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Challenging questions for round Earth, and the explanations


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I couldn't find appropriate thread for this, so posting it here. Please post some challenging questions for round Earth 'theory', and the explanations.

Mine is not quite challenging, but still I found this interesting:

Did you know that kerbal aircraft gets automatically pitched up while in flight? With SSTOs or long-range planes, it's quite noticeable and is sometimes annoying.

Though, we can't get the similar effect IRL. No pilot says that they actually felt like their aircraft is pitching up, and any cruising aircraft doesn't seem to pitch up as well.

So the question is: Why? If the Earth is round, how can't we get any amount of pitch-up?

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Do you mean "why do airplanes flying inside a dense atmosphere retain their orientation relative to the ground, as opposed to satellites orbiting outside the dense atmosphere?"

Edited by LN400
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43 minutes ago, Abastro said:

I couldn't find appropriate thread for this, so posting it here. Please post some challenging questions for round Earth 'theory', and the explanations.

Mine is not quite challenging, but still I found this interesting:

Did you know that kerbal aircraft gets automatically pitched up while in flight? With SSTOs or long-range planes, it's quite noticeable and is sometimes annoying.

Though, we can't get the similar effect IRL. No pilot says that they actually felt like their aircraft is pitching up, and any cruising aircraft doesn't seem to pitch up as well.

So the question is: Why? If the Earth is round, how can't we get any amount of pitch-up?

Please read my edits!

So in KSP the SAS holds to a particular direction vector in the (universal) Unity game space, this is fine except if you fly an exact straight line relative to a round planet surface then it will appear from the point of view of the plane that you are getting further away from the ground (i.e. pulling up).

IRL you cannot hold a particular direction vector because one doesn't really exist (there is no universal coordinate system). IRL you tend to hold an altitude (which of course is alway the same distance from sea level).

 

EDIT: Part of the reason for this is the planes in KSP have grossly overpowered engines, so lift is not really a concern because you can just climb using engine thrust. Thus, you can fly along a straight-line vector (i.e pull up) despite the fact that you're climbing higher and thus generate less lift. IRL most planes rely much more on lift than thrust (except high performance military jets) so if you climb you generate less lift so you fall a little bit, thus you end up keeping a relatively constant altitude.

Another reason is that Kerbin is tiny compared to the Earth, so even if you had a high powered military jet and pointed it along a straight-line vector, the effect would be about 10 times smaller than what you see in KSP.

 

EDIT 2: Actually its not the SAS causing it, it's when the SAS is switched off that a craft will continue along it's heading in universal space. The reasoning above is the saem, but wherever you see a reference to the SAS holding along a vector in universal space, instead pretend I'd said that, with the SAS off, the craft will continue along that vector unless acted on by another force.

Edited by Steel
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If I make a spherical craft and park it on the runway with SAS set to stability assist mode, then wait 6 real world hours (i.e., no time warp, physics and SAS active the whole time), will it slowly roll down the runway as it tries to maintain its orientation with respect to the sun while Kerbin rotates?

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14 minutes ago, HebaruSan said:

If I make a spherical craft and park it on the runway with SAS set to stability assist mode, then wait 6 real world hours (i.e., no time warp, physics and SAS active the whole time), will it slowly roll down the runway as it tries to maintain its orientation with respect to the sun while Kerbin rotates?

No. Ok in my explanation above I said the SAS holds a vector in universal space. Thinking about it I don't think that's the case. It's not the SAS. The SAS holds a vector relative to Kerbin's (or whatever SOI you're in) centre, its only when you turn it off that the craft keeps itself oriented along the vector it's pointed along in universal space, unless its acted on by other forces (which it would be it you leave a spherical thing on the runway with the SAS off). This is the same reason why your craft rotates around if you turn the SAS off in orbit.

Edited by Steel
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1 hour ago, LN400 said:

Do you mean "why do airplanes flying inside a dense atmosphere retain their orientation relative to the ground, as opposed to satellites orbiting outside the dense atmosphere?"

It's similar with that, but I focused on kerbal airplanes: they won't retain their orientation as well. (Rather, gets pitched-up)

So what's the difference between Kerbin and Earth?

1 hour ago, Steel said:

EDIT 2: Actually its not the SAS causing it, it's when the SAS is switched off that a craft will continue along it's heading in universal space. The reasoning above is the saem, but wherever you see a reference to the SAS holding along a vector in universal space, instead pretend I'd said that, with the SAS off, the craft will continue along that vector unless acted on by another force.

As far as I know, the attitude is calculated based on kerbin system under low orbit. It's good approximation for the real world counterpart, which works with this part as well.

Edited by Reusables
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2 minutes ago, Abastro said:

As far as I know, the attitude is calculated based on kerbin system under low orbit. It's good approximation for the real world counterpart, which works with this part as well.

Wait I've lost you, can you clarify what you mean?

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6 minutes ago, Abastro said:

It's similar with that, but I focused on kerbal airplanes: they won't retain their orientation as well. (Rather, gets pitched-up)

Real world aircraft follow air pressure levels (aka flight levels), kerbal planes, when sas is engaged, just keep their orientation in space. They fly straight while Kerbin, being a rather small ball, bends away.

