Heading/Attitude Indicators

[uwochris]

Hey guys,

I have q uestion in regards to the AI and HI principles of operation.

Now, correct me if I am wrong, but it is my understanding that for the HI, the gyro is mounted vertically and spins about its horizontal axis. Although the gyro itself obeys rigidity in space (the axis of rotation stays the same), the gimbals are free to move. As the a/c yaws, the movement of the gimbals causes the compass card to rotate.

As far as the AI is concerned, the gyro is mounted horizontally and spins about its vertical axis. Like the HI, the principle of operation is the rigidity in space principle. The gyro itself does not move, but when the a/c banks or pitches up/down, the gimbals move which cause the horizon bar to move. The horizon bar is attached to the gyro by a pivoted arm which allows it to move up and down only. When you pitch up, there is a downward force on the pivot arm which causes the artificial horizon bar to move down.

First of all, are my understandings correct? Anyone have any pictures of the internal instruments which actually shows the gyros?

Second of all (this Q may seem weird) if it is the gimbals that cause the HI and horizon bar to move, what is the purpose of the gyro itself (i.e. the rotor)? I guess the rotors provide a stable reference, but they do not cause the compass card to move or the horizon bar to move up/down- the gimbals do that. Would it be possible to have a system similar to the gyroscope with no rotor?

I hope someone can clear these misunderstandings up.

 

[mar]

Misunderstandings?

I dunno Chris. I don't see any misunderstanding. Unless we happen to have the same one but I think I'm correct.

Just to add some to what you described: It's my understanding that on those instruments where you can "cage" the gyro that you're essentially locking the gimbals and thus preventing *any* movement so that when a limit of motion is exceeded the instrument won't tumble (which causes wear and tear).

Further, your idea of a gyro without a rotor pretty much describes (or at least hints at) a laser-ring gyro.

A physicist I'm not, but the picture I have in my head of a laser-ring gyro is a triangulated laser and mirror system that is highly accurate and doesn't precess in the same manner as a traditional gyro...

...if at all...

...if I *understand* the whole dam thing at all...

...which is highly doubtful...

I hope someone else chimes in. Good luck.

 

[urflyingme?!]

I think you guys are both right, the Jeppeson flight books have info on it.

Do you guys know anything about "Gymbal Lock" tat they were always yelling about on Apollo 13?
I know that the gymbals are the arms connected to the gyro, but nothing els

 

[Timebuilder]

I think that the gimbal lock they spoke of meant that the normal amount of travel that was designed for the gyros was being exceeded, causing the gimbals to contact stops in the mechanisms.

 

[JetBlast2000]

Hey guys,
Second of all (this Q may seem weird) if it is the gimbals that cause the HI and horizon bar to move, what is the purpose of the gyro itself (i.e. the rotor)? I guess the rotors provide a stable reference, but they do not cause the compass card to move or the horizon bar to move up/down- the gimbals do that. Would it be possible to have a system similar to the gyroscope with no rotor?

The gyroscope resists turning. When it is "gimbaled" ( free to move in a given direction) such that it is free to move either in 1, 2 or 3 dimensions, any surface such as an instrument dial attached to the gyro assembly will remain rigid in space even though the case of the gyro turns. In other words, the AI's Horizon Bar is fixed to the case and moves in relation to the wings. The Horizon Line is stabilized by the gyro (rigidity in space). Thus, the a/c (and the case of the AI) moves about the gyro. The HI operates on the same principal, except the card is fixed to the gyro. It appears the compass card is moving as the a/c turns. The card is actually stable (with the gyro) and the a/c is moving about the compass card. The rotor buckets and the pendulous vanes are the self-erecting mechanisms allowing the gyros to stay stabilized. The "rotor" I think your referring to is the term used to describe the rotating portion of the gyro.
Hope this helps.

 

[DGdaPilot]

So how come the attitude indicator has pendulous vanes, but the heading indicator does not? I have an idea of why but I'm not sure if it's right.
Thanks

 

[DGdaPilot]

Come on. Someone's gotta know!


