DG Precess

DG Precess

<HTML>I was wondering what some of you be getting as normal DG precess - Currently my DG drifts 10 to 20 degrees over a span of about 150 miles or nearly an hour and a half ( with no resetting ) flying a 172 in light to moderate turbulence, usually cruise about 4000 to 8000 ft msl ( incl climb out and descend from fields about 1000 msl ).

If I reset the DG  before it drifts 10 degrees each time, then that means once or twice on a 150 to 200 mile - or 1.5 to 2.0 hour trip - it seems that I've seen better, but after having some gyro trouble a while back I had worse - Just looking for an approx.  benchmark or consensus before I expect more from my DG.

Thanks in advance

Ken Wanagas</HTML>

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Re: DG Precess

<HTML>You did not mention what brand of DG you have but the club I belong to has a sigma tec. I read somewhere, think it was the manual, the drift spec was something like 4 degrees in 10 minutes. It is usually better than that but it has not been unusual to have to reset it several times per hour. More if turns and climbs are performed and less in smooth level cruise.

I own a 150 with an RC Allen DG and it seems quite a bit better but still needs to be  adjusted once or twice per hour.

I don't think you are too far off in stability.

Vin</HTML>

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Re: DG Precess

<HTML>I will have to look in my logbook for the brand of DG I have, I had a yellow tagged one put in last year.  I have to reset mine once or twice an hour and then it usually only off 5 - 10 degrees...</HTML>

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Re: DG Precess

<HTML>Ken, how much vacuum does your vacuum gauge say you're drawing? Do you have a vacuum pump or a venturi? If it's a venturi, you'll be drawing less vacuum during climb because of the reduced airspeed, and you should expect the gyro to precess more with the reduced vacuum.  Just a thought.</HTML>

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Re: DG Precess

<HTML>It's the nature of all high wing Cessna's to throw off your DG due to the magnetic field created by static build up from the wing struts. This brings the magnetic field from the wings, down the struts, then down to the floor. That creates a positive charge just below the wings, and a negative charge up from the floor. And this is of course exactly what causes the polar effect under the panel. No DG can be expected to correct or compensate for this polar effect. It was just very poor planning on Cessna's part. I guess this is why most other manufacturers go with the low wing. Hope this helps,
Rusty</HTML>

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Re: DG Precess

<HTML>Rusty a DG is a Directional Gyro not a magnetic compass. Maybe someone else would like to explain the term precess for Rusty. For anyone that may not be familiar with the person that posted the previous absurd message, do a forum search for author  Rusty.</HTML>

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Re: DG Precess

<HTML>I liked Rusty's explaination....I thought it was pretty entertaining.  It's too bad he couldn't have gone into more detail...such as how the "magnetic field" that develops on "all high wing" Cessnas affects the magnetic compass....Is the 177 Cardinal excluded from this phenomina??  It is a high wing..but no struts...I guess I've been a pretty lucky "high wing Cessna pilot"..flying around in the dark & in clouds with instruments that can't possibly work right because of this phenomina....
...at least my muffler bearings are still in pretty ok shape!
Michael</HTML>

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Re: DG Precess

<HTML>This is the same rusty who got into gay bashing a month ago and left us.  I see the vermon is back.  Too bad.....</HTML>

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Re: DG Precess

<HTML>I'm thinking he still hasn't come to terms with his sexuality.  He lashing out.</HTML>

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Re: DG Precess

<HTML>Thanks -

Vin and Dave K - it looks as though 5 to 10 degrees every  45 minutes or so is reasonable - appreciate the input.

Stan C - I have a vacuum pump - I'm going to replace it because of the Advisory out, but its been good so far and the vac good whenever I have checked the gauge - however also adding a low vac lght soon when I change the pump - after reading othe posts, in the event that the vac's not always good and-or I haven't been looking at the gauge often enough or at the right time to catch an under vac situation.

Rusty - static charge is worth considering because it's been more of a factor than I and many others would have ever thought, including a shop head mechanic who burned down a perfectly good aviation repair shop in part because he momentarily overlooked static charge, I'm not ready to accept your explaination without further info, wheather the wing is on the top or bottom.

I have enjoyed several aircraft - wing on top or bottom (or both), from 28 or so horsepower on up to something that would maintain over 200 KN, - not as many aircraft as others, I'm sure, but enough to enjoy.
When I rented all the time, I would stop by my favorite local airport and ask to rent on a nice afternoon, they would always ask what kind of plane I wanted, and I would always answer - anything with wings, and if that wasn't available then I would seriously consider most anything else that seemed airworthy.

