PPRuNe Forums - View Single Post - Turn Rate Indicator / Turn Coordinator / Looping Error
Old 20th Feb 2014, 02:44
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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)

Last edited by flyer101flyer; 20th Feb 2014 at 16:32.
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