Why does a helicopter lift off the ground?

[A.T.]

For air propelled devices, accelerating the air through it, not that air pushing on the air below it, is what causes the lift.

Agreed. The misunderstanding comes from the ambiguity of the word "pushing". I interpreted it as "applying a force", so a helicopter is pushing the air that it accelerates.

[mod note: split from here: https://www.physicsforums.com/showthread.php?t=762789 ]

 

[russ_watters]

Agreed. The misunderstanding comes from the ambiguity of the word "pushing". I interpreted it as "applying a force", so a helicopter is pushing the air that it accelerates.

I think we're on the same page now except that if we want to go that far we should split that hair as well: the helicopter BOTH pulls and pushes the air that goes through its rotor.

 

[A.T.]

the helicopter BOTH pulls and pushes the air

You are using ambiguous terms again. What does "pull" mean exctly? Exerting an attractive force? The rotor blade cannot do that. There are only repulsive forces in collisions of air molecules and blade.

 

[ModusPwnd]

It gravitationally pulls the air to it. :tongue:Maybe "sucks" would be a better word. Its sucks the air above it by creating a low pressure zone which allows the air above to fill the void.

 

[russ_watters]

You are using ambiguous terms again. What does "pull" mean exctly? Exerting an attractive force? The rotor blade cannot do that. There are only repulsive forces in collisions of air molecules and blade.

There are two ways to see a "pull":

1. The Coanda Effect.
2. Airfoil pressure profiles are typically normalized to atmospheric pressure, so the pressure on the top surface is indeed typically shown as negative.

The common view that an airfoil pushes the air down tends to cause people to miss the fact that the top surface usually produces most of the lift.

 

[A.T.]

Its sucks the air above it by creating a low pressure zone which allows the air above to fill the void.

True but not directly relevant to action/reaction between helicopter and air. The total force by the air on the helicopter is fully accounted for by the repulsive forces in the collisions between helicopter and air.

 

[A.T.]

the top surface usually produces most of the lift.

How is that determined? When there is a pressure difference between two sides of a door, can you tell which side of the door produces the net force on the door?

 

[ModusPwnd]

From his post it looks like it is determined relative to atmospheric pressure.

 

[A.T.]

From his post it looks like it is determined relative to atmospheric pressure.

You mean the difference in pressure to somewhere far away from the wing, is greater above the wing than below the wing? Well, that again is not directly relevant to action/reaction transfer between wing and air, which is local. The aerodynamic force is fully accounted for by the difference in local pressures at different sides of the wing.

 

[russ_watters]

You mean the difference in pressure to somewhere far away from the wing, is greater above the wing than below the wing? Well, that again is not directly relevant to action/reaction transfer between wing and air, which is local. The aerodynamic force is fully accounted for by the difference in local pressures at different sides of the wing.

You seem to be suggesting that the pressures above and below are measured relative to each other. Not only is it not done that way, I don't see a practical way that it could be. No: the pressures are measured relative to freestream.

In any case, this tangent has outlived its usefulness and we should get on it. My point in continuing it was mostly just to point out that it is a tangent and not part of the OP's question.

 

[A.T.]

You seem to be suggesting that the pressures above and below are measured relative to each other.

I didn't say anything about pressure measurements or their practicality. I say that the total aerodynamic force on the wing is fully accounted for by the difference in local pressures at different sides of the wing.

 

[sophiecentaur]

From his post it looks like it is determined relative to atmospheric pressure.

If a pressure difference is involved, it doesn't matter what the reference pressure is (as long as they both use the same). To analyse the motion, it is easiest to use the Earth frame.

 

[sophiecentaur]

You seem to be suggesting that the pressures above and below are measured relative to each other. Not only is it not done that way, I don't see a practical way that it could be. No: the pressures are measured relative to freestream.

In any case, this tangent has outlived its usefulness and we should get on it. My point in continuing it was mostly just to point out that it is a tangent and not part of the OP's question.

Is "freestream" in a wind tunnel or measured with a probe on the flying craft?

 

[olivermsun]

How is that determined? When there is a pressure difference between two sides of a door, can you tell which side of the door produces the net force on the door?

Actually there are some devices than can do that. But unless it's likely to rip the door apart, does it really matter?

 

[A.T.]

When there is a pressure difference between two sides of a door, can you tell which side of the door produces the net force on the door?

Actually there are some devices than can do that.

How?

 

[russ_watters]

I didn't say anything about pressure measurements or their practicality. I say that the total aerodynamic force on the wing is fully accounted for by the difference in local pressures at different sides of the wing.

Difference in pressure between what and what? You understand that whether it is measured or calculated, a differential pressure is between two points, right?

 

[A.T.]

Difference in pressure between what and what?

Between the two sides of the wing.

The local absolute pressure on the wing surface represents the local density of momentum transfer by the collisions of air molecules with the wing, involving only repulsive forces. The surface integral of this absolute pressure on the wing gives you the total aerodynamic force on the wing. No pulling involved here.

 

[olivermsun]

How?

A "pressure sensor." Although, in this case you could argue that the whole door is a (differential) pressure sensor.

Although now that I think about it, I don't see how you could say the force on one side of the door or the other is what "produces" the net force. Isn't it the sum of forces = the differential pressure what produces the "net" force?

 

[russ_watters]

How?

Typically, by identifying what caused the pressure difference; wind? A fan?

 

[russ_watters]

Is "freestream" in a wind tunnel or measured with a probe on the flying craft?

You didn't describe two different methods, but just two different situations that are equivalent: you can reference free stream static pressure with the same probe whether it is in a wind tunnel or outside.

 

[A.T.]

