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Ok, so I have this world(or maybe it will just be a dimension depending on how things pan out) called 'The Weird' that has air in space, like Earthlike air(for the most part). Also, the 'planets' or worldplanes are pretty much flat(barring mountains and the like), at least at the top. It should be noted too that sun(s) and moon(s) orbit these worlds, not the other way around. Another thing, the gravity is kinda funky. So the upper half(or maybe 3/4) of this realm/world is without gravity, and only some planets generate their own there, and the lower portion has gravity, or rather gradients of it. So it starts off at like moon gravity or so, then works its way to Earth gravity, then to like 2x-3x Earth's gravity or higher in some spots as you move to the 'floor'.

So with most of the basic cosmology out of the way, let us move on to the question. Since The Weird has air in space, would it be possible to fly a plane or maybe a chopper to another worldplane instead of needing a rocket or specialized spacecraft? Could flying creatures like birds and insects travel to-and-fro worldplanes?

I am mostly concerned with the zero-G or low gravity section as I know aircraft can fly in Earth gravity and not in (much)heavier Gs and this funny gravity thing might be switched in for all zero-Gs. But if you feel the need to include the other G zones, then feel free as there are worldplanes in all the G zones except for certain distance from the floor.

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  • $\begingroup$ You have omitted the crucial info: how is air distributed in this planet? $\endgroup$
    – L.Dutch
    Apr 1, 2023 at 16:04
  • $\begingroup$ Like this world as a whole or just a single worldplane? @L.Dutch $\endgroup$ Apr 1, 2023 at 16:07
  • $\begingroup$ Pretty much like ours at around sea-level, give or take a bit. That being said, some worldplanes might have diffrent gases than the air around them and there may be pockets of differing gases and/or temperatures or a vacuum bubble, but as a whole, mostly room temp or temperate Earth air. But most of it just lacks the Gs of Earth. @L.Dutch $\endgroup$ Apr 1, 2023 at 16:43
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    $\begingroup$ Keep distances and travel times in mind, and how air itself is moving around the universe. propeller or jet plane might not have enough fuel to go from Earth to Moon in this scenario, but a zeppelin or bumble that makes use of existing air streams might. $\endgroup$ Apr 1, 2023 at 17:35
  • $\begingroup$ In addition to the other answers: Without Gravity, a wing in space would generate significant amounts of Drag for no reason. even trimming the aircraft (which would be possible) to fly straight and level wouldn't be ideal due to the wasted energy. $\endgroup$ Apr 1, 2023 at 19:39

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*Where there is air, you can fly, but be wary of the distances!

It sounds to me like there's nothing special about your "space." It's just more atmosphere. If there's an atmosphere, planes can fly exactly like they fly in our atmosphere.

However, understand how large distances are in space. One of the reasons rockets can travel meaningfully between planets is that, without an atmosphere, rockets can travel very fast. Like 25,000 mph fast. With atmosphere, you have to plow through everything in front of you. Our fastest aircraft have only reached roughly 2,000 mph, so it will take 10x longer to get anywhere, even in a screaming fast jet like a SR-71. Also, it will take fuel. Lots and lots of fuel. Rockets flying through space experience extremely little drag because there's almost no atmosphere. If your atmosphere is dense enough to fly, it's dense enough to slow you down. You would need to run your jet engines steady-state for the entire multi-year journey between planets.

You would need to have fuel along the way. My recommendation for "birds" flying between planets would be to have an entire ecosystem in "space," so that there's fuel to consume. After all, in the end, we're all powered by the great big ball of fire in the sky.

