Servo Setup & Adjustments
28
In the steps 37 through 42 over the next few pages the pushrod hardware will be mounted to the servos
horns and ultimately the pushrods themselves. Each step is well described but lets take a few moments to
cover a few basic points on setting up individual servos. By this time the radio will have been charged
overnight. Recheck that all the servo trims are centered. Each radio manufacturer makes servo horns in
different shapes: round, in a cross and sometimes a star, each giving a selection of hole patterns to choose
from.
What we are looking for is equal travel both in the left and right rotation of the servo, this becomes com-
plicated by pushrods attaching to the servo at an angle. To correct this we recommend an angle for the
hole location. For elevator and collective, we suggest 10-15 degrees to allow for this "linear" geometry to
be set up. A range is given to make it easier for you to just choose an available angle, if this is not pos-
sible, simply lift the servo horn off the servo and press into place one "notch" back ( counter-clockwise ).
Choosing the correct servo horn only involves whether
the particular arm or wheel has the correct hole at the
recommended distance measured from servo center to
hole center, off by 1/2mm is ok. When using cross
horns, many times the remaining arms need to be
trimmed off to avoid binding on another servo like
throttle and collective. When two pushrods need to be on
the same horn but each at an angle, the round wheels are
best suited. For wheels that are not predrilled for offsets,
measure and drill your own holes. Remember to origi-
nally set the wheels or arms centered on the servo.
Futaba
cross horn
JR round
horn
Airtronics
cross horn
10 mm
radius
We strive to have all the control surfaces setup linearly, this means that as
the control stick is moved an equal distance from center that the servo
will move a corresponding equal amount of travel. Although this is not
clear now, this will become very clear when adjusting the throttle and
collective servos movements, commonly called the throttle and pitch
curves. At this time it is worth mentioning the danger of the ATV func-
tion, Adjustable Travel Volume has solved many setup problems while at
the same time has created new ones. Most commonly used for throttle to
easily keep the servo from binding, the thing to remember is to keep the
upper and lower values should have the same. If the final values are
different by more that 10 points then a mistake was made in setting the
mechanical limits. Time to go back and recheck.
90˚
10-15˚
Servo horn shown offset
one notch back on the
output shaft.
The goal in the end after all the servos are mounted is to have the
swashplate sit level or at 90 degrees to the main shaft and have the
swashplate move equally fore, aft and side to side. The swashplate
will also travel up and down as the collective servo is moved, it is
important that in the upper position the washout hub does not
contact the rotor head block while at the same time the swashplate
remains above the top of the frames to avoid cyclic interferrence.
Don't use the pitch gauge until you have installed all the servo
horns and pushrods. Pitch settings usually are the last step in
completing the basic setup for the helicopter just prior to making
the first test flights.
After installing the aileron
and elevator pushrods, the
swashplate should sit level.