Although now that I think about it, I don't see how you could say the force on one side of the door or the other is what "produces" the net force. Isn't it the sum of forces = the differential pressure what produces the "net" force?

Exactly. That's why I don't understand this:

the top surface usually produces most of the lift.

 

[russ_watters]

Exactly. That's why I don't understand this:

Because in reality, there is a reference pressure. You can subtract it out in the math to create a different reference*, but that doesn't actually make it go away in the real world.

*in some cases you might, in others you wouldn't.

 

[russ_watters]

Although now that I think about it, I don't see how you could say the force on one side of the door or the other is what "produces" the net force. Isn't it the sum of forces = the differential pressure what produces the "net" force?

Because something had to add force to one side or take it away from the other. A fan, for example.

Let's simplify: you measure the differential pressure across the escape hatch of a spaceship in orbit. Can you say which side the force is being applied to?

 

[A.T.]

Because in reality, there is a reference pressure.

The aero dynamic force depends only on the difference in pressures between the sides of the wing, not on how these pressures compare to some reference pressure far away from the wing.

 

[olivermsun]

Because something had to add force to one side or take it away from the other. A fan, for example.

So is a fan blowing across one side of the door "pulling" the door or is the static pressure on the other side of the door "pushing" the door? Doesn't it depend on which phenomenon you choose to focus upon?

Let's simplify: you measure the differential pressure across the escape hatch of a spaceship in orbit. Can you say which side the force is being applied to?

I'd have to say air pressure is being applied mostly to the inside surface, since the ambient pressure on the outside is probably pretty small.

 

[A.T.]

Let's simplify: you measure the differential pressure across the escape hatch of a spaceship in orbit.

So, what is the "reference pressure" here, and why?

 

[russ_watters]

The aero dynamic force depends only on the difference in pressures between the sides of the wing, not on how these pressures compare to some reference pressure far away from the wing.

Ignoring it doesn't make it go away. You asked how you can tell which side is most responsible for the lift and the answer is what it is. Not sure what else can be said.

So, what is the "reference pressure" here, and why?

Space is chosen as the reference because it is convenient/useful.

 

[russ_watters]

So is a fan blowing across one side of the door "pulling" the door or is the static pressure on the other side of the door "pushing" the door? Doesn't it depend on which phenomenon you choose to focus upon?

Blowing across? I don't think that situation changes the static pressure on the door.

I was thinking more along the lines of a fan blowing into or out of the room though another opening, either positively or negatively pressurizing it with respect to the other side.

I'd have to say air pressure is being applied mostly to the inside surface, since the ambient pressure on the outside is probably pretty small.

Agreed.

 

[A.T.]

Space is chosen as the reference because it is convenient/useful.

If the choice of reference pressure is so arbitrary, then so is the claim about which side is most responsible for the net force.

 

[sophiecentaur]

If the choice of reference pressure is so arbitrary, then so is the claim about which side is most responsible for the net force.

How does that make sense? It is the difference that counts and that involves both sides; you can't have a 'difference' between one side. They are both "responsible" (if you want to apportion blame lol.)

 

[olivermsun]

Blowing across? I don't think that situation changes the static pressure on the door.

You can change the total pressure on the side of the door with the flow. The static pressure remains on the other side. This is all assuming the door is closed (as with the spacecraft hatch example).

I was thinking more along the lines of a fan blowing into or out of the room though another opening, either positively or negatively pressurizing it with respect to the other side.

Okay.

 

[A.T.]

How does that make sense?

Well, it doesn't to me. Seems arbitrary.

 

[russ_watters]

If the choice of reference pressure is so arbitrary, then so is the claim about which side is most responsible for the net force.

The two things have nothing to do with each other. No matter what you use as a reference, you can observe and quantify which side changes more -- it isn't affected by the choice of reference. That's what "arbitrary" means!

Let's say you have a parcel of still air. Call the pressure 14.7 psi. Call it zero. Call it -47. It doesn't matter: when a wing flies through it, the pressure of the air above the wing goes down and the pressure of the air below the wing goes up, both can be measured or calculated separately and the change in the upper surface is usually greater than the change for the lower surface.

Arbitrarily, let's plug in some numbers:

Let's call our reference pressure -47 psi.

Above the wing, the pressure is measured at -49 and below the wing, the pressure is measured at -46. So the pressure above the wing went down by 2psi and the pressure below it went up by 1psi. So the upper surface contributed 2/3 of the lift.

No matter what you pick for the reference, the answer remains the same.

Something else to consider here is that the wing itself is probably not air tight and certainly doesn't contain a vacuum. The completely separate aerodynamic forces on the top and bottom surfaces need to be considered separately when designing the structure of the wing. IE: even though you might think there is a pressure of 14psi pushing down on the top surface of the wing, the rivets holding the top surface on the wing are under tension, not compression.

 

[russ_watters]

How does that make sense? It is the difference that counts and that involves both sides; you can't have a 'difference' between one side. They are both "responsible" (if you want to apportion blame lol.)

What I think AT isn't getting about which side is "responsible" is that it isn't the difference between the two that you need to look at to see which is "responsible", but the change in each that you need to look at. The change in pressure above the wing is usually larger than the change in pressure below the wing.

For a door to a house after you turn an exhaust fan on in the house, the pressure inside changed and the pressure outside didn't. So it is what is happening inside that causes the force on the door.

 

[sophiecentaur]

This thread had morphed into the same old thread about lift. People seem to assume that the pressure above and below the wing are uniform and static. They are beguiled by the simple wind tunnel idea. What ultimately keeps the plane up there is (has to be) the net downward acceleration of air. The details of air flow around the wing and well beyond it must have an effect.
It is far more complicated than the simple ideas that are being aired here.