Trio Avionics EZ Pilot - v 1.9 - Manual (Page 31)

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7.3 Selecting a Site for the Servo
For most aircraft, it’s relatively easy to find a suitable site for locating the crank arm
servo. The length of the pushrod and, to some extent, the angle it makes with the driven
element are user selectable.
The rod end bearing allows some misalignment, usually about 8 degrees, between the
servo pushrod and the plane of rotation of the crank arm. This limit on angular
displacement often determines the minimum pushrod length. Any side-to-side
movement, such as may be caused by elevator action being mixed with the aileron
movement at the stick, must not jam the rod end bearing. A suitable hard point must be
found, or built, for mounting the servo. The mounting place needs to be as accessible as
possible, and there must be a means of linking to the aileron control system.
The mounting place must be strong and rigid – conceivably a lateral force of up to 50 inch
pounds could be encountered. If, for example, you need to mount the servo on the skin
of an airplane, it will likely be necessary to use additional bracing or a doubler to provide
appropriate rigidity. You do not want the push-pull of the servo to fatigue the metal that
holds it.
When mounting the servo, be careful not
to drill mounting holes into a wing spar or
other load bearing member. It may be
best to construct a wooden, composite or
metal mounting plate, place and bond
machine screws so that they will interface
with the case mounting tabs, and then
secure the assembly into place. Once the
assembly complete, the servo base
mounting tabs will fit down over the
screws. Nuts can be tightened down to
secure it. The long screws shown allow the servo to be raised and lowered simply by
putting additional nuts on them so the servo is elevated above the plate. For a composite
aircraft installation, the bottom of the plate (shown above) should be floxed and the holes
in the plate filled with flox before applying fiberglass layers over it to secure it to the
fiberglass structure.
Usually the servo pushrod will terminate on a control stick, a bellcrank, or perhaps a lever
sticking off of a torque tube. But in some cases (EZ’s for example) it is more convenient
to terminate on an aileron control pushrod, in which case care should be taken to keep
the pushrod from being free to rotate. (To repeat, the pushrod should NOT rotate.)
The reasoning for this is as follows:
The servo pushes or pulls on a saddle bracket mounted on an aileron control pushrod,
the attach point being offset approximately an inch from the centerline of the control
pushrod. If the two pushrods are
not perfectly aligned, the control
pushrod may rotate instead of
moving laterally. Rotating, instead
of moving, has the effect of putting
play, or slack, into the linkage and
will result in “hunting” while tracking.
The control movements are so
Note: A longer aileron crank radius dictates a correspondingly longer radius at the drive end. The
ailerons must travel from aileron stop to aileron stop within the range of movement allowed by
the servo stops which limit crank arm rotation.