Ok, here's another newbie question, that I'm sure once answered by one of you masters, would be a good resource. The guy who trained me to fly, although the flight portion was great, he failed to discuss this all too important facet of RC Flying.

1. What's the difference between running rich and running lean?
2. How can I tell?
3. How does the needle valve work?
4. People who help me tune my plane seem to be listening for something, what is it?
5. Does this play any part in determining RPM's?
6. If so, is there a target RPM range I should be looking for?

Thanks in advance, guys. I know, like always, there will be some very insightful advice to follow.

1 An engine wants to run with a 15% mixture of air and fuel. So (Approximately) 85% air to 15% fuel. If there's too much fuel, the mixture is "Rich", not enought fuel and the mixture is "Lean".

Note: Running an engine Lean can cause damage!

2 As you close the needle valve, the mixture will lean out from its present state, and it will go up in RPM. Once it peaks, back it off a few clicks. Then, point the nose of the plane straight up. There should be a slight increase in RPM. If the RPM goes down, it is too lean.

3 A Needle valve is just that... A needle that goes into a hole to adjust the fuel flow (See Pic)

4 They are probably just listening for anything unusual

5&6 There are mant factors that determine the "target" RPM. Engine size, prop size, type of flying you do etc.

I think it will help to understand what "rich" and "lean" are.

Rich means there is too much fuel in the fuel/air mixture, so there isn't enough Oxygen or time to burn it all. This means that there is unburnt fuel flowing all the way though the engine and out the exhaust. It sounds bad, but that extra fuel means extra lubrication and extra cooling in the engine, and makes for a longer life. But it also cools down combustion which reduces the power output.

Lean is not enough fuel. In this case, all the fuel burns, and frequently some of the oil as well. The engine runs hotter than it should, and gets less lubrication than it should. Engine parts heat up and expand, and in some engines (non-ABC) too much of this can cause serious damage. The lack of lubrication can hurt any engine. And, of course, the lack of fuel burning results in a loss of power.

The needle valve works by having a tapered needle that fits in a small hole. As you screw in the needle, the hole gets closed, reducing the flow of fuel in to the carb.

Ok, so how to tune an engine. Setting the needle is done with a combination of tests. First, the needle is usually backed out a little, richening the mixture a tad (half turn max, maybe less), and the engine is started. (if the weather hasn't changed much since the last run, I don't bother backing the needle, just start it up). If it's a new engine, see the manual for first starting position, it's usually on the rich side.

The engine should start and will usually have a deeper sound. It's common to see fuel coming out of the exhaust. If you put your hand behind the muffler and feel what's coming out, you should get some raw fuel on your hands, and the oil should be light colored, or nearly transparant. The engine might be 4-stroking, which is fireing every other stroke. Yes, they can, and do, do this.

The needle is then slowly turned inwards. I usually turn no more than 1/2 turn at a time when I know I'm way rich, then smaller and smaller adjustments as I get closer to "ideal", including a click at a time at the end. It's important to let the engine "catch up" with your changes. You can't just keep on screwing the needle in, you have to pause between adjustments.

As the need is turned inwards, you will hear the sound of the engine change. first, it breaks out of 4-stroking, though a "middle range" where it's going back and forth, and in to a "clean" 2-stroke where the pitch sounds constant and clear. At this point, you are getting close to peak. If your plane has a lot of power, you can fly at this point, just inside a clean 2-stroke. You'll still be plenty rich for long life, but be getting most of the engine's power potential.

Ok, so you want more power. As you continue to slowly lean the needle, the RPMS will go up slowly. You can hear the rpms going up, that's what guys are listing for. Then, as you keep leaning SLOWLY, a few cliks at a time, you'll reach a point where the RPM doesn't change, or drops slightly. If you keep leaning, the RPM keeps dropping off. At this point, you're too lean, so back out the needle to reach that peak RPM point.

Now, when the plane is flying, the prop "unloads" and the Gforces and drop of fuel level in the tank all combine to lean the engine a bit in flight. So you should never take off at this peak setting. Instead, back the needle just enough so the hear a slight RPM decrease. For most people's ears, that's about 500rpm. Changes smaller than 500 aren't usually detectable by ear, at least I can't hear them unless I'm really paying attention.

Ok, so you've backed off from peak, so you're ready to go? Well, here's a couple of tests you should do to double check. Pick up the airplane and hold it vertically. The engine is now pulling fuel uphill, and will lean out a little. You should hear a slight RPM increase. If you hear a decrease, you are too lean, so back off the needle a tad and repeat. If you don't hear a change at all, I'd still call it slightly lean, so back off a click or two. Another test is to give the fuel line a hard, but short pinch. Again, this breifly leans the engine. You should hear a slight RPM increase. If the engine slows or staggers, back it off a tad.

One the needle is set, you won't need to adjust it much. As the weather changes, you'll need to make small adjustments, but if you find yourself wanting to change the setting by more than maybe 1/2 turn either way, something else is likely wrong in your fuel system.

The exact RPM numbers and needle setting will vary with prop, air density, humidity, fuel, and some other factors. So if you change prop sizes or fuel brand (even at the same nitro, differnet fuels will produce different RPMS and needle settings), you'll want to reset the engine.

