Experimental study of the natural frequencies of rotating thin-walled composite blades

The paper presents an experimental method for detecting the natural frequencies of helicopter-like thin-walled rotating composite blades, and a study of their dependency in the lamination angles and in the rotor angular velocity. The tests were carried out in a vacuum chamber which allowed the isolation of the structural dynamic effects. Both symmetric and antisymmetric box-beam specimens were tested. The blades were excited by an impulse force during rotation and the time history of the resulting strain distributions were recorded. Natural frequencies have been obtained by numerical analysis of the time response. The results demonstrate high repeatability and clear trends of the natural frequencies variation as a function of the blade lamination modes and their angular velocity.