Autopilot install.

Andrew Gideon wrote:
Doug wrote:


But really, you don't need DG coupling. It
makes failure modes MUCH more complicated.

Having an autopilot coupled to a GPS is the cat's pajamas.


At least around here, I spend a nontrivial amount of time on vectors.
That's much easier with an AP connected to the DG.

A vacuum failure that goes unnoticed while one is still on HDG mode would
mean either circles or a bad heading. Not a good thing, but still not the
same as had the AP been using the (now failed) AI.

Can an AP get heading information from the GPS? It can get track; is that
close enough?

The GPS heading information is taken as differences
of position fixes. It is prone to pretty bad errors,
and probably of too low quality for AP reference.

--

Tauno Voipio
tauno voipio (at) iki fi

Tauno Voipio wrote:


The GPS heading information is taken as differences
of position fixes. It is prone to pretty bad errors,
and probably of too low quality for AP reference.


I have flown AP equipped planes that take their steering from the CDI...
which in some cases is driven by a VOR/LOC and in some cases driven by a
GPS. Direct heading info, no.. deviation from desired course... yes.

Same succeptibility to GPS errors.. of course.

Dave

Dave S wrote:


Tauno Voipio wrote:


The GPS heading information is taken as differences
of position fixes. It is prone to pretty bad errors,
and probably of too low quality for AP reference.


I have flown AP equipped planes that take their steering from the CDI...
which in some cases is driven by a VOR/LOC and in some cases driven by a
GPS. Direct heading info, no.. deviation from desired course... yes.


The radial / GPS track following in an autopilot has
three control loops:

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

- next loop controls heading taking reference from
heading gyro,

- outermost loop tracks the navaid course difference.

The outermost loop sets the required heading for the
middle loop, and the middle loop sets the required
roll angle for the innermost loop.

The GPS heading is ill suited to the middle loop
due to the inherent noise enhancing property of
a differencing method. The noise may be attenuated
by filtering, but then the heading reference is
too slow for acceptable control loop stability
and speed.

--

Tauno Voipio
tauno voipio (at) iki fi

snip

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

Wrong....

S-tec AP's do not take any info from the AI. They are rate based not
attitude based and get their roll info from the Turn Coordinator.

The GPS Steer function is a true wonder to behold. Both in simplicity
and accuracy. You know not of which you speak... No offense... :-)

Jon Kraus
'79 Mooney 201
4443H @ TYQ



Tauno Voipio wrote:

Dave S wrote:



Tauno Voipio wrote:


The GPS heading information is taken as differences
of position fixes. It is prone to pretty bad errors,
and probably of too low quality for AP reference.


I have flown AP equipped planes that take their steering from the
CDI... which in some cases is driven by a VOR/LOC and in some cases
driven by a GPS. Direct heading info, no.. deviation from desired
course... yes.



The radial / GPS track following in an autopilot has
three control loops:

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

- next loop controls heading taking reference from
heading gyro,

- outermost loop tracks the navaid course difference.

The outermost loop sets the required heading for the
middle loop, and the middle loop sets the required
roll angle for the innermost loop.

The GPS heading is ill suited to the middle loop
due to the inherent noise enhancing property of
a differencing method. The noise may be attenuated
by filtering, but then the heading reference is
too slow for acceptable control loop stability
and speed.

My autopilot can be coupled to the GPS. It gets it's left and right
from the GPS's OBS (sort of a VOR head for a GPS). In calm air it is
PERFECT. The track lines are straight as they can be. Outflies any VOR,
NDB coupling or anything else. GPS is accurate to a few feet right and
left. In over 2000 hours of flying behind my GPS, it has never failed
on me for any length of time. (A very few lockups and lost signals due
to military jamming, but VERY FEW and never for more than 5 minutes).
GPS coupled autopilot is the way to go. Nothing else even comes close.
Couple your autopilot to the GPS if you can couple to anything. It's
phenmenal.

Jon Kraus wrote:
snip

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

Wrong....

S-tec AP's do not take any info from the AI. They are rate based not
attitude based and get their roll info from the Turn Coordinator.

The GPS Steer function is a true wonder to behold. Both in simplicity
and accuracy. You know not of which you speak... No offense... :-)


It still does not take the short-term heading reference from the GPS,
but it uses the turn coordinator as a replacement for the horizon
roll axis. As is well known, the TC data is part roll speed and part
heading rate, and it can be integrated to create a rudimentary
replacement for the roll reference.

The GPS steering then functions as a setpoint value for the
roll servo. The basic flight dynamics are still there.

The digital signal processing gives an advantage over
the older analog units in thyet it is possible to create
stable long-term (over a couple of seconds) integrators
with the digital way.

--

Tauno Voipio
tauno voipio (at) iki fi

I don't understand GPSS. Why is this better than simply setting a GPS to
follow a CDI, and then setting that CDI to take GPS data?

- Andrew

Andrew Gideon wrote:
I don't understand GPSS. Why is this better than simply setting a GPS to
follow a CDI, and then setting that CDI to take GPS data?

They belong to different layers:

- bottom layer is to keep wings level (or properly banked),
- middle level is to fly a heading,
- top level is to follow a track.

The track following uses heading flying as the tool.
The heading flying uses suitable banking as the tool.

The autopilot can directly control the ailerons
to get the proper bank. For setting a required
banka angle several aileron movements are needed.

The flying of a heading needs several bank angles
to get and keep the heading.

The flying of a track/radial needs several headings
to follow the track.

In control engineering terms, the order of a control
system to directly follow a track with aileron
control will be too high to be realizable.

The CDI difference is what requests heading
changes to follow the track, but to do it
properly, the lower layers (heading control
and roll control) are needed.

HTH

--

Tauno Voipio (MSEE, avionics specialist, CFII)
tauno voipio (at) iki fi

Jon Kraus wrote:
snip

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

Wrong....

S-tec AP's do not take any info from the AI. They are rate based not
attitude based and get their roll info from the Turn Coordinator.

The GPS Steer function is a true wonder to behold. Both in simplicity
and accuracy. You know not of which you speak... No offense... :-)


Yep, watching the STEC track the GNS480 in the GPSS is a true wonder.

Doug wrote:
My autopilot can be coupled to the GPS. It gets it's left and right
from the GPS's OBS (sort of a VOR head for a GPS).

The GPS CDI you mean. The OBS is largely unused in the GPS.

In calm air it is
PERFECT. The track lines are straight as they can be. Outflies any VOR,
NDB coupling or anything else. GPS is accurate to a few feet right and
left. In over 2000 hours of flying behind my GPS, it has never failed
on me for any length of time. (A very few lockups and lost signals due
to military jamming, but VERY FEW and never for more than 5 minutes).
GPS coupled autopilot is the way to go. Nothing else even comes close.
Couple your autopilot to the GPS if you can couple to anything. It's
phenmenal.

The GPSS will track turns in the GPS track better than just following
the CDI deflection.