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I'm not talking about the ball, but instead the plane symbol shown on the instrument:

Turn Coordinator - Credit to Mysid, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Is it showing a roll or is it yaw that it shows? Or is it a mix of the two?

IE, if I do my darndest to fly straight as I bank the wings (uncoordinated flight, so ignore the ball reading), what will I see? If I turn with the rudder, but do my darndest to keep the wings level with ailerons, what will I see?

My interest is trying to understand and gauge (pun intended!) what you can know from this instrument about yaw and or bank if you loose your attitude indicator while in IMC.

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  • $\begingroup$ Which plane shown, sorry? $\endgroup$
    – Koyovis
    Jul 5 at 1:35
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    $\begingroup$ We have some other questions on ASE about turn coordinators which you might find helpful (and may have already addressed this question) $\endgroup$ Jul 5 at 13:52
  • $\begingroup$ @Koyovis I added a picture. The plane symbol shown on the instrument itself. $\endgroup$
    – Azendale
    Jul 5 at 22:25
  • $\begingroup$ What you are looking at is a 2 minute standard rate turn gauge. You roll into the turn and do your darndest to keep the ball centered (with the rudder). This will give a turn rate of three degrees per second, or 180 degrees in 60 seconds. Other experiments can be done in VFR conditions. $\endgroup$ Jul 6 at 0:52
  • $\begingroup$ Does this question help? $\endgroup$
    – Pondlife
    Jul 7 at 4:01
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It's a mix of the two because the gyro is canted at angle such that it is sensitive to both roll rate and and yaw rate (note that it's roll rate, not bank angle).

The result is you start to get a turn indication immediately with the bank (a turn and bank has a slight lag because there is no indication until the heading change has started slightly after the bank), and straight yaw indications without bank are reduced from what they would be in a turn and bank.

In an airplane with a regular turn and bank, if you get bumps that make the nose swing back and forth while the wings remain level, the turn indicator needle will swing from side to side because its gyro doesn't know the airplane isn't banked; it just knows that yaw motions are happening. A turn and bank can be a pain to use in bumpy air in a plane that likes to wag from side to side in turbulence. You can't really use the indication until the slip/skid ball is settled down and centered.

A turn coordinator with its canted gyro does this a lot less because it's only partially sensitive to yaw rates, and because it's also sensitive to roll rates, it gives you an indication of banking into a turn immediately upon dropping the wing.

The overall result is that initial indication is from the roll into or out of the turn, and once the change of bank has stopped, the indication is coming from the yaw rate of the turn itself.

Being sensitive to roll and yaw rates of motion, to work well, a turn coordinator's gyro needs an internal damper to dampen its motion. The turn and bank in my old '68 Cessna Cardinal went bad once when the internal damper failed. The plane symbol would dance around constantly, much worse than a turn and bank needle (which has no damping), making the instrument unusable even though the gyro was still running.

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  • $\begingroup$ Thanks, very helpful answer. Since you say it is both roll rate and yaw rate, is it true to say it would go back to "level wings" if you hold a turn long enough (and consistently enough)? (If so, couldn't that mean that one of the VFR into IMC illusions could be reinforced instead of dispelled by this instrument?) $\endgroup$
    – Azendale
    Jul 5 at 22:35
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    $\begingroup$ Well if you are in a stabilized turn, you have a constant yaw rate and that is being shown as a tilted airplane. It won't go back to level because there is still a continuous acceleration happening, the lateral change in direction. $\endgroup$
    – John K
    Jul 6 at 0:49

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