Continuous pre-heat

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SSR

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SSR
No doubt a topic with various opinions. Tanis says their pre-heat, which heats the oil pan and block separately, can be run continuously in ambient temperatures below 100F. When I started flying 30 years ago, it was commonly said that continuous pre-heat could "cause" corrosion. We were to go to the airport to turn on pre=heat the night before flight, no sooner.

The logic never made sense. To have corrosion, you need moisture, which the engine makes on it's own. I would think a warm engine might evaporate that moisture, which is why I leave the dip stick out on the wing, when the Tanis are plugged in. Hoping the moist air has a place to escape.

Condensation simply can not happen, from ambient air, unless the surface is below the dew point. If the engine is warmer than ambient air, condensation is impossible.

What am I missing? I don't understand the 100F upper limit, which might be revealing. I suppose there could be confined passages with combustion moisture that don't evaporate, but how would heat do harm. Still Tanis seems comfortable taking on the liability of permitting continuous use.
 
SSR said:
I would think a warm engine might evaporate that moisture, which is why I leave the dip stick out on the wing, when the Tanis are plugged in. Hoping the moist air has a place to escape.
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Agreed - however your engine has a crank case breather tube, so no real need to pull the dipstick out. That can create a path for contaminants to enter your oil and be another issue altogether.

SSR said:
Condensation simply can not happen, from ambient air, unless the surface is below the dew point. If the engine is warmer than ambient air, condensation is impossible.
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Yes however your prop acts as a big ol' heat sink, so you may evaporate the moisture from your oil, but it may condense on your crankshaft.

SSR said:
What am I missing? I don't understand the 100F upper limit, which might be revealing. I suppose there could be confined passages with combustion moisture that don't evaporate, but how would heat do harm. Still Tanis seems comfortable taking on the liability of permitting continuous use.
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100*F limit is likely from the possibility of burning or degrading your oil if you plug your engine in above that temp.

SSR said:
No doubt a topic with various opinions. Tanis says their pre-heat, which heats the oil pan and block separately, can be run continuously in ambient temperatures below 100F.
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I leave my Reiff enging heater plugged in almost all of the time (that it's below 40*F), I'm just careful to keep my insulated prop cover on. Otherwise your logic is sound.
 
What does Continental say?
Do not leave an engine-mounted pre-heater system on for more than twenty- four hours prior to flight. Continuous operation of engine-mounted preheater systems may result in aggressive corrosive attack internal to the engine.
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Heat accelerates corrosion. It isn’t always about liquid water. Heat increases the amount of water vapor air can contain. And then there’s the acid component. In a perfect world my engines would be in below freezing temps when not in use.
 
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Stewartb said:
Heat accelerates corrosion.
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500*F-1000*F of heat accelerates corrosion in steel. +/- 70*F has a negligible effect. It feels like a lot to humans, but it's NBD to steel.
Stewartb said:
Heat increases the amount of water vapor air can contain.
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What matters is the amount of water in contact with the steel, doesn't really matter if it's in the air and not condensing. Which it can't condense on the steel of the engine if it's all warm and above the dewpoint. (caveat see above about the prop being a heat sink)
Stewartb said:
In a perfect world my engines would be in below freezing temps when not in use.
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In a perfect world engines wouldn't endure temperature fluctuations - that's what ends up depositing water into/onto the engine. Ever walk into an unheated hangar after the temperature just heated up, but the airplane is cold? Everything is soaked with condensation. Maintaining a steady temperature 24/7 is what would promote longevity i.e a temperature controlled hangar. In lieu of that, plugging it in is a good option.
 
I have been putting a remote thermometer with humidity in the cowl.

Without heat, I am see these days around 30F with 88% RH. With the heat on (no insulated cowl cover), it is 75F 15% RH.

Hmm. If the temp drops to 20F, there will be condensation. But it will also be cold. But the heated temp can sustain a large temp drop (unlikely with heat on) without condensation.

A&P/IA at my field is a proponent of full time heat.
 
SSR said:
When I started flying 30 years ago, it was commonly said that continuous pre-heat could "cause" corrosion.
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As mentioned above, this had more to do with that TCM bulletin that limited the use of engine mounted heaters being applied to all engines than just TCM. However, that same bulletin states to follow the heat system OEM instructions as well. Plus most of the older heaters cycled on/off at certain temps so the engine would go through dew point cycles on a regular basis. Tanis heaters do not cycle and remain on when plugged in so no dew-point cycles. We used Tanis on helicopters which may have 6+ heating elements installed on various components continuously with no issues. Plus Tanis allows continuous operation.

