The Effect of blade torsional elasticity on helicopter flight dynamics

Akel, Ezgi
Helicopter design and test stages require more complex and high fidelity flight dynamic models to aid the design process of the helicopters. In the common approach, rigid rotor blades models are used in the flight dynamics models and since this is a fast modeling method. However, increased flexibility of the rotor blades necessitates the use of elastic blade models to have more accurate prediction of the rotor loads and the control angles. Elasticity brings extra fidelity to the flight dynamics model which is extensively used in design stage of a helicopter. In this study, an in house helicopter flight dynamics code (TOROS) which is in Matlab/Simulink Environment and developed by TUSAS Helicopter department is used as the analysis tool. This tool includes a trim model and a six degree of freedom helicopter model with blades modeled rigidly modeled in flap, lag and torsional direction. The mathematical model of a helicopter generated in in house tool is first validated with the commercial FLIGHTLAB model. In the next stage, elastic blade model is integrated to TOROS. In this study, in the elastic blade model flexibility is added only to the torsional degree of freedom. The added torsional flexibility of the blade accounts for change in the aerodynamic and inertial forces with respect to rigid blade model, resulting in more accurate prediction of the control angles. Tip deflections of the torsionally elastic blade is compared with the results of the finite element based multibody dynamics code Dymore. A finite element modelling capability is added to the in house flight dynamics model to perform trim in level flight, autorotation and transient analyses utilizing both rigid and torsionally elastic blade models. Static longitudinal stability analyses of the helicopter are conducted with rigid and elastic blade models to investigate the effect of elastic blade in the torsional direction on the flight mechanics trim analyses and stability. The result of the study is elastic blades in torsional direction has a direct effect on pitch angle of the blades and hence on collective control. The static stability of the system is also affected in a negative way when the blades are elastic in torsion. Elastic effects should be considered during design process.
Citation Formats
E. Akel, “The Effect of blade torsional elasticity on helicopter flight dynamics,” M.S. - Master of Science, Middle East Technical University, 2017.