Aerial Delivery dynamic model validation by Flight Test
Aerial delivery is a typical operation of military transport aircraft that consists of payload extraction during flight. The operation starts with the aircraft in steady flight and the ramp open. From this moment on, two phases can be distinguished: the payload units movement backwards inside the fuselage and the payload unit release when it reaches the ramp edge. Airworthiness regulations require the critical loads during this process to be combined with atmospheric turbulence loads. For this purpose, a set of scenarios is considered, covering ramp structure, rear fuselage and ramp-to-fuselage junctions. In particular, dynamic loads analyses envisage two situations that are the most severe in each of the two phases: The turbulence encounter during first phase, critical in the instant of time when the payload unit reaches the ramp edge and the aircraft dynamic response due to the ramp spring- back when the payload unit leaves the ramp. In the classical dynamic loads approach, the aircraft incremental dynamic response is linearly superimposed to the steady “1g” loads obtained from steady flight. In the aerial delivery operation, as the cargo payload moves backwards to the ramp edge, the aircraft flight conditions and the steady loads change. For this reason, the classical “incremental + 1g” approach is no longer valid for aerial delivery analyses and an “incremental + pseudo-1g” approach is required. From the structural point of view, the non-linearities in the ramp-fuselage interfaces are one of the most relevant parameters of this study. For this reason, it has been necessary to use a nonlinear approach, different also to the classical linear analyses used for most of dynamic loads calculations. The loads alleviation due to the non-linear Flight Control System is also included in the analyses. In the last years, Airbus DS Military Transport Aircraft has been involved in its large military transport aircraft A400M certification. In addition to the certification loads calculation, this has implied an exhaustive flight test campaign that started in 2013. This paper is devoted to present the A400M aerial delivery dynamic loads calculation methodology and validation by means of flight test. Some of the aerial delivery dynamic loads are among the A400M sizing critical load cases. Therefore the validation of the aerial delivery loads model is a critical issue in the certification path. The aspects detailed above will be covered in this paper by showing: - Different dynamic loads models used for A400M aerial delivery for their use in the linear and non- linear methodologies. - Pseudo-1g approach for quasi-steady flight. - Dynamic response to the payload unit release. Comparison between linear and nonlinear methodologies will show that this is a local effect and that the nonlinear methodology is only required to obtain loads of some specific ramp components (actuators and z-ties). - Aircraft response to a discrete tuned gust produced during the extraction process. - Aerial delivery dynamic loads model validation by flight test. Concluding remarks will highlight how these results constitute a step forward in the understanding dynamic response of the ramp. The paper will end with suggestions for further work in this topic.