DGs and Autopilots

[Andrew Gideon]

How "particular" to an AP are DGs?

We're looking to replace a DG in an aircraft that has an [original] Cessna
Navomatic 300A (I think it is called) autopilot. In the not too distant
future, we'll be upgrading the AP (probably to an STEC unit, likely to the
STEC 20 (we already have the STEC 30 Alt installed)).

Would we need a new DG when we upgrade the AP?

- Andrew

[[email protected]]

Shouldn't. The output of autopilot DGs that heading bugs is
fairly standard. It's a transformer that has a null in its output
when the bug is under the lubber line.

Bill Hale.

How "particular" to an AP are DGs?


We're looking to replace a DG in an aircraft that has an [original]
Cessna
Navomatic 300A (I think it is called) autopilot. In the not too
distant
future, we'll be upgrading the AP (probably to an STEC unit, likely to
the
STEC 20 (we already have the STEC 30 Alt installed)).


Would we need a new DG when we upgrade the AP?


- Andrew

[Andrew Gideon]

wrote:

> Shouldn't. The output of autopilot DGs that heading bugs is
> fairly standard. It's a transformer that has a null in its output
> when the bug is under the lubber line.
>

Oh. I was looking at:

http://www.sigmatek.com/pages/specs/sigma_tek_directional_gyros.htm

and it looks to me like the DGs there are fairly particular for APs. Am I
missing something?

- Andrew

[Ron Natalie]

Andrew Gideon wrote:
> How "particular" to an AP are DGs?
>
> We're looking to replace a DG in an aircraft that has an [original] Cessna
> Navomatic 300A (I think it is called) autopilot. In the not too distant
> future, we'll be upgrading the AP (probably to an STEC unit, likely to the
> STEC 20 (we already have the STEC 30 Alt installed)).
>
> Would we need a new DG when we upgrade the AP?
>
> - Andrew
>
The STEC autopilots are rate based and get their control information
from a special turn coordinator. The DG heading bug is fairly generic
and only plays a factor to tell the DG a desired heading when in that
mode.

[Doug]

The output of the DG to the autopilot is not really standard. A lot of
DG's have no output at all. Get the DG that will be compatible with
your future autopilot. Contact STEC and get the specs. It makes a
difference, you need to make sure you get the right one.

Having said that, I would urge you to consider an autopilot that does
not use a DG. STEC makes them. They work very well. The big advantage
is if you loose vacuum, your all electric autopilot continues to work.
Big safety factor in IMC. Such autopilots have a "wings level" mode
where you point the plane in the direction you want, and then decouple
it. The plane just continues to fly the straight ahead. They also have
a feature where you can make slight corrections ("trim" the autopilot),
and use the autopilot to turn the airplane, by turning a knob on the
autopilot. It works well. Such autopilots couple to both the VOR head
and the GPS (they get their left and right from the indicator dial).
These autopilots are less costly, less complicated, don't require the
more expensive DG and work great. They continue to work without an AI
or DG or vacuum.

[Andrew Gideon]

Andrew Gideon wrote:

>
> How "particular" to an AP are DGs?
>

I found at least a partial answer. When ordering an STEC AP, one specifies
the DG. The STEC is "made to work" (whatever that means).

The avionics person to whom I spoke told me that he'd ordered STEC APs
configured to get HEADING information from DGs previously speaking to ARC
APs. That's my precise upgrade path, so I'm happy.

- Andrew

[Andrew Gideon]

Doug wrote:


> Having said that, I would urge you to consider an autopilot that does
> not use a DG. STEC makes them. They work very well. The big advantage
> is if you loose vacuum, your all electric autopilot continues to work.
> Big safety factor in IMC.

I know what you're saying, and I agree. Our current ARC AP model, and the
STEC model under consideration, are both rate-based. The DG is merely
providing HEADING information (ie. the setting of the bug).

- Andrew

[Thomas Borchert]

Andrew,

> The DG is merely
> providing HEADING information (ie. the setting of the bug).
>

And that is autopilot specific. We had to get a certain model of
Sigmatek DG for it to work with the STEC-50. Also, we now switched to
another model, which also delivers bootstrap heading info like a remote
compass would. We need that for the WX-500 we installed.

--
Thomas Borchert (EDDH)

[Andrew Gideon]

Thomas Borchert wrote:

> Andrew,
>
>> The DG is merely
>> providing HEADING information (ie. the setting of the bug).
>>
>
> And that is autopilot specific.

Apparently.

> We had to get a certain model of
> Sigmatek DG for it to work with the STEC-50.

I was told by a person that installs STECs for us that the STEC can be
"configured" to work with different DGs. He didn't say "all different",
though. We're upgrading from ARC units: Navomatic 300As. According to
him, we merely specify this when we purchase a new STEC.

> Also, we now switched to
> another model, which also delivers bootstrap heading info like a remote
> compass would. We need that for the WX-500 we installed.

Ah, this is something I didn't understand. What is "bootstrap heading
info", and how does the WX-500 (whatever that is {8^) use it? We've a
strikefinder in our aircraft. Does that mean we need "bootstrap heading
info"?

- Andrew

[Thomas Borchert]

Andrew,

> Ah, this is something I didn't understand. What is "bootstrap heading
> info", and how does the WX-500 (whatever that is {8^) use it? We've a
> strikefinder in our aircraft. Does that mean we need "bootstrap heading
> info"?
>

Depends on the model of Strikefinder. In order for the the Strikefinder to
orient its display correctly in relation to where the aircreaft's nose is
pointing, it needs some kind of heading info. The WX-500 (remote
"Strikefinder" with no display, working with Garmin 430 and other
displays) uses the heading info from a "bootstrap compass" as it is
normally found in HSI installations. This compass sends out a compass
signal to auto-align the HSI. It can also be used to align the
Strikefinder displays. Even if the WX-500 is displaying on a GPS, it
cannot use GPS derived heading signals to align itself - the reason being
that the GPS doesn't know your heading, only your track.

If you don't already have an HSI, installing one (including the bootstrap
compass) is really expensive. The "cheap way" to get a bootstrap signal
without actually having a bootstrap compass is to have a DG with a
bootstrap signal output (as made by Sigmatek). Of course, that signal is
not derived from a compass but rather from the manual setting of the DG -
if you don't align it with the compass manually and it precesses out of
the real heading, so will the bootstrap signal.

Hope I'm clear ;-)

--
Thomas Borchert (EDDH)

[Andrew Gideon]

Thomas Borchert wrote:

> Hope I'm clear ;-)

Very! I appreciate the explanation.

- Andrew

[[email protected]]

Fact remains, if they have a bug they are fairly standard unless they
are really
old Mitchells that used a capacitive system.

The century hsi uses a resistive output.

Otherwise, they can be interfaced. The C-300, if it's the one that has
bugs
on the DG is definitely compatible.

The shop might rather sell you a new one.
Bill Hale