# How to read the pressure diagram of an airfoil?

According to most authors, "lift is the result of the difference of pressure on airfoil sides".

My question is not about how this difference is generated, but only about the representation of the pressure around the airfoil. In particular, diagrams like this are frequently seen when talking about airfoils:

Being not an aerodynamics engineer, I'll like to understand what can be deduced from this diagram.

1. Does this diagram indeed shows the variation of pressure on the contour of the airfoil ? Will this vary if the angle of attack or the speed are changed?

2. Is it correct to deduce from this diagram that the points on the red portion undergo a pressure smaller than free airstream static pressure, and those on the blue portion undergo a pressure greater than free airstream static pressure?

3. What is the correct method to obtain the relative pressure at a given point p1? Is it to measure the length of the segment which is perpendicular to the airfoil surface at point p1? What is the scale?

4. On the figure above, is p2 absolute pressure indeed equal to free stream air?

5. Do point p2 and the similar one found on the trailing edge have specific names?

6. What other elements can be deduced from the graphic?

• Commented Jun 24, 2017 at 20:13
• Good questions for a fair understanding of the topic. Commented Jun 24, 2017 at 20:25
• for 1. yes and yes for 5., from my answer on the linked question, they are called "stagnation points" Commented Jun 25, 2017 at 10:56
• @Federico Isn't the $C_p$ at a stagnation point 1? p2 seems to have a $C_p=0$. Commented Jun 25, 2017 at 13:17
• A nice set of questions indeed, and a set of good answers. Although aren't sets of questions frowned upon here, or are these frowns only for low rep users? Commented Jun 26, 2017 at 5:33