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Turn Rate Indicator / Turn Coordinator / Looping Error

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Turn Rate Indicator / Turn Coordinator / Looping Error

Old 21st Nov 2013, 02:29
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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 :

http://www.pprune.org/tech-log/82721...gyro-axis.html

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 02:46.
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Old 21st Nov 2013, 05:22
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Question: why is this a problem? What is wrong with extra sensitivity?
Because the instrument not only overreads under g, it suffers a lot of lag and it's hard to find the wings level position under g loading.

Question: can this really be entirely correct? Why would the direction of rotation be reversed?
Beacuse the turn coordinater is designed to be both yaw and roll sensitive with emphasis in greater sensitivity in yaw. This is to help the pilot coordinate his turns - something I couldn't really see in the very few times I've looked at one. I think it was more of a marketing thing.

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?
Yes, because of the reversed rotation, the turn coordinater under reads under g. This makes it hard to find the wings level position.

AFIAK, the T and S rotates away from the pilot, the turn coordinator towards. There may be manufacurer's variations, but this is the case with the instruments I am (was) familiar with.

To understand how it can effect a recovery, I will describe the partial panel UP recovery we used to teach in the RAF.

1. Check altitude - if below MSA - eject.

2. Unload the g. (To do this, push to 0g, then check back on the control column a little).

3. Roll the aircraft to centre the turn needle. Anticipate about half a rate for lag.

4. Pitch the aircraft until the altimeter needle stops, then check the other way (to account for altimeter lag) a little.

5. Repeat roll and yaw inputs until the aircraft is straight and level. (These should be progressively smaller and you should only need about two or three,)

6. Re-erect the AH.
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Old 21st Nov 2013, 16:35
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Thanks for the reply, Dan--

From your reference to turn "needle", I assume the jets you were instructing in had a turn rate indicator, not a turn coordinator. Please correct me if I'm mistaken here.

I want to learn more about this as I don't understand how the direction of rotation can be reversed with the turn coordinator. It seems to me that this wouldn't work at all. But I don't fully understand the internal mechanism of the instrument and how it differs from a turn rate indicator, other than the fact that the gyro axis is tilted to include roll sensing.

Again my experience has generally been that the combined roll-yaw sensing of the turn coordinator greatly aids the pilot in rolling to wings-level without overshooting into a turn in the opposite direction. But again, I haven't explored the effects of heavy excess G-loads.

Also it's clear enough why an inverted spin, where roll is opposite yaw, is one case where the turn coordinator would give unreliable indications and a turn rate indicator would not.

Re the turn rate indicator-- perhaps it is in the case that in light aircraft where the turn rate for any given bank angle is much higher than in a fast jet, the over-sensitivity due to excess G-loading is much less important and in practical terms it is not usually necessary to unload the G's to 1 or 0 before reading the instrument? I'm going to do some hands-on tests to shed some light on this later this morning.
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Old 21st Nov 2013, 22:07
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observations

Well, that was interesting!

I don't know anything about how the dynamics would be different in faster aircraft, or how older turn rate indicators might have differed from what is currently in use, but here's what I observed:

Aircraft -- 1956 Cessna 172
Age of turn rate indicator (needle and ball style)-- the instrument is of recent manufacture, not original.
Also on panel-- portable piezoelectric 1-axis turn rate indicator (sensing yaw only)

Summary-- pulling extra G's causes more of an increase in turn rate indication in a right turn than in a left turn. Pushing to reduce the G-loading seems to cause more of a decrease in the turn rate indication in a right turn than in a left turn.

Nothing I observed helped me understand why a turn coordinator might be less useful than a turn rate indicator for partial-panel recoveries from unusual attitudes-- except for the inverted spin issue as noted in post #3. I'll repeat these tests some day in an aircraft with a turn coordinator. In general, I still feel like the roll sensing inherent in a turn coordinator usually helps a pilot bring an aircraft smoothly to wings-level with less of a tendency to overshoot into a bank/ turn in the opposite direction.

Based on what I observed, my own partial-panel recovery technique for light aircraft will not include pushing the stick forward to reduce the G-loading to 1 to eliminate errors in the reading of the turn rate indicator or turn coordinator.

(Edit: see post #5 for some more thoughts on possible reasons for the differences between what I observed, and the observations / partial panel recovery techniques that others have posted.)

Details:

Enter coordinated, constant-speed, constant-altitude standard rate right turn at high airspeed. Then pull extra G's.

Turn rate needle shows a strong increase in turn rate, significantly sooner than, and more than, the increase in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate left turn at high airspeed. Then pull extra G's.

Turn rate needle seems to lag in showing an increase in turn rate, compared to the increase in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate right turn at low airspeed. Then push to reduce the G-loading.

Turn rate needle seems to show a strong decrease in turn rate, which seems to be significantly sooner than, and more than, the decrease in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate left turn at low airspeed. Then push to reduce the G-loading.

Turn rate needle seems to show a slight increase in turn rate in some tests, and in other tests, seems to show much less decrease in turn rate than is shown on the piezoelectric sensor.

With this new information in mind, I did some more experiments with spiral dives. The errors in the turn rate indication due to G-loading seemed insignificant compared to the overall dynamics of the spiral dive and the phugoid dynamics that accompanied the entry and recovery to the dive. It still seemed that the basic rule I posted in my first post applied well:

Basic rule for partial-panel recoveries-- use rudder or coordinated rudder/aileron to roll against the deflection of the turn rate indicator. Simultaneously, apply aft stick pressure whenever the airspeed is increasing, and forward stick pressure whenever the airspeed is decreasing, with the amount of stick pressure proportionate to the rate of movement of the airspeed needle (NOT to the actual value of the airspeed.) Once the airspeed is frozen, ease it slowly back to trim speed. Avoid exerting strong aft stick pressure at very low airspeed-- but this situation will rarely arise because the airspeed will never be very low, and rapidly increasing, at the same time unless perhaps the aircraft has whipstalled!

Details of two spiral dive recovery tests:

1) Trim full nose-up which yields 62 MIAS wings-level at 1950 rpms, which yields a constant altitude. Roll to 60 degrees bank, exerting no pitch pressure. Making no pitch inputs, hold the bank angle until the airspeed stabilizes. Airspeed eventually stabilizes at 97 MIAS. Turn rate indicator is pegged. Begin partial-panel recovery.

