Helicopters

"PartyPooper2012" (PartyPooper2012) 06/02/2020 at 08:01 • Filed to: None

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Somehow I stumbled upon a short video of a helicopter hovering and I got stumped.

Helicopter is not touching the ground. Earth is spinning. Yet Helicopter is staying in place. No fancy GPS is used to stay in one spot.

Apparently something to do with air molecules keeping it in place... but that alone isn’t sufficient to explain this.

Are we saying that as long as the universe is filled with air or some sort of gas, Helicopter can take off to infinity and return back to same spot infinite time later?

There has to be some sort of a point where helicopter is far enough away from earth whereby earth’s spin has to matter? What is it? 

DAWRX - The Herb Strikes Back > PartyPooper2012 06/02/2020 at 08:28

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facw > PartyPooper2012 06/02/2020 at 08:28

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Fortunately, air near the surface of the E arth moves along with the surface as the Earth rotates. It would be bad if it didn’t. Here a t around 42°, we are moving at around 770 miles per hour and we would do really poorly if their was effectively a 770 mph wind outside. At the equator, you are moving ~1040 miles per hour so it’s even worse there (this is one of the reasons it’s better to launch spacecraft closer to the equator, as it’s that much less speed you need to add to the craft to get it to orbit).

Helicopters don’t fly high enough for this factor to matter though as even at airliner heights the air isn’t moving all that fast compared to the ground (and is still dominated by wind patterns rather than slipping free of the Earth’s rotation.

The other thing you need to consider, is that upon taking off, the helicopter still carries all of its momentum from being attached to the Earth, even without considering the atmosphere. This means while the Earth’s surface may be moving at 800mph or whatever, the helicopter is also moving at 800mph initially, so if you ignored air friction and gravity, it would stay in roughly the same place over the Earth for some time, until the Earth rotated away enough that the spot on the ground “dropped away” from the helicopter. Of course we do have both gravity and air friction, so nothing so clean happens. Gravity pulls the helicopter back towards the Earth, and the helicop ter must create enough lift to stay “in orbit” for it to hover in one place relative to the surface of the E arth. And of course air friction complicates things immensely since the helicopter is moving within the medium of the air, and the molecules of air are being pulled along due to friction with each other and the ground, as well being moved by the effects of heating and such.

Just Jeepin' > facw 06/02/2020 at 08:30

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The momentum discussion reminds me of this brilliant XKCD comic on why it’s so hard to get to orbit.

https://what-if.xkcd.com/58/

SilentButNotReallyDeadly...killed by G/O Media > PartyPooper2012 06/02/2020 at 08:31

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It all depends on whether it’s a European or African helicopter...

PartyPooper2012 > facw 06/02/2020 at 08:33

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Yep. But at infinity distance away from earth, there would be minimal to no gravity from earth thereby if helicopter was far away enough, would then earth’s spin matter i.e.  in one hour spot under helicopter will have moved 700-1000 miles? So at what point does that happen - what height?

facw > Just Jeepin' 06/02/2020 at 08:34

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I’m sad that Randall seems to have mostly given up on What If, though he does occasionally write and illustrate a similar, though less technical column in the New York Times.

My bird IS the word > PartyPooper2012 06/02/2020 at 08:35

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Going to add to facws post to address your last point, if the universe was filled with gas instead of vacuum, the helicopter would at some point escape earth’s gravity and be thrown like a rock at a vector away from the planet, where friction would eventially slow it to a stop, at which point in time a helicopter would become more like a plane and could travel in any direction.

Although, the earths rotation would,have a decreasing effect the further away so it would depend on how fast the helicopter was "ascending" on whether it got launched or not.

Just Jeepin' > facw 06/02/2020 at 08:44

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I did not know about his NYT work; guess my subscription will be even more useful than I thought!

Thomas Donohue > PartyPooper2012 06/02/2020 at 08:45

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Ignore the other silly responses.

Obviously the helo stays in place because the earth is flat, not spinning. Clearly if earth was round and spinning at 700 mph we’d all be dead already.

PartyPooper2012 > SilentButNotReallyDeadly...killed by G/O Media 06/02/2020 at 08:45

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Cash Rewards > PartyPooper2012 06/02/2020 at 08:45

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http://adsabs.harvard.edu/full/1966SSRv....6..248K

Answer is in there, somewhere

PartyPooper2012 > Thomas Donohue 06/02/2020 at 08:50

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OK... now if earth is flat and not spinning, is it still round like your image or more rectangular like a map?

