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Old 07-21-2010, 11:45 AM   #1
Phil Noel
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Fuel & bearings rusting:

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As this topic was taking the "Did you fly today" thread off topic, I thought I would start a specific thread for it.

Years ago, I did a number of experiments on this issue.

First the static experiment that was brought on by an article in MAN way back when dinosaurs still roamed around Drumheller.

A jar with 0% FAI fuel. another with 15% and a third with 30%. In each was a nail. Then I watched daily to see which accumulated the most rust and in what span of time. To cut to the chase - a very small amount of rust developed in a couple of days on the 30% and a couple of weeks later, it was all rusty. As for the FAI fuel, it took many months to see any rusting of consequence and the 15% was somewhere in between. My conclusion, as long as the metal is immersed in the methanol based fuel - little rusting occurred. Now add nitro, and the amount of rusting increased proportionally.

When I started to use the uniflow tank system with no muffler pressure on my helis, I noticed that the bearings in these HELICOPTER motors seemed to need bearings a lot less frequently then those that I used with muffler pressure. So I did a similar experiment using a tank sealed with a bit of 30% on the bottom with a nail, and another with the same fuel but with some residue from the exhaust trapped inside (as would happen at the end run of the day inside the crankcase). Lo and behold, the nail in the tank with the exhaust residue rusted faster then the other. My conclusion is that the nitric compounds in the exhaust residue also increase the rusting factor.

As a sidenote, I also found that the fuel line in the tanks of the helis that used muffler pressure also deteriorated relatively quickly while the fuel line in the others seemed to last for many years.

So IMHO, the best way to insure long bearing life, specially when using muffler pressured systems, is to scavenge the engine at the end of the flying day by running it at an idle, using a 1/2 oz. or so of FAI fuel.
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Last edited by Phil Noel; 07-21-2010 at 11:49 AM.
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Old 07-21-2010, 01:14 PM   #2
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Re: Fuel & bearings rusting:

The only thing I can add is that unlike the nail in your experiments, the crank cases and related steel parts (crank, bearings) are not completely submerged in liquid.

Anyone having dismantled an engine that was fresh of off flying plane (not being run dry) will notice a table spoon amount of fuel left in the engine. This sits in the bottom of the case, and now we have air around the steel parts as well, which contains water vapour. And its not 100% oil - it is fuel and oil, and residue (from the exhaust, etc)

I have taken apart engines that were sitting in planes for a while, and the bottom of the rear bearing and crank (that were submerged in the fuel left in the case) were rusty as hell, but the stuff up top was mainly discoloured and had just surface oxidation.

It is proven like you stated the acidic nature of the chemicals derived from nitromethane (when it burns) will certainly accelerate any corrosion.

All this is why I do not leave engines wet after flying.

One other thing - which makes HUGE difference is where you are flying, and where you store your models afterwards. High humidity, and frequent, and drastic changes in temperatures can certainly make things worse.

When it comes down to it, I have never ever hurt an engine by running it dry of fuel. You never run it bone dry - there is always some residual oil left on the metal parts. Its just the burnable methyl alcohol you want to get rid of.

Some engine manufacturer's suggest this is a good thing to do. Some fuel manufacturers suggest otherwise (not just Morgan either...). I am going with the engine manufacturer, and my own experiences, over the fuel makers.

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Old 07-21-2010, 03:12 PM   #3
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Re: Fuel & bearings rusting:

For me, its the same reasons as Andrew Mentions. I have had too many rust issues in the past, and they are so easy to deal with if done right.

I never leave fuel in my engine either, and I never shut down the last flight of the day using the throttle.
I always pinch a fuel line (plane) or let it run out while hovering (heli) Some planes I will even fly until they go deadstick.

Even then, there is still some residual fuel in the engine.

Before I go home, and after I made sure the tank is completely pumped out, I always hook up a glowstick and hit it several times with a starter until it no longer burps or cough's. That way I KNOW FOR SURE there is no longer any nitro in the motor. The only thing left in the crankcase is a smidgeon of oil, which will now protect the engine. Some of you may not agree, but this is my preference and I have never had any rusty bearings since I have been doing it this way.
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Old 07-21-2010, 03:34 PM   #4
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Re: Fuel & bearings rusting:

Originally Posted by AJCoholic View Post
The only thing I can add is that unlike the nail in your experiments, the crank cases and related steel parts (crank, bearings) are not completely submerged in liquid....AJC
I guess I should have made it more clear in my second test with the exhaust fumes. The nails where left in tanks that only had a bit of fuel left at the bottom, with the exhaust residue exposed to most of it.
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Old 07-21-2010, 05:11 PM   #5
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Re: Fuel & bearings rusting:

I would like to ask if it makes a difference what type of oil you are using, Castor oil vs Synthetic base. I tend to prefer Castor oil myself, a bit more messy thought. Just wondering if it does.

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Old 07-21-2010, 07:12 PM   #6
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Re: Fuel & bearings rusting:

Perhaps we should ask fuel manufacturers to add more rust inhibitors; thus eliminating the need to run our engines dry after a day of flying. Using YS engines would eliminate the use of exhaust pressure.
Thanks for the information, Phil. Currently I am looking at how the 23 liters of wine is clearing <G>, not at jars of fuel and nails.
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Old 07-22-2010, 12:18 AM   #7
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Re: Fuel & bearings rusting:

Of course a heli engine, unless you store your heli vertical on a wall, is usually left with the crankshaft in the vertical position- nose up- some helis (ie Vario) would have the engine upside down. So there is probably not much liquid fuel touching the bearings, only residue or fumes.
I don't care what the manufacturers of fuel say, I prefer to make sure any liquid fuel is purged before putting the heli away. We live in a higher humidity climate here, and my dehumidifier runs non-stop in the summer removing water from the air. Don't want that affecting the bearings.
Never hear of rusty bearing issues with gassers... no nitro or minimal alcohol content to worry about.
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Old 07-22-2010, 09:01 AM   #8
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Re: Fuel & bearings rusting:

to shed a little light on the reason for nitro increasing the rust factor .nitro can burn without the presence of oxygen .it has 2 oxygen molecules.this is why in the absence of oxygen (blocking off the ports on the motor )if any fuel remains in the motor rust will start.combine that with the methanols ability to absorb moisture .

I think the tried & true method of running it dry for day to day use and storage oil for long term storage is the best way

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