One effective partial-panel recovery technique at this point is to use rudder or coordinated rudder and aileron to roll opposite the turn rate needle deflection, stopping the roll at wings-level (some overshoot is likely--correct back the other way as needed until arriving at wings-level), while simultaneously feeding in forward stick pressure as needed to freeze the airspeed or allow only a slow decrease in airspeed, so that the roll to wings-level doesn't cause the nose to pitch up steeply. Continue to apply forward stick pressure whenever the airspeed is increasing, and aft stick pressure whenever the airspeed is decreasing, until the airspeed is frozen. Then ease the airspeed slowly down to trim speed.

No obvious differences were observed in trials involving left and right turns, nor was there any obvious oversensitivity of the turn rate indicator when the G-load was greater than one or greater than "normal" for the bank angle -- whatever gyro errors may have been present seemed to be relatively insignificant compared to the overall dynamics of the maneuver.

Another experiment, initiating the recovery sooner:

2) Trim full nose-up which yields 62 MIAS wings-level at 1950 rpms, which yields a constant altitude. Roll to 60 degrees bank, exerting no pitch pressure. Making no pitch inputs, hold the bank angle constant as the nose drops. This time begin the partial-panel recovery sooner, at the point of maximum nose-down pitch attitude and maximum rate of increase of airspeed, well before the airspeed peaks or stabilizes.

Again, the turn rate indicator is pegged. One effective partial-panel recovery technique is to use rudder or coordinated rudder and aileron to roll opposite the turn rate needle deflection, stopping the roll at wings-level (some overshoot is likely--correct back the other way as needed until arriving at wings-level), while simultaneously feeding in aft stick pressure as needed to help arrest the ongoing increase in airspeed. At some point before wings-level is reached, the airspeed may begin to decrease again, indicating that the nose has pitched up above horizontal due to the decrease in bank angle. If so, apply forward stick pressure as needed to arrest the increase in airspeed. Continue to apply forward stick pressure whenever the airspeed is increasing, and aft stick pressure whenever the airspeed is decreasing, until the airspeed is frozen. Then ease the airspeed slowly down to trim speed.

Again, no obvious differences were observed in trials involving left and right turns, nor was there any obvious oversensitivity of the turn rate indicator when the G-load was greater than one or greater than "normal" for the bank angle -- whatever gyro errors may have been present seemed to be relatively insignificant compared to the overall dynamics of the maneuver.

In actual practice of course it would be better to initiate the recovery even sooner-- but I wanted to test some extreme cases.

Please don't construe anything in this post to be a critique of some other partial-panel recovery technique for some other aircraft equipped with instruments of an older vintage-- I'm simply reporting what I observed during a couple of hours in the air this morning!

Last edited by flyer101flyer; 9th Dec 2013 at 23:01.
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Old 22nd Nov 2013, 18:33
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Theory of gyros-- turn rate indicators and turn coodinators

* Thinking more generally about turn rate indicators and turn coordinators:

* If the gyro were purely “vertical” in the aircraft’s own reference frame, it would measure yaw rate, not turn rate. A turn is a mix of pitching and yawing-- the steeper the bank angle, the more the pitching. A turn rate indicator with a gyro that stayed nearly “vertical” in the aircraft’s own reference frame could be calibrated to give an accurate turn rate at various bank angles at one specific airspeed or one specific angle-of-attack, but not at all possible airspeeds or all possible angles-of-attack.

* I’ve always assumed that the instrument simply is intended to give a pure yaw indication, with the difference between yaw rate and turn rate being considered negligible at the shallow bank angles typically used in instrument flying. I may be wrong in this assumption.

* Inherent in the design of the turn rate indicator, is that the gyro must be allowed to tilt (precess) in response to yaw. This response is harnessed to drive the movement of the needle or symbolic airplane through a geared mechanism. If the gyro spins clockwise as viewed from the right side of the aircraft, as illustrated here File:Turn indicators.png - Wikipedia, the free encyclopedia, the tilt will be in the direction that tends to keep the gyro somewhat closer to “vertical” relative to the earth, in a coordinated turn. If the gyro spins in the opposite direction, it will tilt in the opposite direction-- further away from “vertical” relative to the earth, in a banked turn.

* If the direction and amount of tilt (precession) in response to yaw is such that the gyro tends to stay rather close to vertical with respect to the earth as the bank angle varies, this will tend to reduce the error in turn rate indication induced by changes in bank angle and airspeed, for any given turn rate. I.e., the errors induced by the fact that a banked turn involves pitching as well as yawing. This may involve designing the gyro in such a way that the amount of tilt (precession) is fairly large. In fact, in the extreme case where the gyro really will stay completely vertical with respect to the earth, the amount of tilt (precession) would need to be equal to the bank angle. Some of the posts in the earlier thread http://www.pprune.org/tech-log/82721...gyro-axis.html seemed to raise this idea-- that the gyro tilts (precesses) so much much that it stays nearly vertical with respect to the earth. Really I’m being completely speculative here-- I have no idea whether turn rate indicators have ever been designed to have large amounts of tilt (precession) for this specific purpose, or not.

* It appears that if the tilting (precession) due to yaw was such that the gyro axis stayed perfectly vertical relative to the earth, the instrument would serve as a perfect turn rate indicator, not a yaw rate indicator. This could never happen perfectly across the whole spectrum of possible airspeeds and bank angles, for any given turn rate. (Because the tilt angle due to precession would need to be equal to the bank angle, and the relationship between bank angle and turn rate varies with airspeed.) The design of the instrument would have to be optimized for some particular target turn rate and bank angle and airspeed. After all, fundamentally, the gyro in a turn rate indicator is not free to float freely in all 3 axis and does not serve as a fixed platform relative to the earth, as does the gyro in an artificial horizon. Still, even if the gyro can't really stay close to vertical with respect to the earth, it seems better that the tilt (precession) of the gyro in response to yaw should be in the direction that keeps the gyro more nearly vertical with respect to the earth, than in the opposite direction, as would be the case if the gyro were spinning the opposite way.

* However, if we design the instrument so that the tilt (precession) in response to yaw is quite large--perhaps in an effort to keep the gyro somewhat close to vertical relative to the earth-- we create some other problems. The more the gyro tilts away from the aircraft’s own “vertical” during a turn, the more that a pitching motion will be sensed as yawing motion. We’ve noted that this pitch sensing may serve a useful purpose in terms of helping to indicate something closer to turn rate than pure yaw rate, in a steady-state turn. However, this pitch sensing will also cause the instrument to show an excessively high turn rate whenever the pilot pulls “extra” G’s, above and beyond the steady-state G-load for the bank angle. If the aircraft is to serve well during a partial-panel recovery from unusual attitudes, this seems like a very poor tradeoff.

