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The determination of dynamic forces acting on a Flapping Wing Micro Aerial Vehicle (FWMAV) is a challenging task due to the unsteady nature of force generation mechanisms. To assure a proper force identification in future researches, this work compares two different methods to obtain the longitudinal forces acting on FWMAVs and discusses their applicability regions. The methods were 1) calculation of forces from the recordings of the FWMAV's position in a free flight condition; 2) direct force measurements in a tethered flight condition in a wind tunnel. The DelFly II is used as the FWMAV test platform in the measurements. During free flight experiments, its position and attitude were recorded at a rate of 200Hz using an external visual tracking system, whose acquired information was then analyzed to obtain the flight states and calculate the forces and moments that act on the platform during flight, under a set of kinematic assumptions. Subsequently, similar flight conditions were tested in the tethered situation. An ATI Nano-17 Titanium force transducer was used to measure time-resolved forces. The results for the most common flight regime of the DelFly, which is a slow forward flight at a high body pitch angle, are presented. It is shown that the tethered force balance tests agree with the free flight data when assessing the aerodynamic forces that are perpendicular to the stroke plane of the flapping wing. However, the forces that act along the stroke plane are coupled with structural dynamic terms, thus affecting the final lift and thrust identification. These results point to inadequate force identification in fixed point force measurements, due to effect the of the dynamic modes of the FWMAV body, thus advising proper cross-comparing between experimental methods.