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Gyroscope question

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Gyroscope question

Old 17th Jun 2019, 06:36
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Gyroscope question

Sorry! Posting another strange sample exam question here that I cannot understand:

Q. The principle of rigidity is used for the operation of the following gyroscopic instruments:
  1. Directional Gyro and Turn indicator.
  2. Directional Gyro and Artificial Horizon.
  3. Artificial Horizon and Turn indicator.
The answer provided is #2 - meaning that the Turn indicator does not use the principle of rigidity. How can that be? I know the turn coordinator is just a ball in tube, but the turn indicator is driven by a gyro, isn't it? And by definition a gyro-driven instrument works by exploiting rigidity? Am I missing something here?

Thanks
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Old 17th Jun 2019, 07:58
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As you say, a strangely worded set of answers. As before, I note that you only quote 3 answers. The current UK CAA exam papers all feature 4 answers (ever since they stopped negative marking for wrong answers and changed the pass mark from 70% to 75%)

As far as I am concerned, the answers you quote aren't using current terminology for the instruments. Generally, the Earth gyro is referred to as the Attitude Indicator or AI, the space gyro is the Direction Indicator or DI. Traditionally, both these two are powered by the vacuum pump, a hangover from the days when they were powered by venturi systems that sucked. The Turn Coordinator, which invariably includes the slip/skid ball is usually an electrically powered gyro, to provide a different source of power should the vacuum system fail.

Can I respectfully suggest that you look at the latest examination preparation publications, available from popular suppliers of pilot equipment?

TOO
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Old 17th Jun 2019, 09:09
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The turn coordinator, but also its predecessor (however you want to call it) use the principle of precession to indicate the direction of the turn. To be able to use the principle of rigidity, the gyro will need to be able to move within its mounting, around all three axis. The gyro in a turn coordinator or indicator can only move around one axis, thereby creating a sensor that picks up movement around the vertical axis of the aircraft. It does this by exploiting precession, as a turning motion around the vertical axis will precess and cause the gyro to cant left or right around the gimbal axis.

What this question is trying to do is make you think about the principles involved, not just whether there is a gyro within the instrument.

As for the terminology, you can have either a turn coordinator or a turn & slip indicator in your aircraft. These names refer to the entire instrument, which incorporates two parts:
1. A slip indicator, which is the curved tube you mentioned.
2. A turn indicator, which uses the gyro as I explained.
In a turn & slip indicator, the gimbal axis is mounted horizontally, while in a turn coordinator it is tilted rearward by 30 degrees. Because of this the turn coordinator will start to indicate a turn once the aircraft starts to roll (the tilted axis causes it to sense direction around the roll axis as well, although the movement around the yaw axis will overrule this once it gets going), whereas the turn & slip indicator will not start to indicate the turn until the aircraft is banked and starting to change direction. Because of this 'delay', turn & slip indicators can cause the unwary instrument pilot to overbank, as he/she will not see the instrument react to the initial movement. Use of an artificial horizon together with the turn & slip indicator can alleviate this problem, but I'm probably digressing....

Edit: the Wikipedia page about these instruments is actually pretty good, see here: https://en.wikipedia.org/wiki/Turn_and_slip_indicator
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Old 17th Jun 2019, 09:33
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I agree with the OddOne, study using the many quality teaching CDs and text books available. There are are too many poor teaching aids on the market which should be avoided. It is correct that all gyros exploit the principles of rigidity. The quoted question is nonsense.

I'm not so exercised about renaming the Artificial Horizon (AH) to a attitude Indicator (AI) which is the case for modern instructing but rarely elsewhere. The instrument displays an artificial horizon after all and is used to set the aircraft attitude in pitch and roll. Manufacturers and retailers in the main continue to use the older term. The change from Direction Indicator (DI) to Heading Indicator (HI) in my view is correct because the heading v direction is not always the same, correctly aligned this instrument displays the heading only not direction of travel.
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Old 17th Jun 2019, 09:45
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Originally Posted by Fl1ingfrog View Post
I agree with the OddOne, study using the many quality teaching CDs and text books available. There are are too many poor teaching aids on the market which should be avoided. It is correct that all gyros exploit the principles of rigidity.
Yes to the first statement, but I respectfully disagree with the last sentence. For a gyroscope to be able to exploit the principle of rigidity, it needs to have three degrees of freedom. The gyro in a turn indicator has only one degree of freedom and is therefore unable to act as either an earth-reference gyro or a space-reference gyro.

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Old 17th Jun 2019, 13:12
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Yes I think the question is trying to find out if you know which gyros are free in all axis, and those which are fixed in one axis.
So B seems to be correct.
.
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Old 17th Jun 2019, 22:48
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BASIC PROPERTIES OF GYROSCOPES have two basic properties: rigidity and precession. Those properties are defined as follows:

1. RIGIDITY The axis of rotation (spin axis) of the gyro wheel tends to remain in a fixed direction in space if no force is applied to it.

2. PRECESSION The axis of rotation has a tendency to turn at a right angle to the direction of an applied force. The idea of maintaining a fixed direction in space is simple to illustrate. When any object is spinning rapidly, it tends to keep its axis pointed always in the same direction. A toy top is a good example. As long as the top is spinning fast, it stays balanced on its point. Because of this gyro action, the spinning top resists the tendency of gravity to change the direction of its axis. You can think of many more examples. A bicycle is easier to balance at high speed than when it is barely moving. At high speed, the bicycle wheels act as gyros, and tend to keep their axes (axles) parallel to the ground. Note that it is easy to move the gyro as long as you keep the axis POINTING in the SAME DIRECTION. The gyro resists only those forces that tend to change the direction of its axis. In a bicycle, since the axis of rotation (the wheel's axles) is horizontal, the wheels resist any force that tends to tilt or turn them to the right or left.


