During this summer, the L-39NG is undergoing certification tests in the Czech Aerospace Research Centre (VZLU). In addition to the strength tests, which we have already informed about, VZLU specialists are also performing ground vibration tests directly at the aircraft´s manufacturer site - in the hangers of Aero Vodochody Aerospace.
During ground vibration tests, the frequencies and vibration modes of the airframe and aircraft control system are being examined. Data from these measurements are then used to set the computational aeroelastic model of the airplane. Correct tuning of the aircraft aeroelastic model and calculation of critical flutter speeds must be performed before the flutter flight tests begin. Flutter is an undesired aeroelastic phenomenon, which may occur based on external or internal forces (flying through gust of wind, droping or launching of weapons, rapid force changes in the airplane control system) and may cause an oscillation of the structure, which can, in the worst case, lead to the destruction of the airplane during the flight. Some examples of flutter can be seen here.
During the vibration test, the aircraft structure is being excited at a number of points (wings, tail, fuselage, weapon pylons, etc.) by electrodynamic vibrators at different amplitudes and frequencies, and the response is measured by accelerometers (in this case more than 200 pieces) installed on the airplane. VZLU uses for vibration tests the French PRODERA mobile station, which allows to perform tests directly at manufacturer´s site.
The reportage of the Czech TV from the testing of the L-39NG can be seen here.