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Re: Autopilot and Stuff

On Sat, 30 May 1998 09:05:50 +1100 gpoole@zeta.org.au (Mr Greg Poole)
>Has anybody had any first hand experience with the fitting and use of 
>Model 30 Autopilots with altitude hold in Velocitys?


I have not had the fortune to install a S-TEC 30 series (or 20 series for
that matter!) is a Velocity. One of the most noticeable 'problems' with
all (Century and S-TEC) two axis autopilots installed so far in the
Velocity has been the pitch stability in altitude hold when going from
one person in the aircraft to two or more. The problem is the pitch
feedback gain from the gyro to the servo.

Because of the light wing loading and the size of the elevator, with only
one person in the plane (the Pilot) a little bit of effort (read servo
torque) goes a long way. When there is a greater load on the elevator,
more "effort" is needed to "lift the nose".

For arguement sake in a normally working autopilot, if during flight with
two or more people, one degree in pitch attitude off of straight and
level produces 1 volt to the servo motor drive and 5 degrees of pitch
attitude off straight and level produces 12 volts.
These voltage changes properly maintain a straight and level aircraft
based on the sensitivety of the nose. Now, remove approximately 50% of
the wing loading to the canard. Now, with the same  "amount" of
correction, but with half the effort needed to maintain straight and
level, the nose will tend to overshoot, thus causing oscillations. That
is why most pilots with altitude hold wind up flying with some amount of
additional weight on the copilots side or even without the altitude hold

The only reasonable solution is to be able to select a predetermined,
fixed gain for single or multiple passengers. Since this gain is 'fixed'
at the factory with a resistor (usually), it would require being able to
externally select the proper value of the resistor for the amount of gain
required. The autopilot folks are not quite yet willing to give the
average pilot the ability to monkey with thier equipment like that. The
average pilots surviving widow's attorney may have something to say and
do about that, if you know what I mean.

I should stress. these are arbitrary numbers. Most autopilot systems now
use PWM, or Pulse Width Modulation, which results in different durations
of pulsed +12 volts to the servo motor. I know that Century does. or did
at least, use +/- variable voltage to the motor for direction and torque

Other than that, both autopilot systems provide very good flying
characteristics. I think those who know me, know I prefer the Century
system because it allows for Glideslope coupling and attitude coupling.
Turn the Century on during climbout and it will maintain attitude.  The
S-Tec won't. Intercept the glideslope with the Century, it will
automatically disengage altitude hold and track the glideslope until the
pilot disengages the autopilot. I have actually flown one (Century
autopilot) down to (not into!) the runway in a Cessna 414 before. It 
'flared and trimmed' the airplane while all we did was manipulate the
throttles! The S-Tec will fly right through the glideslope without pilot

Hope this is of some help!

Safe and Speedy Construction!


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