How to describe the Gyroscopic System

[minitour]

I'm trying to explain to a student pilot. I know about rigidity in space and precession, but what about the actual operation of the instruments: Attitude indicator and DG....

Engine mounted vacuum pump either sucks or blows (heh heh heh) air through the "hoses" into the instrument cases where little "notches" on the gyro wheels catch the air and spin the gyros. A relief valve keeps from too much pressure building in the system and spinning the gyros too fast.

The Attitude indicator is a horizontally "layed down" wheel that spins so when you pitch or bank, it stays in one place and the card moves around it.

The DG is a vertically "stood up" wheel that spins so when you turn, the wheel stays in one place and through mechanical linkage, the card turns.

We need to make sure the DG doesn't "precess" more than 3 degrees in 15 minutes not really due to precession, but more due to the Earth's roration.

Precession on the DG and AI is most "effective" rolling out of 180 degree turns. The AI pitches and banks opposite the turn and the DG should be checked and re-set.

Of course then there's the Rate of turn indicator (wings part of a turn coordinator) which is set up on an electrically driven gyro usually. It spins up (you can actually hear it spinning up when...) when you turn on the master switch...

I think (someone has to help me out here) that the Rate of Turn Indicator is "layed down" like the Artificial Horizon and acts in much the same way except it doesn't relay pitch information and it's only an "indirect" indication of bank angle.

Also, remember to explain that if you had a mean crosswind coming down final and had it slipping with the nose going straight down the centerline and the wing deflected into the wind, the Rate of Turn indicator would indicate wings level because there isn't any "rate of turn", you're going straight...

How'd I do?

-mini

PS.
Thanks for the help with "why" of Precession, but I still don't understand why the force comes out "over there"...seems like to me that If I push on the prop when it hits the top of the arc, it should exert the force straight over the top and not at the right side....

I guess I just need to get a physics book and read up on it...just not getting it...

Thanks though...A Squared, I'll definitely remember that one for when I have to "demonstrate" to a student. Thanks again!

 

[A Squared]

seems like to me that If I push on the prop when it hits the top of the arc, it should exert the force straight over the top and not at the right side....

Well for one thing you are not "pushing on the top of the prop" at all. That's an abstraction, you're actually pushing up with the tail of the airplane, if you want to know where the force is being applied, so I would turn your line of reasoning around and ask why would *you* think that the reaction would be at the top of the prop when you are pushing up on the tail? My point here is to show you the fallacy of thinking that the reaction has to occur where the force is applied, that's a first step to understanding. Does that help clear your mind?

Why does the force appear to be applied "over there"? Well it is not "over there" to the right, that is just another abstraction, it "behaves" like it's "over there" Saying that precession is the same as a force applied 90 degees later is just a shorthand for understanding how it behaves, not an insight into understading *why* it happens.

The precessive forces are a reaction to acceleration, and "over there" is where the acceleration is occuring (at least some of it). By acceleration, I don't mean "increasing speed", that is not what acceleration means Acceleration means changing the velocity of something. Velocity is both rate of motion *and* direction of motion. What you are doing in the gyroscope is changing the *direction* of motion which is acceleration. wheh you rotate a gyro around any axis other than it's spin axis there is acceleration occuring at all points around the circle. The reason there is precession is because some of the reactive forces act in direct opposition to the rotation, and thus are less apparent, and others act orthogonally (at 90 degrees to) the motion, so are more apparent.

Getting back to our taildragger, when we raise the tail, there is acceleration at the top of the prop disk and at the bottom, because the prop tips change direction as the til comes up, the tips at the top of the disk are accelerated forward and the tips at the bottom of the disk are accelerated aft. These accelerations had reactive forces which act aft at the top of the arc and forward at the bottom of the arc. As a result, the forces at the top and bottom of the prop just make it harder to raise the tail. but, like we said, there is accelration happening at all points around the arc. At the left side the motion of the prop tip is being altered forward so the reactive force is aft, and at the right side, the motion of the prop tip is being altered aft, so the reactive force acts forward. Now put those together, pushing aft on the left side and forward on the right side, and you get a turn to the left. It's not that the force is generated at the right side, it is generated all the way around, it's just that the portion of the force which is *not* directly resisting the tail coming up *acts* like it is being applied on the right hand side.

Does that help?

Thanks though...A Squared, I'll definitely remember that one for when I have to "demonstrate" to a student. Thanks again!

Well, tht depends on whether you understand it or not. If you are are sucessful in understanding this explanation, by all means use it for teaching students.

But if you're still not "just not getting it" don't try to use somone else's explanation which you really don't understand. that can only lead to trouble. If you get asked to explain precession by a student "I really don't understand the physics behind it." is a perfectly acceptable answer. As a pilot, you don't have to understand *why* precession happens, just that it *does* and approximately how it behaves.

 

[minitour]

Does that help?

Kinda sorta a little bit more...

...If you get asked to explain precession by a student "I really don't understand the physics behind it." is a perfectly acceptable answer. As a pilot, you don't have to understand *why* precession happens, just that it *does* and approximately how it behaves.

Right, and I totally understand that, but I plan on "getting it" before I start teaching, so...I definitely wanted to write down any explanations that I can...just in case I ever have to use "horse shoes" to get someone to understand it... knowwhatImean?

Thanks again

-mini

 

[DirtyBeech]

an attitude indicator is a gyro that is spun like a merry-go-round. it is layed "flat" so that it can react to pitch and roll forces, but not yaw. any force applid to it's wheel will act 90 degrees ahead in the direction of rotation. any movement of the gyro will direct air through one of the self erecting vanes thus counteracting said movement. i believe that the AI precesses in the opposite direction after a 180 degree steep turn as an overcompensation of the self erecting vanes, but i'm not positive on that.

a heading indicator is a gyro that is spun like a ferris wheel so that it is not sensitive to pitch. again, a force applied acts 90 degrees ahead in the direction of rotation, and that movement is translated via a gear linkage to the upright compass card we all know and love.

a turn coordinator is another gyro that is spun like a ferris wheel, but is mounted on arms with a 30 degree bend in them so that it is sensitive to both yaw and roll. a force applied yada yada yada is then directly translated to the little white airplane your student is now fixating on.

hope that helps. haven't given that lil speech in a couple of years.