Experiments on airfoils at low reynolds numbers

Lift and drag measurements were taken on 34 airfoils at low Reynolds numbers in an attempt to develop a consistent database for use in design studies that require accurate low Reynolds number airfoil data. Prom these data emerged several interesting results related to the behavior of the laminar separation bubbles and their effect on the lift characteristics. A plateau in the lift curve of symmetrical airfoils in the vicinity of an angle of attack of 0 deg was found to be common in the Reynolds number range of 40,000 to 100,000. Through the use of zig-zag type boundary-layer trips, this nonlinearity can be reduced owing to a reduction in the size of the laminar separation bubbles. The influence of laminar separation bubbles was also found to dominate the performance of several high-lift airfoils in the Reynolds number range of 80,000 to 150,000. In particular, hysteresis loops in the lift curve were present, and these are related to the size of the laminar separation bubble as deduced from drag data. The data reveals that some airfoils exhibit both counterclockwise and clockwise hysteresis loops for a given Reynolds number. Moreover, depending on the airfoil, either type of loop can occur first.