Rate sensing gyros
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From: Warks
Rate sensing gyros
Dear wannabes,
I have also posted this on techlog, but thought it was worth putting here as well as many of you will be currently studying this stuff.
There was a question in my JAR ATPL instruments assessment yesterday that went something like this:
For a rate gyro controlling the pitch channel in a AFCS, which axis whould the sensitive axis be aligned with? The correct answer was the pitch axis. Quoting directly from our notes on AFCS:
How can this be true? If the gyro is spinning round the lateral or pitching axis of the aeroplane, how can it detect a pitch? The aeoplane will also be rotating around the same axis, so no force will be applied to the rim of the gyro. Could someone please explain this to me, or are my notes talking nonsense?
Thanks, TB
I have also posted this on techlog, but thought it was worth putting here as well as many of you will be currently studying this stuff.
There was a question in my JAR ATPL instruments assessment yesterday that went something like this:
For a rate gyro controlling the pitch channel in a AFCS, which axis whould the sensitive axis be aligned with? The correct answer was the pitch axis. Quoting directly from our notes on AFCS:
Rate Sensing Gyro. When a gyro is aligned with its sensitive axis parallel to an axis, it will detect any rotation of the aeroplane around that axis.
Thanks, TB
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From: West Mids
Token Bird,
I dont know if this helps but the way I remeber everything about gyros is:
R - T (1)
E I A (2)
S I X (2)
T - D (2)
I know that looks daft but let me explain.
Down the left hand side you have the word REST, and that stands for R = Rate gyro, E = Earth gyro, S = Space gyro, T = Tied gyro.
Then in the middle column you have a "Bed" (Headboard, mattress and foot board), these are the spin axis of the named gyros ie Rate gyro has a horizontal axis, Earth gyro has a vertical axis.
In the next column is the word taxd (You REST in bed because you are TAXD - cheesy but it works for me!).
The letters represent the instrument these gryos are in.
T = Turn Indicator
A = Artificial horizon
X= nothing exists called x!
D = Direction Indicator
Then the last columns are the degrees of freedom.
I hope that makes sense and is of use to you, it certainly helped me pass my JAR Instruments paper!
All the best
TBL
edited for strange words
I dont know if this helps but the way I remeber everything about gyros is:
R - T (1)
E I A (2)
S I X (2)
T - D (2)
I know that looks daft but let me explain.
Down the left hand side you have the word REST, and that stands for R = Rate gyro, E = Earth gyro, S = Space gyro, T = Tied gyro.
Then in the middle column you have a "Bed" (Headboard, mattress and foot board), these are the spin axis of the named gyros ie Rate gyro has a horizontal axis, Earth gyro has a vertical axis.
In the next column is the word taxd (You REST in bed because you are TAXD - cheesy but it works for me!).
The letters represent the instrument these gryos are in.
T = Turn Indicator
A = Artificial horizon
X= nothing exists called x!
D = Direction Indicator
Then the last columns are the degrees of freedom.
I hope that makes sense and is of use to you, it certainly helped me pass my JAR Instruments paper!
All the best
TBL
edited for strange words
Last edited by The Boy Lard; 10th May 2002 at 10:05.
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From: 1060 West Addison
i'm doing this from memory, but you might find that the sensitive axis isn't the spin axis.
think about it, if the spin axis is, for instance, the y-axis, then the x axis or y axis might be measured....
...when i get a minute, i'll have a look at my notes on the subject.
regards,
bgpm
think about it, if the spin axis is, for instance, the y-axis, then the x axis or y axis might be measured....
...when i get a minute, i'll have a look at my notes on the subject.
regards,
bgpm
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From: West Mids
ILS27R,
From memory as I dont have my notes to hand the degrees of precession is one more than the degree of freedom???
ie The rate gyro in the turn indicator has 1 degree of freedom and 2 of precession, earth gyro in AH has 2 of freedom and 3 precession, I THINK thats right.
TBL
From memory as I dont have my notes to hand the degrees of precession is one more than the degree of freedom???
ie The rate gyro in the turn indicator has 1 degree of freedom and 2 of precession, earth gyro in AH has 2 of freedom and 3 precession, I THINK thats right.