If we could stick to conversation and info on flying and such and avoid name calling and "ford vs chevy" kind of personal attacks ( high vs low wing )  I think the world would be a better place - and we would all enjoy aviation a little more, and right now I think that that would be a good thing.

Thanks again all for your info - facts or opinion

Ken Wanagas</HTML>

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Re: DG Precess

<HTML>I agree with you Ken. I got my private in 1977 in a PA28-140, then joined a lub with a 150 and 172. Years later I joind a club with 2 Archers and I now own my own C150H. I like all of them for their own individule "personalities". Like, you I enjoy flying in many different aircraft to see what they are like.

Have a great week everyone!

Vin</HTML>

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Re: DG Precess

<HTML>Before worrying about the DG "precessing", check to ensure your compass is swung right.  Also ensure the DG doesn't "precess" only when the landing light is on (and really the compass is just off 10 degress for some headings and not others).

Mark</HTML>

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Re: DG Precess

<HTML>Mark has a good point.  The compass can be off in different quadrants and by differing amounts.  So depending on which direction you are going it may appear your DG has precessed.  I've had that experience and had the compass recalibrated. My DG is fine.

My vacumn is set on the low side of the range. In my 172M, I find myself resetting the DG about every 15 minutes to 45 minutes.  I would think the conditions of flight (turns, turbulence etc.) have a lot to do with the variations. (Actually, when you said how little you have to reset your DG I was envious.)</HTML>

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Re: DG Precess

<HTML>Mark- I have a SIRS navigator compass and it is swung so its very accurate and reliable, I usually reset my DG comparing it to the compass, GPS heading - usually allowing for cross track observed, and ground section lines ( here in the midwest, generally around Illinois ) locally the magnetic declination is almost negligable and even when flying into Wisconsin flight is usually along the magnetic field lines so its very regular, its just on longer flights and especially Northern Wisconsin the magnetic fields are all over the place which affects the compass and has messed up the section lines many years ago ( I can verify that, being a Land Surveyou for many, many years, in Illinois, Wisconsin, Texas, -- )  and the section lines are anything but cardinal directions, and those that may be are obscured by lots and lots of trees. I usually use the runway headings and verify before takeoff and on arriving into the pattern and on final ( don't want to land on the wrong runway ) . On final I expect to confirm the heading ( and even the airport ) and the DG is just one of those confirmations, It's just unsettling when the DG is substantially different from the numbers I'm seeing at the threshhold.

Years ago I flew rented aircraft and their DG's were quite unreliable, and I can distinctly remember, as a student, on departing a controlled field along Western Illinois the controller questioned my departing heading, I was going to flyoutbound then trac the omni east to head home, - I wasn't going east - On review of the info available I soon decided the DG heading was worthless, the Mississippi river was anything but north and south - more snakelike - and the compass was wildly swinging (turbulence) roughly northeast, I turned about 35 degrees right - to the southeast - and soon was able to get right with the omni also.

I've flown with some pretty unreliable DG's,  so I appreciate one that's close, and a compass thats close also, I now have the luxury of GPS which makes life much easier.  By the way the GPS does affect the compass when its "on" , I just don't want to expect too much out of the DG if it exceeds its capabilities, if it doesn't exceed it's capabilities then I want to get the most reliable reading from the DG, or change it to a unit that is as reliable as can be expected.

Thanks - I will review the electrical load and landing light and all effect on the compass
Ken Wanagas</HTML>

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Re: DG Precess

<HTML>PS -

In Northern Wisconsin the reason for the wild magnetic fluctuation is the iron ore deposites in areas or pockets which render the compass highly questionable, the reason for the water color (rust color) also.

That leaves a highly suspect magnetic indication, almost no ground reference, and a possibly questionable DG (previously I had some pretty unreliable DG indications, but replaced that unit).

The GPS is great, and there are a few VOR stations, but that doesn't leave much backup or redundancy, and I do fly the area sometimes at night, so I'd like to have some confidence im my DG heading before I fly into the area and have a mental picture of about how much I am "expecting" the DG to be in question beforehand.

Again the GPS is great  (Garmin 295) but I don't rely on it exclusively, and it sure is incredible compared to several years ago with only VOR and reconing. ( I don't like the term - "dead reconing" )   

Ken Wanagas</HTML>

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Re: DG Precess

<HTML>Thanks again everyone

Ken Wanagas</HTML>

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Re: DG Precess

<HTML>Ken; I saw a show on Discovery Wings not too long ago.  The term used to be  Deduced Reconing and in later years it was shortened to Dead Reconing.  Just a little tidbit of useless information on this cold (in Iowa) Wednesday morning smile</HTML>

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Re: DG Precess

<HTML>I could get used to "deduced reconing" -

Thanks Ken Wanagas</HTML>

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