A multi-disciplinary design approach for conceptual sizing of advanced rotor blades

Abstract

Rotorcrafts are versatile vehicles with their unique hovering flight capability. However, their forward flight speed limitations and high noise levels are shortened to their usage in much wider areas. Therefore, the rotorcraft industry working on advanced rotorcraft, which are called compound rotorcrafts, development projects increasingly to overcome these problems. The conceptual design phase is the beginning of a development project where the most critical decisions are taken in this stage. So, vehicle-level optimization algorithms are needed for decision-making to lead the project correctly. On the other hand, simplified low-level approaches must be used during conceptual design optimization because of too many design parameters to avoid impractical solution times. Furthermore, rotorcrafts with advanced rotors require advanced design approaches to obtain superior performance, structural, and noise-level characteristics. Therefore, advanced conceptual design approaches are needed to overcome this contradiction. The rotor is the most critical component, which is also the source of the most problems of a rotorcraft such as lack of performance and noise. Therefore, rotor blade optimization is the main issue in the conceptual design phase at the beginning of a project. A multidisciplinary rigid rotor blade design optimization approach that is suitable for the conceptual design, sizing, and evaluation stages of helicopter development processes is suggested. Performance, structural strength of the blade, and noise-level predictions are considered for the objective function. Blade outer surface and structure are represented by a geometrical model in which the chord, thickness ratio, chamber ratio, and twist distributions along the blade radial stations can be defined as linear or nonlinear functions. The distribution of the number of layers for both skin and spar was also defined in the presented model parametrically. Low-level but sufficient fidelity analysis methods were chosen to be able to reduce the computing time. Performance analysis and sizing of the vehicle were obtained by Blade Element Momentum Theory (BEMT) based in-house developed helicopter sizing code called ROTAP. A trim algorithm for compound helicopters that may have additional lifting surfaces and thrust components is suggested. Airfoil Characteristics are calculated by the well-known panel method code Xfoil. Both these codes are modified and embedded in the code developed for this study. Structural analysis was obtained using the 1D FEM approach. Cross-sectional properties of the composite beam are calculated by VABS and displacements under the loads are calculated by GEBT. Reduced FfowcsWilliams-Hawkings equations are used to estimate loading, thickness, and high-speed impulsive noise levels. A hybrid optimization algorithm is suggested to get optimal results. Sequential Quadratic Programming (SQP) can be used to find local optimal points. And then the global optimal point is searched by RSM over local optimal points iteratively. RSM-based surrogate modeling, evaluation, and optimization tool was also developed for manual inspection of the design space. As a case study, multi-objective aerodynamic performance optimization of aircraft propeller is performed.