TBL
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From: Surrey
Lard,
I think a rate gyro has 1 degree of freedom - becuase you dont count the spin axis. However it has 2 planes of freedom becuase this method of classification does count the spin axis.
I think this is what your saying
I think a rate gyro has 1 degree of freedom - becuase you dont count the spin axis. However it has 2 planes of freedom becuase this method of classification does count the spin axis.
I think this is what your saying
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From: Warks
Hmm, precession. Precession is where the force applied gets effectively applied 90 degrees in the direction of spin. The point was that if the spin axis is the pitch axis, then no force would be applied if the aircraft pitched, therefore there would be nothing to precess.
Methinks either my notes are incorrect when they say that the spin axis and the sensitive axis mean the same thing, or the chaps meant to say perpendicular instead of parallel. I shall take it up with the author.
Another idea (a rather bizarre theory I had which is probably nonsense and the product of a deranged mind), is that rate sesning gyros may operate in a similar way to ring laser gyros. That is, with an RLG a pitch would shorten the path of one ray of light and lengthen the other. Is it possible that a pitch would shorten/lengthen the path of the rim of a conventional gyro in the same way, hence enabling a measurement of rate or rotation? If I haven't completely confused you all, discuss!
TB
Methinks either my notes are incorrect when they say that the spin axis and the sensitive axis mean the same thing, or the chaps meant to say perpendicular instead of parallel. I shall take it up with the author.
Another idea (a rather bizarre theory I had which is probably nonsense and the product of a deranged mind), is that rate sesning gyros may operate in a similar way to ring laser gyros. That is, with an RLG a pitch would shorten the path of one ray of light and lengthen the other. Is it possible that a pitch would shorten/lengthen the path of the rim of a conventional gyro in the same way, hence enabling a measurement of rate or rotation? If I haven't completely confused you all, discuss!
TB
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From: U.K.
It is the old exam question language trick again! The sensitive axis of a gyroscope is the axis about which you rotate it to obtain a signal of some sort to indicate that it has been rotated. It is easy to confuse this on a quick read with the gyro's rotational axis. It's all part of the skill of writing exam questions at which the C.A.A. are masters! In this instance the gyro is required to furnish a signal which depends on the rate of pitch of the aircraft. Hence the sensitive axis must be aligned with the axis of pitch of the A/C. This is not the axis around which the gyro rotates but is at 90 degrees to it. It's position depends on the arrangement of the gyro's gimbals and (as it is a rate gyro) the springs restraining the movement of those gimbals. Remember that the restraining springs may be replaced with some form of electro magnetic devices.
I hope this helps.
I hope this helps.
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From: UK
TB,
I think you are a little confused with the concept of the sensitive axis and the spin axis. They will always be at 90 degrees to each other.
Try to imagine this:
A gyro is spinning about an axis that is aligned with the wingtips.
If there were only one degree of freedom which was about the longitudinal axis, this would align the sensitive axis with the fore and aft axis of the aircraft. Think here of a gimbal held in bearings fore and aft.
Any rotation about the sensitive axis would cause the gyro to try and precess about the vertical axis, but there is no freedom to move in this direction. In other words the gimbal would oppose this initial precession because it is fixed in the horizontal plane. By opposing this initial precession it is felt by the gyro as a secondary force being applied, and this force when rotated 90 degrees in the direction of spin will secondarily precess the gyro about its free axis i.e the longitudinal axis.
When the gimbal movement is restrained it is a rate gyro, and if unrestrained it is a rate integrating gyro.
What your notes say is therefore correct. The sensitive axis will sense any rotation about it, which will cause the gyro to precess, and it is the precession which is used as the source of the error signal which is then amplified and used by the AFCS.
I think you are a little confused with the concept of the sensitive axis and the spin axis. They will always be at 90 degrees to each other.
Try to imagine this:
A gyro is spinning about an axis that is aligned with the wingtips.
If there were only one degree of freedom which was about the longitudinal axis, this would align the sensitive axis with the fore and aft axis of the aircraft. Think here of a gimbal held in bearings fore and aft.
Any rotation about the sensitive axis would cause the gyro to try and precess about the vertical axis, but there is no freedom to move in this direction. In other words the gimbal would oppose this initial precession because it is fixed in the horizontal plane. By opposing this initial precession it is felt by the gyro as a secondary force being applied, and this force when rotated 90 degrees in the direction of spin will secondarily precess the gyro about its free axis i.e the longitudinal axis.
