GREAT PLANES Pitts M-12S Instruction Manual page 35

Proper Pushrod Hookup; Avoiding Flutter, Maximizing Servo Output Torque
SERVO ARM
OFFSET
When connecting pushrods and setting up your control throws, it is critically important to use proper pushrod geometry—that is the
distance from the pushrod on the servo arm to the center of the output shaft (servo arm offset) compared to the distance from the
pushrod on the control horn to the pivot point (control horn offset).
One particularly dangerous situation arises when the pushrod on the servo arm is too "far out" and the pushrod on the control horn is too
"close in." This setup is usually chosen by pilots who are trying to achieve maximum, "monster" control throws for 3D fl ight. But with your
pushrods set up this way, any free play (slop) in the linkages or servo will be greatly magnifi ed, possibly causing destructive control surface
fl utter. Additionally, if you have to turn your ATVs way down for "normal" throw, the result will be poor resolution and poor servo holding/
centering capabilities. More importantly, too much force may be transmitted back to the servo, possibly causing control surface blowback,
stripped servo gears or stripped servo arms—the latter two likely causing a crash.
Here is an optimum pushrod setup—the pushrod is "close in" on the servo arm and "far out" on the control horn. This situation gives the
greatest mechanical advantage of the servo over the control surface which will increase the servo's centering capabilities and output
torque, minimize any free play in the system and allow high ATV settings for optimum servo resolution and positive control "feel." Note:
When the pushrod is "close in" on the servo arm, make certain the servo arm can travel through its full range of movement without the
pushrod (or clevis or other type of connector) interfering with the servo arm, output shaft or servo case.
Move the pushrod farther out
on the servo arm...
If the optimum situation doesn't provide enough control throw, the pushrod may be moved inward on the control horn, but it's better to go
farther out on the servo arm because this will introduce less free play than the alternative. Only after moving the pushrod all the way out on
the servo arm, if you still can't get the throw required, you'll have to resort to moving the pushrod closer in on the control horn. Note: If you
have a computer radio, it is always desirable to set your ATVs to 100% (or as near 100% as possible to achieve the control throw required).
If setting up a model that requires extraordinary control surface throw (for 3D fl ying for example), start by "maxing-out" your ATVs (typically
130% -- 140%). Then, the dual rates in your "normal" fl ight mode will still be acceptably high (70% -- 80%) for good servo resolution.
Long
distance
Pushrod far out
on the servo arm...
PREFERRED PUSHROD HOOKUP
"Closest in"
on servo arm
ACCEPTABLE PUSHROD HOOKUP
Note: Your control horn may vary.
Pivot point
EXTREMELY DANGEROUS PUSHROD HOOKUP
...pushrod close in
on the control horn.
"Farthest out"
on control horn
...But leave the pushrod in the farthest out
location on the control horn.
35
CONTROL
HORN OFFSET
Short
distance