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:

enter image description here
From Bernoulli or Newton's Laws for Lift?

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?

    enter image description here

  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?

    enter image description here

  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?

  • 1
    $\begingroup$ Related: How to plot the pressure distribution over an airfoil? $\endgroup$
    – TomMcW
    Jun 24, 2017 at 20:13
  • $\begingroup$ Good questions for a fair understanding of the topic. $\endgroup$ Jun 24, 2017 at 20:25
  • $\begingroup$ for 1. yes and yes for 5., from my answer on the linked question, they are called "stagnation points" $\endgroup$
    – Federico
    Jun 25, 2017 at 10:56
  • 1
    $\begingroup$ @Federico Isn't the $C_p$ at a stagnation point 1? p2 seems to have a $C_p=0$. $\endgroup$
    – Gypaets
    Jun 25, 2017 at 13:17
  • $\begingroup$ 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? $\endgroup$
    – Koyovis
    Jun 26, 2017 at 5:33

1 Answer 1

  1. The drawing in your answer shows the contour of the tips of the local pressure vectors when they are plotted perpendicularly to the local airfoil contour. Yes, this will change with angle of attack. In potential flow theory, the local pressure can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part is constant, the angle-of-attack related part varies linearly with this parameter.
  2. Yes, since the diagram shows the pressure distribution at high angle of attack, the top surface shows mostly suction (= local pressure is smaller than ambient pressure) and all of the bottom surface pressure that is higher than ambient pressure. Static pressure is equivalent to potential energy and total energy is constant, so this means that flow speed is higher than the free flow speed over the red region and lower over the blue region.
  3. Yes, measuring the perpendicular distance between the airfoil surface and the contour line will give you a pressure difference to the ambient pressure, but there is no scale, so all you can do is read relative pressures from that drawing. I recommend to use a different way of plotting pressure, see here for more.
  4. Yes.
  5. P2 has no specific name; here the flow accelerates through the free stream speed. The stagnation point is below the highest distance between airfoil and pressure contour near the nose (in the blue region). That the local pressure at the trailing edge is equal to free stream pressure is a coincidence - at lower angles of attack, the flow will decelerate below the free stream value near the trailing edge, so the blue region would wrap around the trailing edge. Here, flow separation at the trailing edge is imminent, and if the angle of attack is increased a bit, the pressure will turn to suction at the trailing edge.
  6. I think this graph is not quite realistic but is meant to show the general tendency only. More can be seen when inviscid and viscous pressure are plotted together over chord in a cartesian coordinate system.

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