FYI: to "cause" corrosion you need an anode, cathode, and electrolyte. Without any of these 3 corrosion will not form. The relationship of ambient temp to the process is more related to the relative humidity which in-turn has the potential to become the electrolyte in the process. So where the relative humidity is low, ambient temp plays a lesser role regardless of being hot or cold. This is why they store aircraft at Davis-Monthan vs Florida. And the same reason there are lower corrosion rates in frigid climates, low relative humidity.
SSR said:
I don't understand the 100F upper limit,
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I believe the reason is related to the fact Tanis heaters do not cycle on/off and an ambient temp above 100 causes issues with the system. Perhaps a quick call to Tanis tech support will get you an answer?
 
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I have a little heater doodle thing (from Aircraftheaters.com, how embarrassing) that blows warm (not hot!) air on the engine. I put in the cowl plugs and a couple blankets over the nose and the engine in my aircraft stays at about 75 degrees F all winter. Always ready to fly. I basically have a little climate controlled hangar for the engine.
 
Heat degrades most seals and gaskets. Heat evaporates the lighter elements in the oil, thickening it. That's why the 5606 some smear on the nosewheel strut gets so sticky and attracts dust. It's why the oil that leaks out of an engine turns to varnish on the case.

Leaving the dipstick out isn't likely to help much. Some dipstick tubes are actually immersed in the oil when the oil is at max, and no air movement will happen if the stick is out.

Best thing is to fly the thing to to get the oil temp up and keep it there for an hour to drive out moisture. If the cold weather keeps the oil temps down, do something about it.
 
In 20+ years of outdoor parking in Alaska I took comfort in knowing my engines were cold. Now in a hangar, I’ve made engine dehydrators to manage the humidity inside my engines in the 50° hangar. I’m maintaining <10% humidity. Rust potential is near zero. I can’t think of a better solution, and I’ve tried several.
 
My theory (not necessarily sound) for removing the dipstick, has been to allow warmer air to rise and carry moisture away with it.
 
Leave the engine heater on 24/7. The engine temp will be above the dew point, so no condensation.
 
Rust doesn’t require condensation. Rust is a chemical reaction. All chemical reactions happen faster at higher temperatures. Granted, it’s absolute temperature that matters. Still, cooler is better. The best thing would be to freeze-dry the engine after landing. I’m not sure how to make that happen.

There’s a restored P-38 called Glacier Girl. There isn’t a Jungle Girl or even a Desert Girl.
 
Capt. Geoffrey Thorpe said:
The rate is generally exponential with absolute temperature https://en.wikipedia.org/wiki/Arrhenius_equation#Equation
The rule of thumb is the corrosion rate rate doubles about every 10C / 20F.
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This assumes all other variables remain unchanged. As we all know, relative humidity decreases as temperature rises if the absolute humidity remains the same. The assumption that corrosion in/on our engines will increase with increasing temperatures is erroneous without looking at the entirety of all factors involved in the specific areas of corrosion we are concerned with.
 
SSR said:
My theory (not necessarily sound) for removing the dipstick, has been to allow warmer air to rise and carry moisture away with it.
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That's a pretty sound theory in my experience. The place I rent SR-22s from asks that you open the dipstick for a little while after shutdown. There is an impressive amount of steam that leaves the engine when I remember to do that. You can tell when someone forgets to do it because there are water droplets and milky oil underneath the dipstick cap. Don't forget a large component of exhaust (and blowby) is water.
 
robin ardoin said:
This assumes all other variables remain unchanged. As we all know, relative humidity decreases as temperature rises if the absolute humidity remains the same. The assumption that corrosion in/on our engines will increase with increasing temperatures is erroneous without looking at the entirety of all factors involved in the specific areas of corrosion we are concerned with.
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Exactly.

I would like to find references on those effects.

As I posted earlier, measuring under cowl temps, I see around 20 - 30F and 88% RH versus 70F and 16%RH.
 
