Setting the fuel/air mixture of a radio controlled helicopter's engine is often one of the most challenging - and frustrating - tasks that novices run into. Even experienced flyers are sometimes stumped by this one when they get an engine they're not familiar with. I don't have all the answers, but I've created this page in hopes that what I have learned will help others.

Symptoms of poor mixture settings include:

• Difficulty starting the engine
• Difficulty maintaining a consistent idle
• Difficulty transitioning from idle to hovering RPM
• Excessive vibration (often mistaken for clutch or fan imbalance - and vice versa)
• Unsteady tail in a hover (often mistaken for gyro trouble - and vice versa)
• Difficulty transitioning from hover to fast forward flight
• Low power
• Intermittent power loss
• Hovering RPM too high or too low (a governor can hide this, but the problem will manifest itself elsewhere)
• Short glow plug life
• 'Hanging' onto high RPM after engaging throttle hold
• Wagging in fast forward flight (due to vibration and/or excessive rotor RPM)
• Engine quits in flight

Different symptoms will be more evident under different cirumstances:

Starting and Idling

If the idle mixture is too rich, the engine will be difficult to start, and/or it will "load up" if left idling for more than a few seconds. You may also notice an excessive amount of fuel spraying from the exhaust.

The most reliable test for the idle mixture is to pinch the fuel line between your fingers, stopping fuel flow to the carburetor. If the idle mixture is too rich, it will idle even better for several seconds before revving up and dying. If the idle mixture is too lean, the engine will simply quit after just a couple seconds. If the idle mixture is right, the engine will idle normally for just a couple/few seconds before revving up and dying. It takes practice to recognize the difference between 'too rich' and 'just right' during the pinch test. However, if you find that the engine loads up after 10 seconds of idling (without restricting the fuel flow), and if pinching the line for a few seconds improves things, you're too rich.

Hovering

If the hover mixture is too lean, the tail rotor will not hold still, possibly leading you to suspect gyro problems. In extreme cases (less common with modern gyros and tail rotor servos), the tail may swing as much as 90 degrees off heading. If the hover mixture is too rich, the head speed will be low and fuel consumption will be increased.

Listen to the exhaust sound when hovering. A proper mixture will simply drone, with no variation. A slightly rich mixture will go in and out of a "four-stroking" mode, resulting in an exhaust note that sometimes 'burbles,' for lack of a better word. An overly lean mixture will often cause the engine to switch to an 'overheated' mode, during with the exhaust sound becomes quiter and engine power drops off drastically. If you're overheating, these drops will become more and more frequent, and power will decline steadily. The oscillations into four-stroking tend to be very brief, lasting only a fraction of a second, while overheating causes a change in the exhaust sound that lasts a second or more.

At first it may be easy to confuse the sounds of rich and lean mixtures. If you're not sure, it's best to assume it's too lean, and richen things up a bit. If the problem gets worse, at least you won't hurt anything this way. If the problem is reduced, you're on the right track.

You should also look at the smoke trail when you're hovering. It may take some time to get used to the amount of smoke produced by a particular engine/fuel combination, but with practice this can be a useful indicator.

Fast Forward Flight and Vertical Climbs

All of the symptoms described above will be evident in fast forward flight and in full-throttle vertical climbs, but they're more difficult to detect because the helicopter is further away (harder to see) and moving fast (harder to hear).

If the hovering mixture is reasonable, watch the smoke trail as you ease into full throttle. If you see less smoke, your high-end mixture is probably lean. If you see more smoke, your high-end mixture is probably rich.

You should note, however, that YS ST-1 and ST-2 engines running 30% nitro are notoriously intolerant of lean settings at full throttle. They tend to simply quit.

Autorotation

When you hit throttle hold for an autorotation - even just a 'baby auto' from a hover - the engine should settle down to a smooth idle after just a second or so. If the engine remains at a very high idle (often called "hanging"), your idle mixture is probably a bit on the lean side.

Descending

When descending with the rotor disc level, the engine has very little load (possibly none at all, in fact) and thus it tends to overspeed. A lean midrange or low-end mixture will make the overspeeding problem even worse. The smoke trail provides additional evidence - if the smoke disappears during level descents, you need to richen up your midrange and/or idle adjustment. This sort of problem typically goes hand-in-hand with the autorotation problems described above.

Weather

You will inevitably find that after painstakingly perfecting your mixture, it will be off a day or two later. This is because the mixture is determined not only by the amount of fuel flowing into the engine but also by the amount of air flowing into the engine. The fuel intake can be varied by adjusting the valves, and the air intake can be varied by changing the density of the air. Unfortunately, air density is generally out of your control, so you just have to adapt to it.

The air will be less dense on a hot day than on a cool day. In order to keep the air:fuel mixture consistent, you need to restrict the fuel low on hot days, and increase the fuel flow on cold days. It's important to bear in mind that the ideal mixture ratio does not change, you're just varying the fuel flow to compensate for the variation in air density.

You'll find that the engine runs strongest on cold days, when the higher air density allows a great deal of air and fuel to be drawn into the carburetor. On hotter days, the low air density requires you to restrict the fuel flow, which reduces power somewhat.

Temperature Gauge

In addition to all this, I found it very helpful to have a temperature gauge mounted on my engine when I was getting started with RC helicopters. I've tried both the MIP and PC/RC gauges... The PC/RC gauge failed after just a few flights, and was very difficult to read on all but the cloudiest of days, I don't recommend it.

Unfortunately, even with the MIP gauge you have to land the helicopter to read the display safely, but I found that the engine cooled slowly enough that I could still get a pretty good idea of how hot it was running upstairs.

