Abstract: According to the theory on the spacecraft attitude kinetics and on the lamella bending of elastics mechanics, the formulary for the element mass-center displacement of the finite element and for the platform structure mass-center displacement are deduced. It is calculated that the mass-center elastics displacement resulted from deforming of the platform of air-bearing testbed acted on gravity and that the unbalance torque resulted from the displacement. The result shows that the unequal rigidity in different direction of the platform structure causes the unbalance torque when the platform acted on gravity. When rotating around horizontal axis, the change of platform’s unbalance torqueis characterizec by sinusoid, and the torque reaches the top when roll angle or pitch angle and 45°.

Key words: Air-bearing testbed, Finite element, Mass-center, Rigidity, Satellite, Unbalance torque