A Local Analysis of the Fluid Dynamics and Flow Physics of Dynamic Stall on Helicopters

Funded by ARO

Helicopter aerodynamics encompasses a variety of complicated and inherently nonlinear flow phenomena. At high advance ratios the angle of attack of the retreating blade can become large enough for dynamic stall to occur. The rotation of the blades also results in a cyclic surging motion and relative yaw as well as rotational effects. The flow on the advancing blade near the tip can become locally supersonic. Progress in the understanding of dynamic stall for helicopters is hindered by the great complexity of the associated fluid dynamics and the large range of time and length scales, by the fact that in flight measurements are expensive, that flow similarity (Reynolds and Mach number, blade motion) is difficult to accomplish in the laboratory, and that numerical simulations of entire blades are involved and expensive.

Angle of attack distribution on helicopter rotor in forward flight obtained from simplified analysis. The angle of attack for the retreating blade (bottom) is very large to maintain a zero roll moment. Inside the small blue area near the hub the relative wind is negative.

Angle of attack distribution on helicopter rotor in forward flight obtained from simplified analysis. The angle of attack for the retreating blade (bottom) is very large to maintain a zero roll moment. Inside the small blue area near the hub the relative wind is negative.

A combined investigative approach of Computational Fluid Dynamics (CFD) and advanced data analysis is taken to advance the understanding of dynamic stall on helicopters. Instead of carrying out simulations of entire rotor blades simpler “canonical” model problems based on blade section simulations are investigated that capture relevant “subsets” of the fluid dynamics. The main hypothesis behind this approach is that by investigating different mechanisms separately and in combination insight will be gained into the nonlinear interactions.

Implicit large eddy simulations are being carried out for the Sikorsky SSC-A09 airfoil at AoA=13deg and Re=200,000. At this operating point the airfoil stalls at about 9deg angle of attack. Preliminary results for a pitch oscillation with an amplitude of 13deg AoA and a reduced frequency of k=0.31 show the periodic shedding of a dynamic stall vortex and a considerable lift and moment coefficient hysteresis.

SSC-A09 airfoil pitching around quarter-chord point, k=0.31, AoA=13deg; Q=1 iso-surfaces colored by Mach number, 0<M<1

SSC-A09 airfoil pitching around quarter-chord point with k=0.31; Q=1 iso-surfaces colored by Mach number, 0<M<1

Lift and moment coefficient obtained from ILES for Re=200,000 and hybrid simulations for Re=3.5million

Lift and moment coefficient obtained from ILES for Re=200,000 and hybrid simulations for Re=3.5million

  1. Wen, G., and Gross, A., “Numerical Investigation of Helicopter Blade Section Undergoing Time-Periodic Motions,” AIAA-2017-0078, 55th AIAA Aerospace Sciences Meeting, 9-13 January 2017, Grapevine, TX
  2. Gross, A., and Wen, G., “Numerical Investigation of Helicopter Blade Section Undergoing Time-Periodic Motions,” AIAA-2016-4244, 8th AIAA Flow Control Conference, 13-17 June 2016, Washington, D.C.