Progress on an International Collaborative Program on Improved Procedures for the Analysis of Flight Flutter Test Data

In this paper, a TTCP Collaborative Program focussed on the development of improved procedures for flight flutter testing will be described. The Defence organisations of each of the participating nations have programs wherein new underwing stores are cleared on military aircraft typically by a combination of analysis and flight test. Flight flutter testing involves operating the aircraft in regimes which may be hazardous, whilst minimising the risk by prior analyses and by extrapolating sub-critical data to predict the safety of the aircraft at the next speed increment. Such risk minimisation usually implies the penalty of lengthy test programs and can sometimes still not avoid flutter. The collaborative activities that will be described in this paper involve the development of procedures that result in greater confidence in the prediction methods. These may then permit the use of larger speed increments during flight test with a consequent reduction in the total test matrix, without compromising safety. The focus of this collaborative program has been on the development of tools for better predicting flutter boundaries when the system is behaving linearly as well as furthering the state-of-the-art in the identification of system non-linearities. For linear processes, developments will be described that concentrate on better incorporation of uncertainties in the prediction of flutter boundaries. With regards to non-linear systems, aircraft that exhibit limit cycle oscillation (LCO) are of particular concern and system identification techniques for the early identification of non-linearities that may lead to LCO or other non-linear behaviours have been examined and further developed here.