How Much Oil Should You Use In The Fuel?

[David Davis]

When I first got into aeromodelling in the early 1960s we all used fuel containing at least 20-25% castor oil in engines with cast iron pistons in steel cylinders. With the development of synthetic oils and engines which used aluminium, brass and other metals for their internal parts, the percentage of oil in most commercially available fuels has diminished to such a degree that I use fuel containing only 17% oil in most of my two stroke and four stroke engines. For my Laser engines I use a fuel containing 15% synthetic oil.

Having retired to central France last year I joined the local model flying club, Berry Marche Modelisme. The club owns three four-channel ARTF trainers which are used to introduce beginners to radio controlled flight and a glider tug. The trainers are powered by new OS 46AX engines and the tug by an old 25 cc Super Tigre engine.

The club runs all of these engines on fuel which it mixes itself, buying both methanol and oil in bulk. The fuel contains only 11% oil and no nitro at all. I'm assured that the oil is of very high quality and the engines seem to run alright on this mixture.

My colleagues are quite surprised at the amount of oil that I have in my fuel!

I would appreciate any information on this subject from those who are better informed than I am. .

[Jon H]

I (Laser) are looking into this very issue at the moment and we are happy to recommend 10% oil to our customers. There are some proviso's with that, like not using chip fat or some other crummy oil. We are looking at ML70, klotz techniplate (not the low visc versions) etc.

As part of this testing i have been flying my 100 and 180 all year using fuel with 4% ML70 oil, 5% nitro and 1% of model technics SICAL additive. The two engines run clean and if they were going to seize they would have by now. My experiments continue so i dont recommend you try it just yet, but for sure 10-15% is plenty of oil for more or less any modern engine. Even my old plain bearing OS 15fp runs on the 15% oil Laser fuel without any drama at all, and my Nitro car runs on 12% oil at 38000rpm.

The days of 20% oil are long gone IMO and i am going to use my 4% mix in more of my engines next season.

[Engine Doctor]

The Super Tiger was supposed to be run on straigh fuel with only 10% oil called Big Brute. I use fuel in my engines with 17% oil and could easily cut that down as lubricants have improved so much in recent years. John Harper from Laser has posted that they are now using lower oil content and after extensive running engines are fine. Also with less oil the combustion will be far better and cleaner. The biggest killer of engines is storing them wet/ with residue fuel in them .After every session run the engines dry while it's still warm and put a few drops of oil in the carb and spin over. Any methanol left in the engine will absorb moisture from the air and cause bearings to rust.

Sorry John you beat me to it !

Edited By Engine Doctor on 22/11/2016 10:22:43

Edited By Engine Doctor on 22/11/2016 10:28:21

[David Davis]

Thank you gentlemen. When I have exhausted the remaining 25 gallons of fuel I bought from Southern Modelcraft on the way to the ferry, I will start using less oil in my fuel! .

[Nigel R]

Posted by David Davis on 22/11/2016 11:40:25:

Thank you gentlemen. When I have exhausted the remaining 25 gallons of fuel I bought from Southern Modelcraft on the way to the ferry, I will start using less oil in my fuel! .

You could always buy a few gallons of neat methanol to mix in... 2/3 of 15% oil + 1/3 of 0% oil...

[Merco 61]

Mention is made of 10% oil,in straight methanol,for ST 2500 series.Is 10% synthetic the equivalent of 10% castor in all respects? My Tartan 22 glow instructions also specify 10% castor.

I,ve asked this before.What is the exact synthetic oil specification and brand to safely use and who will send by post in UK?

[David Davis]

Good idea SuperNash but I would have to buy 8 gallons, (36 litres) of methanol to put this into effect. That would mean that I would end up with 33 gallons of fuel in the garage! Besides I don't have enough containers!

On the other hand if I were to use half of my existing fuel then keep all of the containers, with some (for me) complicated pouring I could end up with 10% oil in my fuel!

