Consideration of Unsteady Aerodynamics and Boundary-Layer Transition in Rotorcraft Airfoil Design

Open Access
Author:
Oliveira Vieira, Bernardo A
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 05, 2017
Committee Members:
  • Mark D. Maughmer, Dissertation Advisor/Co-Advisor
  • Mark D. Maughmer, Committee Chair/Co-Chair
  • Edward C. Smith, Committee Member
  • Sven Schmitz, Committee Member
  • Christopher D. Rahn, Outside Member
  • Michael P. Kinzel, Special Member
Keywords:
  • Airfoil Design
  • Boundary-Layer Transition
  • Unsteady Aerodynamics
  • Dynamic Stall
  • Helicopter Aerodynamics
  • Transonic Flow
  • Compressible Flow
  • Computational Fluid Dynamics
  • CFD
Abstract:
Traditional rotorcraft airfoil design is based on steady-state aerodynamics, despite the many sources of unsteady-flow in forward flight. At high-thrust and high-speed conditions, the rotor may be susceptible to dynamic stall; consequently, large margins are necessary to prevent fatigue loads on the blades and pitch links, limiting operation under high altitudes, payload, and temperatures, as well as during maneuvers. This work revises typical design requirements and proposes new ways to qualify airfoils in dynamic stall. A number of design studies are conducted to better understand the relation between airfoil shape and dynamic stall behavior. The design manipulations are handled by an inverse-design, conformal mapping method, and unsteady Reynolds-averaged Navier-Stokes equations are used to predict the unsteady aerodynamic performance. In unsteady flow, the occurrence of aerodynamic lags in the development of pressures, boundary-layer separation, and viscous-inviscid interactions suggest more strict requirements than in steady flow. In order to postpone the onset of dynamic stall, the design needs to handle competing leading- and trailing-edge separation mechanisms, which are heavily influenced by shock waves and laminar-turbulent transition effects. It is found that a particular tailoring of the trailing-edge separation development can provide adequate dynamic stall characteristics and minimize penalties in drag and nose-down pitching moments. At the same time, a proper design of the nose shape is required to avoid strong shock waves and prevent premature stall. A proof-of-concept airfoil is developed to improve dynamic stall behavior, while meeting other stringent requirements. Performance calculations using information obtained from comprehensive analysis (RCAS) based on a UH-60A helicopter suggest that an expansion of the operational envelope is possible, while also reducing hover drag, maintaining low pitching moments, and providing reasonable margins to drag rise at the maximum speed of the UH-60A helicopter. Finally, pitching wing calculations are conducted to demonstrate the proposed concepts in three-dimensional flow. The new wing experiences a more favorable dynamic stall inception and considerable decreases in the integrated peak pitching moments compared to traditional designs.