DG precession

[Marty from Sunny Florida]

Hey everyone.

I have a C152 with good vaccuum, but the Directional Gyro precesses at an
abnormally high rate. What's the prognosis? Can it be fixed, or just
replaced? What can I expect to pay for a new DG?

Thanks,
Marty

[Ron Natalie]

Marty from Sunny Florida wrote:
> Hey everyone.
>
> I have a C152 with good vaccuum, but the Directional Gyro precesses at an
> abnormally high rate. What's the prognosis? Can it be fixed, or just
> replaced? What can I expect to pay for a new DG?
>
New, perhaps around $500...should be able to get it overhauled for
about half of that.

[Dave Butler]

Marty from Sunny Florida wrote:
> Hey everyone.
>
> I have a C152 with good vaccuum, but the Directional Gyro precesses at an
> abnormally high rate. What's the prognosis? Can it be fixed, or just
> replaced? What can I expect to pay for a new DG?

Before you spring for a new HI be sure you aren't just seeing a magnetic compass
in need of swinging. Align the HI with a published, known heading from
airnav.com or from the A/FD. Fly around for a bit *without* resetting the HI to
the mag compass. Return to the same runway and check it again. If it's still
correct, have your magnetic compass swung. If not, replace the HI or look for
some reason the HI's not performing correctly. Got vacuum?

Anyway, my main point is that a magnetic compass in need of swinging can look
like HI precession.

[Dale]

In article <1111697079.782652@sj-nntpcache-3>, Dave Butler >
wrote:



> Anyway, my main point is that a magnetic compass in need of swinging can look
> like HI precession.

Hmmm, hadn't thought of that. Was thinking a compass needing swung
would indicate a "fixed" heading...can't see why it would like like the
DG was precessing.

A real quick way to check the DG is to fly a ground track using your
GPS, set the DG to the ground track...see how it does..

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[Dave Butler]

Dale wrote:
> In article <1111697079.782652@sj-nntpcache-3>, Dave Butler >
> wrote:
>
>>Anyway, my main point is that a magnetic compass in need of swinging can look
>>like HI precession.
>
> Hmmm, hadn't thought of that. Was thinking a compass needing swung
> would indicate a "fixed" heading...can't see why it would like like the
> DG was precessing.

Because the mag compass deviation (is that the right word?) is generally
different on different headings, so as you keep changing headings, you keep
resetting your perfectly good HI to match it, all the while muttering about how
your HI is precessing.

> A real quick way to check the DG is to fly a ground track using your
> GPS, set the DG to the ground track...see how it does..

That should work, given a constant wind, I guess. I'd think you'd want to vary
your heading over the duration of the test, though, in case the error only
occurs during heading changes. I'd think that excessive friction in the gyro
bearings might cause an error during turning that doesn't occur during straight
flight, but maybe not.

[George Patterson]

Dave Butler wrote:
>
> Because the mag compass deviation (is that the right word?) is generally
> different on different headings, so as you keep changing headings, you keep
> resetting your perfectly good HI to match it, all the while muttering about how
> your HI is precessing.

One way to test this is to fly one heading for a while, setting the DG to the
magnetic compass. Then change headings. Change headings as often as needed to
duplicate the "precession", but never reset the DG. Now return to the original
heading. If the DG now closely matches the magnetic compass, the compass needs
swinging. If not, the DG is drifting.

George Patterson
Drink up, Socrates -- it's all-natural.

[Dave Butler]

George Patterson wrote:
>
> Dave Butler wrote:
>
>>Because the mag compass deviation (is that the right word?) is generally
>>different on different headings, so as you keep changing headings, you keep
>>resetting your perfectly good HI to match it, all the while muttering about how
>>your HI is precessing.
>
>
> One way to test this is to fly one heading for a while, setting the DG to the
> magnetic compass. Then change headings. Change headings as often as needed to
> duplicate the "precession", but never reset the DG. Now return to the original
> heading. If the DG now closely matches the magnetic compass, the compass needs
> swinging. If not, the DG is drifting.

Yes, that's what I suggested in my first reply to the OP. I suggested using a
published runway heading for additional precision.

[Robbie S.]

Found something interesting on a web site.... I will do this on my C-182 as
the DG precesses like crazy....

http://www.psilabs.com/catalog/aim/dg_error.htm

....Robbie.

"Marty from Sunny Florida" > wrote in message
. ..
> Hey everyone.
>
> I have a C152 with good vaccuum, but the Directional Gyro precesses at an
> abnormally high rate. What's the prognosis? Can it be fixed, or just
> replaced? What can I expect to pay for a new DG?
>
> Thanks,
> Marty
>
>

[[email protected]]

A simple way to test for mag compass error and DG precession is to use your
GPS heading indication while tracking on he ground (i.e. taxiing as opposed
to flying).

Find an airport with some clear apron surface that you an use. Using the
GPS, taxi straight north (hdg 360). Use enough power to keep he vacuum
indication "in the green". Set DG to 360, and note mag compass indication.
Make successive 30 degree turns, according to the GPS. After each, note the
DG and mag compass indications. If the DG fails to track new headings
(within tolerance) you have excessive precession. Mag compass errors can be
used to fill in a correction card, or if they are excessive they indicate a
need to make adjustments.

Doing this on the ground eliminates the issue of wind (unless it is really
howling to the point that the airplane does not track straight while
taxiing).

--
-Elliott Drucker