Could it also be somewhere in between like... oval or rhombus?

Surely it cannot be trapezoid or a triangle... or could it???

MasterMario - Keeper of the V8s > PartyPooper2012 06/02/2020 at 08:54

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The short answer is once you are free of Earth’s gravity.

This is assuming the helicopter is actually a space craft and is only moving vertically directly away from Earth’s center). Once free from Earth’s gravity it would basically stop rotating with the Earth. Even this is way too simplistic as Earth’s gravity technically extends indefinitely, but the effective distance is dependent on the location of other celestial bodies. Plus this excludes A LOT about orbital mechanics and how things move in space.

TheRealBicycleBuck > My bird IS the word 06/02/2020 at 08:55

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To expand on that, the helicopter would be the worst vehicle for traveling through a gas-filled universe. Without gravity to work against, the rotor wouldn’t be able to create a forward motion. It could go up, and if the original design included downward thrust, it could go down, but tilting the rotor for forward flight would result in the helicopter moving “up” and “forward” while rotating around its pitch axis.

An airplane would have problems too. If it were initially in a state of rest, the torque from the engine would cause the airplane to start rolling opposite the rotation of propeller. Once it started moving, the rotation could be controlled, but since the wings provide lift, the forward motion of the airplane would cause the aircraft to move “up” since there’s no gravity to pull the airplane “down.” I suppose some aircraft with symmetrical airfoils could be trimmed to provide “level” flight, but I think it would be easier to start over from scratch. I imagine the design would be quite different from the standard airplane design.

3point8isgreat > PartyPooper2012 06/02/2020 at 08:59

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As the helicopter got to a higher altitude, the Earth beneath it would move faster in relation. Say the Earth’s surface at the point of the helicopter is moving at 1000 mph. As the helicopter takes off it keeps this velocity, but as altitude gets higher the diameter of the circle the helicopter takes becomes larger and larger relative to the diameter of the Earth. So now you have the helicopter going around a BIG circle at 1000 mph, while the point it took off from moves on a smaller circle also at 1000 mph.  So of course the helicopter appears to rotate around its circle compared to the Earth’s surface. As you go higher the difference increases.

This all also requires ignoring a bit of fluids and how wind works. Even without wind, f riction in the air would slow itself down as a parabolic function of distance from the Earth’s surface, with velocity = 0 at infinity. So at high altitude the helicopter would be facing some ridiculous winds to maintain its initial velocity.

Gravity does not play a part as gravity only effects the vertical motion, and the relative position is a horizontal action.

Thomas Donohue > PartyPooper2012 06/02/2020 at 09:07

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It’s Flat!!!

PartyPooper2012 > Thomas Donohue 06/02/2020 at 09:09

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Yes... ok... but a sheet of paper is also flat but its rectangular when viewed from above... so what shape would earth be then? 

nermal > PartyPooper2012 06/02/2020 at 09:10

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Simple. You’ve got the forces of the earth spinning , the top rotors making the helicopter want to spin in one direction, and the back rotor balancing things out.

It’s not the environment, it’s the helicopter’s ability to adapt to the environment and balance itself that makes things work. There’s an important life lesson in there somewhere...

TheRealBicycleBuck > TheRealBicycleBuck 06/02/2020 at 09:10

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I should qualify that the airplane I’m thinking of would be a single-engine propeller driven aircraft. Single-engine turbine-powered would have similar rotation problems. The rotational dynamics of twin or multi-engine aircraft would be different, depending on the number of engines and their rotation direction. Counter-rotating twin engine designs should have fewer rotational problems, but the problems associated with forward motion would be the same due to lift from the wing and the other horizontal surfaces. Then there are the problems associated with lifting bodies...

Grindintosecond > PartyPooper2012 06/02/2020 at 09:11

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Obvious answer is correct answer.

Helicopters work this way because of money. Money makes them fly. Money keeps them from going an y where else.

Thomas Donohue > PartyPooper2012 06/02/2020 at 09:15

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Flat, round- ish.  Some ice around the edges.  That’s it.  Don’t fall off like Shackleton almost did.

