Rotorcraft simulation environments such as training simulators or engineering simulations require high-fidelity models to be suitable for tasks such as pilot training, handling qualities analysis, and flight control system development. Deficits in model fidelity can arise from inaccurate data and unmodeled higher-order dynamic effects. Mitigating these deficits through physics-based modeling usually requires high effort. Therefore, a method is presented that improves a baseline simulation model by a “black-box” (i.e. non-physical) low-order input filter. The baseline can be a linear model or a nonlinear simulation model. Several options for deriving the input filter are elaborated and demonstrated in this paper using data from the CH-47, AH 135, and Bell 412 helicopters. In all cases the simulation fidelity was improved by approximately a factor of two.
This paper presents the validation of a Level-D rotorcraft simulation framework by comparing different quantitative model fidelity metrics. The simulation framework uses a non-linear blade element rotor model to simulate the flight dynamics of rotorcrafts with accurate stability and control characteristics identified from system identification techniques. For this study, frequency-domain system identification is used to generate linear state-space 6-DoF quasi-steady models and an automated process is used to adjust the non-linear simulation framework. Regulatory authorities assess performance models for Level-D simulators by comparing time-domain simulation responses with measured aircraft responses for the same set of control inputs. Over the years, new quantitative fidelity metrics were proposed, like the Maximum Unnoticeable Added Dynamics and the Allowable Error Envelopes. This paper will focus on the blade element rotor model hover modeling process to demonstrate its application using a Bell 412 flight test data package. It will also show how this modeling approach can be used to match both Level-D requirements and the alternative fidelity metrics stated above.
The Applied Vehicle Technology (AVT) Panel of the NATO Science and Technology Organization has recently engaged a Research Task Group on the topic of rotorcraft flight simulation model fidelity. This group aimed to explore a comprehensive set of methods for flight mechanics simulation fidelity enhancement, including training simulation applications. Particular effort was also directed to the metrics used for simulation fidelity model assessment as suitable for the final intent of the model. The work presented in this paper was carried out in the framework of this Research Task Group, AVT-296, which examined seven different approaches; our paper focusses on just one of these. The objective was to assess flight-model renovation methods through four different applications. As a common approach, flight data from various helicopters were used to extract a set of flight dynamics information (state and control derivatives) that were used to compute corrective force and moment terms. The approach consists of a comparison between flight-test and flight mechanics model derivatives to compute delta forces and moments. These delta terms are added to the forces and moments through a linear combination and to generate the additional accelerations needed to capture any lacking dynamics. The derivatives that require updating need to be identified and, of course, will depend on the nature of the modelling deficiency. This paper shows how this method is applied to enhancing the lateral-directional oscillatory characteristics of flight models and how they can be upgraded to achieve higher fidelity for design and development applications but with special attention to meeting the fidelity requirements for flight training simulators.
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