Bio-inspired flying robot (BIFR) which flies by flapping their wings experience continuously oscillating aerodynamic forces. These oscillations in the driving force cause vibrations in the motion of the body around the mean trajectory. That is, a BIFR hovering in place is not exactly stationary in space, rather it is oscillating in almost all directions about the hovering point in space. These oscillations affect the aerodynamic performance of the flier. Assessing the effect of these oscillations on particularly thrust generation in two-wings and four-wings BIFRs is the main objective of this work. To achieve such a goal two experimental setups were considered to measure the average thrust for the two BIFRs. The average thrust is measured over the flapping cycle of the BIFR. In the first experimental setup, the BIFR is installed at the end of a pendulum rod, in place of the pendulum mass. While flapping, the model creates a thrust force which raises the model along the circular trajectory of the pendulum mass to a certain angular position, which is an equilibrium point and it is also stable. Measuring the weight of the BIFR and the equilibrium angle it obtains, it is straightforward to estimate the average thrust by moment balance about the pendulum hinge. This pendulum setup allows the BIFR model to freely oscillate back and forth along the circular trajectory about the equilibrium position. As such, the estimated average thrust includes the effects of these self-induced vibrations. In contrast, we use another setup that relies on a load cell to measure thrust where the model is completely fixed. The thrust measurement of both the BIFRs using the above mentioned setups exhibited an interesting trend. The load cell or the fixed test lead to a higher thrust than the pendulum or the oscillatory test for the two-wings model, showing an opposite behavior for the four-wings model. That is self induced vibration have different effects on the two BIFR models. Note that the aerodynamic mechanisms of thrust generation for two-wings and four-wings are fundamentally different. A two-wings BIFR generates thrust through traditional flapping mechanisms whereas a four-wings model enjoys clapping effect and wing-wing interaction. In the present work, we use motion capture system, aerodynamic modeling and flow visualization to study the underlying physics of the observed different behaviors of the two flapping models.