Why does a helicopter lift off the ground?

[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.

 

[A.T.]

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.

I get what you mean. I just think that your definition of "responsible" is arbitrary. Those discussions about "causes" of lift are bad enough. But this goes a step further and tries to quantify the contribution of opposing forces to the net force on an object.

 

[gianeshwar]

Thank You very much dear friends for an informative discussion!
Through your valuable arguments,my commonsense ideas improved to indepth ones and I got motivation to study and think more.