P.S. Merry Christmas

 

[Tinstaafl]

Both the AI & the DI use the gyro's rigidity in space to provide a reference against which we measure the a/c's movement. As others have said, the gyro stays still and the plane moves around it.

To understand the concept, imagine a super-simplified AI: A gyro spinning on its axle (magically supported in space), a horizontal bar connected to the axle and you hold ruler behind it to compare against. The horizontal bar is used to represent the aircraft, the gyro keeps it still & the ruler represents the horizon. While looking at the bar with the ruler behind it, rotate the whole setup ie pitch up or down. The bar will appear to move against the ruler even though what's happened is the gyro+bar stayed still in space.

You could do the same thought experiment for a DI by substituting a horizontal circular tape marked with 360 degrees connected to the gyro & a hold a pointer vertically against the tape. As you & the pointer move around the gyro then you can measure your angular movement wrt to the fixed gyro/tape combo.

Of course the above wouldn't work in practice & the real items need to be a bit more complicated. A structure is needed to hold the gyro, it needs to be gimballed because the structure is bolted to the aircraft which has to pitch, roll & yaw, and the information needs to be displayed in an orientation that the pilot can see (no good having a nice, simple device if the compass card doesn't face the pilot, or works backwards etc...).

One of the major problems is that we're using a gyro - which is rigid in space - to measure movement w.r.t. the earth's surface - which is rotating in space. That results in the rigid gyro appearing to move over time, when in fact it was still and the earth and everything on it did the moving.

For a DI the solution is to periodically reset the fixed-in-space compass card so that it is realigned with your desired reference on the Earth ie the magnetic pole. A more complicated version applies a continous, slow correction to keep the card aligned eg in an HSI with a remote compass input.

The AI has a problem. It needs to sense Earth vertical in order to show pitch w.r.t. the Earth's horizon. The most common solution is to use gravity since that's always present & for all intents & purposes is always acting straight down. It's easy to do this by mounting the gyro on a hanging platform. Gravity does the rest.

Not the end of the story though. That's fine for keeping the platform vertical, but the gyro & the horizon bar on it are still going to remain fixed in space. No good as you move across the Earth's surface. Think of 'upright' on one side of the planet, compared to upright on the other side...

This means the gyro + horizon bar combination has to have its orientation continually adjusted to keep the correct relationship between it & the Earth's surface/local horizontal. Since it's the gyro that isn't aligned the fix is to have some pendulous covers over vents in the gyro's mechanism. Mount the vents & covers so that ONLY when the gyro is oriented vertically are the vents covered. Get out of vertical alignment & one or more of the vents gets uncovered as the pendulous cover stays where it was. Air escaping from the vents provides a force to re-orient the gyro back to local vertical.

If it's an electrical gyro the use electrical contacts to detect when misaligned & an elecrical motor to move it back.

There's a catch with gravity based correction mechanisms. They can't detect a difference between 'real' gravity and 'apparent' gravity due to turns. The thing will eventually erect to where ever the local 'down' is. This is one of the reasons why DME arcs should be flown as short straight sections instead of continuous turns.




Disclaimer: Typed quickly while the wife is yelling at me to come for xmas dinner. Blame her for typos or incoherence...

 

[sky37d]

How about the EFIS versions. Oh, and why not a non-certified EFIS, as a backup, reference instrument.


I have been looking at the http://www.bluemountainavionics.com/elitemain.php blue mountain EFIS lite, fits in a 3 in hole, and I think it would be a perfect augument to my 'certified' instruments.

 

[DGdaPilot]

good post tinstaafl, but I still don't think anyone has answered my question:

Why doesn't the DG have pendulous vanes? Am I just not putting 2 and 2 together???
Thanks

 

[Tinstaafl]

The DI's gyro is mounted with its axis horizontal wrt the a/c. The aircraft spends the majority of its time wings level so it acts as an acceptable reference. Because the DI's gyro is constrained in its ability to move it's a 'tied gyro'. The AI, on the other hand, is the reference by which the a/c is kept upright. The system of aircraft + various gyros needs an external reference to find upright from somewhere. That's the job of the AI with it's pendulous mechanism to make its gyro an 'Earth tied gyro' - a special case of tied gyros in general.