Experiment & Questions

[Learning2Fly]

The AI is fixed in space over the short term, and adjusts itself to point “down” over the longer term.

According to two credible sources, your statement is inaccurate.

US NAVY Training video

Pilot's Handbook of Aeronautical Knowledge

The gyro does not point to the Earth's core. Not my words, just presenting the troublesome facts. Believe me, I'm having
trouble coming to terms with this.

[Learning2Fly]

Here are my values. The formluas used are to the right. I'm still doing the aircraft altitude calcs, but from
the Earth's curve alone, the pilot must account for about 35,000 feet over about 220 miles of ground travel.

Why does the VSI not show this in rate per minute? Why do the control surfaces not reflect this? The airplane
would have to be in a nose down attitude with appreciable change as the distance increases.

Check my work. Tell me this is all a dream...and we haven't accounted for the ascent values on top of these.

[AirFrame]

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Username checks out.

[Learning2Fly]

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Username checks out.

Does the US Navy and Pilot's Handbook explanation check out as well?

How about the values I just posted? Are you nosing down over 220 miles of ground to correct for 35,000 feet of
ground curve?

Tell me why you're not trimming during level flight or climb to account for the 500 ft/min climb rate? How is this
value fairly constant during ascent or near zero during level flight.

[photofly]

The gyro does not point to the Earth's core.

Where do you imagine the gyro does point?
And how does it point there?

I'm not sure what the cartoon you posted is supposed to represent, or why you posted it.

[photofly]

The gyro does not point to the Earth's core.

Where do you imagine the gyro does point?
And how does it point there?

I'm not sure what the cartoon you posted is supposed to represent, or why you posted it.

Are you nosing down over 220 miles of ground to correct for 35,000 feet of
ground curve?

Yes, you are. But at any sensible ground speed that's such a tiny nose-down pitch rate it's undetectable.

In an airplane with a ground speed of 500 knots it takes 43 hours to circle globe, so the pitch rate is 360 degrees in 43 hours, or 0.13 degrees per minute. Or two thousands of a degree per second.

However if you're flying at an orbital velocity (15nm/second) the nose down pitch rate is 360 degrees in 84 minutes - or 4.3 degrees per minute. Still small, but must be accounted for in spacecraft, as I pointed out.

Alternatively, if you have a really good inertial platform you need to rotate your attitude solution to take account of this.

You also need to take account of the earth's diurnal rotation, which gives an attitude pitch error of 0.25 degrees per minute at the equator, even if you're standing still. Your reference position on the earth is rotating 360 degrees in 24 hours while your theoretically-perfect gyro (without the gravity correction) doesn't. That's why you need the gravity correction. That slowly restores the gyro axis to point towards the centre of the earth.


You can keep asking the same questions, and you'll keep getting the same correct answers

[AirFrame]

The AI is fixed in space over the short term, and adjusts itself to point “down” over the longer term.

According to two credible sources, your statement is inaccurate.
US NAVY Training video
Pilot's Handbook of Aeronautical Knowledge

The gyro does not point to the Earth's core. Not my words, just presenting the troublesome facts. Believe me, I'm having
trouble coming to terms with this.

I believe you're not understanding what your two credible sources are telling you. The image you've included shows exactly what a gyro does, when the earth is outside of the circle. It does not depict what the gyro does when the earth is in the center of that circle. When the earth is in the center of that circle, the gyro will indeed self-correct to point towards the center of the earth.

[Learning2Fly]

I believe you're not understanding what your two credible sources are telling you. The image you've included shows exactly what a gyro does, when the earth is outside of the circle. It does not depict what the gyro does when the earth is in the center of that circle. When the earth is in the center of that circle, the gyro will indeed self-correct to point towards the center of the earth.

Both sources explain the same function

[photofly]

That last diagram is showing a perfect gyro, without the self erecting mechanism, demonstrating "transport error". That's exactly what you DON'T want your Attitude indicator to do. The solution is the self-erecting mechanism, that slowly tilts the gyro axis vertical, as explained, ad-nauseam, in this thread.

How do you think your attitude indicator automatically sets itself to vertical when you start the engine? Compare the Heading Indicator, that you need to align with the compass - but the AI sets itself vertical all by itself. How do you think it does that?

