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Gyro turning errors

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Gyro turning errors

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Old 13th Mar 2018, 13:23
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Gyro turning errors

Just reading an article about it. The horizon slowly moving toward wings level in a prolonged turn. Anybody ever seen this phenomenon, perhaps much earlier in their flying experiences.
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Old 13th Mar 2018, 16:59
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Originally Posted by JammedStab View Post
Just reading an article about it. The horizon slowly moving toward wings level in a prolonged turn. Anybody ever seen this phenomenon, perhaps much earlier in their flying experiences.
Yep. On a forces basic trainer with a 50’s/60’s instrument fit..as I remember it down to “pendulous vanes” or something of a similar nature.
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Old 13th Mar 2018, 17:49
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The air-driven gyro-horizon suffers from acceleration errors. These are due to two design features:

1. The whole gyro case is slightly bottom-heavy, to reduce the time for erection*; and
2. The pendulous vanes, alluded to above, are also, as their name suggests, pendulous and free to swing on their pivots.

The acceleration can be fore and aft, as in an increase or decrease in speed, or sideways, as in the acceleration towards the centre of a circular flight-path (turning flight).

During a longitudinal acceleration, the whole unit will experience a forward-tilting force, due to the inertial lag of the bottom-heavy gyro case. This force is precessed and becomes a sideways-titling force, resulting in an apparent angle of bank.

At the same time, the transverse-mounted vanes will lag rearwards, covering one port and uncovering its opposite counterpart. This will result in a jet of air acting to tilt the bottom of the case sideways. This force is again precessed, resulting in an actual fore-aft tilt, indicating an apparent angle of pitch.

A similar effect occurs due to the centripetal acceleration experienced during a steady turn, the combination of precessed forces acting to show small cyclical, out-of-phase bank and pitch errors, reaching their maxima and minima at 90, 180, 270 and 360 degrees of heading change.

An attempt to minimise these errors is made by slightly off-setting the axis of the gyro.

The classic example of this was during the introduction of the early jet fighters, where the rapid acceleration of a night or IMC take-off could result in an apparent climb to starboard. The pilot might over-correct and then induce a descending turn to port, crashing to the left of the extended centreline.

Most light aircraft are unable to produce longitudinal accelerations sufficient to cause a dangerous effect. The newer electrically-driven gyros and our modern [email protected] have reduced the errors.

*note to self; must investigate further....
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Old 13th Mar 2018, 18:28
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but not moving towards wings level in a prolonged turn, it shouldn't do that.
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Old 13th Mar 2018, 19:41
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actually, I have witnessed that effect thermalling in a glider. I don't know the physics of it and don't even know what horizon it was and I only noticed it once I had rolled out of the thermal and set off for the next one. Horizon was a few degrees off. It sorted itself out again a few minutes later. Ah.. summer of 1995..how I miss thee....
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Old 13th Mar 2018, 21:06
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The devices are 'self-erecting' i.e. designed to level with the horizon themselves without intervention.

What that means is they act as a gyroscopic artificial horizon in the short term and a earth-referenced 'plumb bob' horizon in the long term.

In short, if you hold a coordinated turn long enough the AH will turn towards 'wings level' as the coordinated turn produces a local 'cabin gravity' identical (in direction) to that experienced by the instrument when the aircraft is in a 'wings level' position.

Only a coordinated turn produces this effect, as a fixed angle of pitch or bank does not change the local 'cabin gravity'.

Note that this is a design feature, it is possible to design an AH with no inherent 'self erecting' capability but the precision of the gyroscopes required is much higher if the device is to have any long term stability, more akin to an inertial platform.
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Old 13th Mar 2018, 21:54
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Originally Posted by Lascaille View Post
The devices are 'self-erecting' i.e. designed to level with the horizon themselves without intervention.

What that means is they act as a gyroscopic artificial horizon in the short term and a earth-referenced 'plumb bob' horizon in the long term.

In short, if you hold a coordinated turn long enough the AH will turn towards 'wings level' as the coordinated turn produces a local 'cabin gravity' identical (in direction) to that experienced by the instrument when the aircraft is in a 'wings level' position.
Earlier I posted the same thing, that over time it should align itself with "cabin gravity" since that's the only up/down reference it knows. But after a quick bit of research I changed my mind and deleted it. Because the pendulous vanes are on the gyro case itself, i.e., the innermost part of the instrument that stays fixed in space, inside of all the airplane's free motions around it. So, let's say you're flying North and concerned with rigidity and errors while banking left or right... as far as the inner case, you shouldn't think of it as banking left or right, but rather banking West or East, because it's gonna (mostly) stay fixed in space.

If you start a right turn, the "cabin gravity" banks East (referencing the up direction of it, not down) and starts accumulating a slight bit of that error. Well, as you come through 180 degrees of turn headed South and stay in the right bank, the Cabin G is now banked West and the continued error accumulation cancels the East-banked error accumulated in the first 180 degrees. So every continued half-circle of a turn accumulates an error that cancels the previously accumulated error.

