Electrically driven artificial horizon instrument gyro tied to earths vertical meanin

[akafrank07]

"Electrically driven artificial horizons use the same basic principles as the air-driven instruments, with a gyroscope tied to the earth's vertical and two gimbals."

What does it mean by 'tied to the earths vertical'?

Thanks

[mikelirec]

It means the gyroscope orientation is always going to be fixed perpendicular to the earth's vertical (parallel to the earth surface).

When the aircraft banks, the gyro will maintain it's position paralleled to the surface showing in the instrument the aircraft's relative position.

[Tinstaafl]

It means the gyro has a mechanism that senses when the gyro has wandered or drifted away from the desired orientation and then applies a correction to erect the gyro to that orientation. In this case the orientation is referenced to Earth's vertical via the mechanism being sensitive to gravity.

It has limits though. Stay in a continuous turn for long enough and eventually the mechanism will cause the gyro to orient to the sum of acceleration forces that are acting through it.

The gyro showing attitude changes is not related to it being a tied gyro. It would show attitude changes whether or not it was tied (although its reference would gradually change which is undesirable, hence the use of a tied gyro). Showing attitude changes is making use of the gyro's property of being rigid in space.

[akafrank07]

Thanks for the explanations guys :ok:

"Showing attitude changes is making use of the gyro's property of being rigid in space."
What does rigid in space mean? as i thought that would mean the gyro is always going to be fixed perpendicular to the earth's vertical, though this would contradict what Tinstaafl says; "the gyro has wandered or drifted away from the desired orientation"

Thanks

[keith williams]

What does rigid in space mean?

It means that once the gyro has been accelerated up to a sufficiently high rpm, its spin axis will continue to point in a constant direction in space.

But this will only be really true of a perfectly balanced gyro running on perfectly frictionless bearings. Any imperfections in the balance of the gyro or in its bearings will cause it to drift randomly.

Note also that the term “constant direction in space” does not mean that it will continue point towards the centre of the Earth. To demonstrate this hold you left hand in front of you with your left index finger pointing vertically upwards. Imagine that your left index finger is the spin axis of the gyro. If you move your hand around while keeping you index finger pointing vertically upwards you are demonstrating rigidity.

Now make a fist with your right hand and hold it in front of you to represent the Earth. If you put your left hand directly below the right, your left index finger (the spin axis) will be pointing directly towards the centre of your right hand (the Earth).

Now Move your left hand (the gyro) around your right hand (the Earth) while keeping your left index finger (the spin axis pointing upwards. Does your index finger (the spin axis) continue to point toward the centre of your right hand (the Earth)?

[akafrank07]

No it doesn't, thanks for your reply Keith, though i still can't relate this practically as the spin axis does not revolve around the earth

Thanks

[keith williams]

OK.

Now let's imagine the situation where we start with your left hand (the gyro) directly above the your right hand (the Earth). The spin axis is pointing straight upwards so it is aligned with the local Earth vertical.

Now while keeping your left index finger pointing upwards, move your left hand (the gyro) clockwise 1/4 of the way around your right hand (The Earth). This is the equivalent of flying 1/4 of the way around the Earth.

In this new position your left index finger (the spin axis) is still pointing towards the ceiling, but is now horizontal relative to the surface of the Earth.
This means that your attitude indicator, which was initially vertically aligned has toppled through 90 degrees, to assume a horizontal position. This toppling is caused by the fact that the gyro is rigid relative to a far off fixed point in space, but is not rigid relative to the much closer Earth.

The important thing to note is that the spin axis of the gyro did not actually change its direction relative to space, but it looked as if it did so when compared with the Earth. This process is called Apparent Wander.

To overcome this problem we need to tie the artificial horizon gyro to the Earth's gravitational field. This is done by means of an air driven pendulous erection system or by electric motors and tilt switches. What these actually do is to gradually topple the gyro spin axis so that it remains vertical relative to the Earth. This is done by producing an actual topple rate that is equal and opposite to the apparent topple rate.

[akafrank07]

Thanks a lot for that explanation Keith, that has helped me easily understand apparent wander :)
So i take it rigidity in space in space is a bad thing? as "It means that once the gyro has been accelerated up to a sufficiently high rpm, its spin axis will continue to point in a constant direction in space." and that's not want you'd want as "To overcome this problem we need to tie the artificial horizon gyro to the Earth's gravitational field. This is done by means of an air driven pendulous erection system or by electric motors and tilt switches. What these actually do is to gradually topple the gyro spin axis so that it remains vertical relative to the Earth. This is done by producing an actual topple rate that is equal and opposite to the apparent topple rate."
Would i now be right in my thinking?

