Challenging questions for round Earth, and the explanations

[p1t1o]

6 hours ago, Abastro said:

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?

This is not correct.

The SR-71 gained aeronautically-significant "lift" from its fraction of orbital velocity and it had to be taken into account whilst flying the aircraft.

 

Sorry if Im late to the party, but the obligatory:

This is besides from the fact that a thousand other obvious signs confirm a..."round Earth".

Isnt there some forum rule against pseudoscience?

[Reusables]

6 hours ago, Steel said:

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!

The explanation that I wanted 

This is my explanation as well, I tried hard to avoid saying it by myself. Now I think I induced this one.. 

6 hours ago, Nuke said:

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.

Thanks for clarifying with bridge situation! Actually I got the bridge case previously, I wanted explanations on airplane case. Sorry that my question was unclear.

6 hours ago, Ten Key said:

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.

Yeah, this is it! There are so many other factors shadowing this effect.

Sorry that I needed a complete explanation here, I think that's how science works.

6 hours ago, HebaruSan said:

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.

Much more detailed and accurate explanation!

Besides, planes in KSP which has CoL and CoM nearly matching will pitch up because of this effect as well. I experienced this one.

4 hours ago, PakledHostage said:

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.

Right, I think this is another reason. In KSP, it's hard to make a plane which is aerodynamically stable (holds near-prograde)

2 hours ago, magnemoe said:

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.

I'm concerning about WWI planes. Would they have some kind of autopilot?

2 hours ago, p1t1o said:

This is not correct.

The SR-71 gained aeronautically-significant "lift" from its fraction of orbital velocity and it had to be taken into account whilst flying the aircraft.

If you mean the lift from circumfugal force, then it is not relevant with this question (planes not pitching-up)

Or did they experience that their pitch tends to go upward?

2 hours ago, p1t1o said:

Sorry if Im late to the party, but the obligatory:

This is besides from the fact that a thousand other obvious signs confirm a..."round Earth".

Isnt there some forum rule against pseudoscience?

Well,

Read the title, I *expected* to get plausible explanation for this one. It was shame that what I wanted was actually my own. But certainly, I'm not saying that Earth is flat.

Also, scientific theory doesn't work when there is a single counterexample which can't be explained. Even if there are so many reasons that why Earth should be round, the theory 'Earth is round' is rejected once there is a relevant example which can't be reasonably explained with it. That's why I asked for the explanation for this case as well.

And I think this is science.

 

By the way, anyone with better questions which could be challenging for 'Round Earth'? I want to try to give explanation.

Edited May 6, 2017 by Reusables
By the way part

[HebaruSan]

1 hour ago, Abastro said:

Also, scientific theory doesn't work when there is a single counterexample which can't be explained. Even if there are so many reasons that why Earth should be round, the theory 'Earth is round' is rejected once there is a relevant example which can't be reasonably explained with it. That's why I asked for the explanation for this case as well.

On the contrary, "Science advances one funeral at a time." Any apparent anomaly can be rationalized if your confidence in your preferred theory is strong enough.

Quote

By the way, anyone with better questions which could be challenging for 'Round Earth'? I want to try to give explanation.

Only dumb stuff like, "Why don't people on the other side fall off?"

[Reusables]

3 hours ago, HebaruSan said:

On the contrary, "Science advances one funeral at a time." Any apparent anomaly can be rationalized if your confidence in your preferred theory is strong enough.

How science advance part was just my opinion.

And yes, one can always get up with rational explanation. In my opinion, it gets rejected once it's not plausible for many people and causes other complex problems, but this is just my opinion and might be controversial.

Though without any explanation on an anomaly, it can't remain as scientific theory.

Otherwise, what's the difference between pseudoscience and science? I know the distinction between then is kinda gray, but there should be at least one criteria for the distinction. Is it just that science is done by scientists, while pseudoscience isn't?

[YNM]

Kerbin is really, really small. 600 km radius ? That's the size of moons and asteroids IRL !