TheRealBicycleBuck > PartyPooper2012 06/02/2020 at 09:18

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This line of thought reminds me of The Integral Trees by Larry Niven. He tried to imagine what life would be like in a gas torus which was dense enough to support life.

The people living there were from a colony ship. The trees get their integral shape from differential friction caused by their orientation and movement through the gas torus. It’s a fun read.

Dakotahound > PartyPooper2012 06/02/2020 at 09:31

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Actually, the Earth’s spin will have some effect on a helicopter. Neil deGrasse Tyson recently posted a video explaining how a football, kicked about 50 yards, will deflect by approximately 1/3 inch. I know, this is not the same as a helicopter hovering, but it does show how the Earth’s rotation affects objects near the surface (in this case, the Coriolis Effect).

https://bigthink.com/videos/neil-degrasse-tyson-space-facts

See his explanation at 11:45.

PartyPooper2012 > Dakotahound 06/02/2020 at 09:45

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Yeah. I think that has partly to do with horizontal...ish movement of the ball.

That brings up an interesting point tho... if a helicopter is hovering some short distance away form earth and there is wind to move air molecules.... would earth’s movement be pronounced? 

ttyymmnn > facw 06/02/2020 at 10:08

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facw > ttyymmnn 06/02/2020 at 10:21

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Yep. Incidentally, this relates to a mistake that was made with CDs/DVDs with regards for data. They started at inside of the disc, which is fine for audio/video since every part of the disc is fast enough, but for general data use, it means that you’ve started at the slowest part. There are some advantages to doing that way (allows for those mini CDs), and might be better for durability, but from a pure data transfer speed perspective, it wasn’t  a great idea. I guess it’s all obsolete now.

ttyymmnn > facw 06/02/2020 at 10:24

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Vinyl still sounds better. 

VincentMalamute-Kim > Just Jeepin' 06/02/2020 at 12:01

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In your link, h e says:

“ Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn’t escaped Earth’s gravity at all; it’s experiencing about 90% the pull that we feel on the surface. “

So how come the astronauts in the ISS are floating around?  Aren’t they floating around and not walking?

Just Jeepin' > VincentMalamute-Kim 06/02/2020 at 12:28

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Because they’re falling. F orever!

https://www.wired.com/2011/07/why-do-astronauts-float-around-in-space/

VincentMalamute-Kim > Just Jeepin' 06/02/2020 at 12:55

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oh - because it’s moving, there’s centripetal force opposite the gravitational force and they’re floating.

BoxerFanatic, troublesome iconoclast. > PartyPooper2012 06/02/2020 at 13:02

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gravity is still acting on both the helicopter and the atmosphere.

the fluid viscosity of the air ( and all other gases and liquids on this planet) is also relatively stable to the earth’s rotation, so the planet, atmosphere, everything on the surface, and in the air, are rotating on the earth’s axis as a system. It varies within a margin of plus or minus the earth’s rotational velocity, and it varies in vector angle relative to the earth’s rotational axis, but it is cohesive in general.

This system has a gravitationally captive orbital tidal mass as well, the moon, making a planetary system.

And that whole planetary system, along with 7 others, and a multitude of other objects are revolving around the sun’s gravity well also, constituting our solar system.

Our solar system I revolving around a theoretical massive black hole at the center of our galaxy along with billions of other star systems and other galactic bodies.

Our galaxy is moving relative to other galaxies in our Local Group, in our corner of an expanding universe.

Coming back down to earth...

So in ‘ relative space’ a helicopter hovers. So does a hu mming bird. they maintain relative position to their immediate surroundings.

In objective universal position, we are moving on multiple axes, around multiple gravitational systems, at cosmic levels of speed through a relative concept of space-time.

C osmically, nothing is ever in the same position twice, from moment to moment . We are just in the same position relative to our surroundings. The difference in position is based on the scope of what you consider your surroundings to be. The larger the scope, the larger the difference.

If you dip your finger into a river twice, it may be the same river, but not the same water. (nor is your finger exactly the same the second time, either), you breathe the same atmosphere, but not always a finite air volume, depending on how contained you are... and being too contained will suffocate your respiration as your body converts oxygen content of that air to carbon dioxide. (plants do the opposite for atmospheric balance)

“ Everything is on it’s way to somewhere.”