The term hysteresis, as applied to airfoil aerodynamics, means the difference in C;, Cdy or Cmc/i at a given angle of attack when this angle of attack is approached from a higher and then a lower value. This behavior may be seen in the in the Ci vs a plot. See for example Fig. 12.2. In cases with hysteresis, as the angle of attack is increased from zero to stall, the Cj will usually reach a maximum value and then drop off at some particular a. However, when a is decreased, the C will not retrace its original curve; rather it will stay at the “stall” Ci until a is somewhat below the previous stall value, and then suddenly jump up to rejoin the original Ci vs a curve. Hysteresis in the aerodynamic coefficients with both Reynolds number and angle of attack is common to many of the airfoils tested.
Invariably, hysteresis is a sign of a large, laminar separation which in turn yields high bubble drag. Since this effort concentrated on those airfoils with low bubble drag, the detailed effects of hysteresis were not closely examined. In general, airfoils with hysteresis in the middle of the I2n-envelope of the aircraft should be avoided.
44 Airfoils at Low Speeds