PPRuNe Forums - View Single Post - Turn Rate Indicator / Turn Coordinator / Looping Error
Old 20th Feb 2014, 17:04
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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 donít 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 youíre 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 canít 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?


Last edited by flyer101flyer; 24th Feb 2014 at 22:55.
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