Aerodynamics: if the earth disappeared?

[photofly]

An aerodynamics question, to puzzle over.

Imagine that at some instant, suddenly the earth disappeared and was replaced with air/atmosphere, "all the way down".

What would the effect(s) be on an airplane in flight?

Would any of them be the same for a helicopter in forward flight, or hovering?

What would make the effects more or less noticeable?

When would the effects become apparent?

[Rockie]

If the earth disappeared so too would its gravity. The atmosphere would quickly dissipate into the spacial vacuum leaving the aircraft to continue on its trajectory for the rest of time unless it hit something. The crew of course would die immediately.

Same thing in a helicopter.

[photofly]

Ok. Assume that gravity continues to act unchanged; that the earth is replaced with "enough" air to maintain the same gravity. Air temperature and density around the aircraft remains unchanged. How does the airflow around the aircraft change? What effect does this have on its flight?

[Rockie]

None. Gravity is the same. Air pressure is the same.

The airplane moves through the medium of air and knows nothing at all about the earth except for gravity and the effect it has on the atmosphere. If the earth were to magically disappear to be replaced with nothing but air that exerted the same gravitational influence as the solid earth did, then the airplane would behave exactly the same way because for it nothing has changed.

[photofly]

The airplane moves through the medium of air and knows nothing at all about the earth except for gravity and the effect it has on the atmosphere.

Is that true for all kinds of flight?

[iflyforpie]

Not just the gravity's effect on the air, but the aircraft as well.

Gravity is a force and imparts an acceleration on the aircraft towards the source of gravity. The aircraft must change its energy state to go towards and away from it. That is why even if the earth disappeared and we had no horizon, you still would have very strange effects on the typical aircraft straying into unusual attitudes.

I can see this where I fly all of the time, because I rarely have a good horizon to work with.

[photofly]

I hadn't thought about not having a horizon. Nice catch.

Let's assume the aircraft is flying on autopilot, to maintain level flight. The autopilot maintains attitude reference gyroscopically, or via an AHRS, neither of which is affected by the sudden disappearance of the earth.

[Rockie]

The airplane moves through the medium of air and knows nothing at all about the earth except for gravity and the effect it has on the atmosphere.

Is that true for all kinds of flight?

I hadn't thought about not having a horizon. Nice catch.

Let's assume the aircraft is flying on autopilot, to maintain level flight. The autopilot maintains attitude reference gyroscopically, or via an AHRS, neither of which is affected by the sudden disappearance of the earth.

There is still a horizon because the atmosphere below you in your example is still exerting the same gravitational force the solid earth did. The "horizon" lies 90 degrees in any direction from the center of the gravity's vector. As you fly in any direction you will maintain an altitude reference the altimeter which will behave exactly as it always has because the atmosphere will be exactly the same. Climbing would be the same as now, but descending into exponentially increasing air pressure would be an interesting experience.

[snoopy]

If you remove the earth from the equation a helicopter in the hover will most certainly be affected as there would be no more ground effect. All hovering performance/capabilities would have to be be calculated HOGE (hover out of ground effect).

What on earth (pardon the pun) lead you down this garden path?

Cheers,
Kirsten B.

[Shiny Side Up]

None. Gravity is the same. Air pressure is the same.

The airplane moves through the medium of air and knows nothing at all about the earth except for gravity and the effect it has on the atmosphere. If the earth were to magically disappear to be replaced with nothing but air that exerted the same gravitational influence as the solid earth did, then the airplane would behave exactly the same way because for it nothing has changed.

Of course without the earth's nickel iron core, it also would lose its magnetoshpere and our hypothetical sphere of air would be blasted away by solar wind. Unless we possibly suppose that said air-earth was of approximate density to have a liquid/solid/slushy nitrogen-oxygen core, but that might suppose a relative increase in the radius of earth where there would still be atmospheric conditions at appropriate altitudes where this question might still be relevant, assuming one can conduct flight in areas to avoid new super-air-earth newly formed giant hurricanes that regularly circle it...

[grimey]

http://what-if.xkcd.com/ is probably the best place to ask. Or at least the most entertaining.

http://what-if.xkcd.com/8/

[photofly]

If you remove the earth from the equation a helicopter in the hover will most certainly be affected as there would be no more ground effect. All hovering performance/capabilities would have to be be calculated HOGE (hover out of ground effect).

What on earth (pardon the pun) lead you down this garden path?