* Alternatively, the gyro may be designed in such a way that the tilting (precession) in response to yaw is kept to a minimum--by using stiff centering springs as noted in post #4 in the earlier thread http://www.pprune.org/tech-log/82721...gyro-axis.html. This will keep gyro nearly “vertical” in the aircraft’s reference frame. Now the gyro will be much closer to a pure yaw rate sensor.

* In this case, for any given turn rate, the indicator will tend to read significantly low at a steep bank angle. The calibration marking on the face of the instrument an be arranged to take this into account, but this requires some assumption to be made about what the airspeed will be. A yaw rate sensor can't be calibrated to perfectly indicate turn rate for all possible combinations of airspeed and bank angle. However, this design will also minimize the instrument’s tendency to show an excessively high turn rate when the pilot pulls “extra” G’s, above and beyond the normal steady-state G-load for the bank angle. If the instrument is to serve well in a partial-panel recovery from unusual attitudes, this seems like a very good tradeoff.

* Thinking about the fact that I couldn’t clearly detect a relationship between G-load and gyro sensitivity in my recent tests, while others describe a need to unload the aircraft before reading the gyro-- maybe the design of modern turn rate indicators has evolved to the point where the tilting (precession) of the gyro is fairly small and thus pitching-related errors are minimized. Alternatively, maybe there is an advantage to designing turn rate indicators intended for use in high-speed aircraft to have more tilting (precession) while yawing, for better indication of something close to turn rate (not yaw rate) across a wider range of bank angles, so a test in a high-speed aircraft might yield significantly different results than I observed, even with an instrument of modern design.

* Whether the instrument is a turn rate indicator or turn coordinator, I can’t see any advantage to spinning the gyro the opposite way (counter-clockwise as viewed from the right side of the aircraft). This would cause the gyro to tilt (precess) away from true vertical during yawing, rather than toward true vertical. This would accentuate the error caused by the fact that a turn includes pitch as well as yaw-- if the indicator were calibrated to work well at shallow bank angles, then it would tend to read very much too low at steep bank angles. It’s hard to see why such a design would ever be used. Pitching would be sensed as a reduction in yawing. Pulling “extra” G’s above and beyond the steady-state value for the bank angle, would also result in a reduced indication of turn rate. If some turn coordinators really were designed in this way, it’s not hard to see why they would prove unsuitable for unusual attitude training, as suggested by post # 13 on the earlier thread http://www.pprune.org/tech-log/82721...gyro-axis.html. I don’t see anything inherent in the operation of a turn rate coordinator that requires that the rotor to spin in this way (counterclockwise as viewed from the aircraft’s right side.) This illustration File:Turn indicators.png - Wikipedia, the free encyclopedia suggests that the more common practice is for the rotor to spin the other way (clockwise as viewed from the aircraft’s right side), whether the instrument is a turn rate indicator or turn coordinator. My "research" has been limited to a bit of googling around on the web but I can't find any depiction of a turn coordinator with the gyro spinning counter-clockwise as viewed from the aircraft's right side, so I suspect this is not a common practice.

* One possible advantage to spinning the rotor counter-clockwise (as viewed from the aircraft’s right side) is that the direction of tilt of the needle (turn rate indicator) or symbolic aircraft (turn coordinator) is same as the direction of tilt (precession) of the gyro, rather than opposite as in the design illustrated here File:Turn indicators.png - Wikipedia, the free encyclopedia . This might simplify the operation of the mechanism-- yet it seems simple enough to include a reversal of direction in the gearing.

* As long as the rotor is spinning in the correct direction (clockwise as viewed from the right side of the aircraft), I still think that the combined yaw/roll sensing inherent in a turn coordinator will generally be helpful in partial-panel recoveries from unusual attitudes. The turn coordinator tends to make it easier to stop the roll out of a steep turn right at wings-level without overshooting into a turn in the opposite direction. As noted previously though, the combined roll/yaw sensing does present problems in recovering from an inverted spin.

* One more note-- what is "looping error"? In the older thread http://www.pprune.org/tech-log/82721...gyro-axis.html, post #11 seems to suggest (at least in my reading of it) that looping error is the tendency of the instrument to read too low at steep bank angles because the turn involves pitch as well as yaw, and the gyro is staying near vertical relative to the aircraft (not the earth) and so is sensing yaw, not turn rate. The posts suggests that this is considered when the markings are put on the instrument-- which suggests that the instrument is optimized for some particular airspeed.

On the other hand post # 13 in the same older thread seems to suggest that looping error is the instrument's tendency to over-deflect when the pilot pulls excess G's, because the gyro is significantly tilted relative to the aircraft's own "vertical".

Note that IF the gyro were always tilting (due to precession) exactly as needed to stay perfectly vertical relative to the earth, then the turn rate would be perfectly indicated at all bank angles and airspeeds, so long as there were no upward or downward curvature in the flight path as viewed by an external observer at the same altitude. I.e., as long as the G-loading was not higher or lower than the normal, steady-state value for that bank angle. Letting the gyro tilt relative to the aircraft's axis does NOT introduce an over-sensitivity in the turn rate indication as long as the gyro is not tilted beyond vertical with respect to the earth, and is long as the pilot is not pulling more G's than the normal steady-state value for that bank angle. Rather, the mixing of pitch sensing with yaw sensing is helping the gyro to detect something closer to turn rate, not yaw rate. But, I don't believe it is possible that the gyro behaves this way, consistently tilting to anything close to vertical with respect to the earth over a wide range of airspeeds and bank angles and turn rates.

Note also that as the bank angle is increased beyond 45 degrees, the yaw rate starts to decrease, not increase, so there's no way that precession would continue to drive a further tilt of the gyro. Of course, in the real world the gyro reaches its limit of tilt long before the bank angle reaches 45 degrees...

Last edited by flyer101flyer; 23rd Nov 2013 at 15:54.
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Old 10th Dec 2013, 00:31
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Left-right asymmetry in turn rate indicator response to G-loading/ pitching

This relates to post #4 above. After posting, I started second-guessing the results. Why should there be a difference in the response of the turn rate indicator to pitching or changes in G-loading in left turns versus right turns? In fact I made a post-- now deleted-- suggesting that what I was seeing was a real difference in the aircraft's yaw response to the imposition or relaxing of G-loading, due to gyroscopic effects from the propeller that converted pitching into yawing.

Well-- I flew again and it was very clear that the turn rate indicator in this particular aircraft (see post #4 for details) DOES respond differently to "extra" G's (above and beyond the "normal" amount for the bank angle) in left turns versus right turns. This was clearly NOT a real yaw effect, but an instrument error.

In fact in wings-level flight, "pulling" G's shifted the turn rate indicator needle to show a right turn, and "pushing" to unload to near zero G' shifted the turn rate indicator needle to show a left turn.

