Vertical and horizontal force of a flapping-wing micro air vehicle (MAV) has been measured in slow-speed forward flight using a force balance. Detailed information on kinematics was used to estimate forces using a blade-element analysis. Input variables for this analysis are lift and drag coefficients. These coefficients are usually derived from steady-state measurements of a wing in translational flow. Previous studies on insect flight have shown that this method underestimates forces in flapping flight, mainly because it cannot account for additional lift created by unsteady phenomena. We therefore derived lift and drag coefficients using a concept for delta-wings with stably attached leading-edge vortices. Resulting lift coefficients appeared to be a factor of 2.5 higher than steady-flow coefficients, and match the results from previous (numerical) studies on instantaneous lift coefficients in flapping flight. The present study confirms that a blade-element analysis using force coefficients derived from steady-state wind tunnel measurements underestimates vertical force by a factor of approximately two. The equivalent analysis, using "vortex-lift" enhanced coefficients from a delta-wing analogue, yields very good agreement with force balance measurements, and hence seems to be a good approximation for lift-enhancing flow phenomena when modelling flapping flight.