AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  DG precession          AirTalk.org Forum Index -> Owning Airplanes View previous topic :: View next topic   Author Message Marty from Sunny Florida Guest Posted: Thu Mar 24, 2005 1:39 am    Post subject: 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 Back to top Ron Natalie Guest Posted: Thu Mar 24, 2005 2:14 am    Post subject: Re: DG precession Marty from Sunny Florida wrote: Quote: 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. Back to top Dave Butler Guest Posted: Thu Mar 24, 2005 8:39 pm    Post subject: Re: DG precession Marty from Sunny Florida wrote: Quote: 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. Back to top Dale Guest Posted: Thu Mar 24, 2005 9:08 pm    Post subject: Re: DG precession In article <1111697079.782652@sj-nntpcache-3>, Dave Butler <x@x.x> wrote: Quote: 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 Back to top Dave Butler Guest Posted: Thu Mar 24, 2005 9:30 pm    Post subject: Re: DG precession Dale wrote: Quote: 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. Quote: 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. Back to top George Patterson Guest Posted: Fri Mar 25, 2005 4:44 am    Post subject: Re: DG precession Dave Butler wrote: Quote: 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. Back to top Dave Butler Guest Posted: Fri Mar 25, 2005 1:23 pm    Post subject: Re: DG precession George Patterson wrote: Quote: 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. Back to top Robbie S. Guest Posted: Fri Mar 25, 2005 3:21 pm    Post subject: Re: DG precession 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" <nospam (AT) sorry (DOT) com> wrote Quote: 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 Back to top e.drucker.remove@verizon. Guest Posted: Sat Mar 26, 2005 1:22 am    Post subject: Re: DG precession 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 Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AirTalk.org Forum Index -> Owning Airplanes All times are GMT Page 1 of 1   Jump to: Select a forum Aviation----------------alt.aviationAviation safety EnhancementPlanes, gliders and flying in AustraliaAviation et pilotage (Français)rec.aviationAviation (Aerobatics)Hot air, gas balloons and airshipsHang-gliding, paragliding, foot-launched flightAircraft Design & BuildingInstrument Flight RulesAviation (Classifieds)Military AircraftNaval AviationAviation (Miscellaneous Topics)Owning AirplanesAviators (General Discussions)Aviation related productsAircraft RestorationHelicopters and rotary wing aircraftsFlight SimulationSailplanesAviation (Students)Light aircraftFlying related activities in the UK Airports----------------alt.airportsEdinburgh Airport Glasgow Airport Humberside Airport Luton Airport Aberdeen Airport Glasgow Prestwick Airport  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.