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From: mmallory@netcom.com (Mark Mallory)
Subject: Re: TI errors (was: 17 misconceptions)
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Date: Fri, 18 Dec 1998 02:51:26 GMT
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Julian Scarfe (jas1@scigen.co.uk) wrote:
: Well having looked at my notes on turn indicators, I think it's more serious
: than that.  The turn indicator is a tied gyro sitting along the axis of pitch
: when the aircraft is in level flight. For small rates of yaw, the gimbals
: apply a torque in the yaw axis, with the result that the gyro axis rotates
: about the roll axis (and that displaces the needle).

I don't believe this is correct.  My turn *indicator* appears to have the 
gyro's axis parallel to the engine's crankshaft; ie, the gyro spins about 
the *roll* axis of the airplane, and the needle of the instrument responds 
to pitching motion of the gyro axis.


: For steeper turns, the turn indicator may be pegged on its stop. The gyro axis
: will no longer be close to the pitch axis. If the turn involves a rotation
: about the pitch axis (pitch up), as steep turns do, the corresponding torque
: on the gyro will cause a change in direction of its axis.  Unfortunately, a
: pitch up will make it overread in one direction, and underread in the other.
: Overall, it's extremely difficult to work out where wings level would be.

This is not the case.  In fact, it takes very little movement of the gyro's 
axis (only a few degrees) to "peg" the instrument; as a result, the axis 
never moves very far from it's normal position.  The only real error 
experienced by the TI due to steep banking is a tendency to under-read, 
since the component of angular motion around the yaw axis is less than 
the actual turn rate (but the instrument is probably pegged anyway).  Most 
importantly though, the instrument ALWAYS indicates the proper *direction* 
of the turn, and recovers quickly as when the airplane stops turning.


: For the Goldstein test of this, I suggest putting the aircraft into a shallow
: turn, and then rolling the wings level and comparing AI with TI. The turn
: indicator (or coordinator) should register zero close to the wings level
: position, as you'd hope if partial (limited) panel flight were ever to work.

Actually, the best way to learn about how the thing works is to remove it 
from the panel, and twist and rotate it with your hands with the power 
connected and the gyro spinning; you can feel the reaction torques and 
observe the instrument's indications under "simulated" acrobatic flight 
conditions...