Your diagrams and video are correct, as far as they go, but for simplicity they don't include the self-erection mechanism. Your AI needs to be both perfectly rigid in space and also self-righting, and these two things are contradictory. The self righting mechanism means the gyro is no longer entirely rigid in space but it makes the instrument useable because it automatically corrects for long term errors. Like transport error, and both apparent and real precession.

[photofly]

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[Learning2Fly]

That last diagram is showing a perfect gyro, without the self erecting mechanism, demonstrating "transport error".

No, it's not. Please watch the video

[youtube]https://youtu.be/JnKloSdUJLo?t=1m41s[/youtube]

The Handbook also corroborates the Navy training video.

https://books.google.ca/books?id=0l8WO6 ... e&f=false

[digits_]

How about the values I just posted? Are you nosing down over 220 miles of ground to correct for 35,000 feet of
ground curve?

Depending on your frame of reference: yes (if you compare it to your sketch) or no (if you compare it to earth's surface). Note that nose up or down doesn't really matter. A plane with nose up could still be descending, and a plane with a nose down attitude could still be climbing.

Tell me why you're not trimming during level flight or climb to account for the 500 ft/min climb rate? How is this
value fairly constant during ascent or near zero during level flight.

You can look at it from a different point of view: The airplane only cares about the air density it's flying in. If you trim it out perfectly, the plane will fly for a constant pressure altitude. That means, if it is trimmed, it will -roughly- follow earths surface.
Planes fly in earth's atmosphere, the atmosphere is wrapped around earth. They don't care exactly what path the atmosphere follows. As long as the gravity and density is the same, the plane will try to keep that equilibirum.

Another way to look at it: tie a rock around a string. Swing it around. Does the rock fly away or does it make a circle?
Rock = plane
String = gravity
Centifugal force that makes the rock want to fly out = air density/airplane wings

[digits_]

That last diagram is showing a perfect gyro, without the self erecting mechanism, demonstrating "transport error".

No, it's not. Please watch the video

https://youtu.be/JnKloSdUJLo?t=1m41s

This is not the gyro that is mounted in an airplane. Well, it is, but the self levelling mechanism explained multiple times by photofly is not present in the video clip you linked to. If you put the gyro from that movieclip in the plane, you will see a significant increasing nose down attitude as you keep flying around the world at a level attitude. If you were to follow the gyro from the movieclip with an indicated nose level attitude, you would eventually end up in space (if you are flying a rocket or some fancy fighter jet, as normal airplanes don't get that high)

Did you read photofly's explanation? Did you understand what he is saying?

[Learning2Fly]

This is not the gyro that is mounted in an airplane. Well, it is, but the self levelling mechanism explained multiple times by photofly is not present in the video clip you linked to.

Yes it is, did you watch far enough?




[youtube]https://youtu.be/JnKloSdUJLo?t=4m37s[/youtube]

This afterall is a pilot training video using aircraft instruments. Can either of you provide a dependable source that
supports what you are explaining? Mainly, that the ADI will fix to the Earth's core?

I'm seeing two sources from early times and present times that contradict what you are both saying, so this is a problem
in a formal debate.

[Chris M]

When you hop into a plane with a plain old steam gauge AI, before starting up it often looks like it's drunk. Tilted over and nosed way up or down. When you start up it self levels. Why? And why does it self-level to the horizon even if the ramp you're parked on isn't perfectly flat?

[photofly]

Yes it is, did you watch far enough?

here's what your video says about self-erection mechanisms. I transcribed it:

In order for the gyro instrument to be dependable, drift must be corrected continously. There are several types of correction systems that correct for drift. The principle [behind] all these systems is based on three steps. The direction and degree of drift are measured by electrical or mechanical sensing elements. These elements then control the application of a proper force to the gyro and it precesses back to its normal attitude.

In the pneumatic AI, the sensing elements are mechanical. They are vanes that flop to the side, under the force of local accelerations, including gravity. When the gyro axis isn't vertical (or aligned with the local acceleration) one or other vane swing to the side and open a hole that allows air out of the rotor system. The escaping air exerts a tiny force on the gyro axis. The vanes are designed so the force is at right angles to the direction of error, which causes the gyro to precess back to vertical.

