sure that all of the sticks and trims are
in the neutral position. You can verify
the trims using the display and listening
for the tone change when the trim is
With that task completed, take a
close look at the relationship of the
servo arm and the pushrod attached to
it. For 99% of all setups, you want the
pushrod to connect to the servo arm and
create a 90° angle. You can unscrew the
servo arm screw, pull off the arm, and
reposition it to get it as close as possible.
This ensures that the servo’s motion will
have the same effect (deflection angle)
on the control surface as you go from
one side of neutral to the other.
In the case of elevator control, if you
move the stick 1 inch to either side of
neutral, you should get the same angle
change on the control surface. This
will make the aircraft responsiveness
symmetrical throughout total travel.
This basic starting point is required to
get the most out of future adjustments.
If you find that the servo arm is not
perfectly at 90° to the servo and pushrod,
then you need to go into your sub-trim
menu and adjust the channel. You will
find that this is a very fine adjustment,
and you can be extremely accurate.
Go through the rest of the controls to
make sure they are the same. In the case
of glow or gas-powered models, make
sure the carburetor arm is also at 90° to
the pushrod when at neutral stick or half
throttle. You might even need to make
an adjustment to the carburetor arm
position to achieve this. Check with your
engine instructions for how to do this.
Now that the servos are working in
the proper direction and fine-tuned to
exact center, we want to adjust the total
travel of the servo to get the correct
amount of travel at the flight controls.
Review the travel amounts that are
listed in the instructions that came
with the aircraft. If you don’t have
instructions, then you have to use your
best guess, or ask another modeler who
has the same or a similar airplane, for
recommendations on control surface throw.
After you decide on the amount, it’s
time to do some measuring. There are
several tools out there that you can
purchase to check control throw and
they all have advantages, but for now,
With the radio turned on and
everything centered, check the elevator
position to be sure it is exactly neutral.
If not, adjust the clevis in or out until
it is in neutral. Get your ruler and align
the elevator’s trailing edge on an inch
or millimeter mark so you can measure
how far the control will move with stick
movement. Move the elevator stick
on the transmitter to full up-elevator
and measure how far the control
surface moved. Check this against the
If the travel is too much, try moving
the pushrod connection to the next hole
outward on the control horn on the
elevator. You can also move the pushrod
connection at the servo horn in toward
the center of the servo arm. Either one
or both of these adjustments can be
done to get close to the desired amount
To increase the throw, move the
pushrod connection in the opposite
direction. You want to be roughly the
same distance from the center of the
servo to the pushrod as you are from the
hinge line of the elevator to the pushrod
connection. By trying different positions,
you will find a location that gives you
nearly the desired travel on the elevator.
Now that you are close, you can
use the endpoint adjustment feature
to adjust the travel to the exact
specifications. Most radios will typically
default to 100% travel in both the up
and down side of each channel, which
can be individually adjusted. You might
think that this is maximum available
travel, but most are adjustable up to
125% and some even up to 150%. This
allows you to increase above 100% to
get the travel properly adjusted.
Keep in mind that you want to keep
the endpoint adjustment between 75%
and 120% in order to have the best
resolution for the servos. If you need
more or less throw than these numbers, go
back and change the pushrod connection
point on the servo arm or control horn
as needed, and repeat the process.
Following the radio instructions, access
the endpoint adjustment program and
select the elevator channel. Move the
transmitter stick to give full up-elevator
and measure the travel. If you need
more travel, increase the percentage. If
you need less, decrease it.
Do this for both the up and down side
of the elevator, and once it is completed,
recheck it. When you are happy with the
setting, move to the rudder and throttle.
For glow or gas engines, you want the
full-throttle setting to fully open the
carburetor, but not stall the servo.
Listen for a buzzing sound from the
servo that would indicate that you have
gone past the stop for the carburetor and
the servo is no longer moving. Reduce
the travel until the buzzing stops and
make sure the carburetor barrel is fully
open at full throttle.
For low throttle or idle, you want
to move the stick to low throttle and
completely close off the opening in the
throttle barrel using the sub-trim. Now
open it back up to show approximately
1/16 inch of opening. This should be a
starting point for idle that can be finely
adjusted using the trim lever next to
the stick when you run the engine. For
electric motors, follow the instructions
on the ESC to set the throttle travel.
RB3 Model Aviation JULY 2017 www.ModelAviation.com