Experts, how does artificial horizon work?

[miko2d]

 There is that neat little gadget in a plane  - an artificial horizon. It is modelled on all the sims and is even more usefull in a sim then in the real life, since our view is restricted and we have no gravity feedback. It is easier to check the A/H then try to look around.
 In fact I got used to A/H while flying sims without even noticing it. It became apparent only on my first real flight in T-6. We only had permision for VFR (visual flying only) and the trainer disconnected the device. I caught myself looking at it every few seconds and getting distracted by the discrepancy in it's indications to what I expected to see!
 So I did not have a chance to observe the operation of the A/H in the real plane.

 So how does that device work? Is it just some kind of a simple pendant showing where the "down" is? The pendant would only give the correct indication if the plane flew straight. So if you are unsure which way is up while in the clouds, unloading the controls would be a prerequisite to getting the right indication.
 When flying under Gs, like doing a loop, the down would be wherever the G force pushes. Is that true?
 Of course if the A/H is based on a gyroscope, it would always point in the right direction (disregarding the curvature of the Earth and it's rotation).
 At the same time a gyroscopic device takes time to get up to speed before it becomes operational - a couple of minutes at least.

 So how does it work? Also if you can explain how the slip and other attitude indicators work, many of us would appreciate it.

miko--

[indian]

Miko some are electrically driven gyros some are air driven gyros. They can and will get out of control start spinng realy fast and they will keep on spinng for awhile. They do spool up fairly fast. Newer onse which we wont have here use a ring laser gyro.

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[jedi]

The standard ADI (attitude director indicator) is a little sphere with a gyro in it.  The gyro spins up, and gives the sphere an inertia so that it resists changes to its attitude.  The aircraft basically then "revolves" around the gyro sphere.  "Down" on the sphere is down in real life.

The older ones work on basically the same principle I think, but are subject to various errors as time passes, so that you needed to occasionally "re-erect" the gyro by resetting it to level flight when you know you are IN level flight.

REAL early ones?  Not sure how those worked  

--jedi

[Minotaur]

From my experience with naval navigation and weapons fire control gyros.  The "Gyro" is actually likely to be two gyroscopic wheels.  One gyro spinning inside of the other gyro on a gimble arrangement.  These two gyro wheels at set up with 90 degrees rotational plane offset.  One wheel for the vertical component and one for the the horizontal component.  Both gyro wheels have the same center point.  The outer gyro wheel lighter in weight or slower in rotation to balance gyroscopic vector forces between the two wheels.  A gyroscopically stable platform is created within the gimble.

The airplane rotates in space, but the gyroscopic platform remains stable to the orientation that it was in, at "Spin up".  The deviation between the gimbles position and position of the aircraft position drive the indication. Or, it might be just the force required to keep the gimble stable that is detected, and that detected force used to drive the indication.

Another configuration.  The gyro wheels may be solidly fixed in the horizotal and verticle plane and move exactly as the airplane moves.  Gyroscopic translation vector forces are detected as the plane changes axis and as the gyro's resist these changes.  The magnitude of these forces drive the indication.  By adjusting the indication at "Spin Up", this type can also be fairly accurate.  This is because a change in one direction generally must have an equal change in the opposite direction thus re-centering the indication.

The entire gimble or a gyro wheel might be enclosed.  Sealed completely and under vacuum.  This is too minimize the forces resisting the free movement of the gimble, as well as minimizing the power required to keep the gyro's rotating (wind drag).  In an imperfect application of technooloy vs theory, as Jedi said, the indication must be re-adjusted from time to time.

Crystal clear?        

As far as interpreting phyically what the indication provides in the airplane, you must contact a pilot more knowledgable than myself.  But, I believe it represents the horizon (Artificial Horizon).  Indicating the airplanes position of pitch and rotation, relative to the horizon.

The gyro's are "Spun up" and their "Gyroscopic Plane(s)" established, with the airplane on the ground, as the reference point.

Indian probably knows in great detail how they work in airplanes.  

Merry Christmas Everyone!

Mino

[This message has been edited by Minotaur (edited 12-09-1999).]

[-blk--]

  You guys are sooo close to why you didn't use it in the T-6...

  First off, in the T-6, and in the AH aircraft, they are Vacuum operated.  Basically, we suck air over a set of vanes which cause the gyros to spin.

  An attitude indicator is just a gyroscope with a vertical axis or rotation, that's mounted on some gimbals (more on these in a bit).  We spin the gyro real fast (in the case of AH, with vacuum pressure, but you can use a pressure system, or Electric).  As we probably all know, a spinning gyro will resist any attempts to alter the plane in which it rotates.  Since the AI has the gyro mounted on gimbals, the A/C pitches up, but the Gyro remains level.  All we need to do is set up a system which displays this angle (the roll works the same way BTW).  

