Gyro percession
05-08-2008, 02:38 PM
#11
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I still do not see where precession is playing a roll in the AI or HI. The Handbook of Aeronautical Knowledge and Instrument Flying Handbook each say that the AI and HI rely on rigidity and that precession "error" is apparent. Maybe you have a deeper understanding of physics. I was just going on what the FAA says and explain it in a simple way to a beginning student. Thanks for your comments though.
05-08-2008, 03:49 PM
#12
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No deeper understanding here, just a student like everybody else. Precession is as described, but I need to make some clarifications.
FAA is correct (relief), the DG does not reply on precession to work properly. It relies solely on rigidity in space. It is adversely affected by precession, and must be corrected occasionally using the little knob.
The Attitude Indicator uses rigidity in space to show bank angle, by simply staying put as shown by a fake horizon. As the aircraft rolls it carries the little airplane on the dial into the roll with it, and voila you have bank angle. Pitch indication is accomplished the same way, using rigidity in space.
However, the precession is found to do both "good" and "bad" things to an AI. Good effects are, precession corrects the gyro back to level when it gets tilted by action of the little air valves at the base. "Bad" effects come from the gyro responding to accelerations of the aircraft in erroneous ways momentarily. It precesses in reaction to accelerations placed on it, just as the rate gyro does in the turn coordinator does but this time it is useless information.
In contrast, the Turn Coordinator relies on the "good" kind of precession to do its job, and would not work without precession of that sort. It senses angular acceleration, a fancy term for reaction to the rate of turn of the aircraft around the yaw axis. It also uses rigidity in space. It is technically known as a "rate gyro" because it reacts to rates of turn as well as acceleration (rate of a rate, you could say). That is what prompted me to get fussy about it. Rate gyros are useful because they precess so nicely!
FAA is correct (relief), the DG does not reply on precession to work properly. It relies solely on rigidity in space. It is adversely affected by precession, and must be corrected occasionally using the little knob.
The Attitude Indicator uses rigidity in space to show bank angle, by simply staying put as shown by a fake horizon. As the aircraft rolls it carries the little airplane on the dial into the roll with it, and voila you have bank angle. Pitch indication is accomplished the same way, using rigidity in space.
However, the precession is found to do both "good" and "bad" things to an AI. Good effects are, precession corrects the gyro back to level when it gets tilted by action of the little air valves at the base. "Bad" effects come from the gyro responding to accelerations of the aircraft in erroneous ways momentarily. It precesses in reaction to accelerations placed on it, just as the rate gyro does in the turn coordinator does but this time it is useless information.
In contrast, the Turn Coordinator relies on the "good" kind of precession to do its job, and would not work without precession of that sort. It senses angular acceleration, a fancy term for reaction to the rate of turn of the aircraft around the yaw axis. It also uses rigidity in space. It is technically known as a "rate gyro" because it reacts to rates of turn as well as acceleration (rate of a rate, you could say). That is what prompted me to get fussy about it. Rate gyros are useful because they precess so nicely!
05-08-2008, 05:11 PM
#13
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Cubdriver is correct:
DG: Functions by rigidity in space, Precession is an error.
AI: Functions by rigidity in space, precession is used to correct the horizon to "down". Pendulous vanes (the air valves) allow air flow 90 degrees offset from the needed correction, and precession makes the right correction. If the aircraft spends a significant amount of time in non-level flight, the mechanism will "correct" the horizon to something other than gravity...in this case the precession of the correction mechanism is not helping you.
TC: Precession causes this work.
Another good precesion example: Helicopters. One of the first helicopters was uncontrollable because the controls were rigged logically...ie push forward on the cyclic and the rotor disk tilts forward. Because of precession this caused the force on the disk to be redirected 90 degrees, causing the controls to function 90 degrees off...ie push forward and the helo rolls sideways
These early experiements were obviously entertaining to watch...
Functional helicopters all have controls rigged 90 degrees ahead of where the force actually needs to be applied.
DG: Functions by rigidity in space, Precession is an error.
AI: Functions by rigidity in space, precession is used to correct the horizon to "down". Pendulous vanes (the air valves) allow air flow 90 degrees offset from the needed correction, and precession makes the right correction. If the aircraft spends a significant amount of time in non-level flight, the mechanism will "correct" the horizon to something other than gravity...in this case the precession of the correction mechanism is not helping you.
TC: Precession causes this work.
Another good precesion example: Helicopters. One of the first helicopters was uncontrollable because the controls were rigged logically...ie push forward on the cyclic and the rotor disk tilts forward. Because of precession this caused the force on the disk to be redirected 90 degrees, causing the controls to function 90 degrees off...ie push forward and the helo rolls sideways
These early experiements were obviously entertaining to watch...Functional helicopters all have controls rigged 90 degrees ahead of where the force actually needs to be applied.
05-08-2008, 05:31 PM
#14
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I think we are all in agreement here, just not phrasing the arguments correctly. (or atleast me)
I never said that the turn coordinator did not rely on precession. I was only talking about the AI and HI. I was not accepting prescession's role on the pendulous vanes, but after rick's explanation that makes perfect sense. My main argument all along was that precession was bad for atleast the HI because that's what we have to "correct" when flying. I now have a better understand of the attitude indicator as well.
Thanks for all the comments and I hope that the original poster had his questions answered.
I never said that the turn coordinator did not rely on precession. I was only talking about the AI and HI. I was not accepting prescession's role on the pendulous vanes, but after rick's explanation that makes perfect sense. My main argument all along was that precession was bad for atleast the HI because that's what we have to "correct" when flying. I now have a better understand of the attitude indicator as well.
Thanks for all the comments and I hope that the original poster had his questions answered.
05-08-2008, 06:49 PM
#15
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Position: JS32 FO
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There you go. Rick's got it right. Rigidity in space for the AI and DG, precession for a TC. The Jepp book (especially the Instrument/Commercial Manual) show you what the instrument looks like. The Instrument Flying Handbook also has some diagrams, but the Jepp ones are easier to understand. I always show them to students, and it helps a lot.
