DG Precession - How Much is Too Much?

#1 09-04-2006, 09:38 AM

Just got back from a trip to Minnesota and Montana in my -6, and noticed that the DG seems to precess quite a bit. On a trip where I was basically flying a steady, cross country course with very little maneuvering or significant course changes, I would constantly be having to reset the DG by about 5 degrees every 20 minutes or so, and always in the same direction (DG was reading low compared to whiskey compass).

The DG is a standard RC Allen vacuum powered model, which I bought new but which then sat on the shelf about 1.5-2 years before first flight in late 2004. At the time, I saw their "1 year from date of purchase" warranty and wondered if that was an indicator of poor quality and longevity. Now I am wondering that again.

So, when is precession significant enought to warrant repair or replacement, and is there some horrible failure mode that could occur. other than it just getting gradually worse? And, is there a better option that the RC Allen? I fly VFR only and have GPS, so the DG is really not flight-critical, but on the other hand having an instrument that reads incorrectly is in some ways worse than none at all.

ChiefPilot · #2 09-04-2006, 12:30 PM

5 degrees in 20 minutes sounds pretty reasonable to me; the Archer and Arrows in my flying club are about that or perhaps a bit worse.

-Brad

RV6_flyer · #3 09-04-2006, 07:56 PM

Just got home from the 2006 Homecoming. My vacuum operated DG has been in the airplane since I built it. I put the airplane away with 1,950 hours on the hobbs meter. My DG will precess five (5) degrees in a little over one hour. Maybe I have a good one.

az_gila · #4 09-04-2006, 09:20 PM

Quote:

Originally Posted by RV6_flyer

Just got home from the 2006 Homecoming. My vacuum operated DG has been in the airplane since I built it. I put the airplane away with 1,950 hours on the hobbs meter. My DG will precess five (5) degrees in a little over one hour. Maybe I have a good one.

Gary ... depending on your direction of flight, 15 degrees per hour could happen since that is how much the earth turns under you in one hour, even if you have a perfect DG....

You need to fly 1 hour on each of 8 compass points to calibrate it, and then tell us which is the non-optimum direction of flight....

gil in Tucson

John_RV4 · #5 09-05-2006, 07:27 AM

An interesting excercise is to set your gps to read MC and then fly on a day with light winds. I find that my DG and GPS often agree pretty closely when the whiskey compass does not. Eventually, the compass will come around if I can remain perfectly steady long enough. It takes a surpisingly long time for the compass to settle. That is if the day is smooth enough.

I wonder how many times in days gone by I re-set my DG and in fact only set if farther off course...

YMMV

John

RV505 · #6 09-05-2006, 08:22 AM

I had a buddy that had a aircraft salvage yard.. He would pull gyros out of wrecks

and within 8 moths they would go bad.... Gryos arn't meant to sit.. Ask any gyro house... They'll tell you not to buy them untill they are ready to be used....

John Silver · #7 09-12-2006, 07:34 PM

Gil is right. Your plane, sitting on the ramp and with perfect bearings in the gyro, will drift 15 x the sine of the local latitude. Called apparent drift. Some old DGs had latitude correction knobs that would induce drift to counter the apparent drift. Slaved is nice.

N91CZ · #8 09-13-2006, 07:26 AM

Quote:

Originally Posted by Rutus

Just got back from a trip to Minnesota and Montana in my -6, and noticed that the DG seems to precess quite a bit. On a trip where I was basically flying a steady, cross country course with very little maneuvering or significant course changes, I would constantly be having to reset the DG by about 5 degrees every 20 minutes or so, and always in the same direction (DG was reading low compared to whiskey compass).

The DG is a standard RC Allen vacuum powered model, which I bought new but which then sat on the shelf about 1.5-2 years before first flight in late 2004. At the time, I saw their "1 year from date of purchase" warranty and wondered if that was an indicator of poor quality and longevity. Now I am wondering that again.

So, when is precession significant enought to warrant repair or replacement, and is there some horrible failure mode that could occur. other than it just getting gradually worse? And, is there a better option that the RC Allen? I fly VFR only and have GPS, so the DG is really not flight-critical, but on the other hand having an instrument that reads incorrectly is in some ways worse than none at all.

John,
While building I would run all vacuum instruments with a vacuum pump at least every two weeks to keep the bearings happy. The instrument panel was the first part of the plane I built. Two years after flying started while going to OSH, I recall not having to adjust the RC Allen Gyro even once in the air. Now after nine years and 1,000 hours of use on the instrument it is still holding course when flying a straight line. The amount of precession you are seeing sounds high to me. It is certainly much more than I have seen on the same instrument. I suspect that gyros have a very short shelf life. I don't understand the mechanics of why this is the case. I heard rumors about this before building, so I took precautions and it seems to have paid off. I'm sure the relatively short warranty period is for the potential damage done by inactivity. I would suggest an overhaul if the amount of precession you have bugs you.
Chris