PPRuNe Forums - View Single Post - Turn Rate Indicator / Turn Coordinator / Looping Error
Old 21st Nov 2013, 03:29
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Turn Rate Indicator / Turn Coordinator / Looping Error

I was googling around about some questions I'm interested in and came across this old discussion on PPRuNe :


The idea seems to be that a turn rate indicator precesses while turning in such a way that the gyro remains nearly fixed relative to true vertical-- or at least up to 6 degrees of offset from the airplane's own sense of vertical-- as illustrated here-- Sample/The Turn Indicator

Since the gyro remains aligned somewhat "true" to the earth not the aircraft, any pitching motion will be seen in part as a yawing motion. This leads to "looping error"-- the instrument will over-read when the pilot is pulling excess G's. Therefore the pilot should relax any excess G's before reading the instrument, while recovering from an unusual attitude.

Question: why is this a problem? What is wrong with extra sensitivity? The instrument must still read zero once the wings come to level, if the ball is centered. Does the instrument just get too sensitive to be of any practical use, so that as the aircraft rolls through level at a modest roll rate, the instrument abruptly slams from full-deflection one way to full-deflection the other way, giving the pilot no chance to halt the roll at wings-level?

One poster went on to say :

"A turn co-ordinator (as fitted as a factory item on most spamcans) has the axis of rotation tilted about 30 degrees and rotates the other way. As a result, looping errror is reversed and the instrument under-reads at more than 1g which makes it near to useless when recovering from an unusual position on instruments. For this reason, the club where I instruct has replaced turn co-ordinators with turn and slips on our fleet of PA28s."

Question: can this really be entirely correct? Why would the direction of rotation be reversed? I'm not finding other supporting material on line in support of this assertion-- might it be an error? Does the turn coordinator, just like the turn rate indicator, becomes over-sensitive at excess G-loadings?

Edit: this graphic File:Turn indicators.png - Wikipedia, the free encyclopedia from this wikipedia page File:Turn indicators.png - Wikipedia, the free encyclopedia indicates that the direction of spin is the same with each instrumment.

Question: in practical terms is it in fact the case that the turn coordinator is much less useful than the turn rate indicator, for the specific purpose of partial-panel recovery from unusual attitudes? What are the characteristics that make it so? Does the turn coordinator in fact become less sensitive under excess G-load? If so, why? Or is there some other characteristic that makes it objectionable for recovery from unusual attitudes? In actual practice in my own experience it seems more useful for this purpose, not less.

Thanks for any comments, or links to other sources of information on this.

PS FYI I'm involved in some hands-on explorations with a piezoelectric turn coordinator / turn rate indicator (sensor may be canted to either position) with no moving parts, and am interested in better understanding any strengths or weakness that may exist compared to their mechanical counterparts of yesteryear...

Last edited by flyer101flyer; 21st Nov 2013 at 03:46.
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