# Why does centre of pressure move forward with increasing angle of attack in this airfoil?

As a high school student, I am trying to learn more about airfoils and the theory behind it.

I started off with analyzing the NACA0012 airfoil on Autodesk CFD. I modified the lower surface as shown in the image below (the modified airfoil simulation at Angle of Attack=9 degrees).

I simulated the NACA0012 airfoil and the modified airfoil at Reynolds number 1 million (with inlet velocity of 48.655 m/s, chord length of 0.304 8m (1 foot), air density=1.20473 kg/m3, coeff of viscosity= 1.817×10-5 Pa). My simulations were incompressible flow simulations. I know that the NACA 0012 airfoil has its center of pressure at quarter chord point at low angles of attack by the thin airfoil theory.

In the case of my modified airfoil, I find that the centre of pressure moves from 0.596 chord length to 0.259 chord length as I vary the angle of attack from 0 to 10. I have attached an image of the center of pressure location below. The baseline airfoil is the NACA0012 airfoil and I have measured location of center of pressure from the trailing edge. I also read that beyond the viscous fluid layer, near a solid, the fluid behaves like a frictionless fluid. Based on this theory, I assume that at low angles of attack, the fluid slows down and pressure increases by Bernoulli's principle beyond the viscous layer as shown in the image below. I think this idea reasonably explains why the centre of pressure is closer to the trailing edge:the high pressure fluid exerts more force near the trailing edge as compared to the NACA0012 airfoil, which moves the center of pressure toward the trailing edge. The NACA 0012 airfoil at AOA 0 for comparison.

Based on the viscous boundary layer theory or any other theory that I am unaware of, is it possible to explain the reason for the centre of pressure moving toward the quarter chord point at higher angles of attack? I am thinking that the viscous layer is thicker at higher angles of attack and hence the freestream fluid velocity does not decrease as much as it does at angle of attack zero. I am pretty sure this is incorrect and there is a lot more going on. Would someone mind explaining what other factors are at play or what other topics I need to know to answer this question?

Appreciate any help.

• How do you calculate the centre of pressure - using paper and a pen? Then think about what changes to make it move. Jan 1 at 9:45
• You would probably get a more thorough answer on the Aviation SE site. Jan 1 at 16:46 