PPRuNe Forums - View Single Post - Turn Rate Indicator / Turn Coordinator / Looping Error
Old 10th Dec 2013, 01:31
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flyer101flyer
 
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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

Last edited by flyer101flyer; 10th Dec 2013 at 16:32.
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