Chengjian He

Optimization of rotor blades for combined structural, dynamic, and aerodynamic properties

A helicopter is intrinsically interdisciplinary due to the close coupling among aerodynamics, dynamics, and the blade structural details. Therefore a design optimization with proper interactions among appropriate disciplines (such as structure, dynamics, and aerodynamics) can offer significant benefit to improve rotor performance. This paper studies the integration of structure, dynamics, and aerodynamics in design optimization of helicopter rotor blades. The optimization is performed to minimize the rotor power required and to satisfy design requirements from structure (minimum blade weight and safe stress margin and fatigue life) and dynamics (proper placement of blade natural frequencies and free of flutter). An effort is made to formulate an effective strategy for combining these various requirements in the optimization process. The paper also presents a way for an intelligent phasing of this interdisciplinary optimization to overcome the hurdles due to conflicting demands on the design variables which arise from different disciplines.

Computational Modeling of Variable-Droop Leading Edge in Forward Flight

In recent years, there has been significant interest in on-blade concepts that expand the operating envelope of helicopters without compromising the performance characteristics of the baseline rotor. The variable-droop leading-edge concept is explored in a modified version of the Navier-Stokes solver OVERFLOW. Modifications were made to the solver to allow deforming-grid capability. This concept was explored in dynamic stall tests of the VR-12 and SC1095 helicopter airfoils. The variable-droop leading-edge airfoils have dramatically reduced drag and moment rises associated with dynamic stall. Using the results of these tests, a modified UH-60A rotor incorporating variable-droop leading-edge airfoils was analyzed using loosely coupled computational fluid dynamics and comprehensive structural dynamics with OVERFLOW and DYMORE and compared with the standard UH-60A rotor for high-thrust-case flight 9017. Results show a reduction in the peak negative pitching moment. The rotor efficiency was shown to improve by 2.9% and the 4/rev component of vertical force reduced by 8%. These performance improvements can be improved with an improved droop schedule and by incorporating improved transonic airfoils.

Analytical formulation of optimum rotor interdisciplinary design with a three-dimensional wake

An analytical formulation of optimum rotor interdisciplinary design is presented. A finite-state aeroelastic rotor model, coupling generalized dynamic wake with blade finite elements, is applied to perform the optimum rotor blade design for improved aerodynamic performance and vehicle vibration, while a feasible direction nonlinear optimizer, CONMIN, provides the optimization algorithm. The approach features a systematic rotor aeroelastic model which offers an efficient analytical tool, and retains necessary aerodynamic and blade dynamic building blocks for a sufficient rotor dynamic response analysis. The formulation is well suited for an efficient design sensitivity computation without resorting to finite difference, and thus provides a practical design tool. The results show improved rotor aerodynamic performance and reduced hub vibratory loads for the optimized blade as compared to the advanced rotor of reference design.

Optimum rotor interdisciplinary design with a finite state aeroelastic system

This paper presents an analytical formulation of an optimum rotor interdisciplinary design. A finite-state aeroelastic rotor model, coupling simultaneously a generalized dynamic wake model with blade finite elements, is applied to perform the optimum rotor blade design for improved aerodynamic performance and vehicle vibration. A feasible direction nonlinear optimizer provides the optimization algorithm. The uniqueness of the present approach is the systematic rotor aeroelastic model, which offers an efficient analytical tool, and retains necessary aerodynamic and blade dynamic building blocks for a sufficient rotor dynamic response analysis. The formulation is well-suited for an efficient design sensitivity computation without resorting to finite differencing, and thus provides a practical design tool.

Development of a Reduced Order Model to Study Rotor/Ship Aerodynamic Interaction

Rotor/ship aerodynamic interaction is a challenging problem that significantly affects various aspects of shipboard rotorcraft operations, including performance, stability, control, and loads. Therefore it is essential to accurately capture this interaction for application in both engineering analysis and real time flight simulation. The complicated aerodynamic interaction requires a first principle based solution for needed accuracy. However, CFD is computationally expensive and cannot be directly used in flight simulation and therefore, CFD generated ship airwake has traditionally been applied using table lookup methods in rotorcraft analysis and flight simulation. These methods are limited by the capability of computational systems to store large amounts of pre-generated ship airwake interference data. Moreover, when mutual aerodynamic interaction is considered, the airwake table is governed by a large number of rotor and ship parameters which further dramatically increases the table dimension and the storage overhead and restricts the ability to perform a rigorous rotor/ship interaction study that can capture all the essential aerodynamic phenomena. A POD based Reduced Order Model (ROM) was developed to address the limitations of using a table lookup method. The ROM is an Ordinary Differential Equation (ODE) approximation of the CFD governing equations that can effectively model the fundamental physical phenomena in a computationally efficient manner. The ROM was calibrated to improve its stability and was validated for a 2-D cylinder case where it showed excellent correlation with CFD results. This ROM was further tested for an isolated ship case and one with a coupled rotor disk and ship model. The model shows reasonable predictions for both the cases, though the rotor/ship case is seen to be more complex to model using ROM due to the strong coupling between rotor and ship airwake that creates a highly unsteady aerodynamic environment.

A VPM/CFD Coupling Methodology to Study Rotor/Ship Aerodynamic Interaction

The air disturbance (or airwake) resulting from the interaction of ship motion and air flow, has a significant effect on shipboard rotorcraft operations. It is essential to develop capabilities to accurately model the rotor/ship aerodynamic interactions which can subsequently be used to implement an efficient and accurate virtual dynamic interface (VDI) simulation. The aerodynamic interaction of the rotorcraft and ship involves the mutual interference between the rotors, fuselage and ship structure. The ship airwake produces unsteady loads on the helicopter as a result of shear layers and turbulence. Therefore modeling rotor/ship aerodynamic interaction requires a high fidelity approach due to complex physical mechanisms that drive the flow phenomena. The interactions between rotor and ship have known to be highly non-linear, therefore the airwake due to an isolated ship has different flow characteristics compared to the ship airwake in presence of a rotorcraft. Moreover the coupled airwake has a feedback effect on the rotor wake geometry and vorticity strength which creates a two-way coupling scenario. In this study, a methodology was developed to couple a CFD ship airwake solver with a viscous Vortex Particle Method (VPM) for rotor wake solution to study rotor/ship aerodynamic interaction. The CFD and VPM solvers were used to compute the time accurate ship airwake and rotor wake, respectively, from first principles. This coupling methodology is demonstrated for a helicopter ground effect study. The coupling simulations were subsequently run for helicopter performing shipboard operations. The simulations showed the complex interactions of ship airwake with rotor wake including the ground effect that rotor experiences in the vicinity of the deck.