there isn’t any slop within the attachment
points as well as within the servo, servo arm,
and control horn. Similarly, make certain that
the linkage will not flex under a simulated load.
Never use fully threaded pushrods unless they
are installed in a carbon-fiber or similar sleeve
with metal nuts at each end to firmly secure
the sleeve in place.
With the servos and linkage installed and
properly connected, the next step is to program
each servo. While it may appear obvious that
control-deflection angles must be correct, I
cannot begin to tell you the amount of times
I have flown an aircraft and found that the
right and left ailerons differed with respect to
their maximum travel amounts. Using a digital
angle meter is beneficial, but at a minimum,
the use of any measuring device is sufficient.
Be sure the deflections are identical before
you program the differential so that you know
exactly how much deflection you have and
that each percentage change in the differential
function obtains an accurate deflection from
the desired control surface.
Some manufacturers offer aileron-deflection
amounts that incorporate differential. Set the
maximum travel points within the adjustable
travel volume (ATV) or endpoint adjustment
(EPA) functions so that travel is equal and
matches the maximum amounts given by
the manufacturer. Then, use the differential
program to tune all settings. Never change the
ATV or EPA settings to result in the desired
differential amount as severely different
percentages will change how the servo
responds to a given command.
Testing for differential will differ between
different model types. With a slow-flying scale
model, many sport pilots typically turn by
applying a touch of aileron input followed by
up-elevator. If they don’t use differential, the
increased drag on the down aileron will result
in a yawing tendency in an adverse direction
to the desired turn because a greater amount
of drag exists on the wing opposite the turn.
With an aerobatic model like the 70cc Mamba
bipe, I prefer to perform two different tests.
First, I climb to a high altitude, pull the throttle
to idle, and push to establish a 90-degree
downline. I then apply full left aileron input
and note any tendency for the aircraft’s nose
to move. Another test is to apply maximum
power, pull to a 45-degree upline, and apply
full left aileron input to perform a roll or two.
If the nose wanders to the left, you need to
reduce the downward-moving aileron with
differential; similarly, if the nose wanders to the
right, reduce the amount of up-aileron travel
with differential. The goal is to perform a roll, or
multiple rolls, so that the aircraft will stay on
the same vertical climb. If you prefer to perform
the 45-degree climb test, limit the number of
rolls performed. Airspeed may drop over time,
and depending on the power-to-weight ratio
of your aircraft, this will have an impact on
what differential value you feel is necessary.
Typically, you want more up-aileron
movement compared to downward movement.
Different hinge methods decrease the amount
of adverse yaw, but while most center-hinged
airplanes need differential that results in having
the top aileron move more than its downward
deflection, some composite airplanes that
have top hinges need reverse differential
(the aileron will travel more down than up as
the control-surface area differs from the top
surface to that of the bottom surface). You
can mechanically change the placement of
the linkage attachment point to incorporate
aileron differential into an aircraft. The more you
advance the servo-arm neutral position, the
higher the differential effect on the ailerons.
With an advanced computer radio, the easiest
way to incorporate aileron differential is to
apply a certain percentage in the “aileron
With the assumption that the construction
of the aircraft is structurally sound, with
When an airplane uses dual ailerons concealed within the wing, it is often easier to
program and verify functionality with the servos exposed. If each aileron servo is
connected to a different receiver port, you can program the differential.
Use of an incidence meter, like this one from Hangar 9, allows you to precisely
adjust and verify the wing and stabilizer incidence angle as well as the engine’s
Use of heavy-duty ball links and titanium turnbuckles offer a direct-linkage configuration.