Sub Trim Usage And Mechanical Advantage; Sub Trim; Mechanical Advantage - JR X9303 2.4 Instruction And Programming Manual

SUB TRIM USAGE AND MECHANICAL ADVANTAGE

SUB TRIM

Sub Trims are intended for relatively minor adjustments
to servo linkages and not for major trim adjustments to
the aircraft. Using excessive sub trim percentages can
cause a loss in servo travel in one direction, where the
servo reaches its travel limit and stops moving before the
control stick is fully deflected.
Linkage is 90 degrees to servo arm
Linkage is 90 degrees
to control horn
Servo arm is 90 degrees to servo

MECHANICAL ADVANTAGE

Mechanical Advantage is a very important concept when
dealing with larger aircraft. It refers to the leverage that
the servo can exert on the control surface. Since the
control surfaces are rather large, it is important for the
servo to have enough mechanical advantage or leverage
to control them, regardless of the servo's rated torque.
A large amount of torque is of little value if there is not
enough leverage to use it. Insufficient leverage can lead to
control surface flutter (usually a catastrophic event) and
blow-back, where the air flow pushes the control surface
backwards resulting in mushy or no control at higher
speeds.
There are two ways to increase the mechanical advantage
of the servo. One is to make sure that the control horn
device, whether it be a horn as shown in the illustration
above or a bolt with a ball-link fastener, is long enough.
The horn is the lever that the servo uses to control the
surface. The longer the horn, the more leverage the servo
has.
The second way to increase the mechanical advantage
for the servo is to attach the linkage at the servo arm as
far inward (towards the servo arm retaining screw) as
possible while still providing enough throw. This takes
leverage away from the control surface by providing it
with a shorter lever to work against the servo.
The diagram below illustrates an ideal servo/linkage setup
when the servo is at neutral (no sub trim and digital trims
centered). Notice that the servo arm is positioned at 90˚
or perpendicular to the servo. Also note that the linkage
or rod is attached at 90˚ to both the servo arm and the
control surface horn. This setup will result in the same
amount of throw in both directions (0 differential throw).
If the servo cannot be mounted parallel to the linkage/rod
then just make sure the servo arm is at 90˚ to the control
rod when the servo is at neutral.
Ensure that the attach point is the same distance from the
hinge line for like surfaces (two Ailerons, two Elevators
and two Rudder horns). If the attach points are not the
same distance from the hinge line there will be unequal
throw and it will be more difficult to synchronize the
surfaces for equal deflection. This is especially critical
for the Rudder where two servos are attached to the same
surface – unequal throw will cause the servos to fight one
another causing excessive battery drain, and in severe
cases may cause servo damage.
Always try to use the maximum amount of Travel (100%)
that the radio provides. Don't reduce the percentage of
travel in the radio. Instead, move the linkage further away
from the hinge line at the control surface and/or move
the linkage inward on the servo arm or use a shorter
arm. If you use high percentages of travel, you maintain
resolution (fine movements of the stick result in fine
positive movements of the control surface).
Greater
Mechanical
Advantage
Greater
Mechanical
Advantage
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