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What planetary conditions would make flight easiest for both lighter than air and heavier than air craft together?

The planet is earth like and habitable, but the conditions can be adjusted to suit within those constraints. The atmosphere should be capable of sustaining human life.

By easiest I mean capable of carrying the most weight off of the ground

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    $\begingroup$ This question looks pretty broad as you are asking for three scenarios in your first sentence: 1) easiest for lighter than air, ") easiest for heavier than ar, 3) easier (though not necessarily easiest) for both lighter and heavier air. Please edit this to focus on one scenario. I am voting to put this question on hold until you focus it on one scenario. You can always wait a bit for feedback and then ask another question with a different scenario. $\endgroup$ – Secespitus Sep 27 '17 at 9:16
  • $\begingroup$ @Secespitus A fair point. I have restricted the question accordingly $\endgroup$ – Slarty Sep 27 '17 at 9:29
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    $\begingroup$ Actually, all this talk about gas mixes is completely wrong. Pressure doesn't depend just on gravity, amount (total mass) of gasses is more important factor here. You can have same gravity and much higher pressure with same fractional composition without any problem. $\endgroup$ – M i ech Sep 27 '17 at 11:03
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    $\begingroup$ edited easiest to mean carry the most weight off of the ground $\endgroup$ – Slarty Sep 27 '17 at 11:07
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    $\begingroup$ In case you are worried about atmospheric stability (en.wikipedia.org/wiki/Atmospheric_escape), don't. Venus has over 90 times higher pressure (and only ~90% of Earths gravity), and atmospheric escape there happens due to lack of magnetic field and not because of thermal/hydrodynamic processes. You could probably increase Earth's pressure 1000 times without worrying about atmospheric escape making atmospheric composition unstable - you would however need to adjust orbit, because such increase would cause higher surface temperature and THAT could trigger thermal/hydrodynamic escape... $\endgroup$ – M i ech Sep 27 '17 at 11:25
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High atmospheric density

As pointed out in the comments by @M I ech, gravity and gas mixing has little to do with it.

Heavier than air craft, as in airplanes and helicopters, rely on lift to get off the ground and stay aloft. Lift is created by creating a disparity between the air pressure above the lift surface (wing) and the air pressure below the lift surface. So when the air pressure above the wing is lower than that below the wing then craft is capable of moving upwards. See the Wikipedia entry on lift for more info.

Increasing the atmospheric density would create a higher standard pressure, and therefore make it easier for a heavier-than-air craft to create the needed pressure differential for take off. Incidentally, this is the same situation (but reversed) that requires hypothetical Martian aircraft to be light with large wings.

As for lighter than air craft, like blimps etc, this problem becomes exceedingly easy. Lighter-than-air craft rely on the same principle of buoyancy as boats: by introducing a large volume with a lower density than the medium (air or water) then you can decrease the average density of the craft and it floats. In a situation where the surrounding air is denser, this again means that it is easier to create a volume of lower density and take off.

This is of course assuming a) gravity on the order of 1G (different gravity will have other effects on these forces) and b) hand-wavey sci-fi tech like inertial dampeners or anti-gravity are not options.

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    $\begingroup$ I want to add this bit from a What If-XKCD article about flight in different planets: "When it comes to flying, Titan might be better than Earth. Its atmosphere is thick but its gravity is light, giving it a surface pressure only 50% higher than Earth’s with air four times as dense. Its gravity—lower than that of the Moon—means that flying is easy. Our Cessna could get into the air under pedal power." what-if.xkcd.com/30 $\endgroup$ – M Arif Rahman Winandar Sep 27 '17 at 13:02
  • $\begingroup$ This actually has a down sides for winged aircraft; yes higher density makes more lift at the same speed, but higher density makes more drag making it take more energy to get up to that same speed. So it would be great for blimps and other lighter than air craft, but kind of a mixed bag for winged aircraft. $\endgroup$ – Josh King Sep 27 '17 at 21:33
  • $\begingroup$ Some things are a bit off. Lift isn't created entirely by the air differential on the wings. Higher atmospheric pressure wouldn't lower the pressure differential, what you might mean is the required pressure differential is a smaller percentage of the total pressure. $\endgroup$ – Passer By Sep 28 '17 at 15:20
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The moon Titan in our own solar system seems to have the correct conditions for flight, a relatively low gravitational field and a dense atmosphere (Surface gravity is only .14g while the atmosphere pressure is 1.45 atm).

enter image description here

True colour image of Titan

This means very little energy will be needed to take off (regardless of what sort of engine you are using) and the dense atmosphere would make displacement by a LTA aircraft easy, while also requiring less surface area for lifting surfaces like wings or rotors to work. Indeed, it seems possible for humans to fly on Titan simply by strapping wings on their spacesuits and flapping madly away.

enter image description here

Wingsuit

A small drone has been designed to fly a science mission in Titan, and you can see the small relative size of the wings compared to the fuselage.

enter image description here

Aviatr probe concept

So in general, any place with a low gravity allows you to use les energy to fly, while a thick atmosphere provides more displacement or lift for powered flight. Titan is probably unusual because it combines the two, but the extreme conditions and cold which allow these conditions to exist probably can be replicated in the moons of gas giants in other solar systems.

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