A plasma actuator was used to control leading-edge flow separation and dynamic stall vortex on a periodically oscillated NACA 0015 airfoil. The effectiveness of the actuator was documented through phase-conditioned surface pressure measurements and smoke flow visualization records. The airfoil was driven in a periodic cycle corresponding to α = 15 deg+10deg sinwt. The results presented here are for a reduced frequency of k = ωc/2U ∞ = 0.08. Three cases of control with the plasma actuator were investigated: open-loop control with steady plasma actuation, open-loop control with unsteady plasma actuation, and closed-loop control with steady plasma actuation. For closed-loop control, the actuator was activated in selected portions of the oscillatory cycle based on angle-of-attack feedback. All of the cases investigated exhibited an increase in cycle-integrated lift with improvements in the lift-cycle hysteresis. In two cases, the pitch-moment stall angle was delayed and in one of these, the adverse negative moment peak was significantly reduced.

Separation Control Using Plasma Actuators: Dynamic Stall Vortex Control on Oscillating Airfoil

Dynamic stall is an incredibly rich fluid dynamics problem that manifests itself on an airfoil during rapid, transient motion in which the angle of incidence surpasses the static stall limit. It is an important element of many manmade and natural flyers, including helicopters and supermaneuverable aircraft, and low–Reynolds number flapping-wing birds and insects. The fluid dynamic attributes that accompany dynamic stall include an eruption of vorticity that organizes into a well-defined dynamic stall vortex and massive excursions in aerodynamic loads that can couple with the airfoil structural dynamics. The dynamic stall process is highly sensitive to surface roughness that can influence turbulent transition and to local compressibility effects that occur at free-stream Mach numbers that are otherwise incompressible. Under some conditions, dynamic stall can result in negative aerodynamic damping that leads to limit-cycle growth of structural vibrations and rapid mechanical failure. The mechanisms leading to negative damping have been a principal interest of recent experiments and analysis. Computational fluid dynamic simulations and low-order models have not been good predictors so far. Large-eddy simulation could be a viable approach although it remains computationally intensive. The topic is technologically important owing to the desire to develop next-generation rotorcraft that employ adaptive rotor dynamic stall control.

Dynamic Stall in Pitching Airfoils: Aerodynamic Damping and Compressibility Effects

A computational fluid dynamics (CFD) code and rotorcraft computational structural dynamics (CSD) code are coupled to calculate helicopter rotor airloads across a range of flight conditions. An iterative loose (weak) coupling methodology is used to couple the CFD and CSD codes on a per revolution, periodic basis. The CFD code uses a high fidelity, Navier‐Stokes, overset grid methodology with first principles-based wake capturing. Modifications are made to the CFD code for the aeroelastic analysis. For a UH-60A Blackhawk helicopter, three challenging level flight conditions are computed: 1) high speed, μ = 0.37, with advancing blade negative lift, 2) low speed, μ = 0.15, with blade‐vortex interaction, and 3) high thrust with dynamic stall, μ = 0.24. Results are compared with UH-60A Airloads Program flight test data. For all cases the loose coupling methodology is shown to be stable, convergent, and robust with full coupling of normal force, pitching moment, and chord force. In comparison with flight test data, normal force and pitching moment phase and magnitude are in good agreement. The shapes of the airloads curves are well captured. Overall, the results are a noteworthy improvement over lifting line aerodynamics used in rotorcraft comprehensive codes.

https://rotorcraft.arc.nasa.gov/publications/files/Potsdam2_AHS04.pdf

Rotor Airloads Prediction Using Loose Aerodynamic/Structural Coupling

Compressibility effects on dynamic stall

Parallel domain connectivity algorithm for unsteady flow computations using overlapping and adaptive grids

Flight Testing the UH-60A Airloads Aircraft

: Flight test measurements of the performance of the UH-60 Black Hawk helicopter with both standard and advanced rotors are compared with calculations obtained using the comprehensive helicopter analysis CAMRAD II In general, the calculated power coefficient shows good agreement with the flight test data However, the accuracy of the calculation degrades at high gross weight for all of the configurations The analysis shows fair to good correlation for collective and longitudinal cyclic angles and pitch attitude, and poor to fair correlation for the lateral trim quantities "lateral cyclic angle and roll attitude" The increased solidity of the wide chord blade appears to be a dominant factor in the performance improvement at high gross weight by reducing blade loading and thus delaying stall

https://rotorcraft.arc.nasa.gov/publications/files/Yeo_AHS2002.pdf

William G. Bousman

Papers

Flight Testing the UH-60A Airloads Aircraft

Performance Analysis of a Utility Helicopter with Standard and Advanced Rotors

An Experimental Investigation of the Effects of Aeroelastic Couplings on Aeromechanical Stability of a Hingeless Rotor Helicopter

A Qualitative Examination of Dynamic Stall from Flight Test Data

High Load Conditions Measured on a UH-60A in Maneuvering Flight