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  • $\begingroup$ Would a lack of Gs make the craft/bird unsteady? $\endgroup$ Apr 1, 2023 at 16:36
  • $\begingroup$ No. Stability is all about the center of pressure and center of mass. Mentally, it may need a new way to retain orientation, but flying bodies are no more or less stable in free fall. $\endgroup$
    – Cort Ammon
    Apr 1, 2023 at 19:35
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    $\begingroup$ As a specific example, the Vomit Comet, an aircraft used to practice operations in free fall, does not become less stable just because it is on a parabolic trajectory that yields free fall/weightlessness. There's a very small number of things which operate fundamentally different (fire being one... without gravity/bouyancy making hot air rise, flames are very different in microgravity) $\endgroup$
    – Cort Ammon
    Apr 1, 2023 at 19:55
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    $\begingroup$ It would be more than just a mental way to retain orientation. An airfoil is still going to produce "lift" perpendicular to the direction of travel. So without gravity, the plane would try to fly in a loop. You'd end up using the other control surfaces to cancel out the lift. WIth a helicopter, they fly at an angle to maintain lift and forward (or sideways, whatever) motion. Without gravity, you would orient the "top" of the helicopter toward your destination and fly "up" all the way there. $\endgroup$
    – user34314
    Apr 2, 2023 at 18:48
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    $\begingroup$ @user34314 Airfoils can generate lift, but they're not obliged to. It's all about the shape. In particular, planes designed for acrobatic performances have a symmetric wing that generates no lift unless one has a non-zero angle of attack. It makes it so that the planes can do rolls without suddenly changing the forces on the aircraft. $\endgroup$
    – Cort Ammon
    Apr 2, 2023 at 18:54
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Since there is too much of what would go weird should the system be subjected to the real world's physics, I assume that there is some magic at work that helps stuffs retain their properties, namely air in space, zero gravity between worlds (there is NO place without gravity in the universe, and the smallest object at the farthest place does apply its gravity "here" wherever that is), suns with less mass and apparently no friction vs interplanetary air (also no gravity close to them?), etc. Therefore, the plain answer to your question is a definite YES, as in, you CAN reach the other planet on an ordinary aircraft, but...

But, spacefaring devices would still not be like intra-planetary aircraft, namely because there is place with no gravity, thus no "down" and "up" which have to be accounted for when designing aircraft. Wings would serve no purpose out of gravity, engines would have their thrust vectors off mass center with no compensation from control elements, making a normal airplane start rotating under its own thrust with a sudden inability to normally control the flight. A helicopter with added means of orientation control could do, however. I assume that a "rocket" with twin rotors like on a modern attack helicopter like Ka-52 on its top would be the correct shape for an interplanetary spacecraft within your world set. You can control thrust with copter's normal means of control, you can drive through "space" on the same rotors, you can control turn in zero gravity using rocket's fins (retractable if they spoil travel while in a gravity field), additionally you can use smaller airplane engines to turn your rocket sideways, once in another planet's gravity field, you turn the "bottom" of the rocket towards the planet and start an almost normal helicopter landing.

The main problem with such travel would still be fuel. The space is BIG, your air-filled space might not be that big but still, there would also be currents in intra-planetary air that could easily shun a vehicle off course by too much to miss the destination, there could be rogue asteroids barely seen in space because the ambient light would not let your spacecraft see them before they would have no time to dodge the collision, there might be dragons (Chinese type) afloat in midair that prey on unsuspecting travellers or whatever life that could exist there as well, and all the mentioned and not mentioned dangers would ask for fuel expenditure to avoid. A helicopter's normal flight span is limited in hours, yours might take a day or two worth of fuel, yet still it might be too small to reach the destination. There is a possibility however, use a VERY large rotor that's powered by rocket passengers' muscle force, with zero gravity and low air resistance air-rowing is quite a method to provide a little bit of speed. In fact, such a rotor could well serve as fins when the craft is powered by engine. But, this type of travel would be even slower, and would expend the rocket's food storage together with fuel. Hmm.

Regarding planet gravity of 2-3 Gs at ground level, you can alter the planet's atmospheric density to allow flight in such conditions, and thus design your aircraft to both withstand this gravity construction wise, and fly in dense air using normal aerodynamics. Note however, since the density curve for an ideal gas in a gravity well is a frikking EXPONENT, and for a real gas it's close to that at relatively low pressures, you need to design your gravity wells somehow to allow nonzero values at their edges and not too high values at the bottom, otherwise floating would surpass heavier-than-air flight for both lifeforms and aircraft. After that, the answer to your question of "can ordinary aircraft reach another planet" would shift to a no, as a floater cannot properly move in zero gravity, or would break down while lifting from the dense atmosphere.

Regarding interplanetary travel for lifeforms - the answer is yes, provided they can feed on something mid-flight. Otherwise, they would run out of fuel (fats and other energy within a living body) and die in mid-air. There's a multitude of issues with such a travel however, first the movement in air at zero gravity is something different from flight (close to swimming underwater, in fact), which birds might not be able to learn, while insects and seeds/pollen might reach the other planets by intra-planetary currents; second, the navigation problem - unless your midair is EXTREMELY transparent, your planet dwellers would not be able to see the sun(s), probably the nights if any would also be very weird. Here on Earth, when you look at the sun, there is about ten kilometers of dense air obscuring sunrays, in your world it would be millions of kilometers of dense air, making the intra-planetary medium opaque. There might be more problems, requiring more time to analyze, but I believe this set of problems would first need to be solved before you continue.

