New Flutter Flight Test Identification Method Developed for Large and Flexible Aircraft – MEFAS –

J. ROUBERTIER, AIRBUS-France, Toulouse, France
C. DAUDET, AIRBUS-France, Toulouse, France
F. DESSILLONS, AIRBUS-France, Toulouse, France
A. BUCHARLES, ONERA, Toulouse, France
P. VACHER, ONERA, Toulouse, France

Abstract

The main purpose of flight vibration tests is to open the flight envelope and demonstrate Airworthiness Authorities that the aircraft is really free from flutter and divergent phenomenon. Flight tests results are participating to the aeroelastic demonstration during certification exercise in compliance with JAR 25 chap. 629 requirements; this is achieved with the measure of the frequencies and damping values for all structural modes of interest.

Flight tests results also help to prove that the mathematical model predicts well what will be flight tested later and demonstrate how the model is able to predict the cases not tested. Another really important purpose of the flight vibration tests is to secure the crew and aircraft during the flight domain opening. For that, AIRBUS and ONERA have developed in 1987 a near real time process working with telemetry device, able to get the modal characteristics and particularly the damping values which are the final criterions of safety.

The couples frequency, damping (f, α) were obtained by single transfer functions identification of accelerations measurements versus controlled excitations (external exciters or control surface excitations for frequencies swept up to 20Hz). This procedure needed strong engineering efforts to analyse about twenty transfer functions, proceed afterwards to a modal synthesis and obtain at the end a modal schema for all flight points. This technique reached its limit in the nineties with large four engines A340 aircraft for which quantity of modes where gathered in a small frequency band.

With similar modal deformations and close each others, this amount of modes combined with computers improvements encouraged the test engineers to increase firstly the number of measurements and, in a second stage to develop new identification method able to distinguish and separate close modes.

AIRBUS and ONERA-DCSD in 1999 started again a co-operation research program (MEFAS) for on-line modal analysis improvements with A340-600 aircraft as main industrial target. After masses of developments on pre- processing and identification techniques, it appeared that multi-transfer identification could be the base of further developments. This approach might work in frequency and also time domain, it gives the opportunity to take into account small responses somewhere on the aircraft and then enriching modal results, it appeared helpful and more or less simple for mode separation and recognition.

Another real advantage of such a method is to suppress the long and rather difficult task of modal synthesis. Results are then more consistent with a better reproducibility since human subjective judgement is not predominant any more.