In a right turn, "pulling" extra G's caused the needle to show a higher turn rate, and "pushing" to reduce the G-loading caused the needle to show a lower turn rate.

In a left turn, "pulling" extra G's caused the needle to show a lower turn rate, and "pushing" to reduce the G-loading caused the needle to show a higher turn rate.

In some cases involving a mild (half standard rate) turn, a strong push or pull could actually cause the needle to cross the center line and show a turn in the opposite direction of what was actually occurring. For example, in a half-standard-rate left turn, introducing a net pull of force of 2.5 G's caused the needle to swing right of center and show a half-standard-rate right turn.

This reversal never occurred with stronger (standard rate or more) turns. For example, it was never possible to keep the instrument pegged to one side as the aircraft was rolled through level into a bank in the opposite direction, regardless what G-load was pulled.

(It has been noted to me that this IS possible with some turn rate indicators, presumably featuring a transverse/ spanwise spin axis, a clockwise rotation of spin as viewed from the right side of the aircraft, and fairly weak springs-- the instrument is so over-sensitive to pitching that a strong G-load will keep it fully pegged to one side even as the aircraft is rolled through wings-level into a bank in the opposite direction. In more detail:

"One of the looping error demonstrations we were shown was to roll in to a steep turn then apply G, this would peg the turn needle. Then, while maintaining the G, you would roll back through upright, in to a steep turn in the opposite direction. The needle would remain pegged indicating a turn in the original (wrong) direction. So it wasn't just G causing over reading but it was quite feasible to end up with the needle indicating in the wrong direction as well."

Hence the requirement to relax the G-loading before responding to the turn rate indicator needle, at least when flying with this particular type of turn rate indicator.)

Getting back to the turn rate indicator I've been describing in posts #4 and in the current post:

What does this left-right asymmetry mean about the physical design of this particular instrument? More on this below.

What about practical effects on partial-panel recoveries from unusual attitudes?

This left-right asymmetry does have some effect on recoveries from unusual attitudes. When trying to roll the aircraft to wings-level, pulling extra G's leads the pilot roll the aircraft too far to the left, and pushing to reduce the G-loading leads the pilot to roll the aircraft too far to the right.

However these effects are quite transitory. As long as the pilot is following the recovery technique I outlined in post #4, exerting "push" or "pull" force strictly according to the direction and rate of motion of the airspeed indicator needle, he'll never be "pushing" or "pulling" a strong G-loading for very long. Even if the turn rate indicator error leads the pilot to roll the aircraft beyond level, or stop the roll short of level, during the recovery, the resulting bank angle won't be that large, and it won't be long before the pilot has stopped exerting the extra "pull" or "push" force, at which point the turn rate indicator will guide the pilot to make another roll input to bring the wings closer to truly level. It seems very much more important to concentrate on making pitch inputs to stop the pitch phugoid, then worrying about small errors in the turn rate indicator. Recall again that these errors are really only prominent at very modest turn rates-- for example they will only lead to a wrong-way indication of turn direction if the actual turn rate is in the ball park of half-standard-rate or less, assuming that the pilot won't be pulling more than a couple of G's, as he shouldn't be. IF there is room in the pilot's mind to think about such a thing, it would be worth remembering to go easy on the left roll inputs when the G-low is high, and to go easy on the right roll inputs when the G-load is less than one or suspected to be lower than "normal" for the bank angle. But at least within the flight envelope of this particular aircraft, and given the characteristics of this particular instrument, it seemed one of the less important things for the pilot to be concerned with during the partial-panel recovery.

Again, for clarity, I don't doubt in the least that there are some combinations of aircraft and instrument design where it is very important to reduce the G-load before responding to the turn rate indicator needle-- as noted above, there are cases where the needle will remain pegged to one side even as the aircraft is rolled through wings-level into a turn in the opposite direction, while under heavy G-loading.

Moving on-- so, what does this left-right asymmetry suggest about the design of this particular instrument?

This seems to suggest that on this particular instrument, the gyro axis of spin may be longitudinal (fore-and-aft), not the more common lateral (spanwise) orientation.

In the more common arrangement, where the gyro spin axis is transverse/ spanwise/ lateral, the needle is driven by precession effects, which makes the gyro's spin axis tend to roll or bank in response to yaw. However, in this particular instrument, if the gyro's spin axis really is fore-and-aft or longitudinal, the needle must be driven by the gyro's inherent tendency act like a stable platform and resist yawing, so that the aircraft yaws around the gyro which tends to remains fixed in space until the springs force it to come along. For example, in a right turn, the gyro tends to be "left behind" until the springs force it to come along, and this tendency to be "left behind" results in a counterclockwise rotation of the gyro's spin axis as viewed from above, in the aircraft's reference frame, i.e. by the pilot looking down on the gyro wheel from above. A nose-up pitch torque would rotate the gyro's spin axis in the counterclockwise direction (as viewed from above) through precession, if the gyro wheel's direction of spin is counterclockwise as viewed from behind. In this case, a nose-up pitching motion would affect the gyro much as would a right turn. A nose-down pitching motion would affect the gyro much as would a left turn.

Actually another arrangement is possible-- and maybe more probable. With the same longitudinal or fore-and-aft gyro spin axis, but with a clockwise direction of gyro spin (as viewed from the rear), the mechanism could be geared to respond to the gyro axle's tendency to pitch nose-down and tail-up (due to precession) when a right yaw torque is applied to the gyro. The gyro axle would be connected to the pointer needle in such a way that this nose-down tail-up precession indicates a right turn. If the aircraft were pitching nose-up, this would have effect of rotating the gyro axle nose-down relative to the aircraft-- because the gyro axle tends to stay fixed in space. This would cause a right turn to be indicated. A nose-down pitching would cause a left turn to be indicated.

In the first arrangement, the tendency of the gyro axle to stay fixed in space is used to drive the pointer needle, and precession causes unwanted secondary effects. In the second arrangement, precession is used to drive the pointer needle, and the tendency of the gyro axle to stay fixed in space causes unwanted secondary effects. Since precession is used to drive the pointer needle in the case of the more common design with the transverse/ spanwise/ lateral gyro axle, that is probably the case with this instrument as well-- the second arrangement described above.

Either way, it appears that in the case of this particular instrument, the design cannot encourage the gyro's spin axis to stay in a fixed orientation relative to the earth, as is the case with some of the designs mentioned in the earlier thread noted on post #1. So there is no effort to measure true turn rate rather than yaw rotation rate.

