We experimentally investigate, through direct force and torque measurements, the effect of length of a flexible flap attached to the trailing edge of a rigid airfoil (NACA0015), pitching in a quiescent fluid ambient, on thrust generation. We aim to identify the existence of an optimum length, for different pitching parameters, for a hover-like situation. To this end, we conduct force and torque measurements on a purely pitching flexible airfoil consisting of a 100 \(\mu \) m polyethylene terephthalate flap attached to the trailing edge of rigid airfoil. The length of the flap is varied from \(c_{_F}/c_{_R}\) = 0 (no flexible flap) to 4, where \(c_{_R}\) is the rigid foil chord, in increments of 1. Additionally, one case without any flap is also studied. The results suggest that attaching a flexible flap increases the thrust coefficient over that for the foil without flap. A selection criteria is defined for choosing the optimum flap length for a given set of pitching parameters. This criteria is the maximization of the ratio of time averaged thrust coefficient to the magnitude of the lift coefficient ( \(\overline{C_T}/[\overline{C_L}]\) ). We found that the optimum flap length depends on the pitching parameters.