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  #11  
Old 02-24-2020, 12:27 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Bcone1381 View Post
Think of a cold glass of water on a warm summer day. Moisture collects on the outside of the cold glass.

Dis-similar temps will result in the engine when you plug in your engine heater. Warmer temps will be near the heat source, colder temps will exist further out. If the heat is left on all winter, then places where the temp/dew point match in the engine are susceptible to collecting moisture like your glass of water all winter long and rust.

A metal prop is a cold sink and will cause the front of the crank to be cold. A constant speed prop may get moisture collecting in its hub.

So these are problems to over come or turn it on before flight. There are solutions to turn on the pre-heater via a cell phone connection before going to the airport.

A sump heater heats the oil first and the air inside the engine second, along with other non-submersed parts. Air outside the engine is irrelevant. That leaves the air temp approximately the same as the steel part temps. The air inside the engine heats along with the engine, keeping the dew points aligned and the risk of condensation minimal. The crankcase is the exception, where outside air exposure can make it colder than the internal engine air. However, condensation on the crankcase is of no concern, from a corrosion perspective, as it is aluminum. Any condensation developed on the inside of the case will drip down to the oil pan, where it does no short term damage. I believe you get much more condensation inside the engine after hot shut down then produced from a sump heater. The 200* air inside your engine holds a LOT of moisture and as that air cools down, that moisture must precipitate out as the saturation level shrinks with the cooling temps. Same concept that creates dew on the grass in the early morning.

My experience is that the sump heater uniformly heats the engine and cowl interior assuming plugs and blanket are used, with the exception of the oil itself, being the heat source. No cold air sources to create temp differentials and condensation. I am in Chicago and do a lot of winter flying.

I would never use the cyl bands for more than a couple hours. The intense heat at the band contact area will eventually burn off any oil there, allowing corrossion in that spot. I am not surprised that some find rust bands inside the cyl aligned with the heaters.

Larry
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Last edited by lr172 : 02-24-2020 at 12:53 PM.
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  #12  
Old 02-24-2020, 02:50 PM
Tankerpilot75 Tankerpilot75 is offline
 
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These answers and rational are exactly the same I hear at my airport. Again opinions and justification seem to land on both sides of the original question. Oh well, so be it.
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  #13  
Old 02-24-2020, 05:36 PM
Jetmart Jetmart is offline
 
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I had an Aerostar that had a corrosion problem until I put a Tanis pre-heat on it. Both oil sump and cylinders. Left it plugged in continuously in the winter at Tanis's recommendation. The very important thing to do is remove the oil filler cap. This allows any moisture escape through the tube. I did this summer and winter immediately after last landing of the day with or without using the pre-heat.
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  #14  
Old 02-24-2020, 07:01 PM
BillL BillL is offline
 
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On a side bar, but not completely off topic . . . I have been doing some experiments and calculations for improving the typical crankcase dryer. It is all about dew point temperature. The best I can figure the crankcase dew point at shutdown is around 180F. If it is winter, the ambient dew point is pretty low.

OK, finally the experiment. I took a 12vdc coleman mattress inflator and hooked to my oil filler (IO360) with 10" of 1/4" ID hose and a stopper. It completely replaced the crankcase air in 3 min as confirmed by the exit air dew point being within 1-2 C of the ambient dew point. The main variable is the crummy meter I use to measure humidity. It takes 5 min for a 540.

If this was done at shutdown, there is little chance (probably an understatement) any region of the engine will be close to the dew point. 3 or 5 min is not very long and easy to do during normal post flight activities.

This can put any corrosion worries to rest for leaving a (self regulated) heater on for extended periods. Not sure about other reasons, but that one can be addressed.

I use a phone switch or turn on the night before. A hand in the cooling inlets will find the plenum cavity toasty warm. No way it will have any part below dew point.

But wait, there is more. After the purge (as I call it), a 2 l/min flow will achieve the lowest dew point ( in the crankcase) possible with silica gel in 45 min. Then the dryer can be shut off. Recirculation is not needed and 45 min of drying ambient air will collect less water than a recirculating system that does not use a purge. The desiccant will last quite a while this way before it needs regeneration.

Back to the Op's question . . .
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Last edited by BillL : 02-24-2020 at 07:07 PM.
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  #15  
Old 02-24-2020, 07:59 PM
sibriggs sibriggs is offline
 
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I had a PA23 Apache once. Had both engines zero timed when I bought it. Had it for 8 years here in NH. Installed heating pads on both engines and ran them all during the winter. By year three I ended up paying for tear downs and replacement of cams and lifters as a result of corrosion caused from the freezing hangar temps and moisture. Never again. The heating pad on my RV9 only goes on just before flying.
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  #16  
Old 02-24-2020, 08:02 PM
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Alan Carroll Alan Carroll is offline
 
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I've never understood why cylinder band heaters should cause corrosion inside a cylinder. Surely if the cylinder wall gets hot enough to boil off the oil (>500?F) they will also boil off any water?

Back when I was in a flying club we had three planes that lived in a large unheated hangar. The sump heaters were kept plugged in continuously for months at a time, yet the engines always made TBO or beyond.
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  #17  
Old 02-24-2020, 08:12 PM
Kyle Boatright Kyle Boatright is offline
 
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Quote:
Originally Posted by Alan Carroll View Post
I've never understood why cylinder band heaters should cause corrosion inside a cylinder. Surely if the cylinder wall gets hot enough to boil off the oil (>500?F) they will also boil off any water?

Back when I was in a flying club we had three planes that lived in a large unheated hangar. The sump heaters were kept plugged in continuously for months at a time, yet the engines always made TBO or beyond.
Heat is a conundrum in aircraft engines.

On the plus side, heat evaporates moisture and reduces the relative humidity inside the case.

On the downside, it heats the oil and allows the oil film on internal parts to flow off of the parts faster. Also, if moisture (or other corrosive products) are present, heat speeds the rusting process.

There's plenty of anecdotal evidence that heating the engine over the long term is bad, and there are just as many anecdotes which show heating the engine is good.
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  #18  
Old 02-24-2020, 09:27 PM
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Sam Buchanan Sam Buchanan is offline
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Quote:
Originally Posted by Kyle Boatright View Post
There's plenty of anecdotal evidence that heating the engine over the long term is bad, and there are just as many anecdotes which show heating the engine is good.
Which makes you wonder if in many cases the corrosion was unrelated to heating the engine.....
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  #19  
Old 02-25-2020, 02:06 AM
bobnoffs bobnoffs is offline
 
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a lot of posts about condensation causing rust. i think everyone has seen something rust that you know was never exposed to liquid water. i think i read somewhere that above 65% rel. hum. this becomes a possibility.
after reading a lot of threads about this subject there is a lot of anecdotal info but not much scientific fact for most of it. i have my opinions but it seems everyone has got one and we don't really know what's going on in our engines.
for ex. i have never seen a graph, statistics that show at what point the higher temps.[which cause increased rates of chemical reactions-rust] is offset by lowered rel. humidity because of rise in temp. isn't that what's it's all about?
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  #20  
Old 02-25-2020, 04:53 AM
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rzbill rzbill is offline
 
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If I had time to `get there early` to turn on a heater, I would not need a sump heater in the first place. The sun would be out and the temps usually above freezing by then.

When I get home Friday, the bird gets plugged in for Monday AM departure. If weather is bad, I drive the commute and the engine stays heated until the next Monday.

Like Larry said, the whole cowl interior is warmed. There is no temp differential to create an imagined condensation point.
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