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# Artificial Horizon (Turning & Acceleration Errors)

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# Artificial Horizon (Turning & Acceleration Errors)

9th Jan 2013, 01:36

Join Date: Jun 2010
Location: Belfast
Age: 31
Posts: 98
Artificial Horizon (Turning & Acceleration Errors)

Abstract:
"The artificial horizon experiences the following errors:
1.Turning errors
2.Acceleration errors
3.Real wander of the gyroscope's spin axis away from its alignment with the earth vertical.

Turning and acceleration errors in the artificial horizon gyroscope are caused by lateral acceleration in turns and by the aircraft's acceleration and deceleration forces that induce a false position indication of the gyroscope's vertical axis.

Note: Remember, the artificial horizon is an earth gyroscope with its spin axis in the earth vertical."

I would like to know what it means by lateral acceleration in turns? further into that explanation what/why does these acceleration turns induce a false position indication of the gyroscope's vertical axis?

Thanks
9th Jan 2013, 07:07

Join Date: Apr 2009
Location: down south
Age: 74
Posts: 13,228
I assume you are referring to the air driven variety.

If so, the explanation is simple.
9th Jan 2013, 13:27

Join Date: Nov 2009
Location: uk
Age: 56
Posts: 72
The pendulous vanes which are used to erect the gyro react to the centrifugal force produced in a turn causing it to erect to false datum. Also the 'bottom heaviness' of the gyro case also reacts to the same centrifugal forces causing the following: -

90 degrees of turn - Indicates Climb and underbank.
180 degrees of turn -Indicates Climb and normal bank.
270 degrees of turn - Indicates Climb and overbank.
360 degrees of turn - Indicates normal attitude and normal bank.

The precession rate is only small so not much error.

Thats why holding patterns are the shape they are rather than circular orbits.
9th Jan 2013, 15:00

Join Date: Feb 2011
Location: Europe
Posts: 271
@Jockster
What does it have to do with holding pattern?
In HP you make 180 turns at standart rate and you time your outbound leg.
You dont aim to make it a orbit at all, since it would be a lot harder to hold over the same position and would be very unpleasant for passengers...? Or Am I wrong?
9th Jan 2013, 23:02

Join Date: May 2008
Location: UK
Posts: 1,365
But the acceleration errors are minimal unless you are accelerating an awful lot, I don't think many civilian aircraft will face this trouble.

And it has nothing to do with hold pattern design, the straight legs are so you can take account of the wind, even with a perfect instrument you would have to constantly vary the angle of bank in order to orbit over a fixed point in the real world, something which would be impossible without an FMS doing the flying for you.
13th Jan 2013, 13:23

Join Date: Jun 2010
Location: Belfast
Age: 31
Posts: 98
Thanks for the explanation jockster
14th Jan 2013, 07:12

Join Date: Feb 2001
Location: The Winchester
Posts: 6,082
Just for completeness you definitely can see acceleration errors in run of the mill civil aircraft, I've seen it more than once and by coincidence this comment has just come up in another thread - remember if you're turning, you're accelerating.

....an inherent limitation of gyro technology...VG errors may be caused by shallow turns//..in effect the VG aligns itself with bank angles of less than six degrees but lags due to the slowness of the erection circuit
2nd May 2017, 16:26

Join Date: May 2010
Location: europe
Age: 64
Posts: 643
The pendulous vanes which are used to erect the gyro react to the centrifugal force produced in a turn causing it to erect to false datum. Also the 'bottom heaviness' of the gyro case also reacts to the same centrifugal forces causing the following: -

90 degrees of turn - Indicates Climb and underbank.
180 degrees of turn -Indicates Climb and normal bank.
270 degrees of turn - Indicates Climb and overbank.
360 degrees of turn - Indicates normal attitude and normal bank.
Yes but why, once the turn is established and maintained, would these errors change from indicating underbank to correct bank to overbank if the same centrifugal (or is it centripetal?) force was applied throughout?

Just asking because I really don't understand.
2nd May 2017, 19:35

Join Date: Jan 2011
Location: England
Posts: 649
The errors are not simply due to centrifugal force. The inertia of the pendulous vanes also plays an important part. This subject is too complex to explain in a single post here, but if you follow the attached link you will find an explanation of how these instruments work.

Once you have a good mental picture of how they work, you should be able to think through what happens during a full 360 degree turn.

Having said all of that, most students simply memorize the questions and answers when preparing for the ATPL exams.

www.TheAirlinePilots.com :: View topic - Air Driven Artificial Horizon
3rd May 2017, 08:17

Join Date: May 1999
Location: Bristol, England
Age: 62
Posts: 1,636
Keith is correct, the errors and the cause of them throughout the orbit are too complex to work through in detail.

It may help you to remember that the gyro attempts to re-erect to a false datum in space, not relative to the aircraft.

Initially, and in a right turn, you might expect the pendulous base of the gyro to swing out to the left leading to a roll error (underbank) but as the turn continues and the aircraft direction changes what was a false datum to the left progressively becomes a false datum to the rear and therefore becomes a pitch error. Now put precession into the pot and recall that the gyro is not erected to a single false datum but is continually being re-erected to a changing false datum in space throughout the turn and you will start to get a feel for the complexity of forces.

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