# Trying to understand this pressure distribution over an aerofoil?

can someone please help me understand why the pressure is like that over the top and bottom surface of this aerofoil please?

Also, what do the arrows point out mean? Why are the front and end ones pointing in?

• There are tons of answers about this topic. I personally suggest you to get some nice book about aerodynamics to lern the basics: making a lot of questions won't help you understand the basics rather will confuse you even more. Commented Apr 7, 2023 at 17:25
• @sophit ever try to read a ton of books? Sometimes, it's better to ask someone. Commented Apr 10, 2023 at 18:30
• One very good book is enough and definitely better than many answers from random people on the internet. The questioner clearly lack the basics of aerodynamics, answering this question won't give it any help rather open a Pandora's box of other questions like: why does pressure decrease? Why and what is camber? Why symmetrical airfoil? What's stagnation point? Bernoulli? Why a pointy trailing edge? Turbolent flow? Why does airflow detach? What's momentum? ... Commented Apr 10, 2023 at 19:01
• @Sophit (and others) Instead of sending new users away by telling them to read a ton of books first, why don't we invest in new users by helping them to get the right answer by asking the right questions? That way we foster a healthy community and a healthy stream of new questions. We write our own book here :-) Commented Apr 18, 2023 at 9:33
• @DeltaLima: as said, the questioner clearly lack the basics of aerodynamics and, correct me if I'm wrong, this is not really the right website to obtain them i.e. this is not Wikipedia. Anyway I didn't send anybody away, just suggested to learn the basics and come back with maybe a more precise question ðŸ™‚ Commented Apr 18, 2023 at 10:12

## 2 Answers

There are a few interesting aspects to these seemingly innocuous pressure diagrams that bear closer examination.

Firstly, this is a non symmetrical airfoil with a bit of camber. This is why even the first diagram, at 0 degrees, it is generating more upward that downward force. The difference is more apparent at 6 degrees Angle of Attack.

cambered wings can have zero lift at a negative angle of attack

The second item of note is the pressure gradients at the trailing edge. Interestingly, these might be a result of insufficient airspeed, growing worse with increasing AoA.

Part of what makes wings work is the momentum, mv, of the airflow around the wing. As speed increases, in the exact same manner turbulent flow separates from the transom of a boat, the airflow from the top and bottom of the wing is carried past the trailing edge before merging.

This is what would expect to see with a properly functioning airfoil. The calm lower pressure stagnation zone directly behind the wing would not be creating any buffeting on the trailing edge.

• Thanks Robert, I read that the top is a suction region? Would it be correct to say this means the ambient pressure is lower than on the aerofoil surface then? Commented Apr 15, 2023 at 22:52
• @FlightWatcher the arrows can be confusing. Those pointing inwards are towards higher than ambient surface pressure. Those pointing outwards from the surface point away from pressure lower than ambient. Commented Apr 16, 2023 at 0:22

The arrows represent the direction the pressures are acting. Pointing towards the wing are increased pressures, and those pointing away are where the wing experiences a decreased pressure. At the leading edge, you can conceptualize this as the point where the air collides with the wing and must be deflected away. Meanwhile, due to fluid dynamics principles (e.g. Bernoulli's principle) the high-velocity air moving around the curved surface is related to a decrease in pressure at those locations. In the diagrams, the leading and trailing edges are being pushed inward while the upper and lower surfaces are being pulled outward. Summing up all the inward (increased) and outward (decreased) pressures, and looking at only the vertical component will result in a single upward force felt by the airplane.

• Thanks fencerPTS Commented Apr 15, 2023 at 22:53