When the gimbal movement is restrained it is a rate gyro, and if unrestrained it is a rate integrating gyro.
What your notes say is therefore correct. The sensitive axis will sense any rotation about it, which will cause the gyro to precess, and it is the precession which is used as the source of the error signal which is then amplified and used by the AFCS.
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From: Warks
I am not confused. My notes are incorrect. They clearly state that the spin axis and the sensitive axis are the same thing. This is clearly nonsense, as many people have pointed out. As for it being a standard JAA trick, the question was written by the FTO, not the JAA, and I don't fall for their tricks anyway,
TB
PS. Anyway, I got the question right, I just knew that it contradicted common sense if my notes were correct and the spin axis and the 'sensitive' axis were the same thing. I shall point this out to my instructor and hopefully we will get it fixed in the notes (and pigs might fly!)
TB
PS. Anyway, I got the question right, I just knew that it contradicted common sense if my notes were correct and the spin axis and the 'sensitive' axis were the same thing. I shall point this out to my instructor and hopefully we will get it fixed in the notes (and pigs might fly!)
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From: England
Hello TB,
you do seem to be hellbent on blaming your notes for being wrong but consider the following:These are all taken from your own comments.
quote: Apparently from your notes
--------------------------------------------------------------------------------
"Rate Sensing Gyro. When a gyro is aligned with its sensitive axis parallel to an axis, it will detect any rotation of the aeroplane around that axis."
"Could someone please explain this to me, or are my notes talking nonsense? "
No, I think your notes make sense
"Methinks either my notes are incorrect when they say that the spin axis and the sensitive axis mean the same thing"
No, from the extract you have supplied from your notes they do not say this.
"I am not confused. My notes are incorrect. "
Say it often enough and someone will believe you.
"They clearly state that the spin axis and the sensitive axis are the same thing. This is clearly nonsense"
No, they say that the sensitive axis will produce a precession in response to rotation about it. Says nothing about the spin axis.
Sorry TB, but it is just too easy for people who are struggling to blame anything except themselves. I suggest you book an appointment to speak with your instructor again,
all the best OMH
you do seem to be hellbent on blaming your notes for being wrong but consider the following:These are all taken from your own comments.
quote: Apparently from your notes
--------------------------------------------------------------------------------
"Rate Sensing Gyro. When a gyro is aligned with its sensitive axis parallel to an axis, it will detect any rotation of the aeroplane around that axis."
"Could someone please explain this to me, or are my notes talking nonsense? "
No, I think your notes make sense
"Methinks either my notes are incorrect when they say that the spin axis and the sensitive axis mean the same thing"
No, from the extract you have supplied from your notes they do not say this.
"I am not confused. My notes are incorrect. "
Say it often enough and someone will believe you.
"They clearly state that the spin axis and the sensitive axis are the same thing. This is clearly nonsense"
No, they say that the sensitive axis will produce a precession in response to rotation about it. Says nothing about the spin axis.
Sorry TB, but it is just too easy for people who are struggling to blame anything except themselves. I suggest you book an appointment to speak with your instructor again,
all the best OMH
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From: Warks
omh,
Wow, you're smug aren't you.
For one thing, I am not struggling (unless you consider a 90% average to be evidence of me struggling). For another thing, just because I did not quote the bit of the notes that said that the sensitive axis and the spin axis were the same thing, does not mean that it did not say it. I just didn't happen to quote that bit.
You said:
No you're right, it doesn't say anything about the spin axis in that particular sentence. Sorry for failing to quote the entire book. I'll know better next time. Unless of course you have read all my notes and know for sure that they did not say anywhere that the sensitive axis and the spin axis are the same thing.
When I posted the original quote, I made the fundamental assumption that the spin axis and the sensitive axis were the same and thought you would all also assume it, since I had read it elsewhere in my notes. It was only later that it came to light that they were different, which put the section I quoted in a completely different context and it would therefore be correct. However, the other part of the notes which stated that the spin axis and the sensitive axis are the same, must therefore be incorrect. They cannot both be correct. Do I have to explain this any more times or has it sunk in yet?