The video is describing exactly how the pneumatic mechanism works.

Here is a picture of the rotor vertical and the vane closed:

7DA7B6D2-BC66-4E26-87A1-86CA1E9A58FD.jpeg (78.98 KiB) Viewed 1095 times

And here it is with the rotor off vertical, an error condition, showing the vane open and the hole where the air escapes:

A331875B-841C-4EC3-9DCD-FAEB8EB6009C.jpeg (76.85 KiB) Viewed 1095 times

[digits_]

Yes it is, did you watch far enough?

I didn't watch the full 20 minutes no. On your screenshot, the actual erection mechanism is not shown. Your video does not address your question in as much detail as you are after.

This afterall is a pilot training video using aircraft instruments. Can either of you provide a dependable source that
supports what you are explaining? Mainly, that the ADI will fix to the Earth's core?

I'm seeing two sources from early times and present times that contradict what you are both saying, so this is a problem
in a formal debate.

Another link with a similar discussion, including the pendulous vanes that take care of your problem:
https://aviation.stackexchange.com/ques ... t-accurate

Look at how the pneumatic AI works. It is easiest to understand.

The electric AI works on the same principle, but uses magnetic induction to accomplish the same effect, which is a bit harder to understand.

Regarding dependable source: cut open any pneumatic attitude indicator if you don't believe the schematics here are accurate. Try to understand what exactly is happening with the airflow. If you don't understand how exactly it works, then write down exactly what point you don't understand. Just because you don't fully understand it, doesn't mean the information is wrong.

[photofly]

Actually I need to amend slightly my description of how the self-erection mechanism works, for the same of accuracy.

There are four holes at the bottom of the rotor, and two vanes. When the rotor is vertical all four holes are half-covered by the vanes, so the net erecting force is nil.

When the rotor isn't vertical, one or both of the vanes swings to the side under the influence of gravity (or acceleration), revealing more of one hole and covering more of the hole on the opposite side. If the rotor is off-vertical by more than 1 degree, then one hole is completely covered and the opposite one is completely open.

Here's the vane position when the rotor is actually vertical, covering half of the hole, and (not shown) covering half of the hole on the opposite side of the rotor:

685CE496-E822-4730-B5C1-37FD03267D8B.jpeg (82.5 KiB) Viewed 1090 times

(For some reason that picture is showing at 90 degrees to the correct orientation on my laptop, but correct on my iPad. You will have to make the obvious rotation, if you're seeing it wrong. The plate with the two big screws that's blurred in the foreground should be at the top of the image, not the left.)

[photofly]

Another link with a similar discussion, including the pendulous vanes that take care of your problem:
https://aviation.stackexchange.com/ques ... t-accurate

Look at how the pneumatic AI works. It is easiest to understand.

Many of the answers on that page are at best, unhelpful, and in many places wrong. I would steer clear of it.

[digits_]

Another link with a similar discussion, including the pendulous vanes that take care of your problem:
https://aviation.stackexchange.com/ques ... t-accurate

Look at how the pneumatic AI works. It is easiest to understand.

Many of the answers on that page are at best, unhelpful, and in many places wrong. I would steer clear of it.

You are right. I mainly used it to reference the image and paid attention to the first answer only, but it turns out to be exactly the same as you posted before. So ignore.

[Learning2Fly]

here's what your video says about self-erection mechanisms. I transcribed it:

In order for the gyro instrument to be dependable, drift must be corrected continously. There are several types of correction systems that correct for drift. The principle [behind] all these systems is based on three steps. The direction and degree of drift are measured by electrical or mechanical sensing elements. These elements then control the application of a proper force to the gyro and it precesses back to its normal attitude.

The video is describing exactly how the pneumatic mechanism works.

Here is a picture of the rotor vertical and the vane closed:

And here it is with the rotor off vertical, an error condition, showing the vane open and the hole where the air escapes:

Thank you for the photos, Photofly. I understand that the pulses of air force the gimbal back to the original axis in the event of drift.