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On Earth airplanes can't go to space because of the lack of air: airplane stay airborne because the motion of air around their wings causes and upward force that cancels out gravity; since the air density decreases with height, at a certain point the airplane can't get enough lift to go higher.

In your world this limitation is removed, since there is some sensible atmosphere at high altitude. This means that, as long as the airplane has means of keeping motion with respect to the air, it can get lift force.

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  • $\begingroup$ Will lack of gravity make it go wonky? $\endgroup$ Apr 1, 2023 at 17:25
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    $\begingroup$ Your universe is already wonky. Just write your story... $\endgroup$
    – RonJohn
    Apr 2, 2023 at 19:50
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Flight on Earth uses engines (either jet or prop) to produce thrust and aerodynamic wing surfaces to provide lift against gravity. And that last part is where your universe might have some issues.

An unmodified aircraft will be able to fly through a suitable atmosphere, but will be restricted to regions where there is sufficient gravity to stabilize the flight envelope. If you try to go too fast the generated lift will be too high and the plane will rise into even lower-gravity areas. Without changing the wing structure of the craft you'll have to align the plane's velocity vector correctly by pointing your nose down so that the additional lift translates to 'forward' motion at some angle above the direction the nose is pointed.

So you either fly slowly, thus extending your travel time, or you have to deal with odd flight vectors. Most of your navigation aids will be almost useless since they're calibrated for flight in an entirely different environment. You'll need telescopes and other manual methods to ensure you're getting the flight vector close enough, at least until you can find someone with enough information and technical ability to make adjustments.

And of course there's the fuel issue. The longest commercial flight on Earth at the moment is a little under 10,000 miles (~15,300 km) and just shy of 19 hours in an Airbus A350 variant designed for long-range flight (the A350-900ULR specifically) and carrying 166,000 litres (44,000 gallons) of fuel. You'll get a little more range running semi-ballistic flight paths through the lower gravity at high altitude, but don't plan on doing any really long-range travel that way. Given the right sort of flight path you might double the range, but at the cost of being in the air for four or five times as long.

High gravity is going to be much harder to sell with Earth technology. Not only do our aeroplanes not have the kind of thrust it would take to generate enough lift (except perhaps for some of the more interesting stunting builds) but they're just not built to take higher gravity. Just sitting on the ground in 2G would probably snap the wings off most commercial aircraft, and the framing required to resolve that issue would make the plane too heavy to lift. I'm sure you could build a 2G aeroplane with the right materials, but it's not going to be simple and fuel consumption is going to go up exponentially. For those areas you'll need to build dirigibles or other lighter-than-air craft to stand a chance. And balloons - especially gigantic helium-filled dirigbles - aren't exactly known for speedy travel.

But you know what dirigibles are good at? Staying in the air for as long as you can keep the lift gasses contained. If you have the right kind of material to hold helium indefinitely (which is a bit of handwavium, but so's your entire universe) then you can float around the sky for months without expending a single litre of fuel for lift.

And here's where we can have some fun.

In order for your universe to make a tiny bit of sense there has to be something moving the air around, preventing stagnation and ensuring that the massive volume of air doesn't get depleted. That means that you at least have winds and possibly major air currents that we can use. Start with a large dirigible with some solid internal structure that we can attach wing masts to. If you just pictured a streampunk airship then you're headed in the right direction. Use sails to capture the air currents and ride them whereever you're going. For faster acceleration throw out massive parachute-like sails that act like the spinnaker on a modern racing yacht.

I imagine that there would be "air rivers" filled with lighter-than-air craft travelling between land masses at different levels. Which means air pirates. On hang gliders, probably. Sounds awesome!

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The distances are really big. You need fueling stops frequently (more like an archipelago of planets/moons) or some really big aircraft. @Cort Ammon has a very good answer about distances and fuel requirements.

One other really important consideration for planes in particular is going to be the angle of incidence (the angle that the wings are mounted relative to to the longitudinal axis of the plane). From what I understand of aviation, this angle translates part of the forward acceleration of the plane into downward acceleration to counter gravity. Planes can change their pitch to modify this force, but this angle is the angle used as the "default" for cruising. This angle is usually around 6 degrees for planes designed for long-haul flying, though fighter jets may use lower angles and I believe that stunt planes have no angle of incidence. If you are designing planes to do long-haul flights in microgravity, you will want a much lower angle of incidence so these planes can have almost all of their acceleration be forwards, as there is almost no gravity that they need to fight against to stay aloft between planets. Otherwise, the planes will likely get pushed continuously off-course in a direction perpendicular to their path of flight.

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