Edited By David Davis on 22/11/2016 12:49:32

[Ernie]

25 gallons? that should keep your ED bee going for quite a bit

ernie

[Jon H]

ED has it right with methanol causing corrosion. That is another reason not to run your engines as lean as possible as a richer mix plonks more un burnt fuel into the crankcase. Higher nitro fuels do the same. Methanol is nasty stuff.

I was also going to suggest thinning down the fuel you have but if you do make sure you get good quality methanol. Cheap stuff will have water in it.

Merco, my OS 15fp instructions demanded 20% castor and didnt even give an option to use synthetic. Using the 15% synth Laser fuel has resulted in nothing but improved performance and less (slightly, it is a two stroke still) mess. In fact the model didnt fly for about a year but i recently took it out and it fired up first time, didnt touch the needle, and off it went. For a specific oil ML70 or Klotz 200 techniplate would be my suggestions. If you are in the UK call Model Technics. I am sure they will help you if they can

 

 

Edited By Jon Harper - Laser Engines on 22/11/2016 13:38:15

[Martin Harris - Moderator]

Posted by Jon Harper - Laser Engines on 22/11/2016 13:37:01:

ED has it right with methanol causing corrosion. That is another reason not to run your engines as lean as possible as a richer mix plonks more un burnt fuel into the crankcase.

I'm not following this Jon. Surely more methanol residue equates to more probability of corrosion from the hygroscopic absorption of water vapour in the air - although I'm certainly not advocating running lean...

[David Davis]

Posted by Ernie on 22/11/2016 13:28:52:

25 gallons? that should keep your ED bee going for quite a bit

ernie

I've never owned an ED Bee Ernie!

I've just worked out that if I use half of my existing fuel, say 12 gallons-worth and keep the containers, I could buy six gallons of methanol; the stuff the club uses seems to be alright. Then I could transfer one third of the fuel from the 12 existing "full strength"containers into the 6 empty containers. I would then have 18 containers each one holding two thirds of a gallon of fuel. If I top each container with one third of a gallon of neat methanol I would have 18 gallons of 10% oil fuel.

It took me a long time to work that out with my Grade 6 Maths O Level! .

Edited By David Davis on 22/11/2016 14:05:55

[roger graves]

I used to work for Makita and they were testing oil levels of 200:1 in 2 St petrol engines .

This was to meet Californian proposed laws.

[Martin Harris - Moderator]

I understand that petrol has more natural lubricating properties than methanol so I doubt we'll ever get to those levels!

[Don Fry]

David, no. Open no more that one fuel and one of methanol at a time, and mix the new fuel mix into a third container. That way nearly all your containers have not been opened, and so have not started to deteriorate.

Re corrosion, one of the partial burning products of nitro methane is nitric acid. So methanol draws in water, creating a weak acidic wet coating on yer bearing. Given time will eat skin, so bearings are a doddle.

That's why the problem is less when no nitromethane is used in fuel.

But the problem is minimised by running the engine dry when hot, and oiling.

Has anyone ever managed to pour a gallon down the throat of someone who does these predictive spell checkers as a service to humanity

[Jon H]

Posted by Martin Harris on 22/11/2016 13:51:35:

Posted by Jon Harper - Laser Engines on 22/11/2016 13:37:01:

ED has it right with methanol causing corrosion. That is another reason not to run your engines as lean as possible as a richer mix plonks more un burnt fuel into the crankcase.

I'm not following this Jon. Surely more methanol residue equates to more probability of corrosion from the hygroscopic absorption of water vapour in the air - although I'm certainly not advocating running lean...

Sorry martin i was rushing and got myself muddled with my edits.

It should read as it sounds, run the engines leaned off to max performance and do not run unnecessarily rich. I tune my engines for max rpm and leave them there. If folk aren't comfortable with this, tune the engine for max rpm then richen it as far as you can without loosing rpm. If you loose rpm you are now too rich for optimum combustion so go a bit leaner. You can also lean off slow run needles a long way, which helps when at 1/2 throttle or below.

Dont bother with the nose up test, just fly.