PaulS said:
Don't forget a large component of exhaust (and blowby) is water.
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I've been preaching that for years, but people keep blaming the atmospheric moisture. Burning a gallon of gasoline can produce up to a gallon and a half of water, and some of that squirts past the clearances in a cold engine. Even in a hot engine some gets past, but the heat keeps it as vapor and it is pushed out the crankcase breather by gases following it. That keeps the engine relatively dry inside. You start that engine and run it for five or ten minutes, then shut it down and put the airplane away, you have pumped a bunch of water into that crankcase and the engine suffers terminal corrosion.

Cars get away with short runs because they have tight clearances and PCV systems. You follow a car on a cold day and see the water dripping out of the tailpipe and the condensing vapor behind that pipe, and think about that gallon of gasoline producing all that water. Hydrogen from the fuel, combined with oxygen from the air. H2O.
 
Preheat and continuous heat are such a hot (no pun) topic these days. I see it argued a lot on the flying facebook groups. My concern with Tanis' claim of constant heat is that nobody other than Tanis claims that. How they explain it makes sense, in theory, to us non-scientists, but I wish it would be properly tested or claimed by other manufacturers.

In any case, I don't have a Tanis so I live by the mantra of "preheat before you fly." Any preheat I use will certainly cause condensation, so it's 12-24 hours of heat then a long flight.

I also like popping the oil cap for 15min after a flight. It's a lovely steam display for all 15mins. My favorite mechanic told me that trick, and it really seems to help water escape.
 
50kttailwind said:
My concern with Tanis' claim of constant heat is that nobody other than Tanis claims that.
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FYI: other vendors allow for continuous heat as well. For example, Rieff doesn't prohibit continuous ops either similar to Tanis. It really depends on the heater install and operation parameters. The main issue is if the heater allows dew point cycles within the engine which causes condensation. No internal moisture, no issues.
 
Heat does not cause condensation. Cool causes condensation.

The point that some people make is that chemical reactions (corrosion) occur faster at higher temperatures. But the question is, does the lower RH offset the reaction rate increase.

From my short look, the amount of moisture in the air (grams per volume of air) under my cowl with (75F) or without (30F) heat is the same. BUT, unheated, it is over 80% RH and heated it is less than 16%.

The dew point is the same at about 24F. So unheated at 30F, if the temp drops to 24, there will be condensation. And this is likely during the winter. Heated, it is very unlikely that the under cowl temp will drop some 50 degrees, so very little chance of condensation.

I am sure there are some scientific papers on this, I just cannot dig them up.
 
I don't worry about it and do nothing, no dip stick rituals, no nothing....I preheat the night before if the temps are below 40 degrees F. Here's an engine that was last overhauled in 1997. Pictures are from the top overhaul. So....tell me about your expensive dehydrator? :cool:


....this engine flys once a month whether it needs it or not. ;)
 

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Pinecone said:
The dew point is the same at about 24F. So unheated at 30F, if the temp drops to 24, there will be condensation. And this is likely during the winter. Heated, it is very unlikely that the under cowl temp will drop some 50 degrees, so very little chance of condensation.
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That assumes that enough water has escaped through the breather to get close to the ambient humidity.

When you shut down your crankcase is full of blowby which is about 10-15% water with a dew point on the order of 100Cish. Finding data on crankcase contents is pretty tough since most papers tend to be automotive related and auto engines all have PCV which changes the whole game.
 
Checkout_my_Six said:
I don't worry about it and do nothing, no dip stick rituals, no nothing....I preheat the night before if the temps are below 40 degrees F. Here's an engine that was last overhauled in 1997. Pictures are from the top overhaul. So....tell me about your expensive dehydrator? :cool:


....this engine flys once a month whether it needs it or not. ;)
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Yup. Regular flying, no ground-running or short flights, and the engine will look like that even in Maryland.
 
Capt. Geoffrey Thorpe said:
That assumes that enough water has escaped through the breather to get close to the ambient humidity.

When you shut down your crankcase is full of blowby which is about 10-15% water with a dew point on the order of 100Cish. Finding data on crankcase contents is pretty tough since most papers tend to be automotive related and auto engines all have PCV which changes the whole game.
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I open the oil filler (Continental).

But if dew point is 100C, then you WILL have condensation in the engine.
 
Pinecone said:
But if dew point is 100C, then you WILL have condensation in the engine.
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The thing is to have that engine, all of it, hot enough that there is no liquid water in the case. When water turns to vapor it expands enormously and has to leave via the breather. Whatever is left will be minimal.
 
Let's see, if we take a box the size of an IO-360 (worst case), it is about 13.4 cubic feet or 0.38 cubic meters. Air density of 1.225 kg/cubic meter. So about 0.469 kg of air.

