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Posted: Tue Apr 04, 2006 11:09 am
I will have my instrument stage check soon, and I will have to be able to explain every instrument as far as to how it functions.

A question I have that nobody has been able to explain to me well is why the directional gyro (heading indicator, if you prefer) needs to have a double gimbal? AFAIK, the gyro only needs to sense yaw, so I don't really get why the need for the other gimbal. I have acces to decomissioned and opened instruments at school to see the internal working if that helps.

BTW, I'm talking for a vacuum driven heading indicator, found in late model C172s.

Posted: Tue Apr 04, 2006 11:20 am

Without a double gimbal, rigidity in space would be unattainable.

2H4

Posted: Tue Apr 04, 2006 11:23 am
You need to see the compass in turns AND when you climb and dive. A gimbal for pitch and a gimbal for roll ...

Posted: Tue Apr 04, 2006 12:16 pm

Speaking of rigidity in space, I think I just found the coolest example ever:

2H4

Posted: Wed Apr 05, 2006 12:55 am
 Quoting 2H4 (Reply 1):rigidity in space would be unattainable

 Quoting Dc10guy (Reply 2):A gimbal for pitch and a gimbal for roll

So what you guys mean is that, say, without a gimbal for pitch, the gyro would precess in the roll axis, which is something we certainly don't want, right?

 Quoting 2H4 (Reply 3):I think I just found the coolest example ever:

The first two pics aren't working

Thx for the answers so far

Posted: Wed Apr 05, 2006 1:53 am

 Quoting FLY2HMO (Reply 4):The first two pics aren't working

Here you go:

2H4

Posted: Wed Apr 05, 2006 2:28 am
 Quoting FLY2HMO (Thread starter):the gyro only needs to sense yaw

Actually the DG does not "sense" yaw. It keeps its rigidity in space and you yaw around it. It might be better to say it is immune to yaw, or ignores yaw.

Posted: Wed Apr 05, 2006 3:00 am
 Quoting 2H4 (Reply 5):Here you go:

Wow, that is really impressive. Thanks for the pictures.

 Quoting SlamClick (Reply 6): or ignores yaw

So it ignores pitch too, right?

Posted: Wed Apr 05, 2006 3:12 am
 Quoting FLY2HMO (Reply 7):So it ignores pitch too, right?

I guess you could say that. And roll too. If it remains rigid in space the compass card (for the drum type) would remain stable, paralell to the earth's surface where you are flying. So the part of the card that you pointed north (by setting the heading from your wet compass) would remain pointed north. When you make a turn, you are flying the window and lubber line around to different parts of the card - giving you a glimpse of the reading there. If you head south you get to see the part of the card that says "south."

That fact might help you more instinctively understand lead/lag and northerly turning error if those are still taught and tested. It helped me understand them.

If that doesn't make sense leave it alone and go back and re-read something that did.