SongMan

I know that gyros has two principles.

1. Regidity in space
2. Percession.

I understand the first part but have hard time fully understanding the second. I know that once applied force, the reaction is 90 degrees later. However I can connect this to gyros working in Heading Indicator or A.I. My question is usually, "so what? how does percession help H.I. and A.I. in terms of what they read out?"

Can someone elaborate little more as to this second principle known as percession?

Thanks

atlmsl

First of all it's "precession."

Precession is not a good thing for gyros. It's a byproduct that cannot be avoided, hence being a principle. It is why you must realign your HI and understand the erros of the AI when accelerating/deccelerating and turning. A gyro is effective because of rigidity in space.

Hope this helps.

N0315

precession is used in the turn cordinator. The way I "understand" precession is take regidity in space. The object wants to stay in that place. When things like the plane move it is bound to move the gyro a bit. Like a top. Spin the top and it wants to stay in one place, but it WILL move a little bit if you start moving the table. Poor example, but best I can think of. Thats one reason of several why if you're in IMC keep the turns standard rate. The more you trun and the steeper you turn the more that gyro wants to move off. I could be wrong, so any CFI's that can explain better or tell me I am wrong please do. My understanding of it may be a bit off base.

Ewfflyer

Precession is also a by-product of friction. That's why older gyro's precess a greater amount per given unit of time. Dirt, weak vacuum, bearings getting old, etc... All adds up, but it is amazing if you think about it how well they do work.

SongMan

I'm curious to why they call Precession a principle rather than just calling it an error. It makes it seem like it's necessary or a helpful thing for gyro operation. Wouldn't calling it One main principle and one main error make more sense than calling it two principle?

Also, I have never heard of Attitude Indicator error. This is facinating to me and I would like to learn more. Do you know where I can look this up? or find more about it? Especially the accerating/deccerlerating error.

SongMan

Ok...another question.
Why is it that if Precession affects gyro, Attitude Indicator do not need to be reset on regular basis like Heading Indicator? Does it have to do with how they are mounted? If yes, why can't you mount H.I. in the same way so we can avoid resetting in flight? Perhaps a questions I should ask Engineer but I'm curious to know.

Cubdriver

Au contrare, precession is what they are designed to do. Turn left, precession makes the DG change heading, the turn coordinator and attitude indicator tilt. You are lumping benefical and nonbeneficial effects into one. As EWFflyer said, the actions of dirt and other misgivings cause the gyro to receive impure inputs that are also called precession but are not desirable.

The reason attitude indicators can not be expected to drift is they are spun in the level plane. They constantly correct by the action of gravity, just as a plumb bob corrects itself. If however you feed in some lengthy input you will see that eventually it can't correct fast enough and you will get a tumble. Try it carefully sometime- do a steep turn and just keep flying the turn until the attitude indicator tumbles. It happens sooner in airplanes with clunky old horizons, I can't really say how long it will take in a newer aircraft and it may not happen at all. In older aircraft it only takes maybe 5 turns.

Here's a nice link on this subject.

atlmsl

I'm not following your reasoning.

http://ma3naido.blogspot.com/2007/11...indicator.html

According to the FAA (and my understanding) rigidity in space is the principle they rely upon, not precession. The gyro remains spinning in it's plane, so when the airplane is moved about that plane, it shows a change in pitch/bank. In the actual handbook it goes on to say that precession casuses minor errors in these instruments. Precession is intergral in the turn coordinator, but the original discussion was HI and AI.

atlmsl

After a prolonged turn in the same direction, upon level out the AI may indicate a climbing turn in the opposite direction. That is the result of precession.

As far as why precession is a principle, it's much like a discussion of principles of flight. We all know that lift makes the airplane fly, but you cannot have lift without drag (induced drag). It's an unwanted byproduct that can't be avoided.

Cubdriver

First off, the word "precess" is defined by Mirriam Webster as "movement". It is not defined as error, or anything else. "Precession" is based on the word precess and is usually associated with conical movement around the axis of rotation seen in almost any gyroscope, an example of which is easily seen in a football flying through the air. It is also what causes a gyro to slowly hunt around the axis of rotation, drifting or hunting which makes heading indicators lose their proper setting. But the word also defined more broadly as "where a gyro responds to a torque in one direction with a motion in another direction" to quote the link I put in my post. It is the same thing- we call one type of precession good when it makes instruments respond to movement in an aircraft, while the other is troublesome because it produces instrument error. Here's wiki's view if you would rather hear it from them. It's both types of gyroscope movement, the one where the gyro wobbles (bad) and the one where the gyro shows an indication (good).