And, as i always do, to clarify the terminology 'running lean' means too lean, ie past peak rpm. This is bad. Running an engine at optimum mixture or max rpm is not lean, it is optimum performance. The problem is at this setting most model engines do not have enough surface area to keep themselves cool if run flat out, or if the cooling provided is not good enough. They are also less tolerant of incorrect fuel tank position and so when some fuel is used they become lean, overheat, and stop. In this case the corrective action would be to move the tank, but most just run the engine richer which sort of covers the problem, but you dont get optimum performance throughout the flight and use more fuel, make more mess and spend more money.

[Jon H]

Posted by Martin Harris on 22/11/2016 14:05:07:

I understand that petrol has more natural lubricating properties than methanol so I doubt we'll ever get to those levels!

The lubricity of petrol vs methanol is only an issue in a two stroke where the fuel passes through bearings and such. In a 4 stroke its not important at all as fuel never makes it into the crankcase (assuming correct tuning!).

In theory, we could run far lower oil content in 4 strokes, and that is what i am currently testing.

My petrol prototypes run 50:1 and use about half the fuel of a glow engine so in glow terms they are running on 1% oil without issue.

[Dai Fledermaus]

Jon, your comments are interesting as always.

I seem to remember in a previous thread that you said that unlike conventional wisdom, that engines do not lean out whilst in the air, they in fact get richer. Have I got this right? If so why does this happen?

Also, when adjusting the mixture screw to get optimum revs/maximum performance, do you go from rich to max revs or from lean to max revs? I've always assumed it would be the latter.

[Martin Harris - Moderator]

Posted by Jon Harper - Laser Engines on 22/11/2016 15:40:27:

Posted by Martin Harris on 22/11/2016 14:05:07:

I understand that petrol has more natural lubricating properties than methanol so I doubt we'll ever get to those levels!

The lubricity of petrol vs methanol is only an issue in a two stroke where the fuel passes through bearings and such. In a 4 stroke its not important at all as fuel never makes it into the crankcase (assuming correct tuning!).

In theory, we could run far lower oil content in 4 strokes, and that is what i am currently testing.

My petrol prototypes run 50:1 and use about half the fuel of a glow engine so in glow terms they are running on 1% oil without issue.

Interesting - but doesn't that imply that the petrol lubricity is significant? Or are you running glows on 2% or less now? Whatever, it's food for thought and good that you're investigating the practicality of running on less oil - potentially one in the eye for those electrovangelists that delight in telling us "real" modellers that our engines are horribly messy!

[Jon H]

Engines tend to go rich in flight as they unload but only within certain limits. If you unload it massively (small prop at full chat in a steep dive) they will lean out again but if normal props are used its not an issue.

Tuning is done rich - lean - rich. ie you start rich, lean off, go too lean, then back to optimum again.

The whole process is pretty quick. No more than 10 seconds or so.

In the video below i start off with a 150 set really rich on both needles. I then set up the engine, then faff about with a few tweaks (it has to be perfect!) and then turn off. Its all done in just a few minutes.

[Jon H]

Posted by Martin Harris on 22/11/2016 16:11:45:

Posted by Jon Harper - Laser Engines on 22/11/2016 15:40:27:

Posted by Martin Harris on 22/11/2016 14:05:07:

I understand that petrol has more natural lubricating properties than methanol so I doubt we'll ever get to those levels!

The lubricity of petrol vs methanol is only an issue in a two stroke where the fuel passes through bearings and such. In a 4 stroke its not important at all as fuel never makes it into the crankcase (assuming correct tuning!).

In theory, we could run far lower oil content in 4 strokes, and that is what i am currently testing.

My petrol prototypes run 50:1 and use about half the fuel of a glow engine so in glow terms they are running on 1% oil without issue.

Interesting - but doesn't that imply that the petrol lubricity is significant? Or are you running glows on 2% or less now? Whatever, it's food for thought and good that you're investigating the practicality of running on less oil - potentially one in the eye for those electrovangelists that delight in telling us "real" modellers that our engines are horribly messy!

