With the growing interest in faster helicopters, commercial unmanned vertical lift vehicles, and vertical-axis-wind-turbines, it becomes critical to understand the unsteady loading on the rotor blade so that more efficient designs can be produced. The classic unsteady airfoil theories are tools useful to predict loads on two-dimensional rotor blade sections for moderate surging, pitching, and plunging motions. Specifically, Isaacs' surging airfoil theory assumes a two-dimensional thin flat plate at small angles of attack under a sinusoidally-oscillating freestream in an incompressible inviscid flow. The unsteady lift coefficient is then solved by the potential flow theory with the Kutta condition at the trailing-edge. However, Isaacs' surging airfoil theory has not been fully validated experimentally. As a result, this work is intended to investigate this surging airfoil theory experimentally and evaluate the key parameters in such unsteady motion, the flow physics, and the validity of the various assumptions. Firstly, the wind tunnel at the Aerospace Research Center, the Ohio State University, is modified to generate a sinusoidally-oscillating freestream in the test section with the design of rotating elliptical choke-vanes downstream. The modified unsteady wind tunnel is characterized to understand its unsteady response for a surging airfoil experiment, with the help of various intrusive measurements and analytical and numerical tools. Secondly, a NACA 0018 and a NACA 63-015A airfoil were tested at moderate surging flow conditions in terms of Reynolds number, Mach number, velocity amplitude, and reduced frequency. Surface pressure measurements of the lift frequency response indicated a significant disagreement with Isaacs' theory in terms of the lift overshoot and undershoot, where optical measurements revealed signs of an oscillatory trailing-edge stagnation condition for the surging airfoil which violates the classic Kutta condition in the unsteady potential flow theory. Finally, a hypothesis was formulated about the momentum balance of shear layers at the trailing-edge, which is intended to develop a viscous and unsteady correction to the classic Kutta condition and connect the unsteady stagnation condition with the resulting lift frequency response. A near-wake velocity survey was conducted on a NACA 63-015A airfoil with various flow transition behaviors. Results supported the hypothesis that a dynamic shear layer momentum balance exists near the trailing-edge and its dynamics can be correlated to the measured lift undershoot and overshoot. Overall, the experimental study in this work offers additional insight into the unsteady trailing-edge stagnation condition, and highlights the need to modify the Kutta condition with unsteady and viscous effects for a surging airfoil.