For your information, apparently the previous set of students on this course (none of whom were struggling either, judging from their results) had exactly the same argument about the same question. It even foiled the instructor, all because of this mistaken assumption that the spin axis and the sensitive axis were the same thing. Considering that 1 instructor and 18 student all made the same mistake, I think it damn good evidence that we were taught something in error, unless of course we are all struggling and all trying to blame our notes, including the instructor,
TB
PS. Almost forgot - I think I did already mention in one of my above posts that I got the question right, therefore the argument that I am trying to 'blame' someone else for something makes no sense. The point was that the question caused a big uproar in class, hence I decided to come onto PPRuNe to see if I could get some clarification. Thank you to those of you who did actually make useful comments, the mystery is now solved, but I will think twice about asking any technical questions on this forum again since some people like to use this as an excuse to accuse other people of being dense
PS2. TBL - my classmates liked your aide memoire with the bed. Thanks
PS3. I shared the above information with my instructor, who also agrees that the notes were incorrect. Needless to say I have erased the offending line from my notes
Wow, you're smug aren't you.
For one thing, I am not struggling (unless you consider a 90% average to be evidence of me struggling). For another thing, just because I did not quote the bit of the notes that said that the sensitive axis and the spin axis were the same thing, does not mean that it did not say it. I just didn't happen to quote that bit.
You said:
No, they say that the sensitive axis will produce a precession in response to rotation about it. Says nothing about the spin axis.
When I posted the original quote, I made the fundamental assumption that the spin axis and the sensitive axis were the same and thought you would all also assume it, since I had read it elsewhere in my notes. It was only later that it came to light that they were different, which put the section I quoted in a completely different context and it would therefore be correct. However, the other part of the notes which stated that the spin axis and the sensitive axis are the same, must therefore be incorrect. They cannot both be correct. Do I have to explain this any more times or has it sunk in yet?
For your information, apparently the previous set of students on this course (none of whom were struggling either, judging from their results) had exactly the same argument about the same question. It even foiled the instructor, all because of this mistaken assumption that the spin axis and the sensitive axis were the same thing. Considering that 1 instructor and 18 student all made the same mistake, I think it damn good evidence that we were taught something in error, unless of course we are all struggling and all trying to blame our notes, including the instructor,
TB
PS. Almost forgot - I think I did already mention in one of my above posts that I got the question right, therefore the argument that I am trying to 'blame' someone else for something makes no sense. The point was that the question caused a big uproar in class, hence I decided to come onto PPRuNe to see if I could get some clarification. Thank you to those of you who did actually make useful comments, the mystery is now solved, but I will think twice about asking any technical questions on this forum again since some people like to use this as an excuse to accuse other people of being dense
PS2. TBL - my classmates liked your aide memoire with the bed. Thanks
PS3. I shared the above information with my instructor, who also agrees that the notes were incorrect. Needless to say I have erased the offending line from my notes
Last edited by Token Bird; 14th May 2002 at 15:42.
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From: England
No TB (now there's a thing, TB......a pain in the gut)
I'm not suggesting you're dense, just struggling with the concept of gyro's; but having seen your reaction, perhaps struggling with reality as well.
Look at what you put in your initial statement. You have quoted a part of your notes and then accused that quote as being wrong. Now I wouldn't smugly suggest that all notes are perfect, and there may well be errors in them, but logic would suppose that you would at least quote the incorrect bit of the notes to back up your argument.
Here is another thought though: your hysterical written outburst is alien to the qualities required by the very profession you are trying to enter.
Be COOOOL; Chill a bit; have a beer and then settle down to some good bedtime reading. Oh, and seperate the sensitive from the spin
I'm not suggesting you're dense, just struggling with the concept of gyro's; but having seen your reaction, perhaps struggling with reality as well.
Look at what you put in your initial statement. You have quoted a part of your notes and then accused that quote as being wrong. Now I wouldn't smugly suggest that all notes are perfect, and there may well be errors in them, but logic would suppose that you would at least quote the incorrect bit of the notes to back up your argument.
Here is another thought though: your hysterical written outburst is alien to the qualities required by the very profession you are trying to enter.
