Ground Vibration Tests and Flutter Analysis

For aircraft certification, Leichtwerk AG offers ground vibration tests and aeroelastic analyses for assessing flutter hazards. A record of substantiation to this effect is required for many aircraft categories (according to Section 629 of the certification specifications) and must be performed before the maiden flight.

 

The tests can be carried out locally at the site of the aircraft manufacturer, worldwide. In the USA, Leichtwerk AG is represented by Sonja Englert (www.caro-engineering.com).

The test facilities in use today originate from Professor Dr. Norbert Niedbal, with some significant elements having been modernized by Leichtwerk AG. Professor Niedbal has tested and successfully evaluated over 400 aircraft with this technology over the past 25 years.

Flutter occurs as a result of the interaction between aircraft vibration modes and the subsequent transient aerodynamic forces. Additionally, particularly complex situations can arise from the aircraft's control system (stiffness, play).

In a ground vibration test all the relevant elastic vibration modes of the aircraft are determined experimentally. This involves vibrating the aircraft structure with the use of electromechanical exciters. The reaction of the aircraft is then established via a number of acceleratometers distributed on the aircraft. 

 

Besides knowing the natural frequency of a vibration, it is particularly important to determine the related local amplitudes of vibration modes, as this is what creates the aerodynamic forces which amplifies the vibration in the air.

For this purpose Leichtwerk AG applies a special experimental technique - called phase resonance technique - which has been developed and fine-tuned over decades by the DLR in Göttingen for use with aircraft structures. 

During the ground vibration test, every single vibration mode of the aircraft is systematically isolated, optimally excited and measured. The results reflect the actual features of the aircraft and are not derived from a numerical simulation.

The subsequent flutter analysis simulates the transient aerodynamic forces and calculates the damping of the individual vibration modes via aircraft speed. In particular, this technology enables the assessment of life cycle effects. What happens, for example, if the elastic features change slightly during the aircraft's life cycle, or if the rudder mass is changed after repainting work? It is not sufficient to merely assess the current status of a prototype aircraft.

Finally, the flutter properties have to be assessed in a test flight devised on the basis of the flutter analysis.

At Leichtwerk AG it is also state of the art to perform a numerical simulation of the vibration modes for the purpose of flutter analyses during the development phase of an aircraft, so as to implement potential improvements in the early design stages.