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attitude indicator and centrifugal force

mightypen (Aeronautics) (OP)
16 Aug 11 16:54
I was wondering about the basics of attitude indicators.
My questions are:
How does centrifugal force affect the attitude indicator? When the plane banks, can the indicator tell the difference between centrifugal force and gravity?
What about digital indicators? Can they keep an accurate reading while centrifugal force acts on them?
Can fighter jets' indicators keep an accurate reading while they are banking with multiple g-forces?
Thanks!
mighty
 
IRstuff (Aerospace)
16 Aug 11 17:57
Attitude sensors are generally gyros, and as such, are not affected by linear forces.

TTFN

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Kwan (Aerospace)
17 Aug 11 12:10
There is information about AI errors on this web site.  Scroll down until you see errors.

http://www.pilotfriend.com/training/flight_training/fxd_wing/attitude.htm
debodine (Electrical)
17 Aug 11 12:47
mightypen, if you are speaking of attitude indicators driven by rotating mass gyros, then the link from Kwan covers that information very well.  Note that the errors discussed in the article apply regardless of whether the rotating mass is inside the instrument itself or remotely located elsewhere in the aircraft.

As to your question about digital indicators, keep in mind that digital indictators typically draw their inputs from a remote mounted sensor, and thus the narrow answer to your question is that digital indicators are not affected by these forces.  However, the complete answer that I suspect you wanted to know is that the remote sensor to which they connect and from which they obtain their data will be subject to those forces.

So if the remote sensor is a rotating mass gyro, the same errors apply, although remote mounted sensors normally have superior error correction capability.  That is one of the reasons they are remote mounted...to have the space to include capable error correction circuitry.

If your indicator uses a system as its sensor that instead of rotating mass has accelerometers or magnetometers (usually one or more for each axis - longitudinal, lateral and vertical), the circuitry that processes the combined signals does some thorough error correction before the signal is sent to the indicator for viewing by the pilot.

Hope this helps!
mightypen (Aeronautics) (OP)
17 Aug 11 13:51
thanks debodine and kwan, your answers were very helpful.

however, I'm still not clear on whether there is an attitude indicator that can keep an accurate reading during multiple changes of multiple g-forces, such as a fighter jet may experience in operation.

Also,  by digital, I meant is there an attitude indicator that operates without any gyro at all? (like how an iphone knows which way it is facing at all times). With new digital technology, are we beyond the need for gyros to tell us attitude?

Thanks again!
mighty
rb1957 (Aerospace)
17 Aug 11 13:59
i reckon there is, since fighters seem to be flying quite well.  don't they also have a laser-based INS ?
mightypen (Aeronautics) (OP)
17 Aug 11 14:16
laser-based INS? What's that?
mightypen (Aeronautics) (OP)
17 Aug 11 14:24
nvmd, I looked up INS. Thanks, that's very helpful
IRstuff (Aerospace)
17 Aug 11 14:35
Heading errors during a high-g turn are irrelevant.  The only reason for doing such a turn is because the plane is dogfighting, in which case visual flight rules apply.

In any case, a fighter's attitude displays are different, in that they are NOT derived from a single sensor; the mission computer has total knowledge of all sensor inputs, including GPS, and can generate synthetic data that represents the aircraft's true heading and attitude regardless of the turn conditions.


INS is simply the processor plus gyros plus accels plus GPS plus other ancillary sensors

TTFN

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moon161 (Mechanical)
24 Aug 11 13:22
An ADI doesn't sense 'down' or the direction to the center of the earth, it senses rotations in 3 axes of the a/c from where it started. Not sure how it is corrected as an a/c travels significantly around the curve of the earth.

Thinking of a classical ADI, a horizon and directional gyro, each in a gimbals to allow three axes of rotation. It is quite conceivable and proably not very hard to design the gyro and gimbals such that the CG of the moving assy is always at the intersection of the axes of rotation with necesary precision. As such, linear inertial forces would not cause the gyro to move.

Now Laser ring gyros are something completely different. Look it up.
kl7sg (Aerospace)
24 Aug 11 20:59
Depending on the construction of the attitude indicator, the gyro is effected by external forces such as gravity and lateral accelerations that may be felt during a non-coordinated turn.  The older gyros were mechanicaly aligned with the earth by a system of rotating balls.  Thus, if you were in an extended turn that wasn't coordinated, the gyro alignment would start to change, via the erection mechanism, and the indicated pitch angle would be impacted.  Also, if your turn is not coordinated, a force is present on the gyro that will cause real precession at right angles to the applied force resulting in a change in indicated angle.  More modern mechanical gyros use electronic means and torque motors to keep the gyro level.  During turns, the erection mechanism, would be shut off to minimize the undesirable effects of turns.  

As technology has improved, the undesirable effects of the external applied forces have been greatly improved.  Examples of improved gyros are laser ring and solid state.  But even with these improvements, mother nature still has an effect an attitude indicators.

Also, gyros are impacted by apparent precession, this is when the gyro alignment relative to the earth changes because of the rotation of the earth.  Apparent precession has a large impact on directional gyros and this is corrected by the flux gate correction signal.

Anyway, cheers.
mightypen (Aeronautics) (OP)
25 Aug 11 1:52
thanks everyone! you've been extremely helpful!
mighty

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