11 replies to this topic

[frontiersim]

In the fs realism settings what will gyro drift do? thanks

[Lateral-G]

Imagine such a gyro system being exactly aligned to the horizon and to north. In a perfect world this system, whilst in motion, would be able to calculate, from its inertial sensors, the present orientation and position, assuming the starting co-ordinates are known.
In other words, the sensors measure rates and acceleration, which are integrated over time to relative angles and positions; these are then transformed mathematically. With known starting co-ordinates and the transformed sensor signals true orientation and position can then be calculated.

Alas, there is no perfect world; gyros and accelerometers have an offset, known as bias. The gyro bias for instance, shows itself after integration as an angular drift, increasing rise linearly over time (ramp).

What happens to the accelerometer signal is even worse. Following double integration so as to obtain position the sensor bias now gives rise to a positional drift error raised to the power of 2. The signals with their drifts are transformed and so the drifts are also present in the reference co-ordinate system. This means that the errors of orientation and position are increasing proportionally with the elapsed time.
Fortunately, there are methods to compensate for drifts of this nature.

    
To compensate for sensor drift, a process called augmentation is used, whereby gyro and acceleration errors are compensated by utilising other system states. For instance the angular drift error of a gyro after integration can be compensated, if true angular measurements are taken. Comparing the two shows then the gyro drift.

Assuming that inertial systems are generally operated around an equilibrium position and thus linear acceleration integrates to zero over time, then attitude can be processed via gravitational acceleration, thus giving true angular measurements to compensate gyro drift.

If determination of position is not needed, then inclinometers can be used
(inclinometers are cheaper and provide the same or even better degree of angular resolution).

This can be interpreted as follows :

The gyros are responsible for measuring the dynamic part of system motion, while the inclinometers give us, over a longer period, a precise absolute measurement of attitude. Combining these measurements makes precise attitude information available (in other words, the drift is now observable).
The heading (azimuth) can not be compensated with this method, because the acceleration vector of gravity is parallel to the Yaw-axis of the reference co-ordinate system.

With earth-fixed or low-dynamic systems (gyro compass) geographical north can be found via the earths rotation. For this application high-quality gyros are recommended where the bias is typically less then 0.1 to 0.01 °/h.

In small GA aircraft gyros are mechanical devices that succumb to friction and lose precision over time. Periodically over a flight the pilot has to use the set knob on them to tweak the heading.


-G-

[frontiersim]

thank you very much for that very detailed answer.

[slick]

Even I didn't know that. Great answer. It's true about learning something new everyday  

[sweetfracture3]

I just figured it was in refrence to the heading indicator precessing due to friciton and the like  

[jnb]

Though I doubt I understand it as well as Lateral-G  
Here's a simple analogy.

The analogy is a bullet coming out of the barrel of a rifle. There are spirals (rifling) cast into the interior of the barrel that cause the bullet to spin/twist rapidly in flight. As a result, any problems that may cause the bullet not to fly straight, such as it being heavier on one side or having a slightly deformed shape etc, will be evened out and it will now fly straight.

Similarly, the workings of a gyro instrument are rotating at high speed to eliminate the erroneous input that would come from the forces of the plane not staying still. Eventually though, things catch up and the instrument can get a little out.

At least that's how I remember it being explained to me.

[frontiersim]

jnb, on Mar 3 2007, 08:02 PM, said:

Though I doubt I understand it as well as Lateral-G  
Here's a simple analogy.

The analogy is a bullet coming out of the barrel of a rifle. There are spirals (rifling) cast into the interior of the barrel that cause the bullet to spin/twist rapidly in flight. As a result, any problems that may cause the bullet not to fly straight, such as it being heavier on one side or having a slightly deformed shape etc, will be evened out and it will now fly straight.

Similarly, the workings of a gyro instrument are rotating at high speed to eliminate the erroneous input that would come from the forces of the plane not staying still. Eventually though, things catch up and the instrument can get a little out.

At least that's how I remember it being explained to me.

So this is a realistic thing to enable?

[jnb]

Yes.

