# Gauge Pressure References

1. Jul 23, 2014

### A.T.

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 ]

Last edited by a moderator: Jul 24, 2014
2. Jul 23, 2014

### Staff: Mentor

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.

3. Jul 23, 2014

### A.T.

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.

4. Jul 23, 2014

### 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.

Last edited: Jul 23, 2014
5. Jul 23, 2014

### Staff: Mentor

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.

6. Jul 23, 2014

### A.T.

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.

7. Jul 23, 2014

### A.T.

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?

8. Jul 23, 2014

### ModusPwnd

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

9. Jul 23, 2014

### A.T.

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.

10. Jul 23, 2014

### Staff: Mentor

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.

11. Jul 23, 2014

### A.T.

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.

12. Jul 24, 2014

### sophiecentaur

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.

13. Jul 24, 2014

### sophiecentaur

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

14. Jul 24, 2014

### olivermsun

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

15. Jul 24, 2014

### A.T.

How?

16. Jul 24, 2014

### Staff: Mentor

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

17. Jul 24, 2014

### A.T.

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.

18. Jul 24, 2014

### olivermsun

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?

19. Jul 24, 2014

### Staff: Mentor

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

20. Jul 24, 2014

### Staff: Mentor

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.

21. Jul 24, 2014

### A.T.

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

22. Jul 24, 2014

### Staff: Mentor

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.

23. Jul 24, 2014

### Staff: Mentor

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?

24. Jul 24, 2014

### A.T.

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.

25. Jul 24, 2014

### olivermsun

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?

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.