Above: Both weights
have been secured
inside the ailerons.
Left: If you use two
servos on a given
control surface, ensure
that the linkages
are tight, no slop
exists, and that both
servos move precisely
together at all times.
the receiver has enough channels to mix
these servos to work on one channel, or
by using a servo synchronizer.
Servos can develop slop within
their gear trains. Slop may also come
from the servo arm and/or linkage.
Regardless of where the slop is coming
from, this could result in flutter
because the servo can oscillate!
Often, a servo responds to a slight
bump and a rapid oscillation develops.
When force is applied to the servo,
this will stop but can also cause flutter.
In this instance, counterbalancing a
surface can be beneficial.
A builder can mass balance a control
surface by adding weight in front of
the hinge line on the counterbalance.
Depending on the size of the control
surface, as well as the size of the
aerodynamic counterbalance, a
considerable amount of added weight
may be needed.
The reason for this is simple: the
control surface’s small area of the
counterbalance, located in front of the
hinge line, can add weight and is small
compared with the size of the actual
If you disconnected the servos from
the control surface, the balanced
surface should remain level with the
flying surface. At that point, you should
be able to lift or push the control
surface to any position and it will
return to the neutral position.
Unbalanced Surfaces Fall
I decided to use one JR8911HV
servo per aileron—which gives nearly
500 inch-ounces of servo torque—for
my Desert Aircraft 120cc-powered
CARF-Models Edge 540. CARF-Models advises the builder to partially
balance the elevators and the ailerons
by adding 15 and 20 grams of weight in
each respectively, especially when using
one servo. Any gear slop can result in
flutter at high speeds.
For the semi-counterbalance weight
installation, I gathered the necessary
items which included lead shot
(available online), 3/8-inch-diameter
carbon tube, 1/32-inch balsa sheet, Zap
CA glue, Zap five-minute epoxy, milled
fiber from Bob Violett Models (BVM),
a scale, and a Dremel tool.
After I cut the carbon tube so that
it was slightly smaller than the width
of the counterbalance, I glued 1/32-
inch balsa cut to a 3/8-inch diameter to
132 Model Aviation OC TOBER 2013 www.ModelAviation.com