Cheers,
Kirsten B.

So for the helicopter hovering in ground effect, how would the airflow and pressure pattern around it change, if the earth disappeared? What would the pilot have to do, to maintain the hover?

Do any of the same effects apply to aeroplanes? If so, under what circumstances?

[Redneck_pilot86]

What happens when the aircraft runs out of gas? It can't crash, does it glide forever? Thats pretty efficient, the only change is the lack of earth. I say we get rid of the earth, so we can save money on fuel.

[grimey]

If it runs out of gas, it falls towards the gravitational centre of the earth. And eventually gets crushed by atmospheric pressure or crashes into liquid or solid compounds which has precipitated out of the atmosphere because of the pressure. Either way you're fucked. Before either of those things happen you'll die of inert gas narcosis.

[Shiny Side Up]

If it runs out of gas, it falls towards the gravitational centre of the earth. And eventually gets crushed by atmospheric pressure or crashes into liquid or solid compounds which has precipitated out of the atmosphere because of the pressure. Either way you're fucked. Before either of those things happen you'll die of inert gas narcosis.

Ah! But suppose you had a clean enough airframe and the "solid" part of the earth was far down enough could you dive at sufficient velocity, using a gravitational slingshot and then make escape velocity so you could at least die by exiting the atmosphere, or would the atmosphere be so dense you'd burn up on the attempt? Inquiring minds must know!

[North Shore]

Of course without the earth's nickel iron core, it also would lose its magnetoshpere and our hypothetical sphere of air would be blasted away by solar wind. Unless we possibly suppose that said air-earth was of approximate density to have a liquid/solid/slushy nitrogen-oxygen core, but that might suppose a relative increase in the radius of earth where there would still be atmospheric conditions at appropriate altitudes where this question might still be relevant, assuming one can conduct flight in areas to avoid new super-air-earth newly formed giant hurricanes that regularly circle it...

Ah, ok there, pdw!

[Shiny Side Up]

Hey, I didn't mention the variable tailwinds, which I might add would probably also be present considering the ammount of convective activity in this super sphere that we're flying around in.

[ea306]

Hmmm.... Have I just stepped into an Alice in Wonderland thread?

[CFR]

No "nickel iron core" no magnetic pole, so no having to reset the DI.

[dashx]

All your answers can be found in Washington.

[photofly]

Only snoopy so far has mentioned that if the Earth suddenly disappeared, an aircraft that was in ground effect at the time would experience an abrupt change in aerodynamics. And that was in respect of a helicopter, hovering. What effects would an airplane notice?

[snoopy]

Well, it's not just helicopters that are affected by ground effect - all fixed-wing aircraft are subject to it in one form or another on take-off and landing - the helicopter simply has the unique feature and the ability to choose neither. You still haven't answered my question. What is the point of this random discussion? Is it the narcotics, or is there a purpose?
Cheers,
Kirsten B.

[photofly]

It's not random, it's an interesting thought experiment.

So what is the aerodynamic effect on an aircraft of an instantaneous loss of ground effect? What would the pilot have to do to maintain level flight?

[ruddersup?]

Correct me if I'm wrong but doesn't every aircraft fly because of ground effect. Even a jet at 30,000 feet is related to ground effect. I say an aircraft heavier than air can not fly. Balloons only, ?????

[Meatservo]

If an aircraft was flying in ground effect, and suddenly the ground disappeared, but there was still gravity, the aircraft in question would just start to drift downwards as induced drag slightly increased. I suppose if there was no ground under it, there wouldn't even be a "down" for the aircraft to drift to, so it might only be correct to say that the location of the centre of pressure over the wings or rotor blades would move slightly, and that's about it. I'm guessing almost nothing would happen,

I guess you could see what would happen by flying really low approaching the top of a cliff on a calm day and fly over it. I suppose if you did nothing at all to the controls as the aircraft passed over the edge of the cliff, it would very gradually lose altitude.

As thought experiments go, this one isn't very interesting.

Ever notice how in Sci-fi movies, with very few exceptions the space ships are depicted banking into turns? I've always thought that was silly. There are a couple of recent shows where they don't do that, but mostly they do. Shows a lack of aerodynamic knowledge on the part of the special-effect guys, for sure. Or maybe a lack of "extra-aero-dynamic" knowledge would be more appropriate to say.

Anyway, a more interesting thing to discuss would be whether you guys think a small flying craft on the MOON would need to bank in its turns.