It's a negative gauge pressure and might be visualized as a pull. Similar to the way a vacuum cleaner "pulls" dirt from a rug.So by your definition, a surface exerting an upwards force on the air above it, is actually "pulling down on the air"?
Thank you!It is correct to say that differential pressure (or more correctly, gauge pressure, since it is generally referenced to atmosphere) is used extremely frequently when dealing with the pressure distribution over an airfoil. It is definitely the most common method of doing things, typically in the form of the dimensionless pressure coefficient, ##C_p##. This is done because using gauge pressure is more mathematically compact and lends itself to collapsing the data into relationships that work over a range of free-stream conditions.
Fine. So in the future, i would hope that you will recognize the convention without it needing to be described to you, so we can avoid repeating much of this.That said, gauge pressure is also more physically misleading.
If the person trying to learn about the process in the first place does not understand the distinction, then he or she will note that the upper surface shows a negative pressure, which implies that the air should "pull" on the wing on that side. While this still all works out mathematically, it leads to physically incorrect conclusions in the hands of a novice.
Conventions are personal choices and cannot be right or wrong if both work when applied correctly. You prefer your convention, and I prefer mine and both work and that's fine. Please be ok with that. Or at least pretend to be for the sake of the OP!So, circling back to the concerns about beginners, I still maintain that doing this in terms of gauge pressure right off the bat is misleading and the wrong approach.
So again: something true cannot be wrong just because you prefer a different convention or verbiage. The word "contribution" is used in academic settings. You don't like that. That doesn't make it wrong, but you don't have to use it. But more importantly - again - please try to recognize differences in conventions/wording and not to be so quick to argue over them. It is not helpful to the OP.On top of all of that, the fundamental concept of the top "contributing more" is still wrong. It is a given that the gauge pressure on the top is going to be negative. It is true that in some cases, the integrated force from that upper gauge pressure would be greater than on the lower surface.
Agreed/understood. But it *is* true for the example first given to the OP in post #2, which is a common example because it is common in real life and simple for the very first thing someone learns about lift. This isn't something that should create a disagreement, but rather a stepping stone to the next, deeper point.That is not a universally true fact, however.