Or, "Everything you wanted to know about Nav-o-matic, but were afraid to ask."
DISCLAIMER - I can neither confirm nor deny any knowledge about anything I may say or do
First, realize there is physical reality of control position,
and then there is the autopilot's perception of that reality.
Zeroing the servos means getting those realities to match.
1. MARK CONTROLS FOR STRAIGHT AND LEVEL IN FLIGHT
Step one is to mark the yoke position precisely where you want it to be
during straight and level flight
A small magic marker, or a piece of paper taped above the yoke,
and some marks will do.
Before marking 'straight and level,'
let the airplane stabilize for a bit in level cruise at normal power.
My RSTOL is very sensitive to pitch, so marking off even a quarter inch is way too much!
Now you have marked the target for where the controls should be
when the autopilot is set to straight and level.
2. ZERO OUT THE AUTOPILOT FOR STRAIGHT AND LEVEL
Step two is to allow the autopilot to go to its electronic 'zero' position on the ground
A. Power everything off.
B. GYRO DISABLE
Back out the autopilot control head in the center console.
At its 'back,' facing forward in aircraft, slide the small gyro disable switch to its OPEN
The slide switch part should be away from the wires coming from it.
This temporarily removes any 'error/correction' signals from either the horizon or DG.
C. ACCESS THE SERVOS
The elevator servo is behind the larger access panel in the pilot side tailboom.
The aileron servo above an outboard access panel in the passenger side wing.
D. SPLIT SERVO HALVES
Split each servo in half using a flathead screwdriver.
A long thin small fat head screwdriver works best.
Leave the servo halves electrically connected. Let them dangle.
Each servo has four flathead screws, and splits diagonally.
Don't worry, then wont fall out.
Half of each servo stays with the airframe,
the other half is the electronics, clutch and servo.
E. PHYSICALLY SECURE YOKE TO STRAIGHT AND LEVEL POSTION
This will be the position the autopilot will synch to zero/cruise.
Secure the yoke control in its real-world zero / cruise position.
I use two clamps. Be creative.
Get it held RIGHT on the marks you made in flight
F. MATCH AUTOPILOT AND YOKE TO STRAIGHT AND LEVEL
Now power up the autopilot.
With no 'error/correction/ signal,
you will hear the servos each move (whine) to their own zero position,
Now the servos will be at 'zero/rest' when the controls are in actual cruise position
G. RE ASSEMBLE
Power it all down. Put the servos halves back together.
Double check each of the four screws on each servo.
H. CONFIRM
Power it back up (with gyro-disable switch in the head still set to zero).
After a few second's delay, you will hear the autopilot clutches 'click' engage
and should hold the controls at the cruise position.
If not repeat steps above.
I. RE-ENABLE GYRO SIGNALS AND RE-ASSEMBLE
Power back down, reset the gyro=disable switch in the head, re-assemble, etc.
Straight and level in the autopilot now matches straight and level with the controls!
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MORE ON SERVOS
The servos have two safeties
1. SLIP CLUTCH -
An internal slip clutch, which is what you hear engage with a 'click'
A few seconds AFTER you turn on the autopilot
Inside the half of each servo that comes out, you will see the clutch
It can be tightened and loosened using two allen wrenches
stuck in holes you will see, moving in opposite directions
This tightens and loosens two rotating flat faces against a flat spring in between them.
The clutch should be tight enough for the autopilot to move the controls,
but loose enough that if you force the controls against the autopilot
the clutch will slip, rather than break the shear pin described below
2. SHEAR PIN
A shear pin is located in the toothed gear
that connects each servo to the actual controls, via a small chain drive
If when testing on the ground you hear the servo 'click,' and even rotate,
but it doesn't clamp at all, then the shear pin is probably sheared.
To replace requires getting access to the half of the servo
that normally stays connected to the control by its drive chain.
(The half that controls the aileron can be difficult to remove entirely.
You just need it free enough to undo the pin. No need to totally remove the servo),
Then with the gear itself free, knock out the old shear pin, and replace.
Cessna and autopilot shops may have the precise part. They may not.
Aluminum tubing can also work in a pinch,
Wall thick enough to hold against the clutch rotating, but thin enough to shear if need be
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When I have a chance back at the office, I will review which adjustment pots
on the control box overhead the third row adjust what, and post.
Any questions, feel free to call or email!
David Wartofsky
bigcheese@potomac-airfield.com
301 248 5720