Is that what you're looking for ?

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6 minutes ago, Green Baron said:

Real world aircraft follow air pressure levels (aka flight levels), kerbal planes, when sas is engaged, just keep their orientation in space. They fly straight while Kerbin, being a rather small ball, bends away.

Is that what you're looking for ?

That's what I got wrong, the SAS will actually keep it at a constant altitude (follows a vector relative to the centre of Kerbin, i.e if you set it at 10 degrees above horizon, the SAS will keep you pointed 10 degrees above the horizon) its when the SAS is off that the craft will carry on in a straight line.

EDIT: actually you'll probably still see a small climb with SAS on, it's a PID system so it will be slow to react to keeping your nose pointed at the desired setting, so it will slowly drift upwards

Edited by Steel
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23 minutes ago, Abastro said:

So what's the difference between Kerbin and Earth?

Kerbin's radius is 600 km. The Earth's radius is about 6,400 km. Kerbin's much smaller size makes its curvature far more noticeable. 

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24 minutes ago, Steel said:

Wait I've lost you, can you clarify what you mean?

I mean, it holds its attitude relative to Kerbin system not the solar system without SAS off. So revolution of Kerbin won't affect it. And the same effect should appear on Earth as well.

Just now, Ten Key said:

Kerbin's radius is 600 km. The Earth's radius is about 6,400 km. Kerbin's much smaller size makes its curvature far more noticeable. 

It's smaller on Earth, but it doesn't immediately mean that it is unnoticeable. Many planes fly at least a third of the circumference of Earth, yet they won't notice any pitch-up effect. Consider that only 1 degree of pitch change is already significantly noticeable.

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did you know that bridge builders need to account for the curvature of the earth when dealing with huge spans? loads placed on 2 non-parallel gravity vectors. if the earth were not round these structures would collapse.

1 hour ago, LN400 said:

Do you mean "why do airplanes flying inside a dense atmosphere retain their orientation relative to the ground, as opposed to satellites orbiting outside the dense atmosphere?"

because pilots tend to maintain altitude. since the atmispheric density gradient follows the curvature of the earth, and a pressure measurement determines the altitude, the pilot/autopilot is constantly nosing down to maintain the pressure at the sensor that corresponds with the desired altitude. satellites can hold any orientation they want and are not dependent on their orientation to maintain their altitude.

Edited by Nuke
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Just now, Nuke said:

did you know that bridge builders need to account for the curvature of the earth when dealing with huge spans? loads placed on 2 non-parallel gravity vectors. if the earth were not round these structures would collapse.

Right, the curvature should be noticeable in relatively small scale like that. Many airplanes fly further.

So now, please provide a complete and plausible explanation for this issue.

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17 minutes ago, Abastro said:

I'm asking why IRL planes follows that. They will want to hold their attitude as well.

Ok I think I've got it fairly definitively now:

It will happen on Earth, but as I and others above have said, the Earth is has radius more than 10 time the size of Kerbin so the effect will be much less noticable. Add in that IRL the atmosphere is not totally stationary with respect to the ground (like it is in KSP) so you have wind, thermal pockets and a host of other effects that all affect a plane more strongly than the effect we're talking about, so it's negligible and you can't notice it!

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1 minute ago, Abastro said:

So now, please provide a complete and plausible explanation for this issue.

What you see in KSP is an artifact of how SAS works. Real world airplanes don't have KSP SAS modules installed.

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3 minutes ago, HebaruSan said:

What you see in KSP is an artifact of how SAS works. Real world airplanes don't have KSP SAS modules installed.

 I'm actually not sure it's an SAS artifact, @Abastro makes a fair point that the same effect would be present (minutely) IRL, because it is true than unless there are forces acting (which there are IRL), a plane would fly in a straight line, not with the curvature . See my last post for my attempt at an explanation.

Edited by Steel
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11 minutes ago, Abastro said:

Right, the curvature should be noticeable in relatively small scale like that. Many airplanes fly further.

So now, please provide a complete and plausible explanation for this issue.

see my aircraft explanation i already posted. bridge scenario probibly requires some math im not willing to do and i would probibly get it wrong anyway. it likely entails ensuring the loads on the towers of a suspension bridge are always parallel to their local gravity vectors, even though the towers themselves are not parallel. cables would be tensioned so that the road deck is always level between the towers which would make the entire road deck follow the curvature of the earth. if you had assumed the earth was flat you would have all kinds of leveling issues and lateral forces on the towers would be asymetrical, and force the towers to lean (potentially leading to collapse). a few arc minutes matter when you build megastructures.

Edited by Nuke
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Just now, Steel said:

 I'm actually not sure it's an SAS artifact, @Abastro makes a fair point that the same effect would be present (minutely) IRL. See my last post for my attempt at an explanation.

