First the AI, then the DG, then...?

Yes, but in this instance he needed a DG to drive the autopilot for
flying desired or assigned course headings.

On Sun, 30 Nov 2003 16:35:46 GMT, Kevin wrote:

Jay Honeck wrote:
WOW, sure is! Isn't the new STec DG around $2300?


Yeah, for some reason electric DGs are outrageously more expensive than
vacuum ones.

Anyone know why?


Probally a dumb question, but I have to ask. would a GPS not be more
accurate than a DG ?

"Jay Honeck" wrote in message news:%Hpyb.172161$Dw6.663993@attbi_s02...
Probally a dumb question, but I have to ask. would a GPS not be more
accurate than a DG ?

Yes, but many autopilots are "slaved" to a "heading bug" on your DG. This
set-up pre-dates GPS by decades, and is much cheaper than the (seemingly
simpler) GPS-autopilot connection.

Again, there is no logic to this -- but what else is new?

A DG gives heading, a GPS gives track. Two very different things.

Dan

"G.R. Patterson III" wrote in message ...
David Lesher wrote:

Does garbage really migrate upstream into the expensive toys?

If a dry pump fails, the carbon vanes may turn into powder. When this happens,
the pressure on the instrument side is lower than atmospheric pressure, and you
may wind up with some of this powder being blown back into the instruments. In
normal use, however, garbage does not migrate back upstream.

(And if so, you'd thunk someone would add a inline filter between the
toy and the common vacuum line..)

Some people do this as a preventative measure. Alternatives include prophylactic
replacement of the dry pump to attempt to ensure it never fails, using a wet
pump, and cleaning the instruments before using them after a pump fails. The
filter is cheap insurance.

George Patterson

A failed vac pump isn't likely to send garbage back up the line.
The instruments represent a significant vacuum leak, and the pressure
will drop off almost instantaneously when the pump fails. The filters
between the gyros and pump are more likely to keep garbage from a
failed instrument from gettingn into the pump, failing it as well.

Dan

"Jay Honeck" wrote in message news:E7nyb.369913$Fm2.367716@attbi_s04...
WOW, sure is! Isn't the new STec DG around $2300?

Yeah, for some reason electric DGs are outrageously more expensive than
vacuum ones.

Anyone know why?

So few made. Most general aviation aircraft use vacuum or air-driven
attitude and heading indicators, and an electric turn coordinator to
provide some redundancy if the vacuum fails. Electric DGs and horizons
would be used as backups in much larger airplanes, like the small
electric attitude indicators in the centre of most airliner panels.

Dan

"Jay Honeck" wrote in message news:fXJxb.334160$HS4.2853565@attbi_s01...
A friend of mine says that the gyros inside ride on some sort of a "ruby
bearing" -- and that they eventually wear out. Sometimes the rotor (or
whatever) even falls right off the bearing. When that happens, everything
just stops -- like mine did.

Or something like that. Either way, it's just more money...

Little wee tiny ball bearings. The lube dries out from all
the air going through the instrument, cold weather hardens it,
airborne pollutants get through old/cracked/nonexistent filters
(cigarette smoke is a bad one) and bearings can't take it anymore.

Dan

"Mike Rapoport" wrote in message hlink.net...
How old were your gyros? At 10,000 rpm (just a guess) a gyro revolves
600,000,000 times in 1000hrs.

Mike
MU-2

I think it's a lot higher than that, maybe as much as two or three
or four times. Plenty, anyway.

Dan

Again, there is no logic to this -- but what else is new?

A DG gives heading, a GPS gives track. Two very different things.

True. However, the illogic I was referring to is the incredible difference
in price between an autopilot that is slaved to a DG, versus an autopilot
that is connected to a GPS. The former is MUCH more difficult to achieve
than the latter, yet the GPS connection is much more expensive to buy.

This is due to demand, I suppose?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

(Dan Thomas) wrote in message . com...
"Mike Rapoport" wrote in message hlink.net...
How old were your gyros? At 10,000 rpm (just a guess) a gyro revolves
600,000,000 times in 1000hrs.

Mike
MU-2

I think it's a lot higher than that, maybe as much as two or three
or four times. Plenty, anyway.

Dan

Looked it up just now: 25,000 RPM. Bearings are typically .031" (1/32") diameter.

Dan

Jay Honeck wrote:
Again, there is no logic to this -- but what else is new?
snip

Jay,

I joined this thread a bit late...after my local server flushed the
original message, but I believe your original question was "what could
cause premature gyro failure"?

In my experience, two things:

1) If it fails within a few hours of being new, that's typically because
of manufacturing, long-term storage, or shipping damage -- all of which
can cause the bearings to "flat spot". Apparently the bearings are
*that* soft.

2) If it fails after a few hundred (as opposed to a few thousand) hours,
improper mounting or excessive vibration is usually the culprit. The
problem is most severe in aircraft whose panels aren't shock mounted AND
when those same aircraft don't have their props properly (dynamically)
balanced.

Our gyros were so damn expensive that when we had our panel apart last
time I had the shop replace the mounts. Not only does the panel NOT sag
like it used to, but as we encounter various (normal) harmonics in our
particular engine/prop combination, the shock-mounted instruments simply
don't shake like some of the other fixed mounted units (fuel gauges, etc).

I forget if your airplane has a fixed or shock-mounted panel, but if
it's shock mounted, I'd recommend replacing the shock mounts and getting
the prop redressed / painted, then dynamically balanced for good measure.

HTH,

-Doug

--
--------------------
Doug Vetter, CFIMEIA

http://www.dvcfi.com
--------------------

"Jay Honeck" writes:

Yeah, for some reason electric DGs are outrageously more expensive than
vacuum ones.

Anyone know why?

They ARE direct drive, are they not?

My guess is as follows. The air/vacuum motor is trivial; a series
of vanes and a jet tube from the intake filter. Easy to make go
fast.

But a tiny DC motor is difficult; the rotor windings will want to
fling themselves out at high speeds. The brushes and commutator
will be near watchmaker-skill level.

A new design would have a AC inductor motor with a squirrel-cage
motor, and a small internal inverter. But does anyone make them
like that?


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