At least as far as I understand it... maybe I'll learn some more that causes me to reverse position once again!
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Old 13th Mar 2018, 22:02
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Originally Posted by tommoutrie View Post
actually, I have witnessed that effect thermalling in a glider. I don't know the physics of it and don't even know what horizon it was and I only noticed it once I had rolled out of the thermal and set off for the next one. Horizon was a few degrees off. It sorted itself out again a few minutes later. Ah.. summer of 1995..how I miss thee....
If what I wrote just above is right (and I'm not sure, just supposing) then there's no accumulated continuous bank error. But there are still all the shorter-term errors from Eckhard's detailed post. What was the power source of your gyro, and its overall quality (of the bearings)? Because the weaker those are, the less rigid its rigidity in space will be, and the more susceptible it will be to those errors. I had some time (VFR only!) in a plane with 70-year old clapped out gyros that you could hear grinding like a tractor. No doubt further degraded by acro nearly nearly every flight. Powered by a vacuum pump last overhauled God knows when (if at all). So they were completely worthless and did their own thing.
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Old 14th Mar 2018, 05:53
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Have a look at the following:
FL-107 Gyro Erection Limitations Course
1journey.net/1candle/av/FL-107/FL-107.htm

Item 4 in: False Inflight Topples.

Wish i had read this article when i was working on aircraft.
Sperry Vertcal Gyro's VG-14 showed false horizon when on constant bank angle 20 minutes or so. It was recommended to do figure 8 turns if in long holding pattern.
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Old 14th Mar 2018, 11:06
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Originally Posted by Vessbot View Post
Earlier I posted the same thing, that over time it should align itself with "cabin gravity" since that's the only up/down reference it knows. But after a quick bit of research I changed my mind and deleted it. Because the pendulous vanes are on the gyro case itself, i.e., the innermost part of the instrument that stays fixed in space, inside of all the airplane's free motions around it. So, let's say you're flying North and concerned with rigidity and errors while banking left or right... as far as the inner case, you shouldn't think of it as banking left or right, but rather banking West or East, because it's gonna (mostly) stay fixed in space.

If you start a right turn, the "cabin gravity" banks East (referencing the up direction of it, not down)
I think you're correct but misunderstanding how the 'cabin gravity' affects the vanes. In a 360 degree perfectly coordinated turn each of the pendulous vanes will open and close according to a sinusoidal pattern with each vane's degree of openness being 90 degrees out of phase with the next one. The outcome is a _constant_ force which rotates around the axis of the gyroscope's spin. The force is located below the gyroscope's center of gravity. The outcome - as far as I can see - is a constant force acting to push the base of the gyroscope into a position which results in an equal force being applied by all the vanes. In level flight that will result in zero vane motion over time. In a coordinated turn that will (eventually) result in the equal opening of each vane in turn once the gyroscope has become aligned with cabin gravity - different vanes will open as the aircraft adopts a heading of 90, 180, etc as the gyroscope remains fixed in position. With the turn being coordinated, once the gyroscope has become aligned with cabin gravity the changing bearing of the turn manifests only as yaw, and single axis vertical-rotating gyroscope is not affected by yaw so the bearing component of the turn has no overall effect.

That's probably not very well explained.
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Old 14th Mar 2018, 11:21
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Flying a 737-200 on a 1000 mile leg over the South Pacific. Blue skies. Visited the cabin and got talking, leaving the experienced co-pilot up front. On return to cockpit I noticed the copilot reading a magazine which is why he had not noticed we were gently going off track. I checked the cross track reading on the INS and saw it read 25 miles off track. Noticed the FD 108 artificial horizon showing about 6 degrees from level flight and aircraft heading 30 degrees from flight plan heading.
What happened was slight turbulence had occurred during the time I was down the back but was corrected by the autopilot. The bank angle corrections showed on the ADI. When the turbulence stopped the AP was still in the process of correcting back to level flight with the ADI showing 6 degrees angle of bank. Inside 6 degrees angle of bank the autopilot will automatically go wings level but when the turbulence stopped the angle of bank was close to 6.5 degrees and stayed there. The prolonged angle of bank of 6.5 went unnoticed by the co-pilot and gyro drift started. We manually returned to track while the ADI continued to display slight bank angle for the rest of the flight.

Last edited by Centaurus; 14th Mar 2018 at 11:32.
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Old 14th Mar 2018, 11:47
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Most light aircraft are unable to produce longitudinal accelerations sufficient to cause a dangerous effect
Agree. But over the years I have read accident reports where the Light aircraft have crashed during the initial climb after dark night takeoffs. Because of lack of FDR's in light piston singles and the pilot was inexperienced, it is anyone's guess what caused the aircraft to crash. Mostly it boils down to poor instrument flying skills in IMC rather the pilot being affected by somatogravic illusions.
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