Flight flutter testing and aeroelastic stability of aircraft Altan Kayran Department of Aerospace Engineering, Middle East Technical University, Ankara, Turkey Abstract Purpose – This paper sets out to provide a general review of the flight flutter test techniques utilized in aeroelastic stability flight testing of aircraft, and to highlight the key items involved in flight flutter testing of aircraft, by emphasizing all the main information processed during the flutter stability verification based on flight test data. Design/methodology/approach – Flight flutter test requirements are first reviewed by referencing the relevant civil and military specifications. Excitation systems utilized in flight flutter testing are overviewed by stating the relative advantages and disadvantages of each technique. Flight test procedures followed in a typical flutter flight testing are described for different air speed regimes. Modal estimation methods, both in frequency and time domain, used in flutter prediction are surveyed. Most common flight flutter prediction methods are reviewed. Finally, key considerations for successful flight flutter testing are noted by referencing the related literature. Findings – Online, real time monitoring of flutter stability during flight testing is very crucial, if the flutter character is not known a priori. Techniques such as modal filtering can be used to uncouple response measurements to produce simplified single degree of freedom responses, which could then be analyzed with less-sophisticated algorithms that are more able to run in real time. Frequency domain subspace identification methods combined with time-frequency multiscale wavelet techniques are considered as the most promising modal estimation algorithms to be used in flight flutter testing. Practical implications – This study gives concise but relevant information on the flight flutter stability verification of aircraft to practising engineers. The three important steps used in flight flutter testing – structural excitation, structural response measurement, and stability prediction – are introduced by presenting different techniques for each of the three important steps. Emphasis has been given to the practical advantages and disadvantages of each technique. Originality/value – This paper offers a brief practical guide to all key items involved in flight flutter stability verification of aircraft. Keywords Aerodynamics, Flight dynamics, Aircraft Paper type General review Introduction Aeroelastic flutter is the complex interaction of aerodynamic, elastic, and inertia forces producing an unstable, usually divergent oscillation of the aircraft structure or the component (Fung, 1993). Airworthiness regulations require that stability within the flight envelope of the aircraft be demonstrated by flight flutter tests (MIL-A-8870C, 1993). In spite of the improvements that have taken place in the flutter test techniques, instrumentation, and response data analysis, flutter testing is still a risky test for several reasons. One reason for the risk associated with flutter testing is that sub- critical damping trends cannot be accurately extrapolated to predict stability at higher airspeeds. In addition, aeroelastic stability may change suddenly from a stable condition to one that is unstable with only few differences in airspeed. Thus, one must fly as close as possible to actual flutter speeds to detect the instabilities accurately. Therefore, a careful expansion of the flight envelope is required to avoid possible hazardous situations. For this purpose, a large number of flight tests are usually performed with careful increases in flight test speeds. During the flight flutter tests, aircraft is flied at a range of sub- critical speeds and some form of excitation is applied to the aircraft. The response of the structure is measured at a number of locations on the aircraft, and the data obtained is used to determine the stability at the current test point. Predictions are then made whether it is safe to proceed to the next test point. This process is repeated at various flight conditions and aircraft weight, and CG configurations until the flight envelope is cleared. Common excitation means used in practice include control surface pulses, oscillating control surfaces, thrusters, inertia exciters, aerodynamic vanes, and random atmospheric turbulence (Kehoe, 1995; Brenner et al., 1997; Meijer, 1995; Nunen and Piazzoli, 1979). It is very important that the excitation system must provide adequate excitation over the desired frequency range, and must provide adequate force levels to ensure accurate determination of stability parameter. In practice, the most common response data analysis procedure is to track the variation of the modal damping of the aircraft structure with the air speed. Some examples of other flutter prediction methods include flutter margin (Zimmerman and Weissenburger, 1964), envelope function (Cooper et al., 1993), Nissim and Gilyard (Nissim and Gilyard, 1989; Nissim, 1993), eigenvector orientations (Pidaparti et al., 2001). Cooper (1995) gives an overview of the several modal estimation algorithms, both in time and frequency domain for extracting stability estimates and detecting time-varying and non-linear dynamics. In the present paper, the effect of different excitation systems, modal parameter estimation techniques on the accuracy of the processed flight test data, and other special considerations to achieve a successful flight flutter test are highlighted. The current issue and full text archive of this journal is available at www.emeraldinsight.com/1748-8842.htm Aircraft Engineering and Aerospace Technology: An International Journal 79/5 (2007) 494–506 q Emerald Group Publishing Limited [ISSN 1748-8842] [DOI 10.1108/00022660710780623] 494