Thanks a lot man :ok:

[Tinstaafl]

Not quite right. We want the gyro to be rigid in space otherwise it wouldn't give the steady reference point about which we're trying to measure a displacement. The problem is that as we move across the Earth's surface, that steady reference point that *we* want to be provided by the gyro's rigidity in space changes.

Think of a ball with many spikes sticking straight out from all over the surface. Each of those represents vertical w.r.t to the ball's surface but only for that spot on the ball. Move a bit to the left, or a bit to the right, or forward or back and the local vertical changes. A gyro that is correct for one place would not be oriented correctly for another place around the globe. We could use an enormous number of gyros, each one oriented so that it is correct for a specific spot on the surface, and just refer to the single gyro that is correct for that place. Each one would be rigid in space and appropriate to be used for a reference when you're at it's planned location - but nowhere else. Not exactly practical.

Instead, use a single gyro - rigid in space, remember, so from one moment to moment it seems rock steady so you can measure your movement w.r.t. it. - but have a mechanism that can gradually re-orient the gyro's fixed reference point in space so that it always appears appropriate to local vertical.

The Earth is also rotating so vertical on the surface changes where it's pointing in space. Think of looking straight up at the equator. In the course of 24 hrs that vertical orientation changes w.r.t. to a point in space. Unless you change the orientation of the gyro to keep up with the Earth's rotation it will appear to topple. It's not really. It's still oriented to the same point in space but we on the Earth have moved 360 degrees around it. So again, an adjusting mechanism is needed to constantly change the gyro's orientation to keep up with local vertical as the vertical orientation moves with the globe's rotation.

One last thing, mentioned by Keith: The gyros we make aren't perfect devices. There are errors in orientated introduced through friction, minute imbalances, and forces that get applied through the axle. Those errors need to be corrected too.

[Turbavykas]

It has limits though. Stay in a continuous turn for long enough and eventually the mechanism will cause the gyro to orient to the sum of acceleration forces that are acting through it.

Is that true? Doesn't those error cancel after 360? Can you give any info on this?

[Tinstaafl]

The erection mechanism is unable to determine the source of the forces acting on it. It's design presumes that the force(s) acting on it are in a particular plane ie through the vertical axis of the aircraft, and that any additional forces are sufficiently transient that there will be insufficient time for the mechanism to effect a significant change to the gyro's orientation. Hold those additional forces for long enough though, and the gyro will eventually erect to the vector sum of the forces.

No different to you not being able to distinguish an acceleration due to gravity, or due to a change in motion. Or, in another example, water in a container in a balanced, continuous turn will always be level w.r.t. to the aircraft even though its at an angle w.r.t. the Earth's surface. The water is responding to the sum of the forces acting on it. It doesn't return to 'Earth upright' after turning 360 degrees but will stay an angle w.r.t. the Earth for as long as the balance of forces remain unchanged. The gyro's erection mechanism similarly senses a difference between gyro orientation and the aircraft's normal axis. If there's a difference then it causes a gradual correction to occur to reorient the gyro. If the turn is transient then it's correction is insignificant. Hold the turn for long enough, however...

[Capt Pit Bull]

The erection mechanism is unable to determine the source of the forces acting on it.

It can take a guess based on the size of it.

If there's a difference then it causes a gradual correction to occur to reorient the gyro. If the turn is transient then it's correction is insignificant. Hold the turn for long enough, however...

More about size of the difference rather than duration, bearing in mind the OP was talking about electrical gyros.

If there is a big error then this is probably due to acceleration (a turn is a lateral acceleration) and therefore arrange the gyro to ignore it (by tilt switch design).

[Potential problem: What if the gyro is seriously misaligned, especially during start up? Solution: Give the pilot a manual fast erect control to override the tilt switch cutouts.]

If it is a small error then this is probably due to a gradual topple of the gyro away from the correct orientation. We can detect this and use a torque motor to make a correction.

