Effects of pivot point and spacing on the aerodynamic characteristics of tilting biplane airfoils

The effects of pivot locations (xp/c) and spacing (ds=λc) between the biplane airfoils was studied using the numerical method. SST k-ω turbulence model and U-RANS equations were solved when adopting overset grids. The findings revealed the aerodynamic characteristics of tilting biplane airfoil existed phase lag under five fixed pivot points from LE to TE but if the wall effect in the gap was so strong, the hysteresis of CL,lower significantly collapse. Besides, CM curves drop as pivot moves upward comparing to coincident relative and increasing λ could improve total lift coefficients. These results could be explained from the view of vorticity evolution where LEV, SV and TEV showed diverse structures to induce dynamic stall. Additionally, with rearward movement of the pivot point, the emergence of identical flow structures was delayed, and thus aerodynamic characteristics had the high similarity under different xp/c and λ. Therefore, a concept of effective angle of attack of biplane airfoils was first proposed and its function introduced a new factor λ in order to study the backward shift of pivot point in the centerline.