So, why don't planes do that thing (explained in OP) IRL ? You can see this problem in two ways :

- Game defects. Built-in SAS in-game still reacts to gradual movements; compare to, say, KER or MechJeb's SmartASS. You can try download them and see how it solves your in-game problem. (ADDITION : Basic straight-line movements in game means that they're spatially truly straight (look in RL "defects" below)). Plane autopilot, however, is more precise - for all it's worth, they're cross-referencing to what's basically a gravimeter (I think gyroscope do precess in planes such that they're actually always keep position wrt ground or so ?), and plane autopilot is rather like MechJeb SmartASS where it'll try to lock to an attitude rather than just holding it (which is why moving the control stick while AP is running results in no reaction and when you want to disengage AP you need to neutral the controls, otherwise bad things happen). (EDIT : It turns out that the precession has to be induced - ie. the gyroscope will naturally follow what it really should be - pointing one direction - so errors have to be cleverly induced. Details :

Heading indicator (direction gyro) : https://en.m.wikipedia.org/wiki/Heading_indicator

Gyroscopic autopilot (AP) : https://en.m.wikipedia.org/wiki/Gyroscopic_autopilot

Gyrocompass (actually for ships but the heading indicator is just yet another correction in ways) : https://en.m.wikipedia.org/wiki/Gyrocompass)

- Real world "defects" (or rather, superiority). In game, you can easily climb with engines on to 30 km out of 70 km atmo. IRL, reaching 30 km out of 100 km is really hard... Also, on most KSP planes, you can easily pull a pitch up right after takeoff, and planes will look like keeping attitude during a stall... not the case IRL ! You can't just pitch up, and probably you'll come down nose first if you do ! Point is, real-world wings works significantly more effective (and effecting) compared to KSP-calculated wings. Check out FAR to see how far is it.

And, don't forget 70 km atmo for a 1200 km across ball is really thick compared to 100 km atmo for 13000 km ball...

Edited May 6, 2017 by YNM
Corrections(s)

[magnemoe]

6 hours ago, Abastro said:

I'm concerning about WWI planes. Would they have some kind of autopilot?

If you mean the lift from circumfugal force, then it is not relevant with this question (planes not pitching-up)

Or did they experience that their pitch tends to go upward?

Well,

Read the title, I *expected* to get plausible explanation for this one. It was shame that what I wanted was actually my own. But certainly, I'm not saying that Earth is flat.

Also, scientific theory doesn't work when there is a single counterexample which can't be explained. Even if there are so many reasons that why Earth should be round, the theory 'Earth is round' is rejected once there is a relevant example which can't be reasonably explained with it. That's why I asked for the explanation for this case as well.

And I think this is science.

By the way, anyone with better questions which could be challenging for 'Round Earth'? I want to try to give explanation.

Not sure about WW1 planes but even the simplest plane autopilot would need an attitude control, using an barometer is most relevant here. 
For the SR-71 I assume that it would get extra lift because orbital speed or circumfugal force, you would want to keep constant attitude anyway. 

And you don't need all answers for an scientific theory you can even live with conflicts as long as its good and useful enough. 

Most seafaring civilizations has known earth is round. Non seafaring like the ancient Egyptians thought it was flat. 
No it was not science just an explanation why thing went below the horizon then far away. 

[Green Baron]

Thought this was about aircraft behaviour, not flat earth bs.

For the 10th time or so: Eratosthenes

I don't like linking to wikipedia but for all the doubters it's the fastest way now.

 

Edited May 6, 2017 by Green Baron

[vger]

18 hours ago, Steel said:

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!

You might want to try playing KSP with this mod.
 

20 hours ago, Abastro said:

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?

I don't see why this matters. You wouldn't notice the pitch-up. Without an autopilot aiding you, a plane is going to pitch and yaw all over the place. You would need to make minor corrections all the time to stay your heading. That's just the nature of turbulence. Any shift in air pressure (that's what wind is) will cause you to gain or lose lift, or can push your nose in a new direction. You even even experience this in your car. Find a section of highway that has been built that is absolutely perfectly straight, line up your car with the other end and then try to get there without having to steer. Chances are, the car is going to start drifting to the left or to the right.

 

Edited May 6, 2017 by vger

[Reusables]

5 hours ago, YNM said:

Kerbin is really, really small. 600 km radius ? That's the size of moons and asteroids IRL !

