Engine Tuning
Simon Lockington

A lot of people have emailed me since the last newsletter asking what tips I learned from Curtis in respect to engine tuning. Now I must begin by declaring that I am by NO MEANS an engine expert, but since enough people have asked, I'm going to write how I setup an engine. Take this not as gospel, but as another point of view.

What I had been doing wrong:
Previously, I had been tuning an engine on how it sounded in the air. If it seemed to bog down and had smoke, then I leaned it out. If it made a cackling noise in descents or hover, then I richened it up. While this isn't wholly incorrect, this method should be used in conjunction with others to tune the engine.

You see, you should use a mixture of listening to the engine, looking at the helicopter for shakes and smoke as well as feeling the temperature of the engine. All of these methods will give clues as to what the engine is doing at the current fuel mixture setting.

How I tune now
Let's take as an example tuning my Vigor CS. My CS has an OS 91 SX-H with the OS 60B three needle carb from the OS 61 WC fitted. The needles are bottom, mid-range and high. The top end controls the total amount of fuel passed to the engine, the other needles meter out a percentage of that total flow at each part of the rev range of the engine.

I tune this engine according to the instructions provided by OS for the 60B carb as well as looking at heli for visible vibrations and the quantity of smoke. I also listen to the tone of the engine as well as any other noises and finally, I test the temperature of the engine by touching the backplate.

Tuning the needles:
I tune this carb in accordance with OS instructions. This is to close the mid range needle fully and set the bottom end to a fairly consistent idle and the high end set about two turns out.

I then hover the heli at about 50% power for a couple of minutes to let the engine warm up. Most likely the heli will be running very rich (rough running, fins vibrating, canopy shaking, slow headspeed, slow in throttle response, lots of smoke). I will confirm this by landing the heli and feeling the backplate of the engine. It's most likely that with the heli running so rich, that the backplate will be stone cold. If that's so, then the heli is definately too rich.

To rectify this, I lean the top needle about four clicks. This will be enough to make a noticeable difference. Again, I hover the helicopter for about one minute to allow time for the mixture change to show results. Now, we should notice the engine should be four-stroking less (it will still be four-stroking to some extent), we should also notice that the vibrations in the heli (canopy, fins, etc) will have lessened, but probably not vanished. The engine should also feel a bit more responsive to throttle inputs and the headspeed should have increased slightly from the first hover. Smoke will have lessened a little bit, but will still be flowing in copious amounts. Confirm that the engine is warming up by landing the heli and placing the index finger on the backplate. You should now start to be feeling some warmth, but it is unlikely that it will be very hot.

Again, lean the high end needle 3-4 clicks and hover the helicopter again. You should see a noticeable difference from the previous flight. The vibrations in the heli should have settled down now to either none, or very minimal. The engine should also have reduced it's four-stroking behaviour to either none at all, or very infrequent. The headspeed should be close to normal and the throttle should be quite responsive now. The smoke content shouldn't be as copious as before, but should be constant.
This means that the needles will now be getting close to optimum operating range and so adjustments to the needles should be minimal (ie two clicks instead of four). However, before any further adjustment takes place, you should land the heli and again feel the backplate which is most likely going to be warm now. If it gets to the point where it is uncomfortable to hold your finger to the backplate then the engine is too hot (therefore, too lean). Always leave the needles on the rich side of optimum at this stage.

Now, it's time to set the needles for full throttle. I always do this by flying the heli backwards down wind climbing at about a 45 degree angle. The reason for this is that if the heli has a flame out, it is easy to autorotate the machine back down again. I do this by slowly and smoothly feeding in collective (not stabbing it which may flood the engine and quit it) and listening to how the engine reacts to the throttle and watching both the headspeed and the smoke.
If the heli is very sluggish on the throttle and the headspeed dies back but produces a good amount of smoke, then the high end is too rich. If however, the engine is sluggish, the headspeed dies back and there isn't much smoke, you're most likely too lean. Again, confirm this by landing the helicopter quickly and touch the backplate. At no time should you let the engine become hot enough to make touching the backplate uncomfortable.

Ideally, what you want is for the engine to take the throttle input smoothly with the headspeed not decaying and the smoke output should be consistent. If this is the case, then I fly the machine around at full throttle for a couple of minutes seeing if the engine loads up (if so, it is most likely too rich on the high end) and then land and check the engine temperature. Again, you should be able to comfortably hold your finger to the backplate, it not, then it's too lean, richen the high end needle.

Now, I then hover the helicopter again for a couple of minutes and watch the helicopter again. Now it's most likely to be a fraction lean now that we have set the high needle for full throttle. The heli is likely to run a little rough here and vibrations may have returned. The headspeed is likely to be pretty high and the throttle will be very responsive. The smoke however is likely to be quite low. Land the heli and test the engine temperature. You're likely to find that the engine is quite hot. Now I adjust the mid-range needle to achieve a nice smooth hover. I open this needle four clicks and hover the heli again to ensure that the vibrations have lessend, the engine has smoothed out and the smoke has become consistent. Land the heli and check the temp. Continue to open the mid-range until you have a nice smooth hover.

Again, fly the heli round to ensure that you are happy with the engine settings and that's pretty much it.

Tuning your throttle curve:
One thing that will make tuning your engine difficult is if your throttle curves are not set correctly. If they are set too high for a given pitch point, then the engine will rev too high and cause overspeeding. This will cause the engine to act like it is running lean and the engine will 'cackle' and heat up. Conversely, if it is set too low, then the engine will load up and will appear to be running too rich.

Fuel:
The content of your fuel will have a bearing on your needle settings. Over here in New Zealand, we custom mix our own fuels (ie we buy the nitro, oil and methanol seperately then mix it up). When I moved from 10% nitro/20% Coolpower Oil/70% methanol to 5% nitro/15% oil/80% methanol I had to change my needle settings.

Glowplugs:
Glowplugs have a major effect on how your engine runs. I've experimented with various plugs with different fuel combinations to see what happens. Intially I found that my Webra engines ran best with the hot K&B 4520 plug as opposed to the colder Enya 4. However, I now run my OS 91 engine with the above fuel mix with the cool Enya 4 plug. I've found that the engine runs smoother with this plug than the OS 8. I'm also now running my Webra 91 with the Enya 4 as well. This is my current setup that works for me and my type of flying.
I encourage you to experiment with different types of plugs to see what works best for you.

Different carb styles:
You've seen the way that I tune my OS 91. When my Webra 91 turned up, I tried tuning it the same way as the OS and wondered why in the hell the engine wouldn't run properly. After numerous posts on various forums with other Webra 91 owners we found out that the idle needle is the dominant needle on the Webra's Promix carb and the high needle (it's a two needle carb) is literally the very high section of the range (about the top 20% or so). Tuning this thing was a whole different mindset and if the truth were to be told, I don't think I've got it at it's full potential yet so I'll not comment any further on it. The point is, that different carbs require different methods of tuning. What's important is that you use a combination of the techniques discussed here (Appearance - smoke, headspeed, vibrations / Sound - four-stroking, cackling / Temperature - stone cold, burning) to tune your needles.

Remember, this is just a guide, it's by no means gospel. There are many people out there who know much more about engine tuning than I, but this is what I have used to tune my engines and it seems to work for me.