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2.
Having provisionally set the idle mixture, the next
step is to adjust the mixture for hovering flight.
Hover the model and actuate the throttle to observe
response over the medium speed range. lf the
engine smokes excessiveIy and throttle response is
poor, the mixture is too rich ; in which case, land the
model and turn the High-Speed Needle Valve
clockwise. Do not close the High-Speed NeedIe
Valve more than the recommended initial
adjustment (3 to 4 clicks) at a time, keeping it a little
on the rich side at this stage.
lf, on the other hand, hovering is not stable and
response to the throttIe is over-sensitive, or if the
engine tends to overheat, this indicates that the
mixture is too lean and should be corrected by
turning the High-Speed Needle Valve counter-
clockwise.
When satisfactory hovering flight has been achieved,
land the modeI again and re-check the engine's idle
qualities.
Now adjust the High-Speed Needle Valve to
3.
achieve the best performance when the model is
flying at full throttle. lf, at full throttle, acceleration
is poor, the exhaust unduly smoky and the model
fails to reach expected straight line speed, the
mixture is too rich and the High-Speed Needle
Valve setting will need to be reduced.
lf, however, after smoothly acceIerating to
satisfactory high-speed straight and level flight,
power is lost when the model is puIled up into a
climb, the mixture is too lean. ln this case, land the
model immediately and readjust thw High-Speed
Needle Valve to a richer setting.
4.
Having now found the optimum setting for the
High-Speed Needle Valve, re-check the engine's
idle performance, fine tuning the Idle Mixture
Control Screw, if necessary, to perfect the idle.
For helicopters, good throttle response at medium
r.p.m. (e.g. hovering speeds) is most important,
since this is a power range widely used in
helicopter flight. The optimum fuel / air nixture
strength at medium speeds is dependent on
obtaining balanced adjustment of both the Needle
Valve and the Mixture Control Valve.
After about 10 seconds of idling, open the throttle to
'float' the model. lf the transition is smooth, the idle
mixture is O.K. If the symptoms of either rich or Iean
running are observed, readjust the Idle Mixture
Control Screw accordingly.
26
If both controls are alreday at their optimum setting,
some modification to these settings may be
necessary to achieve satisfactory mid-range throttle
response, but such readjustments should only be
made within the rarge where idling reliability and high
speed performance are not undnly compromised.
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