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#1
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DG precession
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 |
#2
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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. |
#3
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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. |
#4
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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 |
#5
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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. |
#6
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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. |
#7
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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. |
#8
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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 |
#9
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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 |
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