fheppenheimer

My Feedback: (11)

THEORETICAL QUESTION: When switching a well-running engine to a higher % nitro mixture, same oil content, would you expect a need to richen or lean the high end needle valve? richen or lean the low end needle valve?

MY SET-UP: I'm running a Saito 100 and have been using 20/20 all synthetic. I'm switching to a "20-something" blend I've made which is 1/2 all-synthetic 30% heli and 1/2 all-synthetic 15% heli. BTW, the engine runs great on the 20/20 except I can't seem to get the idle quite low enough the plane pulls forward at idle on the ground.

MJD

My Feedback: (1)

When you increase the nitromethane percentage and reduce methanol percentage, the required air:fuel ratio is reduced because nitromethane burns at a much lower ratio than methanol. So with the same needle setting you are now burning a mixture that hass an excess of oxygen = lean. In order to burn the higher nitromethane fuel at the proper mixture ratio (more fuel to air), you need to open the needle valve to richen the mixture. Also note that your idle is now a hair leaner as well, though may not be enough to need a tweak with a change like you are mentioning.

MJD

DarZeelon

F,


The needles do not directly control the RPM of the engine...
If your model creeps forward at idle, if means the thrust the prop produces at the engine's current idle RPM, is greater than the total drag and friction of your model, set on its wheels.

It could be that your engine is idling at too high an RPM, but it can also be that the model is particularly free of drag and friction...
Draggy wheels can be an asset, rather than a liability, if this is the case...

If your idle RPM is too high (your Saito should easily be able to idle between 2,000 and 2,300 RPM; depending on the size of the prop), you must close the throttle further (using the throttle trim), to reduce the RPM - not to change the low-speed needle setting...


You might need to change the setting of this needle, if the mixture setting is either too rich, or too lean to reliably sustain idle.

The low-speed needle setting should be set for optimum transition.
It should be as lean as it can possibly be, without causing the engine to 'bog' on transition from idle (as fast as the servo can open the throttle).


As to the changes you must make, when you increase the percentage of nitro in the fuel; it is a constantly rising function.

The more nitro in the fuel, the more (significantly) fuel your engine needs!


Nitro has a stoichiometric range of 0.5-2.5:1, compared to methanol, which is about 6.5:1 (gasoline/petrol is about 14.7:1).
So, your engine need much more fuel, when running higher nitro percentages.

If it was adjusted perfectly for, say 15% nitro; it will definitely not run OK, on the same needle settings; even if you increase the nitro by just 1 percentage point.

fheppenheimer

My Feedback: (11)

Thanks Dar, very understandable and helpful.

DarZeelon

You're welcome, F..

In addition; despite the fact that Saito and Horizon Hobbies recommend running all-synthetic lube fuels in Saito engines, I recommend that you add 2-3% castor oil to your fuel, for that extra measure of protection.

I use this formula myself and even for YS engines (which people say castor residue can soil the regulator), I recommend doing the same.

For most of their engines YS don't subscribe all-synth and I have never seen anything jam-up, or cake-up from just 2-3% castor oil, even in those engines.
But your engine will get much needed protection and the bearings (a known bugaboo) will last much longer (no rust).