Described below is a procedure which I have found to work on a number of
different types of engines. Using this method should allow you to get a smooth
running engine.
Opening and closing the
throttle (by moving the throttle arm) will increases the amount of fuel and air
flowing into the engine. The ratio of this mixture is automatcally controlled by the
design of the carburetor. However, the mixture can be tuned further by the mixture
control valves found on either side of the carburetor. There are (usually) two
adjustable mixture control valves on the carburetor. The high-speed needle,
and the low-speed needle. Adjusting the low-speed needle will
primarily affect your idle to mid-range throttle response. Adjusting the high-speed
needle will primariliy affect your mid-range to top-end throttle response.
The objective you are trying
to acheive is an engine that will run smoothly at a constant speed without bogging down
through all phases of flight.
A good place to start (using
10 - 35% heli fuel) is with the high-speed needle 1.5 turns out from the fully closed
position, and the low-speed needle 2.5 turns out from the fully closed position.
Your hover pitch should already be set. I favour setting the hover pitch between 5
and 6 degrees at half stick. At half stick the carburetor barrel should also be open
50%. (this has worked for me). The transmitter should also be set up to give equal
and linear throw between zero to half throttle, and from half to full throttle.
Start the engine.
At this point increase
throttle trim just enough to acheive a steady idle.
Let the model warm up for
about 20 - 30 seconds. (Right now
you are warming up the engine.)
Now observe the quality of
the exhaust smoke as you increase the trottle just enough to just lift the model just
above the ground.
If the smoke was really
thick and dense as you increased the throttle with a tendency for the engine to quit then
the mixture is too rich. Close in on the low-speed needle valve
(adjust by turning the low speed needle only a quarter of a turn at a time, it is very sensitive).
If the smoke was really thin
and light as you increased the throttle with a tendency for the engine to quit then the mixture
is too lean. Open up on the low-speed needle valve (adjust by
turning the low speed needle only a quarter of a turn at a time, it is very sensitive)
Complete the above steps.
You should acheive a mixture that allows you to open up the throttle with smooth
response from the engine from idle to hover. Also the exhaust smoke should not be
too heavy or too light.
Now hover the helicopter and
observe the response to collective control.
Increase the throttle to
full open and observe response. If the helicopter responds really sluggish the mixture
is too rich. (or maybe your helicopter is too heavy ;-) Lean out the
mixture by turning in on the high-speed needle.
If your helicopter seems to
respond too quicly and there is a decrease in smoke, the mixture is too lean.
Richen up the mixture by opening up on the high-speed needle.
Check the top-end response
again if the engin boggs down when accelerating straight up then your top-end collective
pitch needs to be decresed. The opposite applies if your engine speeds up when you
increase throttle.
Now land the helicopter and
observe the idle quality again.
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did the engine buzz on the way
down; mixture a little lean |
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was the engine still racing
even wit controls at idle position; mixture really lean |
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engine quit and smoking;
really, really, really lean |
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adjust mixture by way of the
low speed needle if you had any of the above. |
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no buzzing, idling nicely; you
are done adjusting |
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