That's it. The key is running a bit off peak RPM at all times. On my student's planes, I set the needle quite a bit off peak, since they don't need the power on the trainers anyway. It's common to see a nice smoke trail behind the plane with an engine set rich.

If you want to, you can use a tach to set the engine, but the procedure is the same. Just slowly lean while watching the RPM. You have to let the engine and tach settle after each change. As you go too lean, you'll see the RPM drop, then I personally back off 500 to 1000rpm depending on what I'm doing.

Actual RPM ranges for 2-strokes vary quite a bit. But for most sport flying, the prop and fuel selected will normally put the engine in the 12-16k range for a .25-.60 class engine. On my combat engines, I sometimes peak around 20k, 4-strokes normally shouldn't be over 10-12k on the ground for most of them. Larger engines turn slower, smaller engines usually wind up faster.

Kirk,

That's the best explanation I've ever read on this topic! Thanks!

Montague -

Thanks for that information. It was probably the most helpful explanation of anything I've ever heard on any topic. After reading, I feel confident enough that I can do the needle valve adjusting on my plane, without assistance.

Thanks again!

Thanks guys, though it's not new stuff, just long winded

Oh, something I forgot, not all needle valves are created equal. Different hole sizes and tapers and ratchets make some needles much more sensitive than others. For example, the Magnum .15s that I run in combat are known to have really really REALLY sensitive needles. I mean 1 click is too rich, 1 click is right on, 1 click more is too lean. (it doesn't help that I'm pushing the engine for every last RPM I can get out of it). A lot of combat guys using Mags will actually buy an OS .15LA remote needle and put it in line before the Mag carb to make it easier to tune. Sometimes this silly of a thing is the difference between a beginner friendly engine and a real beast. I've never heard a good reason why any engine company has a blunt, sensitive needle, but they exist. Go figure.

And, of course, I totally left out anything to do with low end, but that's for another day

Oh, and while I mentioned the light color of oil coming out in a rich run, I forgot to mention that if you go over lean, the oil coming out will get really dark carmel or brown in color as some of the synthetic oils start to burn. Black gunk coming out the exhaust is a bad thing. Usually, it's fine metal particles. 9 time out of 10, it's coming from a loose muffler, or muffler tail cone, but if it's coming from inside the engine, the engine is wearing itself in a bad way. If you put your finger in the oil and rub your fingers together and see particles or feel grit, your engine is about to be toast. Of course, the exact color of the oil in the exhaust will vary with fuel brand and other factors, so it's not a percise measure of anything.

I'm sorry Kirk but we are going to need the low end needle explanation now.

Pixelator, It's really not that hard once you know what to look for. I learned the same information that Montague perfectly explained. I'm very new to this hobby and it didn't take me any trying to figure out what was explained after you get the plane started. What I did was run the engine full throttle and backed out the needle until it started to sputter like it was about to die. Start from there and slowly turn the needle in and you will hear all the stages that was explained. I learned a similar technique described by Montague from the site below.

http://www.indiarc.com/tune.htm

Thanks for the info that was a big help for me also, One other question. When you are holding the plane vertical is this with it in full power or at idle?


Thanks Again

J. Stewart

Smyrna, Tn

Extremely informative. Great explanation for even the newest of the new.

The only thing I'd add is a safety issue:

MAKE ALL ADJUSTMENTS FROM BEHIND THE PROP!

Secure the plane mechanically, using a rope and tree, a helper, whatever, never attempt this alone unless the plane is firmly attached to something solid. Then quickly go back around to the rear of the plane and carefully make the adjustments. If the engine quits, you have to move back up front, but at least you won't be reaching through the prop to adjust your needle valve.

Good advice from pettit. I never adjust anything from in front of the engine, I think it's not safe.

stewart,
The lift test is done at full throttle. Usually the plane is run up to full, then lifted and pointed upwards, so you are always dealing with the same "pull". Most of the time, you get someone else to hold the TX while this is going on, so they can pull back the throttle if things start going badly. Also, if you do start to slip or whatever, dump the plane on it's nose away from you, don't try to catch a slipping plane. Yes, the plane will likely be damaged, but it's better than getting turned to hamburger.

A lot of guys don't like the lift test, or it's impractical on larger airplanes. So the pinch test makes more sense. But sometimes the fuel lines aren't accessable either. In those cases, most guys just back off peak a little extra, and fly it. Once you've been running a particular engine for a while, you'll get used to it, and find that you don't need the tests as often, you just know when it's about right.

geish,
Yeah, nothing I wrote is new, or anything I came up with, just my rehash of the same old stuff . I have no doubt there are sites out there with even better descriptions. I just never know where they are when I want them

About the low speed settings, it's harder to describe because it varies a lot from carb to carb. I know it's been talked about on here and on other sites a fair bit though. (besides which, I don't have time to write that all out any time soon, I don't think)

Great posting Kirk, thanks!!!

But I second jucava-RCU - we like to know about low-end adjustments soon (like very soon ).

Thanks a lot again,
EK