There is obviously more diversity in the design of these instruments than I had realized. The existence of turn rate indicator gyros that spin in the roll-wise plane, as well as turn rate indicator gyros that spin in the pitch-wise plane, is stated here:

The Laws of Motion [Ch. 19 of See How It Flies]

"Sometimes the rate-of-turn needle is built to spin in the pitch-wise (ZX) plane, in which case the airplane’s yawing motion requires a torque in the roll-wise (YZ) direction. Other models spin in the roll-wise (YZ) plane, in which case yaw requires a torque in the pitch-wise (ZX) direction. In principle, the spin and the torque could be in any pair of planes perpendicular each other and perpendicular to the yaw-wise (XY) plane."

("See How it Flies" aviation training website by John S Dencker)

By the way, two paragraphs later, the author mentions the idea of a design that encourages the gyro axis to stay fixed in space to measure true turn rate rather than yaw rotation rate, as noted in the other earlier thread on the subject as noted in post #1. The clear implication is that this arrangement is NOT universal:

"Many rate gyros incorporate a sneaky trick. They spin around the pitch-wise (ZX) plane, with the top of the gyro spinning toward the rear. They also use a spring that is weak enough to allow the gyro to precess a little in the roll-wise (YZ) direction. In a turn to the left, precession will tilt the gyro a little to the right. That means that during a turn, the gyro’s tilt compensates for the airplane’s bank, leaving the gyro somewhat more aligned with the earth’s vertical axis. The goal, apparently, is to create an instrument that more nearly indicates heading change (relative to the earth’s vertical axis) rather than simply rotation in the airplane’s yaw-wise (XY) plane, which is not exactly horizontal during the turn. Since the relationship between bank angle and rate of turn depends on airspeed, load factor, et cetera, this trick can’t possibly achieve the goal except under special conditions."

An additional note: considering the apparent diversity in design of these instruments, I don't doubt at all that in the case of some turn rate indicator and/ or turn coordinator instruments, the gyro axis is transverse/ spanwise and the direction of spin is counterclockwise as viewed from the right side of the aircraft, rather than the more common spin direction of clockwise as viewed from the right side of the aircraft. I can see this would introduce an under-sensitivity to G-loading which would certainly seem problematic, as suggested in post #2. However nothing I've seen in my limited research suggests that this arrangement is common to most "turn coordinator" instruments as suggested in post #2. Still, it's interesting to think about how such an arrangement might help solve some problems, while possibly introducing other problems. For example the fact that the turn coordinator is sensing roll as well as yaw would seem to aggravate the tendency of the needle to stay stuck at full deflection even as the pilot rolls through wings-level into a turn in the opposite direction while maintaining a substantial G-load. Particularly if the gyro centering springs are fairly weak, not stiff. But this tendency might vanish if the direction of spin were counterclockwise as viewed from the right side of the aircraft. Accuracy in stabilized turns might suffer-- certainly there would be no tendency to indicate true turn rate rather than yaw rate-- but there might be significant advantages as well as disadvantages for partial-panel recovery from unusual attitudes.

Alternatively, if the gyro centering springs were quite stiff, the instrument might be almost uninfluenced by pitching / changes in G-loading, in which case the direction of spin of the gyro wouldn't make much difference. In the absence of rolling, such an instrument ould indicate something close to yaw rate, not true turn rate.

I'll certainly pay some more attention to some of things the next time I'm flying an aircraft with a turn coordinator-- I'll try to see if I can discern via response of the instrument to changes in G-loading, what the direction of spin of the gyro in that particular instrument is. A counter-clockwise spin of the gyro as viewed from the right should cause a decrease in reading when the G-load is increased, while a clockwise spin as viewed from the right should cause an increase in reading when the G-load is increased.

More detailed results of in-flight tests of turn rate indicator as noted at start of this post:

Results--

Wings-level flight:
Pull G's-- turn rate indicator shows a right turn
Push to reduce G-load-- turn rate indicator shows a left turn

Idle power, left turn: pulling extra G's decreases the indicated turn rate
Pushing to reduce G-load increases the indicated turn rate

Idle power, right turn-- pulling extra G's increases the indicated turn rate
Pushing to reduce G-load- decreases the indicated turn rate

Standard rate turn-- can't consistently cause a reversal in indicated turn direction by pulling extra G's or pushing to reduce G-load

1/2 standard rate turn to left-- pull 2.5 G's total-- indicator needle switches to indicate 1/2 standard rate turn to right

Right turn-- half standard rate--pushing to reduce G-loading -- needle moves to indicate a 1/2 standard-rate turn to left

Right turn-- one needle-width less than standard rate-- pushing to reduce G-loading moves needle to centered position

Right turn--standard rate-- pushing to reduce-G-loading-- needle moves about halfway toward centered position

Left turn-- pushing to reduce G-loading always causes an increase in indicated turn rate, despite the fact that the turn rate (and yaw rotation rate?) must actually be decreasing

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Old 20th Feb 2014, 01:44
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Snoop notes on turn coordinator

Re posts 1 and 2-- I recently took note of the behavior of a turn coordinator in flight, in a light plane. In a standard rate turn, a hefty pull on the yoke only increased the indication by about 25%. It 's quite possible that the actual turn rate was increased by this much. A hefty push unloading to perhaps 1/2 G (wild guestimate) seemed to have little effect on the reading in a right turn, and seemed to decrease the indication by about 25% in a left turn. One would expect some decrease-- the turn rate is decreased by "unloading" the wing.

All things considered I didn't see any evidence that the turn coordinator systematically reads significantly low under increased G-load and reads significantly high under reduced G-load, as has been suggested (see for example post #2). When I rolled into a 60-degree bank turn, and then rolled back to level, the instrument seemed to work well during the roll back to level. If I were using the instrument during an partial-panel recovery from an unusual attitude, there wouldn't seem to be any need to do anything special pitch-wise to manage the G-load for the sake of making the instrument read more accurately, as was described in the case of an older style of turn rate indicator in post #2. For example in a steep turn I would simply avoid pulling any excess load beyond what is already being generated in the steep turn with the yoke at trim, and roll toward level.

The roll-rate sensing does cause the instrument to tend to show level before you actually reach level, when rolling briskly out of a steep turn. In the context of recovery from a really unusual attitude, I think this is a good thing-- you slow the roll rate before reaching level and tend not to overshoot into a steep turn in the opposite direction. If you slow the roll rate just as much as is needed to hold the instrument indication at zero bank during the last 20 degrees or so of rollout, you'll arrive smoothly at wings-level with no tendency to overshoot into an opposite turn.