  But here's why you locked the Gyro's...  Basically, those gimbals can only move so much (just like the Gimbals on your joystick can only move so much).  If we try to roll the aircraft 360 degrees, or loop it, we will exceed the mechanical ability of the gyro.  This wreaks havoc with the gyro.  It's spinning there trying to remain in the same plane as always, but we're forcing it to turn, and as such it has to exert all sorts of wierd forces (due to gyroscopic precession) in all sorts of wierd directions, and the gyros will precess rapidly (precessing gyros are the errors induced over time which were mentioned above).  This is called "tumbling" the gyros.  

  Tumbling gyros are a problem for a number of reasons.  First, they are totally unreliable.  Boy can they get screwed up, and it seems like every A/C does it just a little bit differently (I fly two basically identical A/C, and the gyros tumble when we spin them, and I've heard rumors of people actually placing bets on how they'll tumble).  Second, a normal gyro takes forever to get back to normal (after spinning the above mentioned A/C, I've flown for another 2 hours and not had the AI erect itself).  The way the gyro erects itself is through a system of pendulous vanes which open and close using gravity...  Anyway, it takes forever.  Now, many A/C have a quick erect mechanism which will allow you to pull a knob and fix it quickly, and when you caged the gyro in the T-6, that did the same thing (but the gyro really doesn't take to well to that, and the gyro is unusuable while you cage and uncage the gyro...this one can vary a lot depending on the specific system used).  The last reason you caged the gyros in the T-6 is that tumbling them causes a lot of undue wear.  A normal AI should last a few thousand hours or more, a gyro that tumbles a lot may last less than 200.  

  Anyway, that's how an Attitude Indicator works...  Jeppesen has really good explanations of all this in their newest manuals, but I doubt they're on the web anywhere...

blk  (AT)

(edit)--if you want, I'll clear this up, and write a quick section on the toehr instruments if you'd like (I'll do it tomorrow even if you don't like, since I've gotta brush up on how these things work by next Thursday anyway...)

[This message has been edited by -blk-- (edited 12-10-1999).]

[Minotaur]

-blk--;

-blk-- Quote ---  Now, many <snip> have a quick erect mechanism which will allow you to pull a knob and fix it quickly --- End Quote

What type of gyro are we talking about here again?  (muhahaha   )

BTW, very good post.  Thanks for the information!

Merry Christmas Everyone!

Mino

[-blk--]

  The type that has a quick erecting mechanism would generally be found on electric gyros used on higher priced singles up to most Turbine powered A/C.  

  A lot of the T-34s, T-6s, T-28s, P-51s, etc..  out there flying with updated cockpits use this type of AI because it's easy enough to go do aerobatics, then erect the gyro when you want to sue it.  I'll note, howerver, that the guys who do this generally have tons of money and aren't concerned with a gyro replacement every year or two.

  There are also a number of gyros that are both cageable and have a quick erecting mechanism....

  Now, in the world of AH...  The vast majority of the AIs should be cageable (all american A/C should have them, and I'd assume everyone else would as well).  So, what you'd do is you'd use the gyro, then before going into a fight, you'd cage the gyro...  Go do your thing, and on the way out, you'd get the A/C relatively straight and level, then uncage the gyro again.

blk  (AT)

[MRIDGELL]

interseting. been flying 20yrs and can still learn from the techno types    
what i do know-horizons are usless in aerobatic manuvour.in my starduster i cage the thing to keep from screwing it up befor i exceed normal flight attitudes. and when i was flying phantoms, the dern thing lagged so bad i was afraid to trust it. you navagate with instruments, but you do aerobatic manovour by experiance. its timeing and practice. we always taped markers on the side of our cockpit to judge angles with ,by looking side to side. another neat trick is a streamer attached to the wind screen . (even used them on phantoms) to show relative wind.

[Hans]

Well, I am a maintence guy at an FBO and have an associates degree in avionics.

Related story that is cool, but isn't about the Attitude Indicator.

On our aircraft (buisness jets) we get all types.  Directional gyros.  They run the HSI or horizontal situation indicator....tells you pilot types which way your going.  You handle those gyro like they were your fiance.  You move slow and do not jostle them at all.  Most have a big sticker on them that says "handle like eggs".  I like mine scrammbled so.....

Those you let spool down for at least an hour before you remove them from the aircraft.

Why?

The gyros are still spinning in those things at VERY high speeds.  Without that massive aircraft holding them in place they can "get away" from you if you remove them while they are still spun up.

What do they do?  They try to take your arm off.  At the least they fly out of control and go bouncing across the hangar floor.

Even when they are stopped you still need to baby them.  Drop one an inch onto a table and the thing can get out of calibration or even ruined.

[miko2d]

 Thanks, guys.
 Here is what I learned:

 The artificial horizon (attitude indicator) on the WWII-era planes was only usable for cruising/landing in a limited visibility conditions.
 It was not usable during any kind of aerobatic maneuvers.
 Doing some kind of crazy maneuver and then looking at the device to figure out where the hell your nose is pointing was not an option then, like it is in WB now.

miko--