It is well known that the wake characteristics of a flapping foil are the function of the Strouhal number (St). The transition from Bénard-von Kármán (BvK) wake to reverse BvK (rBvK) vortex street indicates drag to thrust transition which takes place beyond St \(\sim \) 0.18 (Schnipper et al. in J Fluid Mech 633:411–423 [1]). We presume that for such flapping flexible appendages (which we model with foil), the Strouhal number would not be the sole criterion for the transition, and the flexural properties would also play a role. To identify the additional parameter governing this transition, if there exists any, we conduct experiments on a flapping rigid foil with flexible filament using the newly built soap film tunnel. The two-dimensional flow is studied using high-speed imaging, and flow visualizations (to understand the wake patterns). We identified that the Strouhal number for transition is lower in the case of flexible foil than that of rigid foil. Moreover, we found that the Strouhal number where transition occurs, reduces with an increase in the flexural rigidity of the filament.