So, why don't planes do that thing (explained in OP) IRL ? You can see this problem in two ways :

- Game defects. Built-in SAS in-game still reacts to gradual movements; compare to, say, KER or MechJeb's SmartASS. You can try download them and see how it solves your in-game problem. ADDITION : Basic straight-line movements in game means that they're spatially truly straight (look in RL "defects" below). Plane autopilot, however, is more precise - for all it's worth, they're cross-referencing to what's basically a gravimeter (I think gyroscope do precess in planes such that they're actually always keep position wrt ground or so ?), and plane autopilot is rather like MechJeb SmartASS where it'll try to lock to an attitude rather than just holding it (which is why moving the control stick while AP is running results in no reaction and when you want to disengage AP you need to neutral the controls, otherwise bad things happen). (EDIT : It turns out that the precession has to be induced - ie. the gyroscope will naturally follow what it really should be - pointing one direction - so errors have to be cleverly induced. Details :

Heading indicator (direction gyro) : https://en.m.wikipedia.org/wiki/Heading_indicator

Gyroscopic autopilot (AP) : en.m.wikipedia.org/wiki/Gyroscopic_autopilot

Gyrocompass (actually for ships but the heading indicator is just yet another correction in ways) : https://en.m.wikipedia.org/wiki/Gyrocompass)

- Real world "defects" (or rather, superiority). In game, you can easily climb with engines on to 30 km out of 70 km atmo. IRL, reaching 30 km out of 100 km is really hard... Also, on most KSP planes, you can easily pull a pitch up right after takeoff, and planes will look like keeping attitude during a stall... not the case IRL ! You can't just pitch up, and probably you'll come down nose first if you do ! Point is, real-world wings works significantly more effective (and effecting) compared to KSP-calculated wings. Check out FAR to see how far is it.

And, don't forget 70 km atmo for a 1200 km across ball is really thick compared to 100 km atmo for 13000 km ball...

Even better! Thanks for more detailed explanation 

4 hours ago, magnemoe said:

And you don't need all answers for an scientific theory you can even live with conflicts as long as its good and useful enough.

In the case, one can just mark it as the point where the theory can't cover. Like high speed region for Newton Physics. It is actially quite plausible explanation. This is just the case that it is highly unlikely for this issue to be an exception of the theory.

Also, some conflicts like singularity is a big crisis for a theory. Do you know how black hole is treated when it's discovered in theory? Usually, these defects have to be explained in any way.

3 hours ago, Green Baron said:

Thought this was about aircraft behaviour, not flat earth bs.

For the 10th time or so: Eratosthenes

I don't like linking to wikipedia but for all the doubters it's the fastest way now.

What...

I don't get how people get to these conclusion.

So I can never examine anything, and should never try to convince things myself? Even though I try to make it clear that Earth is round, those tries to confirm this for many circumstances is just wrong, right?

I'm done, think however you want.

[vger]

11 minutes ago, Abastro said:

Even better! Thanks for more detailed explanation 

In the case, one can just mark it as the point where the theory can't cover. Like high speed region for Newton Physics. It is actially quite plausible explanation. This is just the case that it is highly unlikely for this issue to be an exception of the theory.

Also, some conflicts like singularity is a big crisis for a theory. Do you know how black hole is treated when it's discovered in theory? Usually, these defects have to be explained in any way.

What...

I don't get how people get to these conclusion.

So I can never examine anything, and should never try to convince things myself? Even though I try to make it clear that Earth is round, those tries to confirm this for many circumstances is just wrong, right?

I'm done, think however you want.

Flat-Earth theory is about a lot more than just denying that Earth is round. It requires vast conspiracy theories to make it work. Doctored NASA photos, holographic moon projections, and heck knows what else that I've forgotten. I've read their technical jargon and the simplest least cost effective way for them to be happily satisfied that Earth is indeed round, would require personally crossing Antarctica in a plane. That's not easy or cheap, even by today's standards. And conveniently for them, it's also restricted airspace if I remember correctly.

Edited May 6, 2017 by vger

[Green Baron]

2 hours ago, Abastro said:

In the case, one can just mark it as the point where the theory can't cover. Like high speed region for Newton Physics. It is actially quite plausible explanation. This is just the case that it is highly unlikely for this issue to be an exception of the theory.

Aircraft behaviour is perfectly covered by theory and practice. Did you know that even the most complex aircraft are completely constructed in computers and on paper. Then a prototype is built and it flies and behaves exactly as calculated ? Would be possible if anything of what you call theories had flaws or exceptions ?

You can try that yourself with a model aircraft from balsa, it is actually not difficult. Boys (and girls, sorry dear ladies !) 40 years ago did so. And still a few today i hear :-)

Quote

Also, some conflicts like singularity is a big crisis for a theory. Do you know how black hole is treated when it's discovered in theory? Usually, these defects have to be explained in any way.

Changing subject ? Many here know how black holes are described in theory. And in practice maybe at the end of the year when luckily the first images of the shadow of an event horizon is published. If not, probably next year. There are no defects, only things waiting for discovery.

Quote

What...

I don't get how people get to these conclusion.

