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. ρ = air density σ = rotor solidity ψ = rotor azimuth angle, 0 aft, deg

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.

Blade section normal force and pitching moment were investigated for six rotors operating at transition and high speeds: H-34 in flight and wind tunnel, SA 330 (research Puma), SA 349/2, UH-60A full-scale and BO-105 model (HART-I). The measured data from flight and wind tunnel tests were compared with calculations obtained using the comprehensive analysis CAMRAD II. The calculations were made using two free wake models: rolled-up and multiple-trailer with consolidation models. At transition speed, there is fair to good agreement for the blade section normal force between the test data and analysis for the H-34, research Puma, and SA 349/2 with the rolled-up wake. The calculated airloads differ significantly from the measurements for the UH-60A and BO-105. Better correlation is obtained for the UH-60A and BO-105 by using the multiple-trailer with consolidation wake model. In the high speed condition, the analysis shows generally good agreement with the research Puma flight data in both magnitude and phase. However, poor agreement is obtained for the other rotors examined. The analysis shows that the aerodynamic tip design (chord length and quarter chord location) of the Puma has an important influence on the phase correlation. NotationC T rotor thrust coefficient G strength of trailed vorticity M Mach number N b number of blades r blade radial station r C centroid of vorticity r G moment (radius of gyration) of vorticity R blade radius Re Reynolds number α T PP tip path plane tilt angle, positive forward Γ bound circulation µ advance ratio σ solidity

Blade flap and chord bending and torsion moments are investigated for five rotors operating at transition and high speed: H-34 in flight and wind tunnel, SA 330 (research Puma), SA 349/2, UH-60A full-scale, and BO-105 model (HART-I). The measured data from flight and wind tunnel tests are compared with calculations obtained using the comprehensive analysis CAMRAD II. The calculations were made using two free wake models: rolled-up and multiple-trailer with consolidation models. At transition speed, there is fair to good agreement for the flap and chord bending moments between the test data and analysis for the H-34, research Puma, and SA 349/2. Torsion moment correlation, in general, is fair to good for all the rotors investigated. Better flap bending and torsion moment correlation is obtained for the UH-60A and BO-105 rotors by using the multiple-trailer with consolidation wake model. In the high-speed condition, the analysis shows generally better correlation in magnitude than in phase for the flap bending and torsion moments. However, a significant underprediction of chord bending moment is observed for the research Puma and UH-60A. The poor correlation of the chord bending moment for the UH-60A appears to be caused by both the airloads model (at all radial locations) and the lag damper model (mostly at inboard locations).

A study was conducted to assess the capability of active controls to produce rotor performance improvements. The calculations were conducted using the comprehensive analysis CAMRAD II. The baseline rotor considered was a four-bladed articulated rotor with a VR-12 airfoil. Seven active control concepts were investigated: leading edge slat, variable droop leading edge, oscillatory jet, Gurney flap, individual blade control, active twist, and trailing edge flap. Comparative performance calculations have been conducted for two cases: (1) an airspeed sweep at C T /σ = 0.075 to examine rotor lift-to-drag ratio and (2) thrust sweeps at fixed airspeeds to estimate maximum blade loading capability. The individual blade control, active twist, and trailing edge flap concepts improved the rotor lift-to-drag ratio with a 2/rev harmonic control. The remaining four concepts-leading edge slat, variable droop leading edge, oscillatory jet, and Gurney flap-increased the blade maximum loading capability when these devices were used over the retreating side of the rotor disk.