Experiment & Questions

[photofly]

The video is brief about self-erection mechanisms, but consistent with what I wrote.

From https://www.faa.gov/documentLibrary/med ... 65-15A.pdf

81C837E2-A03B-4187-888B-96C1CD847A35.jpeg (161.45 KiB) Viewed 2274 times

[ahramin]

What would be really strange to me, is if it really did read 176 500 feet. How would the plane magically compensate for the earth's rotation and the orbital speed of the earth travelling about the sun? Not to mention we're way out here on an arm of the milky way as the galaxy spins around. You'd think the centripedal force would be so great that the gyro would collapse in on itself.

[Chris M]

What you don't seem to be grasping is that the AI only cares about the local gravitational field of the Earth. It isn't an astro-navigational instrument that is stabilized relative to the solar system or galaxy. The axis of the AI will always, via the self-erecting mechanism Photofly has so well explained (much thanks by the way, I'm learning a lot here too), point to the center of mass of the Earth. Straight down, in simplified terms.

What this means is that it doesn't matter how far around the globe you travel. The axis of the AI is always pointed directly at the center of the Earth and a local, level flight attitude is always perpedicular to it.

With regards to the diagram you drew, the second point should not still be 15 degrees pitch attitude. You're measuring that angle against an imaginary horizontal plane bisecting the planet. You should be drawing a radial line from the center of the planet and measuring the angle to that, which will get you a lot more than 15 degrees.

[Learning2Fly]

What you don't seem to be grasping is that the AI only cares about the local gravitational field of the Earth. It isn't an astro-navigational instrument that is stabilized relative to the solar system or galaxy. The axis of the AI will always, via the self-erecting mechanism Photofly has so well explained (much thanks by the way, I'm learning a lot here too), point to the center of mass of the Earth. Straight down, in simplified terms.

This is not what the US Navy video explains. The Gyro self corrects to maintain the original phase of the spinning rotors.

However, there are more issues besides this point which I'll be updating shortly.

With regards to the diagram you drew, the second point should not still be 15 degrees pitch attitude. You're measuring that angle against an imaginary horizontal plane bisecting the planet. You should be drawing a radial line from the center of the planet and measuring the angle to that, which will get you a lot more than 15 degrees.

Again, the US Navy video contradicts this paragraph. Once we detail the curve of the earth vs. climbing pitch, and VSI the problem becomes more apparent.

[photofly]

The AI is fixed in space over the short term, and adjusts itself to point “down” over the longer term. “Down” is the only acceleration that’s consistent in direction, so averaging the directions of the accelerations felt by the instrument over a sufficiently long period gives an adequate reference, and enable the instrument to right itself at startup and fix its own precessional errors during operation.

Reasons to erect faster are to recover from topples quicker, and to reduce the effect of errors due to precession and transport effects.

A reason to erect slower is to reduce the compounded error introduced during accelerating flight, that is, during turns, and while the aircraft is speeding up or slowing down, or pulling out of a dive. Speeding up and slowing down, and pulling out of dives doesn’t usually last very long. Turns generate bigger pitch and roll errors.

Slow turns are generally worse for errors than fast ones. In a rapid turn the direction of error reverses itself quickly. In a slow turn there’s enough time for the error to build up.

Attitude errors that occur during turns appear as both both pitch and roll errors. Imagine an airplane flying north that rolls into a right turn. The gyro senses an acceleration to the east and slowly starts to builds a bank error due to the non-vertical local acceleration. After some seconds the aircraft has turned ninety degrees and the initial east tilt error in the gyro appears as a pitch-up error, incorrectly showing a nose up attitude when the aircraft is level. Alternatively the aircraft will be flown nose down if held level according to the AI.

As the aircraft turns through east the error continues to accumulate, the direction of the error always to the inside of the turn. The biggest aggregate error occurs shortly after 180 degrees of turn.

If the aircraft continues to fly a circular path for long enough, eventually the AI error settles down to a steady combined pitch-and-bank error, showing. too much nose-up and under-representing the bank.

I have some graphs somewhere showing how the error varies with rate of turn. An AI that meets TSO-4c has sufficiently small errors still to be useful as an attitude reference during turns.

[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]

...

Username checks out.

[Learning2Fly]

...

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]

...

[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?