Engine humidity

[larryb]

I've posted some of these graphs before, but nothing recently. This one is interesting.

Returning from my last trip I forgot to plug in my engine heater and humidity measuring system. The system is designed to keep the engine at an even 90 degrees F and measure engine and ambient temperature and humidity.  After a couple weeks of sitting I check it, find it offline, and return to the airport to plug it in. So the engine was sitting with it's cowl blanket on, the oil filler open, and around 55 degrees average ambient temperature. The humidity measured inside the engine is the yellow line on the graph. 

What the graph shows is that when I started up the system the humidity inside the engine was 95%. After warming it dropped to 85%. But after 36 hours of heat, humidity gradually declined to 35%.  Ambient humidity is high because it's been raining here the last couple days.

I was shocked to find that after 2 weeks of no heat, with the oil cap open, the humidity was so high. But constant application of mild heat does drive the humidity down to reasonable levels.

Larry

[jetdriven]

The Rh goes down but that automatically goes down as warmer air holds more water.  But what’s the total grains of water per cubic foot? Also corrosion increases as temperature increases, if the water content is the same. 

[larryb]

I think the key finding here can be seen in the RH between 12 AM Tuesday and 6 AM Wednesday. It drops from 80% to 35% with the temperature being a constant 90 degrees F the whole time. So I'd say there is 56% less water molecules per cubic foot at the end of the test period.

[jetdriven]

I guess the next question is determine the corrosability of 35% air at 90° versus 60%or even 80% air at 30°

[A64Pilot]

1 hour ago, jetdriven said:

I guess the next question is determine the corrosability of 35% air at 90° versus 60%or even 80% air at 30°

I think you can find that, I found a graph of corrosion of 2024 and 6061 vs relative humidity, but don’t think it had temp, but the graph curve did increase with RH, but there was a knee in it at 65%, meaning above that corrosion increased quickly, because of that I keep RH in my hangar below 65%, and you can tell the lack of corrosion on your tools etc that if kept lower it does slow almost stop corrosion even on the steel drill press plate etc.

From just raising temp your not removing moisture just raising air’s ability to hold moisture, so it that as effective as removing moisture?

I don’t know honestly, I know if you let the temp drop to the condensation point, that bad, real bad, because then you get liquid water.

Temp greatly influences RH of course, that’s why the dew sets at night, there isn’t more moisture, but the temp drops until the dew point is reached.

I’ve been inside of quite a few engines and honestly haven’t seen any corrosion at all on engines flown regularly, I define regularly as once a week or so, even more than once a month seems to be enough.

I think we place more importance on this engine humidity than we should, if it were a primary driver on engine life, then no Fl airplane engine would last past 5 yrs and ones in the desert would last forever and never have any cam failures, but it seems desert kept airplanes lose just as many cams as Fl airplanes and don’t seem to last any longer, airframes of course are a different story, if kept in the shade desert airplane just don’t seem to have any corrosion where a Fl airplane unless special attention is paid surely will. 

But for example an older Gentleman passed away here not long ago, I think he hadn’t flown his C-140 for two years or so, and while I’ve not looked at it I’m told the cylinders are heavily rusted, engine is “gone”

Different airplane, a twin Comanche sat for I think ten years until the Widow finally sold it, A&P’s bought it, pulled a couple of cylinders on it to inspect he cams, declared them good, put cylinders back on and after a day or so of run ups etc got in and flew away.

Same neighborhood, so what was the difference? Not sure.

 

[A64Pilot]

Here is one article, I haven’t read it yet

https://mti.memberclicks.net/assets/documents/TAC-Bulletins/Non-Member-Egnlish/Bulletin 13 - non-member English.pdf

Just did I think the first paragraph covers us

[GeeBee]

 

4 hours ago, larryb said:

I've posted some of these graphs before, but nothing recently. This one is interesting.

Returning from my last trip I forgot to plug in my engine heater and humidity measuring system. The system is designed to keep the engine at an even 90 degrees F and measure engine and ambient temperature and humidity.  After a couple weeks of sitting I check it, find it offline, and return to the airport to plug it in. So the engine was sitting with it's cowl blanket on, the oil filler open, and around 55 degrees average ambient temperature. The humidity measured inside the engine is the yellow line on the graph. 

What the graph shows is that when I started up the system the humidity inside the engine was 95%. After warming it dropped to 85%. But after 36 hours of heat, humidity gradually declined to 35%.  Ambient humidity is high because it's been raining here the last couple days.

I was shocked to find that after 2 weeks of no heat, with the oil cap open, the humidity was so high. But constant application of mild heat does drive the humidity down to reasonable levels.

Larry

Nice presentation. Some years back Aviation Consumer did a similar study and found that keeping heat on the engine has the effect of lowering RH levels significantly. The best way they found was continuous air blown heat. It was enough for me to buy an AeroTherm Deuce and I keep it on full time during the colder months. In the summer I switch to a desiccant powered system. Others use different methods but the important thing is to use something. Not only is ambient air with high RH a problem, but there is a lot of moisture developed in combustion that remains in the engine after running. Old timers will "vent it off" by taking off the oil cap, but that open the engine up to bugs and other contaminants. 

[Ragsf15e]

I thought there’s no corrosion below freezing?

https://www.researchgate.net/profile/Yu-Panchenko/publication/227242816_Atmospheric_corrosion_of_metals_in_regions_of_cold_and_extremely_cold_climate_a_review/links/543d07d70cf2c432f74236b0/Atmospheric-corrosion-of-metals-in-regions-of-cold-and-extremely-cold-climate-a-review.pdf?origin=publication_detail

ok, maybe not zero.  But much reduced if it’s really cold.  

