# Is a Helicopter on a Geostationary Orbit?

1. Mar 13, 2014

### Master Wayne

Is a helicopter which is stationary relative to the surface on a geostationary orbit? Why, or why not?

2. Mar 13, 2014

### Staff: Mentor

No, it is not in an orbit at all. An orbit is the gravitationally curved path of one object around a point or another body.

3. Mar 13, 2014

### Master Wayne

But if it stays still on the air for a whole day, it's trajectory will be a circle around, and slightly above, the surface of the earth. I can't see the difference from a geostationary satellite.

4. Mar 13, 2014

### Staff: Mentor

The difference is that a satellite is in free fall in its orbit around the Earth. A helicopter is not. As an example, let's replace the helicopter with a blimp floating stationary in the air. The air is a fluid, just like water, so a blimp hovering in the air is similar to a submarine underwater. Neither are in free fall under the influence of gravity, so neither are in an orbit.

Edit: Note that there is no real difference for this discussion whether our helicopter/blimp/sub is in the air or underwater. The surface of the Earth is an arbitrary choice for deciding what a satellite is orbiting around. Just think of Jupiter or other gas giants which have no solid surface.

Last edited: Mar 13, 2014
5. Mar 13, 2014

### Master Wayne

But now look at it this way. What is the centripetal force which keeps the satellite on a circular trajectory around the planet? Its weight.

And what is the centripetal force which keeps the helicopter on a circular trajectory around the planet? Also its weight. The aerodynamic forces (or hydrodynamic, for that matter) which keep the helicopter (or submarine) suspended are not exactly equal in magnitude to its weight. There's a little weight "left", and that acts as the centripetal force.

So when you look at it, both the satellite and the helicopter are spinning around the planet in a circle due to their weight. The fact that there are other forces acting on the helicopter doesn't really matter, as I see it.

6. Mar 13, 2014

### phinds

To come at it a slightly different way from Drakkith's excellent answer, you seem to think that "orbit" has to do with somethings position relative to a spot on the surface of the Earth. It does not. It has to do with the way that the object is interacting with all of earth. According to your understanding everything on the Earth that isn't moving relative to some spot on the ground is in a geostationary orbit. You've got the "geostationary" part right but not the "orbit" part.

7. Mar 13, 2014

### Staff: Mentor

All of which is irrelevant since an orbit is the path due solely to gravity. A helicopter is being held up by lift and cannot take the path that gravity wants it to.

That is incorrect. If the weight of an object is not countered exactly by something such as lift then the object will accelerate downward and fall.

The fact that there are other forces acting on the helicopter is the exact reason that it isn't in orbit. Note that a helicopter follows none of the laws of orbital mechanics, laws which were developed specifically to describe objects in space that orbit with no other forces other than gravity acting on them.

8. Mar 13, 2014

### Master Wayne

Alright, now I got it. It all comes down to the definition of orbit.

If the weight of the object is countered exactly by something such as lift, then the net force on that object is zero. If the net force is zero, then how come the object is traveling in a circle?

If you are sitting in a chair in a room for a whole day, then you will have traveled a full circle after 24 hours. There must be a net force on you for you to do that. The way I have always seen it is that the chair's force on you is slightly smaller than your weight, leaving a small net force pointing towards the center of the planet. Same would go for the helicopter. Don't you agree?

9. Mar 13, 2014

### Staff: Mentor

That's a good question. I actually can't answer it at the moment.

10. Mar 13, 2014

### Master Wayne

Thank you very much for your help!

11. Mar 13, 2014

### ehild

You are right.

ehild

12. Mar 14, 2014

### abitslow

Your motion (or lack of it, relative to the surface of the Earth) is quite accurately explained just by considering the conservation of momentum (for you).

13. Mar 14, 2014

### Master Wayne

Thanks for taking the time to write all of this. I'm aware of everything you said.

However, the question being asked here is really pretty simple: due to the rotation of the Earth, objects lying still on the planet's surface are always describing curved trajectories. For an object to describe a curved trajectory, the net force on it must be different from zero. What force is acting on all of these objects to cause them to move like this? I'm suggesting it's their weight, which is not exactly balanced by "the electromagnetic repulsion (and Pauli exclusion)". What do you suggest?

14. Mar 14, 2014

### rcgldr

It's a small difference. The net force on an object on the surface of the earth is a tiny centripetal force, except at the north or south pole where the net force is zero and the object rotates but does not accelerate towards the center of the earth. At the equator, centripetal acceleration is about 0.03392 m / s^2.

15. Mar 14, 2014

### sophiecentaur

The fact that it is travelling in a circle tells you that the net force is not zero. IF it were, it would carry on in a straight line. Its weight force is much more than enough to hold it onto the surface of the planet because the necessary centripetal force is so tiny. The rest of the weight force is cancelled by the upward force of the ground.

The definition of Orbit must involve the notion of free fall. Once you provide any force (even a rocket engine) then the motion is no longer Orbital. At lease, that definition produced consistency.

16. Mar 14, 2014

### Staff: Mentor

Question: a centripetal force is required to cause an object to move in a uniform circular motion. Since both myself and the Earth's surface are rotating, then both myself and the ground feel the same centripetal force. So, relative to the ground, I am not accelerating at all, correct? And if so, then the force of my weight is exactly cancelled by the ground pushing up on me with nothing left over (relative to the ground). Is all that correct, or have I misunderstood something?

17. Mar 14, 2014

### paisiello2

I think you and the ground experience the same centripetal acceleration, not the same centripetal force since that depends on the relative masses of yourself and the ground.

Otherwise I think you have it right.

18. Mar 14, 2014

### Staff: Mentor

Ah yes, thank you for the correction.

19. Mar 14, 2014

### paisiello2

Now here is a bit of semantics for you:

If an orbit is defined as the path of an object subjected only to gravitational forces, then can anything really be 100% in orbit? Satellites in orbit experience orbital decay because of drag caused by the earth's atmosphere, magnetic field, and/or tidal forces. Objects in space are going to be affected by the solar wind.

So the definition of when something is in orbit or not seems arbitrary.

20. Mar 14, 2014

### FactChecker

The helicopter path is certainly "geostationary". So is my path as I sit on this chair. The definition of "orbit" in Webster's and other common dictionaries do not specify how the orbit path is maintained. In the context of flying vehicles, "orbit" is used to describe a flight path around a point, powered or otherwise. It is clear from the posts here that "orbit" has a more specific understood meaning in physics (at least where an entire planet is at the center).

Last edited: Mar 14, 2014
21. Mar 14, 2014

### FactChecker

One point that I do not see mentioned here is that the only geostationary orbit around the earth is on the plain of the equator (22,236 mi above earth's equator, moving in the direction of earth rotation). Most of the discussion here has been about geosynchronous, not geostationary. For a helicopter to stay in geostationary position in the northern hemisphere, it must do more than provide lift to cancel the gravitational force, it must continually correct toward the North.

22. Mar 14, 2014

### sophiecentaur

An alternative way of putting it would be that the upwards force from the ground will be equal to the weight force minus the centripetal force.

23. Mar 14, 2014

### paisiello2

Is this because any object in "orbit" and only affected by gravity must be in a plane that passes through the center of the earth? And therefore the only true geostationary satellites must be at the equator?

24. Mar 14, 2014

### FactChecker

Yes. And any plain through the center of the earth that is tilted away from the plain through the equator would make an orbit that varies North and South.

25. Mar 14, 2014

### SteamKing

Staff Emeritus
I think you mean 'plane' instead of 'plain'.