A Coupled CFD/CSD Investigation of the Effects of Leading Edge Slat on Rotor Performance

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A coupled Computational Fluid Dynamic (CFD) and Computational Structural

Dynamics (CSD) methodology is extended to analyze the effectiveness of a

leading edge slat (LE-Slat) for mitigating the adverse effects of dynamic stall on

rotor blade aerodynamic and dynamic response. This involved the following improvements

over the existing CFD methodology to handle a multi-element airfoil

rotor: incorporating the so-called Implicit Hole Cutting method for inter-mesh

connectivity, implementing a generalized force transfer routine for transferring

LE-Slat loads onto the main blade, and achieving increased parallelization of the


Initially, the structured overset mesh CFD solver is extensively validated

against available 2-D experimental wind tunnel test cases in steady and unsteady

flight conditions. The solver predicts the measurements with sufficient accuracy

for test cases with both the baseline airfoil and that with two slat configurations,

S-1 and S-6. As expected, the addition of the slat is found to be highly effective

in delaying stall until larger angles for the case of a static airfoil and ameliorating

the effects of dynamic stall for a 2-D pitching airfoil. The 3-D coupled CFD/CSD

model is extensively validated against flight test data of a UH-60A rotor in a high-altitude,

high-thrust flight condition, namely C9017, characterized by distinct dynamic stall events

in the retreating side of the rotor disk.

The validated rotor analysis tool is then used to successfully demonstrate the

effectiveness of a LE-Slat in mitigating (or eliminating) dynamic stall on the rotor

retreating side. The calculations are performed with a modified UH-60A blade

with a 40%-span slatted airfoil section. The addition of the slat is effective in the

mitigation (and/or elimination) of lift and moment stall at outboard stations,

which in turn is accompanied by a reduction of torsional structural loads (upto 73%)

and pitch link loads (upto 62%) as compared to the baseline C9017 values.

The effect of a dynamically moving slat, actuating between slat positions

S-1 and S-6, is thoroughly investigated, firstly on 2-D airfoil dynamic stall, and

then on the UH-60A rotor. Three slat actuation strategies with upto [1, 3, 5]/rev

harmonics, respectively, are considered. However, it is noted that the dynamic

slat does not necessarily result in better rotor performance as compared to a

static slat configuration.

The coupled CFD/CSD platform is further used to successfully demonstrate

the capability of the slat (S-6) to achieve upto 10% higher thrust than C9017,

which is beyond the conventional thrust limit imposed by McHugh's stall boundary.

Stall mitigation due to the slat results in a reduction of torsional load upto

54% and reduction of pitch link load upto 32% as compared to the baseline

C9017 flight test values, even for an increase in thrust of 10%.