Navaid Devices AP-1, Manual

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USER SELECTABLE GPS / LORAN SIGNAL GAIN
The standard error signal voltage for driving a CDI, which is also used for the autopilot,
is 150 mV at 5 miles off course (cross track error) for a GPS or LORAN. Some receivers
also have user selectable tracking gain that can be set to give a 150 mV error signal at
1.25 miles off course, and it may be called APPROACH MODE instead of CRUISE.
Another way of saying the same thing is that it may be possible to set your receiver
tracking sensitivity to 1/4 mile per dot instead of 1 mile per dot. If you have this feature,
try it—most customers prefer the increased tracking accuracy.
ADJUSTING THE SERVO SLIP CLUTCH - SAFETY VS. RESPONSE
The FAA requires autopilots for production aircraft to be set up so that a hard-over
failure will not cause the airplane to bank more than 60 degrees in the first 3 seconds
after recognition of failure. Since it takes about a half second to recognize a failure, the
time period might as well be 3.5 seconds.
For certified installations, the servo slip clutch is specifically set to meet this specification
for each aircraft model. A homebuilder, who is under no such obligation, may elect not to
perform the following test. Or he may perform the test and then deliberately increase the
torque setting to enhance performance under turbulent conditions. But he should at least
understand that there is such a thing as too much autopilot control, and that a fast
response to a hard-over failure can be dangerous. If you prefer a fast response, then you
should set conditions, such as minimum AGL altitude or formation flying, under which
the autopilot will not be engaged.
The worst case roll condition occurs at minimum speed and aft C.G. This could be a slow
airspeed if you are going to track the localizer. But we hope you just use the minimum
cruise speed—the autopilot is not required to work over as large an envelope, and you
can more gracefully afford a hard-over failure at cruise altitude and speed than at that
typically encountered while flying the localizer.
Check the maximum roll rate with the aircraft at aft C.G. and minimum speed. Disengage
the autopilot, then set the TURN CONTROL to full left mm. Hold the same airspeed as
you bank 60 degrees to the right, engage the servo, and measure the time it takes for the
aircraft to get to level. If the aircraft passes through level in less than 3.5 seconds, that is
too fast—the servo Torque Control nut on the output shaft needs to be loosened so that
the wind load on the aileron causes the clutch to slip sooner.
Before making a change to the servo, check for an adequate minimum roll rate with the
aircraft at maximum cruise and forward C.G. Rotate the TURN CONTROL back and
forth to see whether the aileron has adequate control. If the control is too weak, the servo
TORQUE CONTROL NUT needs to be tightened.