At 100F at 100% RH there is about 41 grams of water per kilogram.

So about 19 grams of water. Or 2/3 of a ounce.

Since we used a box shape, the reality is probably about 1/2 that, or 1/3 ounce of water.
 
Pinecone said:
Let's see, if we take a box the size of an IO-360 (worst case), it is about 13.4 cubic feet or 0.38 cubic meters. Air density of 1.225 kg/cubic meter. So about 0.469 kg of air.

At 100F at 100% RH there is about 41 grams of water per kilogram.

So about 19 grams of water. Or 2/3 of a ounce.

Since we used a box shape, the reality is probably about 1/2 that, or 1/3 ounce of water.
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We did this math a few years ago.

Your estimate of 13.4 cubic feet is laughably erroneous. Have you ever had a cylinder off an engine and looked inside? The space is tiny, with little more than room for the crank and rods to move. An IO-360 might have two cubic feet, at the most, of unoccupied space, and likely a lot less than that. Maybe only one cubic foot. Maybe less. The sump, being full of oil, offers little extra volume.

Engine makers make these things tight to keep the thing light and compact. Larger volumes require more wall area and thickness to support that area, and the space taken up in the engine compartment gets too big.

If you take that generous 2 CF and calculate the amount of expansion and contraction of air with the most extreme temperature shifts, you find that there is VERY little air movement into and out of the case, and that small volume of movement carries miniscule amounts of water even if its in the form of fog, 100% RH. And when the environment warms, the air will leave the case again but it won't leave all of the moisture behind.

The only way to find the crankcase volume would be to take an engine and fill it with water until it's right full. Pour it out and measure it. Subtract the oil's volume, and what's left is the crankcase volume.

Most crankcase moisture comes from combustion. Period.
 
Dan Thomas said:
We did this math a few years ago.

Your estimate of 13.4 cubic feet is laughably erroneous. Have you ever had a cylinder off an engine and looked inside? The space is tiny, with little more than room for the crank and rods to move. An IO-360 might have two cubic feet, at the most, of unoccupied space, and likely a lot less than that. Maybe only one cubic foot. Maybe less. The sump, being full of oil, offers little extra volume.
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I said worse case. And then cut the result in half. If that number is small, then the real number is even smaller.

And DUH, ALL the water in the crankcase come from combustion, the starting moisture is pushed out of the combustion gas leakage. :D
 
Pinecone said:
I said worse case. And then cut the result in half. If that number is small, then the real number is even smaller.

And DUH, ALL the water in the crankcase come from combustion, the starting moisture is pushed out of the combustion gas leakage. :D
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Worst case for 13 cubic feet, you mean. One has to start with the right data, and 13 cf is far from right.

Garbage in, garbage out.

A cubic foot holds about 7.5 US gallons. Look at a 5-gallon bucket and realize that is only 2/3 of a cubic foot, then look into that crankcase all crowded with crank and cam and rods and gears, and see what I mean.
 
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Actual volume not withstanding, you are going to have condensation when you shut down. And, unlike moisture from the ambient air. blowby had fun stuff like NO, NO2, and a little sulfur compounds. Not particularly iron friendly.

If you fly once or twice a week, ain't gonna make much difference because you will wear it out before it corrodes. If you fly significantly less frequently, the corrosion will likely win.
 
Capt. Geoffrey Thorpe said:
Actual volume not withstanding, you are going to have condensation when you shut down. And, unlike moisture from the ambient air. blowby had fun stuff like NO, NO2, and a little sulfur compounds. Not particularly iron friendly.
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That is true, but after a flight where the engine reached proper operating temperature for an hour or so, the clearances have closed up and the blowby has decreased enormously, and the moisture (which is water vapor at that point) is driven out by whatever blowby is occurring. Unless the engine is totally worn out (low compression, etc.) or the oil cooler isn't blocked off in cold weather, there will be little moisture left in that engine. The anticorrosion compounds in the oil can easily handle it.

The other byproducts can be corrosive, but in many cases they mix with the oil, along with water, and in the presence of catalysts like metals, they form acids: hydrochloric, sulfuric and nitric acids among them. It's one reason why we change the oil periodically. Again, it's worse in worn-out engines, or in engines that aren't run up to temperature and held there. Ground-running is lethal to engines. As a mechanic, I have seen engines wrecked by it. It's sad. And even after pointing this out to some owners, they persist in doing it.