However the instrument did seem to be very oversensitive to small rolling or yawing motions when near wings-level-- much less so in a standard-rate turn, and even less so in a steeper turn. I suspect that something is not working quite right with this particular instrument-- and this problem may not be uncommon either. I recall seeing something like this in another aircraft. Anyway, the result is that if you were relying on this instrument alone to help you stay exactly wings-level, I think you'd be constantly chasing it back and forth, and likely nauseating everyone in the aircraft, even if you kept your inputs quite small. The combination of extreme sensitivity plus some noticeable lag is really annoying. It would be much easier to rely on the DG, or on a needle-and-ball turn rate indicator. Again though, I strongly suspect that something is not working quite right with this particular instrument, so that the motions are not sufficiently damped.

I tried to see if I could get the instrument to "stick" in a turn indication by maintaining a lot of G as I reversed the turn indication-- no such thing, it showed the reversal of turn direction even before I rolled through level-- as one would expect from the combined sensing of yaw and roll. Of course, "sticking" at full deflection while maintaining G would presumably be associated with an instrument that read too high under strong G load, not too low.

Unlike the particular needle-and-ball turn rate indicator I described in previous posts, when wings-level, pulling extra G's or unloading the G-load to less than one did not create any indication of turn.

All things considered I'd apply the same strategy for unsusual attitude recovery with this turn coordinator as with the needle-and-ball turn rate indicator I experimented with a few months ago:

"Based on what I observed, my own partial-panel recovery technique for light aircraft will not include pushing the stick forward to reduce the G-loading to 1 to eliminate errors in the reading of the turn rate indicator or turn coordinator."

I am well aware that the turn coordinator is particularly problematic an in inverted spin-- this is the one case where the combined yaw and roll sensing is completely unhelpful.

Also see the last few posts above-- there's apparently lot of diversity in the design of these instruments, so the above comments might not pertain to some other turn coordinator of a different design, especially an older design. The ultimate lesson I guess is-- be familiar with the characteristics of the instruments you are flying with.

Anyway, for the specific purpose of recovering from an unusual attitude (other than an inverted spin) without ripping the wings off the aircraft, I'd be happy to be flying with this turn coordinator. On the other hand, for the purpose of keeping the wings near level or precisely maintaining a standard rate turn, a needle-and-ball turn rate indicator might out-perform a turn coordinator in general, and certainly would outperform this particular instrument whenever the wings are near level-- this instrument is very over-sensitive whenever the wings are near level.

The oversensitivity seemed to have more to do with the aircraft being in wings-level flight than with the G-loading being near one-- again, reducing the G-loading while staying in a banked turn didn't cause the indicated turn rate to increase at all. I don't think what i was seeing could be described as "looping" error, or a "reversed" version of looping error, and I don't think it really had anything to do with whether the aircraft was rotating nose-up or nose-down in the pitch axis, or anything to do with the direction of spin of the gyro and whether it was the same or opposite of the most common design of needle-and-ball turn rate indicator. But I could be wrong...

PS I don't see this extreme oversensitivity to yaw and roll motions when the wings are near level, in my piezoelectric turn coordinator. Even when I tilt the sensor axis so that I get combined sensing of yaw and roll. I don't think the issue is something fundamentally inherent to combined yaw/roll sensing. Rather, I think the mechanical instrument is having some problem.

(Quote) (Quote) 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? (End quote)

Yes, because of the reversed rotation, the turn coordinater under reads under g. This makes it hard to find the wings level position.

AFIAK, the T and S rotates away from the pilot, the turn coordinator towards. There may be manufacurer's variations, but this is the case with the instruments I am (was) familiar with. (End quote)

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Old 20th Feb 2014, 05:58
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I vaguely recall the Turn Coordinator was useless over rate one and certainly completely cactus in a spin as it can display a turn in the wrong direction. It was never designed for unusual attitude flying. It was designed as a simple method for a non instrument rated pilot to make rate one turns using rudder alone - no aileron.

Page 3-13 of the Cessna 172N Information manual under the heading of Emergency Descent through Clouds, states in part: "In addition keep hands off the control wheel and steer a straight course with rudder control by monitoring the turn coordinator....Monitor turn coordinator and make corrections by rudder alone....check trend of compass card movement and make cautious corrections with rudder to stop the turn."

Early Turn and Slip indicators were calibrated from Rate One to Rate four turns and are reliable for recovering from spins in either direction as well as other UA's. It is important to unload any G when using the Turn needle to keeps wings level during a pull-out from a dive on limited panel. This is because G forces being applied in a pull out can drag the Turn needle to one side or another if the pull out is made with slight angle of bank present.
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Old 20th Feb 2014, 06:40
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Perhaps within the voluminous post(s), your instrument progresses from something other than this explanation:

Also on panel-- portable piezoelectric 1-axis turn rate indicator (sensing yaw only)

With a 1-axis instrument, I cant really see why virtually any of this is relevant?

There are some folks in SEA using 4D sensors...have you been in contact with them?
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Old 20th Feb 2014, 07:20
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I'm sorry, I don't understand the question (immediately above). I don't know what is meant by a 4-d sensor in this context. A 6-d sensor would detect acceleration as well as rotation, in all 3 axes. We can also sense the direction of the earth's magnetic field in all 3 axes after the manner of the "Bohli compass" which has been used as blind flying instrument long before these solid-state sensors existed, so perhaps we should speak of a 9-d sensor?

To be frank though, I find it more interesting to know just how much can be done with very little. I find it interesting how very much can be accomplished by reference to a simple one-axis rotation rate sensor-- particularly when it is optimized by tilting the sensing axis for mixed yaw / roll sensing.

The piezoelectric turn indicator is really just an electronic analog of a conventional turn rate indicator or turn coordinator (depending on how the gyro is canted.) It's interesting to compare the performance of the piezoelectric and conventional devices, to see what aspects of the observed behavior of the mechanical turn coordinator derive directly from the simple fact that we are sensing yaw as well as roll because we have canted the gyro axis, and what aspects of the observed behavior of the mechanical turn coordinator are due to other specific details in the way it is constructed, and likewise to see what aspects of the observed behavior of the mechanical turn rate indicator are due to specific details in the way it is constructed rather than the fundamental dynamics of flight.

I'm just interested in learning more about how all these devices work-- and in learning the best techniques for partial-panel recovery from unusual attitudes with each of these different devices.

S

Last edited by flyer101flyer; 20th Feb 2014 at 21:34.
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Old 20th Feb 2014, 16:04
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"I vaguely recall the Turn Coordinator was useless over rate one and certainly completely cactus in a spin as it can display a turn in the wrong direction. It was never designed for unusual attitude flying. It was designed as a simple method for a non instrument rated pilot to make rate one turns using rudder alone - no aileron."

