixing and distributing the air-gas mixture. The rough running and rpm drop are a product of the lost efficiency.
Of course, if the engine is tightly governed, it will not drop in rpm as much, but the slight roughness might be noticable, anyway.
The worst times for carb ice are in low power descent into moist air, where the cool carb and throat are just waiting for that moist air to deposit itself on the walls of the carb.
The danger is from carb throat restriction (thanks Whirly!) and also that the newly formed carb throat is ragged and makes for turbulent flow of the air, thus bad for mixing and distributing the air-gas mixture. The rough running and rpm drop are a product of the lost efficiency.
Of course, if the engine is tightly governed, it will not drop in rpm as much, but the slight roughness might be noticable, anyway.
The worst times for carb ice are in low power descent into moist air, where the cool carb and throat are just waiting for that moist air to deposit itself on the walls of the carb.
Last edited by NickLappos; 2nd Mar 2005 at 08:36.
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The question remains: does the governor mask carb ice?
cl12pv2s said: "R22 pilots dont have an accurate way of guaging how open the throttle is"
I take that to mean the pilot may have no way to notice carb ice until it is a big problem.
Going for my first enstrom flight tomorrow. No carb to worry about. Yahoo!
slowrotor
cl12pv2s said: "R22 pilots dont have an accurate way of guaging how open the throttle is"
I take that to mean the pilot may have no way to notice carb ice until it is a big problem.
Going for my first enstrom flight tomorrow. No carb to worry about. Yahoo!
slowrotor
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slowrotor,
The R22 has a carb temperature gauge. You apply as much carb heat as is required to keep it out of the yellow band, or many people say above ten degrees, which is a bit higher. At below 18" MAP, you pull full carb heat, for safety, to allow for the fact that the carb heat probe is not exactly at the position of the throttle butterfly. This system seems to work.
Once when I forgot to apply enough carb heat, I thought I noticed rough engine running in the cruise. I pulled carb heat and all was well. So you still have ways of noticing, if you're alert.
OTOH, flying an aircraft without a carb sounds like a wonderful idea!
The R22 has a carb temperature gauge. You apply as much carb heat as is required to keep it out of the yellow band, or many people say above ten degrees, which is a bit higher. At below 18" MAP, you pull full carb heat, for safety, to allow for the fact that the carb heat probe is not exactly at the position of the throttle butterfly. This system seems to work.
Once when I forgot to apply enough carb heat, I thought I noticed rough engine running in the cruise. I pulled carb heat and all was well. So you still have ways of noticing, if you're alert.
OTOH, flying an aircraft without a carb sounds like a wonderful idea!
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I think we should ask ourselves why the hell we are still using engines that have carburettors, magnetos and use lots of oil.
I dont know of too many cars even in the lower price bracket that use this old technology.
And if a newish car used as much oil as a aircraft piston engine you wouldn't be very happy.
But as pilots we are taught that this is normal, (even up to a quart of oil per hour) what the?
So why the hell is it taking so long to get a new generation of piston engines?
Maybe a new thread should be started on this subject.
I dont know of too many cars even in the lower price bracket that use this old technology.
And if a newish car used as much oil as a aircraft piston engine you wouldn't be very happy.
But as pilots we are taught that this is normal, (even up to a quart of oil per hour) what the?
So why the hell is it taking so long to get a new generation of piston engines?
Maybe a new thread should be started on this subject.
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EC2
This issue was recently done to death by me and a few others in this recent thread
http://www.pprune.org/forums/showthr...hreadid=162835 .
Incidently The Enstrom 28A was injected in the early 1970s. Things seem to have gone backwards.
Why have we still got carbs? Because they are cheap - except in terms of crashes and deaths caused by vaporisation carburettor ice, or the inability of pilots to detect and avoid it.
I have come across several theories on what happens as ice forms and when, and what the governor does.
One is that the R22 was originally designed without the governor, so as designed the pilot would have realised when the throttle hit the stop. This changed with the later addition of the governor. The theory is that the governor does mask ice as the pilot has no idea where the throttle is most of the time. Effectively the manifold becomes blocked with ice. I think this theory is unlikely to be correct as we all know that ice tends to occur when the throttle is closed, not when it is wide open.
