Assessment of advanced flutter flight test techniques and flutter boundary prediction methods

The paper presents an experimental study, conducted at the Israeli Air Force, aimed at the assessment of several advanced flutter flight test techniques that are designed to improve the efficiency and accuracy of flutter tests. These include the auto-regresive moving-avarage (ARMA) and operational modal analysis (OMA) system identification methods, the use of system stability and flutter margin parameters, and the use of atmospheric turbulence versus prescribed flaperon motion as the source of structural response excitation. The methods were evaluated based on data from a dedicated transonic flight test of the F-16 platform. All of the tested methods were able to accurately predict the instability (LCO) onset. The OMA method has the advantage that it does not require a priori knowledge of the instability mechanism (participating modes and approximate flutter frequency). However, it requires delicate identification of the participating modes, thus making it less suitable for real time processing. The ARMA method, on the other hand, is easier to implement, and straight forward to use in real time. Both methods require relatively large data records, of about 60 seconds at each test point. Excitation by atmospheric turbulence was found to be adequate in the current test case, in which the modes are lightly damped, and highly suitable for system identification methods that rely on stochastic inputs. While further assessment is required for cases of highly damped modes and of flight in low turbulence levels, the use of ARMA and OMA methods based on responses to atmospheric turbulence appears to offer an accurate and cost effective flutter testing methodology.