I believe the above comments are in error. The truth is almost exactly the opposite.

See John S Denker's superb flight-training-and-physics website "See How It Flies":

See How It Flies and *6**Angle of Attack Stability, Trim, and Spiral Dives

6.2.4 Recovering From a Spiral Dive

"If you don’t have good outside references, you should not rely on the attitude indicator (artificial horizon). The attitude indicator contains a gyro mounted on ordinary mortal gimbals, which can only accommodate a limited range of pitch and bank angles. A steep spiral can easily cause the gyro to tumble, whereupon it will need several minutes of relatively straight and level flying before it can re-erect itself. Military aircraft have non-tumbling attitude indicators, but you’re not likely to find such things in a rented Skyhawk. Therefore, you should roll the wings level by reference to the rate-of-turn gyro.8 Being a rate gyro (as opposed to a free gyro) it has no gimbals, and therefore can’t possibly suffer from gimbal lock." And "8. That is, the turn needle or turn coordinator, whichever you happen to have."

I'd say that by sensing roll as well as yaw, the "turn coordinator" gives a quicker indication when an aircraft starts rolling into a turn, and also helps a pilot not "overshoot" when rolling back to wings-level with a substantial roll rate, after an accidental excursion to a steep bank. Really I'd say that it's in unusual attitude recovery that the mixed yaw-roll sensing offers the greatest benefits-- other than inverted spins and perhaps other negatively-loaded maneuvers (sustained inverted flight on instruments?!!)

Certainly I've never seen a turn coordinator be disturbed by steep turns-- it may be pegged, but then as the bank angle is reduced, it comes off the peg like it should.

In an upright spin, the direction of roll and yaw are the same, and the turn coordinator reads correctly. Meaning the gyro part of the indicator, not the ball. Forget about the ball in a spin.

I've flown with a 1-axis piezoelectric turn rate indicator which I could mount with the sensing axis either vertical (as in the old-fashioned needle-and-ball indicator) or canted about 35 degrees for mixed roll and yaw sensing (as in the so-called "turn coordinator"). In any aircraft that suffered from appreciable adverse yaw, if you were flying with aileron inputs only (no rudder inputs) the canted orientation dramatically outperformed the vertical orientation. By picking up on the rolling motion, the instrument tended to show very much less of a wrong-way indication as a turn was initiated, and the readings were very much less confusing. On the other hand if you were flying in a normal coordinated manner, or if you were flying with rudder inputs only, there was less difference in the performance of the two instruments. In case you are wondering what on earth is the point, please note that there are some perfectly good aircraft that don't have a rudder at all. But that's getting a bit off topic from what we usually talk about on the PPRuNe forum.

I believe that the over-sensitivity to small yaw or roll motions when flying wings-level that we see in some "turn coordinators" is largely due to a mechanical problem with the particular instrument.

For example: "Damping: Gyro rate instruments such as turn & banks and turn coordinators use silicone or a mechanical dashpot to dampen the needle movements. Over time silicone will eventually wick out making the instrument too sensitive requiring service." (Source: Gyro Tech Tips - Nu-Tek Aircraft Instruments, Inc. - www.Nu-TekInc.com )

And also: "The usefulness is also impaired if the internal dashpot is worn out. In the latter case, the instrument is said to be underdamped and in turbulence will indicate large full-scale deflections to the left and right, all of which are actually roll rate responses. In this condition it may not be possible for the pilot to maintain control of the aircraft in partial-panel operations in instrument meteorological conditions." -- Turn and balance indicator - Wikipedia, the free encyclopedia

On the other hand see also--

"The advantage of a turn coordinator is that it helps you anticipate what actions you need to take. That is, if the airplane has its wings level but is rolling to the right, it will probably be turning to the right pretty soon, so you might want to apply some aileron deflection. The disadvantage has to do with turbulence. Choppy air oftentimes causes the airplane to roll continually left and right. The roll rate can be significant, even if the bank angle never gets very large. The chop has relatively little effect on the heading. In such conditions a plain old rate-of-turn needle gives a more stable indication than a turn coordinator does." -- source *19**The Laws of Motion

By the way I think that when we are dealing with a purely electronic (eg piezoelectric) instrument rather than a mechanical gyro, this problem of an unstable indication in choppy air is significantly reduced.

As to the original intentions behind the design of the "turn coordinator", my understanding is that the idea of a canted gyro axis for mixed sensing of yaw and roll originally came about during the design of autopilots-- possibly using aileron inputs only, no rudder inputs. The autopilot worked so much better with the canted gyro axis, that the logical idea was that perhaps a human pilot would experience the same.

Read about it here CSOBeech - Old Bob's History of the Turn & Bank . However the comment here about the turn-and-bank indicator being unaffected by pitching is not accurate in all cases, as we have been discussing on this thread.

This author mentions not liking the "presentation" on the turn coordinator. To be quite honest I don't either. Even in the case where we do wish to retain the combined yaw-roll sensing by canting the gyro axis, I would prefer a presentation similar to the old-style needle-and-ball-- there is absolutely no ambiguity in this presentation. There is a sense in which the turn "coordinator" seems to move "backwards", when someone is used to looking at an artificial horizon.

Re the quote "The Cessna 172N Information manual under the heading of Emergency Descent through Clouds, states in part: "In addition keep hands off the control wheel and steer a straight course with rudder control by monitoring the turn coordinator....Monitor turn coordinator and make corrections by rudder alone....check trend of compass card movement and make cautious corrections with rudder to stop the turn." "

-- The specific reason the pilots are being told to use rudder only is that it is assumed they are not instrument trained and there is much less tendency to over-control the aircraft using rudder only. You could very easily do the same thing with a turn rate indicator-- there's going to be relatively little difference in the indications of the two instruments in rudder-only flight. In fact as long as the aircraft is near wings-level, this technique would probably work a bit better with a turn rate indicator than with a turn coordinator, because the turn rate indicator is purely sensing yaw, so the initial yaw response to the rudder will be strongly signalled immediately even before any roll starts. This will make the instrument more sensitive, which will help remind the pilot to make small inputs and not overcontrol the aircraft. But if the plane does get in a steeper turn, the turn coordinator will do a better job of helping the pilot anticipate as he is rolling the aircraft back to wings-level, and not go right on through into a turn in the opposite direction.

On the other hand if for some reason you were compelled to fly with ailerons only, and your aircraft had any appreciable amount of adverse yaw, you would be very happy to have a turn coordinator rather than a turn rate indicator. Any roll input is signalled much faster on the turn coordinator-- the turn rate indicator responds to a roll input (plus adverse yaw) with an initial wrong-way indication which can be very confusing in actual practice.

