In message <005901c126e9$5cf85aa0$6e820b18@plstn1.sfba.home.com>, Dave
DeFord <davedeford@home.com> writes
>Hi Folks,
>
>N135TD tends to oscillate slowly in altitude when the stick is left free.
>This is most noticeable when the Navaid autopilot is on, since there is a
I wouldn't touch anything on the aircraft until you are certain that any
control parameters accessible in the Navaid have been optimised. Any
feedback control system tends to oscillate unless the control parameters
have been matched to the system being controlled. In industrial
controllers it is usual to have three control parameters:
Proportional which generates a signal proportional to the difference
between the desired value and the actual value (the error signal). In an
aircraft control system there would also be limits to the commanded
pitch changes.
Integral which feeds in a signal continuously as long as the system is
out of balance. Without an integral term a new controlled state could
not be achieved accurately because the proportional signal would be
needed to generate a changed control signal.
Derivative outputs a signal proportional to the rate of change of the
error signal. Although it is sometimes possible to avoid using
derivative action, this parameter can overcome oscillation.
Kenneth Whiteley
|