Be COOOOL; Chill a bit; have a beer and then settle down to some good bedtime reading. Oh, and seperate the sensitive from the spin
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From: Warks
OK, I think we have agreed that the initial quote I posted is correct, once it became clear that the sensitive axis is not the same thing as the spin axis. Since you insist, here is the relevant quote:
In the Burton book the term 'plane of rotation' is used to mean 'plane of spin', therefore changing the sentence to mean 'the gyro's plane of spin is known as the sensitive axis '.
Two things in the above sentence are incorrect. Firstly, a plane is not the same thing as an axis. So you can't say that the plane of something is the axis of something. They are two different concepts. An axis is the thing it spins around. A plane is a thing it spins in. The plane that something spins in is at right angles to the axis it spins around.
Now, in each plane there are 2 axes, so although you could say that the sensitive axis is in the plane of rotation, that wouldn't tell you which axis of the 2, so therefore the sentence doesn't even make sense as written. If you take it to mean that the axis of spin is known as the sensitive axis, which seems the only possible interpretation, and the interpretation that everyone in my class made, the sentence becomes untrue. Are you happy now?
As for me struggling with the concept of gyro's, I think it's clear from my previous postings that I have no problem with the concept, as well as what I just wrote above. After all, it's not exactly rocket science, and JAR ATPLs aren't exactly the most challenging thing I've done in my academic life. Also I think that the fact that I question things a little deeper rather than blindly believing everything I read is a good thing. I'm glad you acknowledge the fact that notes can be wrong sometimes. In fact, our Instruments instructor has a rubber stamp with 'Bull!!!!' written on it, which he uses to mark the notes which are nonsense. That shows you just how often this happens.
Anyway, as for the 'hysterical written outburst' nonsense, there was nothing hysterical about it, as far as I can see. I was just trying to explain things again in the hope that you might read what I was saying rather than what you wanted to read. As for qualities that are alien to the profession, all the other people who answered my query tried to contribute something to the discussion, however you seemed to join in merely to throw insults, not to contribute to the discussion. I don't think that quality is particularly admirable,
TB
The gyro's plane of rotation is known as the sensitive axis
Two things in the above sentence are incorrect. Firstly, a plane is not the same thing as an axis. So you can't say that the plane of something is the axis of something. They are two different concepts. An axis is the thing it spins around. A plane is a thing it spins in. The plane that something spins in is at right angles to the axis it spins around.
Now, in each plane there are 2 axes, so although you could say that the sensitive axis is in the plane of rotation, that wouldn't tell you which axis of the 2, so therefore the sentence doesn't even make sense as written. If you take it to mean that the axis of spin is known as the sensitive axis, which seems the only possible interpretation, and the interpretation that everyone in my class made, the sentence becomes untrue. Are you happy now?
As for me struggling with the concept of gyro's, I think it's clear from my previous postings that I have no problem with the concept, as well as what I just wrote above. After all, it's not exactly rocket science, and JAR ATPLs aren't exactly the most challenging thing I've done in my academic life. Also I think that the fact that I question things a little deeper rather than blindly believing everything I read is a good thing. I'm glad you acknowledge the fact that notes can be wrong sometimes. In fact, our Instruments instructor has a rubber stamp with 'Bull!!!!' written on it, which he uses to mark the notes which are nonsense. That shows you just how often this happens.
Anyway, as for the 'hysterical written outburst' nonsense, there was nothing hysterical about it, as far as I can see. I was just trying to explain things again in the hope that you might read what I was saying rather than what you wanted to read. As for qualities that are alien to the profession, all the other people who answered my query tried to contribute something to the discussion, however you seemed to join in merely to throw insults, not to contribute to the discussion. I don't think that quality is particularly admirable,
TB
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From: England
Scroggs
I think you're wrong here. Although the thread got a little heated at times, it never really deviated from the subject. As it happens the differences between TB and myself have been effectively removed as the final quote given by TB is inaccurate.
Maybe the author of these notes will amend that particular sentence, if he or she has been following this thread and some good will have come out of this.
I wish you luck TB, whoever and where-ever you are.
I think you're wrong here. Although the thread got a little heated at times, it never really deviated from the subject. As it happens the differences between TB and myself have been effectively removed as the final quote given by TB is inaccurate.
Maybe the author of these notes will amend that particular sentence, if he or she has been following this thread and some good will have come out of this.
I wish you luck TB, whoever and where-ever you are.
