I have this question in my mind. I have been trying to look for this in different books about aerodynamics and I couldn't find the answer. I know I could check the answer experimentally, if I had a wind tunnel or some advanced software, but I don't have access to any of them.

Let's say we have this plot (fig. 1) obtained experimentally by testing an airfoil at a low speed (subsonic) with a wind tunnel. If I were to change the wind speed of the wind tunnel, would the graph change or would it remain the same?

For instance, if the wind velocity was initially 15 m/s and then 25 m/s, would it change or is the CL-AoA relationship constant for a given airfoil?

I just need to understand this, I don't need any example with numbers (I just give the numbers to have an order of magnitude of the velocities of the testing. And assume the airfoil has a regular shape, for instance a NACA0012, which is pretty common for academic examples.

CL vs. AoA (Source: https://en.wikipedia.org/wiki/Lift_coefficient#/media/File:Lift_curve.svg)

  • $\begingroup$ Any wind will change the change the velocity and the angle of attack. Always remember that the angle of attack is the angle of the aerofoil to the relative airflow, not the pitch angle. I'm not sure that I understand your question. $\endgroup$ – Simon Nov 26 '15 at 11:55
  • $\begingroup$ I was meaning that if we somehow fix the angle of attack and the wind speed increases, what happens with the lift coefficient? Now that you told about the pitch angle maybe it doesn't have sense what I asked, but I'm still not sure. @Simon $\endgroup$ – Airman01 Nov 26 '15 at 11:57
  • 2
    $\begingroup$ @Airman01 Lift increases as the airspeed (and therefore dynamic pressure) increases, but the Coefficient remains the same. The graph is a constant, as the Coefficient is there to quantify all of the complex properties of the wing that can't be mathematically quantified $\endgroup$ – Dan Nov 26 '15 at 12:45

Yes, it does vary slightly due to viscous effects.

In inviscid flow, the flow speed would not affect the lift coefficient - angle of attack relation. However, increasing the flow speed will result in a thinner boundary layer and a slightly different shape of the airfoil - boundary layer combination as "seen" by the outer flow. This influence is captured by the Reynolds number. See below for a plot of the venerable NACA 4412 from Abbott and Doenhoffs collection of airfoil data (picture source):

NACA 4412 lift curve and drag polar

Note that the lift coefficient is plotted for Reynolds numbers R of 3, 6 and 9 million.


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