[Potential problem: What if the aircraft undergoes a prolonged gradual acceleration, below the threshold of the tilt switch cut outs, leading to the gyro being driven into the wrong orientation? Solution: Train pilot not to make prolonged turns with small bank angles]

[Tinstaafl]

The electric gyros in the pistons, turboprops and jet I fly can't tell the difference. They're just dumb devices. No discrimination , just 'force experienced = 'x', erect gyro to 'y'.

[Turbavykas]

Tinstaafl (http://www.pprune.org/members/4150-tinstaafl) I think you are not correct. You have to remember that gyro is rigid in space. Using gyro you can detect that you are turning. Gyro compass works using that principle.
Why the all sources I was able to find tell that error cancels itself after 360?
As I understand gyro is rigid in space, bet if you do constant turn inside airplane you try to erect gyro to not a constant place in space. It will be like / and \ on the opposite places of the circle. So that's why errors cancel each other after 360. So if you do constant loop gyro will show small errors but will never wonder off. It think is quite easy to understand if you think about it.

[Tinstaafl]

We're not discussing a gyro compass. We're discussing Earth tied gyros eg attitude indicator.

[Capt Pit Bull]

Hmmm. Well, electrical AHs with vintages of 50+ years incorporate cut outs in the tilt switches. It's really very simple so I would be surprised if its not universal, or nearly so.

But I'm not an avionics engineer so I certainly entertain the possibility I'm wrong. This is interesting, I might do a bit more digging when I have the time.

[Tinstaafl]

Maybe those electric ones I've flown do have cutout switches for beyond certain AoBs.. But even so, in each of them I've flown at any bank I normally use, if I fly a continuous turn for long enough, the AI erects to a new 'vertical'.

As far as I know, pneumatically operated AIs don't have a discrimination mechanism.

[Turbavykas]

erects to a new 'vertical'. But how can gyro erect to a new vertical if you fly a constant loop? That new vertical it's not constant in space but gyro is.

[Tinstaafl]

Because it doesn't sense 'vertical' as we normally think of the term ie a line that is at 90 degrees to the Earth's surface. It responds accelerations such as gravity. Gravity's acceleration is conveniently be aligned with Earth's vertical so during unaccelerated flight or when motionless it works well. Most of our flight time is generally unaccelerated, the correction mechanism's corrections are slow to happen, and accelerated flight is generally transient.

Consider this thought experiment: Imagine some device that could change the direction that gravity acts, so that gravity is no longer aligned with Earth's vertical. What direction do you think the gyro's erection mechanism would cause the gyro to erect to? Can't be Earth's vertical - it doesn't know where 'vertical' is. It only can sense accelerations due to gravity and that's what it aligns to. No different to a plumb bob hanging from a piece of string.

Now replace that gravity device with accelerated motion eg a level continuous balanced turn at, say, 10 degrees AoB. In what direction are the sum of forces felt? Again, not 'vertical' w.r.t. the Earth's surface but at some angle to it (in this case 10 degrees) acting through the normal axis of the aircraft and that's what the erection mechanism senses. Just the same as what you, I & everything else in the aircraft experiences. The alignment of those forces felt within the aircraft don't change through the turn. They're constant for a constant turn.

Once you have that point, remember that the alignment mechanism is continuously adjusting the gyro's alignment during the turn, just like it does as you travel from one point on Earth to another.

By the way, you keep using the term 'loop'. A loop is a pitching manoeuvre in the vertical plane. That will topple many attitude indicators when the limit of it's gymbals are reached. I'm referring to a continuous turn (a horizontal plane manoeuvre) where the gymbal limits are never reached.

[Turbavykas]

I understand very well what you want to say and I am sorry for the "loop". I don't think you are reading what I want to say. Let say you are in continuous turn. Aircraft nose just this second pointing North. Erection mechanism tries to erect gyro to Again, not 'vertical' w.r.t. the Earth's surface but at some angle to it (in this case 10 degrees) and your are perfectly right!!! But what will happen when aircraft nose will point South?!!! Erection mechanism will try to erect gyro but this time to minus!!! -10 degrees. It will be like 10, -10, 10, -10 and so on. The sum of this is 0!!! Gyro bank will lag aircraft bank by 90 degrees and will always show some errors but the gyro will never show level flight!!!

If you put gyro into the rocket and fire it at 60 degrees to the earth acceleration will erect gyro to the local vertical because it's constant in space. But when you do continuous turn the new vertical is not constant in space! And gyro is not happy with that ! :)