I didn't post against you, sorry if you feel offended. I wanted to make clear that flat earth (which was set into the antique before my post) is rather a modern thing than an antique. It was decreed in the Renaissance with reference to the late roman mathematician C. Ptolemaios. The antique world certainly knew that the world was round, though the exact diameter could not be measured until ~250 before christ. It was estimated too small before. Since the Rennaissance it comes up every now and then and today it has a fan base as described by @vger for example, but these are modern cultural excrescences, not ancients.

Quote

So I can never examine anything, and should never try to convince things myself? Even though I try to make it clear that Earth is round, those tries to confirm this for many circumstances is just wrong, right?

Sure ! By all means, go ahead !

Quote

I'm done, think however you want.

Again, sorry if i have upset you, and i will do as asked :-)

gb

Edited May 6, 2017 by Green Baron

[YNM]

2 hours ago, vger said:

I've read their technical jargon and the simplest least cost effective way for them to be happily satisfied that Earth is indeed round, would require personally crossing Antarctica in a plane. That's not easy or cheap, even by today's standards. And conveniently for them, it's also restricted airspace if I remember correctly.

Worse than that, they'll just say the plane turn slowly or moves fastly or so that the "south pole" wasn't actually crossed at any time. (Still remembering "government. meat. puppets." remark here XD)

I don't think south pole is restricted airspace - no reason to flying over a land basically equal to the outer space being restricted - but there's a complete rarity for any planes to actually fly very close to the south pole. Other than routing problems (no two sensible airport will produce a path really close to the south pole unlike the north one), weather is horribly problematic, with what all the wind and turbulence (southern polar wind and jets are the regularly strongest wind anywhere !).

EDIT : Just remembered : ETOPS requirement. North pole is OK because the land near there contains airport (and converted airbases). South pole... no way.

Edited May 6, 2017 by YNM

[Green Baron]

2 hours ago, vger said:

Flat-Earth theory is about a lot more than just denying that Earth is round. It requires vast conspiracy theories to make it work. Doctored NASA photos, holographic moon projections, and heck knows what else that I've forgotten. I've read their technical jargon and the simplest least cost effective way for them to be happily satisfied that Earth is indeed round, would require personally crossing Antarctica in a plane. That's not easy or cheap, even by today's standards. And conveniently for them, it's also restricted airspace if I remember correctly.

Luckily we have two of these poles. The north-pole is frequently crossed on intercontinental flights. Would that do as well ? I would accept it as a surrogate for the south pole, but you never know what excuses come to peoples minds ... :-)

Edit: it might be connected to air traffic regulations: planes are only allowed so far away from the next airport.

Edited May 6, 2017 by Green Baron

[YNM]

4 minutes ago, Green Baron said:

Luckily we have two of these poles. The north-pole is frequently crossed on intercontinental flights. Would that do as well ? I would accept it as a surrogate for the south pole, but you never know what excuses come to peoples minds ... :-)

Have you not see their map of the Flat Earth ?

[Green Baron]

29 minutes ago, YNM said:

Have you not see their map of the Flat Earth ?

Oh, i see :-) Of course, that doesn't work ... how silly from my side :-)

Edit: no, you could use the north pole as well, but it takes a little longer and some preparation. Also you need two groups. Drop a buoy at the pole if no ice.

One group flies directly to the pole (heading 0°) thus crosses it sooner than later. The other one flies a loxodrome, a steady course of lets say 300° aaaand guess where they end up later than sooner ?

Now, both flew a steady but different bearing and arrived at the same spot. How comes ?

 

Edit: correction: 270 has a special case where it doesn't work, the equator. So we take 300 for example to save fuel.

Edited May 6, 2017 by Green Baron

[vger]

34 minutes ago, Green Baron said:

Luckily we have two of these poles. The north-pole is frequently crossed on intercontinental flights. Would that do as well ? I would accept it as a surrogate for the south pole, but you never know what excuses come to peoples minds ... :-)

Edit: it might be connected to air traffic regulations: planes are only allowed so far away from the next airport.

No, unfortunately it wouldn't.

This might be hard to visualize without a diagram. Flat Earther's acknowledge the magnetic north pole. Compasses still work. Trouble is when you map the north pole onto a flat disc with the north pole as the center, it becomes very difficult to use circumnavigation as evidence. Because on a disc, if you tried to fly due West, your compass would slowly turn to stay oriented with the position of the north pole, resulting in you "orbiting" the north pole.  So the only remaining solution is to straight away from the north pole until the compass suddenly flips.