You can't infer from that passage that the turn coordinator was invented to facilitate rudder-only flight. If you pick the right year airplane handbook to search, I bet you would find a very similar comment but with reference to a turn rate indicator. If not, it's only because the older manuals tended to be rather spartan.

By the way that passage is missing one particular small bit of information that would make that emergency technique have much greater probability of a successful outcome. But moving right along...

The name "turn coordinator" is completely confusing-- it has nothing to do with "coordinating" your turns. It actually allows you to better get away with less "coordinated" inputs (no rudder). But it also is quicker to show the initiation of a roll and turn away from wings-level, and also helps you stop a rapid roll out of a steep turn right at wings-level without overshooting into a turn in the opposite direction.

I would suggest that the situation where the canted gyro axis really outperforms the vertical gyro axis most clearly, is when a pilot is caught off guard in a partial-panel situation, perhaps without recent refresher training, and needs to recover from a really steep turn without accidentally rolling into a turn in the opposite direction. The fact that the turn coordinator instrument will start to show a reduced or even zero turn rate as the aircraft approaches wings-level, allows the pilot to confidently apply a substantial roll rate to bring the situation under control swiftly, without fear of overshooting right on past wings-level. In the same situation a needle-and-ball turn rate indicator might stay near pegged and then rapidly cross through centered into a turn in the other direction, as the pilot inadvertently rolls briskly right on past wings-level. In this situation certainly I'd rather have any turn coordinator, no matter how worn out the dashpot/ oversensitive near wings-level, than the particular design of needle-and-ball turn rate indicator described in post #2, where excess G's might cause the needle to "stick" in a strong turn indication even as the aircraft was actually being rolled through wings-level into the opposite bank and turn! (Granted, as noted in recent posts above, by no means do all needle-and-ball turn rate indicators seem to have this problem.) As far as precisely holding heading-- i.e. keeping the wings exactly level-- that's not so important as keeping the aircraft all in one piece. We can use GPS for staying on our intended heading. Or the DG if it's still on line. I guess my opinion is in the minority though-- the preference seems to bash the "turn coordinator".

Except, again, I do think the turn coordinator (canted gyro axis) would serve even better, especially when a relatively inexperienced pilot was caught off guard in a partial-panel situation, if it adopted the simpler display face of the turn rate indicator, where there is no possibility of reading the instrument "backwards". I'm not a fan of the tippy little airplane that too much resembles the artificial horizon line.

Re the quote above " It is important to unload any G when using the Turn needle to keeps wings level during a pull-out from a dive on limited panel. This is because G forces being applied in a pull out can drag the Turn needle to one side or another if the pull out is made with slight angle of bank present."

This has been discussed on this thread already; I surely agree that this would not be good. I've seen no evidence of this when flying with a "turn coordinator", as detailed above, and also when flying with a "turn rate indicator", as detailed in posts 4,5, and 6. It's going to depend on the design of the particular instrument. This issue is due to design features in older instruments which may be absent in newer instruments. For example there's more than one way to orient the gyro in a "turn rate indicator"-- this is detailed in the latter half of post 6. If gyro is mounted in the most common orientation, and rotates in the most common direction of spin, and the centering springs are weak, two things happen-- 1) the gyro tends to stay somewhat fixed relative to the earth so it does a better job of measuring true turn rate rather than yaw rate at least for turns near the intended rate for which the instrument is optimized, and 2) you get these problems where the instrument responds by over-reading during pitching during maneuvers with a high nose-up pitch rate, and may even "stick" in a deflection one way even as the turn rate is reversed, if the high-G load and pitch rate is not relaxed during the reversal. "Looping error". If the springs are stiff this sort of problem is going to be minimized, regardless of the direction of spin of the gyro. With a different instrument design (different gyro orientation), whatever small sensitivity the gyro shows to pitching may be quite different than the "classic" looping error. See post 6 for more, including a link to a reference source. Know the instrument you are flying with...

The long and short of what I've learned since starting this thread, is

a) there is a lot of variety in the design of these instruments. It's worth taking time to explore the characteristics of the instruments you are flying with.

b) "looping error" is not a problem in practice in many instruments of relatively modern design, be they "turn rate indicators" or "turn coordinators". Specific unusual-attitude recovery procedures developed for some types of older instruments may not be appropriate or necessary for other instruments. E.g. the requirement to reduce the G-load before reading the instrument.

c) "turn rate indicators" and "turn coordinators" both have some specific advantages and disadvantages

d) there are a LOT of misconceptions out there about "turn coordinators"

e) there are some specific maintenance issues that can make a turn coordinator become very over-sensitive especially when the aircraft is near wings-level. Turn rate indicators may develop the same problem as well, but the problem may appear more pronounced in a turn coordinator in turbulence or "chop", because roll is detected, not just yaw.

A final thing occurs to me. The faster the flight speed, the lower the turn rate and yaw rate for a given bank angle, so perhaps less tilt in the gyro sensing axis would be appropriate. So that we sense less roll and more yaw. Otherwise the deflections due to roll might totally swamp the deflections due to yaw. Or is it the opposite-- the higher the flight speed and the lower the yaw rate, the more interested we are in having a direct indication of roll direction and rate, so lots of tilt in the sensing axis for lots of roll sensing is a good thing?



S

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PS I spoke to a former USAF instructor pilot (T-37's) yesterday, whose military and civilian career also spanned through C-130's and MD-11's. He was completely unfamiliar with any need to reduce G's before reading the needle-and-ball turn rate indicator, and was also completely unfamiliar with this scenario where under heavy G's, the turn rate indicator may continuously be deflected to one side even as the direction of bank and turn actually goes through a reversal, as the aircraft is rolled through wings-level and beyond. He said "sounds like something from the dustbin of history".

Sounds to me like the designers and manufacturers of these instruments eventually realized it was best to use stiff centering springs, to cut this "looping error" to a minimum.

And I also suspect the issues that have been observed re oversensitivity of the "turn coordinator" in flight conditions close to straight-and-level have nothing to with "looping error", or with the direction of spin of the gyro. "Looping error" should be zero when the aircraft is near wings level and plus 1G. The "wrong" direction of spin should give you something analogous to "looping error" when the aircraft is engaged in a nose-down pitch rotation, producing less than 1 G, or even minus G -- but except for a few very exotic cases, it's impossible to sustain a substantial nose-down pitch rotation for more than a few seconds without heading for something like a downward vertical dive (or sustained inverted flight, in which case any sustained turn will have a nose-down pitch rotation) ... that doesn't have much in common with flight near wings-level at approximately +1 G.

S
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