The next theory is that ice blocks the fuel flow causing the mixture to be too lean to burn. As the jets block and the mixture leans, the power will first increase and then drop to the point when the engine cuts. The governor will mask this power change, The pilot would see big manifold pressure changes as the engine starts to go lean. the engine would stop after the MP reaches 30 inches. Remember, the jet is blocked, not the manifold. This is not what pilots report happens, so is possible but unlikely.
In my opinion, the most likely explanation of what happens, is that ice forms in the descent with the throttle closed. This enrichens the mixture by enhancing the venturi effect to the point when it is too rich to burn. As the mixture gets richer, more fuel is vapourised so a positive feedback cycle is established. As the R22 already burns rich, around 10% fuel/air mix, and 12.5% is too rich to burn, there is not much lattitude to enrich the mixture before the engine cuts. The manifold pressure will not change markedly before the engine cuts. The throttle position is irrelevant if the mixture is too rich to burn, so it will quit without the throttle being wide open. The governor will hardly make any difference, or mask the effects until its too late as the mixture/power graph is very steep at these mixtures, i.e. once it starts, it happens bloody quick. The pilot will get little or no warning. It doesn't take much ice to increase the venturi effect. This seems to fit with pilots reports of what happens. This also agrees with what Nick sez above. The only thing I add is the explanation of how ice upsets the mixture, leading to rapid, terminal loss of efficiency!
My (considerable!) experience of icing problems is not with aero engines but with historic automotive engines so I stand ready to be corrected.
In my opinion, if you get a warning from the governor moving the throttle in an R22, it's probably too late and the engine is about to quit. Watch the gauge and pull the knob.
Whatever the reason, you are better off with fuel injection.
Loads of information on mixtures etc at:
http://www.avweb.com/news/columns/182084-1.html
This issue was recently done to death by me and a few others in this recent thread
http://www.pprune.org/forums/showthr...hreadid=162835 .
Incidently The Enstrom 28A was injected in the early 1970s. Things seem to have gone backwards.
Why have we still got carbs? Because they are cheap - except in terms of crashes and deaths caused by vaporisation carburettor ice, or the inability of pilots to detect and avoid it.
I have come across several theories on what happens as ice forms and when, and what the governor does.
One is that the R22 was originally designed without the governor, so as designed the pilot would have realised when the throttle hit the stop. This changed with the later addition of the governor. The theory is that the governor does mask ice as the pilot has no idea where the throttle is most of the time. Effectively the manifold becomes blocked with ice. I think this theory is unlikely to be correct as we all know that ice tends to occur when the throttle is closed, not when it is wide open.
The next theory is that ice blocks the fuel flow causing the mixture to be too lean to burn. As the jets block and the mixture leans, the power will first increase and then drop to the point when the engine cuts. The governor will mask this power change, The pilot would see big manifold pressure changes as the engine starts to go lean. the engine would stop after the MP reaches 30 inches. Remember, the jet is blocked, not the manifold. This is not what pilots report happens, so is possible but unlikely.
In my opinion, the most likely explanation of what happens, is that ice forms in the descent with the throttle closed. This enrichens the mixture by enhancing the venturi effect to the point when it is too rich to burn. As the mixture gets richer, more fuel is vapourised so a positive feedback cycle is established. As the R22 already burns rich, around 10% fuel/air mix, and 12.5% is too rich to burn, there is not much lattitude to enrich the mixture before the engine cuts. The manifold pressure will not change markedly before the engine cuts. The throttle position is irrelevant if the mixture is too rich to burn, so it will quit without the throttle being wide open. The governor will hardly make any difference, or mask the effects until its too late as the mixture/power graph is very steep at these mixtures, i.e. once it starts, it happens bloody quick. The pilot will get little or no warning. It doesn't take much ice to increase the venturi effect. This seems to fit with pilots reports of what happens. This also agrees with what Nick sez above. The only thing I add is the explanation of how ice upsets the mixture, leading to rapid, terminal loss of efficiency!
My (considerable!) experience of icing problems is not with aero engines but with historic automotive engines so I stand ready to be corrected.
In my opinion, if you get a warning from the governor moving the throttle in an R22, it's probably too late and the engine is about to quit. Watch the gauge and pull the knob.
Whatever the reason, you are better off with fuel injection.
Loads of information on mixtures etc at:
http://www.avweb.com/news/columns/182084-1.html
Last edited by Gaseous; 3rd Mar 2005 at 00:08.
