When setting up the travel of the throttle and/
or choke arms, ensure that you have a clear
view of the carburetor and each corresponding
butterfly. If the aircraft is on the bench, it could
be difficult to see the full motion and you might
feel that they have obtained maximum throttle
when, in reality, the butterfly can move another
5% or 10%.
I prefer to use masking tape or similar
material to cover the open areas. After
the modifications are complete, use a
vacuum and confirm that debris is clear
of the engine before removing the tape
from the cylinders and carburetor.
You could remove the throttle arm
and perform the required modifications
away from the engine, and similarly
remove the engine before working on
the airframe. Keep the engine in a dust-free area. Remember that when using
a drill and a Dremel, dust and particles
On my Hangar 9 Composite QQ Yak
54, I opted to use a high-quality throttle
servo and omit a servo to actuate the
choke arm. I fabricated a pushrod that
was secured to the choke arm using a
4-40 ball link that was long enough to
extend approximately 1/2 inch under the
bottom of the cowling.
If using a pushrod, bend the last 1/4
inch approximately 90° to allow you to
easily grab the choke linkage. If you are
using a titanium turnbuckle in place of
a pushrod, fasten a locknut at the end of
the turnbuckle. For cosmetic purposes,
apply a piece of heat-shrink material
over the linkage.
Some modelers prefer to have a
servo actuate the choke arm instead of
a mechanical linkage. If this is the case,
follow the practices previously outlined.
Proper Programming 101
Because the goal is to obtain a
perfect linkage setup that allows for
linear response in the throttle stick
movement, a few basic fundamentals
must be followed. Begin by making
sure that all subtrim values are at zero
and that you have a clear view of the
carburetor and each butterfly.
Disconnect the throttle or choke
linkage from the corresponding arm and
note the direction of travel from the
servo. In the case of the throttle, move
the throttle stick to 50% and move the
linkage so the throttle butterfly is 50%
open. Secure the arm in place.
Slowly move the throttle stick
to 100% and make any required
adjustments to the adjustable travel
volume (ATV) function (also known
as end-point adjustment), so that no
binding occurs. Do the same for the
Ideally, the ATV value is maximized
at both the high and low settings. If
the percentage is only at 90 (Spektrum
radios offer a maximum value of 150%
and Futaba offers 140%), begin to move
the linkage inward on the servo arm and
readjust the settings.
During the programming process, if
you find that a particular servo does
not return to the same position, study
the linkage setup so no friction exists
and the servo is not trying to “drive” the
pushrod to an angle that is not allowed
by the carburetor.
If you are also using a choke servo,
perform the same steps for the choke.
The only difference is that the choke
servo should be assigned to either a
switch or a dial. Most pilots prefer a
dial to prevent accidental activation of
the choke while the airplane is in flight.
You have now learned about the
importance of proper setup techniques
and how something that could be
overlooked, such as the throttle or choke
linkage, is extremely important for a
reliable result and consistent aircraft
performance. Above all else, remember
to learn from each experience, ask for
advice from fellow skilled pilots, and
enjoy this fine sport!
Until next time, fly hard!
104 Model Aviation APRIL 2017 www.ModelAviation.com