REFLECTOR:Navaid Autopilot

[Brian Michalk]

Where does the Navaid get it's inputs from?

This is a classic coupled oscillation problem.

If you gyro lags the actual orientation of the aircraft, then the Navaid is
always going to correct (overcorrect) for a condition that existed in the
past.

If turbulence upsets our airplane, we want the autopilot to immediately
correct, just like we do.  However, unless there's a good PID algorithm in
the autopilot, it's going to correct nice and easy.  I doubt they have a CPU
controlling this, as it's got to meet a whole other set of rules for
certified installation.  I could be wrong, I don't know if the S-tec is
approved for certified aircraft.  Anyway, it could still be a hardware PID,
and PID's can be difficult to tune.  Back to turbulence.  If he have it be
nice and snappy in turbulence, then when we are in calm air, it will tend to
oscillate, because it is trying to quickly bring the plane back to level.
However, if the gyro is lagging the actual state, then the AP will
overcorrect, causing the oscillation.

It's actually a lot more complicated, but that's the gyst.  Other culprits
are slack in the control surfaces.  Even though some people have added
damping to the mechanical output, slack could have been the original
problem.  All you need to do is move the oscillating frequencies of the
aircraft and control system away from each other such that they tend to damp
each other out.

PID algorithms are pretty cool.
P---- proportional: move a proportional distance towards the target
	The main parameter that makes the PID work.  If I have 10PSI of pressure,
and an air cylinder moved a weight 50% to my target, set the pressure to
15PSI.
I---- integral: the longer I've been off target, move more forcefully
towards the target
	This one keeps one from being "almost" at the target.  If there's friction
in the system, you need a little bit of push to move small distances to get
right on top of the target.
D---- derivative:  The quicker I'm getting to the target (actually
increasing rate of error), move less forcefully.
	This one keeps us from overshooting the target.  If I am a mile away from a
stopsign, going 60miles per hour, my error rate is fairly low, it's the
error from second to second.  At a mile away, it's 1/60.  However as I
approach the stopsign without reducing speed, then the rate of error becomes
1.  I'll overshoot unless I pay attention to the error rate.

 Brian Michalk  <http://www.michalk.com>
Life is what you make of it ... never wish you had done something.
Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
musician, home-brewer, entrepreneur and mostly single


> -----Original Message-----
> From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
> Behalf Of Chuck Jensen
> Sent: Friday, July 11, 2003 7:48 AM
> To: 'reflector@tvbf.org'
> Subject: REFLECTOR:Navaid Autopilot
>
>
> I found the comments about the S-TEC and its oscillation phenomena
> particularly interesting.  I'm running a Naviad AP and Wing
> Leveler and see
> the same type of behavior, especially above approx. 8,000'.  When
> the WL is
> turned on, it rapidly oscillates L/R.
>
> I really didn't have a clue what was wrong and hadn't had it
> looked at yet,
> but now I wonder if this is, in fact, the same thing that occurs in the
> S-TEC.
>
> Has anyone had a similar behavior with the Navaid?   Solution?   Is this
> problem likely similar/same as with the S-TEC?
>
> Chuck
> N27GV
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