Have you flown many planes in KSP? Apologies if that sounds rude, but you're the first person I've seen suggest that SAS holds orientation relative to the horizon. Many pages of counter testimonials are collected here:

I'm thinking about running that runway-roll experiment I mentioned earlier; just need to make sure everything is set up properly so I don't have to re-do an hours-long test multiple times. I don't expect the same result you do.

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22 minutes ago, Abastro said:

I'm asking why IRL planes follows [air pressure levels] that. They will want to hold their attitude as well.

The altimeter in KSP is magic. It knows, with perfect certainty, the distance between the aircraft and sea level at all times. Altimeters in real life work on air pressure-- an airliner flying at 35,000 feet cannot "see" the ground at all with respect to determining its altitude. It only knows the pressure of the air it's flying through at any given moment. 

I am not a pilot. I suspect, to get a fully accurate reply to your question, you'd need to corner one. But I think real life aircraft probably would pitch up slowly over time when traveling long distances, if not for the fact that the pilot/computer is already having to make constant adjustments for air conditions and fuel consumption.

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5 minutes ago, HebaruSan said:

Have you flown many planes in KSP? Apologies if that sounds rude, but you're the first person I've seen suggest that SAS holds orientation relative to the horizon...

Not recently enough apparently! I just fired up KSP and I was in fact misremembering how good the SAS was, I thought it was better! It still is a physical effect (it still happens if you switch off SAS), but the SAS will magnify it!

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46 minutes ago, Abastro said:

I'm asking why IRL planes follows that. They will want to hold their attitude as well.

Ah, oh. It is actually more complicated but i stick to flight levels (dear pilots around, sorry for simplifying and not telling about air spaces, ifr, vfr, transition altitude, qnh and qfe :-)). That is because they measure the altitude with the most precise of all methods: by measuring the air pressure. But as they all would measure different altitudes depending on their start altitude and local air pressure (weather) they all agreed on a single base altitude to switch their instruments to. And that is the standard atmosphere's sea level pressure of 1013,x hPa (or 27.weird inches of mercury). Which means, that if 2 planes are assigned the same flight level (as measured by an instrument set to 1013,2 hPa) they actually do fly on the same altitude. They just sway up and down when flying through high and low pressure areas. On the other hand, if one of them is assigned FL200 and the other FL210 they actually are a thousand feet (edit +/- local weather adjustments) apart vertically.

 

Edited by Green Baron
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1 minute ago, Steel said:

It still is a physical effect (it still happens if you switch off SAS)

It would be, if the plane's rotational inertia was great enough compared to the other forces acting on it. Agreed on that point.

But a plane in flight is a dynamical system, air is constantly whizzing past and pushing against every outer surface with effects varying according to the shape and air speed at each point. And not accidentally---planes are designed with shapes jutting out of them to maximize these effects in a controllable way. Whether it pitches up or down depends mainly on those forces.

In the absence of control inputs, you get whatever the designers decided should happen in a neutral configuration. With control inputs, you get the behavior the [auto]pilots choose. Which is mostly level flight relative to the ground/horizon in the planes we ride on. In KSP, by contrast, SAS gives inputs that attempt to point in the same direction relative to the distant stars, which no sane airplane designer would do IRL.

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3 hours ago, Abastro said:

Right, the curvature should be noticeable in relatively small scale like that. Many airplanes fly further.

So now, please provide a complete and plausible explanation for this issue.

Because airplanes have stability. Very generally speaking, you trim an airplane in level flight to maintain a given cruising speed by trimming the pitch angle. The interaction of the pitching moments of the wing, fuselage, tail, etc work together to maintain that angle of attack relative to the airflow at that steady state airspeed. Add thrust without touching the trim and the airplane won't speed up but will climb instead. Decrease thrust without touching the trim and it won't slow down but will descend instead. All the while it will maintain the same angle of attack relative to the airflow, and the same indicated airspeed. None of this behavior requires an autopilot; it is just how aircraft are designed to work.

Now before someone gets pedantic, there will also be oscillations around the steady state for a range of reasons, but a stable aircraft will behave this way sufficiently well to overcome perturbations and return to the set point. Pitching due rotational inertia as the aircraft flies around the curvature of the earth would only be a very small perturbation, easily overcome by the aircraft's inherent stability.

Edited by PakledHostage
Fixed word that was missing from a sentence
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3 hours ago, Abastro said:

Right, the curvature should be noticeable in relatively small scale like that. Many airplanes fly further.

So now, please provide a complete and plausible explanation for this issue.

Planes has an optimal cruise attitude, its there air resistance is lowest while its still enough air to run the engine efficiency, engine and intakes is therefor optimized for this. 
Pilots know this they set the autopilot to keep this attitude. I assume this practice date back to early autopilots. 
You use an barometer for this, add a lot of damping on it as pressure might change because of weather and you don't want the plane to do radical changes. 
This probably dates back to WW2 or older. 

You are right that an gyroscopic autopilot would have the plane gain attitude, this is something you don't want especially back before they had pressure cabins. 
So i assume they used attitude sensors in some of the first autopilots, I guess they also used normal compasses as both are very simple.

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