Insects that use a Weis-Fogh clap and fling mechanism, where their wings clap together and fling apart, show an increase in thrust per unit muscle mass compared to conventional flapping insects. This has motivated the development of macroscale clapping winged ornithopters with four wings. Most clapping wing ornithopters use electric motors with gears and linkages that are inefficient at the sub-millimeter (meso)scale. Piezoelectric actuators are attractive for Nano Air Vehicles (NAVs) because they have high power density, high efficiency, and new fabrication processes have been developed at this scale. Recently developed piezoelectric T-beam actuators are monolithically fabricated from bulk PZT and function like unimorph actuators without the need to bond passive layers. These bending actuators drive a novel four-winged clapping NAV that produces thrust. This paper studies thrust force generation of a clapping wing NAV using a model-based approach. A three degree of freedom dynamic model of the clapping wing nano air vehicle is derived including unsteady aerodynamic forces and torques. The model is validated using experimental data from a NAV prototype.