One thing you should know about on-board temperature gauges is that the reading depends as much on the placement of the temperature sensor as anything else. It's probably not useful to compare figures with someone else's temperature gauge unless you've got the same type of sensor mounted in exactly the same place. With the MIP gauge and a lug-style sensor mounted on an exaust bolt opposite the muffler, I found that temperatures around 260F yielded the best performance from my OS32sx.

Even if you don't know exactly what temperature to aim for, just having the gauge there gives you another piece of information to consider when making mixture adjustments. With some experimentation you'll notice correlations between different temperature ranges and different operating characteristics. With more experimentation you'll eventually find the temperature range in which the engine runs best. With a little practice you'll see the relationships between different engine behaviors and different temperatures, and as time goes by you'll find yourself using the gauge less and less as you become better able to recognize the other signs of an engine that's too rich or too lean.

Valve Interaction

This has been one of the hardest things for me to understand, and that's partly because it's different for every engine. This is why the same approach to tuning one engine might not work so well when tuning another.

Most of our carburetors have at least two and often three different values to adjust the fuel flow at different throttle ranges. That would be no problem in itself if it were obvious what the throttle range really was for each valve. Unfortunately, you don't get nice overlapping bell curves for each adjustment.

Typically one of the valves will impact all of the throttle range, or most of the throttle range above idle. This is the valve you should use to compensate for temperature changes, since a change in ambient air temperature will impact the mixture actross the entire throttle range.

It's helpful to know where each valve's effective range begins and ends. With the YS 61 ST2, for example, the "midrange" adjustment affects the mixture across the entire throttle range. The "idle" adjustment affects only up to about 20%, at which point the midrange adjustment takes over rather abruptly.

For weeks I thought my idle was too rich because of the way the engine would load up and stumble when I transitioned from idle to a hover. It turned out the idle was way too lean - in order to get a proper hover I had set the midrange richer than was really appropriate. Fattening up the idle and leaning the midrange just a hair smoothed the transition and also cured me of an overspeeding problem I had with descents.

With the OS 32 sx, it's common to drill an air bleed hole to lean out the idle (more on this later). The usual approach is to drill a very small hole in the front of the carb, in positioned so that when the engine reaches about half throttle, the throttle barrel closes off the hole. Intuition suggets that placement of the hole would be important, and that the engine would get noticably richer when the hole is closed off. Closer examination suggests that neither is true. The effect of the air bleed hole is proportional to the ratio between size of the air bleed hole and the size of the regular throttle opening. This means that when you're at an idle, and the air bleed hole is 15% of the size of the regular throttle opening, the air bleed hole will lean out the mixture by roughly 15%. Conversely, when you're at half throttle, the air bleed hole is only 1% of the size of the throttle opening, so closing it off makes a negligible difference.

This is why I think the servo-actuated air-bleed cutoff featured in MHT a while back is unnecessary. You don't need to close off the air bleed to get a rich midrange - the air bleed simply becomes irrelevant by the time the throttle is halfway open. My own Concept works well enough without a servo controlling the mixture, and I know I'm not alone in this.

Carburetors That Suck

Every carburetor sucks. The suck fuel in from one side, they suck air in from another side, and the blow a mixture into the engine. But, some carburetors also really don't work very well. This is the most maddening thing, as there may be no combination of valve adjustments that gives you a proper mixture at idle, hover, and full throttle.

Consider the OS 32sx for example. It's got an idle adjustment and a high-speed (full throttle) adjustment. And yet, when tuned for easy starting and reliable idling, plus optimum full-throttle performance, the midrange is lean and the helicopter behaves like crap in a hover. So what do you do if you want to richen the midrange?

If you richen up the top end, you get a very nice midrange mixture, but full-throttle performance will suffer (see above r.e. valve interaction). If you richen up the bottom end instead, you can get a nice midrange but the engine will be troublesome to start and difficult to idle for extended periods. You're screwed either way.

The solution here is to drill a small "air bleed" hole in the carburator (link to a diagram coming soon...). Now you can "richen" the low-end valve until you get a satisfactory hover, and yet the air bleed will keep the idle lean enough for to quick starts and prolonged idling.

The throttle barrel (the carburetor's main moving part) that came with my YS ST-1 was poorly machined, and had a rich spot around 3/4 throttle that could not be dialed out with the main needle valve or the high-end valve. When transitioning from hover to fast forward flight, the engine would briefly lug down and belch smoke at twice the normal rate. In this case, the only solution was to replace the throttle barrel with a new one. Fortunately, the first replacement barrel I tried had a properly-shaped fuel passage and yielded a consistent mixture from idle to full throttle.

Dealing with a rich idle

Sometimes, a rich idle is difficult to avoid. Before drilling out my OS 32 sx carb, I had to deal with it on a regular basis. I learned a couple things from friends that make it somewhat easier to manage.

First, before attempting to start the engine, take off the glow plug, go to full throttle, and spin the crankshaft for a several seconds. This clears residual fuel from the carburetor and crankcase. Remember to bring the throttle back to an idle before you put the glow plug igniter back on!

Second, hold the transmitter in your left hand, with your thumb over the throttle gimbal. This way you can carry the helicopter in your right hand while still working the throttle to keep the engine from sputtering out.

Third, pinch the fuel line periodically to lean the mixture. This is especially useful when the heli is on the bench or on the ground, and you can pinch with one hand while holding the transmitter and working the throttle with the other hand.

Good Luck

It takes practice to be able to tune an engine by ear. I've been at this for almost five seasons, and I still don't always get it right the first time when someone comes to me with an engine problem. Hopefully what I've written here will help you get thing hang of things just a little bit more quickly.