#Airfoil stall free
When operating at and beyond the clean airfoil's stall angle, the free effector automatically deploys to progressively higher angles with increasing angles of attack. A comparison of the fixed-effector results with those from the free-effector tests shows that the free effector's deployment angle is between 30° and 45°. This is thought to be the main mechanism by which an effector improves both lift and drag. Oil flow visualization on the airfoil with and without the fixed-angle effectors proved that the effector causes the separation point to move aft on the airfoil, as compared to the clean airfoil. Drag tests on both the free-moving and fixed effectors showed a marked improvement in drag at high angles of attack. A progressive increase in the stall angle of attack with increasing effector angle was observed, with diminishing returns beyond the effector angle of 60°. To better understand the aerodynamics and to estimate the deployment angle of the free-moving effector, fixed-angle effectors fabricated out of stiff wood were also tested. The maximum lift coefficient with the effector was the same as that for the clean airfoil, but was maintained over an angle-of-attack range from 12° to almost 20°, resulting in a very gentle stall behavior.
The tests were performed in the NCSU subsonic wind tunnel at a chord Reynolds number of 4 × 10(5). The effector, fabricated from a thin Mylar sheet, is allowed to rotate freely about its leading edge. These observations suggests that the mechanism of stall onset can change from 'LSB burst' to trailing edge separation as airfoil thickness is further increased.A flap mounted on the upper surface of an airfoil, called a 'lift-enhancing effector', has been shown in wind tunnel tests to have a similar function to a bird's covert feathers, which rise off the wing's surface in response to separated flows. However, investigation of the reverse flow region on the suction surface shows tremendous differences between the different airfoils, with the thickest airfoil showing a very large reverse flow region. In all cases, dynamic stall onset occurs immediately following the bursting of the laminar separation bubble. Unsteady aerodynamic loads are compared with the corresponding static values. Dynamic simulations are carried out until the angle of attack goes past the lift stall point. A ramp function is used to smoothly increase the pitch rate from zero to the desired value and then held fixed.
#Airfoil stall skin
Good code-to-code agreement is observed for aerodynamic pressure- and skin friction coefficient distributions.
Results of the static simulations are compared with XFOIL predictions as a sanity check. A static simulation is first carried out with each airfoil set at alpha = 4 degrees. A constant-rate pitch-up motion about the airfoil quarter-chord point is used to study dynamic stall. Three symmetric airfoils are studied with thickness-to-chord ratios of 9%, 12%, and 15%. The investigation is performed for three airfoils from the NACA family at Re c = 2 x 10 5. Large eddy simulations are used to investigate the effects of airfoil geometry, particularly thickness, on inception of dynamic stall.