[A64Pilot]

22 minutes ago, Ragsf15e said:

I thought there’s no corrosion below freezing?

https://www.researchgate.net/profile/Yu-Panchenko/publication/227242816_Atmospheric_corrosion_of_metals_in_regions_of_cold_and_extremely_cold_climate_a_review/links/543d07d70cf2c432f74236b0/Atmospheric-corrosion-of-metals-in-regions-of-cold-and-extremely-cold-climate-a-review.pdf?origin=publication_detail

ok, maybe not zero.  But much reduced if it’s really cold.  

I think just by raising the temp but not actually dehumidifying the air that the increase corrosion rate from higher temps and lower RH probably balance each other out. I can’t prove that though.

The converse of that is if the engine is cold enough for condensation to form then that of course is damaging.

‘Many mariners in colder climates will leave a 60W bulb in the engine compt because the cold engine will condense water from warm air if they don’t. 

[larryb]

To prevent corrosion, what you really want to do is keep the surface of the metal dry. If the water molecules are held in suspension in the air, they are not on the metal and therefore not available to cause corrosion. If the air is warm, and the metal is similarly warm, and the RH is low, then that water vapor will not condense on the surface of the metal. 

[Pinecone]

My plane just has a oil sump heater.

So, before our recent trip to FL, I had the heater one for about 36 hours (he had to slip the departure a day due to weather), cowl plugs in, no blanket (insulated cowl cover ordered).   Outside temps were in the 10 - 15F range, uninsulated, but pretty tight, T-hangar.

Temp was about 69F and RH was 15.6%.   Measured siting next to the oil dipstick at the top of the cowl.

[A64Pilot]

17 hours ago, Pinecone said:

My plane just has a oil sump heater.

So, before our recent trip to FL, I had the heater one for about 36 hours (he had to slip the departure a day due to weather), cowl plugs in, no blanket (insulated cowl cover ordered).   Outside temps were in the 10 - 15F range, uninsulated, but pretty tight, T-hangar.

Temp was about 69F and RH was 15.6%.   Measured siting next to the oil dipstick at the top of the cowl.

 

Yes take very cold air, seal it in an air tight bag, raise its temp 50 F and the RH is a lot less. But the amount of water in suspension is the same. Reason why in cold Wx my nose dries out so bad and my skin too.

Warmer accelerates corrosion, worst is warm and humid, spelled Florida. Cold slows it, so was your motor better off at 10F or 69F? I don’t know interesting question.

If that’s all it takes, all we need to do is slap pan warmers on and plug them in with the float charger and leave them on. I suspect there is more to it than that or it would have been common practice for 50 years, and people who sell pre-heaters would be heavily advertising, buy our heater, stop engine corrosion, I know I would.

A 60W light bulb, a old fashion drop light sitting in the cowl flaps provides a not insignificant amount of heat.

[DXB]

On 12/28/2022 at 12:28 PM, jetdriven said:

The Rh goes down but that automatically goes down as warmer air holds more water.  But what’s the total grains of water per cubic foot? Also corrosion increases as temperature increases, if the water content is the same. 

Warm air holds more water than cold air, but it's relative humidity, not absolute water content that determines corrosion rates. Atmospheric corrosion occurs when the amount of moisture in the air reaches critical humidity, the point at which water no longer evaporates or gets absorbed from the atmosphere. That happens around 80% relative humidity.  Of course increased temperature also accelerates corrosion, but under the 36% relative humidity observed by the OP, the dry air is the dominant effect in providing corrosion protection, far outweighing any corrosive effect of increased temp from the heater. Overall, uniformly heating the oil pan and cylinders together as an open to air system when the plane is sitting over the winter is an excellent safeguard against corrosion.  The only real downside is the corrosion-protective oil film may come off metal parts faster at higher temps, but I suspect the reduced relative humidity still offers a large net benefit under these conditions. 

[A64Pilot]

The oil film and higher storage temps are a red herring, reason is we shut down hot, close to 200 F or so and it stays hot for quite awhile, so any oil that’s  going to drip off does so while it’s hot and thin, if it’s still there after cool down, it’s staying there, I don’t think anyone is preheating to shut down temps.

People often say straight weight 50 is better cause it’s thicker when cold so it clings to engine parts better and longer, but it’s not thicker at shutdown, so it doesn’t really cling better.

So all that we need to do to prevent engine corrosion is to keep it warm?

Has to be more to it than that

 

[DXB]

1 hour ago, A64Pilot said:

The oil film and higher storage temps are a red herring, reason is we shut down hot, close to 200 F or so and it stays hot for quite awhile, so any oil that’s  going to drip off does so while it’s hot and thin, if it’s still there after cool down, it’s staying there, I don’t think anyone is preheating to shut down temps.

People often say straight weight 50 is better cause it’s thicker when cold so it clings to engine parts better and longer, but it’s not thicker at shutdown, so it doesn’t really cling better.

So all that we need to do to prevent engine corrosion is to keep it warm?

Has to be more to it than that

 

There's a lot of oil that settles into the sump the time between the engine reaching ambient temp after running and after sitting for several days.  I can see 1/4 a quart extra increase between the day after running and a week after. The increase is more pronounced when I leave it on the heater in the winter.  There must be some initially coated metal parts getting exposed in that time interval.