Artificial Horizon (Turning & Acceleration Errors)

[akafrank07]

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

[Lightning Mate]

I assume you are referring to the air driven variety.

If so, the explanation is simple.

[Jockster]

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.

[cefey]

@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?

[RTN11]

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.

[akafrank07]

Thanks for the explanation jockster :ok:

[wiggy]

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

[deefer dog]

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.

[keith williams]

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 (http://www.theairlinepilots.com/forum/viewtopic.php?t=905)

[Alex Whittingham]

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.