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An annular lift fan aircraft configuration with high disk loading, low empty weight, and acceptable hover efficiency is assessed using numerical methods. The fans have short and very thin blades.With the fan disk loading increased from 28.37 to 91 lb/ft2, the figure of merit is 0.658 but the power loading drops from 9.03 to 4.65 lb/hp. However, when the aircraft is in ground effect within one aircraft radius to the ground plane, the power loading increases almost 50% to 6.77 lb/hp, which is comparable to helicopters and general aviation aircraft. The design features jet thrusters for forward flight propulsion, where the maximum requirement for jet thrust with the aid of a skirt in transition is 0.448 of the aircraft weight. Turbofan engines with a thrust-to-weight ratio of 0.3 and an augmentation ratio of 1.6-2.0 can satisfy this requirement and enable a top speed of Mach 0.62 (759 km/h) based on drag predictions. Shutters are proposed to close off the annular duct during cruise, and results show that they can close smoothly at the end of transition without causing oscillations of lift and drag. [ABSTRACT FROM AUTHOR]

Comparisons of rotor aerodynamic performance parameters in hover, between rotorcraft comprehensive analysis predictions and measured data, for two model-scale and two full-scale coaxial rotors are presented. The comprehensive analysis predictions are from the U.S. Army’s Rotorcraft Comprehensive Analysis System (RCAS) using a free vortex wake model. Predictions from a computational fluid dynamics analysis are also included for one of the full-scale rotors to provide additional insights. RCAS predictions form sweeps in both the coaxial rotor system thrust and the axial separation distance between the two rotors. The RCAS predictions are mostly in good agreement with both measured data and the computational fluid dynamics predictions, including reflecting the same trends from all sweeps. The RCAS predictions also show that even though the coaxial rotor figure of merit is not strongly dependent on the thrust sharing ratio between the two rotors, the figure of merit of the two individual rotors is highly sensitive to the thrust sharing ratio. [ABSTRACT FROM AUTHOR]

Wind tunnel measurements of the rotor trim, blade airloads, and structural loads of a full-scale UH-60A Black Hawk main rotor are compared with calculations obtained using the comprehensive rotorcraft analysis CAMRAD II and a coupled CAMRAD II/OVERFLOW2 analysis. A speed sweep at constant lift up to an advance ratio of 0.4 and a thrust sweep at constant speed into deep stall are investigated. The coupled analysis shows significant improvement over comprehensive analysis. Normal force phase is better captured for all the test conditions examined. Pitching moment is better predicted, including the magnitude and phase of the two stall events in the fourth quadrant at the deeply stalled condition. Structural loads are, in general, improved with the coupled analysis, but the magnitude of chord bending moment is still significantly underpredicted. As there are three modes around 4 and 5/rev frequencies, the structural responses to the 5/rev airloads due to dynamic stall are magnified and thus accurate analysis of the deeply stalled condition is challenging. [ABSTRACT FROM AUTHOR]