If I understand correctly (and for example on a real plane), on the ground before taxi you align the gyroscopic compass with the magnetic compass. The magnetic compass is consitered to be true in that it always points to magnetic north, and the gyroscopic compass is only considered good if it was calibrated a short time ago with the magnetic compass. The magnetic compass is difficult to use in flight as it doesn't hold still enough to be useful and it is meant to be used on the ground.

If you don't set this checkbox in fs, your instruments will just be correct always, which isn't very realistic.

Edited by jnb, 03 March 2007 - 10:47 PM.

[NegativeBeef]

so how do you correct for the instruments in game?

[Tobe-le-Rone]

jnb, on Mar 4 2007, 05:46 AM, said:

Yes.

If I understand correctly (and for example on a real plane), on the ground before taxi you align the gyroscopic compass with the magnetic compass. The magnetic compass is consitered to be true in that it always points to magnetic north, and the gyroscopic compass is only considered good if it was calibrated a short time ago with the magnetic compass. The magnetic compass is difficult to use in flight as it doesn't hold still enough to be useful and it is meant to be used on the ground.

If you don't set this checkbox in fs, your instruments will just be correct always, which isn't very realistic.

I use PS Panel for 737-NG by Peter Slater. Before flight you have to "align the IRS" or you don't get any readings on your instruments. Would that be the calibration process?

[_NW_]

NegativeBeef, on May 31 2010, 12:47 AM, said:

so how do you correct for the instruments in game?

You can do it by pressing d one time.

[THBatMan8]

Tobe, on May 31 2010, 05:02 AM, said:

jnb, on Mar 4 2007, 05:46 AM, said:

Yes.

If I understand correctly (and for example on a real plane), on the ground before taxi you align the gyroscopic compass with the magnetic compass. The magnetic compass is consitered to be true in that it always points to magnetic north, and the gyroscopic compass is only considered good if it was calibrated a short time ago with the magnetic compass. The magnetic compass is difficult to use in flight as it doesn't hold still enough to be useful and it is meant to be used on the ground.

If you don't set this checkbox in fs, your instruments will just be correct always, which isn't very realistic.

I use PS Panel for 737-NG by Peter Slater. Before flight you have to "align the IRS" or you don't get any readings on your instruments. Would that be the calibration process?

In a sense yes, but airplanes with a INS have a slaved compass card. Basically, when the INS is in alignment mode, it looks for the geographical north pole, or true north, based on the IRU's sensing of the earth's rotation and the local vertical entered in the CDU. When the INS locates the geographical north pole, it uses data stored in it's database to locate the magnetic north pole. When the magnetic north pole is located, the IRU can then feed magnetic variation data to it's respective instruments. Because the INS periodically updates itself with the aircraft's position via dead reckoning every 15 minutes, magnetic data can therefore then be continuously updated in the aircraft's instruments automatically.

All compass cards are subject to gyro drift, but depending on the card the drift may be automatically corrected.

Airplanes with a rotatable compass card are different, because with a rotatable card you'll need to do this on your own. A fixed compass card will always show the same heading, no matter what your actual heading may be.

Edited by Pope_Mischievous, 01 June 2010 - 10:34 PM.

[mdw]

Lateral-G, on 02 March 2007 - 02:01 PM, said:

Imagine such a gyro system being exactly aligned to the horizon and to north. In a perfect world .....................................
................  (edited. for reference.)
-G-

jnb, on 03 March 2007 - 08:02 PM, said:

Though I doubt I understand it as well as Lateral-G  
Here's a simple analogy.

The analogy is a bullet coming out of the barrel of a rifle. There are spirals (rifling) cast into the interior of the barrel that cause.....................................
................  (edited. for reference.)

That was briliant for both of U Lateral-G and jnb...

Lateral-G explanation just where confusion comes from....
jnb, came with simpler explanation, moreover I had modification my own barrel rifle so your explanation was crystal clear..

After that,, I start-over reading "Lateral-G" testimonial,, then I get valuable information... Thanks, for both of you guys... Hope,, we will see on the air real-life someday.. :-)