lml@lancaironline.net Mailing List Archive

A pretty logical question actually. The gyro AI has small "pendulums" hanging from the side of the gyro cage and they sense gravity, deflecting an air jet to bring the gyro "upright" and keep it there. Their response is slow enough that during normal maneuvers they don't affect the attitude. That's also how it initially assumes an upright posture. The short answer is that the gyro isn't referenced to "space", but is referenced to gravity and therefore will follow the Earth's rotation. Presumably one could enter a shallow coordinated bank and keep it long enough for the gyro to eventually show a wings-level indication. I think. One of the failure modes is for the AI to gradually rotate in pitch or roll (I've had it happen) a full 360. This is usually caused by one of the pendulums getting stuck. Neither the yaw rate gyro or the heading indicator have these pendulums. If you have a really good heading gyro you might notice that the precession rate is different going north than when going south because of the effect you mention.

A good question coming from an ES builder - after all, we don't have all those landing gear questions to ask :-)

Gary Casey

ES #157

From: "rtitsworth" <rtitsworth@mindspring.com>
Date: January 1, 2008 12:49:33 PM PST
To: lml@lancaironline.net
Subject: First dumb question of the year...

Happy new year,

So, I was sitting around thinking silly thoughts (when I probably should
have been building) and it occurred to me that gyroscopic instruments
essentially attempt to maintain a constant reference frame (in space) from
which to measure changes in attitude, heading, etc. So, over time,
shouldn't the rotation of the earth cause the gyro to indicate a change
(error) relative to earth's horizon. i.e. An attitude indicator in an
airplane sitting on the equator and facing west (or east) should
progressively pitch up (or down) at about 15 degrees per hour
(360deg/24hrs). That's a bunch.

Likewise, the attitude indicator of a south (or north) facing airplane on
the equator would indicate a roll at the same rate. Likewise, the DG of an
stationary airplane sitting at the north (or south) pole would react
similarly. Likewise, at all other latitudes and orientations a mix of the
above effects would seem to be at play.

Hmmmm. Any insights?

For BrentR, AHRS work differently (by measuring and integrating changes in
accelerations over time???), but it seems they would ultimately be subject
to the same effect?

Rick Titsworth