Think of it this way, the fuel (methanol or petrol) is thinning the oil. If you thin the oil with something that has no lubrication properties at all, you need more oil to compensate than if your fuel has reasonable lubricity. In the case of a model 4 stroke there is no fuel in the crankcase, only oil, so nothing is thinning the lubricant be it glow or petrol powered.

As always, its not quite that simple, but that is the principal.

One further thing to note, my petrol engines are currently (and will always) use bronze bushes in the conrods as they are less prone to damage in the case of a prop strike. i am still using 50:1 without any trouble so all the blurb you read about needing roller bearings to get below 20:1 is a load of old toilet

[Engine Doctor]

Posted by Merco 61 on 22/11/2016 12:34:24:

Mention is made of 10% oil,in straight methanol,for ST 2500 series.Is 10% synthetic the equivalent of 10% castor in all respects? My Tartan 22 glow instructions also specify 10% castor.

I,ve asked this before.What is the exact synthetic oil specification and brand to safely use and who will send by post in UK?

Hi give Dave watts a call at Southern Model Craft he will explain what oil he uses . I would not hesitate to use synthetic over castor for modern engines. Old engines with poor tolerances and lower power out put were quite happy on castor. Imagine using Castrol 30 or Duckhams 20/50 in a modern car or motorcycle ! Thats comparable with using castor in modern aero engine 2 or 4 St . The only good thing about castor oil is that it keeps your skin soft and smells good when used in diesel engines 😀

Edited By Engine Doctor on 23/11/2016 15:44:04

[Manish Chandrayan]

Jon

In that 150 tuning video what Prop is on the engine and what fuel in the tank?

[Jon H]

Its a 16x8 radio active prop and the normal laser 5 model technics fuel.

The RAM props were really good and perform very much like an apc but they dont make them any more as they didnt sell enough to make it worth while. A shame as they really do perform well

[Alan Gorham_]

I think it's a case of horses for courses with regard to oil.

For example, if you have a "budget" engine with un-bushed conrod, plain crankshaft bearings and a ferrous piston and liner then I would always be tempted to use 20% castor in the fuel and perhaps more while running in.

Such engines are rare these days, but some of Enya's 2-strokes are still built like this and if run on 20% castor will last forever.

Modern engines with bushed conrods, ball-raced crankshaft bearings and either ringed or ABC piston/liners can be run on less oil and synthetic is certainly less gummy afterwards.

Cox engines and diesels are again special cases.

Personally I'd be loath to use less oil than the manufacturer recommends and in any case, most easily available commercial fuel that I can buy meets the manufacturers requirements of my engines, so I don't have to get involved in mixing.

For Jon Harper, RAM props are still available, now made by SLEC:

**LINK**

[Jon H]

Posted by Alan Gorham_ on 23/11/2016 16:53:54:

I think it's a case of horses for courses with regard to oil.

For example, if you have a "budget" engine with un-bushed conrod, plain crankshaft bearings and a ferrous piston and liner then I would always be tempted to use 20% castor in the fuel and perhaps more while running in.

Such engines are rare these days, but some of Enya's 2-strokes are still built like this and if run on 20% castor will last forever.

Modern engines with bushed conrods, ball-raced crankshaft bearings and either ringed or ABC piston/liners can be run on less oil and synthetic is certainly less gummy afterwards.

Cox engines and diesels are again special cases.

Personally I'd be loath to use less oil than the manufacturer recommends and in any case, most easily available commercial fuel that I can buy meets the manufacturers requirements of my engines, so I don't have to get involved in mixing.

For Jon Harper, RAM props are still available, now made by SLEC:

**LINK**

My 15 FP is a plain bearing engine, and every glow engine on the market has a bushed conrod although saito do run theirs straight on the ali.

Old engines, like ancient ones are a different story but they are fringe cases and best taken on their own. But in general, any modern (last 20 years at least) engine will run on far less oil than most model fuels contain. I wind up the diesel lovers too by running 20% castor in my plain bearing PAW and 15% synthetic in my Laser Diesels. They all work a treat so...

As for RAM props, the 16x8 is no longer available. I know the guys at RA well as they supply us with mounts. I asked him to make more but he said its not worth it