If you can obtain a gyroscope top, you can do some instructive experiments with it. Hold the gyro top with its axis vertical as shown in figure 3-2 and start it spinning. As long as it is spinning fast, it will stay balanced. You can balance it on a string or on the point of your finger; the axis will stay vertical as long as the top is spinning fast. As we mentioned before, this ability of a gyro to keep its axis fixed in space is called RIGIDITY.

I'm sorry that I can't credit the source of the above, it is not mine, but it is a good simple explanation.
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Old 18th Jun 2019, 04:50
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Thanks all for the very helpful input.

I am not surprised that the terminology in the question is considered obsolete. I am using a current textbook, but also digging through some random questions from a question bank that I am told is the vintage that will be used for the exams in my jurisdiction.

I am using the questions as a learning vehicle - converting any I don't get into a mini project - doing a bit of research and writing-out a detailed description. That seems to be working well for me in solidifying my understanding, but this question had me stumped. Thanks all for helping me to grasp the bigger picture.

This prompts me to introduce some major thread drift...in this process I have done a lot of work on NDBs and VORs, there seems much interest in the questioners about interpreting instrument readings and using them to orient yourself. Reasonable enough priority I guess (even in these days of GPS?!), but I must say, I still don't get how it is that a much more sophisticated VOR is actually so much more useable/useful than a simple NDB! To my simple mind, the NDB is exactly like a lighthouse or other feature that sailors typically use for close-in pilotage, you just keep it on one particular bearing, and travel down that bearing until something else happens - eg another thing starts to bear on a value >x degrees. This works brilliantly because, as long as you have done your prep work properly, you don't need to know exactly where you are at any given moment, just that you are somewhere along a safe line.

My question is (other than the simple differences in accuracy) how is knowing the bearing of an NDB any worse/different from knowing which VOR radial you are on?
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Old 18th Jun 2019, 05:44
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The bearing of an NDB is relative to the Heading you are flying. So wind will make a difference. The bearing to/from a VOR is independent of your heading. When flying an NDB approach, if you just point the arrow to 0/360, you will fly a curved track in most wind conditions. You need to learn to do wind correction to fly a straight line. The VOR keeps you on a radial, and the wind correction is what you use to keep the CDI centered.
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Old 18th Jun 2019, 10:59
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Originally Posted by double_barrel View Post
...how is knowing the bearing of an NDB any worse/different from knowing which VOR radial you are on?
Another thing is that you need two instruments to work to know the bearing of an NDB, unless you use an RMI. With a VOR, just one instrument can provide this information. From a practical point of view, chasing an ADF needle around the dial is more work than keeping a CDI centered (in my view).

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Old 18th Jun 2019, 11:15
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Hmm, yes, you are right of course. I had to think about that! Pointing the needle to 360 just means it's ahead of you and you will start a spiral downwind but always facing straight towards the beacon. So that is not equivalent to keeping a lighthouse on a constant compass bearing.

But if you offset your course, into the wind, to keep it on the same absolute bearing then you will fly straight down its 'radial'. Don't I achieve that if I adjust my course into the wind so that a steady compass course keeps a steady relative bearing, which will not be 0/360?

OK, maybe a VOR is easier!
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Old 18th Jun 2019, 12:12
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The strange thing about Aero-Navigation is that we always fly towards a beacon.
Nautical Navigation involves NOT sailing towards a lighthouse, as they would soon become littered with beached ships. Ships rely upon finding the bearing, then calculating what the bearing should be in ten minutes time. It requires the use of a large chart table, pencils, and geometric instruments.
Our main worry is that many other pilots will also be tracking towards the same beacon, so you need to keep a good lookout, and listen to the radio, when getting close to the overhead.
.
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Old 18th Jun 2019, 12:23
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Originally Posted by scifi View Post
The strange thing about Aero-Navigation is that we always fly towards a beacon.
Nautical Navigation involves NOT sailing towards a lighthouse, as they would soon become littered with beached ships. Ships rely upon finding the bearing, then calculating what the bearing should be in ten minutes time. It requires the use of a large chart table, pencils, and geometric instruments.
Our main worry is that many other pilots will also be tracking towards the same beacon, so you need to keep a good lookout, and listen to the radio, when getting close to the overhead.
.
Actually, it is common to sail towards an object such as a lighthouse. Incredibly accurate and very simple pilotage, accounting for currents and wind, can be done by simply keeping something on a fixed bearing using a hand bearing compass, then when something else comes onto some other pre-defined bearing, you alter course for that. With some good prep you can thread your way through hidden rocks with extreme accuracy and without touching a chart. You can also use transits to fix and maintain a line that is perfectly accurate, takes complete account of wind and current, and does not even require a compass. These traditional methods can be more accurate and simpler than GPS but they do require some preparation and planning and as we get lazy they are less often used.

You can maybe see how I made the mistake of thinking that an NDB was the aviation equivalent of a lighthouse and the ADF was the equivalent of a hand bearing compass!
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