When airspeed is sufficiently fast, the compressibility of the air can alter the performance characteristics of an airfoil. This effect may be evaluated by selecting the "compressibility" option. AeroFoil uses the Karman-Tsien correction. This will change the lift coefficient, Pitch Coefficient, and drag coefficient to a small degree as the velocity increases. This correction is only valid for sub-sonic airflow. A check is made at every angle of attack to ensure that the local surface velocity has not exceeded 1.0 Mach. If this should occur, a warning message is displayed and the Karman-Tsien correction is not applied. This warning will be received most often in high angles of attack with the free stream velocity greater than approximately 0.5 Mach. If received, the warning will be shown on the tabular data. In the various graphical displays, it will show as a discontinuity at and above the point at which the critical Mach number was reached.
If this occurs, either reduce the Mach number or decrease the range of angle of attack.