 

Edit: Oh, and I see you already figured that out.

Edited May 6, 2017 by vger

[magnemoe]

25 minutes ago, vger said:

No, unfortunately it wouldn't.

This might be hard to visualize without a diagram. Flat Earther's acknowledge the magnetic north pole. Compasses still work. Trouble is when you map the north pole onto a flat disc with the north pole as the center, it becomes very difficult to use circumnavigation as evidence. Because on a disc, if you tried to fly due West, your compass would slowly turn to stay oriented with the position of the north pole, resulting in you "orbiting" the north pole.  So the only remaining solution is to straight away from the north pole until the compass suddenly flips.

Edit: Oh, and I see you already figured that out.

And it makes no sense, it made no sense 150 years ago either. 
If you want to circumference the earth by ship the fastest way is to start in southern Argentina and hug the south pole as close as its safe. 
The Volvo ocean race regatta does this from south Africa to Australia as part of the trip. 
If Antarctica was the outer edge of the disc it would be far faster going the Suez, Indonesia and Panama route. Most do as its between the major economical powers and its an more fun trip for sailing around the world but that is off topic. 
No complex or expensive stuff required, you can get an sailing boat capable of crossing the pacific for $50K if you accept an shabby interior. 

Flatt earth advocates is probably 80% trolls and 20% idiots who adjust the world to how they want it to work, you have people willing to blow them-self up for religious reasons after all 

Edited May 6, 2017 by magnemoe

[p1t1o]

17 hours ago, Abastro said:

Or did they experience that their pitch tends to go upward?

smh...Just because the pilot isnt wrestling with the controls to keep from shooting into space, doesn't mean you aren't missing something.

 

17 hours ago, Abastro said:

then it is not relevant with this question

Oh well silly me. In the future, it would be convenient to let us know which answers would be relevant, save us some time.

[CSE]

@Abastro, @magnemoe: even for WWI aircraft, the designer can aim for pitch stability, using faster airflow to push down harder on the tailplane, which will make the plane pitch up. This gives the aircraft negative-feedback response patterns of: pitch down, go down, speed up, pitch up (from faster airspeed); and pitch up, go up, slow down, pitch down (from slower airspeed). This idealised aircraft will oscillate slighty about a steady (air pressure/density) altitude without control inputs, and could in principle fly around the world like this with infinite fuel, attitude approximately parallel to the ground all the way.

There are two ways this can go wrong. First, dramatic pitching can stall the tailplane and spoil the expected relationship between aircraft airspeed and tailplane force.

Second, and perhaps more relevant to a typical KSP craft, you can fit super-powered ultrajet engines and fly eveywhere with the throttle wide open (no, Jeb, no-one in particular, why do you ask?) at the terminal velocity of your design. This spoils the relationship between climbing/descending and airspeed, because when your TWR is more than one, adding a little contribution from gravity doesn't change your terminal velocity by more than a tiny amount.

Edited May 7, 2017 by CSE
typo

[magnemoe]

13 minutes ago, CSE said:

@Abastro, @magnemoe: even for WWI aircraft, the designer can aim for pitch stability, using faster airflow to push down harder on the tailplane, which will make the plane pitch up. This gives the aircraft negative-feedback response patterns of: pitch down, go down, speed up, pitch up (from faster airspeed); and pitch up, go up, slow down, pitch down (from slower airspeed). This idealised aircraft will oscillate slighty about a steady (air pressure/density) altitude without control inputs, and could in principle fly around the world like this with infinite fuel, attitude approximately parallel to the ground all the way.

There are two ways this can go wrong. First, dramatic pitching can stall the tailplane and spoil the expected relationship between aircraft airspeed and tailplane force.

Second, and perhaps more relevant to a typical KSP craft, you can fit super-powered ultrajet engines and fly eveywhere with the throttle wide open (no, Jeb, no-one in particular, why do you ask?) at the terminal velocity of your design. This spoils the relationship between climbing/descending and airspeed, because when your TWR is more than one, adding a little contribution from gravity doesn't change your terminal velocity by more than a tiny amount.

I know of the pitch stability, especially during WW2 it was some planes who was found flying long distances with dead pilot even if autopilot was not used. 
In some cases it was very weird like an spitfire crashing in Norway who was occupied by Germany, far outside of operational range. 

[HebaruSan]

Amazingly, someone seems to have sincerely believed this in real life, and this rebuttal was done in KSP:

 

[kerbiloid]

Also on the topic