From a Lycoming publication:

Some operators are running the engines on the ground in an attempt to prevent rust between infrequent flights. This may harm rather than help the engine if the oil temperature is not brought up to approximately 165˚ F because water and acids from combustion will accumulate in the engine oil. The one best way to get oil temperature to 165˚ F is to fly the aircraft. During the flight, the oil normally gets hot enough to vaporize the water and most acids and eliminate them from the oil. If the engine is merely ground run, the water accumulated in the oil will gradually turn to acid, which is also undesirable. Prolonged ground running in an attempt to bring oil temperature up is not recommended because of inadequate cooling that may result in hot spots in the cylinders, baked and deteriorated ignition harness and brittle oil seals which cause oil leaks.

Pulling on the engine through by hand if it has not been run for a week or more is NOT recommended, and can result in increased wear. Refer to Lycoming Service Letter L180. If the engine is flown so infrequently that it does not accumulate the operating hours recommended for an oil change (25 hours for a pressure screen system and 50 hours for a full-flow filter system), then the oil should be changed at four-month intervals to eliminate water and acids.

From https://www.lycoming.com/content/frequency-flight-and-its-affect-engine

Another good article: https://www.aviationpros.com/home/article/10387461/corrosion-how-does-it-affect-the-internal-engine
 
Dan Thomas said:
is driven out by whatever blowby is occurring.
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Except that blowby is still loaded with water. There is no source of fresh air in an aircraft engine. Volume of the engine, blowby rates, whatever do not matter. Blowby is blowby - some unburned fuel/air mixture and the rest is products of combustion. That's what fills the crankcase. The only thing that makes a difference is that if the engine is hot, you don't get condensation until after it shuts down and it does not accumulate in the oil from run to run.
 
I left the flight school airplanes plugged into the engine heaters for the entire winter and probably some of the fall and spring. I did not use insulation blankets, nor did I worry about removing dipsticks (that would be a major liability in such an environment, someone would forget about it). Sometimes the airplanes sat on the ramp too long and students forced them to start in the worse possible ways.

One of the airplanes, we bought at TBO, then we put another 2,000 hours on it. Then we sold it to another flight school that is probably still running the damn thing. Never had to replace a cylinder or do any bottom or top-end work (other than maybe spark plugs). Most of our repair costs were in tires & fixing jacked up Cessna flap tracks. You put a brand new tire on a flight school airplane and the damn thing is full of flat spots before you even finish installing it.

So anyhow, I'd look at it like this:
1.) If it is too cold for the engine to start, you should probably heat it.
2.) If the electricity is free, then, who cares. Leave the heater plugged in all the time, and make sure you fly it at least once a week.
3.) If the electricity is not free, then don't leave it plugged in all the time, and make sure you fly it at least once a week.

If you cannot fly your airplane once a week, figure out a way to get it done. Get a friend to fly it. Get a partner. Find a way to make sure the airplane is flying. If you cannot do that, be prepared to buy ExpensiveEngineThings on a regular basis.
 
The flight school I went to most times handed me a hair dryer and instructed me to put it in the cowl with cowl plugs installed. Drag a extension cord out to the plane on the ramp. Sometimes they would plug them in to engine heaters but 9 times out of 10 I used the hair dryer. Couple times I walked back into the school after plugging in the dryer and the instructor said "you ready to go" as soon as I walked in. The hair dryer may have run for 5-10 minutes max. They ran the engines to at least 3000 hrs.

Bad habits die hard as I use a hair dryer on my own plane in the hangar in conjunction with a oil pan heater. I do fly my plane multiple times a week so hopefully I won't have much corrosion to deal with.
 
Speaking of electricity and engine heaters, I have small fleet of trucks and each one has dedicated 20amp plug/circuit for each truck. They pull 1500 watts each which is like having a toaster on full time. I doubt that air plane heater pull that much? We burn up plugs on the cords after a winter or less.
 
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Gary Ward said:
Speaking of electricity and engine heaters, I have small fleet of trucks and each one has dedicated 20amp plug/circuit for each truck. They pull 1500 watts each which is like having a toaster on full time. I doubt that air plane heater pull that much? We burn up a plugs on the cords after a winter or less.
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Power draw varies by the type of heater you have. The ones I have (Reiff Hotstrips) are 400W/engine.
 
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