My video also states these properties of the gyro:

"In other words, the gyro maintains its axis in relation to space, and not to the surface of the Earth"

"The gyro maintains its attitude while the Earth turns under it."

"Every six hours, the gyro drifts 90 degrees in relation to the Earth's surface."

At this point we have no formal, credible source that states the gyro/ADI will fixate to the core of the Earth.

Here is a third reference from a current source about gyros behaviour: "Rigid in Space", "point to space in a fixed direction; follow a fixed star"

Source: Seamanship Techniques Shipboard and Marine Operations

https://books.google.ca/books?id=GQ4KAg ... ar&f=false

[Learning2Fly]

When you hop into a plane with a plain old steam gauge AI, before starting up it often looks like it's drunk. Tilted over and nosed way up or down. When you start up it self levels. Why? And why does it self-level to the horizon even if the ramp you're parked on isn't perfectly flat?

I'm not familiar with in-aircraft calibrations. The closest I've come is flying right seat in a 172 and Piper Archer as a guest/student ("fam. flight").

With respect to level D simulators, I've had 4.5 years of experience training inside several types. Beech 1900D, Dash 7, and Dash 8 100, 200, 300.
All of these appeared to have 'drunk' ADI's upon power-up. The ball will suddenly spike up, and fall to the side, then gradual float to a stable position.
At this time, we still had to adjust the horizon bar. I don't recall every scenario, or whether we were on an incline/decline during the power-up,
however a majority of the cases involved adjustment before 'flight'.

The problem that continues to halt us is the three sources stating that a gyro does not move in relation to the Earth's core. This is just one aspect
of many that I'd like to discuss. There is still the issue about VSI, aircraft configuration while in level flight, or climb, and others.

It's my lunch time now, so I'll try to calculate the rest of the table for airplane altitude to show another curious anomaly.

[photofly]

Ok, I'm being trolled. Ha ha, very funny. But it's ok, I have infinite patience, even if not infinite free time.

So, here we go, one more time:

1.An ideal gyroscope is fixed in space, not relative to the earth.
2. A real gyro isn't ideal and doesn't remain fixed in space, at least not for very long. Moreover ...
3. we don't want an instrument that's fixed in space, we *want* an instrument that's fixed relative to the centre of the earth.

So we take a good (but not ideal) gyroscope, and bolt onto it a very cunning self-erecting mechanism. We call it ...(fanfare please!) a gyroscopically stabilized attitude indicator, or AI for short. It's not a gyroscope, it's a instrument with a gyroscope inside it, plus some other stuff that makes it behave the way we want.

If you understand the properties of a pure gyroscope (you say you do), and you understand the properties of the self-erecting mechanism with vanes and air jets (you say you understand that too) then it's just a matter of putting the two systems together in your head to see how the whole thing works. You need that lightbulb moment.



On the subject of "formal" or "credible sources":
You appear to be seeking someone else (other than me, apparently) to tell you how an AI works, instead of seeking understanding in your own head. For my part, I know an AI slowly moves the gyro axis towards the centre of the earth because I observe them doing just that, in operation, multiple times a day. Every time I fly. So in fact does everyone else who uses this forum. Also I have one on my desk, cover off, right now, and I can see how the mechanism works. I don't need to read it anywhere else to believe it. So I can't help you with that part of your struggle. Sorry.

[digits_]

The problem that continues to halt us is the three sources stating that a gyro does not move in relation to the Earth's core. This is just one aspect
of many that I'd like to discuss.

That's where the air pulses come into play. You say you understand, but look at it again. If you'd understand, it would answer your question above.

Gyros are rigid in space.

An attitude indicator is a gyro with the air pulses as mentioned previously to make sure one axis is aligned with gravity.

[Learning2Fly]

I've added the aircraft figures, but they seem incorrect. With a constant pitch angle of 5 degrees over 100 miles, the altitude
already exceeds typical cruise. Units are miles for lengths.



The problem here is that an ascending aircraft, climbing away from the curvature of the Earth does not register on the VSI, or
Altimeter at the rate shown in the chart. Within 50 miles, there is already an apparent change in altitude that exceeds the
typical cruise for most jets.

I'm willing to bet my climb values are wrong; at least I hope so.